Preliminary experimental study of a specular and a diffuse surface for daytime radiative cooling

Abstract

Radiative cooling is a natural phenomenon in which earthbound objects emit energy into outer space through thermal radiation. Emitters should have high spectral reflectivity in the solar radiation band and high spectral absorptivity (emissivity) in the infrared band, especially the atmospheric window band (i.e., 0.3–3 and 8–13 µm), to efficiently utilize radiative cooling. In this work, we proposed two types of surfaces to achieve such spectral properties based on the analysis of selective and broadband radiator cooling performance. One type is a specular surface made by ultra-white glass-plated silver (Ag). The other type is a diffuse surface made by spraying zinc phosphate sodium (NaZnPO4) particles on aluminum (Al) substrate. Sample tests showed that both surfaces can effectively reflect sunlight and strongly exchange heat with outer space via thermal radiation. A radiative cooling apparatus with simple structure was built, and the equilibrium temperatures of the surfaces were tested. Both surfaces achieved consecutive 24 h sub-ambient radiative cooling effect. The equilibrium temperatures of the specular and diffuse surfaces at nighttime were lower than those of ambient air by approximately 9.8 °C and 7.3 °C, respectively. When solar irradiance exceeded 430 W m−2 at noontime, the two surfaces reached equilibrium temperatures of 2.5 °C and 1.5 °C lower than those of the surroundings. Both surfaces showed daytime radiative cooling characteristics and were superior to existing radiative coolers in terms of cost and fabrication.