Energy, cost and LCA results of PV and hybrid PV/T solar systems

PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS Prog. Photovolt: Res. Appl. 2005; 13:235–250 Published online 14 January 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pip.590

Applications

Energy, Cost and LCA Results of PV and Hybrid PV/T Solar Systems Y. Tripanagnostopoulos1,*,y , M. Souliotis1, R. Battisti2 and A. Corrado2 1

Physics Department, University of Patras, Patra 26500, Greece Department of Mechanical and Aeronautical Engineering, University of Rome ‘La Sapienza’, Via Eudossiana, 18 00184, Rome, Italy 2

Hybrid photovoltaic/thermal (PV/T) solar systems provide a simultaneous conversion of solar radiation into electricity and heat. In these devices, the PV modules are mounted together with heat recovery units, by which a circulating fluid allows one to cool them down during their operation. An extensive study on water-cooled PV/ T solar systems has been conducted at the University of Patras, where hybrid prototypes have been experimentally studied. In this paper the electrical and thermal efficiencies are given and the annual energy output under the weather conditions of Patras is calculated for horizontal and tilted building roof installation. In addition, the costs of all system parts are included and the cost payback time is estimated. Finally, the methodology of life cycle assessment (LCA) has been applied to perform an energy and environmental assessment of the analysed system. The goal of this study, carried out at the University of Rome ‘La Sapienza’ by means of SimaPro 51 software, was to verify the benefits of heat recovery. The concepts and results of this work on energy performance, economic aspects and LCA results of modified PV and water-cooled PV/T solar systems, give a clear idea of their application advantages. From the results, the most important conclusion is that PV/T systems are cost effective and of better environmental impact compared with standard PV modules. Copyright # 2005 John Wiley & Sons, Ltd. key words: photovoltaics; hybrid PV/T systems; PV performance; PV/T system performance; life cycle assessment; solar system economics; environmental impact

INTRODUCTION

T

he absorbed solar radiation that is not converted into electricity increases the temperature of photovoltaics, leading to a reduction of their electrical efficiency. Therefore, PV cooling is needed to keep electrical efficiency at a satisfactory level and it can be achieved by water or air heat extraction. Natural or forced air circulation is a simple and low-cost method to remove heat from PV modules, but it is less effective at low latitudes where the ambient air temperature is over 20 C for many months during the year. Water heat extraction is more expensive than air heat extraction, but it can work effectively as the water temperature from * Correspondence to: Y. Tripanagnostopoulos, Physics Department, University of Patras, Patra 26500, Greece. y E-mail: [email protected]

Copyright # 2005 John Wiley & Sons, Ltd.

Received 22 March 2004 Revised 7 September 2004

236

Y. TRIPANAGNOSTOPOULOS ET AL.

mains is lower than 20 C almost all year. In order to avoid pressure and electrical problems, the usual technical solution for PV cooling by water is to circulate it through a heat exchanger in thermal contact with the PV module rear surface. If the heat removal fluid is used not only for PV cooling, but also for other practical applications, hybrid photovoltaic/thermal (PV/T) solar system will be obtained. In these devices PV modules and thermal units are mounted together and the systems is able to convert solar radiation to electricity and heat simultaneously. PV/T systems provide a higher energy output than standard PV modules and can be cost effective if the additional cost of the thermal unit is low. Water-cooled PV/T systems are practical systems for water heating, but, as for both cost reduction and production technology, they are not yet improved enough for widespread commercial applications. The first PV/T solar systems using air or water heat extraction were investigated as alternative to typical PV modules.1–4 Even a PV/T model based on a novel transparent type module was analysed.5 Later, the practical aspects6 and the modelling results7 of liquid type PV/T systems gave raise to the idea of using PV modules for both electrical and thermal conversion of solar radiation. The research on liquid-type hybrid systems was followed by a study on PV/T systems with a water cylindrical storage,8 the work of the PV/T collectors with polymer absorber9 and the performance analysis on PV/T solar systems.10 The investigations on one-, two- and three-dimensional models for PV/T prototypes with water heat extraction11 and also the systems with water circulation in channels attached to PV modules12 are the most important recent studies on water-cooled PV/ T systems. The electrical and thermal output of hybrid PV/T systems can be increased in case of concentrating solar radiation and this was suggested by mounting flat13,14 or curved15,16 reflectors on PV/T modules. Design concepts, prototype construction and test results for water- and air-cooled PV/T systems have been presented at the University of Patras,17 where PV/T systems with and without additional glass cover are experimentally analysed. In the same work the concept of using stationary diffuse reflector to increase the total energy output, instead of specular reflector, is also suggested. The study of a dual-type PV/T system, operating either with water or air heat extraction18 and the results from an economic analysis that compares water-cooled PV/T systems with standard PV and thermal systems19 are two additional investigations of water-cooled PV/T solar systems. Aspects and cost analysis results for standard PV modules20,21 and PV/T systems,22,23 give an idea for the practical use of photovoltaics. The consideration of the environmental impact of PV modules by using life cycle assessment (LCA) methodology has been presented for typical photovoltaic systems,24–29 for comparison of concentrating and non-concentrating PV systems30 as well as for domestic PV/T systems.31 The LCA method has been extensively used at the University of Rome ‘La Sapienza’, starting with a PhD Thesis32 on LCA for photovoltaic systems and following the study on the simplified life cycle analysis in buildings33 and the overview and future outlook of LCA for photovoltaics.34 In addition, the comparison of PV/T systems with standard PV and thermal systems35 confirmed the environmental advantage of PV/T systems. In the present paper, an extended36 analysis is provided, focusing on both standard PV and water-cooled PV/T systems, covering system performance, LCA results and economical aspects. The work is based on the performance of PV/T systems investigated at the University of Patras with the LCA results from a specific software (SimaPro 51) performed at the University of Rome. These results refer to PV and glazed and unglazed PV/T solar systems on horizontal and tilted building roofs and for operation at three temperature levels. In addition, the use of a booster diffuse reflector between the parallel rows for the horizontal installations is suggested and the corresponding results are presented, aiming to achieve more effective PV and PV/T applications. The calculated energy performance, the LCA results and the estimated cost payback time of all systems can be considered useful as guidelines for the application of the standard PV and the new suggested PV/T systems.

EXPERIMENTAL MODELS AND RESULTS Description of tested PV and PV/T models Practical considerations in PV/T system design include the evaluation of the thermal and electrical efficiency improvement with respect to the system cost. The cost of the thermal unit is the same either the PV module is Copyright # 2005 John Wiley & Sons, Ltd.

Prog. Photovolt: Res. Appl. 2005; 13:235–250

HYBRID PV/T SOLAR SYSTEMS

237

Figure 1. Cross-section of the PVT/WATER systems studied

crystalline silicon (c-Si), multi-crystalline silicon (mc-Si) or amorphous silicon (a-Si). Thus the ratio of the additional cost of the mounted thermal unit per PV module area cost is different and is almost double in the case of a-Si compared with c-Si or mc-Si PV modules. 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 the achieved 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), although this solution lowers the PV module electrical output because of the increasing reflection and absorption of the solar radiation. The study of the hybrid PV/T water systems includes outdoor tests for the determination of the steady state electrical efficiency el and the thermal efficiency th. The PV/T experimental models were built at the University of Patras and consisted of commercial type mc-Si PV modules in combination with a laboratory-made water heat recovery unit (HRU). The experimental study is based on two hybrid PV/T module designs36 (Figure 1, left) of same aperture surface area Aa (Aa ¼ 04 m2). The first model has a simple form without additional glazing, named PV/T UNGLAZED or PVT/UNGL (for simplicity we use the term PVT instead of PV/T), while the second model shows an additional glazing (PV/T GLAZED or PVT/GL). These experimental models use a flat heat exchanger with copper sheet as the absorber and copper pipes for the circulation of water. There is also a thermal insulation layer to avoid thermal losses from the non-illuminated side of the system surface. These systems could be installed on a horizontal roof, a tilted roof (Figure 1, right) or on a building fac¸ade. Horizontal and tilted roof installations are more viable at low latitudes, while building fac¸ade (and high tilted roof) installations are more effective for medium- and high-latitude applications because of the lower sun altitude angles. Considering PV/T systems installed on horizontal roofs, a minimum distance between the parallel rows is needed in order to avoid mutual shading of PV modules. We suggest exploiting these areas effectively by using stationary flat diffuse reflectors (REF) placed properly between the PV modules.17,36 These are the PVT/ UNGL þ REF and PVT/GL þ REF systems (in Figure 2 we show the first type). This installation increases solar input on PV modules almost all year, resulting in an increasing of electrical and thermal output. The suggested diffuse reflectors differ from 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 the PV module surface.17 The systems were tested with slope equal to the latitude

Figure 2. Suggested booster diffuse reflectors for horizontal roof installation of PV and PVT systems Copyright # 2005 John Wiley & Sons, Ltd.


238


Table I. Basic characteristics of all systems studied System characteristics

PV PV þ REF PV-TILT PVT/UNGL PVT/UNGL þ REF PVT/UNGL-TILT PVT/GL PVT/GL þ REF PVT/GL-TILT

Heat recovery unit with water circulation (HRU) No No No Yes Yes Yes Yes Yes Yes

Stationary diffuse reflector to increase solar input (REF) No Yes No No Yes No No Yes No

Additional glazing to reduce thermal losses (GL) No No No No No No Yes Yes Yes

Thermal insulation to simulate inclined roof installation (TILT) No No Yes No No Yes No No Yes

of Patras (3825 ). In the experiments with the diffuse reflector, the PV/T systems were tested for variable additional solar radiation to get data for different angles of incidence. The tilted roof integrated systems show an additional thermal insulation on their rear surface, compared with those installed on a horizontal roof, as they are mounted on a tilted roof. The additional thermal protection 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 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 have the same basic system units (PV module, PVT/UNGL and PVT/GL) and are tested with an added thermal insulation on their rear surface, considering the thermal protection of the tilted roof to the attached system thermal losses from this surface. In Table I we present a list of all systems studied regarding their basic characteristics (use of HRU, diffuse reflector, additional glazing and thermal insulation). Experimental results The electrical efficiency el of PV modules depends mainly on the incoming solar radiation and the PV module temperature TPV and is calculated as el ¼ ImVm/GAa, where Im and Vm are the current and the voltage of PV module operating at maximum power and G the irradiance on the system aperture plane. The thermal efficiency th of the PV/T models depends on the incoming solar radiation G, the input fluid temperature Ti and the ambient temperature Ta. During tests for the determination of system thermal efficiency, each PV module was connected with a load (resistance) to adapt its maximum power output and 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 PV/T solar energy systems is calculated by the equation: th ¼ m_ Cp ðTo Ti Þ=GAa , where m_ is the fluid mass flow rate, Cp the fluid specific heat, Ti and To the input and output fluid temperatures and Aa the aperture area of the PV/T model. The thermal efficiency th is calculated as a function of the ratio T/G where T ¼ Ti Ta. In the case of the diffuse reflector, the calculation of thermal and electrical efficiency of the combined system17 is based on the net solar radiation on the PV module surface (not included the radiation from the reflector). This result could be considered as system performance rather than system efficiency and by this calculation we can have a clear idea about the effect of the additional solar input due to the use of diffuse reflectors. The equation37 used for the calculation of PV module temperature TPV, is based on the incoming solar radiation G and the ambient temperature Ta: TPV ¼ 30 þ 00175ðG 300Þ þ 114ðTa 25Þ The electrical efficiency el of the PV/T systems is determined for the operating temperature of the PV modules. In these hybrid systems the PV temperature depends not only on G and Ta, but also on the system operating condition, which is due to the thermal unit mounted on PV module rear surface and depends mainly on the heat Copyright # 2005 John Wiley & Sons, Ltd.



239

extraction fluid temperature. The thermal part affects the PV module temperature and therefore the PV electrical efficiency el can be considered function of a parameter TPV,eff, which corresponds to the PV temperature for the operating conditions of the PV/T systems. The results from the tests regarding the electrical efficiency of PV modules el and also the electrical and thermal (th) efficiency for the PV/T systems for the horizontal roof installation are the following: PVT=UNGL : el ¼ 01659 000094 TPV;eff

PVT=GL : el ¼ 01457 000094 TPV;eff

and th ¼ 055 1199ðT=GÞ

and

th ¼ 071 0904ðT=GÞ

The electrical efficiency of the tested standard type PV module is given by the corresponding equation for the PVT/UNGL model, where the operating PV module temperature is simply determined by the basic equation37 that correlates TPV with the parameters G and Ta. The experimental results for the thermal efficiency of the two PV/T types regarding system installation on an inclined roof were extracted from the tests with the additional thermal insulation on the back of the PV/T systems and are the following: PVT=UNGL-TILT : th ¼ 054 1077ðT=GÞ and

PVT=GL-TILT : th ¼ 070 0835ðT=GÞ

For calculation of the effective value TPV,eff the experimental data from the tests with the PV/T models were used and we observed that TPV,eff can be correlated by the formula: TPV,eff ¼ TPV þ (TPVT Ta). The temperature TPVT corresponds to the thermal unit operating temperature and can be determined approximately by the circulating heat extraction fluid temperature. This relation was also used for the determination of TPV,eff for the tilted building roof PV/T and PV system installation, but the value of TPV for the PVT/UNGL-TILT and PVT/ GL-TILT systems as well as for the standard PV-TILT module was calculated from the modified equation: TPV ¼ 30 þ 00175(G-150) þ 114(Ta-15), which was based on the previously given formula37 for the horizontal roof installation. This modified equation was experimentally validated and corresponds to the increase of PV operating temperature due to the reduced heat losses to the ambient from the rear surface of the PV/T system.

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 a system slope equal to the latitude of Patras for both horizontal and tilted building roof installation. The addition of the diffuse reflector to the PV/T systems results in concentration ratios, calculated as monthly averages, ranging from CR ¼ 100 in December up to CR ¼ 130 in June. The PV temperature, that is used for the determination of the effective value TPV,eff to calculate the electrical efficiency, was based on the additional incoming solar radiation, modifying the basic34 equation to: TPV ¼ 30 þ 00175(G*CR 300) þ 114(Ta 25). We used the measured characteristics of all tested systems and in Table II we present the monthly energy output for the PV modules for horizontal roof installation for the standard mode, for the suggested mode with the diffuse reflector and also for the tilted roof installation. We include the solar radiation G in kW h m2 per month on the PV modules plane, the mean monthly ambient temperature Ta (during sunshine) and the monthly average values of the concentration ratio CR of the diffuse reflector. Regarding the calculated values we include the electrical energy for the standard PV modules (PV-ELECTRIC) as well as with the diffuse reflector (PV þ REF-ELECTRIC) and for the PV modules on a tilted roof (PV-TILT-ELECTRIC). In Figure 3 we present the diagrams for the maximum monthly electrical energy output per m2 and for operation at 25 C, not including the energy reduction from the controllers and inverters. In Figure 4 we give the diagrams for the maximum monthly thermal energy output for operation at 25 C, without considering the energy reduction from the fluid circulation pipes, the heat exchanger and the storage. Copyright # 2005 John Wiley & Sons, Ltd.


240


Table II. Monthly values of standard PV module electrical output, including irradiance, ambient temperature and concentration ratio of the reflector considered Month

Solar radiation (kW h m 2)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

9677 9793 12762 13365 16538 16970 18265 18656 16176 13481 10338 8451

Sum

164472

Ambient temperature ( C) 12 13 14 18 21 27 29 29 25 21 17 14

CR

PV-ELECTRIC (kW h m 2)

PV þ REF-ELECTR (kW h m 2)

PV-TILT-ELECTR (kW h m 2)

105 110 115 120 125 130 125 120 115 110 105 100

1291 1296 1673 1701 2049 2008 2115 2149 1927 1668 1325 1113

1392 1461 1967 2083 2605 2647 2683 2621 2261 1878 1428 1144

1179 1183 1525 1546 1857 1811 1903 1932 1740 1511 1205 1014

20315

24170

18405

Figure 3. Monthly electrical output of all PV/T systems studied for operation at 25 C

Figure 4. Monthly thermal output of all PV/T systems studied for operation at 25 C Copyright # 2005 John Wiley & Sons, Ltd.



241

Table III. Annual electrical and thermal output of all systems studied Electrical and thermal energy output of the systems

PV PV þ REF PV-TILT PVT/UNGL-25 C PVT/UNGL-35 C PVT/UNGL-45 C PVT/UNGL þ REF-25 C PVT/UNGL þ REF-35 C PVT/UNGL þ REF-45 C PVT/GL-25 C PVT/GL-35 C PVT/GL-45 C PVT/GL þ REF-25 C PVT/GL þ REF-35 C PVT/GL þ REF-45 C PVT/UNGL-TILT-25 C PVT/UNGL-TILT-35 C PVT/UNGL-TILT-45 C PVT/GL-TILT-25 C PVT/GL-TILT-35 C PVT/GL-TILT-45 C

Annual electrical output (kW h yr 1) (per m2) 20315 24170 18405 19826 18466 17910 22069 20669 19268 16592 15283 13917 18664 17308 15952 17915 16553 15192 14716 13379 12041

Annual electrical output (kW h yr 1) (BOS 85%) (30 m2) 518033 616335 469328 505563 470883 456705 562760 527060 491334 423096 389717 354884 475932 441354 406776 456833 422102 387396 375258 341165 307046

Final use system electricical efficiency (%) (BOS 85%)

1050 1249 951 1025 954 926 1141 1068 996 857 790 719 965 894 824 926 855 785 761 691 622

Annual thermal output (kW h yr 1) (per m2)

59763 24125 3976 68864 30614 7876 86255 51998 24487 92417 57684 29329 60342 25791 6110 85959 54316 27124

Annual Final use thermal output system thermal (kW h yr 1) efficiency (%) (BOS 85%) (BOS 85%) (30 m2)

15 23957 615188 101388 17 56032 780657 200838 21 99503 13 25949 624419 23 56634 14 70942 747890 15 38721 657671 155805 21 91955 13 85058 691662

3089 1247 205 3559 1582 407 4458 2687 1266 4776 2981 1516 3119 1333 316 4442 2807 1402

The complete systems include the necessary additional components (balance of system, BOS, for the electricity and the BOS hydraulic system for the heat) and therefore the final energy output is reduced due to the electrical and thermal losses. Estimating a minimum energy reduction of about 15% for the electricity (electrical BOS efficiency 85%) for the calculation of the useful electrical energy output. Regarding system thermal part, we consider also an efficiency of 85% for the final heat output (pipes, heat exchanger, etc.) and we take these new values as the final use energies. The annual energy output per m2 of the considered PV modules and of the PV/T systems was based on the experimental performance and was calculated for operation at 25, 35, and 45 C and the results are included in Table III. The calculated values for PV and PV/T systems with BOS of 85% for both electrical and thermal system parts as well as the percentage of final use system efficiency are calculated considering the annual solar input (164472 kW h m2 yr1) on the plane of the PV module for Patras. These results give an idea about the limits of practical use, as the operation of PV/T systems in low-, moderateor high-temperature results in a considerable reduction of the electrical and thermal output of the systems. We must notice that among all PV/T systems, the electrical output increase is obtained by the PVT/UNGL for operation at 25 C and during summer only, because this period the mean ambient temperature is higher than that of the above level (25 C) under the weather conditions of Patras.

PV AND PVT SYSTEM ECONOMIC ANALYSIS The reference case of the study is a system with active surface area of 30 m2 (aperture surface area), an inverter of 3 kWp and a water storage tank (WST) of 1500 l. Considering Greek market prices for system materials and components, a mean cost of mc-Si PV modules is about 700 s/m2, the HRU cost for the PVT/UNGL type systems is 100 s/m2, the HRU cost for the PVT/GL type systems is 200 s/m2 and the additional cost for the diffuse reflector is 30 s/m2. In addition, we consider 1500 s for the cost of electrical BOS (inverter 3 kWp, cables, etc.), Copyright # 2005 John Wiley & Sons, Ltd.


242


Table IV. Cost breakdown and cost payback time of all systems studied System cost 30 m2 installation cost payback time (CPBT) PV PV þ REF PV-TILT PVT/UNGL-25 C PVT/UNGL-35 C PVT/UNGL-45 C PVT/UNGL þ REF-25 C PVT/UNGL þ REF-35 C PVT/UNGL þ REF-45 C PVT/GL-25 C PVT/GL-35 C PVT/GL-45 C PVT/GL þ REF-25 C PVT/GL þ REF-35 C PVT/GL þ REF-45 C PVT/UNGL-TILT-25 C PVT/UNGL-TILT-35 C PVT/UNGL-TILT-45 C PVT/GL-TILT-25 C PVT/GL-TILT-35 C PVT/GL-TILT-45 C

Cost of Cost of Installation PV þ HRU þ REF electrical þ thermal cost (s) (s) bos (s) 21 000 22 500 21 000 24 000 >> >> 25 500 >> >> 27 000 >> >> 28 500 >> >> 24 000 >> >> 27 000 >> >>

1500 1500 1500 4500 >> >> 4500 >> >> 4500 >> >> 4500 >> >> 4500 >> >> 4500 >> >>

1500 1500 900 1500 >> >> 1500 >> >> 1500 >> >> 1500 >> >> 900 >> >> 900 >> >>

Total cost (s)

24 000 25 500 23 400 30 000 >> >> 31 500 >> >> 33 000 >> >> 34 500 >> >> 29 400 >> >> 32 400 >> >>

CPBT for electricity saving (in yr) 258 229 282 119 187 281 111 170 255 105 145 212 103 139 198 118 185 282 105 142 207

CPBT for electricity and gas saving (in yr) 258 241 269 181 238 296 172 223 281 176 219 279 172 211 263 182 242 308 179 221 282

and 3000 s for the cost of the hydraulic BOS for the heat extraction and the water storage (WST of 1500 l, heat exchanger, pipes, pump, etc.). Regarding system installation cost we estimate 50 s/m2 for horizontal building roof installation, 30 s/m2 for tilted building roof installation and 20 s/m2 of PV or PV/T system as additional installation cost for the diffuse reflectors. In Table IV we give the cost breakdown for the considered PV and PV/ T module installations of 30 m2 active surface area, 3 kWp inverter and 1500 l WST. From these values one can observe that the PVT/UNGL-TILT system has the minimum additional cost, compared with the standard PV (by 225%) and to PV-TILT (by 256%) type, while the PVT/GL þ REF is the most expensive system (by 437% regarding the PV and 353% the PV þ REF system). The economic analysis is performed on an annual basis and it assumes that the system outputs replace equivalent amounts of energy for water heating from electricity grid or from burning natural gas. In addition, the user avoids the consumption of electricity produced by the PV and PV/T systems. We calculated the cost payback time (CPBT) considering the number of years that the investment cost is overcome.38 In the calculations we used the initial total system cost that is given in Table IV (Total cost) and the annual energy output in electricity and heat for BOS 85%, presented in Table IV. The cost of the conventional energy source replaced for the electrical and thermal needs is 01 s/kW h for the electricity and 003 s/kW h for the natural gas, considering energy conversion efficiencies for the final use energy 100% and 80% correspondingly. The estimated values for the market discount rate and the conventional energy source interest rate were taken as 005 and 01 respectively. The cost payback time (CPBT, in years) is calculated without considering tax rebate or other cost reduction in order to find out the maximum time to overcome the initial cost. In addition, the maintenance cost is very low and therefore is not included in the calculations. Table IV shows the CPBT for the electricity saving (both PV modules and HRU system replace electricity only) and also the CPBT for the electricity saving by the PV modules and the gas saving by the HRU system. The results highlight that the use of the HRU in PV/T systems, saving either electricity or gas, reduces the CPBT compared with standard PV modules and it is cost effective if the PV/T system operating temperature is low (25 or 35 C). The same is observed with slightly better results in Copyright # 2005 John Wiley & Sons, Ltd.



243

the case of a diffuse reflector. Lower values of CPBT are obtained for the systems with the additional glazing due to their higher thermal energy output. The operation of the unglazed type PV/T systems at 45 C or at higher temperatures, and also of both glazed and unglazed PV/T systems in case that HRU replaces gas, is not cost effective. The use of the diffuse reflector has a positive effect in all cases, although the decrease of CPBT is below 10%.

LCA PV AND PVT SYSTEM STUDY 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 51 software, determining two payback time parameters: the energy payback time (EPBT) and the CO2 payback time (CO2 PBT). As a matter of fact, by producing clean energy during their operation, PV and PV/T systems avoid the primary energy resources consumption and the CO2 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 of the whole system life cycle. Only after those periods does the real environmental benefit start. The EPBT and CO2 PBT values for the analysed systems are given, considering the final use energy output as electricity for standard PV modules and as electricity and heat for the suggested hybrid PV/T systems. As for heat, the results are provided for two heat sources: electricity and gas. Electricity, in spite of its high environmental and energy costs, is widely used for water heating in Greece, Cyprus and other countries that show favorable weather conditions for solar energy applications, while gas is used in many European and other countries. Life cycle assessment (LCA) methodology 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 14040 an LCA study shall be divided into the following main steps:39 1. 2. 3. 4.

Goal and scope definition; LCI (life cycle inventory); LCIA (life cycle impact assessment); Interpretation of the results.

Goal and scope definition System boundaries The performed study focuses on the life cycle of a 3 kWp PV (or PV/T) system, with an active surface of 30 m2. All the analysed systems were modeled taking into account the following sub-parts: * * *

* *

multi-crystalline silicon (mc-Si) photovoltaic modules; electrical balance of system (inverter and cables); mechanical balance of system (BOS): PV module and PV/T system support structures for both horizontal and tilted roof installation and the hydraulic circuit; aluminium reflectors; heat recovery unit (HRU), with or without additional glazed covering;

For all the listed components, the environmental indicators were calculated, from raw material extraction to end of life disposal. Environmental indicators and parameters For LCIA performing, two environmental indicators were used, in order to assess the potential impacts of the environmental flows collected in the inventory phase: global warming potential at 100 years (GWP100) and Copyright # 2005 John Wiley & Sons, Ltd.


244


primary energy resources (PER). The GWP100 value, expressed in kg CO2 eq, is the sum of each greenhouse gas emission multiplied by a weight factor (characterization factor). The characterization factors for GWP100 are taken from the ‘CML 2 baseline 2000’ method, implemented in the database of SimaPro 51 software. This method provides very recent characterization factors (year 2000) for GWP100, developed by the Intergovernmental Panel on Climate Change. A sensitivity analysis was performed to evaluate the results using a different set of characterization factors from the ‘Eco-indicator 95’ method. The comparison between the two methods shows differences of 1–2% in the LCA results. Greenhouse gas effects on the global warming phenomenon depend on the chosen time horizon. A sensitivity analysis was performed in order to assess the difference in the study results using a time horizon of 500 years (characterization factors for GWP500 indicator40). Differences are in the range of 5–8%. As for primary energy requirement, PER indicator is expressed in MJ LHV (low heating value) and the characterization factors are the low heating values of the fuels used both as materials and for the production of thermal and electrical energy. The characterization factors for PER are taken from ‘Ecoindicator 95’ method, in the database of SimaPro 51 software. LCA results: contribution and sensitivity analyses In the LCI step, the environmental data for all the components of the system are collected, carefully verified and put into the model built in SimaPro 51 software. Table V summarizes, for each component, the type and the amount of material for the 3 kWp reference system. The inventory data collected in the LCI step must be processed in the LCIA phase, the outputs of which are the values of the aggregate indicators GWP100 and PER for all life cycle phases. The first task of LCIA is the contribution analysis, with the aim of highlighting the bottlenecks in the product life cycle, thus finding out the phases characterized by the highest environmental burdens. In the first contribution analysis, the life cycle of a 3 kWp PV system on a horizontal roof is assessed. A road transportation of 100 km by means of a 28 t total load truck was considered for the distribution phase. As for the system disposal at the end of its technical life, an uncontrolled disposal via landfill was taken into account (average transportation distance 50 km). The results suggest that the main contribution (more than 99%) to the total impacts comes from the PV system itself, i.e., from the production of all its components, including mechanical and electrical BOS. Despite Table V. Amounts of materials used in PV and PV/T systems PV and PV/T system component

Sub-component and material

Multi-crystalline silicon photovoltaic module

Amount (kg)

PV cells (including cell contacts) Glazed covering (low iron glass) Lamination material (ethylen vinyl acetate) Aluminium frame Electrical balance of system Steel Copper Plastic (polyVinyl chloride) Reflectors Aluminium (diffuse reflector material) Galvanized iron (for reflector installation) Heat recovery unit Copper sheet Copper pipe Thermal insulation (polyurethane) Collector frame (aluminum profiles) Collector back cover (aluminium sheet) Only for glazed HRU: glazed covering (low iron glass) Only for glazed HRU: additional collector frame for glazed covering (aluminium) Mechanical balance of system Galvanized iron rods (support structure for horizontal roof) (support structure and hydraulic circuit) Galvanized iron rods (support structure for tilted roof) Aluminium (support structure for tilted roof) Pipes for water circulation (galvanized iron) 1500-l water storage tank (galvanized iron) Heat exchanger (copper tubes for storage tank)


21 225 39 45 10 6 4 90 90 45 105 30 60 30 375 45 120 90 30 60 350 25



245

disposal phase contribution is almost negligible, a sensitivity analysis was performed in order to estimate the potential benefits of a ‘controlled’ system disposal for a PVT/GL þ REF system on a horizontal roof. In this scenario, recycling of the materials contained in the following components was taken into account: BOS (both mechanical and electrical), hydraulic circuit, HRU and reflectors. Owing to material losses during system disassembling, a recycling rate of 80% was considered and the landfilling of the remaining 20% is assumed. PV modules’ recycling was not considered because, so far, an industrial process has not been developed yet, even though some LCA data exist on PV modules recycling.41 With the aim of quantifying the recycling benefits, an open loop recycling approach was applied, using the ‘system expansion’ methodology. According to this methodology, described in ISO 14040 standards, the system boundaries shall be extended in order to include the product of the recycling process, which is the recycled material. Since recycled materials displace equal amounts of primary ones, the impacts related to the displaced amount of primary materials are subtracted from the total impacts, while the impacts associated to the recycling process are added. In the controlled disposal scenario, thanks to recovery and recycling, the life cycle impacts are quite lower, about 90% of the values for an uncontrolled disposal by landfill. This is true especially for the analysed PV/T system, that shows high contents of aluminium, copper and steel, for which the recycled material has a much lower impact than the primary one. As for the system production, by means of an ad hoc contribution analysis performed only for the PV system production phase, we found out that nearly the whole of the impacts (96–97%) are due to PV modules production, with barely significant contributions from the other system components, such as support structures or electric and electronic devices. It is interesting to perform the same contribution analysis for the most complicated system, PV/T with glazed covering and aluminium reflectors (PVT/GL þ REF, Table VI). PV modules share of the total impacts is, in that case, considerably lower, between 60% and 65% and relevant contributions come from the additional components needed for heat recovering and reflection. The glazed HRU impacts come from copper (pipes and heat exchanger), aluminium (collector frame and collector back cover), glass (glazed covering) and polyurethane foam (insulation). The impacts of the reflectors are due to their high aluminium content, while for the mechanical BOS most of the impact is due to the hydraulic circuit, mode of copper (heat exchanger in the storage tank) and galvanized iron (connecting pipes and water storage tank). As to aluminium products, recycled aluminium content of 30% was assumed. The environmental impacts for the whole life cycle of all the analysed systems (on both horizontal and tilted roofs) are summarized in Figure 5. Energy saving and pay back time calculation Only the environmental impacts for the whole life cycle of the PV systems have been analysed so far. It has not been yet considered that, during their operation, PV and PV/T systems produce clean electricity and heat, thereby displacing conventional energy. Therefore, environmental benefits due to avoided environmental impacts are associated to the system operation phase. The benefit quantification depends on two parameters: the amount of electrical and thermal energy produced by the system in a given period (e.g., one year); and the conventional source partially displaced by PV and PV/T systems. Table VI. Primary energy resources and global warming potential at 100 yr for a 3 kWp PV/T system (with glazed covering and aluminium reflectors) on horizontal roof (from cradle to ‘gate’), distribution and disposal are excluded the analysed systems System component

PV modules BOS (electrical) BOS (mechanical) HRU glazed Reflectors Total


Primary energy resources (GJ LHV)

Global warming potential 100 yr (t CO2

9130 093 1564 3050 990

821 0059 109 253 092

14827

1281

eq)


246


Figure 5. Primary energy resources and global warming potential at 100 yr for the systems analysed

Grid electricity impacts depend on the energy source mix, which regarding PER is 65, 99 and 188 MJLHV and GWP100 is 06, 09 and 14 kg CO2 eq for Europe, Italy and Greece. The European average electricity mix has been considered for the calculation. It shows the lowest values between the considered scenarios and, therefore, the assessed benefits are consequently the lowest. Thus, the choice of this electricity mix assures considerably conservative results. As to the displaced conventional heat source, two scenarios are investigated: natural gas and electricity. The chosen electricity mix is again the European mix and an efficiency of 100% was considered for the electric boiler (conservative approach for assessing the avoided impacts). The impacts for the production of a given heat amount by means of natural gas and electric boilers is 48 and 65 MJLHV in PER and 03 and 06 kg CO2 eq in GWP100, respectively. Basing on system energy output and on displaced conventional sources, the avoided impacts (or environmental savings) are summarized in Table VII. Matching the environmental savings with the previously assessed impacts, the values of the energy and CO2 payback times may be calculated. These values represent the time period needed for the benefits obtained in the use phase to be equal to the impacts related to the whole life cycle of the analysed systems and are summarized in Table VIII. Interpretation of LCA results In order to fully understand the energy and environmental effectiveness of PV systems improved configurations, we focus our attention on horizontal roof systems. The use of diffuse reflectors (suitable only for horizontal roofs) means higher impacts due to their aluminium content and to the galvanized iron needed for reflector installation. The annual savings are higher as well, since reflected radiation enables both heat and electricity production to be increased, thereby overcoming the additional, with the result of lower PBTs by 69% and 74% for energy and CO2 respectively. As for PV/T systems, the potential benefits of heat recovery via module cooling depend on the following parameters: presence of the glazed covering in the HRU; conventional heat source partially displaced; and system operating temperature. Focusing on the first parameter, it should be observed that both glazed and unglazed PV/T systems allow environmental savings that are much higher than the corresponding impacts, thus lowering the PBT values by 586, 444% (PVT/UNGL) and 621, 518% (PVT/GL). As a matter of fact, the use of a glazed covering slightly increases life cycle impacts, while it enables the PV/T collector to enhance heat production, despite a small decrease in electricity output. The influence of the conventional heat source is of major importance as well, because, when electricity is displaced as heat source, the savings are definitely higher, thereby obtaining significantly lower PBT values. As to the third parameter, the system operating temperature, it should be highlighted that, in any case, higher temperatures mean higher PBT values, since environmental savings decrease with increasing operating temperature, due to the corresponding lowering of thermal and electrical energy outputs. Copyright # 2005 John Wiley & Sons, Ltd.



247

Table VII. Primary energy resources (PER) and global warming potential (GWP) at 100 yr avoided in a year thanks to energy produced during system operation System environmental performance

PV PV þ REF PV-TILT PVT/UNGL-25 C PVT/UNGL-35 C PVT/UNGL-45 C PVT/UNGL þ REF-25 C PVT/UNGL þ REF-35 C PVT/UNGL þ REF-45 C PVT/GL-25 C PVT/GL-35 C PVT/GL-45 C PVT/GL þ REF-25 C PVT/GL þ REF-35 C PVT/GL þ REF-45 C PVT/UNGL-TILT-25 C PVT/UNGL-TILT-35 C PVT/UNGL-TILT-45 C PVT/GL-TILT-25 C PVT/GL-TILT-35 C PVT/GL-TILT-45 C

PER avoided for electricity saving (GJ LHV/yr) 3340 3980 3030 13100 7010 3600 15000 8440 4470 16900 11100 6320 18300 12300 7450 12900 6970 3510 16600 11100 6450

GWP at 100 yr avoided for electricity saving (t CO2 eq/yr) 311 371 282 1220 653 336 1390 786 416 1570 1030 589 1700 1150 694 1200 649 327 1540 1040 600

PER avoided for electricity and heat by gas saving (GJ LHV/yr) 3340 3980 3030 10500 5970 3430 12000 7120 4130 13200 8830 5260 14300 9850 6190 10300 5860 3240 12900 8800 5270

GWP at 100 yr avoided for electricity and heat by gas saving (t CO2 eq/yr) 311 371 282 764 469 305 869 553 356 917 635 402 998 710 470 739 452 280 888 623 394

Table VIII. EPBT and CO2 PBT values for all systems studied System EPBT and CO2PBT results PV PV þ REF PV-TILT PVT/UNGL-25 C PVT/UNGL-35 C PVT/UNGL-45 C PVT/UNGL þ REF-25 C PVT/UNGL þ REF-35 C PVT/UNGL þ REF-45 C PVT/GL-25 C PVT/GL-35 C PVT/GL-45 C PVT/GL þ REF-25 C PVT/GL þ REF-35 C PVT/GL þ REF-45 C PVT/UNGL-TILT-25 C PVT/UNGL-TILT-35 C PVT/UNGL-TILT-45 C PVT/GL-TILT-25 C PVT/GL-TILT-35 C PVT/GL-TILT-45 C

EPBT for replacing electricity only (yr) 29 27 32 10 19 36 09 17 31 08 13 22 08 12 20 10 19 38 08 13 22


CO2 PBT for replacing electricity only (yr) 27 25 31 09 17 33 09 15 29 08 12 20 08 11 19 10 18 35 08 12 20

EPBT for replacing CO2 PBT for replacing electricity and gas (yr) electricity and gas (yr) 29 27 32 12 22 38 12 20 34 11 16 26 10 15 24 13 23 41 11 16 27

27 25 31 15 24 37 14 22 34 13 19 30 13 18 27 16 25 41 14 20 31


248


LCA main conclusions The highest contributions to the total impacts come from PV modules, aluminium (for REF and HRU) and copper (for HRU and hydraulic circuit). 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 4 yr, while PV systems life time could be assumed to be 15–25 yr.27,42 Besides, LCA results underline that the proposed improved configurations for PV systems (heat recovery by liquid cooling and use of diffuse reflectors) enable the energy output to be significantly increased. The higher energy production from the improved systems (PVT and PVT þ REF) and the consequent energy savings, overcome the increased impacts due to the additional system components (REF and HRU). Thus, the proposed configurations show lower values for the PBTs. Finally, it becomes clear that the use of less additional aluminium for the frame of PVT/GL-type systems compared with separate standard PV modules and glazed-type thermal collectors is a positive step, although less aluminium in the frames and the reflectors could lead to a further environmental improvement. The system operating temperature was shown to be a chief parameter of the study. Therefore, some of the configurations analysed have a lower environmental performance than the reference system (PV system), in case of a system operating temperature of 45 C, for example, the electricity output is much lower with respect to the PV system, because of the higher working temperature. Additionally, the heat production is not high enough to compensate 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 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 and on the other side, though, thanks to the greenhouse effect inside the collector, the amount of heat recovered is greatly increased and the result of this two opposite effects is positive, thereby achieving lower PBTs. The conventional heat source partially displaced by system energy outputs is of main importance as well, since the environmental benefits due to system operation are higher when it replaces heat generated via an electric boiler. The best case is the PV/T with glazing (with or without reflectors) operating at the lowest temperature (25 C). It shows PBT values of 08 yr, which means a reduction of more than 70% with respect to the simple PV system.

CONCLUSIONS We have calculated the energy output for operation of all hybrid photovoltaic/thermal systems in 25 C, 35 C and 45 C and the results show that both electrical and thermal energies decrease with temperature rise, although higher temperature values are more effective in practical thermal applications. The PV/T systems with additional glazing are of lower electrical output, but of sufficiently higher thermal output because of the greenhouse effect. The use of a diffuse reflector between parallel rows of PV/T systems increases both electrical and thermal output and can be considered an effective system modification. The installation on a tilted building roof reduces electrical output but increases thermal output. We calculated the cost pay back time (CPBT) for the experimentally tested PV and PV/T systems and we found interest results regarding the effect of using the heat recovery unit (HRU) to heat water, where a reduction up to about 50% can be achieved in the CPBT value. The results showed that the unglazed type PV/T systems (PVT/UNGL) present lower values of CPBT for operation in low temperatures (25 and 35 C). The PV/T systems are also effective in the reduction of CPBT for operation at 45 C when replacing electricity. The application of the diffuse reflector is positive up to about 10% in the reduction of CPBT. In tilted roof installed systems the conversion of solar radiation into heat is more efficient (lower thermal losses) in contrary to the conversion to electricity (higher cell temperature) than in the horizontal roofs, but regarding cost, they have almost same value of CPBT. The best results for energy payback time (EPBT) and CO2 payback time (CO2 PBT) have been pointed out for system operating with a temperature of 25 C, while the performance is satisfactory for 45 C, except of the unglazed-type PV/T system (PVT/UNGL). Regarding the contribution of the stationary diffuse reflector, the solar input increase effect is positive in all cases. Estimating all the extracted results we notice that the system Copyright # 2005 John Wiley & Sons, Ltd.



249

that combines the higher total energy output with the lower values of CPBT, EPBT and CO2PBT is the glazedtype PV/T system (PVT/GL). This system can be used on horizontal or tilted building roofs, with better performance for the horizontal roofs and in addition these systems can be combined with diffuse reflectors (PVT/ GLþREF) between the parallel rows of the PV/T systems. Concluding, the heat extraction from the PV modules results in cost-effective solar devices, that are of positive performance regarding their environmental impact, compared with standard PV modules. This advantage of the hybrid PV/T solar systems makes them attractive for a wider application of photovoltaics, providing heat apart from electricity, and therefore increasing the total efficiency of the converted solar radiation into useful energy.

REFERENCES 1. Kern EC, Russel MC. Combined photovoltaics and thermal hybrid collector systems. Proceedings of the of 13th IEEE Photovoltaic Specialists, Washington DC, USA, 1978; 1153–1157. 2. Hendrie SD. Evaluation of combined photovoltaic/thermal collectors. Proceedings of the International ISES Conference, Atlanta, Georgia, USA, 28 May–1 June 1979; 3: 1865–1869. 3. Florschuetz LW. Extension of the Hottel–Whillier model to the analysis of combined photovoltaic/thermal flat plate collectors. Solar Energy 1979, 22: 361–366. 4. Raghuraman P. Analytical predictions of liquid and air photovoltaic/thermal, flat–plate collector performance. Journal of Solar Energy Enginnering 1981; 103: 291–298. 5. Lalovic B. A hybrid amorphous silicon photovoltaic and thermal solar collector. Solar Cells 1986–1987; 19: 131–138. 6. Garg HP, Agarwal PK. Some aspects of a PV/T collector/forced circulation flat plate solar water heater with solar cells. Energy Conversion and Management 1995; 36: 87–99. 7. Bergene T, Lovvik OM. Model calculations on a flat-plate solar heat collector with integrated solar cells. Solar Energy 1995; 55: 453–462. 8. Huang BJ, Lin TH, Hung WC, Sun FS. Performance evaluation of solar photovoltaic/thermal systems. Solar Energy 2001; 70: 443–448. 9. Sandness B, Rekstad J. A photovoltaic/thermal (PV/T) collector with a polymer absorber plate-experimental study and analytical model. Solar Energy 2002; 72(1): 63–73. 10. Chow TT. Performance analysis of photovoltaic-thermal collector by explicit dynamic model. Solar Energy 2003; 75: 143–152. 11. Zondag HA, De Vries DW, Van Helden WGJ, Van Zolingen RJC, Van Steenhoven AA. The thermal and electrical yield of a PV-thermal collector. Solar Energy 2002; 72: 113–128. 12. Zondag HA, de Vries DW, Van Helden WGJ, Van Zolingen RJC, Van Steenhoven AA. The yield of different combined PV-thermal collector designs. Solar Energy 2003; 74: 235–269. 13. Sharan SN, Mathur SS, Kandpal TC. Economic evaluation of concentrator-photovoltaic systems. Solar and Wind Technology 1985; 2: 195–200. 14. Al Baali AA. Improving the power of a solar panel by cooling and light concentrating. Solar and Wind Technology 1986; 3: 241–245. 15. Brogren M, Nostell P, Karlsson B. Optical efficiency of a PV-thermal hybrid CPC module for high latitudes. Solar Energy 2000; 69: 173–185. 16. Brogren M, Karlsson B, Werner A, Roos A. Design and evaluation of low-concentrating, stationary, parabolic reflectors for wall-integration of water-cooled photovoltaic-thermal hybrid modules. Proceedings of the International Conference on PV in Europe 7–11 October, Rome, Italy, 2002; 551–555. 17. Tripanagnostopoulos Y, Nousia Th, Souliotis M, Yianoulis P. Hybrid photovoltaic/thermal solar systems. Solar Energy 2002; 72(3): 217–234. 18. Tripanagnostopoulos Y, Tzavellas D, Zoulia I, Chortatou M. Hybrid PV/T systems with dual heat extraction operation. Proceedings of the 17th PV Solar Energy Conference, Munich, Germany, 22–26 Oct 2001; 2515–2518. 19. Tselepis S, Tripanagnostopoulos Y. Economic analysis of hybrid photovoltaic/thermal solar systems and comparison with standard PV modules. Proceedings of the International Conference on PV in Europe 7–11 October, Rome, Italy, 2002; 856–859. 20. Evtuhov V. Parametric cost analysis of photovoltaic systems. Solar Energy 1979; 22: 427–433. Copyright # 2005 John Wiley & Sons, Ltd.


250


21. Hynes KM, Pearsall NM, Shaw M, Crick FJ. An assessment of the energy requirements of PV cladding systems. Proceedings of the 13th European Photovoltaic Solar Energy Conference, Nice, France, 23–27 October 1995; 2202– 2205. 22. Leenders F, Schaap AB, van der Ree BGC, van der Helden WGJ. Technology review on PV/thermal concepts. Proceedings of the 16th European PV Solar Energy Conference, Glasgow, U.K., 1–5 May, 2000; 1976–1980. 23. Kalogirou SA. Use of TRNSYS for modelling and simulation of a hybrid pv-thermal solar system for Cyprus. Renewable Energy 2001; 23: 247–260. 24. Hynes KM, Baumann AE, Hill R. An assessment of the environmental impacts of thin film cadmium telluride modules based on life cycle analysis. Proceedings of the 1st World Conference On PV energy conversion, 5–9 December, Hawaii, 1994; 958–961. 25. Keoleian GA, Lewis GMcD. Application of life-cycle energy analysis to photovoltaic module design. Progress in Photovoltaics: Research and Applications 1997; 5: 287–300. 26. Alsema EA, Frankl P, Kato K. Energy pay-back time of photovoltaic energy systems: present status and prospects. Proceedings of the 2nd World Conference and Exhibition on Photovoltaic Solar Energy Conversion, 6–10 July, Vienna, Austria, 1998; 2125–2130. 27. Kato K, Murata A, Sakuta K. Energy pay-back time and life-cycle CO2 emission of residential PV power system with silicon PV module. Progress in Photovoltaics: Research and Applications 1998; 6: 105–115. 28. Dones R, Frischknecht R. Life-cycle assessment of photovoltaic systems: results of Swiss studies on energy chains. Progress in Photovoltaics: Research and Applications 1998; 6: 117–125. 29. Alsema EA, Energy pay-back time and CO2 emissions of PV systems. Progress in Photovoltaics: Research and Applications 2000; 8: 17–25. 30. Wheldon A, Bentley R, Whitfield G, Tweddell T, Weatherby C. Payback times for energy and carbon dioxide: comparison of concentrating and non-concentrating PV systems. Proceedings of the 16th European Photovoltaic Solar Energy Conference, Glasgow, U.K., 1–5 May, 2000; 2622–2625. 31. Coventry JS, Lovegrove K. Development of an approach to compare the ‘value’ of electric and thermal output from a domestic PV/thermal system. Solar Energy 2003; 75: 63–72. 32. Frankl P. Analisi del ciclo di vita di sistemi fotovoltaici (LCA of photovoltaic systems), PhD. dissertation thesis, Universita` di Roma ‘La Sapienza’, Rome, May 1996, available at the Dipartimento di Meccanica e Aeronautica, Universita` di Roma ‘La Sapienza’, Rome, or at the Biblioteca Nazionale, Rome. 33. Frankl P, Masini A, Gamberale M, Toccaceli D. Simplified life-cycle analysis of PV systems in buildings: present situation and future trends. Progress in Photovoltaics: Research and Applications, 1998; 137–146. 34. Frankl P. Life cycle assessment (LCA) of PV systems–overview and future outlook. Proceedings of the International Conference on PV in Europe, Rome, Italy, 7–11 October 2002; 588–592. 35. Frankl P, Gamberale M, Battisti R. Life cycle assessment of a PV cogenerative system: comparison with a solar thermal and a PV system. Proceedings of the 16th European PV Solar Energy Conference, Glasgow, U.K., 1–5 May 2000; 1910– 1913. 36. Tripanagnostopoulos Y, Souliotis M, Battisti R, Corrado A. Application aspects of hybrid PV/T solar systems. Proceedings of the ISES 03 World Congress, Goteborg, Sweden, 14–19 June 2003, CD-ROM, paper P2 65. 37. Lasnier F, Ang TG. Photovoltaic Enginnering Handbook, Adam Hilger: Bristol, 1990; 258. 38. Duffie JA, Beckman WA. Solar Engineering of Thermal Processes. Wiley: Canada, 1991. 39. International Organization for Standardization, ISO 14040:1997, Environmental management—Life cycle assessment— Principles and framework, 1997. 40. Leiden University, Institute of Environmental Sciences (CML), Department of Industrial Ecology, Characterisation factors, available on line, http://www.leidenuniv.nl/cml/ssp/index.html. 41. Frisson L, et al. Recent improvements in industrial PV module recycling. Proceedings of the of the 16th European PV Solar Energy Conference and Exhibition, Glasgow, UK, 1–5 May 2000, CD-ROM, paper OA35. 42. Watt ME, Johnson AJ, Ellis M, Outhred HR. Life-cycle air emissions from PV power systems. Progress in Photovoltaics: Research and Applications, 1998; 127–136.

