Considerations of passive radiative cooling

Radiative cooling is a passive cooling method, which pumps the waste heat of terrestrial objects to the cold universe. Although much effort has been devoted to radiative cooling, the understanding of cooling properties should be further improved. This paper presents some considerations for radiative cooling to strengthen the understanding of the mechanism. First, it is generally recognized that the broadband cooler can get a higher maximal cooling power and the selective cooler can achieve a maximal temperature reduction, but this characteristic will be destroyed by non-radiative heat transfer between the cooler and surroundings, which means broadband materials will outperform selective materials across the board, violating the general viewpoint. Second, the maximal temperature reduction of the grey body cooler is independent of emissivity when the non-radiative heat transfer process is suppressed, indicating that only using maximal temperature reduction can not fully reflect the real cooling performance of the grey body cooler. Third, the thermal requirement of the potential applications should be highly considered during material design. If improving heat dissipation is always needed, the broadband cooler is the best choice. Otherwise, radiative cooler with dynamic emissivity modulation is the better one to adjust the dynamic requirement of heat dissipation and insulation.