A self-adaptive film for passive radiative cooling and solar heating regulation

Abstract

Although passive radiative cooling has received great attention recently because it can minimize the absorption of thermal energy and emit its own thermal energy into cold outer space, the existing designs inevitably increase the heating load to offset the unwanted cooling on winter days and thus significantly affect the application scope. Herein, we designed a self-adaptive film with a sandwich structure, based on a thermochromic poly(N-isopropylacrylamide) (PNIPAm) hydrogel and polyvinylidene fluoride (PVDF) film with high infrared emissivity. This film exhibits large visible light reflectance/transmittance modulation (ΔR_vis = 70.0% and ΔT_vis = 86.3%) and a high longwave infrared emissivity of 0.96. Outdoor tests demonstrated that the PVDF@PNIPAm film self-adaptively realizes highly efficient sub-ambient radiative cooling of 1.8–3.7 °C during hot daytime and above-ambient solar heating of 4.3–5.8 °C during cold daytime (ΔT_{cooling–heating} = ∼9.5 °C), which is favorable for cooling/heating applications in different geographic locations and various weather conditions.