Investigating the effective radiative cooling performance of random dielectric microsphere coatings

Abstract

Dielectric microsphere coatings are gaining increasing interest for passive daytime radiative cooling (PDRC) owing to their potential in mass production and low cost. Understanding the relationship between sphere parameters and their mechanisms of infrared thermal emission and solar reflection may greatly widen the design of microsphere coatings suitable for PDRC applications. In this work, the relationship between microsphere parameters (radius r, fill rate f, thickness t and environment) and PDRC performance is studied. Dielectric sphere in air can scatter more light in the solar spectrum. Maximum solar reflectance (R¯solar) can be obtained when r = 0.5 μm and thermal emittance (ε¯LWIR) in the atmosphere's long-wave infrared (LWIR) transmission window is mainly determined by the effective thickness of dielectric material due to the small sphere compared with infrared wavelengths. The optimal cooling performance based on the single-size sphere can be obtained as R¯solar = 0.949 and ε¯LWIR = 0.919 when r = 0.5 μm, t = 300 μm and f = 0.6. Furtherly by controlling the two-size sphere distribution, the cooling performance at t = 300 μm can be enhanced as R¯solar = 0.961 when r1 - r2 = 0.5 μm - 0.2 μm, and ε¯LWIR is almost the same. In addition, R¯solar can be further enhanced as large as 0.986 on the Al substrate at t = 300 μm, resulting in a net cooling power of 80 W/m2.