Superhydrophobic bilayer coating for passive daytime radiative cooling

Abstract

Passive radiative cooling is an energy-free cooling method by exchanging thermal radiation with the cold universe through the transparent atmospheric window. Spectrum tailoring of the radiative cooler is the key to daytime radiative cooling in previously reported works. In addition, radiative coolers with large-scale fabrication and self-cleaning characteristics should be further developed to improve their industrial applicability. Herein, we propose a bilayer radiative cooling coating with the superhydrophobic property and a scalable process, by covering TiO2/acrylic resin paint with a silica/poly(vinylidene fluoride-co-hexafluoropropylene) (SiO2/P(VdF-HFP)) composite masking layer. The strong Mie scattering in TiO2/acrylic resin paint contributes to high solar reflection, while the SiO2/P(VdF-HFP) masking layer is responsible for superhydrophobicity and synergetic solar reflection in the ultraviolet band, resulting in an effective solar reflectivity of 94.0 % with an average emissivity of 97.1 % and superhydrophobicity with a water contact angle of 158.9◦. Moreover, the as-fabricated coating can be cooled to nearly 5.8 ◦C below the temperature of commercial white paint and 2.7 ◦C below the local ambient temperature under average solar irradiance of over 700 W m−2. In addition, yearly energy saving of 29.0 %–55.9 % can be achieved after the coating is applied to buildings in Phoenix, Hong Kong, Singapore, Guangzhou, and Riyadh.