Spectrally selective coating for the combination of solar thermal and sky radiative cooling

Sky radiative cooling (RC) has drawn much attention recently for its potential as a passive cooling method for energy harvesting and thermal management. Sky RC primarily relies on the “atmospheric window” with a high transparency for electromagnetic wave from 8 to 13 μm, allowing some of the heat to effectively escape to the colder universe. In this study, a spectrally selective coating is proposed and designed for the combination of daytime solar thermal (ST) conversion and nocturnal sky RC. Optical simulation is conducted, and results show that the proposed coating has a high solar absorptivity of approximately 0.92 and a strong thermal emissivity only in the “atmospheric window” with an average value of 0.86. Moreover, the performance of the ST conversion and the nocturnal sky RC of the proposed coating is evaluated by thermal analysis. The predicted results show that the proposed coating can be passively cooled to approximately 10 °C below ambient air temperature during night-time and can be heated to 73.4 °C higher than ambient air temperature by solar radiation. In summary, this work provides a new way of hybrid utilisation for solar energy and nocturnal sky RC.