Solar Collectors. Solar Energy R&D in the European Community. Test Methods and Design Guidelines. Solar Energy Applications to Dwellings

Transcription

1 Solar Energy R&D in the European Community Series A Volume O Solar Energy Applications to Dwellings Solar Collectors Test Methods and Design Guidelines D. Reidel Publishing Company Dordrecht / Boston / Lancaster for the Commission of the European Communities

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5 Solar Collectors

6 Solar Energy R&D in the European Community Series A: Solar Energy Applications to Dwellings Volume 6

7 Solar Energy R&D in the European Community Series A Volume 6 Solar Energy Applications to Dwellings Solar Collectors Test Methods and Design Guidelines by W.B.GILLETT Sir William Halcrow and Partners, Swindon, U.K. and J. E. MOON Solar Energy Unit, University College, Cardiff, U.K. PAR-. ; ' : -\ 'r-'ioth. N.C.. L LLM?* CL D. Reidel Publishing rnmphny ^ A MEMBER OF THE KLUWER ACADEMIC PUBLISHERS CROUP H B Dordrecht / Boston / Lancaster for the Commission of the European Communities

8 Library of Congress Cataloging in Publication Data Gillett, W. B. Solar collectors. ne (Solar energy R&D in the European Community. Series A, Solar energy applications to dwellings ; v. 6) Bibliography: p. 1. Solar collectors-testing. 2. Solar collectors-design and construction. I. Moon, J. E. II. Commission of the European Communities. Directorate-General for Science, Research, and Development. III. Title. IV. Series. TJ812.G ISBN This publication was prepared under contract for the Commission of the European Communities Directorate-General Science, Research and Development, Brussels Publication arrangements by Commission of the European Communities Directorate-General Information Market and Innovation, Luxembourg EUR ECSC, EEC, EAEC, Brussels and Luxembourg LEGAL NOTICE Neither the Commission of the European Communities nor any person acting on behalf of the Commission is responsible for the use which might be made of the following information. Published by D. Reidel Publishing Company P.O. Box 17, 3300 AA Dordrecht, Holland Sold and distributed in the U.S.A. and Canada by Kluwer Academic Publishers, 190 Old Derby Street, Hingham, MA 02043, U.S.A. In all other countries, sold and distributed by Kluwer Academic Publishers Group, P.O. Box 322, 3300 AH Dordrecht, Holland All Rights Reserved No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the copyright owner. Printed in The Netherlands

9 CONTENTS Preface Acknowledgements The CEC Solar Collector Testing Group Participation in the CEC Solar Collector Testing Group Recent CEC Publications in the Field of Solar Energy Applications to Dwellings vi vii ix xi xii EXECUTIVE SUMMARY 1 Part One COLLECTOR TEST METHOD RECOMMENDATIONS Chapter 1 Introduction to Collector Testing 13 2 Solar Collector Performance 15 3 Units and Symbols 21 4 Definitions 25 5 Collector Mounting and Location 33 6 The Test Installation 39 7 Instrumentation for Use in Collector Testing 47 8 Collector Performance Analysis 81 9 Optical Performance Measurements Thermal Performance Tests for Water Heating Collectors Thermal Performance Tests for Air Heating Collectors Collector Testing using Oil as the Heat Transfer Fluid Durability and Reliability Test Methods 149 References for Collector Testing 161 Part Two GUIDELINES FOR SOLAR COLLECTOR DESIGN Chapter 1A Introduction to the Design Guidelines General Collector Design Considerations Collector Design Problems Observed in Solar Heating Installations Good Collector Design Features Materials Considerations Thermal Performance Conclusions and Recommendations on Collector Design 255 Bibliography 259 Appendix I Solar Simulator Design 263 II_ Recommended Nomenclature 277 III Test Format Sheets 283 IV Properties of Water and Air for Analysis of Collector Test Results 299 V Inspection Report Format Sheets for the Reliability and Durability of Solar Collectors 303

10 PREFACE The performance rating of solar collectors for heating applications is a complex task. This book provides a comprehensive review of the most relevant ways and means for collector testing; it is based on eight years of extensive work under the European Communities' Solar Energy R&D Programme. In a concerted action with 20 laboratories across Europe, the Commission of the European Communities conducted a series of test runs on common collector types for this purpose. On this basis the recommendations contained in this book were compiled. In the course of the programme, an impressive record of experience was accumulated which made it possible to draw up some general guidelines for collector design. They are also presented as a separate part of this book. The programme for solar collector testing was first initiated at the Commission by Dr. E. Aranovitch under the direction of Dr. A. Strub. The E.C. Joint Research Centre, Ispra Establishment, was a major participant. Later on, the Commission entrusted the University College Cardiff with the coordination of the European Solar Collector Testing Group. The quality of results achieved is largely due to the excellence and competence of the two coordinators: Mr. W. Gillet and Mr. J. Moon who also undertook to write this book under the direction of Prof. B. Brinkworth. It should be mentioned that not all the symbols in this book correspond to the latest recommendations on solar radiation symbols which were recently introduced by the E.C. Commission. Here the reader should also refer to the symbols published in the "European Solar Radiation Atlas". I express my hope that many designers, producers and appliers of solar energy conversion systems will make use of the information which we are able to present in this important reference book. It is also expected that the recommendations will be considered by the international committees who have competence for establishing standards in this field. W. Palz Head of the Solar Energy R&D Programme Commission of the European Communities

11 ACKNOWLEDGEMENTS The authors would like to thank the many experts who have contributed their experience to the compilation of this book, including both past and present members of the CEC Collector Testing Group. We are particularly grateful to A. Derrick, who produced the first CEC Recommendations for European Solar Collector Test Methods on which much of this book is based. Many specialists have given freely of their time to assist with the preparation of this document, but we would like to express particular thanks to M. Antinucci, E. Aranovitch, B. J. Brinkworth, J. L. Chevalier, A. A. Green, A. J. Th. M. Wijsman and S. J. Wozniak for their help with refining the final text. In addition to the individuals mentioned above, valuable contributions were received from H. Birnbreier (D), C. Boffa (I), J. Bougard (B), C. Boussemaere (B), D. Braggion (I), J. Cardi (F), B. Cross (UK), B. Devin (F), R. Dietz (F), W. Dutrg (B), D. Gilliaert (JRC), K. Gindele (D), J. Gregan (Ir), H. Hettinger (JRC), A. Hoess (D), W. Ley (D), C. den Ouden (NL), P.V. Pedersen (Dk), J. Renoville (Lux), M. Reuss (D), G. Riesch (JRC), C. Roumengous (JRC), D. Royer (F), W. Scholkopf (D), F. Simonis (NL), H. Stein (D), S. Svendsen (Dk), R. Torrenti (F), M. Tsamparlis (Gr) and R. de Vaan (NL). We would like to express particular thanks also to Mrs. D. Price for her help and painstaking work in the preparation of the typescript. This publication was produced under the direction of W. Palz and T. C. Steemers of the Commission of the European Communities DG XII, and of E. Aranovitch of the CEC Joint Research Centre, Ispra.

12 Figures The majority of the illustrations in this publication have been prepared with the assistance of the staff of the Solar Energy Unit at University College, Cardiff. In addition the authors are indebted to the following organisations for their kind permission to use photographic and other material for illustrations. Building Research Establishment, Garston, England: Figures: 15.6, 16.15, 16.16, CSTB, Sophia Antipolis, France: Figures: 16.1, 16.2, 16.4, 16.7, 16.10, 16.13, 16.17, 16.19, 16.20, E.C. Joint Research Centre, Ispra, Italy: Figure Katholieke Universiteit, Leuven, Belgium: Figure 4.2. K.F.A. Juelich, W. Germany: Figure 5.2. Philips, Eindhoven, Netherlands: Figures 15.3, Phoebus, Catania, Italy: Figures 10.6, Politecnico di Torino, Italy: Figure Thermal Insulation Laboratory, Lyngby, Denmark: Figure TNO, Delft, Netherlands: Figures 10.7, TUV, Muenchen, W. Germany: Figure 13.4.

13 THE CEC SOLAR COLLECTOR TESTING GROUP Solar Collector Testing forms one of the concerted actions within the solar energy R&D programme of the Commission of the European Communities. Approximately 20 laboratories participate in a collaborative group, which has been actively engaged in the development of solar collector test methods since Thermal performance test methods are studied by carrying out "round robin" tests on commercially available collectors, and the experience gained is then collated and published. All types of thermal collector are of interest to the group, including flat plate liquid and air heating collectors, evacuated, concentrating, and high temperature collectors. Durability and reliability aspects of solar collectors, which are at least as important as thermal performance from a cost-effectiveness point of view, are also studied. Collectors are monitored during long term ageing outdoors, durability inspections are performed on collectors in working systems, and laboratory test methods are used. These studies have been used to provide a basis for guidelines on collector design, and a set of recommended procedures for testing commercial collectors. Methods are under development for testing domestic solar water heating systems, but experience is insufficient at the present time to permit recommendations to be included in this publication. The technical direction of the Group is provided by the CEC Joint Research Centre at Ispra, where extensive indoor and outdoor test facilities have been established. The European Solar Test Installation (ESTI) at Ispra, which includes a range of solar simulators and accelerated ageing test facilities, provides essential support for the work of the Collector Testing Group. Participants in the Group come from laboratories in each of the countries in the European Community. Some laboratories specialise in thermal performance testing, some in durability testing and some are involved in both. The names of the participating laboratories from each country are listed overleaf, and their locations are indicated on the map below.

14 The work of the Collector Testing Group is directed by T.C. Steemers and E. Aranovitch for the Commission of the European Communities. Locations of Laboratories Participating in the Collector Testing Group

15 PARTICIPATION IN THE CEC SOLAR COLLECTOR TESTING GROUP BELGIUM: DENMARK: E.C. : FRANCE: GERMANY: GREECE: IRELAND: ITALY: LUXEMBOURG: NETHERLANDS: UNITED KINGDOM: CRES, Faculté Polytechnique de Mons; Katholieke Universiteit, Leuven. Technical University of Denmark, Copenhagen. Joint Research Centre, Ispra, Italy. CEA, Saclay & Cadarache; Ecole Nationale Supérieure des Mines, Sophia Antipolis; CSTB, Sophia Antipolis; CETIAT, Lyon; Electricity de France (until 1981); Ecole Supfirieure d'ingenieurs de Marseilles (until 1979). KFA, Juelich; BLL Technische Universitaet Muenchen; Universitaet Stuttgart; TUV ev Bayern, Muenchen; Ludwig-Maximilians-Universitaet Muenchen, DFVLR, Cologne (until 1982); Brown Boveri & Cie, Heidelberg (until 1982). University of Athens. Institute for Industrial Research & Standards and University College, Dublin. CRAIES, Verona; Politecnico di Torino; Phoebus, Catania, Sicily; Fiat, Torino (until 1979); Zanussi, Pordenone (until 1981). GRADEL S.A., Steinfort. TPD-TNO-TH, Delft; Philips, Eindhoven. Building Research Establishment, Garston; University College Cardiff.

16 RECENT CEC PUBLICATIONS IN THE FIELD OF SOLAR ENERGY APPLICATIONS TO DWELLINGS Palz, W. and Steemers, T.C. "Proceedings of the EC Contractors Meeting held in Athens (Greece) November 1981", EUR 7664, Reidel (1982). Palz, W. and den Ouden, C. "Proceedings of the EC Contractors Meeting held in Meersburg (F.R.G.) June 1982", EUR 8046, Reidel (1983). Turrent, D., Baker, N., Steemers, T.C. and Palz, W. "Solar Thermal Energy in Europe: An Assessment Study", EUR 8473, Reidel (1983). Palz, W and Steemers, T.C. "Solar Houses in Europe: How they have Worked", EUR 7109, Pergamon Press (1981). Turrent, D., Godoy, R. and Ferraro, R. "Solar Water Heating", EUR 8003, Commission of the European Communities (1981). Godoy, R., Turrent, D. and Ferraro, R. "Solar Space Heating", EUR 8004, Commission of the European Communities (1982). Ferraro, P., Godoy, R. and Turrent, D. "Monitoring Solar Heating Systems: A Practical Handbook", EUR 8005, Pergamon Press (1983). Aranovitch, E. and Gillett, W.B. "Workshop on Solar Simulators Proceedings", CEC Joint Research Centre, Ispra, Italy (1982). Moon, J.E. "Results and Analysis of a Round Robin Test Series using Solar Simulators", EUR 8006, Commission of the European Communities, DGXII, Brussels (1982). Moon, J.E. "Results and Analysis of a Round Robin Series using an Evacuated Tubular Collector", EUR 8757, Commission of the European Communities, DGXII, Brussels (1983). Van Galen, E. "Recommendations for European Solar Storage Test Methods (Sensible Heat and Latent Heat Storage Devices)" (to be published). Further information on the work of the CEC on Solar Energy Applications to Dwellings may be obtained from T.C. Steemers, Commission of the European Communities, DG XII, 200 Rue de la Loi, B1049 Brussels, Belgium.

17 EXECUTIVE SUMMARY E.l INTRODUCTION Collaborative work on the development of solar collector test methods for use in Europe began in 1975 with a meeting at the Joint Research Centre, Ispra, to establish the CEC Solar Collector Testing Group. The Group has enjoyed active participation by experts from each of the countries in the European Community, and has tested a wide range of commercially available collectors. In 1978 a Group co-ordinator was appointed to analyse the results and test procedures from the different countries, and to draw together the combined experience of the Group. To do this, discussions were held with each participating laboratory and at the regular 6 monthly meetings of the experts' Group. The first set of recommendations from the Group on test facilities, instrumentation and test procedures was published by the CEC in January Work has continued since 1980 and the test methods have been developed further. Part One of this book contains the latest compilation of recommendations agreed by the Group. In several EC countries these recommendations have already been incorporated into national standards. The Format Sheets developed by the Group for the presentation of test results have also been widely adopted. These Formats are included as Appendices to this book. The Collector Testing Group has also been concerned for a long time with the durability and reliability of collectors. A survey of collectors in working systems was therefore carried out in 1981 to identify the most common failure modes. The results, which were obtained from inspections of more than 2500 m 2 of collectors in 85 systems spread between the countries of the EC, led to the recommendations for collector durability test methods which are included in Part One of this book. This survey also provided a major input to the guidelines on collector design which are presented in Part Two. E.2 SOLAR COLLECTOR PERFORMANCE The thermal performance of a solar collector may be expressed in the form of a linear performance characteristic relating the rates of useful heat output per unit aperture area (Q/A ), the solar input (G) and a the heat losses as:

18 Q/Aa = n 0 G - U (Tm - Ta) Eqn El. Alternatively, this may be cast in the form of an efficiency equation as: n = ri o U (Tm - Ta)/G Eqn E2. = n - UT* Eqn E3. where T* is called the reduced temperature difference. In this way, the performance of a collector may be defined by only two coefficients no and U. The value of the optical coefficient no. is determined by the solar absorptance of the absorber surface, the solar transmittance of the cover and the efficiency of the absorber as a heat exchanger. The value of the overall heat loss coefficient U depends mainly on the insulating properties of the front cover system, with contributions from losses through the back and sides. Performance characteristics for some typical collectors are given in Figure E.l. Single glazed (matt black) Double glazed (matt black ) Selective (single glazed) Evacuated tubes Unglazed T* (Km? W" 1 ) Figure E.l Typical Instantaneous Efficiency Curves

19 It can be seen from Figure E.l that measured performance characteristics exhibit varying degrees of curvature. This is primarily caused by heat loss coefficients which increase with increasing temperature. The recommended equation for a curved collector performance characteristic is: n = no - ait* - a 2 G(T*) 2 Eqn E4. This characteristic, or its linear equivalent, can be used to compare collectors and to calculate the performance of solar heating systems, since it takes into account the collector temperature and the most important weather parameters (i.e. solar irradiance and air temperature). However, it does not include other important variables such as wind speed, solar radiation incidence angle, and the percentage of diffuse solar radiation. Ways to determine the influence of these parameters, which have a second order effect on the efficiency of most collectors, are presented in Part One of this book. E.3 THERMAL PERFORMANCE TESTS FOR COLLECTORS E.3.1 Introduction The main reason for establishing the Collector Testing Group in 1975 was to develop thermal performance test procedures which would produce repeatable and accurate results wherever they were used. Several procedures were investigated, and it became clear that very careful attention to instrumentation was needed because of the small temperature differences and flowrates involved. Detailed recommendations are therefore given in this book on test facility design and the use of instrumentation. Much of the early work of the Group was directed towards steady state outdoor efficiency testing, but in many parts of Europe such tests can be performed only on a few days in summer when the sky is sufficiently clear. In Northern Europe, most testing in recent years has therefore been carried out with indoor solar simulators to overcome the problems associated with variable conditions outdoors. For laboratories without access to solar simulators, two alternative test procedures have been developed for use in variable outdoor conditions. One involves a mixed indoor/outdoor test in which only the optical coefficient no is determined outdoors. Another approach is to record data rapidly during variable outdoor weather conditions and to process the data in such a way as to deduce the steady state collector parameters.

20 E.3.2 Outdoor Steady State Efficiency Test A steady state test is carried out by mounting a collector outdoors on a clear day and supplying it with fluid at about four well-controlled inlet temperatures, spaced evenly over the typical operating temperature range of the collector. The rate of heat collection is deduced from the fluid mass flow rate and temperature rise, and the collection efficiency is then computed using records of solar irradiance measured during the test. Experience has shown that about 16 data points, each taking between 30 and 60 minutes to measure, are required to give reliable values of no an d U, and that to minimise the scatter of results caused by diffuse solar irradiance and other environmental variables, data should be considered only when the global solar irradiance at the collector aperture is above 600 W m~ 2. The main problem with this test method is that of finding steady outdoor weather conditions in which it can be used. It is reasonably suitable for use in Southern Europe, but in the North many laboratories find that they can test on only a few days per year if they follow these requirements. E.3.3 Steady State Efficiency Test in a Solar Simulator Solar Simulators have been constructed for collector testing in most EC countries. In general, the results obtained in simulators agree well with those obtained outdoors, but there is a need in some simulators to correct performance test results to take account of unrepresentative thermal irradiance and diffuse solar irradiance conditions. The recommended methods of measurement for solar irradiance and air temperature in solar simulators are not the same as those used outdoors because of the spatial variations which occur indoors. However, in most other respects, testing in a solar simulator is similar to steady-state outdoor testing. E.3.4 Indoor Heat Loss and Combined Indoor/Outdoor Tests Combined indoor/outdoor methods were developed primarily for use in Northern Europe where the weather is unsuitable for steady state outdoor testing. The procedure is the same as that used for steady state outdoor testing, but outdoor measurements are made only with a mean collector fluid temperature equal to the local ambient air temperature in order to determine no at the point where T* is zero. This greatly reduces the time required for outdoor testing. The heat losses are then determined indoors by supplying the collector with

21 fluid at about three well controlled inlet temperatures spaced over the typical operating range and measuring the fluid temperature drop on passing through the collector. Combined indoor/outdoor test results have been shown to agree well with steady state outdoor test results for good collectors. However, studies have shown that this procedu're is. only valid for collectors which have a heat loss coefficient that does not vary significantly with temperature. It is unfortunate that the method tends to under-predict the heat loss coefficient and hence to over-predict the efficiency for poor collectors. E.3.5 Transient Outdoor Tests Three test methods have been developed for use in variable outdoor conditions, but none has become very widely used yet. The results obtained with the methods appear to agree well with those from steady state outdoor testing, but where solar simulators are available, simulator testing is now preferred by most laboratories. E.3.6 Air Collector Testing Methods for testing air heating collectors are still under development in Europe. Preliminary recommendations are included in this book, but work is continuing on the subjects of: E.3.7 air leakage accuracy of air flowrate measurements accuracy of air temperature measurements. Oil Heating Collector Testing Collectors which heat the fluid above 100 C generally use oil as the heat transfer fluid. Additional precautions are needed when testing collectors with oil. It is also necessary to take into account the variations in fluid properties with temperature. E.4 DURABILITY AND RELIABILITY TESTS FOR COLLECTORS E.4.1 Qualification Tests Many tests have been identified as potentially suitable for solar collectors, but most are rather expensive to perform and several have been shown to provide little useful information. Taking account of the current level of experience in this field, which it must be recognised

22 is still rather limited, the Group has identified the following shortlist of qualification tests which appear likely to be appropriate for use in Europe and to be reasonably inexpensive. It is perhaps worth emphasising that this list is not exhaustive and that it may well be shown, as experience develops, that other tests will be devised. (a) Internal pressure test of absorber fluid passageways (b) High temperature stagnation test (c) External thermal shock test (d) Rain penetration test of collector module. E.4.2 Natural Ageing With many of the simple (first generation) flat plate collectors durability problems have been identified using the simplest test of all, which is to leave the collector outdoors to age naturally. Natural ageing is of course the absolute reference against which all laboratory tests should be compared, but with a good collector it should not be expected to cause any noticeable changes for many years. E.4.3 Accelerated Ageing It is in the field of accelerated ageing that perhaps the widest range of possible test methods exists, and also the most expensive test methods. There is already widespread international experience of the resistance of most materials to standard ageing tests, and the main task is to ensure that the implications of this experience are reflected in collector design. However, combinations of materials, new manufacturing processes and unusual combinations of humidity, temperature and pollutants can still lead to collector failure, even when tests on individual materials might indicate that designs are satisfactory. The Group therefore considered it important to identify suitable accelerated ageing tests for complete collector modules. Work is still in progress in this field, and opinions vary concerning the most suitable test methods. However, it is generally agreed that one of the most useful accelerated ageing tests is that of exposure to salt mist. E.5 GENERAL COLLECTOR DESIGN CONSIDERATIONS Solar collectors can be designed in many ways. They can incorporate many different materials and be manufactured

23 using a variety of techniques. At this relatively early stage in the development of the solar heating industry, there are few mass produced collectors on the European market, and significant improvements in collector design are still anticipated. The improved designs are not expected to show major increases in collection efficiency however, but rather to have longer lifetimes and lower installed costs. Because of the wide range of design possibilities and the continuing search for new ideas and approaches to the development of durable collectors, it would not be appropriate to recommend any specific design. Instead, the approach is to draw attention to the design requirements and to indicate which design features appear to have been successful and which have failed. General guidelines are given on the design of each of the main components of a flat plate collector, namely: Absorber Transparent Cover Insulation Enclosure and Mountings Sealants and Gaskets. Problems which have been experienced in the field with each of these components are discussed, and reference is made to satisfactory and good design features associated with each component. In addition to the guidelines associated with each collector component, special consideration is given to other aspects of collector design which need to be taken into account, namely: Thermal expansion Weather resistance (wind, rain, snow, freezing) Ventilation and condensation Installation site (shelter, access, tilt, aesthetics). The information contained in the guidelines has been gathered from European experts, from published literature and Codes of Practice, and from studies of working solar heating systems in the countries of the European Community. Not all of it is new. The merit of the compilation is that it represents a considered view of experts from the different countries of the Community. E6 MATERIALS CONSIDERATIONS IN COLLECTOR DESIGN A collector designer needs to take into account several factors which involve the physical properties of

24 materials. In addition he needs to consider many other constraints including costs, availability of materials, his manufacturing capabilities, and local experience. Despite the mistakes made in many first generation collector designs, the experience of the last few years provides a basis for comments on the suitability of some materials for use in solar collectors. New materials will undoubtedly continue to appear in the market place and these will cause differences in design between the collectors currently available and those which will be on the market in a few years' time. Nevertheless, it is expected that many of today's common building materials will continue to be used in solar collectors for some years to come, and their properties are therefore discussed in Part Two of the book. The main properties considered are: Corrosion resistance Thermal expansion Thermal conductivity Mass Specific heat capacity Tables of values are included for the more common materials, and general comments are given on their suitability for use in each of the collector components. Typical values are presented for the optical properties of the commonly used cover materials and absorber surfaces, although it should be borne in mind that these vary depending on the substrates used and the quality of application. E7 PERFORMANCE CONSIDERATIONS IN COLLECTOR DESIGN As indicated in E2, the thermal performance of a collector, which depends on the materials used for each component, and on the component designs, may be characterised by only two parameters: an optical coefficient no anc * an overall heat loss coefficient U. In terms of these parameters, the task of the collector designer can be concisely defined as: "to maximise the value of the optical constant no> whilst at the same time minimising the overall heat loss coefficient U". In addition, he should ensure that initial performance levels will be maintained throughout the lifetime of the collector. The design guidelines include a brief review of the main properties of each collector component which have a direct influence on thermal performance as follows:

25 Cover: Absorber: Enclosure: Insulation: Solar transmittance Thermal transmittance Solar absorptance of the surface Thermal emittance of the surface Fluid passageway (and fin) design Collector efficiency factor (F 1 ) Thermal bridges Ventilation Cover spacing Conductivity Thickness Many studies have been carried out to examine the heat transfer processes which take place in a solar collector, and these are presented in engineering texts. It is not appropriate to reproduce these in detail in this book. Instead, the conclusions from the analyses are presented in practical terms, giving generalised guidance to the collector designer. A bibliography of engineering texts is given for those designers who wish to study the subject to greater depth. E8 CONCLUSIONS The Collector Testing Group of the European Communities has made extensive evaluations of methods of testing solar collectors. Methods for determining thermal performance are recommended, which are suitable for use in the full range of climatic conditions encountered in Europe. These methods are now fairly comprehensive, but developments are still needed, notably in the determination of incidence angle effects and of flow-rate and leakage in air-heating collectors. Tests for durability and reliability of collectors are in an earlier stage of development. Recommendations are given for initial qualification tests to demonstrate the capacity of the collector to resist such influences as internal pressures, temperature at stagnation, thermal shock and rain penetration. Tests for long-term durability await further studies of the correlation between the response to laboratory tests and degradation observed in service. The design requirements for flat plate collectors are now quite well understood, and the designer has a wide range of materials from which to choose. At the present time there is no single design solution which dominates the market, and there appears to be little likelihood of such domination in the near future.

26 The range of materials actually used for collectors in Europe is currently rather limited, but there are many alternatives which might become more widely used in future. The guidelines included in this book are based on field experience with "first generation" collectors, but are sufficiently general to apply to most systems. They can therefore be applied also to new solar collector designs. The costs of solar collectors will become lower as the levels of production are increased. High production levels permit greater investments in machinery, and open up economic possibilities for using materials which cannot be easily employed in small batch production processes. Future development work on collector design can be expected to pursue three main objectives, which are (a) lower installed costs, (b) better collection efficiencies at high temperature, and (c) better durability. -10-

27 PART ONE COLLECTOR TEST METHOD RECOMMENDATIONS

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29 CHAPTER 1 INTRODUCTION TO COLLECTOR TESTING The recommendations given in Part One of this document are based on practical experience gained by the participants of the CEC Collector Testing Group during the development of their test facilities, and on the findings of 'round robin' test series involving a variety of collectors carried out in two R & D programmes directed by the CEC in the period The units used are those of the Systeme International and the definitions of terms are consistent with the recommendations of other published codes as far as possible. The Units used in this document are defined in Chapter 3, and additional units which are used in connection with solar heating systems are given in Appendix II. The participants originally considered the direct adoption of other published collector test methods for use in Europe, but concluded that additional recommendations and test procedures were required in order to make collector testing a reasonable proposition in European weather conditions. In 1980 a set of Recommendations for European Solar Collector Test Methods was published which mainly covered the outdoor testing of flat plate liquid heating collectors. However, much new work has been done since then, and this substantially revised publication now includes procedures for testing in solar simulators, and testing with other heat transfer fluids. Durability and reliability tests are also included, though these are still under development. The performance of solar heating systems depends largely on the performance of the solar collectors employed, and it is therefore particularly important for manufacturers and customers to know how well a collector will perform. The measurement of collector efficiencies has been shown to require specialised facilities and ' careful experimental practices if it is to be performed accurately. As a result, a number of countries have established working groups to develop national Standards in the field of solar collector testing. In 1981 the International Standards Organisation (ISO) also began work on solar heating Standards, and a sub-committee of ISO/TC 180 has begun work on the drafting of an international Standard on collector testing. The experience of participants in the CEC Collector Testing Group has already been employed in the drafting of national Standards in several countries, and is also contributing to the work of the ISO. 13-

30 The main measurements required to establish the quality of a collector are those relating to collection efficiency, and test methods for this are given in Chapter 10. Other tests which permit further characterisation of collector performance and which may be used by manufacturers to develop their products are discussed in Chapters 9, 11, 12 and 13. In most European countries at least one laboratory has been established for carrying out tests on solar collectors, but it is anticipated that new facilities will be needed as the solar heating industry becomes more firmly established. Detailed guidelines for the establishment and operation of collector test facilities are given in Chapters 5, 6, and 7. Specific recommendations for the design of solar simulators are given in Appendix I

31 CHAPTER 2 SOLAR COLLECTOR PERFORMANCE 2.1 GENERAL A solar collector is required to absorb solar radiation and to transfer the absorbed energy into a heat transfer fluid with a minimum of heat loss. In assessing the performance of a collector it is therefore important both to determine its ability to absorb solar radiation and to characterise its heat losses. The principal components and losses are shown in Figure 2.1. [SOLAR RADIATION INPUT! Front h Side heat losses heat losses Figure 2.1 The Flat Plate Solar Collector The ability of a collector to absorb solar radiation is largely determined by the optical properties of its cover and absorber surface. However, there aire also losses, which may be considered as input losses, associated with the transfer of heat from the absorber surface into the heat transfer fluid. These are influenced by the design of the absorber fluid passageways. A collector loses heat from its front, its back and its sides. The back and side losses from a flat plate collector can be minimised by the use of insulation. The heat losses from the front of a flat plate collector are usually the largest component of the overall heat losses. They occur in the form of convection and thermal radiation from the front cover to the environment and can be reduced by designing the

32 collector in such a way that the temperature of the front cover is kept low. This can be achieved by minimising the heat transfer between the absorber and the outer cover. The convective heat transfer between the absorber and the outer cover can be reduced by using a multiple glazing system or by evacuating the space over the absorber. The radiative heat transfer from the absorber to the cover can be reduced by the use of selective surfaces. Further advice on how to design a collector in order to achieve a good thermal performance, whilst at the same time ensuring a long operating life, is given in Part Two. Heat transfer analyses which allow the performance of a collector to be optimised are included in Chapter COLLECTOR EFFICIENCY The performance of a flat plate collector can be expressed simply as Heat output = solar input - heat losses Eqn. 2.1 The heat output may be measured as a temperature rise in the fluid flowing through the collector and can be expressed as Heat output = m c (Te - Ti) Eqn. 2.2 The solar input to the heat transfer fluid is less than the product of the solar irradiance (G) and the collector aperture (Aa), because the transmittance (T) of the collector cover, the absorptance (a) of the absorber surface, and the collector efficiency factor (F 1 ) each reduce the energy reaching the fluid. Solar input = Aa F'ta G Eqn. 2.3 The heat losses may, to a first approximation, be assumed to increase linearly with the difference in temperature between the fluid and its surroundings. They may therefore be expressed in terms of a constant heat loss coefficient (U) as Heat losses = Aa U(Tm - Ta) Eqn. 2.4 By combining the above equations, the performance characteristic of a flat plate collector becomes: m c (Te - Ti) = Aa F'xa G - Aa U(Tm - Ta) Eqn. 2.5

33 The product F'TCX is often written as n 0 and known as ETA ZERO. The efficiency of a collector n may be defined as the rate of heat output expressed as a fraction of the incident solar flux in steady state conditions, and may be determined by dividing Equation 2.5 throughout by the flux AaG to give 10 - U(Tm - Ta)/G Eqn. 2.6 This equation is widely known as the 'Hottel-Whillier-Bliss equation' or the 'Efficiency Curve 1 for a collector. For most flat plate collectors the efficiency curve can be reasonably well approximated by the straight line relationship of Equation 2.6, and consequently the efficiency characteristics of a collector can be presented in terms of only two on tne coefficients, no and U. The intercept no efficiency axis represents the maximum collection efficiency condition when there are no heat losses from the collector. The slope U of the curve is the overall heat loss coefficient of the collector. F" >N o c 0) "<J it o> i_ o u 0) "o o Intercept T] 0 ^~ Slope U ^^^^ Reduced temperature difference (Tm-Ta)/G Figure 2.2 Collector Efficiency Curve - 17-

34 In Chapter 8 it is shown that there are several other environmental parameters which influence the efficiency with which a collector will operate. However, for most practical purposes a standard collector efficiency test under specified environmental conditions is sufficient to determine the two characteristic coefficients and to allow a system designer to make system performance estimates. Test methods to meet this need are discussed in Chapter 10. For the optimisation of collector design, and for evaluating the suitability of new materials, it may be necessary to consider certain aspects of collector performance in more detail. Additional test methods and measurement procedures which may be useful for more detailed characterisation of collector performance are included in Chapters 9 and ETA ZERO Theoretical values of no ma y be calculated as the product of the transmittance of the glazing (x), the absorptance of the absorber (a), and the collector efficiency factor (F'). However, when the performance of a collector is measured its efficiency is determined using some value of collector area to relate the thermal output to the incident solar irradiance. Measured values of collector efficiency are therefore directly proportional to the reference area used, and consequently measured values of rig are also dependent on the reference area used. The reference area recommended in this document is the Aperture Area of the collector defined in Section Where test results are related to the Gross Area of the collector, the resulting values of no will generally be lower than those related to the Aperture Area. Similarly, variations in the value of no ca n be caused by results which are referred to the temperature of the fluid entering the collector, rather than to the mean fluid temperature in the collector as recommended in this document. It is shown in Section 8.9 that when the fluid inlet temperature is used as reference, the collector efficiency factor F 1 must be replaced by the collector flow factor F and this again results in a lower value of no- Typical collector efficiency curves based on aperture area and mean fluid temperature are shown in Figure 10.1 where it can be seen that typical values of no f r flat plate collectors lie in the range 0.7 to 0.8, depending on the glazing system used.

35 2.4 COLLECTOR HEAT LOSSES The heat losses through the back and sides of most flat plate collectors can be reduced to less than 1 W/m 2 K by appropriate insulation and absorber mounting techniques. The losses through the front of a collector are mainly controlled by the heat transfer between the absorber and the cover. Typically, good single glazed matt black collectors will exhibit heat loss coefficients in the range from 6 to 8 W/m 2 K and selectively coated collectors or double glazed matt black collectors will exhibit heat loss coefficients in the range from A to 6 W/m 2 K. The heat loss coefficient of a collector increases with collector operating temperature and with the local wind speed, as discussed in Chapter COLLECTOR EFFICIENCY FACTOR (F') The heat transfer efficiency of a collector is affected by the absorber fluid passageway design, the sizing of the fins between the passageways, and by the convective heat transfer into the fluid, as discussed in Chapter 19. Typical values for F' lie in the region of 0.9, but in poorly designed absorbers F 1 can have a very much lower value. Collectors with a poor value of F' will exhibit low measured values of overall heat loss coefficient and of no- The measured heat loss coefficient is actually the product of the collector efficiency factor F 1 and the overall heat loss coefficient which would occur from a similar collector with an absorber which is at temperature Tm: U = F'UL Eqn. 2.7 An indication of the value of F 1 can usually be estimated from measurements of no, if the values of x and a are known. 2.6 ADVANCED COLLECTOR PERFORMANCE A number of advanced collector designs have been developed, and some of these are now becoming commercially available. In order to improve collector performance, designers have mainly concentrated on two approaches: (a) to use evacuated tubes around the absorber to reduce heat losses, and (b) to add specular or diffuse reflectors to increase the solar input. -19-

36 The test methods included in this document have been shown to give adequate results for evacuated tubular collectors, but more work is required before recommendations can be made concerning the performance of concentrating collectors. The performance of collectors containing heat pipes depends on the heat pipe design, and may generally be expected to differ slightly from that of conventional collectors. The variations in performance can mainly be attributed to the operation of the heat pipe. Generally, the efficiency of a heat pipe varies with the power which it is transferring, and there is a maximum operating temperature above which the fluid in a heat pipe will not operate. Analysis of the performance of heat pipe collectors is outside the scope of this document. Concentrating collectors which employ specular reflectors to increase the solar irradiance at the absorber are usually only able to focus direct solar radiation, and must be tracked around during the day to follow the sun. Their collection efficiencies are generally greater than those of flat plate collectors at high temperatures in clear sky conditions. However, their long term performance, when cloudy days are taken into account, nay not be so good. A full analysis of the performance of concentrating collectors is outside the scope of this document. Diffuse reflectors are sometimes used to improve the performance of evacuated tubular collectors, and experience indicates that they can produce a significant increase in energy collection. Where flat plate collectors are used at a steep tilt angle with respect to the horizontal, their performance can also be improved by using a flat solar radiation reflecting surface in front of the collector. A considerable amount of research and development effort has been directed in recent years towards the optimisation of reflector systems for solar collectors. One of the major results of this work has been the development of Compound Parabolic Concentrator (CPC) systems which have relatively large focal planes and can concentrate both direct and diffuse solar radiation onto absorbers. At the present time such systems are not widely used in Europe but they have considerable potential for higher temperature solar heating applications. Further information on CPC collectors will be found in Reference (1). 20-

37 CHAPTER 3 UNITS AND SYMBOLS 3.1 GENERAL RECOMMENDATIONS Units The system of units for solar energy quantities should be the Systeme International d'unites, (S.I.), detailed in Reference (2). S.I. Units for parameters relating to solar energy are given in Appendix II Symbols The recommendations given in Appendix II have been drafted taking into consideration the recommendations of other European scientific organisations and also the recommendations of the Committee on Education and Standardisation of the International Solar Energy Society (3). However it is recognised that work is still in progress in the European Community on the subject of units, and the recommendations given may need to be revised in the future. 21-

38 3.2 DOCUMENT NOMENCLATURE Symbol Meaning Units ao,ai,a 2 Algebraic constants A Aperture area of collector m 2 a r A g Gross area of collector m 2 AM Air mass A P Absorber plate area of collector m 2 Cf Specific heat capacity of heat transfer fluid J/kg K C CT Effective thermal capacity of collector Effective thermal capacity of the total loop including the collector J/K J/K D Date D-M-Y E Voltage output V e F F' FR Error of curve fit Radiation view factor Collector efficiency factor Collector flow factor G Solar irradiance W/m 2 G, Diffuse solar irradiance W/m 2 G L Longwave irradiance (X > 4pm) W/m 2 LT Local time hours K(v) Incidence angle modifier m Mass flow of heat transfer fluid kg/s N Number of data points n Day number (1st January =1) P Fluid heating power input W Q Useful power extracted from collector W Q Power loss of collector W

39 Symbol Meaning Units R t T T T. 1 m T s U u u V f AP At AT a a Thermometer resistance at K Thermometer resistance at temperature T Time Absolute temperature Ambient or surrounding air temperature Collector outlet (exit) temperature Collector inlet temperature Mean temperature of heat transfer fluid Atmospheric or equivalent sky radiation temperature Reduced temperature difference Measured overall heat loss coefficient of collector Overall heat loss coefficient of a collector with uniform absorber temperature = Tm Surrounding air speed Fluid capacity of the collector Pressure difference between fluid inlet and outlet Time interval Temperature difference between fluid outlet and inlet (Te-Ti) Hemispherical absorptance Azimuth angle of a plane (measured from South, West positive) Inclination angle of a plane with respect to horizontal Solar declination Hemispherical enittance Wavelength a a s K or C C C C K or C K m 2 K/W W/m 2 K W/m 2 K m/s m 3 Pa s K degrees degrees degrees m 23-

40 Symbol Meaning Units n Collector thermal efficiency no Eta Zero (n at T* = 0) no Mean of measured eta zero values $ Latitude degrees v(b,a) Angle of incidence of direct solar radiation degrees a Stefan-Boltzmann Constant W/m 2 K 1 * o Standard deviation of curve fit error p Density of heat transfer fluid kg/m 3 x Collector time constant s T to Transmittance Hour angle (15 per hour, measured from solar noon, afternoon positive) degrees -24-

41 CHAPTER 4 DEFINITIONS 4.1 COLLECTORS Solar Collector A solar collector is a device which absorbs solar radiation, converts it into heat and passes this heat on to a heat transfer fluid Flat Plate Collector A flat plate collector is a solar collector whose aperture area is essentially identical to the area of the absorber surface, that employs no concentration, and in which the absorbing surface is essentially planar Concentrating Collector A concentrating collector is a solar collector which uses reflectors, lenses or other optical elements to concentrate the solar energy incident at the aperture onto an absorber, whose area is smaller than the aperture area Liquid Heating Collector A liquid heating collector is a solar collector which employs a liquid as the heat transfer fluid Air Heating Collector An air heating collector is a solar collector which employs air as the heat transfer fluid High Temperature Collector A high temperature collector is a solar collector which is designed for regular operation at temperatures greater than 100 C Evacuated Tubular Collector An evacuated tubular collector is a solar collector which employs a cylindrical evacuated envelope to suppress convective heat transfer in the region of the absorber. 25-

42 4.1.8 Heat Pipe A heat pipe is a device for transferring heat by means of evaporation and condensation of a fluid in a sealed system. Heat pipes may be used as components of a solar collector Heat Transfer Fluid The heat transfer fluid is the medium by which the solar energy absorbed by a collector is removed from the collector Absorber The absorber is that part of a solar collector which converts the incident solar radiation into heat and from which the heat is removed by the transfer fluid. If an absorbing liquid is used then this may constitute both the absorber and the heat transfer fluid Selective Surface (Absorber) An absorber is considered to have a selective surface if it absorbs a large proportion of incident solar radiation while simultaneously exhibiting a low emittance at longer wavelengths Aperture Cover The aperture cover is the transparent part of a solar collector, normally positioned at the aperture, which is used to reduce the heat loss from the absorber, and to provide some protection from the weather Aperture Area of Collector (Aa)* The aperture area of a collector is the opening or projected area of a collector through which the unconcentrated solar energy is admitted Gross Area of Collector (Ag)* The gross area of a solar collector is the overall projected area of the collector including its enclosure. * Note: The recommended reference area for the presentation of collector test results is that defined in 4.1.IS, However, in the UK the definition has been modified to "include any glazing bars or supports over the absorber". In France and Greece the Gross Area is used as a reference for collector test results, following the practice in the USA. -26-

43 4.2 RADIATION AND SOLAR ANGLES A.2.1 Radiation Radiation is the emission or transfer of energy in the form of electromagnetic waves. A.2.2 Radiant Flux Radiant flux is power emitted, transferred or received in the form of radiation Irradiance (measured in W/m 2 ) The irradiance at a surface is the ratio of the radiant flux incident on the surface to the area of that surface. (Solar irradiance is sometimes termed "incident solar radiation intensity", "instantaneous insolation", or "incident radiant flux density".) Irradiation (measured in J/m 2 ) The irradiation of a surface is the time integral of the irradiance at that surface. (Irradiation is sometimes termed radiant exposure.) Solar Radiation Solar radiation is the radiation emitted by the sun. (Approximately all of the solar energy incident at the earth's surface is at wavelengths less than 4.0 um, and is often termed shortwave radiation.) Direct Solar Radiation Direct solar radiation is the solar radiation received at a surface from the solid angle subtended by the sun's disk Diffuse Solar Radiation Diffuse solar radiation is the solar radiation received at a surface from a solid angle of 2IT with the exception of the solid angle subtended by the sun's disk Global Solar Radiation Global solar radiation is the sum of the direct and diffuse solar radiation incident on a surface from a solid angle of 2TT. -27-

44 4.2.9 Solar Constant The solar constant is the solar irradiance at the outer edge of the earth's atmosphere when the earth-sun distance is at the average value of 150 x 10 6 km. The solar constant is defined to be kw/m 2 on a surface perpendicular to the direction of the sun Radiation (Thermal Radiation) Radiation which is emitted by warm bodies is often called thermal radiation. Surfaces at typical solar collector temperatures (0 C to 200 C) emit radiation at wavelengths in the range of 2.5 pm to 50 pm Terrestrial Radiation (Atmospheric and Ground Radiation) Terrestrial Radiation is the radiation emitted by gases and particles in the atmosphere and by the ground, at wavelengths greater than 4 pm Sky Temperature The terrestrial radiation received at a surface may be expressed in terms of an equivalent black body radiation temperature, i.e., the sky temperature Visible Radiation Visible radiation is radiation with wavelengths that stimulate the optic nerves. Visible radiation lies approximately within a wavelength band of 0.38 pm to 0.76 pm Angle of Incidence of Direct Solar Radiation The angle of incidence of direct solar radiation is the angle between the direct solar radiation beam and the outward drawn normal from the plane of the collector aperture Solar Altitude The solar altitude is the angle between the direction of the sun and the horizontal at the point of observation Solar Azimuth The solar azimuth is the horizontally projected angle between the direction of the sun and due South at the point of observation. The angle is measured clockwise from South.

45 A.2.17 Solar Declination The solar declination is the angular position of the sun at solar noon with respect to the plane of the equator (North positive) Solar Hour Angle The solar hour angle is an equivalent angle (0 to 360 ) for the time of day, with each hour equalling 15 of longitude and solar noon being zero (e.g. u = 37.5 for 1430 hrs. solar time) Solar Noon For any given location solar noon is the local time of day when the sun is at its highest altitude. 4.3 GENERAL CONCEPTS Instantaneous (quasi-steady-state) Efficiency The instantaneous (quasi-steady-state) efficiency of a solar collector is defined as the ratio of the average useful power extracted from the collector to the average solar radiation flux incident at the aperture under specified quasi-steady-state conditions Eta Zero (no) Eta Zero for a collector is the instantaneous efficiency of the collector when the mean fluid temperature is equal to the ambient air temperature, under specified quasi-steady-state conditions. (Eta Zero is sometimes termed the "Effective Tau Alpha Coefficient", the "Conversion Factor" or the "Zero Loss Efficiency".) Collector Efficiency Factor (F 1 ) The collector efficiency factor for a collector is the ratio of the true value of no to the ideal value of no which could be achieved if the entire absorber were at the mean fluid temperature of the collector. 29-

46 4.3.4 Time Constant The time constant of a collector is the time period required for the temperature of the fluid leaving the collector to achieve 63.2% of its ultimate temperature change, following a step increase or decrease in solar irradiance Useful Power The useful power extracted from a collector is the thermal power extracted from the collector by the heat transfer fluid Quasi-Steady-State Quasi-steady-state describes the state of the solar collector test when the flowrate and temperature of the heat transfer fluid entering the collector are substantially constant and the variations in the outlet temperature of the heat transfer fluid are due only to small variations of the solar radiation flux incident at the collector aperture. (See Section 10.2 for a quantitative definition.) Surrounding Air Temperature The surrounding air temperature is the mean temperature of the air within 10 m of the collector, measured by a sensor which is shielded from solar radiation and placed at least 1 m above the local ground surface Surrounding Air Speed The surrounding air speed is the speed of the ambient air passing over the collector at a distance of 50 mm above its aperture Wind Speed The wind speed is the speed of the air measured in accordance with the recommendations of the World Meteorological Organisation (4), normally measured ten metres above ground level Mean Fluid Temperature The mean fluid temperature is defined as the average of the fluid temperatures at the inlet and outlet of the collector. The mean fluid temperature is normally determined by measurement of the fluid temperature at the inlet to the collector and the fluid temperature rise or drop across the collector. The mean fluid temperature is then determined from the following equation: Tm = Ti (Te - Ti) Eqn

47 Air Mass The air mass is the length of path through the atmosphere traversed by the direct solar beam, expressed as a multiple of the path to a point at sea level with the sun at zenith. (The latter is called AMI, extraterrestrial is called AMO). The Air Mass is equal to the cosecant of the solar altitude for solar altitudes of greater than 10 degrees. (Air mass values less than 5.6) -o- AMO AM1 /) n >) / / rr garth's Surface AMlKl^CosecX (for K > 10 c Figure 4.1 Definition of Air Mass

48

49 CHAPTER 5 COLLECTOR MOUNTING AND LOCATION 5.1 GENERAL The way in which a collector is mounted has been shown to influence the results from thermal performance tests. In this section recommendations are made concerning the structure on which collectors should be mounted during testing, and the local environment surrounding both the back and front surfaces of the collector. These recommendations apply to outdoor testing, indoor heat loss testing and testing in solar simulators Safety The collector should be mounted in a manner such as to ensure safety to personnel. Due consideration should be paid to the likelihood of glass failure and the leakage of hot liquids, etc. Mountings outdoors should be able to withstand the effects of wind gusts Collector Size As far as possible full size collector modules should be tested because scaling may influence results. For example, the edge losses of small collectors may significantly reduce their overall performance. Liquid heating collectors are normally between 1-3 m 2 in size, but air heating collector modules may be larger Collector Mounting The collector mounting should in no way obstruct the aperture of the collector, and the mounting structure should not significantly affect the back or side insulation. Unless otherwise specified (for example, when the collector is part of an integrated roof array), an open mounting structure is recommended which allows air to circulate freely around the front and back of the collector, and the collector should be mounted such that the lower edge is greater than 0.5 m above the local ground surface Tilt Angle In order to facilitate international comparisons of test results it is recommended that the collector should be mounted such that the angle of tilt of the aperture from the horizontal is 45 ±

50 In order to test collectors at other tilt angles recommended by manufacturers or specified for actual installations, test frames allowing fixed tilt angles between 0 and 60 may be required. For many collectors the influence of tilt angle is small, but it can be an important variable for specialised collectors such as those incorporating heat pipes. Figure 5.1 Open Rack Mounting 34-

51 5.2 FIELD OF VIEW OUTDOORS Collector Orientation The collector may be mounted in a fixed position facing South but this will result in the time available for testing being restricted by the acceptable range of incidence angles. A more versatile approach is to move the collector to follow the sun in azimuth using manual or automatic tracking Shading from Direct Solar Irradiance The collector should be located such that a shadow will not be cast onto the collector at any time during the test period Diffuse and Reflected Solar Irradiance For the purposes of test results analysis, solar irradiance not coming directly from the sun's disk is usually assumed to come isotropically from the hemispherical field of view of the collector. In order to minimise the errors resulting from this approximation the collector should be located where there will be no significant solar radiation reflected onto it from surrounding buildings or surfaces during the tests, and where there will be no significant obstructions in the field of view. The reflectance of most rough surfaces such as grass, weathered concrete or chippings is not usually high enough to cause problems during collector testing. Surfaces to be avoided in the collector's field of view include large expanses of glass, metal or water. Not more than ^ 5% of the collector's field of view should be obstructed, and it is particularly important to avoid buildings or large obstructions subtending an angle of greater than ^ 15 degrees with the horizontal in front of the collectors. Radiation reflected onto the back of the collector is less important than that reflected onto the front for most collectors. However, the field of view behind the collector can significantly influence the performance of collectors with little back insulation, and evacuated tubular collectors Thermal Irradiance The temperature of surfaces adjacent to the collector should be as close as possible to that of the ambient air in order to minimise the need for performance corrections when normalising test results to reference conditions. For example, the field of view of the collector should not include chimneys, cooling towers or hot exhausts. -35-

52 The performance of some collectors, such as unglazed collectors, is particularly sensitive to the levels of thermal irradiance. The effect of thermal irradiance on collector performance is discussed in Chapter Wind The performance of many collectors is sensitive to air speeds over the collector in the range 0-3 m/s. In order to maximise the reproducibility of results, collectors should be mounted such that air with a mean speed of between 3 m/s and 8 m/s will freely pass over the aperture, back and sides of the collector. The use of artificial wind generators may be necessary in some locations, to achieve these wind speeds. Warm currents of air, such as those which rise up the walls of a building, should not be allowed to pass over the collector. Where collectors are tested on the roof of a building, they should be located at least 2 metres away from the roof edge. Collectors designed for integration into a roof may have their backs protected from the wind, though this must be reported with the test results. Figure 5.2 Typical Outdoor Collector Test Facility 36-

53 5.3 FIELD OF VIEW INDOORS (including solar simulators) Air Temperature The temperature of the air indoors can have significant spatial variations if it is not continuously moved by fans or an air-conditioning system. Provision should be made to ensure that the air temperature over the full field of view of the collector and at the outlet of the artificial wind generator does not deviate from the mean surrounding air temperature by more than ±1 C during a test period Solar Irradiance In most solar simulators the simulated beam approximates to direct solar irradiance only. In order to simplify the measurement of simulated irradiance it is sometimes useful to minimise reflected irradiance, and this can be achieved by painting all surfaces in the test chamber with a dark (low reflectance) paint. For heat loss testing, where zero solar irradiance is assumed, the shortwave irradiance (wavelengths < 4 pm) should be less than 2 W/m 2. This can be confirmed by the use of a pyranometer, but a zero check with a light-tight box is necessary when using pyranometers at very low irradiance levels (see Section ) Thermal Irradiance Unless special provision has been made to maintain all the internal surfaces in the test chamber at ambient air temperature, it is possible that a collector may have surfaces with a range of temperatures in its field of view. As far as possible the collector should be shielded from hot surfaces such as radiators, air conditioning ducts and machinery, and from cold surfaces such as windows and external walls. Shielding is important both in front of and behind the collector. In order to minimise the size of performance corrections, it is recommended that the. thermal irradiance in the plane of the collector aperture should not exceed that from a black body at ambient temperature by more than 100 W/m 2. This level is assumed to apply to the total integrated irradiance for all wavelengths greater than 4 pm, including that from a solar simulator (if present) and all the surfaces in the field of view of the collector. The thermal radiation emitted by hot surfaces (such as those of ducts around the simulator lamps) may be minimised by the use of low omittance materials such as polished aluminium. Care should be taken however to minimise the reflection of thermal radiation by polished metal surfaces onto the collector under test. -37-

54 5.3.4 Wind The performance of collectors under zero wind speed conditions is sometimes studied indoors, but zero wind is a very difficult condition to maintain. To ensure reproducible test results an artificial wind generator should be used to produce a flow of air over the aperture, back and sides of the collector. The average speed of the air flowing over the collector should lie between 3 m/s and 8 m/s when measured in the plane of the collector at a distance of 50 mm from the surface of the cover, and at no point over the collector aperture should the speed deviate from the mean by more than ± 25%. The speed at any point over the collector aperture should remain steady and the temperature of the air leaving the wind generator should lie within ±1 C of the ambient air temperature. Collectors designed for integration into a roof may have their backs protected from the wind, though this must be reported with the test results. Figure 5.3 Portable Test Loop with Wind Generator

55 CHAPTER 6 THE TEST INSTALLATION The recommendations given in this section apply to test installations employing water based heat transfer fluids. Additional recommendations concerning air collector testing and the use of oil based fluids are given in Chapters 11 and GENERAL CONSIDERATIONS The heat transfer fluid may be circulated through either a closed loop (Figure 6.1) or an open loop (Figure 6.2 ) arrangement. An open loop arrangement has the advantage that a constant head device can help to minimise fluctuations in flowrate, but the disadvantage that air can be more easily entrained in the fluid. A closed loop generally requires the use of a more expensive circulating pump and a flow control valve in order to maintain a steady mass flowrate. Pressure relief valves should be used as a safety precaution in closed loop arrangements. Fluid loops requiring a continuous supply of fluid are not recommended. 6.2 HEAT TRANSFER The heat transfer fluid used for collector testing may be water or a fluid recommended by the collector manufacturer. The specific heat capacity and density of the fluid used should be known to within ± 1% over the range of fluid temperatures used during the tests. Values for water are given in Appendix IV. Some fluids may need to be changed periodically to ensure that their properties remain well defined. 39-

56 c Anemometer Temperature Transducer Te 6 Air Vent Surrounding Air Temperature Screen Pyranometer w I View A- Pyranometer positioned at Collector Midheight o Hi o I- 1 o 01 (D (D en Temperature Transducer T Sight Glass Temperature Regulator Flowmeter Bypass Valve Heater/cooler > Safety Valve f o a j Flow,..,. ' Control. '" er, Valve «200uml Pump Expansion Tank

57 *1 C H rt> Temperature Transducer Te IZZZSSSZL. Pyranometer X fn Surrounding Air Temperature Screen View A : Pyranometer positioned at Collector Midheight o Hi 05 3 o o (D 3 H n> CO f O o Sight Glass Heater/cooler

58 6.3 PIPEWORK AND FITTINGS The piping used in the loop should be resistant to corrosion and suitable for operation at temperatures up to 95 C. If non aqueous fluids are used, then compatibility with system materials should be confirmed. Pipe lengths should generally be kept short. In particular, the piping between the outlet of the fluid temperature regulator and the inlet to the collector should be minimised, to reduce the effects of the environment on the fluid inlet temperature. This section of pipe should be insulated to ensure a rate of heat loss of less than 0.2 W/K and be protected by a reflective weatherproof coating. Pipework between the temperature sensing points and the collector (inlet and outlet) should be protected with insulation and reflective weatherproof covers extending beyond the positions of the temperature sensors, such that the calculated temperature gain or loss along either pipe does not exceed 0.01 K under test conditions. Flow mixing devices such as pipe bends are required immediately upstream of temperature sensors (see Section 7.3). A short length of transparent tube should be installed in the fluid loop such that air bubbles and any other contaminants will be observed if present. The transparent tube should be placed close to the collector inlet but without influencing the fluid inlet temperature control or temperature measurements. A variable area flowmeter is convenient for this purpose as it simultaneously gives an independent visual indication of the flowrate. An air separator and air vent should be placed at the outlet of the collector, and at other points in the system where air can accumulate. Filters should be placed upstream of the fluid measuring device, the pump and elsewhere, in accordance with normal practices (a nominal filter size of 200 urn is usually adequate). -42-

59 6.4 THE PUMP AND FLOW CONTROL DEVICES The pump should be located in the fluid loop in such a position that the heat which is dissipated in the fluid does not impair either the control of the panel inlet temperature or the measurement of the fluid temperature rise through the collector. With some types of pump, a simple by-pass loop and manually controlled needle valve may provide adequate flow control. Where necessary a proprietary flow control device may be added to stabilise the mass flowrate. The pump and flow controller should be capable of maintaining the mass flowrate through the collector stable to within ±1%, despite temperature variations, at any chosen inlet temperature in the operating range. 6.5 TEMPERATURE REGULATION OF THE FLUID One of the most important features of a collector test loop is that it should be capable of maintaining a constant collector inlet temperature at any temperature level chosen in the operating range. Since the rate of energy collection in the collector is deduced by measuring instantaneous values of the fluid inlet and outlet temperatures it follows that small variations in inlet temperature could lead to errors in the rates of energy collection deduced. It is particularly important to avoid any drift in the collector inlet temperature, and fluctuations in the inlet temperature should not exceed ±0.1 K. It is also important to be able to change rapidly (say within 15 minutes) from one inlet temperature to another, in order to minimise the time wasted between test points. Various methods of inlet temperature control have been found satisfactory. Some of these are described in the following sections Two Heat Exchangers It is possible to make rapid changes in inlet temperature and to achieve precise inlet temperature control by using two heat exchangers in series in the system. One heat exchanger is used to cool the fluid below the desired temperature, and a second is used to bring the temperature up to the desired value. -43-

60 Fluid Heater Fluid Cooler T o = Collector wwws \MMJU Figure 6.3 Two Heat Exchangers Mixing The controlled mixing of fluid from a high temperature reservoir and a low temperature reservoir may be used to provide temperature regulation. This method also permits rapid changes from one collector inlet temperature to another. - Mixing valve J^Henl exchanger Rj ^ I f?\\, L In line temp, controller Flow Controller Pump In line filter Figure 6.4 A Loop Using Controlled Mixing

61 6.5.3 Thermostatic Bath A thermostatic bath may be used to control the temperature of the fluid but this should be continuously stirred to ensure uniformity of the temperature. In order to prevent overheating of the bath during outdoor tests it may be necessary to pre-cool the fluid before returning it to the thermostatic bath. The use of a single thermostatic bath with a pre-cooler may restrict the number of test points which can be obtained in any given day because of the time required to change the bath temperature from one value to another Inline Temperature Control An inline electric heater with feedback control may be used to provide temperature control of the fluid close to the collector inlet. Boiling of the fluid on the surface of the electric element may occur if the power dissipation in the element is high, and hence it is usual to make only small temperature corrections in this way. In many test facilities an inline heater with feedback control is required in addition to one of the devices described in Sections in order to meet the required inlet temperature stability (see Section 7.3.2). Such electric heaters are particularly useful where a long pipe between the temperature controller and the collector inlet cannot be avoided. i_ D *» O l- Ol a E.0) measured temperature rise error transit time of fluid in collector Time Figure 6.5 Depression of Measured Temperature Rise by Drifting Fluid Inlet Temperature in Steady State Weather Conditions -45-

62

63 CHAPTER 7 INSTRUMENTATION FOR USE IN COLLECTOR TESTING The test methods used for the determination of the thermal performance of liquid heating solar collectors require the measurement of experimental variables as shown in Table 7.1. In the light of the experience gained during the European collaborative collector testing programme, recommendations are made in this Chapter with regard to the measurement of these variables. Further information is given in the World Meteorological Organisation Guide (4) and in national Standards. When testing indoors under simulated solar radiation, additional information is required. This is discussed in Section When testing under 'transient' conditions and when using other heat transfer fluids, differences in the measurement of variables may be required. These are discussed in Chapters 10, 11 and 12.

64 Experimental Variable tits Precision (a) Local Time (b) Solar Irradiance (c) Angle of Incidence (measured or calculated) (d) (e) (f) (g) (h) Thermal Irradiance Surrounding Air Speed Fluid Inlet Temperature* Ambient Air Temperature Difference between Fluid Outlet Temperature and Fluid Inlet Temperature (i) Mass Flowrate of Heat Transfer Fluid h W/m 2 (degrees) W/m 2 m/s (j) Aperture Area of Collector m 2 C C K kg/s (k) Fluid Capacity of Collector kg ±0.01 h ± 5 ± 10 W/m 2 ± 0.5 m/s ± 0.02 C ± 0.2 C (1) Pressure Drop across the Collector Pa ± 1% Notes t (1) 'Precision refers to the repeatability error, or the uncertainty involved in interpreting the reading of the measurement. Accuracy refers to the maximum permissible bias error, or the difference between the measured value and the true value. For all variables other than Fluid Inlet Temperature the requirements for accuracy are the same as those for precision in collector testing. * (2) Fluid Inlet Temperature A high level of precision is needed in order to enable small drifts in fluid inlet temperature to be detected (see Section 7.3.2). The accuracy requirement for fluid inlet temperature however is only to.lk. ± ± ± ± 0.1 K 1% 0.1% 10% Table 7.1 Measurement Accuracies Required for Collector Efficiency Testing -48-

65 7.1 SOLAR RADIATION MEASUREMENT The measurement of solar radiation can be made by means of several types of instrument, but the calibration of the instrument should be traceable to the World Radiometric Reference (WRR) maintained at the World Radiation Centre in Davos, Switzerland (5). Most countries maintain a reference pyrheliometer which has been calibrated at Davos, and use this to provide calibrated pyranometers for general use. Before 1981, instruments were calibrated against the International Pyrheliometric Scale (IPS 1956). Measurements made on the IPS Scale can be converted to WRR by multiplying them by Solar Radiation Measuring Instruments PYRANOMETER A pyranometer is an instrument for measuring the solar irradiance from a solid angle of 2TT on a plane surface. When the solar radiation coming from the solid angle of the sun's disk is obscured from the instrument, a pyranometer can be used to determine the diffuse solar irradiance on a plane surface. SOLARIMETER A solarimeter is a specific type of pyranometer based upon the Moll-Gorczynski thermopile design. PYRHELIOMETER A pyrheliometer is an instrument normally used to measure the total irradiance including all wavelengths from a small solid angle. When orientated towards the sun, a pyrheliometer can be used to determine the direct solar irradiance. PYRRADIOMETER A pyrradiometer is an instrument for measuring the total irradiance of all wavelengths from a solid angle of 2TT on a plane surface Pyranometers Pyranometers (some types of which are termed solarimeters) have been classified by the World Meteorological Organisation. These classifications require that the pyranometers have the characteristics given in Table 7.2. Pyranometers have a hemispherical field of view and are therefore able to measure total solar irradiance, i.e. the sum of direct, diffuse and ground reflected solar radiation. -49-

66 Characteristic Secondary Standard First Class Second Class Resolution (smallest detectable change in W/m 2 +1 ±5 ±10 Stability (percentage of scale, change/year) full ±1 ±2 ±5 Cosine response (percentage deviation from ideal at 10 solar elevation on a clear day) Azimuth response (percentage deviation from the mean at 10 solar elevation on a clear day) <±3 <±3 <±7 <±15 <±5 <±10 Temperature response (percentage maximum error due to change of ambient temperature within the operating range) ±1 ±2 ±5 Non-linearity (percentage of full scale) ± ±5 Spectral sensitivity (percentage deviation from mean absorptance 0.3 to 3 Jim) Response time (99% response) ±2 ±5 ±10 <25 s <1 min <4 min Table 7.2 Classification of Pyranometers (from ref. 4) Only pyranometers meeting the WM0 secondary standard or 1st Class classifications should be used for collector testing. Those most commonly used in Europe are manufactured by Kipp and Zonen in the Netherlands (6) and by Eppley in the the USA (7). The accuracy of photovoltaic pyranometers has not yet been shown to meet the requirements of collector testing Mounting, Maintenance and Calibration of Pyranometers The pyranometer should be mounted such that its detector is located in the plane of the collector aperture and to one side of the collector, at the mid height. The mounting position is shown pictorially in Figures 6.1 and 6.2.

67 If the detector of the pyranometer is non-circular as in the Kipp & Zonen CM5 instrument, the orientation should be such that the cables from the pyranometer point North when the pyranometer faces South. Pyranometers are primarily designed for use in a horizontal position and are fitted with a large white disc to shade the instrument body from solar radiation. When pyranometers are used in a tilted position additional protection is required to shade the back of the instrument from diffuse solar radiation. Pyranometers should be well ventilated with ambient air on all surfaces both in front and behind. Any heating of the instrument body is likely to cause measurement errors. The cables should be shielded from direct solar radiation and screened from electromagnetic interference. Prior to collector testing the glass dome of the pyranometer should be cleaned and checked to ensure that it is free of condensation. If present, the desiccant in the instrument should be checked to ensure that it is dry. Figure 7.1 Solar Radiation Measurement Instruments Pyranometers should be calibrated in accordance with the recommendations of the WMO (4) to the World Radiometric Reference and recalibrated annually either by the manufacturer, by a national meteorological laboratory or by the World Radiation Centre in Davos, Switzerland. -si-

68 The output of some pyranometers is affected by the temperature of the pyranometer body. (The calibration constant of the Kipp and Zonen CM5 pyranometer, for example, varies by approximately 0.1 to 0.2%/K). Where calibrations are temperature dependent, measurements should be corrected for this effect using the temperature coefficient given by the manufacturer and the reference temperature at which the instrument calibration was made. In order to measure the total irradiance at the collector aperture including ground reflected irradiance it is recommended that pyranometers be used in the plane of the collector (i.e. usually tilted at 45 ). The variation of most pyranometer calibrations with tilt angle is small (< 1%), but if possible the pyranometer should be calibrated at the tilt angle for which it will be used. The zero offset of a pyranometer may be checked by placing a light-tight box over it. A pyranometer will not always give a zero reading outdoors at night because of the low values of effective sky temperature which sometimes occur. Low sky temperatures depress the zero reading of pyranometers Diffuse Solar Radiation Measurement USE OF SHADE DISC The diffuse solar irradiance in the plane of a collector may be determined by shading the detector of the pyranometer from the direct solar radiation in the solid angle subtended by the sun's disc. This may be accomplished by using an opaque disc of approximately 50 mm diameter attached to a slender rod on a direct line between the detector and the sun. The disc should be held at a distance such that just the dome of the pyranometer is shaded. One pyranometer may be used to measure both diffuse and total solar irradiance by making the diffuse measurements before and after a test period. USE OF SHADE RING As an alternative to a shade disc, a second pyranometer may be mounted adjacent to the first and fitted with a shade ring of a suitable width such that the detector is shielded from direct solar radiation during the period in which tests are performed. Shade rings are commercially available from instrument manufacturers, or may be constructed in accordance with meteorological guidelines (8). -52-

69 An example of a shade ring is shown in Figure 7.2. Its outer surface is white, its inner surface is black and it is mounted with its axis tilted at an angle to the horizontal equal to the latitude, such that the plane of the ring is parallel to the plane defined by the daily motion of the sun. The pyranometer is placed with its detector on the axis of the shade ring (either tilted to the same angle as a collector or horizontally), and the ring is moved every few days, following the declination of the sun, to keep the pyranometer in shadow. SHADE RING CORRECTION FACTOR The reading from a pyranometer with a shade ring should be increased to allow for the region of sky obscured by the ring. The obscured fraction s may be found for a ring of radius R, and width w when it is located with its centre at declination <5 0 by using the following relations to find A and B: A = R tan 6 0 ~ J Eqn. 7.1 B = R tan j Eqn. 7.2 The values of A and B may then be used to define &i and 6 2> which in turn permit the calculation of s: A sin6i = /(R2 +A 2) A and tanfij = ^ Eqn. 7.3 R R sin6 2 = /(RZ+AZ) and tan6 2 = Eqn. 7.4 For a horizontally placed pyranometer at latitude <J>, S(6n) = = - fsin'if-^ll + sin6 /(cos 2 (.-sin 2 6) TT _ [COSlflJ ~l 6 2 sinif cos 2 6 cos 1 (-tan^ta^) Eqn J *, T -53-

70 For a pyranometer mounted at a tilt angle 6 and pointing South at latitude $, s(6o) " ira+cosb) cosb sin sin6 cost cos(»-b) s. n6/ os2 _ s. n2 T COScjl sin(<j>-b)cos^6cos i (-tanc(>tan6) TT(1+COSB) sin sin 6 cos((f>-8) + sin6/(cos 2 ((j>-b)-sin 2 6) sin(((i-b)cos 2 6cos~ 1 (-tan(4> B) tani) J c where the limits are: if «!, <5 2 * 0, a=6 1 ( b = 6 2, Eqn. 7.6 c = 0, d = 0 if «! < 0, 6 2 >0, a = «!, b = 0, c = 0, d=6 2 if «i, a = 0, b = 0, c=6 1 ( d=6 2 Figure 7.2 Kipp Pyranometer with Shade Ring -54-

71 USE OF PYRHELIOMETER Diffuse radiation may also be determined from the difference between global radiation measurements made with a pyranometer and direct radiation measurements made with a pyrheliometer pointed at the sun. The direct solar irradiance measurement needs to be corrected for the incidence angle between the solar beam and the plane of the pyranometer detector Angle of Incidence of Direct Solar Radiation REQUIRED ACCURACY The angle of incidence of direct solar radiation at the collector plane should be determined to ensure that it is less than 40 during the collector test period. For most purposes an accuracy of ± 5 is adequate although ± 1 is desirable when measuring incidence angle modifiers. DETERMINATION BY CALCULATION The angle of incidence v(b,a) may be determined by calculation (9) knowing the time of the collector test, the collector tilt angle, the collector azimuth angle and the geographical position of the test site, using the relation: cosv = sin6sini(icosb - sin6cos< >sinbcosci + cos6cosc(icosbcosu) + cos6sin<j>sinbcosacosω Eqn cos6sinbsinasinosinu where the solar declination 6 for day number n of the year is given by: 6= 23.45sin 360(284 + n)l Vm 7 o 333 j Eqn. 7.8 MEASUREMENT OF THE ANGLE OF INCIDENCE A device for measuring the angle of incidence can be produced by mounting a pointer normal to a flat plate. Graduated concentric rings may then be used to measure the length of the shadow cast by the pointer, and hence the incidence angle. The device should be positioned in the plane of the collector at one side. A typical measuring device is shown in Figure 7.3.

72 Figure 7.3 Angle of Incidence Indicator -56-

73 7.2 THERMAL RADIATION MEASUREMENT Thermal radiation measurements are less important than solar radiation measurements for the determination of collector efficiencies, and are usually neglected outdoors. However, in solar simulators and indoor heat loss testing it is often necessary to make performance corrections for excess thermal irradiance when comparing results with those from outdoor testing Thermal Radiation Measuring Instruments PYRGEOMETER Thermal irradiance (wavelengths > 4 um) from a hemispherical field of view may be measured using a pyrgeometer. This is an instrument rather like a pyranometer, but protected by a dome which is transparent only to thermal radiation in place of the glass domes used on a pyranometer. Figure 7.4 Eppley Pyrgeometer PYRRADIOMETER A pyrradiometer usually consists of a thermopile either unprotected or covered by a very thin dome made of polyethylene which is almost transparent to both solar and thermal radiation. Thermal irradiance can therefore be deduced from the difference between the reading from a pyrradiometer and that from a pyranometer. The accuracy of the deduced measurement is unlikely to be better than about ±10%. -57-

74 7.2.2 Measurement of Thermal Irradiance Outdoors The variations of thermal irradiance outdoors are not normally taken into account for collector testing. However, a pyrgeometer or a pyrradiometer may be mounted in the plane of the collector aperture and to one side at the mid height, to determine the thermal irradiance at the collector aperture. Measurements to an accuracy of better than ± 15 W/m 2 are unlikely to be achieved outdoors Determination of Thermal Irradiance Indoors MEASUREMENT The thermal irradiance may be measured using a pyrgeometer, or a pyrradiometer and a pyranometer, as indicated above for outdoor measurements. For indoor measurements it should be possible to achieve an accuracy of ±10 W/m 2. CALCULATION Provided that all sources and sinks of thermal radiation in the field of view of the collector can be identified, the thermal irradiance at the collector aperture can be calculated using temperature measurements, surface emittance measurements and radiation view factors. The thermal irradiance incident on a collector surface (designated 1), from a hotter surface (designated 2) is given by oe 2 F 12 T 2 "* Or, more usefully, the extra thermal irradiance compared with that which would have come if surface 2 had been at ambient temperature is given by: G L = af 12 (e 2 T2 1 * - Ta 4 ) Eqn. 7.9 Radiation view factors are given in text books on radiation heat transfer such as Reference (10). For example, using the simple case of radiation exchange between two co-axial discs of radii ri and r 2 spaced h apart, the view factor F{ 2 is given as x - {x 2-4(R 2 /R!) 2 } % Eqn where Ri = rj/h, R 2\ /TJ.2 2 = r 2 /h and x = 1 + (1 + R 2 2 )/Rj The thermal irradiance at the collector aperture can also be calculated from a series of measurements made for small solid angles in the field of view. Such measurements can be made using a pyrheliometer with and without a glass filter to identify the thermal component of the total irradiance. -58-

75 7.3 TEMPERATURE MEASUREMENT Three temperature measurements are required for solar collector testing. These are the fluid inlet temperature of the collector, the fluid temperature difference between the outlet and inlet of the collector, and the ambient air temperature. The required accuracy and the environment for these measurements are different and hence the transducer and associated equipment may be different Temperature Measurement Instruments MERCURY IN GLASS THERMOMETERS Mercury in glass thermometers are required mainly for calibration purposes and are available graduated at 0.1 C or 0.05 C intervals. The range of calibration reference thermometers is usually small (typically 30 C), and hence more than one thermometer may be needed. Most thermometers are calibrated for total immersion in the fluid and hence errors will be introduced if the thermometer is used when only partially immersed. Good thermal contact with the fluid is essential. Calibration reference thermometers should be regularly re-calibrated over their operating range by a recognised standards laboratory. One disadvantage of mercury in glass thermometers is that a "hard copy" of the measurement is not usually produced, so frequent visual monitoring is necessary. PLATINUM RESISTANCE THERMOMETERS Platinum resistance thermometers (PRTs) have an especially stable and reproducible resistance/ temperature relationship. For this reason PRTs offer an accurate method of measuring temperature. The temperature resistance relationship given by: for a PRT is Ro" " 1 + ai a 2(T/100-1)(T/100)] Eqn where RT is the thermometer resistance at temperature T C, Ro is the thermometer resistance at 0 C, and a; and a 2 are constants for the individual thermometer. -59-

76 Calibration of the thermometer, associated connecting leads, bridge circuit and readout device should be performed approximately once per year in a temperature controlled water or oil bath to obtain either a calibration curve or determine the constants a x and a 2 in equation Figure 7.5 Platinum Resistance Thermometer THERMOCOUPLES Thermocouples are widely used for solar collector testing. A thermocouple is formed by joining together two wires made of dissimilar metals. When the free ends of the wires are held at a reference temperature, usually 0 C, a voltage E is generated which is a nonlinear function of temperature (T). This function may be approximated by: E = a 0 + a x T + a 2 T 2 Eqn or T = b 0 + b x E + b 2 E 2 Eqn Suitable pairs of thermocouple materials are Copper/Constantan, Iron/Constantan and Chromel/Alumel. As the voltage generated by a thermocouple is derived from both the junctions and the wires close to the junctions in which there is a temperature gradient, care must be taken in both their construction and use if high accuracy is to be ensured. Junctions should not be placed directly into water, but should be protected by a suitable dielectric compound and encapsulated in a metal sheath to provide protection from corrosion, and to ensure that the wires are not subjected to stresses which might change the calibration of the junction. Most suppliers of thermocouple materials offer a wide range of encapsulated junctions which are suitable for use in collector testing. -60-

77 The connections between a thermocouple and the readout device will produce unwanted voltages unless compensated for, either thermally or electrically, as shown schematically in Figures 7.6 and 7.7 respectively. Thermocouple* - "-". Junction MoteriolA^-^ > Material S~~~-^ \ / V A / \ Copper Thermal Reference Device Copper Recordi Instrumi ;nt Figure 7.6 An Arrangement for Thermal Compensation of the Junction between the Recording Instrument and Thermocouple Copper Compensating voltage generator Thermocouple Junction Recording Instrument Figure 7.7 An Arrangement for Electronic Compensation of the Junctions between the Recording Instrument and Thermocouple -61

78 All connecting leads should be screened from electromagnetic interference, and suitable connections should be used to minimise the risk of unwanted voltages. Proprietary plugs and sockets are available on the market for the common thermocouple systems, but to avoid electrical noise it is important to keep these clean. Heat leakage along connecting leads can be minimised by using thin leads, insulation, and adequate depths of immersion. A thermocouple should be calibrated against a reference thermometer over the range of temperatures for which it is to be used, and a calibration curve obtained. Annual checks at a few selected points on the calibration curve should be sufficient to verify the calibration. QUARTZ THERMOMETERS Quartz thermometers employ a precisely cut quartz crystal which has a stable and repeatable relationship between resonant frequency and temperature. Each probe is factory calibrated and supplied with its own calibration module for use in the thermometer unit. Quartz thermometers are capable of very high accuracy but are rather expensive. OTHER TEMPERATURE SENSORS Several other temperature measuring devices are available on the market, but there is little experience with their use for collector testing Measurement of Heat Transfer Fluid Inlet Temperature (Ti) REQUIRED ACCURACY The temperature of the heat transfer fluid at the collector inlet need only be measured to an accuracy of ± 0.1 C, but in order to check that the temperature is not drifting with time a very much better resolution of the temperature signal to ± 0.02 C is required. This resolution is needed for all temperatures used for collector testing, (i.e. over the range from 0 C to 100 C) which is a particularly demanding signal for recording by data logger as it requires a resolution of one part in 4,000 or a 12 bit digital system (see Section ). MOUNTING OF SENSORS The transducer should be mounted at no more than 200 millimetres from the collector inlet. If it is necessary to position it outside this range a test should be made to verify that the fluid temperature measurement is not affected. -62-

79 To ensure mixing of the fluid at the position of temperature measurement, a bend in the pipework, an orifice or a fluid mixing device should be placed upstream of the transducer, and the transducer probe should point upstream as shown in Figure 7.8. Temperature Transducer ITe.AT) It -Pipework bend or mixing device <200mm Temperature Transducer (Ti.AT Solar Collector Pipework bend or mixing device Figure 7. 8 Recommended Transducer Positions for Measuring the Heat Transfer Fluid Inlet and Outlet Temperatures Measurement of Heat Transfer Fluid Temperature Difference (AT) REQUIRED ACCURACY The temperature difference between the collector outlet and inlet needs to be measured to an accuracy of ± 0.1 K in order to achieve ±1% accuracy for temperature differences of 10 K. Accuracies approaching ±0.02 K can however be achieved with modern well matched and calibrated transducers, and hence it is possible to measure temperature differences down to 1 or 2 K with a reasonable accuracy. TRANSDUCER ARRANGEMENTS AND CALIBRATION CHECKS Platinum Resistance Thermometers Platinum resistance thermometers (PRTs) may be arranged differentially to measure temperature difference. A matched pair of PRTs is required with some instruments, whilst in others the matching can be accomplished during calibration by a simple adjustment to the equipment. A zero check for PRTs should be performed regularly by placing both PRTs in a well stirred fluid bath at several temperatures spaced over the operating range. -63-

80 Thermopiles Thermocouples may be connected in series to provide a higher output voltage, and arranged differentially as shown in Figure 7.9 to measure temperature differences. In order to provide sufficient resolution, the minimum number of junctions recommended for such an arrangement is 6 (3 by 3). The thermocouples in a thermopile electrically insulated from each other. should be Differential thermopiles may be calibrated by placing the cold junctions in one well stirred fluid bath and the hot junctions in another, each with a reference thermometer. For high accuracy, it is preferable to perform the calibrations using the same readout instrumentation as is used for collector testing. The relationship between voltage and temperature for a thermocouple varies with absolute temperature, as shown in Equation 7.12, and it is therefore important to calibrate differential thermopiles as a function of absolute temperature. For example: with Chromel/Alumel thermocouples the output rises from 40 uv/ C at 0 C to 41 uv/ C at 70 C, giving 2.5% difference in calibration over the temperature range used for collector testing. A zero check for the differential thermopile arrangement should be made by placing both thermopiles in the same well stirred fluid bath, at temperatures in the normal test range. Alternatively the check may be performed in the collector test configuration by replacing the collector with a short piece of well-insulated piping. Recording Instrument Figure 7.9 A Thermopile Arrangement for Measuring the Temperature Difference across the Solar Collector -64-

81 7.3.4 Measurement of Surrounding Air Temperature (Ta) REQUIRED ACCURACY The ambient or surrounding air temperature should be measured to an accuracy of ±0.5 C. MOUNTING OF SENSORS For outdoor measurements the transducer should be shaded from direct and reflected solar radiation by means of a white painted, well ventilated shelter, such as a meteorological screen, or by two concentric vertical metal pipes. The shelter should be placed at the mid-height of the collector but at least 1 metre above the local ground surface to ensure that it is away from the influences of ground heating. The shelter should be positioned to one side of the collector within 10 metres of it. If air is forced over the collector by a wind generator, the air temperature should be measured in the outlet of the wind generator and checks made to ensure that this temperature does not deviate from the surrounding air temperature by more than ±1 C. m^ -White Paint ^-Concentric metal tubes -Temperature Sensor Figure 7.10 Ambient Air Temperature Sensor -65-

82 7.4 FLUID FLOWRATE MEASUREMENT Flowrate Measuring Instruments WEIGHING DEVICES A simple and accurate method of determining the fluid flowrate is to divert the fluid, downstream of the collector, into a vessel and to measure the mass of fluid delivered during a measured time period. When performing such measurements at high temperature, weighing vessels should be covered to minimise errors caused by evaporation. METERING PUMPS Metering pumps provide an accurate method of producing and measuring the fluid flowrate for collector testing. The principal disadvantage of metering pumps is their high cost. VARIABLE AREA METER (Rotameters) Variable area meters are simple, reliable and widely used for flowrate measurement. The flowrate is determined by the position of a float in a tapered glass tube. The measurement is dependent upon the viscosity and density of the fluid, which may vary considerably over the range of fluid temperatures and flowrates required for collector testing. Determination of the fluid flowrate to within ±1% may therefore be difficult using this type of flow meter. However, meters of this type can provide a suitable visual back-up to other devices, and could be used as the length of transparent pipe recommended in Section 6.2 for checking the presence of bubbles and contaminants. TURBINE METERS Turbine meters measure volumetric flowrate and hence the density of the fluid must be known as a function of temperature. Turbine meters are widely used for collector testing since accuracies of within ±1% may be achieved if the device is suitably calibrated over the flowrate range. The calibration of turbine meters should be checked frequently, for example before each new collector test, to ensure that the required accuracy of ± 1% is maintained. In order to protect these rather delicate instruments, special attention should be given to ensuring that the fluid is free of entrained air and contaminants.

83 UjiXi*, Figure 7.11 Turbine Flow Meter POSITIVE DISPLACEMENT METERS Positive displacement meters measure volumetric flowrate and hence the density of the fluid must be known as a function of temperature. In some models the fluid displaces a piston as it flows through the meter and hence an oscillating pressure drop may be generated. Some types of positive displacement meter totalise the flow, and hence are not recommended, as variations in the flowrate about the mean value are not immediately apparent. VORTEX SHEDDING METERS Vortex shedding meters utilise the phenomenon that a bluff body in a fluid stream produces regular pulsed vortices whose frequency of shedding is directly proportional to the fluid velocity. These vortices are easily sensed and counted and can result in high accuracy metering (±1%). However, these devices are very expensive and are not at present used to any great extent for solar collector testing. -67-

84 MAGNETIC FLOW METERS AND ULTRASOUND METERS Both magnetic flow meters and ultrasound flow meters measure the volume flowrate, and hence the density of the fluid must be known as a function of temperature. These types of meter are unaffected by the viscosity of the fluid and are capable of accuracies within ±1%. The accuracy of these meters deteriorates as the flowrate decreases, therefore transducers with a low flowrate threshold are recommended Measurement of Collector Fluid Mass Flowrate REQUIRED ACCURACY The mass flowrate of the heat transfer fluid should be measured to an accuracy of within ±1%. In order to ensure that this accuracy is maintained, it is recommended that the flow meter be calibrated before each series of collector tests, using the heat transfer fluid specified for the testing of the collector. The output signal from a flow meter is usually either in the form of a proportional voltage or a proportional frequency. The ratio of frequency to volume flowrate should be high enough to provide the required accuracy of ±1% when the flow is measured over one minute intervals. If it can be verified that the test facility pump will produce a repeatable flowrate constant to ±1%, an initial measurement of the flowrate for each test may be sufficient. CALIBRATION The flow meter should be calibrated over the range of fluid flowrates and temperatures to be used during collector testing. The recommended method of calibration is to collect the fluid downstream of the flowrate transducer over a period of approximately 300 seconds. The mass of the fluid collected should then be accurately determined and the average flowrate over the period obtained. It is important that during this period the flowrate is maintained constant to within ±1%. This may be achieved by means of a constant head device. -68-

85 7.5 CALORIMETRIC FLOWRATE MEASUREMENT Systems are under development in the European Community for measuring the calorimetric flowrate directly, i.e. the product of mass flowrate, specific heat capacity and temperature rise of a fluid. The advantage of these systems is that the specific heat capacity and density of the fluid used during the test do not need to be known explicitly. Note: An implied assumption in the use of these devices is that the specific heat capacity of the fluid in the electrical heater is the same as that of the fluid in the collector, although the temperature in the collector may differ by a few degrees from that in the electrical heater. Such devices may consist of an inline electric heater with variable input power, situated downstream of the collector in the fluid loop. Accurate temperature transducers immediately upstream and downstream of the heater measure the temperature rise of the fluid across the heater. A wattmeter measures the power input to the heater. The whole device is usually encased in approximately 100 mm of insulation. -I T Fluid Inlet Insulation (100mm minimum) Tj / / / * / / / / / / / Electrical Heater / / / / / / ^ ^ I Fluid Outlet Variable Electrical Power Supply Figure 7.12 Calorimetric Flowrate Measurement Device 69-

86 The design of the heater unit is important if steady temperature rises are to be achieved. The electric element should have a large surface area to avoid surface boiling, and baffles should be used to produce good fluid mixing. In some designs the fluid is passed around the unit in an annular chamber or thermal guard before entering the heated section, in order to minimise heat losses. The device may be operated by varying the power input into the heater until the temperature rise across the heater is the same as that across the collector. If the heat loss from the device is negligible, the power input to the heater as measured by the wattmeter is equal to the useful power extracted by the collector. In practice, however, a correction is generally applied to account for heat losses from the device. Alternatively a constant power input to the device may be used, and the useful power extracted by the collector determined by comparing the fluid temperature rises across the collector and heater.

87 7.6 AIR SPEED MEASUREMENT Air Speed Measuring Instruments CUP ANEMOMETERS A cup anemometer is a device for measuring the total horizontal component of air speed and is suitable for outdoor collector testing when no forced air movement is employed. A cup anemometer does not indicate the direction of the air movement. Figure 7.13 Cup Anemometer VANE ANEMOMETERS (AIR METERS) A vane anemometer consists of a number of vanes supported on short radial arms and mounted on a spindle. It is a directional transducer which measures the air speed in the direction of the spindle axis. Figure 7.14 Vane Anemometer 71-

88 HOT WIRE/HOT THERMISTOR ANEMOMETERS These instruments contain a hot element which is cooled by the air. The resistance of the element changes with temperature and hence with air speed. Some models are suitable for outdoor and indoor testing. Hand held devices are especially suited for determing the air speed at various positions around the collector. Most models are directional transducers. Thermistor Figure 7.15 Hot Thermistor Anemometer VORTEX ANEMOMETERS Vortex anemometers, which sense the formation frequencies of vortices created by the air flowing past an obstruction, may be used for measuring the air speed both indoors and outdoors. Most models are directional transducers. PRESSURE TUBES (PITOT TUBES) Pressure tube anemometers are suitable for the measurement of air speed both indoors and outdoors. The pressure tube is a directional transducer. -72-

89 7.6.2 Measurement of Surrounding Air Speed The heat losses from a collector increase with the air speed over the collector but the influence of wind direction is not well understood. Measurements of wind direction are therefore not used for collector testing. The relationship between the meteorological wind speed and the air speed over the collector depends on the location of the test facility, so meteorological wind speed is not a useful parameter for collector testing. By using the wind speed measured over the collector it is possible to define clearly the conditions in which the tests were performed. REQUIRED ACCURACY The speed of the surrounding air over the front surface of the collector should be measured to an accuracy of ±0.5 m/s for both indoor and outdoor testing. Under outdoor conditions the surrounding air speed is seldom constant and gusting frequently occurs. The measurement of an average air speed is therefore required during the test period. This may be obtained either by an arithmetic average of sampled values or by a time integration over the test period. MOUNTING OF SENSORS For outdoor and indoor testing the air speed may vary from one end of the collector to the other. A series of measurements should therefore be taken at a distance of 50 mm in front of the collector aperture, at equally spaced positions over the collector area. An average value should then be determined. Non-directional measurements may be made using a directional transducer if a vane is used to keep the transducer pointing in the direction of the air movement. When testing outdoors in unsteady wind conditions it may be necessary to obscure part of the collector aperture from solar radiation in order to take sample measurements of the air speed. For this procedure the transducer and its mounting should obscure less than 5% of the collector aperture for less than 10% of the test period. Alternatively an anemometer may be fixed to the middle of a board of the same size and mounting arrangement as the test collector and placed nearby. Air speed measurements indoors in stable conditions should be made before and after performance test points to avoid obscuring the collector aperture. CALIBRATION The anemometer should be recalibrated at yearly intervals. -73-

90 7.7 PRESSURE DROP MEASUREMENT INSTRUMENTS Pressure Drop Measurement Instruments MANOMETERS Liquid manometers are suitable for the measurement of pressure differences. Consideration should be given to ensuring that the density of the fluid in the manometer is accurately known and that the height of the fluid columns may be determined to a sufficient accuracy. For example, inclining the manometer will enhance its resolution. DIFFERENTIAL PRESSURE TRANSDUCERS Differential pressure transducers are suitable for the measurement of pressure differences. As these devices normally provide an electrical output they are suitable for use with data logging systems. These devices tend to be temperature sensitive, so extra precautions may be required if they are used outdoors Measurement of Collector Fluid Pressure Drop GENERAL REQUIREMENTS The pressure drop across a single collector is usually less than 200 Pa but some types of collector have a larger pressure drop. Detailed pressure drop data may be required when designing the pipework for an array of collectors. The fluid pressure drop between the collector inlet and outlet should be determined by means of static pressure tappings close to the collector inlet and outlet. The configuration of the static pressure tapping and the method by which it is produced may influence the accuracy of the measurement more than the type of transducer used. SIZE AND POSITION OF PRESSURE TAPPINGS The static pressure tapping should be installed normal to the flow. (A tapping inclined towards the flow will measure a higher pressure). The size of the tapping should be small. A hole diameter of 1 mm is typical. The edge of the tapping should be square and free from burrs. Failure to remove the burrs may result in pressure measurement-errors.

91 A smooth straight piece of pipe should be incorporated before the pressure tappings at the collector inlet and outlet. A straight pipe length of 10 pipe diameters is usually adequate. 7.8 COLLECTOR APERTURE AREA MEASUREMENT The aperture area of the collector should be measured to an accuracy of ±0.1Z. The area recommended for use with collector test results is the transparent area of the aperture, as defined in Other definitions such as that used in the UK (8), where glazing bars and intermediate supports over the absorber are included in the aperture area, should be clearly stated in the test report. The aperture area for evacuated tube collectors should be determined from the product of the length of transparent tube, the pitch and the number of tubes. 7.9 COLLECTOR FLUID CAPACITY MEASUREMENT The fluid capacity of the collector, expressed as a mass of the heat transfer fluid used for the test, should be measured to an accuracy of at least ± 10%. Measurements may be made either by weighing the collector when empty and when filled with fluid, or by filling and emptying the collector to determine the mass of fluid which it will contain HUMIDITY MEASUREMENT The measurement of humidity is of particular importance when testing air heating collectors because of its effect on the specific heat capacity and mass flowrate of the heat transfer medium. For 'open' test loops which take air from the vicinity of the test collector it is adequate to measure the relative humidity outside the loop. In other cases, and for 'closed' loops, it is necessary to measure the humidity of the air inside the loop ductwork. -75-

92 Humidity Measuring Instruments A number of different types of instrument are available for the direct measurement or the calculation of humidity: (a) Wet and dry bulb thermometers (b) Dew point hygrometers (c) Mechanical hygrometers (d) Electrical hygrometers (a) to (c) are usually direct readout instruments only. Electrical hygrometers, that use resistance or capacitance transducers, can provide an electrical output for data logging purposes Accuracy and Calibration It is desirable to ventilate humidity measuring instruments. Direct readout instruments typically have an accuracy of ±5%. Electrical instruments typically have accuracies of ±2% in the 20-80% humidity range but diminishing accuracies at higher humidities. Instruments should be recalibrated annually. Figure 7.16 Electrical Hygrometer

93 7.11 OVERALL CALIBRATION CHECK FOR CALORIMETRIC MEASUREMENTS Summary In any collector testing installation it is important to measure the fluid flowrate and the temperature rise or drop of the fluid accurately as it passes through the collector. Calibration of the transducers and instrumentation which measure these parameters is often both time consuming and expensive. An overall check on the flowrate and temperature difference measuring instruments can be made in situ by replacing the collector in the test loop by a reference heat source. This check is not a substitute for full calibrations, but may reduce the frequency at which full calibrations need to be made Apparatus The equipment required in addition to that already available in the test facility consists of an electric fluid heater in a small insulated vessel, a wattmeter, a stabilised electrical supply with an accuracy of <±1% and a variable transformer. The heater should have a large surface area to ensure that boiling does not occur on its surface Procedure The heater is installed in place of a collector in the test facility and the pipe fittings insulated in the same way as for collector testing. The heater unit should be shielded from solar radiation if necessary. The flowrate and temperature measuring devices should be installed in their usual positions in the test loop. The thermal output from the reference heat source is determined from the product of the fluid flowrate and temperature rise at approximately four electrical input powers in the range typical of the output from the collectors to be tested (e.g. from zero to 1500 W). Approximately five fluid inlet temperatures spaced equally over the range used for collector testing (e.g C) are employed. The heat loss coefficient for the reference heat source is derived from the measurements made with zero electrical input. The sums of the heat losses and the measured heat gains are then compared with the electrical inputs for each fluid inlet temperature. Any errors should be investigated, and for this it may be helpful to plot the error as a function of fluid temperature rise, and as a function of fluid temperature above ambient. Individual instrument recalibrations ^ should be made before further collector testing is v u ^'"' performed, if calibration errors are suspected. 'r <^- %

94 7.12 DATA RECORDING General The quality of collector test results depends on the design and maintenance of the system used for measuring the test parameters. A measurement system includes transducers, signal conditioning units and readout equipment. Each of these three elements should be regularly calibrated and, where possible, the three should be calibrated together. The choice in data recording systems becomes wider each year, and a review of available equipment was published in 1983 by Ferraro (11). With the recent availability of microcomputers and instrument interfaces it is now feasible to record, analyse and print out test results using a single set of equipment, and this is becoming an increasingly common approach. However, many laboratories retain a visual indication of the current status of parameters for the experimenter to monitor during a test, either as a computer display or by the use of digital indicators. In addition it is often found useful to retain a chart recorder to indicate the stability of key parameters during steady state testing. The steady state criteria can be programmed into the computer for the selection of acceptable test points, but unless the facility is completely automated a visual indication is also provided for the test engineer Digital Systems In choosing a data recording system there are many factors to consider including cost, compatibility with existing equipment and suitability for the instruments in use. Furthermore it is particularly important to recognise the high demands made on a data logger by the fluid inlet temperature signal. This has to range from about 10 C to 90 C, and in order to detect small drifts in inlet temperature a resolution of ±0.02 K is desirable at any point in the temperature range. This represents the need for a resolution of 1 in 4000 which requires a 12 bit digital system. Many laboratories use one of the more common 16 bit digital systems in order to ensure adequate precision in their measurements, but it is clear that the cheaper 8 bit systems are not adequate to detect inlet temperature drift Signal Averaging Data recorders for transient testing, and in some cases for steady state testing, must be able to accommodate small variations in transducer signals, and this implies the use of averaging techniques. The rate it which a signal should be sampled is related by statistical

95 theory to the confidence in average values deduced from the readings. With this problem in mind, several facilities have moved towards the use of analogue integrators which can be sampled at a lower frequency and thus minimise the quantity of stored data. The quality of the integrator should be good enough to maintain the required accuracy of the parameter whose signal it is integrating. Figure 7.17 Data Logger with Cassette Tape for Transfer to Computer Figure 7.18 Multiple Trace Chart Recorder for Monitoring Steady State Test Conditions -79-

96

97 CHAPTER 8 COLLECTOR PERFORMANCE ANALYSIS 8.1 INTRODUCTION The performance of a flat plate solar collector is usually expressed in the form of a linear characteristic, as discussed in Chapter 2, implying an assumption that the optical and heat transfer coefficients are constant: Q = Aa F'xa G - Aa U(Tm - Ta) Eqn. 8.1 or, in terms of efficiency: n = no - U(Tm - Ta)/G = no - UT* Eqn. 8.2 The performance of collectors is in fact very much more complex than this simple relation suggests, but for many purposes the linear characteristic is sufficient. For detailed analyses of collector performance, the reader is referred to specialised textbooks such as those by Duffie and Beckman (9) or by Garg (40). In this chapter a brief analysis of collector performance is given in order to provide a background for the test methods recommended in this document, and to indicate some of their limitations. 8.2 COLLECTOR EFFICIENCY The efficiency of a collector may be defined as the instantaneous rate of heat output expressed as a proportion of the solar #flux incident at the collector aperture. The output Q is usually measured as the product of the mass flowrate of fluid, its specific heat capacity and the temperature rise across the collector: Q = m c f (Te - Ti) Eqn. 8.3 This equation has any meaning only in steady state conditions, because there is a time delay while fluid is passing through the collector. If the conditions are varying, then the difference between inlet and outlet temperatures is not directly related to the current rate of heat collection.

98 The steady state efficiency of a collector is influenced by many external variables, of which the most important for any given average fluid temperature are: (1) Wind speed (in some cases also direction) (2) Surrounding air temperature (in front of and behind the collector) (3) Solar irradiance (incidence angle, percentage diffuse and power level) (4) Thermal irradiance. Collector efficiency is also influenced by the properties of the heat transfer fluid used, and by the flowrate. The magnitude of the changes in efficiency caused by variations in these variables depends on the collector design, but may be estimated and taken into account if necessary, as discussed in Sections 8.3 to HEAT LOSSES FROM A FLAT PLATE COLLECTOR In most flat plate collectors, the back and side heat losses are reduced by insulation to little more than 10% of the total heat losses. Second order effects of back and side heat losses can therefore be neglected in most cases. Heat losses in the forward direction can be considered as having to flow through two elements of thermal resistance connected in series. The thermal resistance between the absorber and the outer cover is usually higher than that between the cover and the environment, and hence overall heat losses are more significantly affected by collector design changes which alter this internal heat flow than they are by environmental changes. HOT IABSORBEFOWWWV'I High resistance to heat flow COOL COVER l w/ww 1 ENVIRONMENT! Low resistance to heat flow Flow of heat losses Figure 8.1 Collector Heat Losses -82-

99 Heat is transferred from the absorber to the cover by both natural convection and radiation. (The rate of convective heat transfer can be reduced by evacuation or by the use of honeycomb materials, and the rate of radiative heat transfer can be reduced by means of selective surfaces, as discussed in Chapter 19). However, both heat transfer mechanisms are temperature dependent, such that the heat transfer coefficient between the absorber and the cover increases as the mean collector temperature increases. External heat transfer between the cover and the environment also takes place by means of a combination of convection and radiation. The dominant mode here is forced convection, except at very low wind speeds. Hence it can be seen that the overall heat loss coefficient for a collector increases with temperature. A full analysis of collector heat losses is given by Duffie and Beckman (9), but for practical purposes it is sufficient to make an approximation and to assume a linear variation in the overall heat loss coefficient for a collector, of the form: U = a 1 + a 2 (Tm - Ta) Eqn. 8.4 which leads to a curved collector performance characteristic: no ai T* - a 2 G(T*)2 Eqn THE EFFECT OF WIND ON COLLECTOR HEAT LOSSES The heat losses from most collectors are significantly influenced by the wind speed ever the collector, unless the performance is rendered insensitive to external influences by having a very high internal heat resistance, as in the case of evacuated collectors. A typical example of the sensitivity of a single glazed matt black collector to wind is shown in Figure 8.2, where it can be seen that the efficiency drops markedly as the wind speed increases from zero to 3 m/s, but subsequent increases have a less significant effect. The direction of the wind may also affect the performance of some collectors, but information on this effect is not available. For evacuated tubular collectors, where the overall heat loss coefficient is less than 2 W/m 2 K, the dominant losses may occur in the manifold regions, and pipe connections should-therefore be well insulated in order to ensure optimum performance. 83-

100 O.Oi T* IK m 2 W" 1 ) Figure 8.2 Single Glazed Matt Black Collector Performance as a Function of Wind Speed 8.5 THE INFLUENCE OF SURROUNDING AIR TEMPERATURE ON COLLECTOR EFFICIENCY The rate of heat loss from a collector depends primarily on the difference between the temperature of the fluid in the collector and that of the surrounding air. Hence as the air temperature rises, so the efficiency will increase for a given mean collector operating temperature. This effect is included in the variation of efficiency with the reduced temperature difference T*, which is proportional to the temperature difference (Tm-Ta). -84-

101 8.6 SOLAR IRRADIANCE EFFECTS ON COLLECTOR EFFICIENCY Irradiance Level In the simple linear characteristic of Equation 8.2 the influence of irradiance level does not appear. However, it was shown in Section 8.3 that the overall heat loss coefficient increases with temperature. From this it follows that the value of U corresponding to a high value of (Tm-Ta) will be higher than that for a low value of (Tm-Ta). Now a given T* value may be derived from high (Tm-Ta) at high irradiance or from low (Tm-Ta) at low irradiance. Hence the heat loss coefficient and therefore the efficiency can be seen to vary with irradiance level for a given value of T*. The extent to which the efficiency of a collector varies with irradiance level is directly related to the temperature dependence of its heat loss coefficient. Evacuated tubular collectors, for which there is little variation in heat loss coefficient with temperature, show little variation in their efficiency curves with irradiance level T* (K m2 W" 1 ) Figure 8.3 The Influence of Irradiance Level on Single Glazed Matt Black Collector Performance -85-

102 8.6.2 Diffuse Irradiance The influence of diffuse irradiance on collector efficiency depends on the cover system of the collector, and its transmission characteristics. The value of transmittance for diffuse solar irradiance is lower than for direct solar irradiance in most collectors, and hence the collector efficiency decreases with increasing diffuse irradiance percentages Incidence Angle Effects As discussed in Section 10.7, the transmittance of flat collector covers decreases as the incidence angle (the angle between the incident beam and the surface normal) increases. This effect, which results from the optical properties of the cover material, may be enhanced by shading of the absorber at shallow incidence angles, and the combined result is a decrease in collector efficiency with increasing incidence angles Effective Normal Irradiance For the purposes of solar heating system modelling, collector performance is often expressed as: Q = Aa[rio b K <8) Gb + no d Gd - U(Tm - Ta) ] Eqn. 8.6 where n 0 t> an d nod are the effective values of F'ax for beam and diffuse solar irradiance respectively, and the direct beam irradiance Gb and the diffuse irradiance Gd are computed for each time step in the model. A similar approach can be followed for outdoor collector testing, where there is usually some diffuse irradiance present. In the case of collector testing however, the incidence angle modifier K(v) is held near to unity by moving the collector. If the optical properties of the collector are known, then the simple collector performance characteristic given in Equation 8.2 can be derived from tests under a variety of diffuse and direct irradiance conditions by calculating the effective normal irradiance G' for each test point, using the relation (ax) Gb + (ax), Gd G' = -,, - Eqn. 8.7 n (ax) 0 This approach has not yet been widely adopted for collectors whose K(v) cannot be calculated from the optical properties of its components, but it is incorporated in the British Standard Draft for Development (8). -86-

103 8.7 THERMAL IRRADIANCE EFFECTS ON COLLECTOR EFFICIENCY The effect of thermal irradiance on collector performance depends on the collector design. In unglazed collectors it is very important because the radiation exchange takes place directly with the absorber, but some evacuated collectors are very insensitive to thermal irradiance. In outdoor conditions the atmospheric thermal irradiance level on a horizontal surface may be correlated approximately with the ambient air temperature and the sky conditions. A comparison of such correlations has been made by Green (43), from whom the correlations shown in Figure 8.4 were obtained. It can be seen from this figure that the atmospheric thermal irradiance in clear sky conditions can vary from about 60 to about 120 W/m 2 below that from a blackbody at ambient temperature. In overcast conditions the atmosphere approximates closely to a blackbody cavity at ambient temperature AMBIENT TEMPERATURE (K) o Unsworth & Monteith Equation: G L = 1.06aTa"*-119 (W/m 2 ) n Swinbank Equation: G L = 9.37xlO" 6 ota 6 (W/m 2 ) Figure 8.4 The Variation of Atmospheric Thermal Irradiance with Ambient Air Temperature -87-

104 It can be shown (9,41) that, for collectors with thermally opaque covers (such as glass), the thermal radiation exchange is reduced approximately in the ratio of the collector overall front heat loss coefficient (U-F'Uback) to the heat loss coefficient between the cover and the environment (F'Uca), such that: Q = Aatnu G - U(Tm - Ta) - u e (ata 1 * - G )] Eqn. 8.8, (U - F'Uback) where: u = ^ n r ^ >- e = cover emittance In outdoor conditions the variations in thermal irradiance do not usually affect the efficiency of glazed collectors by more than ±1%, but can influence unglazed collectors by more than 10%. The correction suggested in Equation 8.8 is more important for use with solar simulator test results, where a consistently higher level of thermal irradiance than in outdoor testing may occur. Correction of test results to a reference value of thermal irradiance is proposed in the British Standard Draft for Development (8), whilst other standards (42) specify a limit to the thermal irradiance level in solar simulators. Results from tests performed in the solar simulators of the CEC Collector Testing Group are corrected to a reference value of thermal irradiance equal to 50 W/m 2 less than that of a blackbody cavity at ambient air temperature. Outdoor data are not corrected, mainly because measurements of outdoor thermal irradiance are seldom made. In the absence of a well defined alternative, this reference condition is recommended for general use in connection with Equation 8.8, for correcting solar simulator test results to representative outdoor operating conditions. 88-

105 8.8 CONVENTIONS FOR PRESENTING COLLECTOR EFFICIENCY Fluid Temperature The CEC Collector Testing Group have agreed to present collector efficiency in the form of the characteristic defined in Equation 8.2 which uses the mean fluid temperature in the collector. With this approach the perfcin.ance for most collectors is only weakly affected by the fluid flowrate used. Where results are presented in terms of fluid inlet temperature (as is common in the USA), a collector flow factor F_ is used such that: = F R a - F R U (Ti Ta)/G Eqn. 8.9 In this case the flow factor F can be related to the collector efficiency factor F' using the relation: F" = m c F' Aa U T expc-f' Aa U /m c ) Eqn The resulting values of n, no and U are all lower when plotted in terms of (Ti-Ta) instead of (Tm-Ta). It is therefore important to ensure that compatible data are used when comparing collector test results Collector Area A number of countries have decided to use the gross collector area for the presentation of the collector efficiency curve, rather than the collector aperture area recommended in this document. There are arguments in favour of each convention, and the final decision is arbitrary. However, values of n, riu and U are all smaller when referenced to the gross collector area than when referenced to the aperture area. Those in favour of using the gross area usually argue that it can be easily understood and measured. Those in favour of using the aperture area suggest that it is more representative for interpretation in a mathematical model, and furthermore that it does not penalise the collector with good edge insulation in the way that using the gross area does. Values of nu and U can be easily converted from one convention to the other, since both are inversely proportional to the -reference area. 89-

106 8.8.3 Solar Irradiance Level Collectors are usually tested outdoors at irradiance levels of greater than about 600 W/m 2, and little attention is paid to the actual levels used. Mixed indoor/outdoor results are usually assumed to be largely independent of irradiance level, and are simply calculated at the "typical value" of 800 W/m 2 for convenience. As advanced collectors become more common in the market place, it appears likely that performance nay need to be presented for different values of solar irradiance. Heat pipe collectors have already been shown to exhibit significantly different performances for different irradiance levels. At present, however, no widely accepted conventions for efficiency curve presentation as a function of irradiance levels have been established Curve Fitting The Format Sheets given in Appendix III require a first order and/or a second order fit to the collector efficiency curve. This is generally done by means of a regression or least squares fitting routine, on a computer or calculator. For the linear characteristic, fitting routines are widely available to determine the best estimates for no and U. For the relationship n = no - *i T* - a 2 G (T*) 2 Eqn a routine should be used to minimise the sum S of the squares of the errors: i = N S = Eqn where N is the number of data points, and e is the error of the curve fit defined for each collector efficiency measurement (i) as: e = \ no ai (T*l a 2 G (T*). 2 Eqn I The fitting routine should provide the best fit to no> aj, and a 2 for the measurements made. Where only few data sets are available, it may be that the standard error associated with the value of a 2 is larger than the value itself. In this case a first order fit to the data is likely to be a more satisfactory approach. Statistical routines for calculating the standard errors of coefficients obtained from curve fitting are available on many computers. 90-

107 Experience has shown that quite different combinations of values of aj and a 2 can be fitted to collector characteristics which look very similar when plotted in the recommended format. When comparing results from collector tests, it is therefore recommended that the curves themselves be compared, or that a table of efficiency values at particular values of T* be compared rather than values of a 1 and a 2. For example, for water heating collectors it'has been found useful to compare the efficiencies at values of T* equal to 0, 0.02, 0.04, 0.06 and ACCURACY OF COLLECTOR TEST RESULTS General The user of the results from a collector test needs to understand the level of accuracy of the data which he receives from a test laboratory. Uncertainty concerning the results of a collector test may be caused by: (a) (b) (c) Random experimental error Calibration (bias) errors in the measurements Test results obtained in operating conditions which do not meet the requirements for a test data point (d) Random Errors Test method unsuitable for the collector under test. Values for the accuracy to which each measuring instrument should be calibrated are recommended in Table 7.1, taking account of both the current state of the art in calibration and the requirements for accuracy in the test results. Provided that these recommendations are followed, experience within the CEC Collector Testing Group suggests that random errors can be reduced to no more than about ±0.02 in typical values of no anc l about ±0.5 U/m 2 in typical values of U, when following any of the steady state efficiency test methods recommended in Chapter 10 for conventional flat plate collectors.

108 8.9.3 Calibration Errors Errors in instrument calibrations are passed directly through the analysis of collector test results, to give errors in the collector efficiencies deduced. In the format sheets given in Appendix III, test laboratories are asked to indicate the accuracy of their instrument calibrations and the dates when the most recent calibrations were obtained. Recommendations concerning instrument calibration procedures and the accuracies which may be expected from them are given in Chapter 7. Most laboratories should have no difficulty in meeting the accuracy requirements specified for collector testing, provided that they follow these recommendations Errors Caused by Testing in Unsuitable Operating Conditions One of the most important elements in a collector test facility is the equipment which ensures that the collector is supplied with fluid at a constant inlet temperature. Small drifts in collector inlet temperature can cause large errors in measurements of collector efficiency, as discussed in Chapter 6. Minimum levels of irradiance are specified for outdoor testing, with a view to reducing the likelihood of significant errors in the efficiencies produced. If the tests are carried out under conditions other than those recommended, then extra precautions may be needed in order to maintain the accuracy of the results produced Suitability of Test Methods The operating conditions specified for the outdoor and solar simulator steady state test methods usually ensure that representative test results are obtained directly. Normalisation of results using a computer model becomes necessary if high levels of diffuse irradiance are present during outdoor testing or if there are unusual wind conditions. However, it is usually sufficient simply to carry out the tests in the specified conditions. The test method which has the greatest potential for giving unrepresentative collector efficiency values is the mixed indoor/outdoor test. As shown in Section 10.4, the efficiencies are deduced in this method from independently measured values of ri 0 and collector heat losses, using the equation: n = no - A^G Eqn

109 This method of deducing the efficiency at different temperatures and irradiance conditions has been shown to be satisfactory only if the collector heat loss coefficient U and the other heat transfer mechanisms which influence the value of F' do not change with temperature (41). In practice the method works reasonably well for collectors with values of F' near unity and heat loss coefficients which vary little with temperature. However, for poor collectors with a low value of F', the mixed indoor/outdoor test method will indicate values of collector efficiency which are higher than those which would really occur in practice. (Note: It has been argued that the mixed indoor/outdoor test method should not be used because the mean absorber temperature is higher than the mean fluid temperature during normal operation, but lower than the mean fluid temperature when heat losses are being measured. However, analysis of the heat transfer mechanisms in a collector does not support this argument unless the resistances to heat flow are sensitive to direction. Unrepresentative results from mixed indoor/outdoor tests are primarily caused by variations in F' and U with temperature.) In practice, there are several types of collector which cannot be easily tested using the mixed indoor/outdoor test method. Collectors with heat pipes in their absorbers exhibit collection efficiency characteristics which are sensitive to irradiance level, operating temperature and the direction of heat flow. Some collector designs which employ heat transfer oils also have temperature sensitive efficiency characteristics because the heat transfer properties of the oil vary with temperature, causing variations in F'. These collectors should be tested in a solar simulator or using the outdoor steady state test procedure, taking account of the influence of solar irradiance level discussed in Section

110

111 CHAPTER 9 OPTICAL PERFORMANCE MEASUREMENTS 9.1 INTRODUCTION When evaluating the performance of solar collectors or developing new designs, it is important to be able to measure more than simply the overall thermal performance of complete collector modules. In this chapter, procedures are recommended for measuring the optical properties of absorbers and covers. These procedures are somewhat specialised, and may employ relatively expensive instruments. The absorptance of absorbers and the transmittance of covers for solar radiation can be measured using relatively well established techniques, provided that appropriate calibration standards are available. However, thermal emittance measurement for absorbers and thermal transmittance measurement for covers are less well established, and work is continuing on the development of new equipment for making thermal radiation measurements. Solar absorptance and transmittance measurements can be used to predict the value of Eta Zero for a collector, and thermal emittance and transmittance measurements can be used to predict the heat losses from the front of a collector (see Chapter 8). 9.2 SOLAR ABSORPTANCE OF ABSORBER SURFACES Introduction The solar absorptance a of an absorber directly influences the solar energy absorbed by the collector, and is therefore an important design parameter. Its value may vary spatially over the absorber area', and an average value is therefore required for performance prediction purposes. Most absorbers may be expected to have a solar absorptance of greater than 0.9 for all wavelengths in the solar spectrum, and where this is the case, the results are relatively insensitive to the method of measurement used. However, some selective surfaces exhibit spectral variations in absorptance within the solar wavelength range, and require detailed spectral measurements in order to determine the true solar absorptance. Two standards (17, 18) have been published on solar absorptance measurement, but collaborative work by the CEC Collector Testing Group has shown that further standardisation is required to achieve the desired levels of accuracy in solar absorptance measurements. 95-

112 9.2.2 Spectrophotometer Measurements A spectrophotometer with an integrating sphere may be used to determine the hemispherical reflectance of an absorber surface for a near-normal beam of incident radiation, as a function of wavelength in the range from 0.3 to 4 um. Spectrophotometers without an integrating sphere are not suitable for solar absorptance measurement. (Note: Many spectrophotometers employed to measure properties over the solar wavelength region are unable to work at wavelengths greater than about 2.5 um. The power available from the sun between 2.5 and 4 um may usually be neglected when determining the optical properties of solar absorbers.) Some integrating spheres employ a centrally mounted specimen, and measurements can then be made for a range of incidence angles. However, for most solar collectors, only the range of incidence angles from normal incidence to about 60 is of interest because the transmission of the cover is very low for incidence angles of greater than about 60. For opaque absorbers, the absorptance may be obtained directly from the relationship: a(x) = l-p(x) Eqn. 9.1 and the solar absorptance is obtained by weighting the measured spectral absorptance values with an appropriate solar spectrum using the relationship: a s J a(a) Gx dx Eqn. 9.2 Gx dx The variation of irradiance Gx with wavelength given by Moon (19) for Air Mass 2 solar radiation is widely used for this calculation. This spectrum applies for clear sky conditions. In some countries, such as Belgium, irradiance spectra for cloudy skies have been standardised (20). A useful set of values for solving Equation 9.2 is included in ASTM E424-71, (17) and these are reproduced here for convenience in Table 9.1.

113 Wavelength nm Relative Energy % Wavelength nm Relative Energy % Wavelength nm Relative Energy % Table 9.1 Solar Energy Sp< ectrum (from Reference 17) Measurements made by the CEC Collector Testing Group have shown that the use of "selected ordinates" for performing this computation (as proposed in ASTM E424-71) can cause significant errors when working with selective absorber surfaces, and this approach is therefore not recommended. International discussions continue concerning the desirability of using an Air Mass 1.5 instead of the Air Mass 2 solar spectrum, but for most absorbers the effect of such a change is very smal1. An important aspect of the measurement is the reference surface used to calibrate the spectrophotometer. Most laboratories employ BaS0 4 reference standards, but these are not easily checked except by national standards laboratories such as the NPL in the United Kingdom (21) and the PTB in W. Germany (22). With the usual approach employing calibrations at high reflectance and a zero reading, the accuracy of reflectance measurements for highly absorbing surfaces depends on the linearity of the instrument. Calibrated reference surfaces, with a value of absorptance close to that of the specimens under study, will enable more accurate measurements, and these can be purchased from standards laboratories such as the NPL. -97-

114 An important disadvantage of using conventional spectrophotometers is that small surface samples are required. In order to determine the average absorptance of a solar absorber it may therefore be necessary to cut a number of small pieces out of the absorber. This approach is often unsuitable for monitoring degradation of surfaces Solar Simulator Measurements Commercial instruments are available which contain a lamp to simulate solar radiation and an integrating chamber to measure total hemispherical reflectance (23). The accuracy of the measurements depends on the spectral output of the lamp employed and the quality of the integrating chamber, but good results can usually be achieved with "grey" (non-selective) absorbers provided that a well calibrated reference surface is used to calibrate the instrument. These instruments are usually portable and can therefore be used to make a large number of measurements spaced over the absorber area to give a spatial average value. They are also attractive for degradation monitoring. Instruments which do not contain integrating spheres or chambers will not detect diffusely reflected radiation. The resulting reflectance measurements will therefore be too low and the deduced absorptances too high. Such instruments are not suitable for measuring the absorptance of absorber surfaces. 98-

115 9.3 THERMAL EMITTANCE OF ABSORBER SURFACES Introduction The thermal emittance e of an absorber influences the heat losses from a collector, and should be minimised to ensure good collector performance at elevated temperatures. The thermal emittances of matt black paints are typically about 0.9, but a selective absorber may have an average thermal emittance of only 0.1 for temperatures in the range 0 to 200 C. The value of absorber emittance needed for heat transfer calculations is the total hemispherical emittance of the surface (eh) weighted in accordance with the blackbody emission spectrum for the specified temperature. This property should not be confused with the near-normal emittance of the surface (en) which would be higher for dielectric surfaces and lower for metallic surfaces. The thermal emittance of a surface may be expected to vary with temperature, and for some selective surfaces used in evacuated collectors the variation can be quite important for temperatures in the range from 100 C to 200 C. However, for most surfaces there is little variation with temperature in the range 0 to 100 C. With low conductivity materials it is sometimes difficult to define the surface temperature, particularly if the surface is partially transparent in the infra-red, because radiation may be absorbed and emitted at different depths in the surface and consequently at different temperatures. Fortunately this effect is small for most absorber surfaces. A number of standards have been published on thermal emittance measurement (18, 24, 25, 26, 27) and a wide variety of techniques is reported in the literature. However, further work is required before a single technique can be recommended for general use Spectrophotometer Measurements Several models of spectrophotometer are available for making measurements in the appropriate wavelength range for thermal emission at temperatures in the range C, (i.e. 2.5 pm to 50 pm). However, most instruments are equipped to make only specular reflectance measurements and these provide little information concerning the hemispherical emittance of the surface. Integrating spheres are needed for hemispherical property measurements, but these are not widely available for use with thermal radiation, and where they do exist they are suitable for use only in the range from 2.5 pm to 20 pm. Thus very few laboratories are able to perform spectral measurements of the thermal emittance of absorbers.

116 When using a spectrophotometer it is important to calibrate it using surfaces which are similar in character to those of the specimens. Absorbers can exhibit local values of thermal emittance over a wide range, from (say) 0.02 to 0.95, and may have either highly specular or highly diffuse reflectance properties. A number of reference surfaces to cover the range of emittance values is therefore desirable for calibration purposes, but again these are not widely available. Gold is often used as a low emittance standard, and reference surfaces can be calibrated at standards laboratories such as NPL (21) and PTB (22). Where spectral data can be obtained, the emittance should be determined by weighting them with a blackbody spectrum for a reference temperature such as 50 C. The use of selected ordinate techniques for determining the emittance is not recommended. An alternative spectral approach is to use a double beam I-R spectrophotometer, which compares the flux from the specimen with that from a blackbody at the same temperature. However, instruments of this type are usually equipped to measure only the near normal emittance of the surface (24,25) Infra-red Heat Source and Detector The near normal reflectance of a surface can be determined using a thermal heat source and a detector such as a thermopile or heat flux meter (26). Several devices are available, including portable units which permit an average emittance to be determined from a series of readings made over the absorber area (28). Figure 9.1 Portable Thermal Emittance Measuring Instrument 100-

117 9.3.4 Calorimetric Measurements The required hemispherical emittance parameter may be determined by involving the specimen surface in a heat transfer process in which all other variables are well defined (18). For example, an electrically heated specimen may be suspended in an evacuated chamber which is maintained at the temperature of liquid nitrogen, and its emittance determined by heat transfer analysis. Such measurements can be quite accurate, but are slow to perform Comparison between Fluxes from Specimen and Standard" Comparisons may be made by detecting the total thermal flux emitted by the specimen and by a standard surface when the two are at the same temperature. A pyro-electric detector and an optical chopper, or simply a thermopile detector may be used for these comparisons. Such comparisons may be made for a number of angles with respect to the surface normal, in order to determine the hemispherical emittance of the specimen. Figure 9.2 Optical Properties Measurement Laboratory 101-

118 9.4 SOLAR AND THERMAL TRANSMITTANCE OF COVER MATERIALS Solar Transmittance Measurement The solar transmittance of collector covers can be determined accurately by a number of methods for specular, homogeneous materials like glass and acrylic sheet, but more care is needed with diffusing materials such as glass reinforced polyester, and with non-uniform materials such as ribbed or corrugated plastics. The transmittance of a collector cover varies more strongly with incidence angle than the absorptance of a surface, and measurements as a function of incidence angle are therefore important. The transmittance of collector covers for normal incidence solar radiation may be considerably higher than that for diffuse solar radiation. Methods for measuring the solar transmittance of covers are well established (17, 29, 30). SPECTROPHOTOMETER METHODS The transmittance of a cover may be measured for near-normal incidence radiation using an integrating sphere, and in some instruments a range of incidence angles can be accommodated by altering the sphere geometry (use of wedges, etc.) The recommendations of Section 9.2.2, covering the reference spectrum and calculation procedures for solar absorptance also apply to solar transmittance measurements. SUN/SIMULATOR & DETECTOR METHODS Approximate measurements of solar transmittance are sometimes made using the sun itself and a pyranometer, or a lamp and detector (17). When making such measurements, care is required to ensure that (a) the spectral response of the detector does not influence the results, (b) the geometry and cosine response of the instrument are such that diffusely transmitted radiation is properly measured, (c) stray reflections are eliminated, and (d) that the detector is not affected in some other way by the presence of the cover sample, such as by a change in incident thermal irradiance or in wind direction Thermal Transmittance Measurement The thermal transmittance of a collector cover is less frequently measured than its solar transmittance. Indeed, for glass it can usually be assumed that the thermal transmittance is zero. Most equipment simply measures the normal transmittance of a direct beam of radiation, and is unable to integrate diffusely 102-

119 transmitted radiation. Further work is required to identify an appropriate method for measuring the transmittance for diffuse thermal radiation, which is the parameter required for calculating the performance of a collector cover. SPECTROPHOTOMETER METHODS Many spectrophotometers are designed to measure thermal transmittance spectra for chemical studies, but they do not collect diffusely transmitted radiation and therefore give an unrepresentatively low thermal transmittance value. As discussed in Section 9.3.2, integrating spheres for infra-red measurements are not widely available. Where facilities exist the general recommendations given in Section apply. OTHER METHODS Several of the methods described in Section 9.3 could be used to measure the direct transmittance of a cover, but the assessment of the measurements is usually complicated by the emission of thermal radiation from the cover itself. Such methods cannot be recommended at present

120

121 CHAPTER 10 THERMAL PERFORMANCE TESTS FOR WATER HEATING COLLECTORS 10.1 INTRODUCTION The most well established procedure for determining the efficiency of a solar collector is to measure its heat output under steady state operating conditions outdoors. This approach has been widely used for many years, not only in Europe, but also in many other countries, and a detailed procedure for steady state efficiency measurement is given in Section However, whilst this procedure works well and gives good reproducible results on clear days, it is not at all suitable for use on cloudy or overcast days. Over much of Europe clear days occur infrequently, and consequently a number of alternative test procedures have been developed. One of the first means suggested for reducing the number of clear days required for testing was to measure the heat loss coefficient for the collector indoors, and to use outdoor steady state testing only for determining Eta Zero. This method was developed by the BSE in W. Germany (12) and has now been adopted as the German Standard DIN 4757/4 (13). It has been shown to give test results which are in good agreement with steady state outdoor efficiency measurements for collectors which exhibit an almost linear efficiency characteristic, and for collectors with an F' factor close to unity. However, as shown in Chapter 8, the mixed indoor/outdoor test procedure outlined in Section 10.4 can give misleading results for some collector designs, and care is required in the interpretation of results obtained using this method. An alternative way to carry out steady state collector testing is to use a solar simulator. The equipment costs for this are relatively high compared with those for an outdoor test loop, but the running costs for most of Europe are very much lower because time is not wasted as it is in outdoor testing. Indeed, over much of Northern Europe, solar simulator testing has to a large extent replaced outdoor testing in recent years. Testing in a simulator is not fully representative of outdoor conditions for several reasons, and a separate test procedure for use in simulators is therefore outlined in these recommendations. Guidance on the design of solar simulators is given in Appendix I. The choice between the efficiency test methods outlined here is usually determined by the facilities available and the location. Steady state outdoor tests are mainly

122 used in Southern Europe where the weather is sufficiently steady for much of the summer. Where a solar simulator is available, it will often provide the most rapid means of testing, but there are few countries with more than two solar simulators suitable for collector testing. It is unlikely that simulators will completely replace outdoor testing because when evaluating new collector designs it is still wise to check measurements made in a simulator against those obtained outdoors. Such checks are particularly important if the new collector contains reflectors or spectrally selective elements. An alternative to solar simulator testing in regions where steady state conditions do not often occur is to test under variable irradiance or "transient" conditions. Three approaches to testing in transient conditions have been shown to be worthy of further development, and these are presented in Section The test procedures outlined in this chapter are for collectors which employ water based heat transfer fluids. Procedures for testing collectors which heat air or oil based fluids are presented separately in Chapters 11 and 12. The test procedures given here are adequate for most practical purposes, but involve a number of approximations in the interpretation of their results. The analysis of collector performance is discussed in Chapter 8.

123 10.2 OUTDOOR STEADY STATE EFFICIENCY TEST Summary The basis of this method was first published by the National Bureau of Standards (NBS) in 1974 (14), and later as an ASHRAE standard (15). The objective is to determine an efficiency curve outdoors in the environment for which the collector is designed. Test conditions have to be restricted in order that the influence on the collector performance of variations in wind, solar radiation intensity, angle of incidence of direct solar radiation, and ambient temperature will remain small. Variations in these parameters are not taken into consideration in the computation of the instantaneous efficiency in steady state tests. As a result of these restrictions, tests can take several weeks to complete in Central and Northern Europe where suitable climatic conditions cannot be relied upon Test Installation The collector should be mounted in accordance with the recommendations given in Chapter 5, and coupled to a test loop which conforms with the recommendations given in Chapter 6. The heat transfer fluid should flow from the bottom to the top of the collector, or as recommended by the manufacturer Pre-conditioning of the Collector The collector should be visually inspected and any damage recorded. The collector aperture cover should be thoroughly cleaned. Before each day's testing period it may be necessary to expel moisture that has formed on the collector components. Unless the collector is known to be dry, the heat transfer fluid should be circulated at approximately 80 C for 30 minutes or as long as is necessary to dry out the insulation and collector enclosure. The collector pipework should be vented of trapped air by means of an air valve or by circulating the fluid at a high flowrate, as necessary. The fluid should be inspected for entrained air or particles, by means of the transparent tube built into the fluid loop pipework. Any contaminants should be removed

124 Test Procedure The collector should be tested over its operating temperature range under clear sky conditions in order to determine its efficiency characteristic. Data points which satisfy the requirements given below should be obtained for at least 4 fluid inlet temperatures spaced evenly over the operating range of the collector. One inlet temperature should be selected such that the mean fluid temperature in the collector lies within ±3 K of the ambient air temperature, in order to obtain an accurate determination of Eta Zero. With water as the heat transfer fluid, 70 C is usually adequate as a maximum temperature. At least 4 independent data points should be obtained for each fluid inlet temperature, to give a total of 16 data points. If test conditions permit, an equal number of data points should be taken before and after solar noon for each fluid inlet temperature. During a test, measurements should be made as explained in Section These may then be used to identify test periods from which satisfactory data points can be derived Measurements The following measurements should be obtained in accordance with the recommendations given in Chapter 7: (a) (b) The aperture area The fluid capacity (c) The global solar irradiance at the collector aperture (d) The diffuse solar irradiance at the collector aperture (e) (f) (g) (h) The angle of incidence of direct solar radiation The surrounding air speed The surrounding air temperature The temperature of the heat transfer fluid at the collector inlet (i) The temperature rise of the fluid between the inlet and outlet of the collector (j) The flowrate of the heat transfer fluid. 108-

125 Test Period The test period for a steady state data point typically contains a time period of approximately 15 minutes with the correct fluid inlet temperature, followed by a steady state test period of approximately 15 minutes. The length of the required steady state test period depends on the thermal capacity C of the collector and the thermal flow rate mcf of the fluid through the collector. In order to ensure that a steady state will be reached the test period should be greater than about 4 times the period defined by C/mcf. For many collectors, however, the ratio C/mcf can be approximated by the ratio of the fluid capacity of the collector to the fluid mass flowrate, and the required length for a steady state test period then becomes about 4 times the period taken for fluid to pass through the collector (i.e. 4 times the fluid transit time). For example: A collector with a fluid capacity of 3 Jt/m 2, and a fluid flowrate of 1 i/min.m 2 will have a fluid transit time of 3 minutes and will need a steady state test period of at least 12 minutes. A collector may be considered to have been operating in steady state conditions over a given test period if none of the experimental parameters deviate from their mean values over the test period by more than the limits given in Table For the purposes of establishing that a steady state exists, average values of each parameter taken over successive periods of 30 seconds should be compared with the mean value over the test period. Note: The temperature difference between the collector inlet and outlet is included in Table 10.1, although it depends on the other parameters, because of the need to ensure that the heat transfer between the fluid and the thermal mass of the collector has also reached steady state conditions. Parameter Deviation from the Mean Value over the Test Period Total Solar Irradiance ± 50 W/m 2 Surrounding Air Temperature ± 1 K Fluid Mass Flowrate ± 1 % Collector Fluid Inlet Temperature ± 0.1K Temperature Difference between Collector Inlet and Outlet i 0.1 R Table 10.1 Permitted Deviations of Measured Parameters during a Steady State Period

126 Test Conditions The total solar irradiance at the plane of the collector aperture should be greater than 600 W/m 2. The angle of incidence of direct solar radiation at the collector aperture should be less than 40 for conventional collectors. However, much lower angles may be required for particular designs. In order to characterise collector performance at other angles, an incidence angle modifier is usually determined (see Section 10.7). The average value of the surrounding air speed, taking account of spatial variations over the collector and temporal variations during the test period, should lie between 3 m/s and 8 m/s. Unless otherwise recommended, the fluid flowrate should be set at approximately 0.02 kg/s per square metre of collector aperture. It should be held stable to within ±1% of the set value during each test period, and should not vary by more than ±10% of the set value from one test period to another. In some collectors the recommended fluid flowrate may be close to the transition region between laminar and turbulent flow. This may cause instability of the internal heat transfer coefficient and hence variations in measurements of collector efficiency. In order to characterise such a collector in a reproducible way, it may be necessary to use a higher flowrate, but this should be clearly stated with the test results. The use of fluid temperature difference measurements of less than 1.5 K should be avoided because of the associated problems of instrument accuracy Computation and Presentation of Results The measurements should be collated to produce a set of data points which meet the required test conditions, including those for steady state operation. The instantaneous efficiency r\ may be calculated from the following equation: 11 Aa G Eqn where Q may be calculated from: Q = il c f AT Eqn and the aperture area Aa is defined in Section

127 Provided that the incidence angle is less than 40, the use of an incidence angle modifier as discussed in Section 10.7 can usually be avoided. Further discussions of incidence angle modifiers are presented in Section 8.6. A value of c^ appropriate to the mean fluid temperature should be used. If m is obtained from volumetric flowrate measurement then the density should be determined for the temperature of the fluid in the flow meter. The instantaneous efficiency should be presented graphically as a function of the reduced temperature difference T* using the format sheet given in Appendix III. The reduced temperature difference is defined by the relation: T* = (Tm - T S) qn _ 1Q3 where: Tm = (Ti + ) Eqn Graphical presentation should be made by statistical curve fitting using the least squares method as detailed in Chapter 8 to obtain an instantaneous efficiency curve of the form: n = no - a : T* - a 2 G(T*) 2 Eqn Typical instantaneous efficiency curves for a single glazed collector, a double glazed collector, an unglazed collector, a selective surface collector and an evacuated tubular collector are shown in Figure The associated conditions present during testing should be recorded in the format sheets (Appendix III). These are an important part of the test results because they provide a record of the conditions for which the efficiency curve applies. in-

128 Single glazed (matt black) Double glazed (matt black ) Selective (single glazed) Evacuated tubes Unglazed M T* IK m 2 W" 1 ) Figure 10.1 Typical Instantaneous Efficiency Curves 112-

129 10.3 STEADY STATE EFFICIENCY TEST IN A SOLAR SIMULATOR Summary The performance of most collectors is better in direct solar radiation than in diffuse, and there is little experience with diffuse solar simulation. This test method is therefore written for use only in simulators where a near normal incidence beam of simulated solar radiation can be directed at the collector. In practice it is difficult to produce a uniform beam of simulated solar radiation, and a mean irradiance level has therefore to be measured over the collector aperture. It is both expensive and technically difficult to produce an indoor simulation of the low thermal irradiance presented by the sky outdoors. The collector performance data measured in a simulator are therefore corrected, using known or assumed physical properties of the collector materials and measured collector parameters, so that they are equivalent to the results that would be obtained in typical outdoor weather conditions. The test results which incorporate the thermal irradiance corrections should then be equivalent to those produced by steady state outdoor testing The Solar Simulator A simulator should have the following characteristics: The lamps should be capable of producing a mean irradiance over the collector aperture of at least 600 W/m 2. Values in the range 300 to 1000 W/m 2 may also be used for specialised tests, provided that the accuracy requirements given in Table 10.1 can be achieved, and the irradiance values are noted in the test report. The mean irradiance over the collector aperture should not vary by more than ±50 W/m 2 during a test period. At any time the irradiance at a point on the collector aperture should not differ from the mean irradiance over the aperture by more than ±20%. In order that results can be compared with those from outdoor tests, at least 90% of the simulated solar radiation should have an angle of incidence of less than 40 at the collector aperture. However, a simulator beam which has far less divergence than this is to be preferred since it permits the effects of collector incidence angle modifiers to be more easily identified. The spectral distribution of the simulated solar radiation should be approximately equivalent to that of the Air Mass 2 solar spectrum. An approximate distribution, which is likely to be adequate for testing many collectors, is given in Table

130 Waveband (um) % of Irradiance Table 10.2 Spectral Distribution of Simulated Solar Radiation Where collectors contain spectrally selective absorbers or covers, a check should be made to establish the effect of the differences in spectrum on the da) product for the collector. The effective values of (xa) under the simulator and under Air Mass 2 solar radiation should not differ by more than ±1%. Effective (xa) = it Urn x(a).a(a).g da 3 mn 4 Hm G A da 3 W* Eqn The thermal irradiance at the collector aperture should not exceed that of a blackbody cavity at ambient air temperature by more than 100 W/m 2. Guidance on the design of solar simulator test facilities is given in Appendix I Test Installation Section 5.3 describes collector mounting and location requirements for indoor testing. The general considerations of Chapter 6 broadly apply to simulator test installations. The collector tilt angle should be such as to receive a near normal incidence beam of simulated solar radiation. The tilt angle should be at (or corrected to) 45 ±5, or as recommended by the manufacturer. Non standard tilt angles will therefore require a simulator array that has freedom of tilt to maintain normal incidence. A wind generator should be used with a solar simulator to produce an air flow in accordance with Section

131 Pre-conditioning of the Collector The procedure outlined in Section should be followed Test Procedure The collector should be tested over its operating temperature range in approximately the same way as for outdoor testing. Eight test points should be adequate for testing in solar simulators provided that at least A separate inlet temperatures are used and that adequate time is allowed for temperatures to stabilise. Where possible it is considered desirable to proceed up and then back down through the T* range. Fluid inlet temperatures should be selected such that the mean fluid temperatures in the collector will be evenly spaced over the range from within ±3 K of the ambient air temperature, at the time of the test, to the maximum temperature selected for the collector. The latter should be at least 70 C for water heating collectors unless otherwise specified by the manufacturer. During a test, measurements should be made as described in Section These may then be used to identify test periods from which satisfactory data points can be derived Measurements Measurements should be made as recommended in Section 10.2 unless otherwise described below. MEASUREMENT OF IRRADIANCE OF SIMULATED SOLAR RADIATION Simulated solar irradiation usually varies spatially over the collector aperture as well as varying with time during a test. It is therefore necessary to employ a procedure for integrating the irradiance over the collector aperture, and to measure the irradiance at one or more positions in the collector plane during a test. Time variations in irradiance are usually caused by fluctuations in the electricity supply, and changes in lamp output with temperature and running time. Some lamps take more than 30 minutes to reach a stable working condition when warming up from cold. Pyranometers may be used to measure the irradiance of simulated solar radiation, but their bodies should be shaded and cooled with air to maintain the instruments in the same thermal conditions as those in which they were calibrated. Alternatively, other types of radiation detector may be used, provided they have been

132 calibrated for simulated solar radiation. Details of the instruments and the methods used to calibrate them should be reported with the test results. The distribution of irradiance over the collector aperture should be measured using a grid of maximum spacing 100 mm, and the spatial mean deduced by simple averaging. The irradiance should also be monitored during the test by one or more fixed instruments, adjacent to the collector in the plane of the aperture. The number of fixed instruments should be sufficient for the time variations measured by them to be representative of the time variations over the whole aperture. The readings from the fixed instruments should be correlated with the spatial irradiance measurements to provide values of the mean irradiance over the collector, for calculating collector efficiencies. The mean irradiance over the collector aperture at any time during a test period may be deduced from the mean of the readings of the fixed radiometers at that time. For this calculation the ratio of the spatial mean obtained at the start of the test period to the mean of the readings of the fixed radiometers at the start of the test period is usually employed. MEASUREMENT OF THERMAL IRRADIANCE For the outdoor method (Section 10.2) the influence of thermal radiation is implicit and the thermal irradiance need not be measured. The thermal irradiance in a solar simulator, however, is likely to be different from that typical of outdoors and should be measured in order to enable corrections to be applied. The thermal irradiance may be measured directly or indirectly, or deduced from temperature and emittance measurements (see Section 7.2). The thermal irradiance at the collector should be determined to an accuracy and precision of ±10 W/m 2, and presented with the solar simulator test results. AMBIENT ROOM TEMPERATURE Careful consideration should be given to the proper measurement of Ta in simulators. A mean of several measured values may be necessary. Transducers should be shielded in order to minimise radiation exchange. This may be achieved by placing each transducer inside two vertical concentric metal pipes. One transducer and shield should be positioned at the collector mid-height within 2 metres of the collector, and one should also be positioned in the outlet of the wind generator (see Section 7.3.A)

133 Test Period The test period may be determined in the same way as for outdoor steady state testing (Section ). The more stable environment of an indoor test facility may allow steady state conditions to be maintained more easily than outdoors, but adequate time should still be allowed to ensure proper steady state operation of the collector as discussed in Section Test Conditions The test conditions described for outdoor testing (Section ) should be observed with the following additions: The thermal irradiance in the plane of the collector aperture should not exceed that from a blackbody cavity at ambient air temperature by more than 100 W/m 2. The air issuing from the wind generator should not differ in temperature from ambient air by more than ±1K. Unlike the situation in outdoor testing, the wind speed indoors is under the control of the test engineer. It is therefore recommended that a wind speed of 4 m/s be used indoors Computation and Presentation of Results The analysis presented in Section for outdoor testing is generally applicable to solar simulator tests, and the results should be presented on the format sheets shown in Appendix III. The scatter of results from simulator testing can be expected to be less than from outdoor tests, but a bias can be caused by imperfect simulation of diffuse solar irradiance and thermal irradiance conditions. The effect of differences in diffuse and thermal irradiance depends on the collector design. Further information showing ways of correcting performance data to reference conditions is given in Chapter

134 10.4 INDOOR HEAT LOSS & COMBINED INDOOR/OUTDOOR TESTS Summary The overall heat loss coefficient for a collector is an important thermal parameter, which can be determined indoors without the need for steady outdoor weather conditions. It is important to note however that for some collectors the heat loss coefficient determined indoors is not the same as that which would be deduced from outdoor collector efficiency measurements. The reasons for this difference are discussed in Chapter 8. In order to minimise the need for clear sky outdoor conditions, indoor heat loss measurements are sometimes combined with measurements of Eta Zero made outdoors in accordance with Section 10.2 to derive a collector efficiency characteristic. This procedure is outlined by the BSE (12). Recommendations are given below for indoor heat loss measurement, and for the computation of a collector efficiency characteristic by combining indoor heat loss measurements with measurements of Eta Zero made outdoors Test Installation for Heat Loss Testing OVERALL HEAT LOSSES The collector should be mounted in accordance with the recommendations given in Chapter 5, and coupled to a test loop which conforms with the recommendations of Chapter 6 in all respects except the direction of fluid flow through the collector. For the temperature profile of the absorber to be as uniform as possible and similar to that present during outdoor measurements, the heat transfer fluid should flow from the top to the bottom of the collector during heat loss tests. (In the case of collectors with symmetrical absorbers this may be achieved by inverting the collector and using the same collector pipework). BACK AND SIDE LOSSES The back and side losses may be important for validating a model of collector performance. These may be measured using the procedure given below, while the front surface of the collector is covered with a thick layer of insulation. For most simple collectors a cover with an overall conductance of approximately 0.2 W/m 2 K should be adequate. This measurement is unlikely to be appropriate for high performance collectors having low overall heat losses. 118-

135 Pre-conditioning of the Collector The procedure outlined in Section should be followed Test Procedure The collector should be tested over its operating temperature range under conditions of negligible shortwave irradiance in order to determine the variation of its heat loss coefficient with temperature. Data points which satisfy the requirements given below should be obtained for at least 4 fluid inlet temperatures spaced over the operating temperature range of the collector. Measurements should be made at a temperature near to the maximum operating temperature (70 C is usually adequate for water heating collectors), and as near as is reasonable to the temperature of the ambient air, taking into account the limitations of instrument accuracy. At least 2 independent data points should be obtained for each fluid inlet temperature, to give a total of 8 data points. During a test, measurements should be made as explained below. These may then be used to identify test periods from which satisfactory data points can be derived Measurements The measurements required are all those listed in Section with the exception of those for solar radiation. A check should be made using a pyranometer to ensure that the total shortwave irradiance in the collector aperture is less than 2 W/m 2. A zero check on the pyranometer should be employed before performing this measurement, by covering the pyranometer with a light tight box Test Period The test period may be determined in accordance with the recommendations of Section for outdoor steady state testing, with the obvious difference that the solar irradiance requirements do not apply Test Conditions The test conditions outlined in Section for solar simulator testing should be observed, with the exception that the shortwave irradiance (\ < 4 pm) should be less than 2 W/m 2 during each test period. 119-

136 800r 1200 * E 16 U Vjf>^ \o*e - <L--' " * o S'inq \e 9 \oxed l*o»* b ( ^ " _Do U.tM e _ q laz_ed_l-ojtblocm.or.selec<iv_ejsingie_ a La-A).--. 0> I Evacuated tube (Tm-TaU j i i i i i J _l I I I L (K 60 Figure 10.2 Typical Heat Loss Characteristics for Different Collector Types

137 Computation and Presentation of Results HEAT LOSS RATE AND BEAT LOSS COEFFICIENT The overall heat loss rate for the collector may be calculated from the equation: Ql = 4 c f (Ti - Te) Eqn 10.7 The value of Cf should be appropriate for the mean fluid temperature in the collector. If m is obtained from a volumetric flow meter then the fluid density should be determined for the temperature in the flow meter. The overall heat loss coefficient for the collector may be determined from the equation: U = Q*/Aa(Tm - Ta) Eqn This should be presented as a function of (Tm - Ta) as shown in the Format sheets included in Appendix III. Typical heat loss curves are shown in Figure MIXED INDOOR/OUTDOOR TEST Eta Zero (n ) o The value of rig m ay be determined from steady state outdoor measurements of collector efficiency made in accordance with Section 10.2, using the equation: n ~ m cf (Te Aa G Ti) where Te + Ti 2 = Ta Eqn Most outdoor measurements will not exactly meet the requirement that the mean fluid temperature should be equal to the ambient air temperature. Results may be corrected back to this condition using the measured value of the overall heat loss coefficient for near ambient temperature operation. 121-

138 The appropriate equation is: 10 - ih c f (Te - Ti), U(Tm - Ta) ln ln r + A 3 a a E q n where U is the value of the heat loss coefficient when Tin = Ta. A simple arithmetic mean value of n should be determined from the 4 data points measured. 0 Collector' Efficiency Values of collector efficiency may be determined using the equation n = no - J^Q Eqn where the Qi values are those measured in the heat loss tests at mean collector temperature differences (Tm - Ta), and G is set at 800 W/m 2. The efficiencies thus determined are the values corresponding to reduced temperature differences (Tm - Ta)/G, with C set at 800 W/m 2. These efficiency values may be presented as a collector efficiency characteristic using the format sheets included in Appendix III. (Note: Refer to Chapter 8 for further analysis showing that the performance of some collectors should not be interpreted using the simple model outlined above). 122-

139 10.5 OUTDOOR TRANSIENT TEST METHODS Summary Transient collector test methods were first developed in European laboratories in an attempt to simplify the requirements of the fluid loop used for testing. Some methods employed steady solar irradiance and varying fluid temperatures, whilst others employed step changes in solar irradiance. Step changes were produced either by switching lamps on and off in solar simulators or by the use of an opaque shield outdoors. With some of the methods it was possible to determine both the effective thermal capacity of a collector and its steady state efficiency characteristic, but most of these methods have not been widely used and are not included in these recommendations. More recently, transient test methods have been developed in order to permit collector testing outdoors under varying weather conditions. Three such methods employ essentially the same test facilities and procedures as are used for conventional outdoor steady state efficiency measurements, but they are operated under variable weather conditions, and the results are analysed differently. In the British Standard DD77 method (8), the efficiency characteristic is derived from a response function for the collector, which is determined by analysing rapidly sampled performance measurements for a number of fluid inlet temperatures. In an approach proposed by Talarek (31), the steady state collector performance equation with a thermal capacity term is integrated over a long time period such that the magnitude of the transient term becomes small in comparison with the other terms. A similar integrating approach is proposed by Boussemaere (32), but in this case the integration time is shorter and the transient term is used to determine values for the efficiency and for the effective thermal capacity of the collector simultaneously. Each of these three methods has been evaluated to only a limited extent, but they all have the potential for increasing the number of days on which outdoor tests can be performed in any given year. Preliminary results from each of the methods indicate good agreement with steady state test results, and further development of the methods is anticipated in laboratories which do not possess solar simulators The Collector Response Function Approach The method is based on a simple, linear collector performance characteristic, which relates the heat output directly to -the solar input for fixed values of 123-

140 fluid inlet temperature and ambient air temperature. A unique transfer function is assumed to exist, by which the heat output at any time can be related to the solar irradiance over a previous period of time. The collector is set up and operated as if it were being tested in accordance with Section Measurements are sampled rapidly (say every 2 seconds) and averaged to give a continuous series of readings for consecutive time intervals (typically one minute averages). For each time interval the data are processed to give average values of: Collector power output Q(t) (W) Solar irradiance G(t) (W/m 2 ) Temperature difference [Ti-TaJ(t) (K) In any time intprval (j), it is assumed that the collector output Q(j) has been caused by the irradiance over a previous time period. This period is expressed as N of the time intervals defined above (typically N lies between 5 and 10) and the irradiance for each of these N intervals is given a weighting kn, [Note: kn is typically large for intervals near to j and decreases to zero at time j-n as shown in Figure 10.3]. The collector performance characteristic may then be written: N Q(j) = Aa F" no I kn Gn(j) - Aa F" U [Ti - Ta](j) n=l Eqn where, N I kn = 1 Eqn n=l By collecting data for approximately one hour at each of 4 values of collector inlet temperature, a large number j of data sets will be obtained (typically 240 data sets). These data may be used to find a best fit to the N+2 unknowns (kn, F" no and F" U) in Equation The method of solution recommended in the British Standard Draft for Development involves a refinement of the conventional least squares method of fitting coefficients to a set of simultaneous equations, known as the "method of instrumental variables". A full explanation of the method together with a computer program for performing the calculations is given in reference (8).

141 Integral Methods The performance of a collector with a finite thermal capacity may be written as Q(t) = Aa no G(t) - Aa U[Tm - Ta](t) - C ^ Eqn APPROACH A Equation is integrated over long time periods when the variations in Tm are small. The term involving the differential (dtm/dt) is then small compared with the other terms and may be neglected. Preliminary studies (31) indicate that a wide range of variations in irradiance can be accommodated by integrating in this way, but further work is required in order to determine the integration times, and the limits for variations in G and Tm, which need to be specified in order to ensure acceptable test results. APPROACH B The value of the collector heat loss coefficient U is determined indoors in accordance with Section The integrated form of Equation is then used to determine rig and C from test results obtained in general accordance with Section 10.2 but where the irradiance and fluid inlet temperatures are allowed to vary. Preliminary studies (32) indicate that a forced variation in Tm of approximately 6 K/hr is typically required in order to ensure adequate resolution of the variables, but further work is required to substantiate this and to determine the acceptable limits for variations in solar irradiance Equivalent Normal Irradiance The use of transient test methods is often motivated by a desire for testing at low irradiance levels, and this inevitably leads to the need to accommodate diffuse solar irradiance variations. Procedures for deducing Equivalent Normal Irradiance are discussed in Chapter 8, and are included in the British Standard Draft for Development (8). These are likely to be usefully incorporated into the integral methods discussed in Section

142 G N <ii E o Q_ ^ Time kn m N Time Figure 10.3 Transient Data Analysis by the Response Function Method

143 10.6 DETERMINATION OF THE EFFECTIVE THERMAL CAPACITY AND THE TIME CONSTANT OF A COLLECTOR Summary The effective thermal capacity and the time constant of a collector are important parameters which determine its transient performance. A collector can usually be considered as a combination of masses, each at a different temperature. When a collector is operating, each collector component responds differently to a change in operating conditions, so it is useful to consider an effective thermal capacity for the whole collector. Unfortunately, the effective thermal capacity depends on the operating conditions and is not a collector parameter with a unique value. Several different test methods have been used to measure the effective thermal capacity of collectors and it has been shown that similar results can be obtained by using quite different methods (16). The method below is recommended because it requires only conventional collector testing facilities, does not employ time derivatives (which are inherently difficult to obtain accurately) and has been shown to give reproducible results. Just as there is no unique value of effective thermal capacity, there is no unique overall time constant for a collector. For most collectors, the dominant influence on the response time is the fluid transit time, and hence the first order response varies with the fluid flowrate. Other collector components respond with different times to give an effective overall time constant which depends on the operating conditions Test Installation The collector should be mounted in accordance with the recommendations of Chapter 5 and coupled to a test loop which conforms with the recommendations of Chapter 6 for both thermal capacity and time constant measurement. Effective thermal capacity tests may be carried out indoors because only heat loss measurements are employed. The collector time constant may be determined in steady state, clear sky conditions outdoors, or in a solar simulator Effective Thermal Capacity Test Procedure The fluid should be circulated from the top to the bottom of the collector, with a constant inlet temperature, using a flowrate similar to that defined for collector efficiency testing until steady state conditions are reached. 127-

144 The fluid inlet temperature should be raised rapidly by about 10 K, and measurements made continuously until steady state conditions are achieved again. This process may be performed three or four times and an arithmetic mean value of the effective thermal capacity deduced. MEASUREMENTS The following quantities should be measured: (a) (b) (c) (d) Temperature difference between collector inlet and outlet Fluid mass flowrate Collector fluid inlet temperature Surrounding air temperature When testing collectors having a low thermal capacity, the sampling frequency selected for measuring the fluid temperatures may need to be greater than that usually used for collector efficiency testing, in order to adequately follow the transient behaviour of the collector Computation of Effective Thermal Capacity The transient behaviour of the collector between the two indoor steady states 1 and 2 is assumed to be represented by the following equation: C HF 5 = " m c f AT " Aa U (Tm " Ta) Eqn AT = Te - Ti (negative) Integrating Equation over the period between the two steady states, we obtain: C(Tm - Tm ) = -[ 2 m q= AT dt - Aa U [ 2 (Tm - Ta) dt 1 J tx ^ it, Eqn AT Since Tm = Ti + -K we may express (Tm - Ta) as: (Tm - Ta) = (Ti - Ta) + ^ Eqn

145 Combining Equations and and rearranging, the following equation for the collector thermal capacity is obtained: C = t 2 m c f Al AT dt - Aa U ti (Ti - Ta) dt + % Jtj (Tm - Tm ) 2 1 AT dt J tx Eqn From the test records (Ti-Ta) and AT should be plotted as a function of time. The areas under the curves, between the two steady states, are: 2 f t 2 J Ti - Ta) dt and AT dt respectively. t X The heat transfer coefficient U of the collector may already have been determined during indoor collector heat loss testing. However, Aa U may be obtained directly from the two steady states since, in steady state, Equation gives: 0 = -m c f AT - Aa U (Tm - Ta) Eqn and hence: Aa U = (,*- ff) E( * ṉ 10-2 Aa U should be evaluated for both steady states, and an arithmetic mean value taken. It follows therefore that all the terms in Equation may be determined and a value of the effective thermal capacity obtained Collector Time Constant Test Procedure Testing should be performed either under clear sky conditions or in a solar simulator with a solar irradiance at the plane of the collector aperture of greater than 600 W/m 2. The heat transfer fluid should be circulated through the collector at the same flowrate as that used during collector thermal efficiency tests. The aperture of the collector should be shielded from the solar radiation by means of a solar reflecting cover, and the collector fluid inlet temperature set approximately equal to the ambient air temperature

146 When a steady state has been reached, the cover should be removed and measurements continued until steady state conditions have been achieved again. For the purpose of this test a steady state condition may be assumed to exist when the fluid outlet temperature varies by less than 0.05 C per minute. The following quantities should be measured (a) (b) Collector fluid inlet temperature Collector fluid outlet temperature (or Te - Ti) (c) Surrounding air temperature Computation of Collector Time Constant The difference between the temperature of the fluid at the collector outlet and that of the surrounding air (Te - Ta) should be plotted against time, beginning with the initial steady state condition (Te - Ta) 0 and continuing until the second steady state has been achieved at a higher temperature condition (Te - Ta) 2. The time constant T of the collector is defined as the time taken for the collector outlet temperature to rise by of the total increase from (Te - Ta) 0 to (Te - Ta) 2, following the step increase in solar irradiance at time zero. If the response time of the temperature sensors is significant when compared with that measured for the collector, then it should be taken into account in calculating the test results. (Te-Ta) It-Iah ^ ^ _ ITe-Tol. _ ' T [(T e -Tal2- Te-T a ).] 1_ I Figure 10.4 Collector Time Constant Time 130-

147 10.7 DETERMINATION OF INCIDENCE ANGLE MODIFIERS Summary Incidence angle modifiers are used to define the efficiency of a collector when the direct beam of solar radiation is not incident normal to the plane of the collector aperture.- The magnitude of the modifier at any given angle depends on the transmittance of the collector cover, the absorptance of the absorber, and the shading of the absorber by the sides of the collector and any other fixing or glazing bars. For simple geometric configurations, the modifier can be calculated from the collector dimensions and from optical property measurements for its component materials. However, for more complex geometries or when non-homogeneous materials are employed, calculation becomes impractical, and measurement procedures are therefore under development Shading Modifier The amount of shading of the absorber caused by the sides of the collector and any glazing bars depends on the spacing between the absorber and cover, and on the size of the absorber relative to that of the aperture. A shading modifier may be defined as the ratio of the area of absorber which is irradiated by the solar beam to the area of the collector aperture Cover Transmittance Modifier The variation of transmittance with angle can be measured for homogeneous planar materials, as described in Section 9.4. For common materials such as glass, transmittance curves are already available in the literature (9). Most common collector cover materials show only small variations in transmittance for incidence angles in the range from normal to Absorber Surface Modifier The angular dependence of the absorptance of a surface depends very largely on the nature and structure of the surface. For most matt surfaces the angular dependence is very small for incidence angles in the range from normal to 60. For specular, selective and non-flat surfaces the absorptance may vary more strongly and should be measured using the methods discussed in Section Calculation of the Incidence Angle Modifier For any given angle, the incidence angle modifier may be calculated as the product of the shading, cover transmittance, and absorber surface modifiers defined

148 above. The resulting incidence angle modifier K(v) may be written. K(v) = ^ I jj»j Eqn Measurement of the Incidence Angle Modifier Instantaneous collector efficiency measurements made using either the outdoor steady state test given in Section 10.2 or the solar simulator test given in Section 10.3 may be used to determine the incidence angle modifier. The measurements should be made with the mean fluid temperature in the collector approximately equal to the surrounding air temperature, at normal incidence and at five other incidence angles concentrated around the region of the characteristic which has the greatest curvature. The incidence angle modifier for each angle v may then be determined as the ratio n 0 (v)/n 0 (0 ). The incidence angle v should be measured or calculated in accordance with the recommendations of Section Because of the cosine form of most collector incidence angle modifiers it is important to make measurements at incidence angles greater than 50 and collectors usually need to be moved to achieve these angles. Pyranometers do not have good accuracy at high incidence angles, and the irradiance on the collector should therefore be checked using measurements from a pyranometer or pyrheliometer placed normal to the solar beam, as well as by measurements from a pyranometer in the plane of the collector aperture. At the present time, results of incidence angle modifier measurements will only be approximate because no account is taken of the influence of diffuse irradiance outdoors, and of poor parallelism and uniformity in simulators. Further development of the methods is required in order to define satisfactory ways of taking these effects into account Presentation of Results The incidence angle modifier is usually presented as a function of angle, in the form of a curve (Figure 10.5). (Note: For asymmetric collectors,such as tubular collectors, it may be necessary to determine incidence angle modifiers for each axis of symmetry. The procedures outlined above may be used for incidence angles relative to both axes.) 132-

149 1.0 Single glazed cover fco.8 T3 O a 0-6 o> c o a; <u o _c 0.2 Double glazed cover Angle of incidence (degrees) Figure 10.5 Typical Incidence Angle Modifiers 133-

150 10.8 DETERMINATION OF THE PRESSURE DROP ACROSS A COLLECTOR Summary The pressure drop across a collector may be of importance to designers of solar collector systems. The fluid normally used in the collector should be employed for the tests. In order that a representative range of pressure drops may be determined, a number of fluid flowrates should be used Test Installation The collector should be mounted in accordance with the recommendations of Chapter 5, and coupled to a test loop which conforms broadly with the recommendations of Chapter 6, although less instrumentation is required than for collector efficiency testing. The heat transfer fluid should normally flow from the bottom to the top of the collector, and particular attention should be paid to the selection of appropriate pipe fittings at the collector entry and exit ports, as discussed in Section Pre-conditioning of the Collector The fluid should be inspected to ensure that it is clean. The collector should be vented of air by means of an air bleed valve or other suitable means, such as by increasing the fluid flowrate for a short period to remove air from the collector Test Procedure The pressure drop between the collector inlet and outlet connections should be determined with the collector and its fluid at ambient air temperature, and for flowrates which span the range likely to be used in a solar heating system. In the absence of specific flowrate recommendations by the collector supplier, pressure drop measurements should be made over the range of flowrates from kg/s to 0.05 kg/s per square metre of collector aperture area. At least 5 measurements should be made at values spaced equally over the flowrate range. 134-

151 Measurements The following measurements should be obtained in accordance with the recommendations given in Chapter 7. (a) (b) (c) The fluid temperature at the collector inlet The fluid flowrate The pressure drop between the collector inlet and outlet connections Fittings Pressure Drop Check The fittings used to measure the fluid pressure may themselves cause a drop in pressure. A zero check on the pressure drop should be made by removing the collector from the fluid loop and repeating the tests with the pressure measuring fittings directly connected together Test Conditions The fluid flowrate should be held constant to within ±1% of the nominal value during test measurements. The fluid inlet temperature should be held constant to within ±5 C during test measurements. The test should be carried out with the collector at a temperature which lies within ±10 C of that of the surrounding air. Pressure drop tests at other temperatures may be important for oil based fluids Computation and Presentation of the Results The pressure drop should be presented graphically as a function of the fluid flowrate for each of the tests performed, using the format sheet given in Appendix III. 135-

152

153 CHAPTER 11 THERMAL PERFORMANCE TESTS FOR AIR HEATING COLLECTORS 11.1 INTRODUCTION A number of collector testing standards have included air heating collector test methods for several years (12, 15), but little experience of using these in Europe has been reported. Work is now in progress by the CEC Collector Testing Group to develop air collector test methods which are suitable for the European climate. The methods under investigation are based on those discussed in Chapter 10, which have been developed for testing liquid heating collectors. Because of the current lack of experience with air collector testing in Europe, the recommendations given in this chapter should be treated as provisional. The recommendations for collector mounting given in Chapter 5 are generally applicable to air collector testing. Test loops for air collector testing are still under development in Europe, and methods for measuring air flow and temperature to the required accuracy are being investigated. Provisional recommendations on these topics are therefore included in this Chapter rather than in Chapter THE TEST INSTALLATION An air collector test facility can be designed for either open loop or closed loop operation. The arrangement of components in an air collector test loop is an important consideration, because the following design requirements have to be met simultaneously: (a) Adequate entry lengths are needed upstream of flow meters and of the tappings used to measure the pressure drop across the collector. (b) Adequate air mixing is required for temperature measurements. (c) (d) The mean pressure in the collector itself may need to be controlled with respect to the local atmospheric pressure in order to minimise or investigate the effects of air leakage. The collector inlet air temperature must be held stable. (e) (f) The mass flowrate of the air must be held stable. The test loop should be free from leaks

154 c ri to > W X B T3 Key- Flow straightener == Mixing gauzes 1 Insulation '//// d = diameter of Juctwork Extra insulation between, 65d mm, collector and temperature \ iod mm,,, ^FLOWMETER i '///./////in i W;/;;;A 7LV///& Jmzzzzzzzo Te SENSOR VARIABLE FAN ^ DAMPER Q-ICONTROLLERI ^ Smooth walled ducting for flowmeter O Hi 3 > 1-1 o m o n o n H n> CD VARIABLE \ FAN I & - = H 1 TEMPERATURE SENSOR ifor flowmeter^ 1 correction ' FLOWMETER 10d mm ICONTROLLE HEATER ^ if poss T; SENSOR Flowmeter Calibration Arrangement INLET NOZZLE V H FLOWMETER, n Flowmeter with its own pipework under calibration VARIABLE FAN -or o o

155 An example of an air collector test loop designed to meet the requirements listed above is shown in Figure This loop has two fans to permit the pressure in the collector to be controlled independently of the flowrate by adjusting the fan speeds and the position of the damper. Two flow meters are included in order to enable estimates of leakage to be made. Entry length requirements for air flow measurement are given in a number of well known standards (33,34), but the values suggested in Figure 11.1 are somewhat less than those which are sometimes specified, because of the use of flow straighteners. If bends or elbows are included in the loop then entry lengths will need to be increased. Further information on the minimum entry and exit lengths which are needed to give acceptable air collector test results will become available when more experience has been obtained with air collector testing. It is likely that a typical test loop will be required to provide flowrates in the range to 0.1 kg/s, for pressure drops in the range 5 Pa to 300 Pa and temperatures in the range from 10 C to 100 C. To achieve the required heating of inlet air in an open loop system, an inline heater of approximately 8 kw rating is likely to be required, with a 500 W proportionally controlled heating element using feedback from the inlet temperature sensor to stabilise the collector inlet temperature. A cooler which can reduce the inlet temperature to approximately 10 K below the air temperature may also be required in order to make no measurements. A closed loop requires a smaller heater INSTRUMENTATION FOR AIR COLLECTOR TESTING For most parameters, the recommendations given in Chapter 7 should be followed. However, special attention needs to be paid to flowrate and fluid temperature measurement when testing air collectors Air Mass Flowrate Measurement An accuracy of better than ±2% is desirable for air mass flowrates in the range to 0.1 kg/s and over a temperature range from 10 C to 100 C in the collector. Ways of achieving this accuracy are being investigated, and it seems likely that the use of reference nozzles to calibrate the orifice plates might permit such accuracies. There is, as yet, insufficient experience to support detailed recommendations regarding air flow measurement, other than to refer to existing standards (33,34).

156 The flowrate should be measured at both the inlet and outlet of the collector in order to estimate the rate of air leakage. The calibrations of the two meters can be compared by connecting them together with a well sealed pipe Air Temperature Measurement The relatively low flowrates used in air collectors result in temperature variations across the ducts, and some form of mixing device is therefore required upstream of each temperature sensor. Proprietary air mixing elements are commercially available for this, or alternatively a series of baffles made of wire mesh or gauze may be used to mix the air. The need for perfect mixing can be reduced by placing several temperature sensors across the duct in order to deduce a spatially averaged temperature. However, special attention needs to be given to ensuring that the measured temperature is a true mean. Figure 11.2 illustrates two approaches currently under development for measuring the average temperature. Alternatively a large number of sensors may be spaced over the cross section of the duct. The air temperature will change if the air pressure is changed (e.g. by passing it through a flow restrictor). As far as possible the inlet and outlet temperatures should therefore be measured at the same pressure, and this will be aided by using the same sized ducts for the collector entry and exit pipework. The need to provide good mixing of the air before the outlet temperature sensor results generally in a larger distance between the collector and the sensor than in water heating collector test loops. Extra insulation of the ductwork should be used between the collector and the temperature sensors, in air collector test loops. Figure 11.2 Temperature sensor array Temperature sensor array with sensors positioned with sensors positioned around an experimentally to measure in annuli verified mean temperature having the same crossregion, sectional area

157 Figure 11.3 Air Flowrate Calibration Nozzle The temperature sensors and their calibration procedures described in Chapter 7 are generally suitable for air temperature measurement PRE-CONDITIONING OF THE COLLECTOR The collector should be visually inspected and any damage recorded. The collector aperture should be thoroughly cleaned. Before each day's testing, checks should be made to ensure that the collector is dry, and if necessary it should be supplied with hot air to remove condensation and to dry out the inside cf the collector AIR LEAKAGE IN THE COLLECTOR By varying the fan speeds and damper settings the amount of air leakage into or out of the collector should be measured as a function of mean collector pressure. Leakage may be deduced from the difference in mass flow between collector inlet and outlet. This assessment of leakage should be carried out at ambient temperature, and at high temperature. Leakage can also be investigated by the use of smoke COLLECTOR PRESSURE DROP The collector pressure drop is a far more important design parameter for an air collector than it is for a water heating collector, and should be measured for several flowrates over a range from about 0.01 to 0.05 kg/s m 2. Testing need not be continued however for flowrates which produce a pressure drop across the -141-

158 collector of greater than about 300 Pa/m 2, since it is unlikely that collectors will be used with higher pressure drops than this. In order to give representative operating conditions for pressure drop measurements, it is recommended that they be made with a mean air temperature in the collector of about 45 C EFFICIENCY TEST PROCEDURE The collector should be tested in steady state conditions either outdoors or using a solar simulator, in order to determine its efficiency characteristics. Three efficiency characteristics should be determined: one for the nominal collector mass flowrate stated by the manufacturer, one at a lower flowrate and one at a higher flowrate. If no recommendations are made by the manufacturer then the values should be 0.015, and kg/s m 2. For each mass flowrate, at least four fluid inlet temperatures (spaced evenly over the operating temperature range of the collector) should be used to obtain collector efficiency measurements in operating conditions which meet the requirements given below. The number of independent data points measured at each fluid inlet temperature should be selected on the basis of the scatter of results with respect to the best fit to an efficiency characteristic. No more than two independent measurements at each fluid inlet temperature are required if they are found to agree to within less than ±2%. Unless otherwise specified, all efficiency measurements should be carried out with the mean air pressure in the collector maintained near to atmospheric pressure in order to minimise air leakage. Where manufacturers specify that their collectors are designed to work only in overpressure or only in underpressure, then tests in the specified conditions may be performed. The test conditions used should be clearly stated with the test results, and the influence of any air leakage quantified as far as possible MEASUREMENTS The following measurements should be made in accordance with the recommendations of Chapter 7 and of Section (a) The aperture area (b) The global solar irradiance at the collector aperture (c) The diffuse solar irradiance at the collector aperture

159 (d) (e) (f) (g) The angle of incidence of direct solar radiation The surrounding air speed The surrounding air temperature The temperature of the air at the collector inlet (h) The temperature rise of the air between the collector inlet and outlet (i) (j) (k) The mass flowrate of the air at the collector inlet and at the collector outlet (Note: To meet the specified accuracy it may be necessary to measure the temperature of the air entering the flow meters). The atmospheric pressure The static pressure of the air at the collector inlet and at the collector outlet (1) The humidity of the air entering the collector TEST PERIOD Until more experience becomes available, the test period defined in Section should be used for air collector testing TEST CONDITIONS Because of the difficulty of making accurate measurements of air mass flowrate and temperature, the global solar irradiance in the plane of the collector aperture should normally be greater than 600 W/m 2 in order to ensure sufficient accuracy in collector efficiency values. Lower levels of irradiance should only be used if" the accuracy of the air mass flowrate and temperature measurements is adequate. The angle of incidence of direct solar radiation at the collector aperture should be less than 40 for conventional collectors. A smaller range of angles may need to be specified for some designs. The average surrounding air speed should be between 3 m/s and 8 m/s during each test period COMPUTATION & PRESENTATION OF RESULTS The measurements should be collated to produce a set of data points which meet the required test conditions. The instantaneous efficiency the following equation.: (TI) may be calculated from n = A A E<^

160 where Q = m c f (T -T.) When there is air leakage into or out of the collector, there is a choice between using the inlet or outlet flow rate measurement for analysing collector efficiency. Where the difference between these measurements is small, the mean of the mass flowrate measurements made at the collector outlet and the collector inlet should be used. Where only one flowrate measurement is used, the choice should be clearly stated with the results, and the anticipated influence of leakage on the results should be quantified as accurately as possible. The presentation of test results for air collectors is still under discussion. However, since the performance of air collectors is far more dependent on flowrate than is the case with liquid heating collectors, there is a good argument for presenting the results in terms of fluid inlet temperature. It is also common for air collectors to operate in an open loop where the air entering the collector is at ambient temperature. The implications for the measurement of no anc * U of using Ti instead of Tin are discussed in Chapter 8. Values for the density and specific heat capacity of air as functions of temperature, pressure and humidity are given in Appendix IV. Until experience permits the development of new format sheets for air collector testing, the instantaneous efficiency should be presented graphically for each flowrate using the format sheets given in Appendix III as far as possible. ^P\ ^S-T^J' Figure 11.4 An Outdoor Air Collector Test Loop

161 CHAPTER 12 COLLECTOR TESTING USING OIL AS THE HEAT TRANSFER FLUID INTRODUCTION There is only limited experience of collector testing with oils, but this is already enough to show that some extra precautions are required as indicated below THE TEST INSTALLATION Safety Some heat transfer oils are flammable, and many give off unpleasant vapours. An oil loop should therefore be substantially sealed from the atmosphere and in a well ventilated area Inlet Temperature Control Oils generally exhibit poorer heat transfer properties than water, and therefore require heat exchangers with larger surface areas in order to ensure proper collector inlet temperature control. The permitted maximum heat flux on electric heaters is usually specified by the fluid suppliers Temperature Measurement The poor heat transfer properties of oils should also be taken into account when designing fluid mixers for temperature measurement. Significant variations in temperature may occur across a pipe which is carrying heat transfer oil, and special devices for fluid mixing may be necessary in order to ensure that a true bulk mean temperature is measured Fluid Flowrate Measurement The density of oils varies more strongly with temperature than that of water, and hence it is most important to measure the temperature at which volumetric flowrate measurements are made. In order to avoid the complication of volumetric measurements, it is possible to divert the flow into a weighing tank, but maintenance of the vapour seal between the fluid and the atmosphere is then quite a difficult problem. In order to avoid both problems, some laboratories are developing calorimetric flowrate measuring devices like those described in Section 7.5. A small heater in the flow meter raises the temperature of the fluid passing through it, and the rate of heat collection in the collector is deduced from the heat input to the flow meter by comparing the temperature rise across the collector with that across the flow meter.

162 This approach has several advantages, including the elimination of any need to know the specific heat capacity of the fluid. It is attractive to find ways of avoiding the use of specific heat capacity data, for several reasons. The values vary not only with temperature but also with age for some oils, and equipment for measuring specific heats is not widely available which can make the measurements rather expensive Elimination of Moisture & Air from Oil Fluids Moisture in oil loops is a problem only if the system spends a long time at temperatures below 100 C. Periodic heating above the boiling point of water is desirable in order to ensure that the oil remains dry. Air in the oil will affect its heat transfer properties as well as its density and specific heat capacity. Air should be removed by means of a proprietary separator, or a large vessel in which the oil has adequate settling time Fluid Pumping The pressure drop in an oil loop is often greater than in a water loop because of the viscosity of the oil, particularly at low temperatures. When selecting a pump, attention should be paid to the following: (a) (b) Ability to pump static and dynamic heads at low temperature (say 5 C) Resistance of the pump to high temperatures. (Note: It is possible to heat the fluid after the pump and to cool it again in the pump return line, but this approach is not particularly attractive) (c) Compatibility of all seals and pipe connections with the oils used TEST CONDITIONS In order to achieve good internal heat transfer, it is usual to employ higher fluid velocities with oils than are necessary with water. Where flowrates are not specified by the collector supplier, a value of m Cf/Aa = 84 W/m 2 K is recommended. 146-

163 12.4 COMPUTATION & PRESENTATION OF RESULTS The strong temperature dependence of the heat transfer properties of some oils may cause the collector efficiency factor F' to vary significantly with temperature. This may cause the performance of the collector to deviate significantly from the simple Hottel-Whillier-Bliss model, and efficiencies will be seen to vary with solar irradiance. Methods of presenting results to accommodate these effects are still under development, but further analytical explanations of the phenomena involved are given in Chapter 8. Figure 12.1 An Oil Loop for Indoor Testing 147-

164

165 CHAPTER 13 DURABILITY & RELIABILITY TEST METHODS 13.1 INTRODUCTION It is widely recognised that the durability and reliability (D & R) of collectors are of great importance when the overall quality of a collector is being assessed. Work on the development of appropriate tests for assessing these aspects of collectors has, however, been slow to reach maturity, and as a consequence the recommendations given in this chapter should be treated as provisional. Collectors are required to resist a number of influences which can be clearly identified and quantified, such as high internal fluid pressures, high temperatures and rain penetration. Tests to establish whether or not a collector is able to resist these influences are commonly called "Qualification Tests". Several examples of such tests are outlined in this chapter, together with a discussion of the need to carry out such tests in an appropriate sequence. In addition to their ability to resist extreme conditions, collectors must exhibit resistance to natural weathering. All over Europe exposure sites have been established where collectors can be mounted for study under natural weathering conditions, and work is in progress to develop ways of using the results from these exposure tests to predict collector lifetimes. Two approaches are outlined in this chapter for assessing the ability of collectors to resist weathering. One is to predict long term weathering resistance from a short period of natural exposure, and the other is to Use laboratory tests which accelerate the ageing processes. 149-

166 13.2 CHOICE OF SUITABLE DURABILITY & RELIABILITY TESTS Many tests have been identified as potentially suitable for solar collectors, but most are rather expensive to perform and several have been shown to provide little useful information. Taking account of the current level of experience in this field, which it must be recognised is still rather limited, it is possible to identify the following shortlist of qualification tests which appear likely to be appropriate for use in Europe and to be reasonably inexpensive. It is perhaps worth emphasising that this list is not exhaustive and that it may well be shown, as experience develops, that other tests can be more useful than those listed here. (a) (b) (c) (d) Internal pressure test of absorber fluid passageways High temperature stagnation test External thermal shock test Rain penetration test of collector module. With many of the simple (first generation) flat plate collectors, durability problems have been identified using the simplest test of all, which is to leave the collector outdoors to age naturally. Natural ageing is of course the absolute reference against which all laboratory tests should be compared, but with a good collector it should not be expected to cause any noticeable changes for many years. To expose a collector outdoors without any fluid flow is one way of performing high temperature stagnation tests, if the local climatic conditions are suitable. However, over most of Europe it is not an approach which can be expected to lead to reproducible and well defined test results TEST SEQUENCE The sequence in which qualification tests are performed may have an influence on whether or not a collector passes any given test. For example, small distortions caused by high temperature operation may lead to rain penetration in a collector which was previously completely watertight. The list given in Section 13.2 is in a suitable sequence. (Note: It may be interesting to carry out rain penetration tests before and after the high temperature stagnation test in order to confirm whether or not the high temperatures caused distortion of the collector).

167 13.4 SAMPLE SIZE In durability and reliability testing, a sample batch of similar collectors should be tested in order to investigate the probability of failure as well as the possible failure mechanisms. This is particularly important during the early stages of development of a product, when small batch production methods are likely to result in minor manufacturing differences between collectors of the same nominal design. Figure 13.1 Indoor Facility for Durability and Reliability Testing

168 13.5 QUALIFICATION TESTS The test methods outlined below are still under development by the CEC Collector Testing Group, and format sheets for the presentation of results have not yet been finalised. The procedures are therefore provisional Internal Pressure Test of Absorber Fluid Passageways OBJECTIVE The absorber is pressure tested to ensure that it can withstand the pressures which it might meet in service. Where the absorber resistance is expected to be significantly reduced at higher temperatures, such as with absorbers made of organic materials, then a pressure test at a high temperature is proposed. APPARATUS AND PROCEDURE The collector is filled with its heat transfer fluid (usually water) and attached to a means of supplying hydraulic pressure such as a compressor or hand pump. The pump should be fitted with a pressure gauge and a safety valve in order to ensure that only pressures up to the desired value can be supplied. The test pressure is applied and maintained for 15 minutes. The pump is then removed and the pressure in the system is monitored for a further 15 minutes. TEST CONDITIONS For most metallic absorbers, the tests can be carried out at ambient temperature and using a pressure of 1.5 times the maximum collector operating pressure (i.e. either the pressure in the collector when the system pressure relief valve starts to relieve, or the set pressure of the relief valve, whichever is the higher). Where the pressure resistance of the absorber could vary with temperature, as for example with absorbers made of organic materials, the pressure test should be carried out at the maximum temperature which the absorber could reach when working at maximum pressure in a system. If this is the collector stagnation temperature, then its value can be deduced from the collector performance characteristic measured in accordance with the procedures given in Chapter 10. The required temperature may be deduced from the value of T* when n = 0, by setting the irradiance at 1000 W/m 2 and the ambient temperature at 30 C. High temperature pressure tests should be performed for an extended period in order to check for creep in the materials

169 SAFETY Adequate precautions should be provided to protect personnel in the event of failure during a pressure test. Pneumatic tests should not be used for absorbers, other than for very low pressure leakage tests, because of the risk of dangerous failures. RESULTS Leakage, swelling and distortion should all be investigated, and reported together with the values of pressures and temperatures used for the test High Temperature Stagnation Test OBJECTIVE When collectors are first installed or for some reason drained of fluid they may experience high irradiance levels and approach very high stagnation temperatures. This test is intended to ensure that the collector can withstand such conditions without failure. APPARATUS AND PROCEDURE The collector is mounted in accordance with the recommendations of Chapter 5, either outdoors or in a solar simulator, but is not filled with fluid. One of its fluid pipes is sealed to prevent cooling by natural circulation of air, but the other is left open to permit free expansion of air in the absorber. A temperature sensor is attached to the absorber to monitor its temperature during the test. The qualification test is performed for a minimum of 8 hours under the stagnation conditions, and the collector is subsequently dismantled and inspected for signs of failure. This test does not give an indication of the ageing lifetime of the collector, and further tests may be needed to investigate how long the collector will last before serious failure occurs. As a qualification test, the results simply indicate whether or not the collector is susceptible to failure under high temperature conditions. TEST CONDITIONS In a solar simulator the conditions should be established as described in Section 10.3 for a collector efficiency test, but with the simulated solar irradiance set at a value between 900 and 1000 W/m 2, the surrounding air temperature at a value in the range 15 to 30 C, and zero wind speed. If the test is performed outdoors, it may be convenient to allow the test to run for a longer time, but at lower irradiance levels. The exposure outdoors should be

170 continued until at least 30 hours have been experienced at irradiance levels of greater than 800 W/n 2, with the surrounding air temperature at a value in the range 15 to 30 C. RESULTS The collector should be inspected and, if possible, dismantled so that all parts can be examined for degradation, shrinkage, outgassing and distortion. The results should be reported together with the values of irradiance, surrounding air temperature, wind speed and absorber temperature recorded during the test External Thermal Shock Test OBJECTIVE Collectors may from time to time be exposed to sudden rain storms on hot sunny days, causing a severe external thermal shock. This test is intended to check the capability of a collector to withstand such thermal shocks without failure. APPARATUS AND PROCEDURE The collector is mounted in accordance with the recommendations of Chapter 5, either outdoors or in a solar simulator, but is not filled with fluid. One of its fluid pipes is sealed to prevent cooling by natural circulation of air, but the other is left open to permit free expansion of air in the absorber. A temperature sensor is attached to the absorber to monitor its temperature during the test. An array of water jets is arranged above the collector to provide a uniform spray of water over the collector aperture (see Section ). The collector is maintained in steady state operating conditions for a period of one hour before the water sprays are turned on. It is then cooled by the water sprays for 15 minutes before being dismantled and inspected. TEST CONDITIONS The steady state operating conditions should be a solar (or simulated solar) irradiance in the range 900 W/m 2 to 1000 W/m 2, and a surrounding air temperature in the range 15 to 30 C. The water spray should have a temperature in the range 10 C C to 30 C and a flowrate in the range 0.04 to 0.06 t/s per square metre of collector aperture. RESULTS The collector should be examined and any cracking, distortion, condensation or water penetration reported

171 The measured values of solar irradiance, surrounding air temperature, absorber temperature, water temperature and water flowrate should also be reported Rain Penetration Test OBJECTIVE This test is intended to ensure that collectors are substantially resistant to rain penetration. They should not permit the entry of either free falling rain or wind driven rain. Collectors may have ventilation holes and drain holes, but these should be adequately shielded from driving rain. APPARATUS AND PROCEDURE There are many ways of simulating rain (35, 36) but there is insufficient experience available to permit detailed recommendations here. For the purposes of testing collector modules it may in some cases be adequate simply to spray in a direction approximately normal to the collector aperture. For other modules, spraying on the sides and back, and the use of simultaneous wind loading (by suction, etc.) may also yield useful information. The most straightforward test employs no wind, and a water spray which is approximately normal to the collector aperture when the collector is mounted in accordance with the recommendations of Chapter 5. The test can be performed indoors or outdoors, under any solar irradiance level including zero. The collector should have its fluid inlet and outlet pipes sealed, and should be weighed before the test. The water should be sprayed on the collector for a test period of four hours. External surfaces of the collector should then be dried and the collector re-weighed. TEST CONDITIONS The collector should be at approximately the same temperature as the surrounding air, and sprayed with water having a temperature in the range 5 to 30 C. The spray flowrate should be in the range 0.04 to 0.06 je./s per square metre of collector aperture. RESULTS The weights of the collector before and after the test should be reported together with any visible signs of water penetration found when the collector is dismantled. 155-

172 Figure 13.2 A Rain Penetration Test Facility Figure 13.3 Pressure Test for Absorbers

173 13.6 NATURAL AGEING The processes of ageing can be quite complex, involving combinations of many environmental factors. Ageing mechanisms vary with location, particularly with regard to the influences of atmospheric pollutants such as salt and sulphur dioxide, and this location dependence makes it difficult to predict lifetimes in one location from tests in another. However, despite these drawbacks, much can be learned from quite short periods of natural ageing outdoors. Work is in progress in an attempt to find ways of defining an adequate period of natural exposure of a collector for the purposes of carrying out a short term ageing test. Periods of high solar irradiance and some rain need to be included, though tests can sometimes be accelerated by using water sprays to induce thermal shocks and to simulate rain. It is already clear that one year of natural ageing in most European countries is likely to give a sufficiently wide range of environmental conditions to show up most basic faults in a prototype collector. On the basis of current experience, all suppliers of collectors would be well advised to place their collectors outdoors without fluid to age naturally for this period. With further experience it may be possible to reduce the period to 6 months or less if measurements of the environmental parameters are recorded during the exposure period. One year of exposure will not necessarily give a good indication of the resistance of the collector to long term corrosion or erosion processes, but durability experience from other fields where the same materials have been employed may also be used to help designers. Alternatively, accelerated ageing tests may be appropriate, as discussed in Section Figure 13.4 Outdoor Natural Exposure Site with Collectors in Dry Stagnation 157-

174 13.7 ACCELERATED AGEING It is in the field of accelerated ageing that perhaps the widest range of possible test methods exists, and also the most expensive test methods. There is already widespread international experience of the resistance of most materials to standard ageing tests, and the main task is to ensure that the implications of this experience are reflected in collector design. However, combinations of materials, new manufacturing processes and unusual combinations of humidity, temperature and pollutants can still lead to collector failure, even when tests on individual materials might indicate that designs are satisfactory. It is therefore important to identify suitable accelerated ageing tests for complete collector modules. Work is still in progress in this field, and opinions vary concerning the most suitable test methods. However, it is generally agreed that one of the most useful accelerated ageing tests is that of exposure to salt mist Salt Mist Exposure Test APPARATUS AND PROCEDURE A number of standards apply to salt mist testing (37, 38, 39) and can be used to specify the chamber in which it should be carried out and the composition of the salt mist. Before the components are placed in a test chamber and exposed to a salt mist, the surface coatings which are being tested should be scratched. Several scratches, approximately 0.5 mm wide, 60 mm long and deep enough to expose the substrate should be made in a square pattern near to the centre of exposed surfaces. The whole collector or the components being tested should be placed in the test chamber, following common corrosion practices, and avoiding contact with metal parts of the chamber. Salt mist exposure for about 500 hours is likely to be appropriate for collector modules. After salt mist exposure, the components should be washed clean and dried before a "sticky tape test" is carried out on the scratched surfaces. Either Scotch 600 or Tesa 104 tape should be placed over the scratches, left for about 10 minutes and then removed to test the adhesion of the coating after salt mist exposure.

175 TEST CONDITIONS These should largely follow the standards given above, although some variation in chamber temperature may be acceptable in the range C. RESULTS Oxidation or other degradation in the region of the sticky tape test should be reported together with any other signs of corrosion or failure. 159-

176

177 REFERENCES FOR COLLECTOR TESTING 1. Welford, W. T. and Winston, R. The Optics of Non-imaging Concentrators, Light"and Solar Energy, Academic Press (1978). 2. SI Le Systeme International d'unites, Offilib, F 75005, BIPM, Paris, France. 3. Units and Symbols in Solar Energy, Solar Energy, 21, pp (1978). 4. Guide to Meteorological Instruments and Observing Practices. World Meteorological Organisation, Geneva, Switzerland (1983). 5. World Radiation Centre, Postfach 173, Dorfstrasse 33, CH-7260 Davos Dorf, Switzerland. 6. Kipp & Zonen, PO Box 507, 2600 AM Delft, Netherlands. 7. The Eppley Laboratory Inc., 12 Sheffield Avenue, Newport, Rhode Island 02840, USA. 8. British Standard Draft for Development DD77:1982 Methods of Test for the Thermal Performance of Solar Collectors, British Standards Institution, 2 Park Street, London W1A 2BS, England (1982). 9. Duffie, J.A. and Beckman, W.A. Solar Engineering of Thermal Processes, Wiley (1980). 10. Siegel, R. and Howell, J.R. Thermal Radiation Heat Transfer, McGraw-Hill (1972). 11. Ferraro, R., Godoy, R. and Turrent, D. Monitoring Solar Heating Systems, Commission of the European Communities, Pergamon (1983). 12. Usability of Solar Collectors (A). Solar Collector Efficiency Test, Bundesverband Solarenergie (B.S.E.), Kruppstrasse 5, D-4300 Essen 1, W. Germany (1978). 13. DIN 4757/4, Determination of Efficiency, Thermal Capacity and Pressure Drop of Solar Collectors. Beuth Verlag, Berlin, W. Germany (1982). 14. NBSIR , Method of Testing for Rating Solar Collectors Based on Thermal Performance, National Bureau of Standards, US Department of Commerce, Washington DC 20234, USA (1974). 161-

178 15. ASHRAE 93-77, Methods of Testing to Determine the Thermal Performance of Solar Collectors, American Society of Heating, Refrigerating and Air conditioning Engineers, 345 East 47th Street, New York, N.Y , USA (1978). 16. Bougard, J. Determination of the Thermal Capacity of CEC 4 Collector by Several Methods, Faculte Polytechnique de Mons, Belgium (1979). 17. ASTM E424-71, Solar Energy Transmittance and Reflectance (Terrestrial) of Sheet Materials. American Society for Testing Materials, 1916 Race St., Philadelphia, Pa 19103, USA (1971). 18. ASTM E Calorimetric Determination of Hemispherical Emittance and the Ratio of Solar Absorptance to Hemispherical Emittance using Solar Simulation, American Society for Testing Materials 1916 Race St., Philadelphia, Pa 19103, USA (1980). 19. Moon, P. Proposed Standard Radiation Curves for Engineering Use, Jnl. Franklin Institute, Vol. 230, pp (1940). 20. NBN 894, Photometric Functional Characteristics of Transparent and Translucent Materials for Glazing - Purposes, Belgian Standards Institute, Ave, de la Brabanconne 29, 1040 Brussels, Belgium (1971). 21. National Physical Laboratory, Teddington, Middlesex, England. 22. Physikalisch-Technische Bundesanstalt, Bundesallee 100, 3300 Braunschweig, W. Germany. 23. Elan Informatique, Rue des Cosmonautes, Z.I. du Palays, Toulouse, France. 24. ASTM E , Test for Normal Spectral Emittance at Elevated Temperatures, American Society for Testing Materials, 1916 Race St., Philadelphia, Pa 19103, USA (1980). 25. ASTM E , Test for Normal Spectral Emittance at Elevated Temperatures of Non-conducting Specimens, American Society for Testing Materials, 1916 Race St., Philadelphia, Pa 19103, USA (1980). 26. ASTM E , Test for Total Normal Emittance of Surfaces using Inspection-meter Techniques, American Society for Testing Materials, 1916 Race St., Philadelphia, Pa 19103, USA (1980). 27. BS 4892, Guide to the Measurement of Thermal Radiation by Means of the Thermopile Radiometer, British Standards Institution, 2 Park Street, London W1A 2BS, England (1973)

179 28. Devices and Services Inc., Dennis Road, Suite 405, Dallas, Texas 75229, USA. 29. DIN 67507, Light Transmittance, Radiant Transmittance and Total Energy Transmittance of Glazings" Beuth Verlag, Berlin, W. Germany (1980). 30. DIN 5036/Part 3, Radiometric and Photometric Properties of Materials; Methods of Measurement for Photometric and Spectral Radiometric Characteristics, Beuth Verlag, Berlin, W. Germany (1979). 31. Talarek, H.D. and Stein, H.J. Standardised Test Procedures for Solar Collectors"! Evaluation, Status and Trends, Solar Forum, Hamburg, W. Germany (1980). 32. Boussemaere, C. Flat Plate Collector Outdoor Testing under Transient Conditions"! Faculte Polytechnique de Mons, Belgium (May 1981). 33. BS 1042 Part 1:1964 (Amended 1968), Methods for the Measurement of Fluid Flow in Pipes Part 1. Orifice Plates, Nozzles and Venturi Tubes, British Standards Institution, 2 Park Street, London W1A 2BS England (1968). 34. DIN 1952, Flow Measurements with Standardised Nozzles, Orifices and Venturi Nozzles, Beuth Verlag, Berlin, W. Germany (July 1982). 35. ASTM E , Test for Water Penetration of Exterior Windows, Curtain Walls and Doors by Uniform Static Air Pressure Difference, American Society for Testing Materials, 1916 Race St., Philadelphia, Pa 19103, USA (1975). 36. DIN EN86, Methods of Testing Windows; Water Tightness Test under Static Pressure, Beuth Verlag, Berlin, W. Germany (1981). 37. ANSI-ASTM B , Salt Spray (Fog) Testing, American Society for Testing Materials, 1916 Race St., Philadelphia, Pa 19103, USA (1979). 38. IEC : Basic Environmental Testing Procedures, Part 2 Test Ka:Salt Mist, Central Office of IEC, 3 Rue de Varembe, 1211 Geneva, 20, Switzerland. 39. MIL STD 310 B Salt Mist Exposure. 40. Garg, H.P. Treatise on Solar Energy, Vol.1, Wiley (1982)

180 41. Gillett, W.B. The Equivalence of Outdoor and Mixed Indoor /Outdoor Solar Collector Testing"! Solar Energy 25, pp (1980). 42. AFNOR P50-501, Liquid Circulation Solar Collectors. Determination of Thermal Performance, Association Francaise de Normalisation, Tour Europe, Cedex /, Paris, France (1980). 43. Green, A.A. and Gillett, W.B. The Significance of Longwave Radiation in Flat Plate Solar Collecto~r Testing, Proc! IEE Conf. Future Energy Concepts, London, England (1979). 44. Mayhew, Y.R. and Rogers, G.F.C. Thermodynamic and Transport Properties of Fluids, Blackwell (1969). 45. Yass, K. and Curtis, H.B. Low Cost Air Mass 2 Solar Simulator, NASA TM X-3059 (June 1974). 46. Thorn Lighting Ltd., Gt. Cambridge Road, Enfield, Middlesex EN1 1VL, England. 47. Aranovitch, E. and Gillett, W.B. Workshop on Solar Simulators, CEC Joint Research Centre, Ispra, Italy (February 1982). 48. Robinson, N. Solar Radiation, Elsevier (1966). 49. Kraus, K., Hahne, E. and Sahns, J. Laboratory Tests for Flat Plate Solar Collectors, SUN II, Proc. ISES Congress, Atlanta, USA, Vol. 1, pp (May 1979). 50. Green, A.A., Kenna, J.P. and Rawcliffe, R.W. The Influence of Wind Speed on Solar Collector Performance, Proc. IEE Cont. Future Energy Concepts, London, England (1981)

181 PART TWO GUIDELINES FOR SOLAR COLLECTOR DESIGN

182

183 CHAPTER 14 INTRODUCTION TO THE DESIGN GUIDELINES 14.1 BACKGROUND Solar collectors can be designed in many ways. They can incorporate many different materials and be manufactured using a variety of techniques. At this relatively early stage in the development of the solar heating industry, there are few mass produced collectors on the European market, and significant improvements in collector design are still anticipated. The improved designs are not expected to show major increases in collection efficiency however, but rather to have longer lifetimes and lower installed costs. Because of the wide range of design possibilities and the continuing search for new ideas and approaches to the development of durable collectors, it would not be appropriate here to recommend any specific design. Instead, the approach is to draw attention to the design requirements and to indicate which design features appear to have been successful and which have failed. The information contained in these guidelines has been gathered from European experts, from published literature and Codes of Practice, and from studies of working solar heating systems in the countries of the European Community. Much of it is not new. The merit of the compilation is that it represents a considered view of experts from the different countries of the Community. A major input to the guidelines has been a survey of the collectors in eighty-five solar heating systems spread throughout the countries of the European Community. Each of these systems was visited by experts and the condition of over 2500 m 2 of collector was reported using a standardised inspection reporting format (see Appendix V). This has served to illustrate many common design faults in the "first generation" of solar collectors as well as to point towards the more successful design features. The common faults are discussed in Chapter 16, where it can be seen that they are similar to the faults found in many other fields of technology. Many of the collector failures found in the survey could have been predicted from a study of the collector designs, using advice from general engineering Codes of Practice

184 The benefit of bringing this experience together is therefore not to provide a new engineering approach to collector design, but rather to draw attention to many straightforward and some specialised design considerations which should not be overlooked in the quest for new or advanced collector concepts. These guidelines do not include detailed information concerning "second generation" solar collectors, such as those containing evacuated tubes or reflectors. The information presented here is generally applicable to these new types as well as to existing designs, but additional considerations are necessary to accommodate the higher temperatures and different configurations of components often associated with advanced collectors SOLAR HEATING SYSTEMS The most common application for solar thermal collectors in Europe is for heating domestic hot water (DHW) in private dwellings, but they are also widely used for heating swimming pools and to a much lesser extent for space heating in houses. Industrial applications to provide process heating have been the subject of several experimental studies but experience with such applications is limited. The design requirements for a collector can be influenced by the system in which it is to be used. For example, the materials used in the fluid passageways of a collector are particularly important, because they must be compatible with the heat transfer fluid used, and with local regulations. The water quality and the local water regulations vary from one region to another. These variations restrict the use of some collector materials and heat transfer fluids. The system designs and the regions in which the collector may be used should therefore be taken into account at an early stage of the design Water Heating Systems In some systems, known as direct systems, the water which will ultimately be consumed is circulated through the collector as shown in Figure The absorber and all pipework, valves and pumps in direct systems for domestic water heating should be made of materials which are suitable for contact with potable water. Collectors need to drain easily for freeze protection in winter, and to allow free release of steam in the event of boiling

185 flat plate collector auxiliary heater hot.water cylinder Figure 14.1 Direct Forced Circulation System with Drain Back" Indirect Systems Many solar heating systems employ an "indirect" fluid circuit in which a separate heat transfer fluid is used to carry heat from the collector to the heat storage system. Such systems allow a wider choice of materials in the solar absorber and system pipework because anti-freeze, corrosion inhibitors and biocides can be added to the fluid. Biocides are generally used in circuits where the expansion vessel is open to atmosphere because the operating temperatures of solar heating systems permit micro-organisms to grow in the fluid for most of the year in Europe. A closed (sealed and pressurised) system is sometimes used, and can give greater flexibility of component layout, as well as extending the life of antifreeze fluids which need to be protected from oxidation. The collector passageways need to be sufficiently strong to withstand the maximum system pressure at high temperature stagnation conditions in closed systems. Pressure relief valves should always be installed in such systems to protect components from over-pressure, as shown in Figure Most countries have regulations to protect consumers from toxic fluid additives in the event of heat transfer fluid leakage. In some countries ethylene glycol is prohibited, though propylene glycol may be used as an 169-

186 antifreeze. The design of heat exchangers is also restricted in some regions, on the grounds of safety for the system users. expansion vessel Hat plate collector/ pressure relief valve cold mains water supply to taps -auxiliary heater Figure 14.2 Lt_J hot water cylinder Indirect System with Closed Primary Circuit Natural Circulation (Thermosiphon) Both direct and indirect solar water heating systems can be designed to operate using natural circulation in the collector circuit. The main requirement for the absorber in such a system is that it should have a low resistance to fluid flow. This effectively implies a need for an arrangement of fluid passageways with large bore pipes and few sharp bends. Parallel risers are often used in collectors for natural circulation systems, to minimise the pressure drop. Collectors in natural circulation systems have traditionally been mounted below the level of the preheat store, as shown in Figure 14.3, and for domestic water heating an integral unit is often used as shown in Figure However, collectors may be installed at almost the same ~ level as the preheat tank if a non-return valve with a very low opening pressure is used. Direct systems may be damaged by freezing, and may therefore need to be drained down for the winter period in Europe. 170-

187 n lop-up and. I. expansion i. I tank I - = " ' vent pipe cold mains water supply to taps L f g g g i - hot water cylinder Figure 14.3 Indirect Thermosiphon System Figure 14.4 A Packaged Thermosiphon Solar Water Heater -171

188

189 CHAPTER 15 GENERAL COLLECTOR DESIGN CONSIDERATIONS 15.1 INTRODUCTION There are various types of solar thermal collector but the most commonly used is known as a flat plate collector. A typical flat plate collector for heating liquids, employing an absorber with a header and riser tube arrangement, is shown in Figure In other designs there may be a single "serpentine" tube in place of the headers and risers. A very simple configuration which has been used for low temperature applications such as swimming pools employs only a single plate for the absorber, and fluid is allowed to trickle over its surface. Because of the low temperatures involved in swimming pool heating it is common, even when more conventional absorbers are employed, to omit the glazing. Unglazed collectors, such as those used for swimming pool heating, require durable absorber materials which are able to withstand direct exposure to the weather. Flat plate collectors are also used for heating air, but the number of air heating collectors in use in Europe is very small compared with the number used for water heating. Experience with the optimisation of air heating collector design is limited, and consequently the design of air collectors is not discussed in detail in these guidelines. Heat transfer fluid passageways headers and risers).sealant/gasket Transparent cover \ \V V> \S. : Yv vs,"\ \v,, \ \\\ w \\ w -v*. \ \\ Blackened ^_ absorber plate Casing- Insulation " X^-^ZT^ "Sk ^ ~%. ^fesa \. Fluid inlet Figure 15.1 A Typical Flat Plate Collector

190 Some water heating collectors, like the example shown in Figure 15.1, are self contained units, which can be attached to existing roofs, walls or free standing frameworks. Others are integrated into the structure of the roof as shown in Figure Concentrating collectors are used for solar heating applications in some parts of the world, but not very widely in Europe. This is largely because there is a high percentage of diffuse solar irradiance in Europe, and lenses and reflectors camiot bring more than a small fraction of diffuse solar radiation to a focus. Some types of concentrating collector also have the disadvantage that they need to be moved to follow the sun during the day. This can make them expensive and rather difficult to build and maintain. Figure 15.2 A Direct Solar Water Heating System Employing 25 m 2 of Collector Built into the Roof of a Student Residence Several types of advanced collector are now being manufactured. These include designs which can produce a small concentration of both direct and diffuse solar radiation, known as compound parabolic concentrators (CPC collectors), and designs which employ an evacuated enclosure around the absorber to reduce heat losses (Figure 15.3).

191 Figure 15.3 Evacuated Tubular Collector Array The designer of a solar collector has several aspects of his product to optimise, including appearance, weight, durability, thermal performance and cost. Consequently there are many designs available in the market place. The main components of a liquid heating solar collector are: (1) A blackened absorber plate which absorbs solar radiation, converts it to heat and conducts the heat to the fluid passageways. (2) Fluid passageways in which a heat transfer fluid flows. These should be in good thermal contact with the absorber plate so that heat is transferred efficiently to the fluid for subsequent transfer to the heat storage system. (3) One or more transparent covers to insulate the absorber from the cooler ambient air and to shelter it from the wind. A cover also reduces the loss of heat by thermal radiation from the collector and provides weatherproofing. (4) Insulation behind the absorber to reduce heat losses from the back of the collector. (5) An enclosure to hold the components of the collector in their correct positions and to protect the absorber and insulation materials from the weather

192 (6) A sealant or gasket between the cover and the casing. Basic design requirements for each of these collector components are discussed in this chapter. In addition it is also important to optimise the thermal performance of the collector. This aspect is dealt with in more detail in Chapter THE ABSORBER Materials An absorber may be made from any of a wide range of materials, or in some cases from more than one material. Copper, stainless steel, mild steel, aluminium and plastics are all used. The fluid passageways of the absorber may consist of tubes bonded to an absorbing plate or may form an integral part of the absorber (see Figure 15.4). The material used for the fluid passageways should be compatible, from the point of view of corrosion, with the other components in the system and with the heat transfer fluid. (a) o o o o (d) Tubes bonded to an absorber plate. Corrugated ahceta bonded together. (b) o o o o ( e ) Tubes bonded to a shsped absorber plate give increased contact area for themal Corrugated sheet bonded to a flat conduction. sheet. (c) o o o o Integral tube and plate. Two sheets bonded together to produce a sandwich configuration. Figure 15.A Cross Sections of Different Absorber Designs

193 All the absorber materials should also be able to withstand the maximum stagnation temperature which could be reached by the collector in service (i.e. the highest temperature which might be achieved on a clear day when there is no fluid in the collector). Experience has shown that simple mechanical clamping of tubes to an absorber plate usually produces an absorber having a poor efficiency. A good thermal bond is required for tube and plate designs in order to ensure good heat transfer from the absorbing surface into the fluid. Brazing, welding or use of a high temperature solder will provide this. It is particularly important to select a bonding system which can resist both high temperatures and thermal cycling. For example, with advanced absorber surface coatings (selective surfaces) it is not appropriate to use some soft solders because of their high stagnation temperatures Fluid Passageways The fluid in a collector usually flows from the bottom to the top, either through an array of parallel tubes (risers), or through a single tube which passes over the absorber along a serpentine path. The fluid tubes should always be arranged so that air will rise freely up and out of the absorber. Tube arrangements should also permit the fluid to be easily drained from the collector to prevent freezing damage. If parallel risers are used then the manifolds at the top and bottom of the absorber should have a diameter which is greater than the diameter of the risers, in order to encourage an even flow distribution. header Figure 15.5 Pipe Diameters risers 177-

194 Absorbers should be able to withstand both the maximum and minimum pressures which they could experience in service. This is particularly important for collectors which may be used in closed systems at high pressures, and for collectors which may be located in parts of a circuit which sometimes experience fluid pressures which are below atmospheric pressure Absorber Surfaces Matt black paints have been widely used as absorber surfaces for many years because they are relatively cheap, simple to apply and may be easily repaired. Some form of pre-treatment of the plate surface is usually necessary to ensure satisfactory paint adhesion during the repeated thermal cycling which absorbers experience in service. When choosing a paint it is wise to confirm with the manufacturer that the pre-treatment and finish are suitable for the expected service conditions. Thick layers of primer and/or paint should be avoided since they provide a resistance to the flow of heat into the absorber. Most paints unfortunately have the disadvantage that they are strong emitters of thermal (infra-red) radiation, and at high temperatures they produce significant heat losses from the front of the collector. Collector heat losses may be substantially reduced by the use of "selective surfaces" which have a low emittance for thermal radiation, but a high absorptance for solar radiation. A selective surface usually consists of a very thin layer (less than 2 um thick) of solar radiation absorbing material, deposited onto a highly polished metallic substrate which has a high reflectance and therefore a low emittance for thermal radiation. Well known selective surfaces include chromium oxide electro-deposited onto polished nickel, black nickel oxide on bright nickel, black copper oxide on copper, and tin oxide on an enamel substrate. Most of these surfaces require specialist facilities for their preparation, but adhesive metal films coated with a selective surface are available for application to solar absorbers. Good selective surfaces may be expected to have an average solar absorptance of greater than 0.95 over a typical collector area, and an average thermal emittance of less than 0.2. In well designed single glazed collectors a good selective surface may be expected to produce absorber stagnation temperatures of up to 200 C, whilst a matt black paint might not produce more than 150 C. Very much higher stagnation temperatures may occur in evacuated collectors

195 15.3 THE TRANSPARENT COVER Most flat plate collectors incorporate at least one transparent cover made of glass or plastic. The cover protects the absorber and the insulation from the weather, and reduces the heat losses from the front of the collector. Collector covers are required to perform essentially the same functions as those of glass in a greenhouse. They should exhibit a high transmittance for solar radiation (wavelengths 0.3 to 2.5 um) in order to maximise the solar input to the absorber, but should not transmit the thermal radiation of wavelengths greater than about 3 um, which is emitted by the hot absorber. The main difference between the requirements for a greenhouse cover and those for a collector cover is that the collector cover is required to operate at significantly higher temperatures. Under stagnation conditions at low wind speeds the temperature of a collector cover can approach 100 C, whilst inner glazings in a multiple cover system can experience even higher temperatures Glass Glass is a widely used cover material, having most of the required properties. Its main disadvantages are that it is brittle, has a high density, and is relatively expensive. Some glass is made from sand with a high iron content which gives it a green appearance. Low iron glasses give better performance in solar collector applications because of their higher solar transmittance. Toughened glass is becoming more popular for collector covers because it avoids the safety problems. In addition it can be used in thinner sheets which reduces the weight and increases the transmittance. The costs of toughened glass have recently become more competitive with alternative materials. Glass should be separated from metal supports by thermal insulation when used in a solar collector, in order to minimise the risks of thermal stress cracking caused by cold edges around a hot sheet. The insulation should be flexible and allow the glass some freedom of movement, but it should not become soft when heated. Not all mastic sealants used for building applications are sufficiently resistant to high temperature for use in solar collectors. 179-

196 Thermal insulation between glass and casing \ / /* t* 'A * *~ Outer glazing * # *? f s- t* Inner glazing Absorber fl/wwl - Figure 15.6 Avoidance of Thermal Stress in Cover Glass by Insulating it from the Collector Casing Plastics A number of plastic cover materials have been developed specially for solar collectors, where the following design requirements can be identified: (i) (ii) (iii) (iv) (v) Resistance to degradation by sunlight (particularly by u-v radiation). Resistance to chemical degradation and stress cracking when in contact with rainwater, atmospheric pollutants, detergents used for cleaning, bird excreta, paints, sealants, etc. Tolerance of low and high temperatures (say -20 C to 100 C) without undue embrittlement, softening or creep. Good impact resistance, with a hard surface to resist accidental or malicious damage, and abrasion during cleaning. Appropriate fire resistance for use roof. Plastics are lighter and less brittle than glass, but their solar transmittances are higher than that of glass only when they are used in the form of thin films. However, many have a tendency to attract dirt and to creep or sag when warm. Because of the high coefficients of expansion which plastic materials exhibit, special fixings are usually required, and thin plastic sheets must be supported in the middle or tensioned to prevent sagging. on 180-

197 Cover Size The maximum size of a collector cover is limited by the wind and snow loads which it can withstand, and these vary of course with the cover material, its thickness and its method of support. The difficulty of installation and replacement should be considered when selecting cover sizes. Advice can usually be obtained from the material manufacturers, but typical maximum sizes for glass at a tilt of 45 in Northern European conditions are of the order of 0.6 m 2 for 4 mm thickness and 1.4 m 2 for 6 mm thickness. Toughened glass may be used in larger sheets, again depending on the thickness selected Cover to Absorber Spacing The spacing between the cover and the absorber is typically between 20 and 40 mm. A larger spacing than this will cause the shadows cast by the sides of the collector to become too large, and reduce collector performance over the day. A smaller spacing will increase the heat transfer between the absorber and the cover and hence the overall collector heat losses. Additional layers of glazing (double glazing) will reduce collector heat losses but will also reduce the overall transmittance of the cover system. A multiple glazing system may improve the overall performance of the collector if suitably designed (see Chapter 19) THE INSULATION Heat losses from the back and sides of a collector may be reduced by the use of insulation. An optimum thickness may be determined on the basis of cost effectiveness, but it is usually appropriate to reduce the back and side losses to a level at which they are small relative to the losses from the front. The required thermal conductance is typically in the range from 0.5 to 1 W/m 2 K for simple flat plate collectors which is approximately equivalent to between 40 and 80 mm of glass fibre insulation Temperature Resistance The insulation material should be resistant to the maximum stagnation temperature of the collector components with which it comes into contact. For example, insulation close to the absorber should be able to withstand about 150 C in matt black collectors and about 200 C when selectively coated absorbers are used. At these high temperatures the insulation should not shrink, melt, or give off vapours ("outgas") which could condense on the collector cover and reduce its solar transmittance. In order to reduce the costs, a thin layer of temperature-resistant insulation is sometimes

198 used in contact with the absorber, and the remaining thickness filled with a less expensive, low temperature insulating material, as shown in Figure In addition, a thin thermal radiation reflecting foil is sometimes used with an air gap between the absorber and the foil to reduce back heat losses and protect the insulation. When an air gap is used, it should be sealed around the edges of the absorber to prevent natural circulation of air inside the enclosure. Absorber Low temperature insulation High temperature insulation Reflective foil Figure 15.7 The Use of Appropriate Insulating Materials When integrating a collector into a roof, the insulation should be positioned to protect the other roofing materials from extremes of temperature which might cause them to dry out, crack and become weakened. The installation of a collector should not reduce the overall insulation levels of the roof itself Water Resistance and Durability Because of the risks of water penetration in solar collectors it may be considered preferable to employ a closed cell insulation material which can be easily dried out. Water may accumulate from condensation, enter during driving rainstorms or be released at the system commissioning stage from a poorly made pipe connection. Provided that the casing is designed with drain holes and adequate ventilation fibrous insulation materials have been shown to give satisfactory performance. Other considerations relevant to the choice of insulation material are resistance to infestation by insects, to the growth of micro-organisms, moulds and fungi, and to fire.

199 15.5 THE ENCLOSURE AND MOUNTINGS The absorber, insulation and collector cover may either be assembled in a self-contained box or incorporated directly into the roof of a building using the roof timbers to provide support. In either case the assembly should be designed to permit differential thermal expansion between components in each module and between modules. Thermal bridges between the hot absorber and the casing should be minimised, and fastenings should be carefully selected to prevent corrosion between dissimilar materials. The absorber should be insulated at the back and the sides such that it is not possible for air to circulate between the absorber and the insulation, inside the enclosure. Where glazing bars are required to support the cover system, they should be small in section to minimise shading of the absorber and thermal bridging to the environment Wind and Weather Resistance Collectors may be installed on a sloping roof, a wall, or a free standing framework. They should always be securely mounted to withstand both lateral and uplifting wind forces, and mountings should be sufficiently strong to withstand vibration and wind gusts. It is important to check that the supporting structure itself is strong enough to withstand all the wind, snow and other loads imposed by the presence of the collectors and their mountings. When installing collectors on a tiled roof, all penetration holes should be completely sealed with adequate flashing to ensure that they will remain weathertight. Most roofing materials such as slates and tiles move over each other to some extent. Adequate provision should therefore be made for the movement of roofing materials when mounting collectors on a roof. Any penetration of the underlay (water resistant membrane) should be completely resealed Condensation and Ventilation For a large number of collector designs it is not practicable to seal the collector enclosure completely from the atmosphere. The range of operating temperatures is so great that the pressure variations inside the collector make sealing difficult. Adequate ventilation is therefore required to permit the removal of condensation. Drain holes at the bottom of the collector are advisable to ensure that a build up of condensed water cannot occur. Drainage and ventilation holes should be located or shielded to minimise the entry of driving rain and snow, and should be fitted with a gauze to inhibit the entry of insects

200 Evacuated Enclosures As discussed in Chapter 19, the heat losses from a collector can be greatly reduced by evacuating the enclosure around the absorber. The shape of the enclosure may then be determined largely by the need to withstand the pressure imposed on it by the atmosphere. A common choice is that of a cylinder, as in the "evacuated tubular collector". Other evacuated enclosures have also been produced. The basic principles outlined in this chapter also apply to evacuated collectors, although these advanced products require specialist attention to several aspects. Some models employ an evacuated enclosure made entirely of glass, whilst others have a glass to metal seal within the enclosure. An evacuated enclosure which will last for 15 to 20 years under thermal cycling conditions is not easy to produce, and more experience is required before generalised recommendations can be made on evacuated collector design Installation and Maintenance The collector enclosure may need to be designed, to withstand not only the operating conditions on the roof, but also the loads imposed during handling and transportation before installation. Where glass is used as the cover, the enclosure should be either sufficiently stiff to protect the cover from breaking during installation or suitable for glazing in situ (i.e. when the rest of the collector has been installed). The magnitude of wind gust forces during installation should not be overlooked as these will limit the size of module which can be safely installed in windy locations SEALANTS AND GASKETS The integrity and long term durability of a collector depends strongly on the design of the joints and on the quality of the sealants used around the glazing and around the fluid inlet and outlet pipes. In a well designed joint, the stresses are spread over a large area of sealant. The sealants need to be resistant to the temperatures involved, and to weathering. They should remain flexible to permit thermal movement of the collector components throughout the expected lifetime of the collector, and must remain firmly in place. Most low temperature mastic materials used for conventional building sealing applications are inadequate to withstand the temperatures and movements experienced in solar collectors. Wherever feasible, sealants should be provided with a cover strip to protect them from exposure to solar radiation, and from attack by birds and insects.

201 CHAPTER 16 COLLECTOR DESIGN PROBLEMS OBSERVED IN SOLAR HEATING INSTALLATIONS 16.1 INTRODUCTION In order to identify the problems experienced with collectors in the field, a survey of solar heating systems was carried out during 1981 and 1982, with the help of CEC contractors and their colleagues from throughout the European Community. A standard Reporting Format was used for all the system inspections (see Appendix V), and results were collated and analysed at University College, Cardiff. The survey included systems from all 10 countries of the European Community. A total of 85 systems were inspected, of which 9 were CEC Solar Pilot Test Facilities. The total number of collector modules involved in the survey was approximately 2500, and 69 different models of collector were found. All but 4 of the systems used water (or an antifreeze solution) as the heat transfer medium. Of the remainder, 3 used oil and 1 used air. Most of the systems, 54 in all, were confined to heating domestic hot water; 13 heated domestic hot water and provided some space heating; 10 (all at one site) provided air conditioning; 7 of the Solar Pilot Test Facilities simulated space heating only; and 1 system heated a swimming pool. The age of the installations ranged from only a few months to well over 5 years, with an average age of 27 months. The survey was primarily concerned with the assessment of collector durability and performance, but general comments on overall system performance and owner satisfaction were also obtained. In this chapter, the results of the survey are presented for each collector component, and the problems associated with particular materials or fabrication techniques are discussed. It will be seen that the general design guidelines given in Chapter 15 are illustrated and important points emphasised by examples from working installations. In interpreting the results of this survey, it should be recognised that many of the systems were "first generation" prototypes and furthermore, that the inspecting engineers were not specifically trained for the job. Some Formats were completed in great detail, whilst others contained only a brief indication of problem areas. One unfortunate consequence of having some incomplete reports is that it has not been possible to present statistical data in terms of the total number of collector modules 185-

202 concerned. However, statistical data are presented in terms of the total number of installations involved, where appropriate. The extent of the comments in the text has been influenced by the number of reports received which described the particular problem, and the seriousness with which it was viewed by the inspectors THE COVER Materials Used The range of cover materials encountered in the survey is summarised in Table The most common cover material was 4 mm float glass, although glass thicknesses varied from 3 mm to 6.5 mm. A few installations used 'low iron' glass. Tempered glass, textured glass, and wire reinforced glass in patent glazing were also used. Where two glass covers were employed, the inner cover was usually thinner than the outer cover. Thicknesses of 3 mm for the inner cover with 4 mm for the outer cover were typical for collectors with an aperture area of approximately 1 m 2. COVER CATEGORY INSTALLATIONS Single Glass 60(71%) Single G.R.P (Glass Reinforced Polyester) 8 (9%) Double Glass 6 (7%) Single Rigid Plastic 5 (6%) Double Rigid Plastic & Plastic Film (inner) 3 (4%) Single Glass & Evacuated Tubes 1 (1%) Evacuated Tubes 1 (1%) No Cover 1 (1%) Table 16.1 Cover Materials Reported in CEC Survey The majority of the G.R.P. covers were flat and were installed in the same way as glass. Non-reinforced rigid plastic covers (including acrylic sheet) were usually moulded into a convex shape to increase their rigidity, and some collectors contained a thin plastic film as an inner glazing. One collector design which was used to heat an outdoor swimming pool had no cover at all. 186-

203 Cover Problems DIRT ON COVERS Dirt and dust were often observed on the outer surfaces of covers, mainly near the bottom of the collector where it was retained by the enclosing frame. Dirt was also attracted to sticky deposits, such as degrading sealants or bird droppings, if these had been allowed to accumulate. Some dirt was also observed on the inner surfaces of covers, but amounts were usually small. At least some dirt was reported on 72% of the inspected installations, but less than half of these were considered to be dirty enough to impair the collector performance. Dirt was considered to be a major problem only in dusty and industrial areas where covers required regular cleaning. Comments Most of the loose dirt will be washed off naturally from time to time by rain. The accumulation of dirt and dust on the outer surface of the cover can be reduced to some extent by designing the enclosure without a 'lip' along the bottom edge of the cover. Sticky deposits of grease or excess sealant should be cleaned from the cover after installation to minimise the potential for dirt to accumulate. Collectors should be installed as far away as possible from chimneys and ventilation outlets. Periodic cleaning of collector covers should be anticipated when an installation is designed, and appropriate access provided. DEPOSITS ON THE INNER SURFACE OF COVERS Particle and film deposits were sometimes observed on the inside of the cover. These were thought to be the condensed products of outgassing from other materials used in the collector. The problem was not a major one for the inspected installations and was reported for only 8% of them. Comments Some 'binders ' used in glass fibre and mineral wool insulation, and certain types of expanded foam, are known to outgas at typical collector operating temperatures and should therefore be avoided. Resins in timber can also lead to internal deposits. In the case of GRP enclosures, outgassing can be caused by thermal degradation of the releasing agent. To avoid this, the enclosure should be cleaned before assembly.

204 Figure 16.1 Dirt and Outgassing Condensates on a Collector Cover CONDENSATION Some condensation was reported in 67% of the inspected installations, and almost half of these were considered to have a problem that required attention. The condensation usually cleared as the collector temperature increased, and generally disappeared completely by mid morning. Condensation was sometimes accentuated by leaking covers and enclosures, and by retention of water within the enclosure. Many collectors were reported to have inadequate provision for ventilation and for drainage of condensed water. Some collectors had been modified and given larger ventilation holes, which resulted in a marked improvement in the clearing of condensation. Comments In most cases, where condensation disappeared with rising temperatures, the daily system performance was probably not unduly affected. However, humidity cycling is likely to accelerate general degradation of the collector components, and condensation should therefore be minimised by providing adequate ventilation and drainage.

205 Figure 16.2 An Example of Severe Condensation BREAKAGE (Glass Covers) Glass covers are particularly susceptible to breakage, and 27% of the inspected installations that used glass as a cover material had experienced at least one cover failure. Failures were sometimes caused by accident during installation and maintenance, and in a very few cases by vandalism. However, the main cause of failure was believed to be thermal stress in the glass. Stress concentrations around brackets, joints, and fixings also initiated cracking and eventual cover failure. Some covers were reported to have shattered because the enclosure was designed with inadequate provision for thermal expansion

206 Figure 16.3 Cover Failure due to Thermal Stress Comments Inadequate thermal insulation around the edges of a hot glass cover results in steep temperature gradients between the centre and the edges, and a high potential for failure due to thermal stress. Similar effects can be produced in rigid plastic covers, where restricted space for expansion and steep temperature gradients will cause crazing. Sealants and gaskets must be able to accommodate expansion and contraction of the cover and enclosure over the range of temperatures from below freezing up to the maximum collector stagnation temperature. Where covers are secured to the enclosure by means of a cover strip and screws, then the distance between the screw and the edge of the cover should be sufficiently large to accommodate the thermal expansion of the cover. Significantly larger spaces for expansion are required for plastic than for glass. AGEING (Plastic Covers) Some signs of ageing were found in approximately 75% of the installations which employed plastic covers. Discolouration, and sometimes crazing were reported in almost half of the installations with plastic covers. The use of protective film coatings such as PVF on the outer surface of G.R.P. sheeting appeared to reduce this degradation. 190-

207 Comments The reduction in transmittance due to discolouration of plastic covers is not always as great as appearances might suggest. Some thin film plastic materials such as polyvinyl fluoride are known to become brittle with age, when used in collectors as an inner glazing, but this was not observed in the survey. However, the collectors which employed inner film glazing had not been operating for very long. Figure 16.4 An Example of a Degraded Acrylic Cover SAGGING Sagging was reported for many of the flat plastic covers. When the collectors became hot, the covers expanded, sagged and often remained permanently distorted. In some instances the covers were found to be resting on the absorber surface and to have pulled away from the enclosure. Plastic covers moulded into convex or rippled shapes did not appear to suffer from these problems. No serious sagging of thin film inner covers was observed, but it should be noted that the collectors in the survey which employed such covers were not very old. Ripples in thin film covers can detract from the appearance of a collector, as shown in Figure A few glass covers sagged because they were installed with too great a span for the glass thickness and collector tilt. One glass cover pulled away from the casing because of this, and collapsed.

208 Comments Figure 16.5 Horizontally Mounted Collectors with Sagging Plastic Covers Adequate cover supports are an important collector design feature. Plastic covers undergo both thermal expansion and creep, and should be supported accordingly. Guidance on appropriate cover thicknesses and edge support details to withstand wind and snow loading is usually available from cover material suppliers. Figure 16.6 Ripples in Thin Film Covers

209 16.3 THE ENCLOSURE AND COVER ASSEMBLY Materials Used A variety of materials were used to construct the collector enclosures, as shown in Table The designs of enclosure and cover assembly varied considerably between collectors. Most of the single module designs used a separate metal frame to hold the cover with mastics, silicone sealants and/or rubber (EPDM or Neoprene) gaskets to provide watertightness with some degree of flexibility. The frame around the cover was usually attached to the main enclosure with fastenings, and was seated on additional rubber gaskets. Figure 16.7 shows a typical arrangement. Silicone sealants and EPDM or Neoprene gaskets were most common, but other sealing materials were used including butyl rubber. In most collectors the material used for the enclosure sides was also employed in the cover assembly, but the back panel was sometimes of a totally different material. MATERIAL ENCLOSURE SIDES AND/OR COVER ASSEMBLY ENCLOSURE BACK Aluminium Galvanised Steel Stainless Steel G.R.P. Timber Concrete Plastic Masonry/Plaster Fibre Board Plastic Foam (high density) Open 56% (Extrusions) 16% 8% 6% 8% (Building Structure) 4% (Building Structure) 4% (Extrusion) 2% (Walls) 1% 42% (Sheet) 28% (Sheet) 8% 3% (Plywood) 4% 2% (Sheet) 3% 2% 1% 8% Table 16.2 Enclosure Material in the CEC Survey (Note: 85 installations. Some collectors used more than one material) s Reported -193-

210 Glazing Seal Cover retaining clip Soft spacer Casing Figure 16.7 Example of a Cover/Enclosure Assembly Arrangement Collectors with plastic covers did not always require a separate glazing frame. Fastenings were applied directly through the cover edge which, in some cases, was moulded to give adequate support and improved sealing. A typical arrangement is shown in Figure Transparent Plastic Cover Acrylic or G.R.P) Fastenings and Mastic Sealant G.R.P. Casing Figure lb.8 Example of a Cover/Enclosure Assembly for a Collector with Plastic Covers

211 Several of the collectors which had been built in situ into the roof used parts ot the roof structure to provide the enclosure framework as shown in Figure Flexible Moulding Glazing Absorber Capping Strip Hardwood /Sealant Figure 16.9 Insulation Reflective Foil Softwood Roof Bearers Typical Arrangement for Collectors built in situ into the Root One of the collector arrays in the survey was mounted vertically on a wall, and used the building structure as the enclosure. The vertical collectors were used for space heating, with air as the heat transfer fluid Enclosure Problems Reported LEAKAGE Leaking assemblies were reported in 17% of the installations inspected, and almost half of these were considered to have a serious problem. The problems were usually attributed by the inspector to either poor design of the assembly or to poor fitting of gaskets and seals. Extruded rubber gaskets had in several cases loosened and allowed leakage into the enclosure. In some cases enclosure deformation appeared to have initiated the problem, whilst in others overtight fittings and local clamping arrangements had caused cover failure during thermal cycling. Other evidence of leakage was attributed to holes where fastenings were missing, apertures for pipework, and to the use of permeable materials such as concrete for the enclosure. Comments Careful attention to design details is required to ensure that a cover assembly will remain leakproof. Its leak resistance should not be 'put at risk by handling during installation, or subsequently if the cover has to be replaced. When a rubber gasket is used, it should be manufactured or bonded together such that it will provide a continuous seal around the perimeter of the cover. 195-

212 DEGRADATION OF SEALANTS Visible degradation of sealants was round in 16% of the installations, but in most cases was not severe enough to permit leakage. Softening, powdering and embrittlement of sealants were all reported. Sometimes the sealants became sticky and then attracted dust and dirt. Many covers had deposits of mastic sealant that had not been cleaned off after assembly, and these also attracted dirt. Some preformed seals became loose, as shown in Figure Figure Examples of Seals that have become Loose

213 Comments Collector sealants have to withstand higher temperatures than are usually experienced in building sealant applications. Low temperature expanded foam seals shrink on heating, and some self-adhesive gaskets are not designed for operation above ambient air temperatures. It is most important to use sealing materials which are designed for high temperature operation. White or transparent sealants are generally to be preferred (because they will not reach such high temperatures as black sealants when exposed to high levels of solar irradiance) provided that they have adequate protection against degradation by solar radiation (particularly u-v radiation). CORROSION Corrosion of both the assembly and its fastenings was observed in 15% of the installations. However, since most of the installations were not of a great age, the corroded components had not yet suffered a significant reduction in structural integrity. Damage to surface coatings such as galvanising and paint had allowed steel surfaces to rust, particularly at assembly corners, at welded joints, and on brackets and fittings. Fastenings made of materials which were incompatible with the enclosure materials, such as steel screws in aluminium extrusions, were found in several installations, and these had caused localised corrosion. Comments The science of corrosion is comprehensively documented, but all too often, even the basic rules are disregarded. It is particularly important to isolate the more noble metals such as copper from less noble metals such as aluminium in order to avoid galvanic corrosion. Careful attention to corrosion protection is important in design, in order to ensure both a long working life for the collector and an acceptable appearance. DIRT AND GRIT Cover assemblies with complex geometries sometimes allowed dirt and grit to lie trapped in recesses and joints. This trapped dirt was unsightly. The retention of dirt, ice and snow was particularly common where cover assembly frames protruded significantly over the cover, along the bottom edge of the collector. Comments Recesses which trap both dirt and water can be expected in the long term to suffer from corrosion, and eventually to lead to leakage. Sealants, which can adequately resist moisture and rain, may not always be able to resist long term exposure to stagnant water. Freezing damage can also occur where water lies trapped in a recess

214 16.4 THE ABSORBER Materials Used The inspected installations used a variety of absorber materials and surface coatings, as shown in Table ABSORBER MATERIAL Mild Steel Aluminium Copper Stainless Steel (usually only the tubes) Plastic Rubber Composite Metals INSTALLATIONS 29 (34%) 28 (33%) 24 (28%) 9 (10%) 2 (2%) 1 (1%) 1 d%) ABSORBER SURFACE INSTALLATIONS Matt Black 56 (66%) Selective Surface (Deposited) 24 (28%) Selective Foil 3 (4%) Coloured Material (Black Plastic, Rubber) 2 (2%) TABLE 16.3 Absorber Materials and Surface Coatings Reported in the CEC Survey (Note: 85 installations. Some collectors used more than one material) 198-

215 ABSORBER DESIGNS Most absorbers were of a single material, with black paint or a selective surface applied. Some of the fin and tube types had tubes made of a different material to the fins (e.g. stainless steel or copper tubes with aluminium sheet fins). The most common fin and tube arrangement employed shaped sheet fins into which the tubes had been fitted. Sometimes thermal contact relied merely on a 'tight' fit and in other cases a bonding material such as a solder or heat transfer grease was applied between the surfaces (see Figure 16.11). Absorber fin Riser tube Figure Tubes Fitted into a Shaped Sheet Fin Some absorbers were of the 'sandwich' type, comprising two shaped metal sheets (usually steel) spot welded together to leave internal fluid channels, as shown in Figure Section through absorber Figure A 'Sandwich' Absorber 199-

216 Other absorber configurations included: - A serpentine riser mounted on a single absorber sheet - A single coiled tube without an absorber sheet - Aluminium 'Roll Bond' (two sheets cold welded together) - A plastic extrusion with many internal fluid channels - A fin and heat pipe (evacuated tubular collector). ABSORBER SURFACES Black chrome was the most popular selective surface, although tin oxide also figured prominently in the survey because each of the CEC Solar Pilot Test Facilities contained an array of collectors using this surface. The more recently installed collectors had a higher proportion of selectively coated absorbers Absorber Problems Reported CORROSION Corrosion of the absorber surface was reported in 33% of the inspected installations, and in about one third of these was regarded as severe considering their young age. The corrosion was sometimes evident throughout the array but more commonly it was a localised problem on a few collectors. In most cases, local corrosion was attributed to problems associated with the particular collector, such as poor assembly or local damage which had led to water penetration. Pitting of aluminium absorber surfaces (producing aluminium oxide as a white powder), and rusting of unprotected steel were both reported. Bimetallic corrosion between risers and fins made of incompatible materials was also observed in a number of cases. Some internal corrosion of steel absorbers was reported following evidence of discolouration in the heat transfer fluid. Comments Absorber surfaces need to be corrosion resistant unless the collector is evacuated, because of the almost unavoidable presence of condensation from time to time. In ventilated collectors it should also be noted that atmospheric pollutants such as SO or chlorides may enter the space between the absorber and the cover

217 Great care needs to be exercised in design and installation if mixed metals are to be used. The collector fluid circuit (primary circuit) may need to be a closed loop, and appropriate corrosion inhibitors should be added to the fluid when using absorbers with steel or aluminium waterways. All swarf, especially that of noble metals such as copper, should be excluded from the system. LEAKAGE Leaking absorbers were reported in 18% of the inspected installations, with around half of the cases regarded as severe. Local corrosion, especially at the edges of sandwich absorbers, and generally poor assembly of components were the main causes. Spot welds were also an occasional source of failure. Leaks usually led to other problems such as air locks and system failure, and to saturation of the insulation. Comments Corrosion is often initiated where materials have been stressed either by forming or by welding. It is important to select alloys and material thicknesses which can withstand the production processes used, and to employ quality control checks during production. Potentially leaky absorbers can sometimes be identified by an overpressure test before assembly. The number of leaking absorbers reported here emphasises the importance of quality control in absorber manufacture. The costs of such control during manufacture are very much less than those of repairing or replacing collectors which have been installed in a system. DEPOSITS ON THE ABSORBER SURFACE Dirt and dust deposits were reported on the absorber surfaces of 24% of the inspected installations, with around one quarter of the cases regarded as severe. These resulted mainly from failed or poorly designed cover and enclosure assemblies. Sticky deposits were occasionally observed on the absorber surface. These were formed either as a result of outgassing of collector components or by resins from timber frames. Where water had run over the absorber surface following leakage or excessive condensation, this often left a stained surface. Comments In dusty areas it is important that collectors be protected against dust ingress as well as water penetration. The presence of dirt on selective surfaces will usually increase the thermal emittance and hence degrade the thermal performance. Dirt penetration through ventilation holes can be reduced by means of baffles, and the absorber surface can be protected by diverting the ventilation air towards the insulation immediately after entry

218 SURFACE DETERIORATION Peeling and/or flaking of the surface coatings was evident in 15% of the installations, being quite severe in about one third of these. This occurred with both the painted and deposited surfaces, but was usually of a very localised nature and probably initiated by damage during manufacture and assembly. Fine crazing of some selective surfaces was also reported. Comments The main cause of peeling and flaking was probably inadequate surface preparation prior to coating. Temperature resistant paints with their appropriate primer coatings should be used for absorbers. Surfaces should be thoroughly de-greased before coatings are applied. DEFORMATION Deformed absorbers were identified in approximately 10% of the inspected installations, with about half of the cases being severe. The bad cases mainly involved poorly designed prototypes and 'cheap' collectors. Comments Deformation may occur as a result of differential thermal expansion, for example where a lightweight absorber is bonded to expanded foam insulation. It may also be caused by the relief of stresses which have been imposed during manufacture. A high temperature stagnation test will usually indicate whether or not a new absorber design is likely to deform in service THE INSULATION Materials Used The insulation materials from the inspected installations were of three main types, as shown in Table Combinations of materials were common. In several designs, glass fibre or mineral wool was placed between the absorber and expanded foam back insulation. Side insulation was often different from the back insulation, or sometimes omitted altogether. Aluminium foil was used in many of the collectors, to protect and contain the insulation

219 INSULATION MATERIAL INSTALLATIONS Expanded Plastic Foam 43 (51%) Mineral Wool 27 (32%) Glass Fibre Mat 18 (21%) Other (concrete, building fabric) 3 (4%) Table 16.4 Insulation Materials Reported in CEC Survey (Note: 85 installations. Some collectors used move than one material) Polyurethane was the most common form of expanded foam. 'Binders' were believed to have been used in many of the fibre insulation materials. In many cases it was not possible to inspect or identify the insulation. Manufacturers' information has been used to complete Table 16.4 in the cases where no other data were available Insulation Problems Reported DEGRADATION Degradation of the insulation was reported in 16% of the inspected installations. This was usually observed as discolouration, sometimes with powdering indicating embr it t lenient. Loosening of some fibre insulation materials occurred, usually at the sides of the enclosure, and sometimes partly obscured the absorber. Aluminium foil coverings were seen to have become loose in a few cases. Destruction of insulation by mice was reported in one installation, where the collectors were an integral part of the wall of a building. Comments Some polyurethane based foams discolour in the presence of sunlight, even when placed behind glass. The effect of this and of exposure to high temperature is that the foams become brittle, their cell walls break down and their thermal conductivity increases. It is recommended that polyurethane based foams should not be used in direct contact with absorbers. Outgassing of insulation binders may have serious effects on cover and absorber surface performance 3 but does not usually affect the performance of the insulation itself

220 Figure Degradation of Foam Insulation due to Contact with Absorber WATER PENETRATION Water in the insulation was reported in 6% of the installations. It was probably present in many others, where water was known to have entered the enclosures. Comments Clearly many collectors operate well with absorbent, fibrous insulation materials. However, a large number of designers prefer to choose insulation materials which do not absorb liquid water, such as closed cell rigid foams, provided that they can adequately withstand temperature and fire. Water in a collector is always a potential source of degradation, and every effort should be made at the design stage to ensure that water penetration into a collector is minimised, particularly into the insulation. Once water has been absorbed by a fibrous insulation material, it is likely to remain in the collector for a considerable time, especially in winter, and may produce irreversible compaction of the insulation. 204-

221 16.6 MOUNTINGS Systems Used (1) Modules mounted (Figure 16.14) on free standing frames (2) Modules mounted on top of structure (Figure 16.16) a sloping roof (3) Integrated into the building structure to form part of the roof (Figure 16.17). Collectors on free standing frames were usually located on a flat roof or on the ground beside a building, but were sometimes placed above a sloping roof. Free standing collectors were generally more exposed than others since the rear of the collector was open to the wind and weather. Access was rather easier than for modules mounted on sloping roofs, so their inspection, maintenance and repair were comparatively straightforward. However, the extra mounting structure imposed additional costs and further design considerations. Figure Collectors Mounted on Free Standing Frames The collectors in the inspected installations were installed in one of three ways:

222 \ m i Figure Collector Mounting & Flashing Detail The installation of collector modules on sloping roofs normally involved the cutting and/or removal of roof tiles to allow fixing of the mounting brackets to the main roof structure. Flashing (zinc or lead) was then required around the mounting points to ensure weathertightness (Figure 16.15). Once installed, the collectors were often difficult to inspect and maintain. Figure Collectors Mounted on a Sloping Roof -206-

223 Arrays which had been built as an integral part of the roof structure were mainly observed in new buildings, and the overall impression of these installations was aesthetically appealing. In several installations, the collector array replaced the whole of the south facing roof, thus minimising the flashing problems around the perimeter of the collector area, although weathertight seals were still required between collector modules. In some systems the collectors were installed from inside the roof allowing easy access for repair and maintenance. >W:' <S*M Figure Example of a Collector Array Built as an Integral Part of a Roof A few installations had separate single storey buildings (e.g. stores, boiler house) incorporating the collector array. These permitted easy access to the collectors, with minimal disruption to the users (Figure 16.18). Figure Example of a Separate Building incorporating a Collector Array -207-

224 Mounting Problems Reported DEGRADATION Degradation of mounting brackets and supporting frames was observed in 29% of the inspected installations. This was often merely the rusting of a mild steel framework, on which the protective coating (usually galvanised) was damaged by rough handling during installation. Nuts and bolts tightened down directly onto a coated surface also initiated corrosion. Some cases were fairly severe and would eventually require replacement of the brackets to ensure safety. The degraded frameworks were unsightly. There were many cases of dissimilar metals being used to connect the collector to the mounting brackets, causing local corrosion. Some brackets and fastenings were thought by the inspectors to be inadequate in size and quantity to ensure safety. Comments There are many texts which advise on the problems of corrosion when dissimilar materials are in contact outdoors. Unfortunately, even the basic rules are often disregarded. In the case of mild steel frameworks, periodic painting is probably an adequate remedy for the age-old problem of rusting, but the costs of painting may significantly reduce the cost-effectiveness of a solar heating system. LEAKAGE AROUND FLASHINGS Leaking flashings were reported in 6% of the installations, with half of these actually allowing water to enter the building. Wind had lifted part of the lead flashing in one case, and the other cases were attributed to poor workmanship. Comments It is particularly important to provide good quality flashing details where the integrity of the building is involved as well as that of the collector. All seals and flashings need to be resistant to driving rain, and should be able to continue to prevent water penetration through several years of thermal movement. OTHER PROBLEMS Other problems reported occasionally were: Water, snow and ice trapped behind mounted on roofs Timber drying out behind integral collectors collectors roof -208-

225 Rotting of mounting timbers Mounting frames too weak and of inadequate rigidity Poor siting (incorrect tilt and orientation; shading by roof or other objects; too close to the ground, resulting in mud splashes, shading from grass, and easy access for insects, etc.) Figure Collectors Mounted too Close to the Ground 16.7 SYSTEM COMPONENTS, CONNECTIONS & PIPEWORK Background The Inspection Reporting Format was primarily concerned with details of the collectors themselves, but included a small section in which other system problems and failures could be reported. The main problems reported are briefly outlined here because they are to some extent relevant to collector design and installation practices

226 System Problems Reported LEAKING FITTINGS Leaking pipe fittings were observed in 25% of the inspected installations, with around half of these being severe cases. A variety of causes were identified, including poor assembly, lack of provision for thermal expansion, poor soldering and local corrosion. The leaks often led to air locks and eventual system failure. Compression fittings were observed to pull loose when sited in long straight pipe runs, because of the movement caused by thermal expansion and contraction. This problem was sometimes exacerbated by poor pipe alignment, which caused additional stresses on the pipe fittings. Local corrosion was mainly in the form of galvanic action between pipes and fittings made of dissimilar materials, but also resulted from damage to protective coatings such as galvanising. Comments In most respects, standard plumbing practice can be applied to solar heating system pipework design. Many of the problems found in the survey arose simply because basic rules had not been followed. Provision for thermal expansion is particularly important when designing pipework for large arrays of collectors. POOR INSULATION Pipe insulation which was either in poor condition or inadequate was observed in 20% of the inspected installations, with most cases regarded as severe. In many cases absorbent insulation materials had been used with ineffective weather protection, leading to broken and wet insulation. Insulation often became loose, and was either blown away by wind or removed by birds. Many installations had not been completely insulated, and had gaps of uninsulated pipework close to the collector. Unused collector entry pipes and other small pipe fittings were rarely insulated. Comments Pipe insulation for external use is expensive, but experience suggests that attempts to save money by the use of cheaper insulating materials are seldom successful. External insulation can be protected to some extent from excessive temperature cycling by painting with white or aluminium coloured paint, but a better approach is to employ polished metal shielding.

227 Figure Degradation of Pipe Insulation AIRLOCKS Flow distribution problems were reported to have occurred during the commissioning stage in 20% of the installations, and a few cases were never satisfactorily cured. Leaks and bad plumbing geometry were the major causes. Some collector configurations appeared to retain air pockets and were difficult to refill after they had been drained down. Air bleeds and header tanks were sometimes added to overcome initial problems. Comments All pipework should rise continuously to an air bleed, such that air can always escape easily from the system. Where possible, an open air vent is to be preferred to an automatic air bleed valve. Absorbers with horizontal header pipes should be slightly tilted if possible to permit air to escape freely. The siting of overflow pipes and air vents is particularly important when antifreeze fluids are used, because the outflow of such a fluid could cause serious degradation of building materials such as bitumen on flat roofs. 211-

228 NOISE Airlocks, thermal expansion, and pumps occasionally produced noises that caused annoyance to users. Comments Similar problems occur in conventional central heating systems and can be avoided by close adherence to plumbing codes of practice. COMPONENT FAILURE Some form of component failure or problem was reported in 28% of the systems inspected, but most problems became apparent during the commissioning period and were rectified without difficulty. Examples of the types of problem reported were: Pump too small Pump failure Inappropriately placed temperature sensors for pump control Temperature sensors that became loose and ineffective Insensitive controllers Inappropriately set controllers Failure of automatic drainback

229 CHAPTER 17 GOOD COLLECTOR DESIGN FEATURES 17.1 INTRODUCTION In the previous chapter, some of the problems and failures which had been identified during a survey of collector installations were discussed. As part of this survey the system inspectors were also asked to note good design features and to make recommendations. This was not easy because good features are seldom obvious, particularly when the collectors are not very old. The absence of problems does not automatically suggest good design, and a degree of judgement was therefore demanded of the inspectors. Most inspectors were cautious in identifying good design features, but some made suggestions on how to improve particular collectors and installations. These suggestions are included in this chapter together with a discussion of the more general requirements for good collector design and installation GENERAL CONSTRUCTION Good collectors can be made from many materials, and can be produced at very different costs depending on the experience of the manufacturer and the production facilities which are available. It is therefore inappropriate merely to recommend particular materials for building a "good" collector. What is important is to choose a combination of materials which are compatible with each other, with the system and with the materials in the building. A discussion of the properties of some commonly used materials is given in Chapter 18. The design requirements for collector components were outlined in Chapter 15, but good collector design requires more than the assembly of good components. In order to ensure a long and reliable working life, a collector design should take into account each of the aspects described in the following sections

230 17.3 THERMAL EXPANSION In a well designed collector, all the components should be mounted with freedom to expand and contract over the full range of temperatures from below freezing in winter to the highest stagnation temperatures which can occur in mid-summer. In calculating the necessary provision for expansion, it may be necessary to take into account different temperatures as well as different coefficients of expansion. For example, the absorber will usually be hotter than the other collector components. For design purposes it is usually adequate to assume maximum stagnation temperatures of 150 C for matt black absorbers, 200 C for selectively coated absorbers and 100 C for covers, in simple flat plate collectors. Expansion coefficients for materials commonly used in collectors are given in Table Thermal shock conditions should also be considered, since a sudden rain storm for example can rapidly cool the cover whilst the rest of the collector is still hot. Test procedures which involve subjecting a collector to an external thermal shock are under development within the European Community, and development tests to confirm that a design is satisfactory can be expected to play an increasing role in the design process. Where connected components cannot be made of materials with similar expansion coefficients, then gaskets, and/or sealants should be used to minimise the stress resulting from differential expansion. Flexible sealant /gasket Space for lateral expansion of absorber and cover No thermal bridge (metallic connector between absorber and casing." casing Absorber retained in position by insulating grommet Figure 17.1 Provision for Expansion and Absorber Securing System

231 Expansion of the Cover and Cover Assembly Since these components are likely to have very different expansion characteristics, the selection of gaskets and sealants becomes very important. Where specialised products such as silicone sealants are to be used, advice from the suppliers should be sought to select the appropriate type for the materials and temperatures involved, and to ensure that the correct preparation is given to the surfaces concerned Expansion of the Absorber and Adjoining Pipework The high temperatures that may be reached by the absorber necessitate freedom of movement and adequate space between the components and the collector enclosure. Rigid fixings that restrict this freedom could cause the absorber to deform and the pipework to loosen at connections. Thermal bridges between the absorber and the collector enclosure should not be allowed to form during expansion Expansion of the System Pipework The pipework between collectors in an array and between the array and the storage tanks often contains long straight lengths of pipe. With long pipes there can be very significant changes in length as a result of temperature cycling, and adequate provision for this is essential in order to protect the absorbers. Rigid mountings for long pipe lengths should be avoided, and weather resistant flexible couplings used where necessary. Figure 17.2 Flexible Pipe Couplings (insulation removed) -215-

232 17.4 WEATHER RESISTANCE The collector enclosure is required to protect the absorber and insulation from the effects of the weather. The collector mountings are required to support the collector in all weather conditions, and the external pipework with its insulation must also be weather resistant. In addition to the problems associated with high temperature stagnation conditions, which were discussed in the previous Section, the main hazards which need to be taken into account when designing a weather resistant collector and mounting system are: Wind loading (including lift, drag and vibration) Snow loading Hail (large hailstones in some regions) Driving rain Wind driven powder snow Dust, sand and dirt in windy conditions Freezing conditions Wind Loading Wind resistance is important from the point of view of general safety, as well as for the performance of the solar heating system. Collector mountings should be sufficiently strong to withstand the wind conditions expected in the area of the installation, and covers should be correctly sized and securely fixed to resist lifting by the wind. Values of wind speed for use in building design are available on a regional basis in most national building codes of practice. The effects of wind on a collector depend significantly on how and where it is installed. The uplift forces vary with position on a roof, because of boundary layer effects, and eddies which form near the eaves and around protrusions such as chimneys. The lift on the cover of a collector which is laid into a roof is likely to be greater than that on a collector which is held above the roof surface on a framework. In contrast, the drag forces are likely to be greater on a raised collector, than on one which is mounted onto the roof itself. Thin plastic covers are generally poor at resisting wind loads. They vibrate easily and can become noisy with age if the material stretches and becomes loose

233 Snow Loading Suppliers of cover materials are usually able to provide data on the snow load resistance of their products. In designing the cover mountings, it is sometimes possible to provide a free bottom edge such that rain and snow will run off. This is a good design feature as it also minimises the build up of dirt on the cover (Figure 17.3). Solutions allowing run off' of water and snow and minimising the accumulation of dirt at the edge Shaped flexible moulding/sealant Figure 17.3 Tapered cover assembly with flexible gasket /sealant Detail of Bottom Edge of Collectors Hail For most of Europe, hail does not present a serious design problem for collector covers. Where large hailstones do occur from time to time, then toughened glass or impact resistant plastic material should be used. Flexible grommet Side casing Figure 17.4 Pipe Entry with Grommet Detail -217-

234 Driven Rain and Snow These should not form serious hazards for a well designed collector with non-absorbing insulation and with condensate drainage holes. As far as possible, however, ventilation holes and the points of entry for pipes and temperature sensors should be protected from driving rain and snow. Pipework entries should be fitted with grommets, and ventilation holes should be shielded Dust and Dirt When collectors are installed in a dusty area or an industrial area where the air is heavily polluted, then ventilation can result in an accumulation of dirt on the absorber surface. To minimise the ingress of dirt, ventilation holes should be shielded with fine gauze and baffles (which may also be useful to inhibit entry of insects) and placed below the absorber Freezing Conditions As discussed in Chapter 15, the absorber should be designed for easy fluid draindown so that it can be protected from freezing damage. For this reason, serpentine tube absorbers should always be installed in an "S" rather than a "W" arrangement, as shown in Figure The enclosure and cover assembly should be designed without any cavities in which water can lie trapped, for in the event of freezing conditions, trapped water will freeze and could cause structural damage. Correct Incorrect Figure 17.5 Correct and Incorrect Orientation of Serpentine Absorbers

235 17.5 VENTILATION AND AVOIDANCE OF CONDENSATION Ventilation of a working collector will cause heat losses and from the point of view of thermal performance in dry conditions should be minimised. One way to avoid condensation is simply to evacuate the collector and then seal it. If a non-evacuated collector is to be sealed, then care needs to be taken at the manufacturing stage to eliminate moisture from the collector materials, or to incorporate a desiccant so that condensation cannot occur on the inside of the cover under any atmospheric conditions. Furthermore, some means of accommodating the internal pressure changes which will result from the temperature changes in the collector nee'ds to be provided, for example by the use of an easily deformed cover. In European conditions, unsealed collectors experience a wide range of internal relative humidity levels, resulting in condensation when the temperature of the cover or the absorber falls below the dew point. The extent to which condensation remains in a collector can be minimised by the use of drainage and ventilation holes. A layer of condensation will reduce the transmittance of the cover, and moisture in the insulation will reduce its effectiveness, so it is good practice to provide ventilation for the space between the cover and the absorber, as well as to ventilate the insulation. The effects of condensation on thermal performance can be significant, but the main reason for attempting to minimise the amount of condensation inside a collector is to protect its components from corrosion and degradation. Drainage holes > 5mm t Figures 17.6 Ventilation Air Flow and Drainage Holes -219-

236 There are many approaches to overcoming the problem of collector ventilation, but little quantitative data on which to base a design. The usual practice is to place ventilation holes near to the top and the bottom such that warm air slowly rises up and out of the collector, taking excess moisture with it. The holes should be protected with gauze or fitted with "ventilation plugs" containing a large number of small holes, in order to inhibit the entry of insects. They should also be shielded as far as possible from driving rain and snow. The required size of holes depends on the collector design, and is perhaps best determined by a series of development tests. Collector side casing Figure 17.7 Ventilation Plugs As a general principle, the holes at the bottom of the collector should be made reasonably large to ensure that they remain free from debris, to provide drainage. The control of ventilation rates should be effected by altering the size of the upper (air exit) holes EASE OF REPAIR AND MAINTENANCE Collectors should as far as possible be installed where they can be easily cleaned, and it should also be reasonably easy to gain access to pipe connections, insulation and temperature sensors. Where collectors are built into a roof, it is desirable to have all plumbing accessible from inside the roof space. Where modules are mounted on a roof, it is wise to minimise external plumbing. 220-

237 Designers should recognise that problems may occur, even with good collectors. All collectors should therefore be designed so that they can be easily repaired or replaced. Firstly, the removal of a module from an array should be a relatively straightforward procedure, such that repairs or module replacement, if necessary, can be carried out with minimal labour costs. Secondly, where it is cost effective to build a collector which can be dismantled for component replacement, it may be preferable to screw the cover assembly to the main collector enclosure rather than to rivet or permanently bond the two together. Similarly, it should be possible to remove the absorber and insulation without cutting or removing rivets from the main collector enclosure. Silicone sealants are attractive for providing weather protection in collectors, but a wide variety of silicone sealants is commercially available. Specialist advice should therefore be sought to identify the appropriate type for use with the collector materials selected. Most types will provide an excellent weatherproof seal, but some can also be relatively easily removed and renewed should it become necessary to dismantle a collector. Notwithstanding the points made above emphasising that collectors should be easy to repair, all installations should be designed for minimum maintenance and a long service life. It should not be overlooked that all maintenance expenses will reduce the cost effectiveness of a solar heating system CHOICE OF SITE FOR A COLLECTOR INSTALLATION The following factors should be taken into account when choosing a suitable site for an array of collectors Availability of Solar Irradiation The optimum azimuth is usually South, although for much of Europe the performance of a system is only weakly influenced by deviations of up to 45 from South. The collector tilt angle is also not a very sensitive parameter in the European climate, because of the high percentage of diffuse solar irradiance. Collectors should, as far as possible, be installed where they will have an unobstructed field of view. Shadows caused by hills, trees and buildings can be predicted using a theodolite and sun path diagrams or a proprietary shadow indicator. Where an array of collectors is being installed, care should be taken to ensure that they do not shade one another. Collectors mounted at ground level may become shaded by vegetation in the summer months

238 Shelter from Wind The performance of most collectors decreases as the wind speed increases, so it is preferable to choose a site which is sheltered from the prevailing wind, provided that no significant reduction in solar irradiation will result Rain Run Off Some roofing materials are incompatible with materials used for collector enclosures. When rainwater runs over them it picks up chemicals which can subsequently corrode the collector. Compatibility between roofing and collector materials should be confirmed before installation. Rain run off from copper pipes, for example, can accelerate corrosion on aluminium or galvanised roofing and guttering, and cause unsightly local decay of organic coatings such as mosses or lichens which may otherwise be an attractive feature of older roofs Shelter from Snow Collectors themselves should be able to resist snow loads, but they may also need to be protected from falling snow if they are mounted below a higher building or roof. In locations which experience heavy snowfall regularly, collectors should be installed in such a way that snow can slip from their covers without accumulation Cleanliness A collector should not be sited near a ventilation outlet or a chimney where it may quickly gather dirt on its cover. Arrays sited at ground level may become dirty more quickly, but are relatively easy to clean and maintain, compared with roof mounted systems Security When choosing a site it is important to ensure that any failure of a glass cover or of a fluid pipe will not cause a safety hazard, either to passers-by or to inhabitants of the dwelling served by the system. Collectors mounted near the ground are more susceptible to damage by animals or children, but the hazard of falling glass may be more serious from a roof mounted installation Easy and Safe Access The costs of installation, cleaning and maintenance will be greatly reduced if it is easy to gain access to both the outer covers and the pipework of a collector array. Installations on flat roofs are often attractive in this regard. Where collectors are built into a roof, it is

239 preferable that the absorbers, insulation and plumbing should be accessible from within the roof space. Access for cover cleaning is an important consideration Collector Tilt Angle It is very important to ensure that adequate tilt is provided, to allow air to rise freely out of a collector and to permit drain down. Tilt angles of less than about 15 to the horizontal are likely to cause problems with airlocks and draindown. Where an array of collectors is installed with many modules placed side by side, a steady rise should be used on the header pipes, and the collectors themselves may need to be tilted slightly to one side to free airlocks and permit draindown. It is also important to equalise the lengths of the feed and return pipes to each collector, as shown in Figure 17.8, in order to ensure a uniform fluid flow distribution in the array. _ongle exaggerated Figure 17.8 Array Plumbing Arrangements Appearance Attention should always be paid to the appearance of the installation. This may influence the materials used for the collector, its means of support, and the site chosen for the collector array. There is seldom any good reason why a collector array should not be designed to suit the existing architecture and features of the building on which it is sited. Some examples of attractive solutions are shown in Figures 17.9 to

240 , vf.t, : i...; «.., -itj.l...-ait5!»3j»yi *;.' r^b'^j> Figure 17.9 Houses Incorporating Evacuated Tubular Collectors -224-

241 Figure Large Flat Plate Collector Arrays

242 ^^Ari 1,**.' '$_._. r-*. J7ak^Bra«n#s»i Figure Flat Plate Collectors in Multiple Housing Developments 226-

243 »^,<wa Figure Collectors Integrated into Older Buildings (Retro-fit) -227-

244

245 CHAPTER 18 MATERIALS CONSIDERATIONS 18.1 INTRODUCTION The design guidelines given in previous chapters require a collector designer to take into account several factors which involve the physical properties of materials. In addition he will need to consider many other constraints including costs, availability of materials, his manufacturing capabilities, and local experience. Despite the mistakes made in many first generation collector designs, the experience of the last few years provides a basis for comments on the suitability of some materials for use in solar collectors. New materials will undoubtedly continue to appear in the market place, and these will cause differences in design between the collectors currently available and those which will be on the market in a few years' time. Nevertheless, it is expected that many of today's common building materials will continue to be used in solar collectors for some years to come, and their properties are therefore discussed in this chapter PHYSICAL PROPERTIES OF MATERIALS Information is included in this section on the properties of many of the materials which are currently used in the construction of solar collectors. The data apply to the general families of materials discussed; they are approximate only, but will be found useful for conceptual design purposes. The reader is urged to obtain detailed data from his materials suppliers before finalising any specific collector design Corrosion Resistance One of the most important aspects of collector design is to ensure that the materials used are compatible with one another and with the heat transfer fluids and building materials with which they may come into contact. The two main areas where corrosion may occur in a collector are (a) in the absorber fluid passageways and (b) on the surfaces of the absorber, the collector enclosure and the collector mountings. The materials generally used for absorbers are copper, mild steel, stainless steel, aluminium or plastics. Direct systems can usually employ copper, stainless steel or plastic fluid passageways without the need for

246 precautions against corrosion. Mild steel and aluminium fluid passageways should always be protected by using an indirect system with appropriate corrosion inhibitors. Condensation inside the enclosure, and rain on the outside provide moisture which can facilitate galvanic corrosion between dissimilar metals. It is particularly important therefore to protect the less noble metals which might otherwise become corroded. The tendency for metals to behave in this way can be predicted from a galvanic table such as that shown in Table CORRODED END - ANODIC Group 1 Magnesium Aluminium Duralumin Group 2 Zinc Cadmium Group 3 Iron Chromium Iron (active) Chromium-nickel Iron (active) Group 4 Soft Solder Tin Lead Group 5 Nickel Brasses Bronzes Nickel-copper Alloys Copper Group 6 Group 7 Chromium Iron (passive) Chromium-nickel Iron (passive) Silver Solder Group 8 Silver Gold Platinum PROTECTED END - CATHODIC TABLE 18.1 Galvanic Table

247 Metals which appear in the same groups in the galvanic table do not have a strong tendency to corrode one another, under most conditions. However, materials which are far apart in the table tend to react in the presence of an electrolyte, and the metal higher up in the list becomes corroded. For example a coupling between aluminium and copper will promote corrosion of the aluminium. (Note: A common absorber arrangement consists of copper tubes, bonded to aluminium fins. This would suffer from corrosion unless the absorber is kept dry.) Materials such as zinc and cadmium are used as coatings on steel in order to minimise corrosion problems. Zinc is used to galvanise steel in order to minimise general oxidation, but it can be seen from Table 18.1 that it can afford little protection in the presence of copper. Cadmium coatings are often used on steel bolts when these are employed for fixing together aluminium components. The cadmium in this case helps to protect the aluminium from corrosion caused by the steel bolts. If dissimilar metals must come close to each other where, for example, a copper pipe passes through an aluminium collector enclosure, then an insulating barrier such as a rubber grommet should be used to keep the two materials apart. In this case it is also important to ensure that rain will not drip from the copper onto the aluminium as this could initiate pitting corrosion. Pitting corrosion may be caused where very small amounts of a noble metal dissolved in an electrolyte are deposited onto a less noble metal. Copper ions in water will readily cause pitting corrosion of aluminium. Not all plastics are compatible with one another, and some metals are incompatible with some plastics. It is always wise to seek expert advice when choosing plastic materials for the manufacture of collector components Thermal Expansion When designing the tolerances in the sizes of collector components and the spacings for accommodating differential thermal expansion, it is important to recognise not only the differences in the expansion coefficients given in Table 18.2, but also the different temperatures involved. Components with very different thermal expansion coefficients should not be fixed firmly together. Serious distortion can occur if, for example, an expanded plastic foam having a high coefficient of thermal expansion is bonded to a thin sheet of a material which has a low coefficient of thermal expansion

248 Material Thermal Conductivity W/m K Density kg/m 3 Specific Heat Capacity kj/kg K Linear Coeff of Expansion xl0~ 6 m/m K Metals Aluminium Brass Copper Mild Steel Stainless Steel Non-Metals Brick Concrete Glass Rubber Wood Insulators Glass Fibre Mineral Wool Expanded Polystyrene Polyurethane Foam Plastics PMMA (Acrylic) GRP (35% glass) PTFE (Teflon) PVF (Tedlar) PVC Polyester (Mylar) Polypropylene Polycarbonate Table 18.2 Approximate Prop erties of Common Solar C ollector Materials Thermal Conductivity The thermal conductivity of the absorber material is particularly important because it influences the fundamental absorber design. As discussed in detail in Chapter 19, the spacing between the fluid passageways and the thickness of the absorber "fin" between those passageways should be optimised, taking into account the thermal conductivity of the absorber material and the material costs. The thermal conductivities of the most commonly used absorber materials are given in Table 18.2.

249 Mass The mass of a collector is important when it is being handled during manufacture, distribution and installation, as well as during its working life. In order to minimise installation costs it is important at the design stage to consider the size and mass of the final product. The mass per unit area of an installed collector may also be important when considering roof loading, and this is particularly important when a collector is designed for mounting above an existing roof cladding system. The densities of many of the common collector materials are included in Table 18.2, from which the mass per square metre of collector can be deduced once the dimensions of the collector components have been finalised. The overall mass of a collector can be reduced by selecting plastic covers in place of glass covers and plastic enclosure materials instead of metals Specific Heat Capacity In most solar water heating systems, both computer modelling and experience indicate that the thermal capacity of the solar collector has little influence on the long term thermal performance. However, as the use of solar collectors increases and the scope of applications becomes wider, it may become a more important thermal performance parameter. The specific heat capacities of the more commonly used solar collector materials are included in Table CHOICE OF MATERIALS FOR COLLECTOR COMPONENTS The survey of installed collectors discussed in Chapter 16 and the general experience currently available in Europe permit some generalised comments on the relative advantages and disadvantages of using some of the more common building materials in solar collectors. The lists of materials discussed are by no means exhaustive, and it should be anticipated that other materials may also prove to be suitable Choice of Cover Material The most commonly used cover material in the first generation of solar collectors was float glass. However, tempered glass and plastics have become increasingly used in more recent designs

250 Tempered glass has the advantages that it is more tolerant of temperature gradients and has a greater impact resistance than float glass. Because of its greater strength, the thickness of a tempered glass sheet can be less than that of a float glass sheet of the same area giving it the potential to be lighter and to have a higher solar transmittance. GLASS SHEET SIZING The maximum recommended span (metres) of float glass supported on two opposite edges Glass Thickness mm 20 Angli e of Inclination from the 50 Horizontal The maximum recommended area (square metres) of a square piece of float glass (aspect ratio 1:1) supported on all four edges. For an aspect ratio of 2:1 the area may be multiplied by 1.13, and for an aspect ratio of 3:1 by Glass Thickness mm 20 Angl e of I nclination from the 50 Horizontal Table 18.3 Maximum Size Guidelines for Float Glass

251 The maximum span which can be used with glass sheets depends on the tilt angle, the glass thickness and the glazing technique used, as well as on the local climatic conditions. In most countries there are national codes which regulate the sizes of glass sheets used in buildings, but unfortunately these regulations are not the same in all countries. Manufacturers also give guidelines on glass sizing, although in order to protect their reputations the manufacturers are sometimes rather conservative. Some glass manufacturers' guidelines are included here in Table These provide an indication of the maximum sizes which can be used with sheets of different thicknesses, but in certain circumstances it may be permissible to exceed these values. Sizes should be confirmed using national building codes where applicable. PLASTIC SHEET SIZING Maximum size guidelines are also provided by suppliers for the different plastic glazing materials, but special attention needs to be paid to the effects of temperature and creep when sizing plastic glazing. For acrylic sheets (PMMA), it is advisable to pre-shape the cover into a convex dome so that it will have extra rigidity to withstand wind and snow loading. Plastic sheet materials such as GRP should be used with relatively small spans between supports to prevent sagging, so collectors employing plastic sheet covers often require intermediate glazing bars. When using thin film plastic covers it is important to employ a fixing technique which ensures that the film will remain.in tension. OPTICAL PROPERTIES The solar transmittance of a cover material is an important design parameter. Approximate values of transmittance for wavelengths in the range from 0.3 to 2.5 pm, weighted in accordance with an average solar spectrum (Air Mass 1.5 or 2), are given in Table 18.4 for some of the more common glazing materials. Also included in this table are approximate values for thermal transmittance, weighted in accordance with a blackbody spectrum for temperatures in the range C.

252 Material Solar (AM2) Thickness Transmittance (mm) (±0.02) Thermal (0-100 C) Transmittance (±0.02) Float (high Glass iron) II If Float Glass (low iron) II II PMMA (Acrylic) II II Glass Reinforce d Polyester (35% glass) PTFE (Teflon) PVF (Tedlar) PVC Polycarbonate Polyester (Mylar) Table 18.4 App Of roximate Transmittance Properties Collector Cover Materials (Note: Values for plastics vary considerably with material specifications) GENERAL COMMENTS ON COMMON COVER MATERIALS Float Glass - Poor resistance to impact and thermal stresses - Solar transmittance depends on iron content of glass - Rather heavy - Good scratch resistance - Widely available. Tempered Glass - More resistant than float glass to impact and temperature gradients - Cannot be cut after toughening. Therefore usually needs to be supplied by collector manufacturers rather than bought locally by installers. - Thinner and therefore lighter sheets can be used than for float glass. 236-

253 PMMA (Acrylic) - Advisable to use as pre-shaped covers (convex dome) to resist wind and snow loads - Crazing and cracking may occur if subjected to large temperature gradients and stresses - Poor scratch resistance - Tendency to attract dirt and dust - Good impact resistance. Glass Reinforced Polyester (GRP) - Should be used with small spans between glazing bars or given pre-formed convex curvature to prevent sagging - The polyester will discolour (go yellow) in the presence of u-v radiation if not protected, so the external surface should be coated with u-v protection such as a layer of PVF (Tedlar) - Properties vary with the proportion of glass used - Good impact resistance - Appearance changes with age. This is usually caused by light scattering and may not greatly reduce the solar transmittance. PTFE Film (Teflon) - Very high solar transmittance - Suitable for use as an inner glazing only - Rather difficult to fix and handle; tends to form ripples which may look unsightly - Strong tendency to attract dust. PVF Film (Tedlar) - Used to coat GRP for protection against u-v radiation because of its good durability - Can be mounted using clipping or adhesives, and will heat-shrink to form tight cover - Developed for use as solar collector glazing. PVC (rigid) - Poor scratch resistance - Low temperature applications only, so it is unsuitable as an inner glazing. Advisable to use pre-shaped form to resist wind and snow loads. Polycarbonate - Scratch resistant and tough - Sometimes used as anti-vandal glazing - A u-v protected grade should be used. Polyester Film (Mylar) - Suitable for use as an inner glazing only

254 Choice of Enclosure Material The collector enclosure often provides the only structural stiffness in a collector. It holds together the cover, the absorber and the insulation. Some of the most common designs employ aluminium extrusions to form the sides of the enclosure, and these are used to support the other collector components. Other frequently used enclosure designs employ a "box", made of GRP or metal, to contain the insulation and absorber. Such designs usually require a separate glazing frame to attach the cover. Where the collector is integrated into a building, then materials such as concrete, bricks, wood, etc. may also form part of the enclosure. When selecting materials for the enclosure and the collector mounting brackets and fastenings (e.g. bolts and screws), special reference should be made to Section to ensure that the materials involved are corrosion compatible. GENERAL COMMENTS ON COMMON ENCLOSURE MATERIALS Aluminium - Good external weathering resistance - Easily corroded in the presence of copper, steel or agressive water (depending on the alloy) - Light and strong for structural components - Many alloys available with a wide range of properties. GRP (Glass Reinforced Polyester) - Manufacturing process allows a range of variations in design (the mould can be changed cheaply) - Can be produced in a range of colours - Shape and thickness can be selected to provide structural support where required. Galvanised Mild Steel - Should be protected from contact with copper components, and from water dripping from copper pipes - Fixings, holes, bolts, welded areas, etc. should be protected from corrosion, by painting or coating - The lifetime of galvanised steel varies considerably with location; the life can be short in industrial and coastal areas. Stainless Steel - Many types exist, some of which are resistant to corrosion under outdoor conditions. - Regions near to welds may require additional corrosion protection

255 Timber - Should be protected from high temperatures since these may reduce its strength and cause it to dry out - Some types exude resins at elevated temperatures - Not recommended for use as an external material for the collector enclosure, because it needs regular attention (painting or coating with preservative) and therefore incurs high maintenance costs Choice of Absorber Material Most absorbers for use in glazed collectors are made of metal, although rubber and plastics are used, especially for unglazed collectors. As discussed in Chapter 19, the design of the absorber fluid passageways should take into account the thermal conductivity of the absorber material. The fluid passageways need to be made of (or lined with) a material that is compatible with the heat transfer fluid used in the system. In order to reduce costs, whilst maintaining both corrosion resistance and thermal performance, some absorbers are made of more than one material. GENERAL COMMENTS ON COMMON ABSORBER MATERIALS Mild Steel - Usually employed in indirect systems with corrosion inhibitors added - Requires external corrosion protection such as galvanising - Should not be connected directly to copper components - Usually formed into a "sandwich absorber" because of its low thermal conductivity. Aluminium - Widely used for absorber fins because of its high thermal conductivity - Fluid passageways are often made of a different material, such as stainless steel or copper, because the corrosion resistance of aluminium is poor in the presence of water containing copper or chloride ions - Some aluminium absorbers are assembled using extrusions - Pitting corrosion and bi-metallic corrosion are the main sources of failure in aluminium absorbers. It is very important to select an appropriate alloy

256 Copper - High thermal conductivity - Reasonably easy to fabricate (Note: High temperature solders should be used with selectively coated absorbers) - Good corrosion resistance, although it can cause corrosion in other components such as mild steel pipework or tanks - The main disadvantage of copper is its relatively high cost compared with some other materials. Stainless Steel - Generally good resistance to corrosion if an appropriate type is selected - Not very easy to fabricate and poor thermal conductivity, consequently it is often used only in the form of tubes for the fluid passageways Choice of Absorber Surface The main choice for the designer is whether to use a matt black coating or a selective surface. The choice is likely to depend mainly on the application for which the collector is intended, the costs involved and the coating facilities available, because the solar absorptance and durability of modern selective coatings are at least as good as those of matt black paints. The range of selective coatings available is very wide, but only a limited number are in general commercial use in Europe. The methods of application include a) Electrodeposition b) Chemical conversion c) Adhesive foil d) Spray painting Most absorber surfaces can be applied to metallic substrates if appropriate primer coatings are laid down first. For good durability, the substrate usually needs to be degreased and coated with appropriate primer paints. An acid etching coating is usually used on aluminium. Advice should be obtained from the paint manufacturers concerning the selection of primer coatings for use with their products. Typical values for the optical properties of commonly available absorber coatings are given in Table However, in order to ensure that the optical properties are uniform over the absorber area it is important to include quality control checks during manufacture. Selective surfaces and paints should be protected during the manufacturing process, because small scratches and finger marks can initiate local corrosion later during the operating life of the collector

257 Surface Application and Substrate Solar Thermal (AM2) (0-100 C) Absorptance Emittance ±0.04 Black Chrome Electrodeposition on Copper, Aluminium, Stainless Steel, Mild Steel or Copper Black Nickel Electrodeposition on 0.90 Mild Steel Chemical Conversion of 0.97 Adhesive Metal Foil Surface Black Cobalt Electrodeposition on 0.95 Steel Anodic Aluminium Chemical Conversion of Aluminium Sheet Surface Tin Oxide Coated Black Enamel Spray onto Steel Copper Oxide Chemical Conversion of 0.87 Copper Sheet Surface Blue Stainless Steel Chemical Conversion of 0.89 Stainless Steel Sheet Surface 0.20 Selective Paints Spray onto Most Surfaces M5.90 M5.25 Table 18.5 Approximate Optical Properties of Absorber Surfaces GENERAL COMMENTS ON COMMON ABSORBER SURFACES Black Chromium - increasingly used in recent years - good durability in glazed collectors on most metal substrates, if a sub-layer of nickel is used. Black Nickel - has been used for many years - made by electrodeposition or chemical conversion - commercially available on adhesive foil for application to many substrates. Durability of the adhesive should be confirmed for each substrate

258 Black Cobalt - not yet widely used - may become more common in second generation collectors. Tin Oxide on Enamel - good durability on steel absorbers has been demonstrated. Anodic Alvminivm - has been demonstrated satisfactory in evacuated collectors - durability less satisfactory in common flat plate collectors. Copper Oxide - Changes chemical structure with age, resulting in lower solar absorptance and higher thermal emittance - Used widely in the early 1970s, but less widely used today. Blue Stainless Steel - Has demonstrated satisfactory performance - Straightforward chemical conversion process for use with stainless steel absorbers. Selective Paints - Several commercial products available - Little European experience of durability - Provide an attractive option for the future if they are shown to exhibit good durability. Matt Black Paints - Very high solar absorptances are possible (>0.96), but most paints also exhibit undesirably high thermal emittances (>0.9) - Some stoved enamels and acrylic paints have demonstrated good long term durability - It is important to confirm resistance to thermal cycling and humidity. High temperature curing paints are usually better than air drying types Choice of Insulation Material When choosing insulation materials, the three most important factors other than cost are their resistance to temperature, durability in the presence of moisture, and thermal conductivity. Closed cell foams may resist temporary exposure to water better than fibrous materials or open cell foams, but few materials will perform well when wet.

259 An air gap behind the absorber, together with an infra-red reflecting foil on the insulation, may be used to protect insulation materials from exposure to high temperatures. However, if this solution is adopted, it is important to seal the air gap around the absorber to ensure that the space behind the absorber cannot act as a "chimney". Lower temperature insulating foams may also be protected from direct contact with a hot absorber by using an intermediate layer of higher temperature insulating material such as mineral wool or glass fibre. Many insulating materials outgas at elevated temperatures. Glass fibre and mineral wool formulations without organic binders are less likely to outgas during high temperature collector stagnation than those with binders. Generally, selective surface collectors require insulation materials which are able to withstand temperatures of up to ^200 C (matt black surface collectors ^150 C). Where outgassing is foreseen as a problem, it can be minimised by heating the insulation to drive off the vapours, before collector assembly. Foam insulations should not normally be made to adhere directly either to the absorber or the collector enclosure, because this can cause serious distortion due to differential expansion at elevated temperatures. Foam insulation materials require adequate space for expansion, which can be quite high, as indicated in Table GENERAL COMMENTS ON COMMON INSULATION MATERIALS Glass Fibre - Good temperature resistance and widely used - Better without an organic binder in collectors with selective absorbers, in order to minimise outgassing - Settles with age; unless used in a sealed collector it should be supported, ventilated, and prevented from blocking drainage or ventilation holes. Mineral Wool (fibre) - Widely used as an alternative to glass fibre. Polyurethane Foam - This name is used for several materials including poly-isocyanurate foam - Excellent insulation properties - Fire resistance depends on formulation - Tendency to degrade on exposure to solar radiation and to temperatures of greater than about 120 C C (depending on formulation) - Outgasses at elevated temperatures; therefore best used either with an airgap and a reflecting foil, or with an intermediate layer of high temperature insulation such as glass fibre or mineral wool -243-

260 - May be injected into a collector (foamed in situ), but this can result in distortion of the collector at high temperatures because of differential expansion - Provision for expansion is needed. Expanded Polystyrene - A low temperature insulation which will shrink and may melt if in contact with an absorber under stagnation conditions - Should be protected with an intermediate layer of high temperature insulation material such as glass fibre or mineral wool - Relatively poor fire resistance Choice of Sealants and Gaskets The temperature range experienced by sealants in solar collectors is wider than that normally associated with sealant applications in buildings, and consequently some 'building mastics' may be inadequate for use in solar collectors. Sealants should be chemically compatible with the materials of the collector, such as plastic glazing, GRP enclosures, aluminium glazing bars, etc., and able to accommodate the differential expansion involved. Most types of expanded foam gasket should not be used because they will shrink at elevated temperatures. It is important to check on the temperature range of the gasket materials, because their names tend to be used rather loosely to represent materials which have very different physical properties. The lifetimes of most sealants and gaskets can be increased by shielding them from direct exposure to solar radiation. This is commonly done by adding a metal glazing strip over the glazing seal. White or colourless sealants should be selected where possible because their stagnation temperatures will be lower than those of black sealants. GENERAL COMMENTS ON COMMON SEALANTS AND GASKETS Silicone Sealants - A range is available. It is important to choose a type to suit the design of joint and its materials - Generally durable over a wide range of temperatures - Important to prepare surfaces thoroughly before applying the sealant - Widely used as a general purpose synthetic rubber with good resistance to atmospheric ageing. EPDM Gaskets - Commonly used in collectors - Have good weathering resistance - Sometimes incorrectly referred to as Neoprene.

261 CHAPTER 19 THERMAL PERFORMANCE 19.1 INTRODUCTION The thermal performance of a collector depends on the materials used for each component, and on the component designs. For most purposes, collector performance can be presented using an efficiency curve, as discussed in Chapters 2 and 8. The most simple efficiency curve takes the form of a linear relationship between collector efficiency and the reduced temperature difference T*, and allows the performance of a collector to be characterised by only two parameters: an optical constant no anc * an overall heat loss coefficient U. In terms of this simplified characteristic, the task of the collector designer can be concisely defined as: " to maximise the value of the optical constant n 0» whilst at the same time minimising the overall heat loss coefficient U ". In addition, he should ensure that initial performance levels will be maintained throughout the lifetime of the collector. Many studies have been carried out to examine the heat transfer processes which take place in a solar collector, and these are presented in engineering texts, such as that by Duffie and Beckman. However, it would be inappropriate to reproduce these in detail here. Instead, the conclusions from the analyses are presented in practical terms, giving generalised guidance to the collector designer. Clearly, such an approach will not satisfy all designers, and those who wish to study the subject in greater depth should consult the engineering texts listed in the Bibliography. Some readers may be a little surprised to find this chapter on thermal performance near the end of the Design Guidelines, and to find that it is relatively short. This is intended to reflect the emphasis which has come to be placed, within the European Community, on the durability and costs of collectors. Unless advanced concepts are being incorporated, such as evacuated enclosures or heat pipes, then the thermal performance aspects of collector design are relatively straightforward. Nevertheless, thermal performance considerations are important, and need to be taken into account early in the design of a new collector. The information in this chapter is limited, as in earlier chapters, with regard to air collectors and 'second generation' collectors, and no specific guidelines are provided for these. However, relevant texts are included in the Bibliography, and many of the performance considerations discussed in connection with flat plate liquid heating collectors also apply to air heating collectors and 'second generation' collectors

262 19.2 COVER DESIGN From the point of view of thermal performance, the important properties of a cover are its transmittance for solar radiation (wavelengths 0.3 to 2.5 pm) and its transmittance for thermal radiation (wavelengths A to 50 pm). In order to ensure that a collector will have a high value of Eta Zero, the solar transmittance of its cover(s) should be as high as possible, and for a collector to have low heat losses the thermal transmittance of its cover(s) should be as low as possible. The transmittance characteristics of low iron glass, a material which is well suited for use as a collector cover, are shown in Figure U-V visible l-r - Solar radiation ] Transmittance of low iron glass Blackbody radiation at-vsut (wavelength urn} Figure 19.1 Transmittance Characteristics of Low Iron Glass The solar transmittance of some covers can be increased by the use of anti-reflection coatings, and by reducing their thickness provided that the material is strong enough to withstand the wind and snow loads discussed in earlier chapters. Multiple cover systems are sometimes used to reduce the front heat losses of a collector, but the solar transmittance of several covers is lower than that of a single cover. In order to provide a reasonably high solar transmittance with a multiple cover system, thin film plastic materials with high solar transmittances may be used for the inner covers. A thick plastic or glass sheet is usually retained for the outer cover, in order to withstand the wind and snow loads.

263 A cover having a high absorptance for thermal radiation, such as glass, will provide a good thermal barrier in front of the absorber. However, because it absorbs the thermal radiation emitted by the absorber, its own temperature rises and it loses heat to the surroundings. To reduce the amount of thermal radiation absorbed by the cover, an infra-red reflective coating such as indium oxide can be applied to the inner surface of the cover. This type of coating is not used very widely in flat plate collectors, because it reduces the solar transmittance of the cover and is quite expensive. The physical and optical properties of commonly used collector materials are discussed in Chapter ABSORBER DESIGN Solar absorbers could in principle be made from most engineering materials, provided that the designs were tailored to suit the thermal properties of the materials involved. However, most absorbers are made from a limited number of common metals and plastics, as discussed in Chapter 18. The factors which have the greatest influence on the thermal performance of an absorber are the optical properties of its surface, and the arrangement of the absorber fins and fluid passageways Absorber Surface The absorber surface influences both the fraction of solar radiation absorbed by a collector (Eta Zero) and the heat losses from the collector (U). In order to ensure a high value of Eta Zero, the absorber must exhibit a high absorptance for solar radiation (wavelengths 0.3 to 2.5 urn), whilst to minimise collector heat losses the absorber surface must exhibit a low thermal emittance (wavelengths 4 to 50 urn). When a collector is operating, its absorber emits thermal radiation which is absorbed by the collector cover, causing heat to be lost to the surroundings. The thermal radiation emitted by the absorber can be reduced by the use of an absorber surface which has a low emittance for thermal radiation. A good selective absorber surface has a low thermal emittance but a high solar absorptance, as shown in Figure The top priority when choosing an absorber surface is to have a high solar absorptance, and few designers would select a surface having a solar absorptance of less than about 0.9. The importance of the thermal emittance

264 depends on the temperatures at which the collector is designed to operate. For a low temperature unglazed collector there is little advantage in using a selective absorber surface, but for high temperatures (>80 C) the thermal emittance of the absorber is very important. For water heating applications, a design value of thermal emittance averaged over the whole absorber area might be about 0.2, since in practice it is quite difficult to manufacture materials having uniform optical properties over a large area. Values of solar absorptance and of thermal emittance for commonly used absorber surfaces are given in Chapter 18. Absorptance of matt black absorber surface \^_ Absorptance of selective \ absorber surface \ \ Blackbody radiation "> at>~50 C I wavelength iim ) Figure 19.2 Characteristics of Typical Absorber Surfaces Fins and Fluid Passageways Most absorbers in flat plate collectors can be considered to consist of a number of fluid passageways connected to fins. A common example is shown in Figure 19.3(a) where the absorber contains several circular tubes joined together by metal fins. For other designs, the fluid passageways may be extended and the fin area made very small, as is the case in the "sandwich" absorber shown in Figure 19.3(b). The arrangement for any particular absorber should be selected to suit the materials involved, and the production costs are also important, as discussed in earlier chapters. HEAT TRANSFER TO THE FLUID The fluid passageway design should take into account the heat transfer properties of the fluid used. The heat transfer coefficient (h) between the fluid and the walls of the fluid passageways depends mainly on the heat transfer properties of the fluid and on its velocity.

265 Fluid passageways should have a large contact area with the fluid, and the fluid velocities should be as high as possible if oils are used, because of their poor heat transfer properties compared with those of water. Increased velocities, however, also produce an increase in the pressure drop around the fluid circuit, so a compromise is required. The size of the passageways and the number of parallel tubes across the absorber influence the fluid velocities and the pressure drop across the absorber. When designing the passageways it is important to confirm that the local velocities at entry and exit will not be high enough to cause re-circulation within the absorber or stagnant regions of low fluid flow. Provision for a continuous rise in absorber fluid passageways to aid venting is an important design consideration. A uniform flow distribution over most absorbers, including "sandwich" absorbers, can usually be encouraged by providing adequately sized header manifolds along the top and bottom edges of the absorber. Headers of adequate size will balance out the pressure differences over the absorber. W a. Fin and Tube Absorber IE 3nE 3nE b. Sandwich Absorber Figure 19.3 Absorber Configurations -249-

266 HEAT TRANSFER IN FINS AND TUBES : COLLECTOR EFFICIENCY FACTOR F' The quality of a fin and tube absorber can be quickly judged by the collector efficiency factor (F 1 ) of the collector in which it is used. This factor is affected by the absorber materials, the tube sizes, the tube spacing, the fin thickness and the fluid flowing through the absorber. Detailed derivations of this factor will be found in the text by Duffie and Beckman (1980), on which the brief summary given here has been based. The efficiency of a fin (F), which may be determined using classical heat transfer theory, is usually expressed in terms of the overall heat loss coefficient of the absorber (U L ) and the fin dimensions (shown in Figure 19.3a): = tanh m[(w-d)/2] * m(w-d)/2 Eqn where m = /U T /k& Eqn The variations of (k6) with absorber fin thickness are shown in Figure 19.4 for three of the metals which are commonly used in solar absorbers. The variation of F with absorber configuration is shown in Figure Plote thickness I mm) Figure 19.4 Variations of k<$ with Absorber Fin Thickness

267 c LL 0.7 ft w i"0 \ 0.6 Figure 19.5 i \ ,W-D. ror 1 2 W k& Fin Efficiency (F) for Fin and Tube Collectors Once the fin efficiency has been determined, it is possible to calculate the collector efficiency factor F' for a fin and tube absorber with a good thermal bond between the fins and the tubes. F 1 may be approximated by: F' = 1 w [(D+(W-D)F) + wu L TrDh Eqn From equation 19.3 it can be seen that the value of F' depends on two terms. The first term in the denominator reflects the sizes of the tubes and fins, and the second contains the ratio of the overall absorber heat loss coefficient (UL) to the heat transfer coefficient (h) between the fluid and the fluid passageway walls. The value of h varies with the fluid and its flow rate, and for precise calculations, correlations are recommended in the literature between the Nusselt, Reynolds and Prandtl numbers. In some cases it may also be necessary to take into account the differences between the heat transfer in the entry region and that in the region where the flow has become fully developed. 251-

268 However, for design purposes the following approximate values of h may usually be assumed: h < 200 W/m 2 K for oils and very low water velocities 200 < h < 400 W/m 2 K for parallel tube and sandwich absorbers h > 800 W/m 2 K for serpentine tubes with high fluid velocities When designing a new absorber it is necessary to estimate the value of the overall heat loss coefficient for the complete collector in order to determine the collector efficiency factor (F 1 ). Fortunately, however, the value of F' is not a strong function of U L in most collectors, so approximations can be made. For absorber design purposes, a value of UL = 4 W/m 2 K may be assumed for single glazed collectors with a selective absorber, and UL = 8 W/m 2 K may be assumed for single glazed collectors with a matt black absorber. EXAMPLE: Design an all copper fin and tube absorber with an overall width of 600 mm for use in a single glazed aolleator with a selective surface. A number of possibilities should be investigated by the designer, in order to find the most cost effective solution. In this example, the value of F 1 will be determined for the following arrangement: 4 parallel tubes of diameter 10 mm Fin thickness 0.5 mm Water flow rates typical for domestic solar water heating systems. i) Find values of the parameters: W = 0.15 m, D = 0.01 m, 6 = m, h * 300 W/m 2 K, UL= 4 W/m 2 K ii) Find k6 ^ 0.2 W/K from Figure 19.4 iii) Find m(w-d)/2 = iv) Find F -v from Figure 19.5 v) Find the two terms in Equation 19.3 to derive F' W '= D+(W-D)F [dr] = Collector Efficiency Factor F' = 0.92 This value of F' is reasonably good for a fin and tube collector. The designer would be likely to accept a lower value only if he could make a significant saving in cost by changing his design

269 HEAT TRANSFER IN SANDWICH ABSORBERS In most "sandwich" absorbers, the area of the absorber which is welded may be considered to act as a fin, but the welded area usually represents only a small proportion of the total absorber area. If the fins are neglected, and the absorber is made of reasonably thin metal with a good thermal conductivity, then the dominant resistance to the flow of heat into the fluid is the convective heat transfer coefficient (h) between the fluid and the walls of the absorber fluid passageways. The value of F' for "sandwich" absorbers can thus be seen to depend primarily on the ratio of h to Uj., and typical values of F 1 for such designs are greater than When "sandwich" absorbers do show poor performance, the reason is usually either an air lock or poor flow distribution over the absorber area. The use of headers to distribute the flow more uniformly across the full absorber area should minimise flow distribution problems, and a continuous rise in the fluid passageways between entry and exit should minimise airlock problems ENCLOSURE DESIGN The enclosure is an important part of a collector, which can significantly influence its thermal performance. The aspects discussed below should be taken into account, whether the collector is a self contained 'box' or integrated into the structure of a building Thermal Bridges Heat transfer between the hot regions of a collector and the surroundings should be minimised. Thermal insulation should therefore be provided around the edges of the absorber as well as at the back, and absorber mountings should not provide paths for heat to be conducted to the surroundings. The collector enclosure should also be insulated from the air in the space between the absorber and the cover. Grommets should be provided to insulate the fluid entry and exit pipes from direct contact with metallic enclosure materials Ventilation It has been explained in earlier chapters that some ventilation is required in most collectors in order to ensure that condensation can be effectively removed. However, the ventilation will also take heat from the collector, and its flowrate should therefore be no more than is required to remove condensation. There are no well established guidelines concerning ventilation rates, so the sizes and locations of ventilation holes should be determined for each new design by experiment

270 Cover Spacing The convective heat transfer between the cover and the absorber depends on the distance between them, and hence the overall collector heat losses also depend on this spacing. Many studies of natural convection are documented in the literature, and the detailed results depend on the temperatures and the tilt angles of the surfaces involved. However, for most practical purposes it is sufficient to assume that the collector heat losses are relatively insensitive to the spacing between the cover and the absorber, provided that this spacing is greater than about 15 mm. Most designers choose a spacing of between about 20 mm and 40 mm. The spacing should not be too large because of the shading of the absorber which this would cause at shallow solar incidence angles INSULATION Most of the insulation in a collector is placed behind the absorber, to reduce back heat losses. The amount required can be optimised with regard to cost effectiveness, taking account of the front heat loss coefficient and the anticipated collector operating temperature. However, for design purposes it is often considered reasonable to provide sufficient back insulation to ensure that the back heat losses are no more than about 10% of the heat losses through the front of the collector. For example: i) Back insulation consisting of 67 mm of glass fibre, with a conductivity of 0.04 W/m K, might be used to give a back heat loss coefficient of 0.6 W/m 2 K for a single glazed matt black collector with a front heat loss coefficient of 6 W/m 2 K. ii) Back insulation consisting of 133 mm of glass fibre, with a conductivity of 0.04 W/m K, might be used to give a back heat loss coefficient of 0.3 W/m 2 K for a good selectively coated collector with a front heat loss coefficient of 3 W/m 2 K. Depending on the collector design, the insulation around the edges of the absorber should be as effective as that at the back. However, less insulation is required around the edges of the air gap between the absorber and the cover, because the air in this space will have a lower temperature than the absorber. There should be no thermal bridges between the absorber and the collector enclosure, which may require absorber mountings made of insulating materials. The fluid inlet and outlet pipes should also be well insulated.

271 CHAPTER 20 CONCLUSIONS AND RECOMMENDATIONS ON COLLECTOR DESIGN 20.1 CONCLUSIONS The design requirements for flat plate collectors are now quite well understood, and the designer has a wide range of materials from which to choose. At the present time there is no single design solution which dominates the market, and there appears to be little likelihood of such domination in the near future. The range of materials currently used for collectors in Europe is rather limited, but there are many alternatives which might become more widely used in future. The guidelines relating to corrosion resistance, weather resistance, thermal expansion and condensation, which are presented in Chapters 17 and 18, were derived from field experience with "first generation" collectors, but are sufficiently general to apply to most building systems. They can therefore be applied also to new solar collector designs. Future development work on collector design can be expected to pursue three main objectives, which are (a) lower installed costs and, (b) better collection efficiencies at high temperature, and (c) better durability. The costs of solar collectors will become lower as the levels of production are increased. High production levels permit greater investments in machinery, and open up economic possibilities for using materials such as plastics and tempered glass which cannot be easily employed in small batch production processes. Absorber surfaces, including both matt black and selective surfaces, are already commercially available with very high values of solar absorptance (> 96%), and covers are available with high values of solar transmittance (>90%), so there is little potential for improving the values of Eta Zero for flat plate collectors. Greater solar energy input may however be achieved by further work on the development of lenses and reflectors. For this, most attention seems likely to be given to compound parabolic concentrator (CPC) designs. In order to reduce heat losses at high temperature, further development of evacuated collectors can be expected, and there is also scope for further work on the use of thin plastic films in multiple glazing systems and honeycombs to reduce heat losses from absorbers

272 The process of collector design involves a compromise between building science and cost, with a need to ensure that proper attention is given to both the thermal performance and the durability of the final product. In these guidelines, more attention has been given to durability than to thermal performance, because experience from working systems indicates that many designers have given insufficient thought to durability in the past. However, most durability problems found in collectors are, in principle, relatively easy to avoid, and longer lifetimes should therefore be anticipated for "second generation" solar collectors RECOMMENDATIONS Durability and Cost Effectiveness The cost effectiveness of a solar heating system depends strongly on the lifetime and maintenance costs of its solar collectors. It is therefore important for collector designers to ensure both that the materials used are of suitable quality and that designs are appropriate for the combinations of materials selected Mass Production and Cost Effectiveness To design a durable solar collector is not difficult in principle, but may be assisted by a good knowledge of potential problem areas and recognised design solutions. It may be encouraging, therefore, that European manufacture is being undertaken increasingly by companies who have some direct contact with researchers. As the market grows and mass production becomes more widely used, costs may be expected to decrease, and the quality of future collectors may be better assured, because mass production processes are well suited to organised quality control checking Durability Testing Many of the design faults found in "first generation" solar heating systems could have been predicted at the design stage, but others could also have been identified by durability tests. Manufacturers and consumers would benefit if prototype collector designs were subjected to the tests discussed in Chapter 13, before the collectors were put onto the market

273 During production, a number of quality control checks should be carried out on all collectors, such as an absorber leak test. Other qualification tests might be carried out on a regular basis, using samples from the production line Thermal Performance Testing It is important for designers to obtain thermal performance test results for their collectors, both when new and after some outdoor exposure. However, quality control of collector manufacture can usually be performed more economically by component performance monitoring than by full collector testing. For example, the optical properties of absorber surfaces can be checked using simple optical equipment Further Work on Collector Ventilation Insufficient information is available to provide practical guidelines for manufacturers, with regard to collector ventilation. Further work is required on ventilation rates and ventilation hole design, to optimise collector ventilation with regard to both collector thermal performance and condensation removal for various climatic zones Further Work on Absorber Flow Distribution The design of absorber fluid passageways is usually performed in the absence of detailed engineering guidelines. As a result, some absorbers (particularly "sandwich" absorbers) can suffer from a poor flow distribution. Furthermore, collectors with horizontal header manifolds are often connected together, and as a result the flow distribution in the array becomes poor. Further work is required to produce practical guidelines for designers, which will show how to avoid flow distribution problems. 257-

274

275 BIBLIOGRAPHY SOLAR ENGINEERING TEXTBOOKS Coulson, K.L. Solar and Terrestrial Radiation: Academic Press (1975). Duffie, J.A. and Beckman, W.A. Solar Engineering of Thermal Processes: Wiley (1980). Meinel, A.B. and Meinel, M.P. Addison-Wesley (1976). Applied Solar Energy: Robinson, N. Solar Radiation: Elsevier (1966). Sayigh, A.A.M. Solar Energy Engineering: Academic Press (1977). GENERAL DANISH LANGUAGE Bogen om Alternative Energikilder: Politikens Forlag (1977). Lura, A.E., Bogen om Solenergi: Esbensen, T.V. and Lawaetz, H., Clausen Boger, Aschenhoug, Denmark (1978). Byg et Solvarmeanlaeg: (1978). Energihandbogen: Denmark (1981). Kraegpoth, K., Teknisk Forlag Organisationen for Vedvarende Energi, Modul - Solvarmeanlaeg: Sol, 0., Organisationen for Vedvarende Energi, Denmark (1981). Solenergi/Vindkraft - En Handbog: Herforth, C. and Nybroe, C., Informations Forlag (1976). Solvarme - Vejledning i Projektering og Udforelse af Anlaeg: Teknologisk Instituts Forlag (1980)

276 DUTCH LANGUAGE Eindeloze Energie, Alternatieven yoor de Samenleying: Lysen, E., Het Spectrum - Utrecht/Antwerpen, Netherlands (1977). De Zonneboiler (de Invloed van Diverse Variabelen op het Thermisch Rendement): Wijsman, A.J.Th.M. en den Ouden, C., TPD/TNO-TH Rapport Nr Netherlands (1979). Zonne - Energie: Rau, H., Kluwer Technische Boeken, B.V., Deventer, Netherlands (1979). Zonne - Energie: Bouw Zelf uw Installatie (Warmwater voorziening) Vertaling uit het Engels : Kiely, C., d Muiderkring, B.V., Bussum, Netherlands (1978). Zonne - Energiesystemen, Zonneboilers een Informatief OverzichF: TVVL, ECN, Bouwcentrun, Netherlands (1980) ENGLISH LANGUAGE BS5918:1980 Code of Practice for Solar Heating Systems for Domestic Hot Water: British Standards Institution, 2 Park Street, London W1A 2BS, England (1980). Practical Solar Heating: McCartney, K. and Ford, B., Prism Press (1978). Reliability and Performance of Solar Collector Systems: BRE Digest 254, BRE, Garston, Watford WD2 7JR, England (1981). Solar Energy and Building: Press (1977). Szokolay, S.V., Academic Solar Energy - A UK Assessment: UK-ISES, 19 Albemarle Street, London W1X 3HA, England (1976). Solar Energy for Man: (1972). Brinkworth, B.J., Compton Press Solar Energy - Its Potential Contribution within the UK: Energy Paper No. 16 HMSO, England (1976). Solar Heating Systems for the UK: England (1979). Wozniak, S.J., HMSO, Sun Power (2nd Ed): McVeigh, J.C., Pergamon Press (1983) Solar Energy for Ireland: Lawlor, E., Government Publications, Dublin, Ireland (1975). The Climate of Ireland: Rohan, P.K., Irish Meteorological Service, Dublin, Ireland (1975)

277 FRENCH LANGUAGE Bricolez Mieux - Devenez un bon Utilisateur d'energie Solaire: Thevenin, J., Ed. Eyrolles (1981). Calcul D'Installations Solaires a Eau (Eau Chaude Sanitaire, Chauffage des Habitations)": Chateauminois, Mandineau, et Roux, Ed. Edisud (1980). Des Capteurs Solaires Pour Votre Maison, Choisir, Installer, Entretenirl Kut, D. et Hare, G., Ed. Moniteur (1981). Eau Chaude Solaire : Guide de 1'Installateur: Chateauminois, Fogelman, Mandineau, et Roux, Ed. Edisud (1981). Eau Chaude et Chauffage Solaires: Espic, R., Isoardi, J.P., et Moreau, M., Sedit, Paris, France (1978). Fabrication Artisanale de Capteurs Solaires: Cabriol, T., Ed. Edisud (1981). Guide des Installations Solaires dans 1'habitat: AFEDES, EETI, Paris, France (1980). Le Chauffe-Eau Solaire: Roux, D., Ed. Edisud. Cabriol, T., Pelissou, A. et Les Equipetnents Solaires Actifs dans le Batiment. Memento et Catalogue! COMES, CATED, Paris, France (1980). Memosol Memento d'heliotechnique: (1979). AFEDES, EETI, Paris Savoir Acheter, Sayoir Utiliser l'energie Solaire: Morel, H.J.F., Villardonnel, France (1979). GERMAN LANGUAGE DIN 4757 Parts 1-3 Solar heating plants, solar collectors, definitions, safety requirements, test of stagnation temperature: Beuth Verlag, Berlin, W. Germany (1980). Gebrauchstauglichkeit von Sonnenheizunganlagen; Richtlinien und Hinweise: Bundesverband Solarenergie (BSE), Kruppstr. 5, D-4300 Essen 1, W. Germany (1980). Heliotechnik: Rau, H; Pfriemer, Muenchen, W. Germany (1976). 261-

278 Informationswerk Sonnenenergie: Teil 1-4, Pfriemer, Muenchen, W. Germany (197 7). Solartechnik: Lehner, G. et al; Expert-Verlag, Grafenau, W. Germany (1981). Sonnenenergie in Theorie und Praxis: Winkler, J.P; Mueller-Verlag, Karlsruhe, W. Germany (1979). Technische Solarenergienutzung: Korzen, W.A; Mueller-Verlag, Karlsruhe, W. Germany (1981). Wege zum energiesparenden Wohnhaus: Hoerster, H; Philips Fachbuecher, Hamburg, W. Germany (1980). GREEK LANGUAGE EfrAPMOTEZ HAAKHE ENEPTEIAE: Bazeos, E. Athens (1981). HlliES MOPOEZ ENEPTEIAS Vols. I and II: Proc. First National Conference on Renewable Energies, University of Salonika (1982 ITALIAN LANGUAGE Impianti Solari Attivi: Manuale di Calcolo: Lazzarin, R., Franco Muzzio et C. Editore. II Clima Come Elemento di Progetto dell 'Edilizia: Gruppo Energia Solare dell 'Universita di Napoli, Lignori Editore, Napoli, Italy (1977). Impieghi dell 'Energia Solare: (1976). Robotti, A.C., UTET Utilizzazione dell 'Energia Solare e Irraggiamento Verso 1'Infinito: Gaudenzi, P., Ulrico Hoepli, Milano, Italy (1980). 262-

279 APPENDIX I SOLAR SIMULATOR DESIGN

280

281 SOLAR SIMULATOR DESIGN AI.l INTRODUCTION In many parts of the world, and in Northern Europe in particular, steady conditions of fine weather are an infrequent occurrence, and the idea of using an artificial sun to study solar collectors is therefore attractive. For research and development purposes there are also many advantages in being able to reproduce test conditions exactly, and this is not easy to do outdoors. One of the earliest solar simulators to be used for solar collector testing was the tungsten halogen simulator at the NASA Lewis Research Center in the USA (45). Experience with this simulator was used as a basis for the solar simulator test procedure included in ASHRAE (15). The simulator appeared to work well despite its very large number of lamps (143 in all) and their rather short life (35-80 hrs). Early tungsten halogen simulators were also installed at CEA and EDF in France, but these employed larger power linear tungsten lamps with a lower colour temperature and longer life. The main problem with these simulators was their large output of thermal (infra-red) radiation. One of the more advanced solar simulators to be used for solar collector testing is the Xenon simulator at DFVLR in Cologne. This was originally designed as a space simulator; it has very sophisticated optics and a fully automatic control system. Its beam uniformity is better than ±4%, its spectrum can be adjusted to suit the test specifications and it has a small climatic chamber in which the collector can be placed. Its main disadvantage is that the test area is limited to about 1.3 m square. New simulators are still being built in many countries around the world. Some are constructed by commercial concerns, and details of their designs are not made public, but the majority of simulator designers are pleased to share their experiences. A high proportion of the well known simulators employ the Compact Source Iodide (CSI) lamp made by Thorn Lighting Ltd. (46), which was first selected for the solar simulator at University College, Cardiff in The performance of the CSI lamp was discussed in detail at a Workshop on Solar Simulators in the Joint Research Centre at Ispra in February 1982 (47). A more recent development, also announced at this workshop, is the Vortek lamp from Canada. At present there is little European experience with this lamp, but it shows great promise as a source of simulated solar radiation

282 Radiation simulators are built for several purposes other than for testing the thermal performance of solar collectors, such as to test the resistance of materials to ultra-violet light, and to determine the electrical output characteristics of photovoltaic devices. However, each application demands a rather different type of facility. The most important decision in the design of a solar simulator involves the choice of a suitable lamp, and for this a compromise has to be reached, taking account of the different aspects discussed in this chapter. The main constraints which are likely to cause the design of solar simulators to vary from one laboratory to another are (a) the size and shape of the laboratory space available, (b) the electrical power supplies, (c) the range of collector designs for which testing is planned and (d) the finance available for construction and for later development of the facility. AI.2 SIMULATED SPECTRUM The spectrum of the direct solar radiation at the Earth's surface varies with the air mass through which the radiation has passed, and with the humidity and turbidity of the atmosphere. Diffuse solar radiation has a different spectral distribution from that of the direct, so the spectrum of global solar radiation could be said to vary with the percentage of diffuse irradiance present. Typical variations in the spectrum of direct solar radiation are given by Robinson (48). Once it has been recognised that the spectrum of solar radiation varies widely during the day, it follows that there is little point in attempting to simulate any given spectral distribution very accurately for the purposes of obtaining typical collector performance data. Indeed, for testing most thermal collectors, any spectrum which is a rough approximation to the solar AM 1.5 or AM 2 spectrum will give reasonable results, provided that the proportion of the total energy in the wavelength range below the cut-off of typical selective surfaces is the same in both the simulated and the actual solar spectra. It is also important that the lamp output at wavelengths greater than about 2.5 urn should be very small, to prevent unrepresentative heating of collector covers. This requires not only a lamp with a high colour temperature, so that the "light" emitted does not extend beyond 2.5 urn, but also a low heat output from the lamp cover glasses and housing. The heat output can be minimised by having a small lamp housing, or by using cooled sheets of glass between the lamps and the collector to absorb thermal radiation (49). The thermal -266-

283 radiation flux can also be reduced by placing the lamps a large distance away from the test plane. A typical distance to give reasonable results might be 4 m or greater, and methods for calculating the appropriate view factor are discussed in Section One of the more common simulator lamps is the Thorn CSI, and its spectrum is compared with that of AM2 solar radiation in Figure AI.l. The CSI lamp has lines in its spectrum which may give problems if it is used to study photovoltaic or organic materials, but these are relatively unimportant for thermal collector testing. Another well established lamp for solar simulation is the Xenon arc. This has peaks at about 0.9 um in its spectrum but these can be removed by filtering. in o CE C.S.I Lamp Spectrum Solar Spectrum UJ cr: 1.1* 1.M I.» I.3«2.1) 2.M WAVELENGTH (MICRONS) Figure AI.l Comparison between the CSI Lamp Spectrum and the Solar Spectrum By convoluting radiation spectra with measured absorptance and transmittance spectra it is possible to determine the total absorptances and transmittances which would be measured if materials were tested under different forms of radiation. The results of such calculations for some typical solar collector materials are given in Table AI.l where it can be seen that they behave similarly under both solar radiation and the light from a CSI lamp

284 Acrylic sheet Glass sheet G.R.P. sheet (T) (T) (T) AMI AM CSI Black nickel foil Oxidised stainless steel Black chrome Black paint (a) (a) (a) (a) Table AI.l Mean Absorptance and Transmittance Data for Solar and CSI Spectra AI.3 SIMULATED IRRADIANCE LEVELS Most simulators are used at irradiance levels in the region of W/m 2, and some have the capability of providing variable irradiance levels. The minimum collector efficiency (or maximum T* value) which can be measured in a given simulator is limited by the maximum temperature available in the fluid loop and the minimum irradiance level available. For example, with a typical ambient temperature of 20 C, and a peak water loop operating temperature of 90 C C, the maximum value of T* which can be obtained when G = 1000 W/m 2 is 0.07 Km 2 /W. High irradiance levels are usually used for collector testing, because the errors in many of the other variables measured during testing are less significant under high irradiance conditions. Some types of lamp will not operate properly at reduced voltages, and this nay restrict their usefulness in solar simulators. The CSI lamp can be reduced in power to about 50% of its radiation output (^70% of its electrical input power), without significant problems other than a very slight shift in the output spectrum. Unfortunately this shift is detectable with a silicon solar cell, so if silicon cells are used for irradiance measurement, then new cell calibrations are required for each irradiance level. In some simulators the mean irradiance level can be altered by switching lamps off, but this usually impairs the uniformity of the irradiance. 268-

285 AI.4 SIMULATED BEAM UNIFORMITY For most flat plate collector testing, the uniformity of the beam appears to be relatively unimportant because the collectors operate as straightforward integrators. However, the beam uniformity is more important for tubular collectors or those with any reflecting or concentrating elements. By using sophisticated optics, the Xenon simulator at DFVLR achieves a uniformity of ±4%, and a similar result (<±5%) has been demonstrated with a CSI simulator at TNO-TH in Delft. However, such results can only be achieved with CSI lamps by electrically trimming the power to each lamp, since the lamps vary in their individual outputs by up to ±15% as a result of manufacturing tolerances in the ballast resistors and capacitors and in the lamps themselves. Without individual lamp trimming, the bean at Cardiff had a standard deviation of about 110 W/m 2 when the mean irradiance was 790 W/m 2 over a typical 2.1 m x 1.3 m test area. Very good uniformity has been reported for the Vortek simulator where a single lamp has its beam spread by reflectors over a 2.4m x 2.4m test area with a uniformity of ±5% (47). In most simulators, however, there has to be a compromise between uniformity and parallelism in the beam. With an array of lamps, good uniformity can be obtained by spreading the beam from each lamp, but this results in a wider range of incidence angles than may be desirable. For the CSI lamp, a range of lamp cover glasses is available to give different amounts of beam spreading. AI.5 SIMULATED BEAM PARALLELISM The ideal simulated beam would, of course, have a divergence angle of approximately 0.5 degrees like that of the sun, and be perfectly uniform, but this is very difficult to achieve in practice. Analysis of collector test results becomes more complicated as the beam divergence increases, because the transmittance of collector covers is a function of incidence angle. For flat plate collectors, however, the effect on performance of incidence angles ranging between 0 and 40 is small (see Figure 10.5), so most simulators will give reasonable results. Problems occur when tubular or concentrating collectors are to be tested, because their reflecting surfaces are more sensitive to the incidence angles of the irradiance. A number of laboratories are now beginning to look seriously at methods for measuring collector incidence angle modifiers in solar simulators. For these -269-

286 measurements a divergent beam presents two problems. Firstly, there is uncertainty concerning the mean incidence angle being measured, and secondly, the non-uniformity of irradiance over the collector increases as the incidence angle is increased, if the beam itself is divergent and the distance between the collector and the lamps is not everywhere the same when the collector is inclined to the direction of the simulator beam. One way to reduce the problem of variations in irradiance levels when testing at different incidence angles is to mount the lamps on individually rotating shafts, as shown in Figure AI.2. Different incidence angles can then be simulated by moving the collector across a plane parallel to the lamp array, and at the same time rotating the lamps. Procedures for making reliable incidence angle modifier measurements are still under development (see Section 10.7). Lamp Array Lamp Array A A A A A Lamp Array A A A A A Prefered Design for Incidence Angle v lollgcior Normal Incidence Poor Uniformity at Incidence Angle Figure AI.2 Rotating Lamp Array for Incidence Angle Modifier Measurement AI.6 LAMP LIFE AND STABILITY The power output from most lamps decreases with age, and a long life lamp is therefore attractive from a performance view-point as well as for economic reasons. The early tungsten halogen lamps used in the NASA Lewis simulator were cheap, but their life was only about 35 hours. The life of the Vortek lamp is also rather short (50 hrs), but this takes only a few minutes to change, in contrast to the NASA Lewis facility for which relamping was a fairly major task. The CSI lamp has a nominal life of 1000 hrs, but significantly longer operating lives have been achieved in several solar simulators. Stability is important in the short term because steady

287 state conditions for an hour or two at a time are required for testing. In this regard it is unfortunate that the CSI lamp tends to amplify fluctuations in the mains voltage by up to about 40Z, and some measure of mains stabilisation is therefore beneficial for CSI lamp simulators. It is also important to cool CSI lamps to maintain stable operation indoors, and a flow of ambient air with a mean velocity of greater than about 1 m/s appears to be required. With a multiple lamp array it is important to monitor the entire beam area to accommodate variations in uniformity with time. This is usually accomplished by regular mapping of the irradiance over the surface of a collector under test (see Section ). AI.7 THERMAL RADIATION IN SIMULATORS The thermal radiation flux in a solar simulator comes from all the surfaces in the enclosure including the hot lamps. The thermal irradiance from the lamps will depend on the lamps themselves, their mountings and the distance between the lamps and the collector. The thermal radiation from the lamps can be calculated using radiation view factors, as discussed in Section 7.2.3, if the temperatures and emittances of the lamps and their housings are known. It can also be measured, but this is not easy because it is usually small in comparison with the simulated solar radiation. The thermal radiation flux outdoors on clear days can be as much as 120 W/m 2 less than that of a blackbody cavity at ambient temperature, and on overcast days the flux approaches that of a blackbody cavity at the ambient air temperature (see Figure 8.4). The low thermal radiation flux on clear days outdoors poses a design problem for solar simulators because the temperatures of the walls and ceiling in the simulator would need to be reduced to below ambient temperature to simulate cold clear sky conditions, even if there were no thermal radiation emitted by the lamps. In practice, there are very few simulators with facilities for cooling the surfaces in the field of view of the collector. The CSI simulator at CETIAT in France has cooled walls; the simulator at the University of Stuttgart, which employs Osram-Siccatherm infra-red emitters, uses three sheets of glass with a special cooling circuit to minimise the thermal radiation output; and the simulator at CSIRO in Australia has the real atmosphere as its field of view, because it is used outdoors to supplement the natural solar radiation. The most commonly accepted approach, however, is to determine the thermal radiation flux in the simulator -271-

288 and then apply, if necessary, a mathematical correction to the measurements of collector performance (see Section 8.7). AI.8 AIR TEMPERATURE AND WIND SPEED IN SIMULATORS It is well known that collector results obtained in still air conditions show a significantly reduced heat loss coefficient because a boundary layer of warm air becomes established over the collector surface. Simulators are therefore usually fitted with an artificial wind generator. The rate of energy input to most simulator laboratories is so large that some form of air conditioning is required to maintain steady temperatures, and this is also used to mix the air in the testing zone. Even when equipment is installed to mix the air in a solar simulator it is not unusual to find significant temperature differences between one part of the laboratory and another, and it is therefore important to select an appropriate location for measuring the 'ambient' temperature which will be used for analyses of collector performance. One commonly used location is in the outlet of the wind generator, before the air becomes heated by passing over the collector surface. An alternative is to measure the air temperature beside or behind the collector, using a shaded detector. In this case it is advisable to arrange for air to be drawn over the detector by means of a small fan. It is recommended in Section that an average wind speed of A m/s be used for collector testing in solar simulators. Because the artificial wind is usually produced at one end of the collector, its velocity decreases as it passes across the collector, and there is a significant variation in wind speed over the collector surface. A spatially averaged wind speed, determined from a series of measurements made with a movable anemometer, is generally used for collector testing, as discussed in Section and Reference (50). AI.9 SIMULATOR GEOMETRY Solar simulators may be used to study building components with vertical surfaces, such as Trombe Walls, and also to study horizontal surfaces such as solar ponds. However, where simulators are used to test collectors, a standard inclination of about 45 is usually preferred

289 Some types of lamp are designed to operate within only a limited range of inclinations, and to select such a lamp would limit the versatility of the solar simulator. The CSI lamp has the advantage that it will operate at any inclination and can therefore be used to simulate a full 90 of variation in solar altitude. There are many ways of mounting an array of lamps for solar simulation purposes, but whatever the approach selected it is useful to consider making provision for (a) testing at different collector tilt angles, and (b) measuring bi-axial incidence angle modifiers. If the lamps are mounted to rotate about a series of shafts,as indicated in Figure AI.2, then in order to permit the measurement of bi-axial incidence angle modifiers it may be necessary to rotate the entire shaft support structure through 90 degrees in the plane of the lamp array. In order to test collectors at different tilt angles with respect to the horizontal, the whole lamp array will also need to be tilted. The required geometry to permit these measurements is indicated in Figure AI.3. It is sometimes necessary to be able to rotate both the collector and the lamps in order to measure incidence angle modifiers in a laboratory of small size. However, care should be taken not to confuse incidence angle effects with those caused by operating the collector at different tilt angles. The ability to move the lamp array to ground level, or to be able to climb easily to the array, is important for maintenance purposes, lamp cleaning and characterisation of the simulator

290 Tilt angle a) Beam at Normal Incidence to Collector with Variations in Collector Tilt Angle Lamp Array n n n *\ n Lamp Array Collector b) Bi-axial Incidence Angle Measurements Figure AI.3 Simulator Configurations Required for Collector Testing

291 ALIO THE SIMULATOR CHAMBER Reference has been made in this chapter to the need for well mixed air in the vicinity of a collector under test, and to the subject of thermal radiation in simulators. When testing at different incidence angles by moving the lamps and the collector, there is a significant risk of the results being confused by changes in the amount of simulated radiation reflected onto the collector and by changes in thermal irradiance. These problems can be minimised by painting all surfaces in the simulator with a dark paint having a low reflectance, and by controlling the temperature of as many as possible of the surfaces which fall within the field of view of the collector (see Section 5.3). Figure AI.4 A Versatile Solar Simulator Lamp Array -275-

292

293 APPENDIX II RECOMMENDED NOMENCLATURE

294

295 QUANTITY SYMBOL UNITS Absorptance Air Mass Area Conductivity Density Efficiency Electrical Power Emittance Energy Overall Heat Transfer Coefficient Surface Heat Transfer Coefficient Mass Mass flowrate Pressure Reflectance Specific Heat Capacity Stefan-Boltzmann Constant Temperature Temperature Difference Thermal Capacity Thermal Power Transmittance Wavelength a AM A k P ri P E Q u h m m P P c 0 T AT C Q T X W/m K kg/m 3 J W/m 2 K W/m 2 K kg kg/s Pa J/kg K W/m^K 1 * K or C K J/K W Recommended Nomenclature - General

296 QUANTITY SYMBOL CONVENTION UNITS Trradiance, Solar Irrad: Lance, Longwave Irradiation, Solar Irradiation, Longwave Radiant Energy Radiant Flux Radiant Intensity Solar Solar Solar Solar Angle Angle Angle Altitude Angle Azimuth Angle Hour Angle Declination of Incidence of Reflection of Refraction G G L H H L Q * I Y * u> 6 v(b,o) V V R _ (A > 4un) - (\ > 4 ym) (see definition) (clockwise from South) (0 to 360 from Solar Noon) (North positive) W/m 2 W/m 2 J/m 2 J/m 2 J W W/sr degrees degrees degrees degrees degrees degrees degrees Recommended Nome: nclature for Radiation Quantities and Related Sol ar Angles QUANTITY SYMBOL UNITS Humidity, Absolute Humidity, Relative Temperature, Dry Bulb Temperature, Wet Bulb Temperature, Dew Point Vapour Pressure Wind Speed Surrounding Air Speed T T' T X D e u u K or C K or C K or C Pa m/s m/s Recommended Nomenclature for Meteorological Parameters

297 QUANTITY SYMBOL UNIT Altitude (above sea level) Azimuth (of surface) Latitude Longitude Tilt from horizontal m degrees degrees degrees degrees Nomenclature for Surface Location and Orientation QUANTITY SUBSCRIPT QUANTITY SUBSCRIPT Absorber plate P Inlet i Ambient a Longwave (Thermal) L Aperture a Loss l Atmospheric A Mean m Beam (Direct) b Net Exchange X Blackbody b Normal n Convective c Outlet (Exit) e Diffuse d Reflected p Fluid f Spectral X Ground G Total T Gross E Horizontal h Recommended Subscripts

298

299 APPENDIX III TEST FORMAT SHEETS The format sheets in this section are currently being used within the European Community for presenting the results of solar collector performance tests.

300

301 Commission of the European Communities COLLECTOR AUTHOR INSTITUTE SOLAR COLLECTOR TESTING PROGRAMME Performance Tests Format Sheets DATE

302 EUROPEAN SOLAR COLLECTOR TESTING PROGRAMME FORMAT SHEETS TESTS TO CEC RECOMMENDATIONS COLLECTOR REFERENCE: TESTS PERFORMED BY: Address Date Tel Telex. l. Description of Solar Collector (please complete as applicable) 1.1 NAME OF COLLECTOR AND MANUFACTURER 1.2 COLLECTOR TYPE FLAT PLATE EVACUATED TUBE OTHER " - Aperture dimensions (or tube length) - Number of covers Cover materials Cover thickness mm - Number of tubes Tube diameter Tube pitch 1.3 HEAT TRANSFER MEDIUM WATER OIL AIR OTHER ] [ - Specification (additives, etc) - Alternative acceptable h.t. fluids 1.4 ABSORBER - Material - Surface treatment - Construction type - Weight empty kg Fluid content kg - Dimensions 1.5 THERMAL INSULATION AND CASING' - Th^rmal^.ir^u.lation^ Thickness mm - Material - Casing^ Material - Total weight of collector without fluid kg - Gross dimensions

303 1.6 LIMITATIONS - Maximum temperature of operation - Maximum pressure - Other limitations 1.7 SCHEMA OF SOLAR COLLECTOR 1.8 COMMENTS ON COLLECTOR DESIGN

304 2.Instantaneous Efficiency1 2.1 METHOD OUTDOOR STEADY STATE INDOOR STEADY STATE COMBINED INDOOR/OUTDOOR OUTDOOR 'TRANSIENT' 2.2 SCHEMA OF TEST LOOP

305 2.3 PHOTO OF SOLAR COLLECTOR TEST RIG

306 2.4 INSTANTANEOUS EFFICIENCY CURVE The reduced temperature difference T* is defined by T* = ^ ^- (K m 2 /W) The instantaneous efficiency n is defined by n = Aperture area used for curve T\ T*i T'lKm'w-') Linear fit to data T\ = T^ - a, j * Second order (it to data 1\ = T) 0 _ a, T*-a 2 GIT*) 2 I =

307 2.5 INSTANTANEOUS EFFICIENCY: MEASURED AND DERIVED DATA LATITUDE LONGITUDE APERTURE AREA m 1 COLLECTOR TILT FLUID c f J/kg K COLLECTOR AZIMUTH FIELD OF VIEW (SKY) TEMP LOCAL TIME AT SOLAR NOON DATE LT G Gd/G To «Js Ti Te-Tfl Tm m T* T / D-M-Yr Hrs-Mins W m" 2 % C m s - ' C K C kgs" 1 K m 2 W"

308 I 3. Heat Losses 3.1 OVERALL HEAT LOSS RATE Qt (wj 800 indoor outdoor J I (Tm-Talj IK) The heat losses Q are expressed as a function of the temperature difference (Tm-Ta) 3.2 HEAT LOSS COEFFICIENT U IWnT 2 K-') 12-10" i (Tm-Talf IKI Aa(Tm-Ta),

309 3.3 HEAT LOSSES: MEASURED AND DERIVED DATA indoor outdoor I APERTURE AREA m J COLLECTOR TILT FLUID Cf.J/kg K DATE D-M-Yr LT Hrs-Mins To C Us I»J-' T C T -T, K T m C m kgs" 1 (Tm-To)^ K / W U Wm^K" 1

310 I 4. Pressure Drop AP I Pa I n Mass flowrate (kgs-'i Fluid Temperature 5. Other Methods or Special Remarks (Give a short description of methods and essential results)

311 6. Instrumentation and Calibration! 6.1 RADIATION MEASUREMENT - Instrumentation - Date of last calibration - Institution providing the calibration - Are the results temperature compensated? - Are the results corrected for tilt? - Accuracy of alignment to plane of collector ±. - Overall accuracy of radiation measurement data ±. 7.2 FLUID MASS FLOWRATE (m) - Instrumentation - Date of last calibration - Method of calibration (temp, range, flowrate range, accuracy, etc.)... - Overall accuracy of flowrate data 6.3 AMBIENT TEMPERATURE (Ta) - Instrumentation - Date of last calibration - Method of calibration (reference instrument, procedure, etc.) Overall accuracy of ambient temperature data ± K

312 6.4 FLUID INLET TEMPERATURE (Ti) - Instrumentation - Date of last calibration - Method of calibration (reference instrument, procedure, etc. - Overall accuracy of fluid inlet temperature 6.5 INLET TEMPERATURE CONTROL - Instrumentation Stability ± K/hr 6.6 DIFFERENTIAL FLUID TEMPERATURE (Te-Ti) - Instrumentation - Date of last calibration - Method of calibration (reference instrument, temperature range, etc.). Overall accuracy of differential temperature measurement

313 6.7 SURROUNDING AIR SPEED (u ) - Instrumentation - Date of last calibration - Method of calibration - Method of measurement - Artificial wind direction - Overall accuracy of wind data ± m/s 6.8 DATA RECORDING - Instrumentation 6.9 PRESSURE DROP (AP) - Instrumentation - Date of last calibration - Method of calibration (reference instrument, procedure, etc.) - Overall accuracy of pressure drop data ± Pa 6.10 SOLAR SIMULATION - Lamp type - Excess thermal radiation - Corrections applied - Beam incidence angles - Corrections applied - Comments (uniformity, etc)

314 17. Nomenclature] Symbol Meaning Units Aa Aperture area of collector m 2 Ag Gross area of collector m 2 Cf Specific heat capacity of fluid J/kg K G Solar irradiance at collector aperture W/m 2 Gd Diffuse solar irradiance W/m 2 LT Local time Hours-Mins m Mass flowrate of heat transfer fluid kg/s Q Overall heat loss rate W Q Useful power extracted from collector W Ta Ambient air temperature C Te Collector outlet (exit) temperature C Ti Collector inlet temperature C Tm Mean temperature of fluid in collector C Ts Field of view (sky) temperature K T* Reduced temperature difference Km 2 /W us Surrounding air speed m/s U Heat loss coefficient W/m 2 K AP Pressure drop across collector Pa n n' Collector thermal efficiency (from measured data) Adjusted or corrected efficiency (outdoor equivalent) n 0 Efficiency when T* = 0

315 APPENDIX IV PROPERTIES OF WATER AND AIR FOR ANALYSIS OF COLLECTOR TEST RESULTS

316

317 A.IV.l PROPERTIES OF WATER from reference (38) Specific Kinematic Dynamic Temperature Density Heat Capacity Viscosity Viscosity T( C) pxl0~ 3 (kg/m 3 ) c (kj kg" 1 K -1 ) vxl0 6 (m 2 /s) uxl0 6 (Ns/m 2 ) A.IV.2 PROPERTIES OF AIR DENSITY Tables of data for air are available from many sources, but for reasonable accuracy in air collector testing it is appropriate to assume the following reference conditions for DRY AIR: P o = Pa T o = K P o = kg m -3 Density of Moist Air The density of moist air ( p v) may be derived from the reference data given above using the relation: PV = T o P o T Q P 6(l+x) (5+x) 301-

318 1 O /\i r o / where: 6 = rr- = )&'Q^qA = (dimensionless) a and x = specific humidity of air (kg water/kg dry air) SPECIFIC HEAT CAPACITY The specific heat capacity of dry air at constant pressure (cp)a varies between and kj/kg K over the temperature range from 0 to 100 C. This represents a change of approximately 0.5% which at the present levels of air collector testing accuracy is negligible. The specific heat capacity of water vapour (cp)v varies from to over the range from 0 to 100 C. This represents a change of approximately 1.5%. Where high humidity conditions occur at high temperatures, this could cause variations of a similar magnitude in the specific heat capacity of moist air, but under common European weather conditions the variation in the specific heat capacity of water vapour is usually small enough to permit the use of an average value. (Typical values of specific humidity are less than 0.1 for most European weather conditions, which would limit the variations in the specific heat capacity of moist air to no more than ^ 0.15%). Specific Heat Capacity of Moist Air The specific heat capacity of moist air (cp )w may be calculated using the relation:,., _ (c P )a + x (c P )v (c P )w - (1 + X) where x = specific humidity of air (kg water/kg dry air), and for most purposes the following average values may be assumed (c p )a = kj/kg K (c p )w = 1.87 kj/kg K -302-

319 APPENDIX V INSPECTION REPORT FORMAT SHEETS FOR THE RELIABILITY AND DURABILITY OF SOLAR COLLECTORS

320

321 Commission of the European Communities Solar Collector Testing Group Inspection report Reliability and durability of flat plate solar collectors name and address of installation author of report date institute

322 FLAT PLATE SOLAR COLLECTORS RELIABILITY AND DURABILITY HOW TO USE THIS INSPECTION FORMAT This inspection format is designed to allow useful information to be gathered on good design features of solar collectors, and their problems and failures in operating solar heating systems. The format includes several pages requesting general information of the inspected collector and the solar system in which it is working. It is important to give detailed information of the collector construction, the e materials used, and the installation. Information on surface treatment, sealing materials and gaskets is of special interest in order to help identi good design features. Small drawings and photographs of items such as the cover and the enclosure assembly, the flashing system, the pipework & connections, etc. would also be of help in evaluating collector construction. A blank sheet is included on the last page, which may be photocopied and used for extending particular sections of the inspection report. Before the inspection it may be helpful to study the checklists on pages 5, 6 & 8. These have been drawn up from previous collector inspection experience and analysis, and most previous problems and failures have been included. The questions on page.4- should also be studied before the inspection. Problems and failures'observed at the inspection should be referred to using the notation shown in the checklist on pages Future inspections of a solar collector installation may be reported by using pages 5 to 9, with the appropriate group cover labelled "Re-inspection, date: ". Only charges and new occurrances should be reported at these later inspections. Following the inspection, pages 7 & 9 should be used to more fully describe the problems, failures and good design features. Here one should also try to identify potential causes and possible remedies in production and installation. It is.important to record any part of the collector where no problems or failures were observed. A description of the successful details, and an explanation of why there were no problems, would be helpful and provide information for subsequent recommendations and design & installation guides. These inspection reports will be ultimately used to produce reports on collector reliability and durability, and provide information on the positive aspects of collector design, manufacture and installation. This format has been jointly designed by the I.E.A. and the C.E.C., and may therefore be used when reporting to either or both grcuos. The addropriats group cover should be used for identification.

323 FLAT PLATE SOLAR COLLECTORS l RELIABILITY AND DURABILITY Name & address of installation: Author of report: Institute: Date & time of inspection: DESCRIPTION OF THE SOLAR SYSTEM Description of location: (e.g. rural area, heavy industrial area, near a sea coast, near chemical works, etc.). Type of system: (water heating, space heating; etc.) Collector type used: (modules on roof, integrated modules or site-built collector) Manufacturer of collector: Installation contractor: Date of installation: Is there a guarantee for the reliability of the collector?: Total panel area & number of collectors: Orientation 4 tilt of collectors: Heat transfer medium, additives, flow rate: Safety system: (including freezing & boiling protection) Maximum pressure allowed in collector:

324 FLAT PLATE SOLAR COLLECTORS RELIABILITY AND DURABILITY SYSTEM FLOW DIAGRAM: DESCRIPTION OF THE SYSTEM SHORT EXPLANATION OF SYSTEM OPERATING MODES:

325 FLAT PLATE SOLAR COLLECTORS 3 RELIABILITY AND DURABILITY DESCRIPTION OF THE SOLAR COLLECTOR DIAGRAM OF COLLECTOR (including cross sectional view & dimensions] PHOTOGRAPHS OF COLLECTOR ARRAY

326 FLAT PLATE SOLAR COLLECTORS RELIABILITY AND DURABILITY 4 Materials and manufacturing processes used for the collector Cover,material & thickness: Cover, method of sealing, sealants and gaskets used: Absorber, materials and-dimensions of plate & tubes, fabrication method: Absorber Coating: (details if possible) Insulation back, materials & thickness: Insulation sides, material i thickness: Collector enclosure, materials of back and edges: Collector enclosure, method'of assembly and sealants used: Installation of the collector Method of col lector attachment to roof: Method of sealing (flashing) collector into roof, material used: Method of sealing (flashing) between collectors, materials used: Pipework between collectors, materials, connections, insulation used: Pipework to storage, materials, connections, insulation used: Operation of the system Have there been any problems with the system controller? (e.g. placing of sensors): Have there been any problems with the pump?: Has the system suffered from air locks?: If there have been any interruptions in the operating of the system, have these affected the collectors? : Was the collector stagnation protected during construction?: Have the collectors ever been repaired?: Are there any plans for modification or repair in the future?:

327 FLAT PLATE SOLAR COLLECTORS 5 RELIABILITY AND DURABILITY CHECKLIST PROBLEMS & FAILURES FOUND OURING THE COLLECTOR INSPECTION To investigate this please use the following checklist Problems & failures known from experience and previous collector inspectio ns have been listed as a guide. OBSERVED, NOT OBSERVED and NOT POSSIBLE TO INSPECT should be refered to by V, X and ' /, respectively. The number of collectors in the array which suffer from any given problem or failure should be indicated in the frequency column of the checklist. Reference number Problems & Failures of the collector Observed: V Not observed: X Mot inspected: / Frequency etc. Cover Condensation on the inside of the cover Qutgassing, deposits inside the cover Dirt on cover surface Ageing (discolouration, cracking, etc.) Breakage or collapse of cover Sagging of cover 2. Absorber etc. Dirt or dust on absorber Corrosion on absorber surface Peeling 4 flaking of absorber surface Leakage of absorber Deformation of absorber Deposits or condensation on absorber 3 Cover enclosure assembly (sealants & ciaskets) etc. Cover loose or slipped from enclosure Assembly is leaking Degradation of sealants

328 FLAT PLATE SOLAR COLLECTORS 6 RELIABILITY AND DURABILITY Reference number Problems 4 Failures of the collector Observed: V Mot observed X Mot inspected: / Frequency 4. The Insulation etc. Degradation or expansion of Water in the insulation Others (specify) insulation etc. Enclosure Rain leakage other than through cover seals Corrosion- of the enclosure & fastenings Others (SDecify) etc. Mounting of collector Leaks into house. Rotten timber Failure of collector mountings Leakage of flashing Others (specify) 1_ " etc. Connections and piping Leaking pipes or pipe connections Poor pipe insulation Problems with thermal expansion Bad soldering or welding Problems with flow distribution (airlocks, etc.) Others (specify) etc. Other comdonents When problems or failures are ooserved at the inspection, a detailed description and evaluation should be given on page 7. When no proolems or failures are observed for particular comdonents, then a detailed description & evaluation of the good design features should be given on page 9.

329 FLAT PLATE SOLAR COLLECTORS 7 RELIABILITY AND DURABILITY Comments on the problems and failures of the collector Discuss causes & severity of each problem or failure using the Reference Number from the checklist. Suggest how such problems & failures might be avoided in future and how the existing collectors might be repaired. Please indicate if you think that any of the failures might have been predicted by using either (a) materials tests, or (b) durability tests of a collector module.

330 FLAT PLATE SOLAR COLLECTORS RELIABILITY AND DURABILITY CHECKLIST OF DESIGN FEATURES A. Condensation and ventilation Does the collector have ventilation holes?: Has condensation been prevented by suitable ventilation?: Does the collector enclosure have drain holes?: YES/NO B. Weather protection Does the cover & enclosure allow rain & snow to run off easily?: Does the cover withstand high wind loads, hail, rain, etc.?: Are the mountings adequate to withstand high wind loads?: Are all components protected adequately against weather & corrosion?: C. Internal protection Does the absorber have a durable absorber surface?: Are the cover and absorber protected from insulation outgassing?: Will the insulation materials be able to withstand the expected stagnation temperatures?: Does the system employ internal corrosion protection?: Are special provisions made for air venting?: D. General construction Does the design include provision for thermal expansion?: Has the collector been generally well designed & constructed?: Has the collector been properly installed?: Is the collector easy to maintain and repair?: Are all materials and components suitable?: E. Performance Is the collector operating satisfactorily?: Is the system reliable?: Is the collector performance affectea by observed problems 4 failures?: Are the users pleased with the system performance?: Is a satisfactory service life expected?: ExDected lifetime frcm the date of installation (please indicate below): Less than 3 years: j~~j years: f~j years: 3.-5 years: [~~J 10 - '15 years: [T More than 20 years: ~J, j Detailed description and evaluation of good design features should be given on page 9.

331 FLAT PLATE SOLAR COLLECTORS 9 RELIABILITY AND DURABILITY Detailed description and evaluation of the good design features Please discuss and describe all good design features in conjunction with the Checklists on pages 5, 6 & 8.

332 FLAT PLATE SOLAR COLLECTORS RELIABILITY AND DURABILITY

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334

335

336 Solar Energy R&D in the European Community Series A: Solar Energy Applications to Dwellings Series B: Thermo-Mechanical Solar Power Plants Series C: Photovoltaic Power Generation Series D: Photochemical, Photoelectrochemical and Photobiological Processes Series E: Energy from Biomass Series F: Solar Radiation Data Series C: Wind Energy Series H: Solar Energy in Agriculture and Industry D. Reidel Publishing Company Dordrecht / Boston / Lancaster for the Commission of the European Communities