Long-term performance evaluation of liquid-based photovoltaic thermal (PVT) modules with overheating-prevention technique

Laboratory and long-term field tests were conducted to evaluate the overall performance and building applicability of liquid-based photovoltaic thermal (PVT) modules with overheating-prevention techniques in buildings. Three types of flat PVT modules were designed and manufactured: the glazed PVT module with a transparent film covering the photovoltaic (PV) cells, glazed PVT module with a glass covering the PV cells, and unglazed PVT module with a glass covering the PV cells. Based on the thermal and electrical performance results, the glazed PVT module with a transparent film covering the PV cells exhibited a maximum overall efficiency of approximately 71.1%. This could be attributed to less heat losses induced by glazing and less reflectivity caused by film covering the PV cells. Less reflectivity and greater cooling allowed the unglazed PVT module to have the higher electrical efficiency. Thirteen glazed PVT modules with overheating-prevention techniques were installed on the roof of a residential building and field tested for one year. The roof-integrated PVT system with an average overall efficiency of 27.4% produced 8,187.1 kWh heat and 4,430.1 kWh electricity. In terms of electricity performance evaluation, the monthly average performance ratio (PR) ranged from 55.4% to 63.2%, which was lower than that of conventional building-integrated photovoltaic module because of the lower transmittance induced by glazing, greater PV panel temperature caused by glazing and high temperature of working fluid, and low incoming solar irradiation owing to shading. The overheating-prevention device operated well, and no overheating-related failure occurred during field testing.