SOLAR ABSORBERCOATINGS FOR UNGLAZED FACADES

Transcription

1 SOLAR ABSORBERCOATINGS FOR UNGLAZED FACADES Michael Köhl, Markus Heck Fraunhofer Institute for Solar Energy Systems, ISE, Oltmannsstr. 5, Freiburg, Germany Phone Number +49 (761) , Fax Number +49 (761) , address Mike@ise.fhg.de Ueli Frei Solar Energy Testing and Research Group, ITR, Oberseestr. 10 CH-8640 Rapperswil,Switzerland Phone Number , Fax Number , address Markus Arntzen, Interpane E&B, Interpane E & B, Sohnreystraße 21, D Lauenförde, Germany, Phone Number: +49/5273/ , Fax Number: 05273/ , adress: markus.arntzen@interpane-eub.de Angel Morales, CIEMAT-FER, Avda. Complutense 22, ES Madrid, Spain Phone Number: , Fax Number: , adress: angel@ciemat.es Boris Orel, Leon Kaluza, NIC, Kemijski Institut - National Institut of Chemistry, Hajdrihova 19, PO - Box 30, SLO Ljubljana Slovenia, Phone Number: , Fax Number: , boris.orel@ki.si Richard Versluis Building and Construction Research, TNO, P.O.Box 49 visiting address: Schoemakerstraat VK Delft 2600 AA Delft (the Netherlands), Phone Number , Fax Number , address R.Versluis@bouw.tno.nl ABSTRACT - The thermal performance of buildings has improved enormously due to the new regulations in most EU countries during the past decades. The energy needs for heating and cooling of low-energy buildings achieved a level close to self-sufficiency. A major share of the existing building stock in Europe will be used for more than 20 years and renovation of these buildings towards a low energy level is a major challenge for the coming decades. Here exists an ideal application for unglazed solar facades allowing direct solar energy conversion and well-controlled energy management. The thermal energy could be used either for air-heating systems or desiccant cooling devices, or stored in heat storage units. Solar facades offer the opportunity to cover more heating and cooling demand with solar energy, thus enabling a large reduction of the primary energy consumption in the built environment. Moreover, solar air systems offer the possibility to improve the indoor environment without increasing the energy demand and at low extra costs. The most important drawbacks for the widespread introduction of unglazed solar facades lay in the black appearance, the low performance, non-industrialised production and poor durability. Spectrally selective surfaces are needed in order to enhance the performance. An increase of the energy gain up to more than 100% could be achieved. A minor reduction in solar absorptance (20% in maximum) due to appropriately achieved coloration is necessary for acceptance by owners and architects. New coating technologies offer a large-scale production in an environmentally friendly process, but reliable and well-accepted test methods to judge their durability and performance in this new application are lacking. Research capacity for the three most promising approaches for a technological development of coloured, durable spectrally selective absorber coatings for unglazed outdoor application for mass-production or individual application (sputtering, sol-gel technique and selective paints), highly experienced industrial companies in this field, world-wide experience on durability assessment and service lifetime prediction, and experts on the integration of solar facades are brought together in the EU-Joule project Solar building facades. This will enable a mass-production orientated breakthrough for suitable absorber coatings applied to well-defined products, which fulfil all the building integration requirements aiming at low extra costs.

2 The state of the art of the project and the most promising results in coating development are reported. Outdoor-monitoring of samples with different thermal emittance and climatic ambient and micro-climatic conditions yielded not only a base for durability testing but also an indication of the orientation-dependent yearly performance of unglazed facade collectors. Objectives of the project The main objectives are: The development of coloured, durable spectrally selective absorber coatings for unglazed outdoor application for mass-production or individual application. The industrialised production of solar facades by improving the welding and roll-forming technology for a high-quality product with a well-predicted and long service lifetime. Well-accepted products for the retrofit and new building market. Guidelines for the building integration. 1. INTRODUCTION Sol-gel synthesis in combination with dip-coating or slip-casting technique is quite useful for the preparation of various types of thin films and coatings with different properties. Surprisingly there are no commercially available sol-gel solar absorber coatings with spectral selectivity. Conversely, optically transparent antireflecting sol-gel coatings for solar energy applications are entering the market. There are no obstacles from the fundamental point of view that spectrally selective solar absorber coatings could not be made with spectral selectivity comparable to sputtered or electrochemically deposited solar absorber coatings. Technologies of the sol-gel coatings deposition exist in Europe which gives basis for the deposition of novel coatings which are developing in the frame of this project. Nevertheless, before this project has been started there were virtually no research done in this field. We focused our studies on sol-gel multicomponent coatings based on transition metal oxides of Mn, Cu, Fe and Co because of their black colour and weak IR absorption, and on metal/ceramic cermets. Thickness insensitive spectrally selective (TISS) paint coatings for solar facades have not been made yet albeit attempts were done in the past in view of obtaining disguising paints for thermal IR range. TISS paints for solar facade need to have extremely good weather resistance assuring long exploitation period. In addition, the application should be done with the existing technologies (spraying, slip-casting) and should be free of pollutants. From these reasons, water based silicone resin binders combined with bronze pigments represent the best choice for such coatings. 2. DEVELOPMENT OF FACADE ABSORBER COATINGS 2.1. Development of durable sputtered absorber coatings Based on the known optical properties of several materials, calculations of the reflectance spectra of different types of layer systems have been made. Aim was to evaluate the behaviour of the resulting absorptance with the shift of the color for multi layer systems. From the calculated reflectance spectra, the absorptance in the solar region was calculated for several thickness of the top layer. The maximum of reflectance is shifting with the thickness of the top layer. It is possible to shift the maximum of reflectance through the whole visible region. Within certain limits, the resulting emissivity of the absorber is not affected. Changing the base layers will lead to a different shape of the reflectance curve but does not change the general behaviour of the system. 1) Standard coating: Changing the top layer thickness of the coating, the optimum performance of 95 % absorptance is reached with a blue coloured coating. Shifting the colour to red will lead to a decrease in absorptance of about 10 %. Emissivity will not change. 2) Optimised coating: keeping the absorbing first layers and changing the top layer by using standard values of silicon-dioxide leads to a better overall performance of the absorber coating. Using the absorbing layers of the sunselect coating, two top layer materials are tested right now. The base layers had been produced on the production machine and then the overcoat was done in the lab coater. Two materials were used as top layers with an index of refraction (in the visible range) between 1.5 and 1.7. We produced series with varying thickness to optimise the absorptance. The samples are now tested for degradation behaviour. The achieved absorptance is between 93 % and 94 % for the non-optimised systems, the emissivity is about 5% for all samples. Samples of the standard coating with additional interlayers and changed stoechiometric composition were prepared on the production machine providing large area samples for outdoor testing Development of durable absorber coatings made with pigments in paints The main objective of the work done upto now was to find out which of the resin binders currently use for

3 making paints for building walls could be considered for making thickness-insensitive solar selective (TISS) paint coatings. We have found two resin binders: Dural, based on chlorinated rubber and Colofas, which is based on polyacrylate resin which are investigated now. Chlorinated rubber resin (Dural) was used throughout this work as a resin binder for paints. Dural is currently used as a binder for coatings to protect building facades and other objects exposed to severe weather conditions therefore fulfilling high demands for solar facade coatings. We used coated Al pigments which exhibit different colours and accordingly their s values vary from Paints were prepared by varying the pigment-tovolume-concentration (PVC) ratio of the black pigment incorporated in Dural resin into which the chromatic pigments were added in different proportions. Lowering the content of black pigment concurrently decreases the s values. values vary in the same way showing the lowest values for high loadings of chromatic pigments, however, the variations are not as high as found for s values where the differences between the highest in the lowest as values are nearly 10 %. This proves that the addition of black pigment is prerequisite for obtaining acceptable s / values. It is realistic to expect that the optimised paints will exhibit s > 0.8 and < 0.5. In future resin binders with less intensive phonon absorption will be used (Colofas for example) while the s values will be enhanced with the increased amount of black pigment. We may sum up, that chromatic pigments are very promising for TISS paints for facade coatings because they combine aesthetic appearance with moderate solar energy conversion properties Development of absorber coatings made with solgel technology a) Spinel-type coatings Our investigations of black spinel films with a composition Cu(Fe)Mn 2 O 4 with or without silica or zirconia underlayer and the corresponding particulate films prepared via the Stöber synthesis revealed that Cu(Fe)Mn 2 O 4 /SiO 2 coatings, albeit having high solar absorptances, exhibited notoriously too high thermal emittances due to the SiO 2 addition. The use of zirconia as a substitute for the silica in Cu(Fe)Mn 2 O 4 coatings seemed promising and attempts were made for their preparation. When we experimented with Cu(Fe)Mn 2 O 4 films we found that under more severe conditions of thermal hydrolysis the Cu-, Fe- and Mnacetate precursors reacted in thixotropic gels which after the dissolution with ethanol allowed to dip-coat Cu/Fe/Mn-oxide mixed films. The main advantage of these type of coatings is their excellent adherence to metal substrate attained without the silica or zirconia addition. At the same time their optical properties were improved because films exhibit much weaker absorption peaks in thermal IR range but despite their black colour they exhibited a reddish hue due to the Fe addition. Therefore, the covering efficiency and corresponding s values of mixed Cu/Fe/Mn-oxide films are still not adequate because mixed oxides are formed instead of Cu(Fe)Mn 2 O 4 spinel. To avoid reddish hue of Cu/Fe/Mn-oxide films we prepared Cu(Co)Mn 2 O 4 spinels because they exhibit less intensive modes in the IR spectral range and in addition to that, the electronic structure of Co (inter band transitions) makes possible to achieve higher absorption in the visible spectral range as compared to that of mixed Cu/Fe/Mn-oxides. From these reasons we prepared Cu(Co)Mn 2 O 4 spinels following the same solgel route as that used for making mixed Cu/Fe/Mnoxide films. Results revealed that reaction of the acetate precursors resulted in Cu(Co)Mn2O4 spinel films and powders with even better optical properties as that of Co(Fe)Mn2O4 spinel films with the added silica or zirconia. First attempts showed that films were relatively thin when dip-coated only once and 2-3 layers were needed to reach s 0.9, but values were below few %. Experiments made with films having the addition of hydroxypropylcelulose (HPC) which served as a thickening agent confirmed that thicker films could be obtained by using just a single dip-coating cycle. b) CERMET-type solar absorber coatings produced by sol-gel processes Sol-gel technique is used to produce layers of pure or mixed dielectric materials, but it is possible to introduce metallic salts in the precursor solution to prepare cermets or composites. This procedure is widely used in coloured coatings for building glazing with metals such as cobalt iron or other transition metals. In order to produce cermets for solar applications it is necessary to have metal atoms in their reduced state, not metallic ions. There are three sol-gel applications that incorporate metallic ions to gels: catalysts, materials for non-linear optical properties and coloured glasses. These materials present the same structure than solar selective absorbers, but metallic content is quite low (<1-2%). During this work, several materials (Cu/SiO 2, Pt/SiO 2, Ni/Al 2 O 3, Pt/Al 2 O 3, Ag/TiO 2 ) have been produced using different metal particles and dielectric matrices with similar results.

4 3. DURABILITY ASSESSMENT Monitoring of the stress levels for facades in outdoor durability test sites One of the basic requirements to understand the service condition of a material is the measurement of the microclimate. The appropriate design of any accelerated ageing test is only possible, if microclimatic long term data is available. If the material will be used in different climates, the belonging microclimates must be determined. The influence of the macroclimate is often underestimated. Urban, industrial or seaclimate is influencing the microclimate drastically. To evaluate the microclimate of facade elements, adequate test samples have been designed. A minimum sample size is needed for appropriate results. Also the thermal characteristics must be comparable to typical designs of real solar facades. The microclimatic data must first of all represent the typical use, but also a wide range of typical performance of facade elements. Therefore test samples with different coatings are used: Black paint Solar selective surface (black chrome) The substrate material of the test samples is stainless steel (1.4301). The black chrome coating is applied by the Swiss company Energie Solaire SA. The black paint samples were prepared by a stove enamelling finish. The samples have been distributed to the following participating countries: Germany Netherlands Spain Switzerland All samples have been equipped with PT-100 temperature sensors and thermal insulation on the back side. The samples have been monitored for about 6 month. Under 4 different orientations (south vertical, south 45, east and west vertical), 3 different facade samples (low, medium and high solar selective surfaces) are exposed. Besides the sample temperature and wetness, a number of environmental parameters are measured. The outdoor-monitoring at different test-sites yields an estimate of the variation of stress and performance of the solar facade coatings at different climates. The frequency distribution of the solar irradiation for August 1999 in Freiburg and Rapperswil is presented in Figure 2: Insolation south-oriented with two different inclination angles at ISE (Germany) and SPF (Switzerland)2: Generally the results are in good agreement, as could be expected for theses locations, demonstrating the reliability of the experimental set-up and the monitoring. Figure 1: Temperature frequency distribution of spectrally selective solar facades south-oriented with two different inclination angles at ISE (Germany) and SPF (Switzerland) shows the stagnation temperatures of an unglazed selective solar facade coating, which could be reached during summertime, with different tilt angles. 100 Frequency [h] , Temperature [ C] ISE (45 ) ISE (90 ) SPF (45 ) SPF (90 ) Figure 1: Temperature frequency distribution of spectrally selective solar facades south-oriented with two different inclination angles at ISE (Germany) and SPF (Switzerland) 100 log Frequency [h] , Insolation [W/m²] ISE (45 ) ISE (90 ) SPF (45 ) SPF (90 ) Figure 2: Insolation south-oriented with two different inclination angles at ISE (Germany) and SPF (Switzerland)

5 4. BUILDING INTEGRATION The dissemination of unglazed solar facades is stimulated by fundamental research on the performance and product requirements for integration and application. Both constructional and durability aspects (buildability) as well as cost related and functional requirements need to be taken into account. There is a high potential for unglazed solar facades for air-heating systems as well as for solar assisted cooling systems (such as desiccant systems). Performance is determined by a large number of interrelated and dynamic factors such as energy demand, energy supply, temperature level, storage etc. It must be recognised that building regulations, climatic conditions and comfort standards which vary from country to country have their impact not only on the constructional and durability requirements of the products but that they may influence the performance for various applications as well. Figure 3: Performance of solar assisted DEC.

6 Yearly energy yield (GJ/m 2 ) Absorbtivity/emissivity ratio (-) Figure 4: Yearly energy yield of an unglazed collector for low temperature applications Performance and applications are furthermore influenced by the temperature level of the energy supply and/or energy storage. The temperature level of the energy supply is directly related to the spectral selectivity of the absorber coatings. A high absorptance/emittance ratio is favourable, but from an architectural point of view colour, shape and appearance of the absorber are equally important. Colouring decreases the performance of the application: the influence of this decrease (varying from application to application) on the performance is also subject of investigation of this project. Performance requirements for three applications are analysed: pre-heating of ventilation air for offices, DEC cooling system and DHW system. Figure 3: Performance of solar assisted DEC. shows the results of the performance analysis of a solar assisted DEC cooling system and the potential of a solar assisted DEC cooling system for an office building with high cooling demand in the summer. The overall coefficient of performance of the solar assisted DEC system is 2.5. Figure 4: Yearly energy yield of an unglazed collector for low temperature applications shows an example of the yearly energy yield of an unglazed solar collector for low temperature applications. 5. REFERENCES L. Kaluza, B. Orel: Spectroscopic Studies of Mn-, Cu-, Fe-Mixed Oxide Thin Films and Powders Prepared via Sol-Gel Route, Book of Abstracts, Spectroscopy in Theory and Practice, 11th International Symposium, Bled, April 11-15, L. Kaluza, A. Surca, B. Orel, G. Drazic: Structural and IR spectroscopic Analysis of Sol-Gel Processed CuFeMnO 4 Spinel and CuFeMnO 2 /Silica Films for Solar Absorbers, submitted to Journal of Sol-Gel Science and Technology. L. Kaluza, B. Orel, M. Köhl: Sol-Gel Derived Cu(Co)Mn 2 O 4 Spinel Coatings for Solar Absorbers: Structural and Optical Properties, submitted to Sol. Energy Mater. & Sol. Cells. 6. ACKNOWLEDGEMENT This work is partly funded by the EU under contract Contract No. JOR3-CT :