Investigating the effective radiative cooling performance of random dielectric microsphere coatings

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) over bar (solar)) can be obtained when r = 0.5 mu m and thermal emittance (<(epsilon)overbar>(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) over bar (solar) = 0.949 and (epsilon) over bar (LWIR) = 0.919 when r = 0.5 mu m, t = 300 mu m and f = 0.6. Furtherly by controlling the two-size sphere distribution, the cooling performance at t = 300 mu m can be enhanced as (R) over bar (solar) = 0.961 when r(1) - r(2) = 0.5 mu m - 0.2 mu m, and (epsilon) over bar (LWIR) is almost the same. In addition, (R) over bar (solar) can be further enhanced as large as 0.986 on the Al substrate at t = 300 mu m, resulting in a net cooling power of 80 W/m(2). (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

Dielectric microsphere coatings are increasingly being utilized for passive daytime radiative cooling due to their potential in mass production and low cost. By studying the relationship between microsphere parameters and PDRC performance, it is possible to achieve optimal cooling performance.