HEAT PUMP FUNDAMENTALS

Transcription

1 HEAT PUMP FUNDAMENTALS

2 NATO ADVANCED STUDY INSTITUTES SERIES Proceedings of the Advanced Study Institute Programme, which aims at the dissemination of advanced knowledge and the formation of contacts among scientists from different countries. The series is published by an international board of publishers in conjunction with NATO Scientific Affairs Division A Life Sciences Plenum Publishing Corporation B Physics London and New York C Mathematical and D. Reidel Publishing Company Physical Sciences Dordrecht and Boston D Behavioural and Martinus Nijhoff Publishers Social Sciences The Hague, Boston and London E Applied Sciences F Computer and Springer-Verlag Systems Sciences Berlin, Heidelberg, New York G Ecological Sciences Series E: Applied Sciences - No. 53

3 HEAT PUMP FUNDAMENTALS Proceedings of the NATO Advanced Study Institute on Heat Pump Fundamentals, Espinho, Spain, September 1-12, 1980 edited by J. Berghmans Associate Professor Catholic University of Leuven Mechanical Engineering Department Leuven. Belgium 1983 Martinus Nijhoff Publishers The Hague / Boston / London

4 Distributors: for the United States and Canada Kluwer Boston, Inc. 190 Old Derby Street Hingham, MA USA for all other countries Kluwer Academic Publishers Group Distribution Center P.O. Box AH Dordrecht The Netherlands I.ibnry of (..n~<... Cual..~in~ in Publ;<o,;o" 1)0'0 NATO Advanced Study Institute on Heat Pump Fundamentals (1980, Espinho, Portugal) Heat pump fundamentals. (NATO advanced study institutes series. Series E, Applied Sciences; no. 53) 1. Heat pumps--congresses. I. Berghmans, J. II. Title. III. ~r1es, NAro advanced study institutes series. ~r1es E, Applied sciences; v. 53. TJ262.N '5 82~245l7 ISBN-I}: 978' e ISBN-13: 978'94-009""68,8'9 DOl: '0,100']/ <>0<) Copyright 1983 by Martinus Nijhoff Publishers, The Hague. Softcover reprint ofthe hardcover 1st edition 1983 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, mechanical, photocopying, recording, or otherwise, without the prior wrirren permission of the publishers, Martinus Nijhoff Publishers, P. O. Box 566, 2501 CN The Hague, The Netherlands.

5 v TABLE OF CONTENTS Preface J. Berghmans Introduction J. Berghmans vii ix H.F. Sullivan Energy analysis - Thermodynamic performance criteria 1 H.F. Sullivan Principles of vapor compression heat pumps 14 J. Durandet Vapour compression heat pump fluids - properties and selection 34 P. Paikert Calculation and optimization of condensers and evaporators for heat pumps J.H. Grimm Heat pump compressors F.W. Ahrens Heat pump modeling, simulation and design E. Macchi Prime movers for vapour compression heat pumps A. Rojey, J. Durandet Heat pump operating with a non-azeotropic fluid mixture P.A. Kew Developments in vapour compression heat pumps

6 VI P.A. Kew The industrial application of heat pumps J. Berghmans Domestic heat pump applications H. Auracher, H. Glaser, K. Stephan Thermodynamics of mixtures and elementary processes in absorption heat pumps H. Glaser, H. Auracher Theoretical cycles of absorption heat pumps K. Stephan Absorption heat transformer cycles K. Stephan Working substances for absorption heat pumps and transformers I.E. Smith Heat pump absorbers A.M.S. Qasrawi Absorption heat pump generators A.M.S. Qasrawi Absorption heat pump design H. van der Ree, P.A. Oostendorp Resorption heat pumps G. Cohen, A. Rojey Absorption heat pump developments

7 VII PREFACE This book contains the texts of the lectures which were given at the Nato Advanced Study Institute on Advanced Heat Pumns which was held at Espinho, Portugal in September A previous NATO Advanced Study Institute on the topic of heat pumps had been held in The significance of heat pumps with respect to energy conservation was the main topic of this Institute. In 1980 it was felt that considerable research had to be done in order to be able to produce more energy efficient, less costly and more widely ap~licable heat ~umos. This requires a good understanding of the functioning of the types of heat pumps available. The simultaneous coverage of the basic fundamentals of heat pumps of different drive in one lecture series therefore was the goal of the 1980 Advanced Study Institute. Only a few lectures were devoted to heat pump applications. The lectures on heat pump applications were intended to ~ive only a short overview. They were supplemented by lectures on the latest developments on vapour compression as well as sorption systems. Because of time limitations the number of topics which could be treated in depth had to be restricted. It is hoped however that the present book will provide the reader with a clear insip,ht into the phenomena upon which the operation of mechanically and thermally driven heat pumps is based and thus may assist him in his research.

8 \TIll In the first place the editor wishes to thank all the authors for their collaboration. The fine efforts of mrs. F. Decoster who typed the mannscript and mr. J. Beutels who assisted in the editing are gratefully acknowledged. Particular thanks are also due to dr. T. Kester whose assistance was invaluable when organizing the Institute. J. BERGHMANS Heverlee, Belgium September 1982.

9 IX INTRODUCTION J. Berghmans Katholieke Universiteit Leuven, Heverlee, Belgium It has become co~on practice now to call a heat pump any device which extracts heat from a source at low temperature and gives off this heat at a higher temperature level where it can be used beneficially. It should be pointed out that the purpose of the device is to create heat at a high temperature level. If it is the purpose to extract heat from a low temperature source, the device is called a refrigeration system. In most processes the production of heat at a high temperature level can be traced back directly to the consumption of primary energy (fossil or nuclear fuels, wind or solar energy... ). A heat pump only upgrades heat and does not necessarily directly require a source of primary energy. Thus the heat pump shows to be an important instrument in the fight for energy conservation. This also explains the sudden breakthrough of heat pumps in several areas since the advent of the energy crisis. However the second law of thermodynamics requires the presence of a third energy source in order for the heat pump to be able to achieve the "unnatural" transfer of heat from 1m. to high temperature. If this auxiliary source delivers mechanical energy then the heat pump is of the mechanically driven type. If the auxiliary reservoir delivers heat then the heat pump is of the thermally driven type. Almost all heat pumps belong to one of these two classes. First of all it should be pointed out that there is a fundamental difference between mechanically and thermally driven heat pumps. The former rely on high grade energy. The efficiency with which this driving energy can be produced from primary energy is low.

10 x Primary energy conservation with. mechanically driven heat pumps therefore very often is difficult to achieve unless special heat recuperation techniques can be applied. Thermally driven heat pu~s on the other hand can be operated directly by means of heat, without the inefficient conversion to mechanical energy. In addition there is some added flexibility from the side of the temperature level of the auxiliary heat source. It is possible in fact to drive a heat pump with waste heat: i.e. with a source at a temperature lower than the temperature of the useful heat to be produced (the heat transformer). To arrive at a good understanding of the nature of the different heat pumps, the thermodynamic aspects of the upgrading of heat are discussed in the first two lectures of this course. In particular it will be shown how the concept of exergy can be used to compare heatin8 systems. The following lectures are devoted to mechanically driven heat pumps. First the thermodynamic cycles of vapour compression heat pumps are presented and analysed. It is shown how the performance of the heat pump depends upon the parameters defining the thermodynamic cycle (source temperatures, compressor performance.. ). This is followed by a discussion of the requirements which the working fluid of a vapour compression heat pump has to satisfy. Suitable choices of working fluids are presented. Once the thermodynamic cycle is defined, the different components of a vapour compression heat pump are dealt with. In particular evaporator, condenser and compressor are discussed in detail. Special attention is given to the sizing and optimisation of the first two components. The compressors used in heat pumps are discussed subsequently. The elements leading to the choice of a particular compressor are presented. Vapour compression heat pumps presently are encountered in a multitude of applications. Many of these are characterized by widely varying temperatures of the heat sources (e.g. domestic heating). A good design has to take such changes into account. For this reason considerable attention is given in this course to heat pump optimiation and design. An important step towards optimization of a heat pump system is the modeling of the heat pump itself. It is by means of accurate models of the heat pump system that off-design performance can be predicted. Presently most vapour compression heat pumps, as they are used in the domestic sector for instance, are driven by electric motors.

11 XI Due to the low efficiency with which electricity is produced it is difficult to achieve primary energy conservation with these heat pu~ps compared to alternative heating systems (gas, oil boilers). Obviously the heat pump compressor can be driven by other prime movers than electric motors. In this course possible ~rime movers for vapour compression heat pumps are discussed. Special attention is also given to heat pum~s driven by internal combustion engines (gas, oil). Such heat pumps make it possible to achieve high primary energy efficiency by recuperation of engine waste heat. Improved performance of vapour compression heat pumps can be achieved also by decreasing the temperature differences occurring in condenser and eva~orator. This can be achieved by the use of non-azeotropic mixtures as working fluids. The advantages of this technique are discussed. Only the most interesting applications of heat pumps in the domestic as well as in the industrial sector are described. The problems involved are briefly mentioned. The temperature at which most heat pumps are able to operate is too low for most industrial processes (e.g. process stea~ production). For domestic applications the problems are located at the low temperature source (e.g. outside air) and the use of costly electricity. In recent years thermally driven heat pumps have received considerable interest due to a number of features : - lack of compressor; - possibility for direct firing; - high primary energy efficiency; - long life expectancy due to the lack of components with moving parts; - no vibration or noise problems. For these reasons intense research efforts are beinr undertaken presently to develop such heat pumps. Most attention presently is going to the absorption heat pump. The phenomenon of vapour absorption in a liquid has been applied successfully in refrigeration equipment. However only very few absorption heat pumps presently are available on the market. The transition from cooling device to heating device, in contrast with vapour compression systems, has not been achieved thus far. It is the purpose of this Institute to provide researchers with a better insight in the processes which determine the performance of absorption heat pumps in order to promote the large scale

12 XII application of these devices. A profound understanding of the operation of an absorption heat pump requires acquaintance with the thermodynamics of binary mixtures. Based upon the properties of these mixtures, thermodynamic absorption cycles are developed and discussed. Distinction here is made with heat transformer cycles, the latter being characterized by the utilization of low temperature heat as driving heat source. Special attention is devoted to the requirements to be filled by the workinp, pairs suited for absorotion heat pumps and transformers. Absorption heat pumps, in addition to condensers and evaporators, require components such as : generators, rectifyers and absorbers. The principles underlying the operation of each of these components are discussed in detail. It is shown that design and optimization of these items are very complex and require extensive knowledge of the thermodynamic and thermal properties of the working pairs. Such knowledge often is lacking. Furthermore it will become apparent how a number of limitations orlglnating from the working pair influence the performance of the heat pump. It is felt that more suitable working pairs have to be developed. Absorption systems basically consist of a series of components in which heat and mass transfer occur. The size of these components necessary to achieve the required power level often is very large and leads to costly devices. Therefore it can be stated that optimization of heat and mass transfer together with the development of more suitable working pairs are the main areas in which research to promote the development of absorption systems should be performed. Resorption systems offer interesting prospects to be used as heating systems. The principles of operation of these systems are explained. Recent developments in this field are discussed. Throughout these lectures the need for further research and development work will be pointed out. This need is particularly urgent in the field of sorption heat pumps, where even basic problems as the choice of working pairs have not received conclusive answers. The enormous energy conservation potential which these heat pumps offer should be sufficient stimulation to undertake this task.