Performance evaluation of daytime radiative cooling under different clear sky conditions

Abstract

Daytime radiative cooling (DRC) is a new concept of passive cooling that can achieve sub-ambient cooling phenomenon even under direct sunshine at daytime, which has attracted considerable attention and has been demonstrated by various spectral-selective emitters in many studies in recent years. In addition to spectral-selective emitters, sky condition affects the performance of DRC. Here, the performance of the DRC under different clear sky conditions was investigated. A field experimental demonstration was conducted on the basis of a silica (SiO2) mirror emitter that was fabricated by depositing 150-nm-thick silver (Ag) and 50-nm-thick SiO2 on a 500-μm-thick SiO2 substrate. Outdoor experimental results show that the SiO2 mirror emitter maintains an average temperature of approximately 5.9 °C below ambient temperature over the testing period in Beijing, China (116°E, 40°N) but barely achieves daytime radiative cooling in Hefei, China (117°E, 32°N), which implies that DRC is more feasible in dry Beijing than in moist Hefei. Moreover, the effect of solar radiation and the precipitable water vapor on the performance of DRC was numerically explored. The results show that the sub-ambient cooling performance of DRC will be decreased rapidly and then destroyed with increased solar radiation and the precipitable water vapor even in a clear sky condition. This initial investigation provides a reference for the regional/seasonal applicability of DRC.