A dual-layer polymer-based film for all-day sub-ambient radiative sky cooling

Abstract

Radiative sky cooling (RSC) is a promising eco-friendly technique that requires no energy input for sub-ambient cooling. A radiative cooler with high solar reflection and strong thermal emission is the key to achieving sub-ambient cooling effect. Recently, polymer-based coolers have attracted much attention due to their excellent radiative properties and flexibility. Herein, the parasitic absorption of functional groups is applied to select polymer materials for radiative cooling, and a dual-layer film consisting of ethylene-tetra-fluoro-ethylene (ETFE) film and silver layer is fabricated for all-day sub-ambient RSC. Optical characterization shows that the fabricated ETFE cooler exhibits a high AM1.5 spectra-weighed solar reflectivity of 94% and has an average emissivity of nearly 0.83 in the atmospheric window (i.e.,8–13 μm). Besides, thermal performance tests reveal that the cooler's temperature is on average 3.0 °C lower than ambient air during daytime in Hefei, and is approximately 1.6 °C below ambient air even at noon. Additionally, the thermal performance prediction also indicates that the ETFE cooler is a good candidate for sub-ambient RSC.