Both sub-ambient and above-ambient conditions: a comprehensive approach for the efficient use of radiative cooling

Abstract

In real-world radiative cooling applications, cooling surface temperatures may periodically fluctuate between sub- and above-ambient conditions. Traditional radiative cooling surfaces with ‘static’ spectral properties cannot realize high-efficiency cooling owing to different spectral requirements for different working scenarios. Herein, we report an infrared self-adaptive radiative cooling (ISRC) approach to selectively regulate emission spectra in the range out of the atmospheric window, resulting in a broadband emitter or an atmospheric window-selective emitter. A bilayer structure that consists of an upper microporous SiO₂ fiber layer and a bottom poly(N-isopropylacrylamide) hydrogel layer was developed. Through directional transportation of a broadband emission liquid (i.e., water) in thermo-response hydrogels, the switch of the spectra between selective infrared emissions (∼0.85) under the sub-ambient cooling condition and broadband emissions (∼0.92) under the above-ambient cooling condition was achieved. Improved temperature reductions of ∼4.1 °C (sub-ambient condition) and ∼12.4 °C (above-ambient condition) were measured compared to ‘static’ spectral radiative coolers. In addition, we implemented the simultaneous maximum improvement of daytime photovoltaic (12%) and nighttime thermoelectric (80%) power with the ISRC for round-the-clock electricity generation. The proposed ISRC approach demonstrates a comprehensive way to the efficient use of radiative cooling.