Modeling of energy and exergy efficiencies in high vacuum flat plate photovoltaic–thermal (PV–T) collectors

This work deals with the performance evaluation of novel flat photovoltaic–thermal (PV–T) modules under vacuum. Through a 1D (dimensional) steady-state-energy-balance numerical model developed in MATLAB, two different layouts are studied: the first consisting of a photovoltaic (PV) cell installed just below the glass encapsulating the flat panel, and the second where the PV cell is placed on the selective solar absorber (SSA). In both cases the thermal and electrical efficiencies have been evaluated at different SSA operating temperatures, in the range of 323 K to 423 K. The analysis has been conducted at different energy bandgap (Ebg) of the PV cell and assuming a variable transmittance or emittance of the PV cell, depending on the design. The two systems efficiency comparison has been carried out at the same operating temperature. Overall, this work highlights the importance of high vacuum insulation, which guarantees the reduction of convective thermal losses, and shows that the maximum energy is produced for PV cells with Ebg≈1.5-1.7 eV, depending on layout and operating temperature, by including the thermal output in the PV–T​ optimization. The energy and exergy efficiencies obtainable using the proposed PV–T​ systems are considerably improved compared to the results previously reported in the literature.