Performance assessment of a trifunctional system integrating solar PV, solar thermal, and radiative sky cooling

Radiative cooling (RC) with the outer space as a natural heat sink has stimulated widespread attention in the research community and has achieved rapid developments in recent years. However, most available radiative coolers exhibit low power density and long payback periods. To overcome such shortcomings, a cost-effective solution that integrates RC into a solar photovoltaic/thermal (PV/T) collector as a secondary function was proposed. In this study, a trifunctional photovoltaic-photothermic-radiative cooling (PV-PT-RC) system was developed. The proposed system could convert solar energy into electricity and/or heat during daytime and offer cooling energy at night through RC. A mathematical model was built to assess the performance of the PV-PT-RC system quantitatively and investigate the key performance indicators of the system numerically. Moreover, a practical-scale PV-PT-RC testing system was built, and experiments were performed to verify the effectiveness of the numerical model. Results revealed that the mean relative errors are less than 5% for the electrical power, aluminum plate temperature, and water temperature in the tank and 6.83% for the cooling power, thereby proving that the mathematical model can accurately assess the performance of the hybrid system. On the basis of the verified model, the overall performance of the system was examined under different insulation thicknesses, initial water temperatures in the tank, packing factors, panel emissivity values, and tank volumes. Furthermore, the results of the annual performance analysis suggested that the annual electrical, heat and cooling gains of the system in Eastern China are 479.67, 2369.07, and 1432.49 MJ, respectively.