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 pel of the corresponding PV modules of all systems and the thermal efficiency pth of the PVT/AIR models. The electrical efficiency pel of PV modules depends on their temperature (TPV) and the incoming solar radiation and it is calculated by the measured data as: pel=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. Test results showed that the pel for PV, PVT/UNGL and PVT/TFMS type models was: r|el=0.1659-0.00094Tpv, while for PVT/GL type models was: pel=0.1457-0.00094TPV. The value of TPV for the standard module was calculated by the equation: TPV=30+0.0175(G-300)+1.14(Ta-25) (Lasnier and Ang, 1990) that correlates TPV with the parameters G and Ta. The TPV 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.

The thermal efficiency nth of the PVT/AIR models depends on the incoming solar radiation (G), the input air temperature (Ti) and the ambient temperature (Ta). 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 i"|th of the tested hybrid PVT/AIR solar energy systems is calculated for the operating conditions with the lowest thermal losses (T=Ta) by the equation: i"|th = m Cp(T0-Ti)/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 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 i"|th (for T=Ta) of PVT+REF systems varies from a minimum value for December (CR=1.0) up to the maximum value for June (CR=1.3).