Radiative cooling: A coating for windows that also cools

A Korean team has proposed an approach for passive radiative cooling. The team, which includes professors Junsuk Rho and Jin Kon Kim of Pohang Universiy of Science and Technology (Pohang, South Korea; www.postech.ac.kr) and professor Heon Lee of Korea University (Seoul, South Korea; www.korea.ac.kr), says its work demonstrates that a simple and low-cost single membrane can be used to produce efficient daytime radiative cooling, whereas many previous methods have required the precise design of multilayers and patterns. “We believe our approach is suitable for the mass production of a low-cost radiative cooler which would help reduce global energy consumption,” the researchers say.

The researchers used porous anodic-aluminum-oxide (AAO) coated with thin layers of SiO₂. The AAO shows near perfect spectral emissivity in the atmospheric window in the mid-infrared spectrum (8–13µm). According to the team, conventional AAO has a large extinction coefficient over 10µm, and by itself does not produce the required near perfect spectral emissivity band over the entire atmospheric window.

The SiO₂-coated AAO membrane shows an average cooling flux of 65.6 W/m² during the daytime, and a maximum cooling of 6.1°C below ambient temperature under direct sunlight. The final design exhibited a high reflectance of 0.86 in the solar spectral region, and an average emissivity of 0.96 in the atmospheric window.