Heat loss and energy efficiency investigation of vacuum flat plate photovoltaic/thermal collectors

Solar energy photovoltaic/thermal (PV/T) technology is significant for achieving carbon neutrality. However, when the temperature difference between the PV/T module and the ambient environment increases, the thermal efficiency significantly decreases due to high heat losses. In this study, a novel vacuum flat plate PV/T collector with spectral selectivity is proposed, and the effects of vacuum environment and absorber emissivity on the convective, conductive, and radiative heat losses in the PV/T collector are comprehensively analyzed. The results indicate that the convective and conductive heat losses in the PV/T collector are relatively reduced by about 97 % when the air gap pressure is reduced to 0.10 Pa. When the emissivity of the PV/T absorber decreases from 0.9 to 0.1, the radiation heat loss relatively reduces by over 70 %. Compared to the conventional PV/T collector, the novel PV/T collector demonstrates the most significant performance improvement in winter. The thermal efficiency of the novel PV/T collector with an inlet water temperature of 50 °C can maintain approximately 50 % throughout the year. Even when the inlet water temperature increases to 90 °C, the novel PV/T collector can still be maintained at a thermal efficiency of 34 % in winter, whereas conventional PV/T collectors cannot collect heat in any season.