Application aspects of hybrid PVT/AIR solar systems

Transcription

1 Application aspects of hybrid PVT/AIR solar systems Y. Tripanagnostopoulos 1, M. Souliotis 1, R. Battisti 2 and A. Corrado 2 1 Physics Department, University of Patras, Patra 26500, Greece Tel/Fax: , 1 yiantrip@physics.upatras.gr, 2 Dept of Mechanics and Aeronautics, Univ. of Rome La Sapienza, Rome 00184, Italy Tel: , Fax: , 2 riccardo.battisti@uniroma1.it Hybrid PV/T systems with air heat extraction are an alternative and cost effective solution to building integrated PV systems, because of their easier construction and operation. These systems are usually consisted of PV modules with air channel at their rear surface, where ambient air is circulating in the channel for PV cooling and the extracted heat can be used for building thermal needs. To increase the system thermal efficiency, an additional glazing is necessary, but it has as result the decrease of the PV module electrical output from the additional optical losses of the solar radiation. An extensive study on air cooled PV/T solar systems has been conducted at the University of Patras, where hybrid PVT/AIR prototypes have been experimentally studied in their standard form and also with a low cost modification. The methodology of Life Cycle Assessment (LCA) has been used to do an energetic and environmental assessment of the heat recovery system by the University of Rome La Sapienza, implementing a specific software for LCA, SimaPro 5.1. In this paper we provide electrical and thermal energy output results for PV and PVT/AIR systems, analyzing them with respect to their performance improvements and environmental impact, considering their construction and operation requirements. PVT/AIR system concept The temperature of PV modules increases due to the absorbed solar radiation that is not converted into electricity causing a decrease in their efficiency. This undesirable effect can be partially avoided by applying a heat recovery unit with a fluid circulation. In hybrid Photovoltaic/Thermal (PV/T) solar systems the reduction of PV module temperature can be combined with a useful fluid heating. Hybrid PV/T systems can simultaneously provide electrical and thermal energy, thus achieving a higher energy conversion rate of the absorbed solar radiation. These systems consist of PV modules coupled to heat extraction devices, in which air or water of lower temperature than that of PV modules is heated whilst at the same time the PV module temperature is reduced. In PV/T system application electricity is of priority and therefore the operation of the PV modules at low temperatures keeps cell electrical efficiency at a sufficient level. This demand limits the effective operation range of PV/T system thermal unit in low temperatures and the extracted heat can be mainly used for low temperature thermal needs (space heating and natural ventilation of buildings, air or water preheating, etc). Hybrid PV/T systems with air heat extraction (for simplicity PVT/AIR) are more extensively studied, mainly as an alternative and cost effective solution to building integrated PV systems, because of their easier construction and operation. In typical BIPV applications the increase of PV module temperature results to the increase of undesirable heat transfer to the building, mainly during summer. Air cooled hybrid PV/T systems are usually consisted of PV modules with air channel at their rear surface and usually ambient air is circulating in the channel for achieving both PV cooling and thermal energy output, which 1

2 can be used for building thermal needs. In PVT/AIR systems the thermal unit for the heat extraction, the necessary pump and the external pipes for air circulation constitute the complete system that extracts the heat from PV module and brings it to the final use. To increase the system operating temperature, an additional glazing is necessary, but it has as result the decrease of the PV module electrical output from the additional optical losses of the solar radiation. Theoretical and experimental studies are referred to hybrid PV/T systems, with most of them including work on air heat extraction from the PV modules. Among the recent works we can notice the papers of Brinkworth et al (1997), on design and performance of building integrated hybrid PVT/AIR systems and of Hegazy (2000), who compares four PV/T air collectors. We also could refer the work of Eicker et al (2000), with the monitoring results from a BIPV PV/T system that operates during winter for space heating and during summer for active cooling and of Bazilian et al (2001) for the practical use of several PV/T systems with air heat extraction in the built environment. The building integrated PVs is going to be a sector of a wider PV module application and Lee et al (2001), Ito and Miura (2003) and Chow et al (2003), give interesting results on air cooled BIPV modules. University of Patras has been involved in the research of PV/T systems with work on water and air cooled photovoltaics towards the increase of electrical and thermal output of BIPV PVT/AIR systems. The work aims to air heat extraction improvements with modifications in PVT/AIR systems (Tripanagnostopoulos et al, 2000, 2001a). In addition, improved PV/T systems with dual (air or water) heat extraction operation (Tripanagnostopoulos et al, 2001b) and modeling results confirming the improvements of a modified air cooled PV/T model (Tripanagnostopoulos et al, 2002a) are recently presented. The electrical output of PV/T systems is of priority, as the cost of PV modules is some times higher than the thermal unit. The different performance of the two subsystems regarding temperature affects system cost and optimised modifications for both electrical and thermal efficient operation must be considered. The consideration of the environmental impact of PV modules by using Life Cycle Assessment (LCA) methodology has been presented for typical photovoltaic systems by several authors. LCA has been extensively used at University of Rome La Sapienza, starting with the PhD Thesis of Frankl (1996) on LCA for photovoltaic systems and followed by the study on the simplified Life-Cycle analysis in buildings (Frankl et al,1998), on the overview and future outlook of LCA for photovoltaics (Frankl, 2002) and also on the comparison of PV/T systems with standard PV and thermal systems confirming the environmental advantage of PV/T systems (Frankl et al, 2000). In the present paper we give results for system energy performance and environmental aspects by the LCA method for standard PV and air-cooled PV/T systems. The work is based on the combined evaluation, for PVT/AIR systems, of both the energy assessment, that is experimentally investigated at the University of Patras, and the environmental assessment, in terms of LCA results, performed at the University of Rome with the aid of a specific software (SimaPro 5.1). These results are referred to typical PV modules and to glazed and unglazed PV/T solar systems for horizontal and tilted building roof installation, including also modified systems. The use of a booster diffuse reflector between the parallel rows for the horizontal installations is also suggested to increase the solar input to the PV and PVT/AIR systems and the corresponding results are presented. The calculated energy performance and the LCA results can be considered useful as guidelines for the application of the studied standard PV and PV/T systems as well as the modified ones. 2

3 Experimental models In hybrid PVT/AIR systems the thermal unit for air heat extraction, the necessary pump and the external pipes for fluid circulation constitute the complete system that extracts the heat from PV modules and transfers it to the final use. Practical considerations in PVT/AIR system design include the evaluation of the thermal and electrical efficiency improvement with respect to the system cost. The hybrid PV/T systems consisting of PV modules freely exposed to ambient temperature without any thermal protection have high top thermal losses and therefore their operating temperature is not high. To increase the system operating temperature, an additional transparent cover is necessary (like the glazing of the typical solar thermal collectors), but this has as result the decrease of the PV module electrical output because of the increasing reflection and absorption of the solar radiation. In most PVT/AIR systems the air circulates through a channel formed between the rear PV surface and the system thermal insulation, and in some other systems through channels on both PV module sides, in series or in parallel flow. The usual heat extraction mode is the direct air heating from PV module rear surface by natural on forced convection and the thermal efficiency depends on channel depth, air flow mode and air flow rate. Small channel depth and high flow rate increase heat extraction, but increase also pressure drop, which reduces the system net electrical output for the forced air flow, because of the increase of the fan power. In applications with natural air circulation, the small channel depth reduces air flow and this results to an increase of PV module temperature. In these systems large depth of air channel (minimum 0.1 m) is necessary (Bhargava et al, 1991). The aim of our research is to design efficient PVT/AIR systems based on low cost modifications. The main concepts are concerned with the reduction of PV module temperature, the improved air heat extraction by the circulating air and the reduction of heat losses by using thermal insulation on system back side and edges. A low cost modification was investigated in University of Patras, by which satisfactory air heating, reduced PV module temperature and low increase of the opposite channel wall temperature can be achieved (Tripanagnostopoulos et al, 2000b). This modification consists of a thin flat metallic sheet (TFMS) placed inside the system air channel and along the air flow. This TFMS element doubles the heat exchanging surface area in the air channel, aiming also to the reduction of the heat transfer to the back side of the PVT/AIR system. The experimental models were constructed in the University of Patras and consisted of commercial type mc-si PV modules in combination with a laboratory made air heat recovery unit (HRU). The experimental study is based on two hybrid PVT/AIR module designs (Tripanagnostopoulos et al, 2002) of same aperture surface area A a (A a = 0.4 m 2 ). The first model is that of the simpler form without additional glazing, named PV/T UNGLAZED or PVT/UNGL and the second model is that with the additional glazing, the PV/T GLAZED or PVT/GL. These experimental models have an air duct of 10 cm width and a thermal insulation layer to avoid thermal losses from the non-illuminated system surface sides. We consider that these systems can be installed on the horizontal roof, on the tilted roof or on the façade of a building. Horizontal and tilted roof installation are more interesting at low latitudes, while building façade (and high tilted roof) installation are more effective for medium and high latitude applications because of the lower sun altitude angles. In the case of tilted roof system installation, the PV/T systems are placed one beside the other and on the southern building inclined roof surface. The tilted roof integrated systems are additionally thermally insulated on their rear surface, compared to the ones installed on horizontal roof, as they are attached on the tilted roof. The additional thermal protection 3

4 increases the thermal efficiency of the system, but the lower thermal losses keep the PV temperature at a higher level, operating therefore with reduced electrical efficiency. The systems that are considered for installation on the tilted building roof are the standard PV modules and the usual type PVT/AIR systems. The experimental models that simulate the PV and PV/T systems for the tilted roof installation, are the PV-TILT, PVT/UNGL-TILT and PVT/GL-TILT models. They are consisted of the same basic system units (PV, PVT/UNGL and PVT/GL) and were tested with an additional thermal insulation on their rear surface, considering the thermal protection of the tilted roof to the attached system thermal losses from this surface. Considering PV/T solar systems installed on horizontal building roof the parallel rows keep a distance from one row to the other, in order to avoid PV module shading. We suggest the use of stationary flat diffuse reflectors (REF) placed properly between the PV modules from the higher part of the one row of them to the lower part of the next row. This installation increases solar input on PV modules almost all year, resulting to an increase of electrical and thermal output of the PV/T systems. The systems that combine the PVT/AIR modules with diffuse reflector are the models PVT/UNGL+REF and PVT/GL+REF. The suggested diffuse reflectors differ to the specular reflectors, as they avoid the illumination differences on module surface and the reduction of the electrical efficiency, because they provide an almost uniform distribution of reflected solar radiation on PV module surface. The systems were tested with slope equal to the latitude of Patras (38.25 o ). In the experiments with the diffuse reflector, the PVT/AIR systems were tested for variable additional solar radiation (concentration ratio CR=1.0 to CR=1.5) to get data for different angles of incidence between system and sun. Aiming to the increase of PVT/AIR performance three prototypes with the TFMS modification were constructed and tested by University of Patras. These models were all of unglazed type and were distinguished in PVT/TFMS, the simple type for horizontal roof, the PVT/TFMS+REF, the system with the diffuse reflector and the PVT/TFMS-TILT, the system for tilted roof installation. The systems consisted of the same basic unit (mc-si PV module and HRU of 10 cm air channel) with an additional thin aluminium sheet in the middle of the air channel and parallel to the air flow. Experimental results The study of the standard PV system and the hybrid PVT/AIR systems includes outdoor tests for the determination of the steady state electrical efficiency el of the corresponding PV modules of all systems and the thermal efficiency th of the PVT/AIR models. The electrical efficiency el of PV modules depends on their temperature (T PV ) and the incoming solar radiation and it is calculated by the measured data as: el =I m V m /GA a, where I m and V m are the current and the voltage of PV module operating at maximum power and G the irradiance on the system aperture plane. Test results showed that the el for PV, PVT/UNGL and PVT/TFMS type models was: el = T PV, while for PVT/GL type models was: el = T PV. The value of T PV for the standard module was calculated by the equation: T PV = (G-300)+1.14(T a -25) (Lasnier and Ang, 1990) that correlates T PV with the parameters G and T a. The T PV of the corresponding modules of PV+REF, PV-TILT and of all PVT/AIR systems was based on modified formulas of the above equation, which give approximately the PV module temperature and are presented in Table 1. These modified formulas were experimentally validated and correspond to the increase of PV operating temperature due to the reduced heat losses to the ambient. 4

5 Table 1. PV temperature and Thermal efficiency of all PVT/AIR systems SYSTEMS PV temperature Thermal efficiency (T i =T a ) PV T PV = (G-300)+1.14(T a -25) - PV+REF T PV = (G*CR-300)+1.14(T a -25) - PV-TILT T PV = (G-150)+1.14(T a -15) - PVT/UNGL T PV = (G-200)+1.14(T a -25) th =0.36 PVT/UNGL+REF T PV = (G-200)+1.14(T a -25) th =0.56 (CR=1.3) PVT/UNGL-TILT T PV = (G-200)+1.14(T a -20) th =0.37 PVT/GL T PV = (G-200)+1.14(T a -25) th =0.59 PVT/GL+REF T PV = (G-200)+1.14(T a -25) th =0.71 (CR=1.3) PVT/GL-TILT T PV = (G-200)+1.14(T a -20) th =0.60 PVT/TFMS T PV = (G-250)+1.14(T a -25) th =0.41 PVT/TFMS+REF T PV = (G-250)+1.14(T a -25) th =0.57 (CR=1.3) PVT/TFMS-TILT T PV = (G-250)+1.14(T a -20) th =0.42 The thermal efficiency th of the PVT/AIR models depends on the incoming solar radiation (G), the input air temperature (T i ) and the ambient temperature (T a ). During tests for the determination of system thermal efficiency, the PV modules were connected with a load to simulate real system operation and to avoid PV module overheating by the solar radiation that is converted into heat instead of electricity. The steady state thermal efficiency th of the tested hybrid PVT/AIR solar energy systems is calculated for the operating conditions with the lowest thermal losses (T i =T a ) by the equation: th = m& C p (T o -T i )/GA a, where m& is the fluid mass flow rate, C p the fluid specific heat, T i and T o the input and output fluid temperatures and A a the aperture area of the PV/T model. The results are presented in Table 1 for all studied systems. The experimental results for the thermal efficiency of the PV-TILT and PVT/AIR-TILT type systems were extracted from the tests where an additional thermal insulation sheet was mounted on the back of these systems, to simulate the tilted roof. In the calculation of the electrical and thermal output of the compound systems PV+REF and PVT/AIR+REF, we considered the net solar radiation on the aperture surface of PV modules and not the additional on the reflector, in order to have direct comparative results with the standard installation mode of the systems. The calculation of thermal efficiency th (for T i =T a ) of PVT+REF systems varies from a minimum value for December (CR=1.0) up to the maximum value for June (CR=1.3). Annual performance of PV and PV/T systems The electrical efficiency from the tests of standard PV modules and also the electrical and thermal efficiencies of all PV/T models were used to calculate the monthly energy output and from them the annual values for the weather conditions of Patras. In the calculations we considered PV and PV/T system slope equal to the latitude of Patras for both horizontal and tilted building roof installation. The complete systems include the necessary additional components (Balance Of System, BOS, for the electricity and the BOS for the heated air circulation) and therefore the final energy output is reduced due to the electrical and thermal losses from these systems. Estimating a minimum energy reduction of about 15% for the PV electrical energy that is converted in electricity (inverter), we take a value for BOS equal to 85% to calculate the final electrical energy output of the PV and PV/T systems. Regarding system thermal part, we consider also efficiency 85% for the final heat output (tubes, fan, etc) and we take these new values as the final use energies. The annual energy output per m 2 of the considered PV modules and of the PV/T systems are 5

6 included in Table 2. The calculated values for PV and PV/T systems with BOS of 85% for both electrical and thermal system parts are calculated considering the annual solar input ( kwhm -2 yr -1 ) on the plane of the PV module for Patras. The main scenarios for the practical use of the heated air are the following: 1. The heated air is used for twelve months. This consideration is for reference purposes and corresponds to application of annual needs in heated air, as for example in some industries. 2. The heated air is used for six months (November to April), while the rest six months (May to October) the heated air is ejected to the ambient, cooling the PV modules only. This consideration corresponds to typical PVT/AIR applications, as the space heating of buildings. 3. The heated air is used twelve months, six months (November to April) for the effective use of air (e.g. for space heating of buildings) and six months (May to October) for water preheating through heat exchanger. The thermal output in water preheating is lower than that of the air heating only, as there are additional thermal losses in the airwater heat exchanger. From the given results we observe that the total energy output (electrical plus thermal) of PVT/AIR systems is higher than that of the standard PV modules, while regarding only the electrical output, it is higher only of PVT/UNGL-TILT and PVT/TFMS-TILT systems. The suggested TFMS modification in the air channel results to higher total energy output compared to the PVT/UNGL type systems and to higher electrical output compared to PVT/GL type systems. The practical use of the heated air only for six months is not efficient enough (almost 40% of that from the reference mode of the 12 months). In the case of water preheating for the rest six months, the total thermal energy output can be considered satisfactory (about 75% to that of the reference mode of the 12 months). Table 2. Annual electrical and thermal output of all studied PV and PVT/AIR systems ENERGY OF SYSTEMS ANNUAL ELECTR KWh yr -1 (per m 2 ) 12 MO THERM kwh yr -1 (per m 2 ) 12 MO ELECTRIC kwh yr -1 (BOS 85%) (30 m 2 ) 12 MO THERMAL kwh yr -1 (BOS 85%) (30 m 2 ) 6 MO THERMAL kwh yr -1 (BOS 85%) (30 m 2 ) PV PV+REF PV-TILT MO AIR+ 6 MO WATER THERMAL kwh yr -1 (BOS 85%) (30 m 2 ) PVT/UNGL PVT/UNGL+REF PVT/UNGL-TILT PVT/GL PVT/GL+REF PVT/GL-TILT PVT/TFMS PVT/TFMS+REF PVT/TFMS-TILT

7 LCA methodology A Life Cycle Assessment study has been carried out at the Department of Mechanical and Aeronautical Engineering of the University of Rome La Sapienza, using SimaPro 5.1 software. Since each modification of the PV system (glazed covering, heat recovery) on one side leads to a higher energy output, but, on the other side, it requires new components and materials with their energy content, the main aim of the LCA study is to investigate the effectiveness of these modifications. The energy and environmental impacts and savings of all the systems have been assessed by means of two aggregate indicators: Global Warming Potential at 100 years (GWP100) and consumption of Primary Energy Resources (PER). The characterization factors for both the indicators are taken from Eco-indicator 95 method, implemented in the database of SimaPro 5.1 software. We focused our attention on these values because of their relevance and importance in environmental and energy saving strategies. The comparison of the results and the effectiveness of the modification have been evaluated using two pay back time parameters: the Energy Pay Back Time (EPBT) and the CO 2 Pay Back Time (CO 2 PBT). As a matter of fact, by producing clean energy during their operation, PV and PVT systems avoid the Primary Energy Resources consumption and the CO 2 emissions related to conventional energy sources. The PBT parameters are the outputs of an environmental cost benefit analysis and they estimate the time period needed for the benefits obtained in the use phase to equal the impacts related to the whole life cycle of the analyzed systems. Only after those periods the real environmental benefit starts. Table 3. Amounts of materials used in PV and PVT systems (3 kw p, 30 m 2 ) PV AND PV/T SYSTEM COMPONENT Sub-component and material Amount (kg) Multi-crystalline silicon photovoltaic module Electrical Balance Of System Reflectors Heat Recovery Unit Mechanical Balance Of System (support structure and air circuit) PV cells (including cell contacts) 21 glazed covering (low iron glass) 225 lamination material (ethylen vinyl acetate) 39 aluminium frame 45 steel 10 copper 6 plastic (Poly Vinyl Chloride, PVC) 4 aluminium (diffuse reflector material) 90 galvanized iron (for reflector installation) 90 thermal insulation (polyurethane) 30 collector frame (aluminum profiles) 120 collector back cover (aluminium sheet) 30 ONLY FOR PVT/TFMS SYSTEMS (aluminium sheet) 9 ONLY FOR GLAZED HRU: glazed covering (low iron glass) 375 ONLY FOR GLAZED HRU: additional collector frame for glazed covering (aluminium) 45 galvanized iron rods (support structure for horizontal roof) 120 galvanized iron rods (support structure for tilted roof) 90 aluminium (support structure for tilted roof) 30 pipes for air circulation (galvanized iron) 60 copper 9 Fan for air circulation steel 12 Plastic (PVC) 3 heat exchanger (copper) 60 7

8 In the paper, the EPBT and CO 2 PBT values for the analysed systems are given, considering the final use energy output as electricity for standard PV modules and as electricity and gas as heat for the suggested hybrid PVT systems. Life Cycle Assessment (LCA) methodology aims at assessing the potential environmental impacts of a product or a service during its whole life cycle, according to ISO international standards (1997). The performed study focuses on the life cycle of a 3 kw p PV (or PV/T) system, with an overall active surface of 30 m 2. All the analyzed systems were modeled taking into account the following sub-parts: multi-crystalline silicon (mc-si) PV modules; mechanical Balance Of System (BOS); electrical BOS (inverter and cables); PV module support structures for both horizontal and tilted roof installation; Heat Recovery Unit (HRU), with or without additional glazed covering, and, finally, the air circulation circuit. Consistently with the LCA approach, for all the listed components, the environmental indicators were calculated, from raw material extraction to end of life disposal. Table 3 summarizes, for each component, the type and the amount of material for the 3 kw p reference system. Energy savings and Pay Back Time results In spite of the material and energy requirement needed to produce and install PV and PVT systems (and to treat their components at the end of their technical life), during their operation, they produce clean electricity and heat, thereby displacing conventional energy. Therefore, environmental benefits due to avoided environmental impacts are associated to the system operation phase. We used the data from electricity and heat production by PV and PVT systems (Table 2) to achieve the following, assuming that PV systems electrical output displaces conventional grid electricity considering the European average electricity mix for the calculation. Basing on system energy output and on displaced conventional sources, the environmental cost of the systems, in a life cycle perspective, can be matched with the environmental savings obtained thanks to their clean operation phase. The values of the energy and CO 2 Pay Back Times may be calculated, representing the time period needed for the benefits obtained in the use phase to equal the impacts related to the whole life cycle of the analyzed systems and are summarized in Table 4. Regarding PBT values, it should be noted that they are, in any case, considerably lower than the expected lifespan of the systems. From these results we observe that the highest PBT values are about 3 years and 3 months, while PV systems lifespan could be assumed to be nearly one order of magnitude higher. As a matter of fact, the most conservative assessments (Kato et al,1998) indicate expected life periods of years, while other sources (Travaglini et al, 2000), thanks to aging tests conducted on operating plants, suggest a lifespan of more than 30 years. Besides, LCA results underline that the proposed improved configurations for PV systems (heat recovery by air cooling and TFMS modification) enable the energy output to be significantly increased. The higher energy production from improved PV systems and the consequent energy savings, overcome the increased impacts due to the additional system components (HRU). Thus, the proposed configurations show lower values for the PBTs. Additionally, the heat production compensates the impacts due to the HRU. When the HRU of the PVT system is equipped with a glazed covering, though, the increase in thermal energy production allows a considerable lowering of the PBT values. Concluding, the use of a glazed covering lowers the electrical output because of the reflection and absorption from the glazing but, on the other side, thanks to the greenhouse effect inside the collector, the amount of heat recovered is widely increased and the result of this two opposite effects is positive, thereby achieving lower PBTs. It is noticeable that the better performance of the studied systems is achieved the more the thermal energy demand is constant during the year, even though, as underlined in the previous parts of 8

9 the paper, the 12 months air scenario is somewhat ideal, since referred to strictly particular industrial cases. The most interesting scenario for domestic applications (in spite of the increased material requirement for the heat exchanger) is the combination of air and water heat recovering systems, that leads to lower the environmental PBT in all the analysed configurations. Table 4. EPBT and CO 2 PBT values for all studied systems SYSTEMS EPBT (12 Mo air CO 2 PBT (12 Mo air EPBT (6 Mo air CO 2 PBT (6 Mo air EPBT (6 Mo air+ 6 Mo water CO 2 PBT (6 Mo air+ 6 Mo water PV PV+REF PV-TILT PVT/UNGL PVT/UNGL+REF PVT/UNGL-TILT PVT/GL PVT/GL+REF PVT/GL-TILT PVT/TFMS PVT/TFMS+REF PVT/TFMS-TILT Conclusions Hybrid Photovoltaic/Thermal solar systems with air heat extraction were developed by University of Patras, aiming to the increase of the total efficiency of photovoltaics by providing simultaneously electrical and thermal output. We calculated the energy output for operation and the Energy Pay Back Time (EPBT) and CO 2 Pay Back Time (CO 2 PBT) of all studied systems, considering the corresponding materials of the horizontal and tilted building roof installation of systems. Estimating all together the extracted results we notice that the system that combines the higher total energy output with the lower values of EPBT and CO 2 PBT are the PVT/GL and the PVT/TFMS both considered in the configuration with reflectors. These systems can be used on horizontal or tilted building roofs, with better performance for the horizontal roofs. The mounting of the thin flat metallic sheet inside the air channel (TFMS modification) gives higher electrical and thermal output compared to the similar unglazed type of PVT/AIR systems. The addition of the booster diffuse reflectors is positive in all cases although the reduction of EPBT and CO 2 PBT is small. Concluding, the heat extraction from the PV modules results to cost effective solar devices, that are of positive performance regarding their environmental impact, compared to standard PV modules. The advantages of the hybrid PV/T solar systems makes them attractive for a wider application of photovoltaics, providing heat apart of electricity and increasing therefore the total efficiency of the converted solar radiation into useful energy. References Bazilian M., Leeders F., van der Ree B.G.C. and Prasad D. Photovoltaic cogeneration in the built environment. Solar Energy 71, pp (2001) Bhargava A.K., Garg H.P. and Agarwal R.K. Study of a hybrid solar system solar air heater combined with solar cells. Energy Convers. Mgmt, 31, 5, pp (1991) Brinkworth B.J., Cross B.M., Marshall R.H. and Hongxing Yang. Thermal regulation of photovoltaic cladding. Solar Energy 61, pp (1997) 9

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