Scalable aqueous processing-based radiative cooling coatings for heat dissipation applications

Abstract

Passive daytime radiative cooling (PDRC) is an electricity-free cooling technology that has received great interest for its’ environmental benefits. Current organic polymer PDRC materials often require harmful organic solvents for dispersion and inorganic dielectric PDRC materials require costly vacuum processing or have poor flexibility and water-repellence. In addition, most PDRC applications focus on refrigeration at sub-ambient temperatures using a selective emittance spectrum. Here a scalable aqueous-based processing method utilizing commercially available hollow dielectric microspheres is reported to create dielectric PDRC coatings with broadband emittance spectra for the heat dissipation of outdoor devices used above-ambient temperatures. A high solar reflectance and thermal infrared emittance of 0.93 at a thickness of 500 μm can be achieved due to the multi-interface reflectivity between the dielectric material and air in the hollow stacked microspheres. The max temperature (∼ 46 °C) of the outdoor device at a heat density of 5000 W m−2 can drop 20 °C at a solar intensity of ∼ 925 W m−2 compared to an Al metal wall (∼ 66 °C), which is only 4 °C higher than the ambient temperature of 42 °C. More importantly, the aqueous paint does not require any volatile organic compounds and has great stability, water repellence, and excellent flexibility.