Numerical and experimental study of a novel vacuum Photovoltaic/thermal (PV/T) collector for efficient solar energy harvesting

Photovoltaic/thermal (PV/T) technology can generate electricity and heat simultaneously and improve solar energy harvesting efficiency. However, flat-plate PV/T collectors have strong heat loss in cold conditions and the collected heat is low-grade thermal energy, which limits the practical application of the PV/T collectors on a large scale. Herein, the vacuum environment is introduced to the flat-plate PV/T collector, conductive and convective heat loss are blocked, improving the efficiency and operating temperature of the PV/T collector. In this paper, a novel design of the vacuum PV/T collector has been proposed and fabricated, which maintains both the upper and lower space of the absorber in a vacuum state without additional glass cover causing energy loss. Performance comparison between the vacuum PV/T collector and the air-gap PV/T collector has been conducted, demonstrating that the vacuum condition can decrease the heat loss coefficient by 16.08%. In addition, a combined thermal-electrical mathematical model is constructed and validated. The predicted results reveal that the annual thermal energy outputs of the vacuum PV/T collectors without and with ideal low emissivity coating are 124.88% and 346.58% higher than that of the air-gap PV/T collector when the operating temperature is 50 degrees C. In summary, the vacuum PV/T collector exhibits great potential in improving thermal performance while slightly reducing the output of electrical energy. This work contributes to extending the working time of the PV/ T collector and guides the design of the new generation of highly efficient PV/T collectors.