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 CO2 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 15П25 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 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.

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 CO2 Pay Back Time (CO2 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 CO2PBT 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 CO2PBT 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.

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