Sub-ambient daytime radiative cooling by silica-coated porous anodic aluminum oxide

An energy-free daytime radiative cooler based on silica-coated porous anodic aluminum oxide is proposed, optimized, and experimentally realized. It is shown that a simple thin silica layer coating on porous anodic aluminum oxide can produce the spectral emissivity in the atmospheric window (8-13μm) required for sub-ambient cooling effect under direct sunlight. The final design, optimized using effective medium theory, exhibits high reflectance of 0.86 in the solar spectral region, and a substantial average emissivity of 0.96 in the atmospheric window. The fabricated centimeter-scale radiative cooler demonstrates a maximum cooling of 6.1 °C below ambient during the daytime. We believe that the proposed approach is a promising way to produce inexpensive and efficient radiative coolers.