Abstract
Passive radiative cooling is a promoting way for thermal energy management. High reflectance of solar light and high emittance in the atmospheric window (8–13 μm) are acquired for effective radiative cooling. In this work, we demonstrate a daytime passive radiative cooler using chemically fabricated porous anodic aluminum oxide (AAO) membranes. Effective medium theory (EMT) has been applied to analyzing the optical properties of the air-doped porous medium. The composite structure is specifically optimized so that it has a high absorbance (emittance) in the far-infrared atmospheric window and nearly no loss in the solar spectrum. The calculated emittance is well reproduced in the experiment by our AAO sample. The fabricated porous membrane shows a potential cooling power density of 64 W/m2 at ambient temperature (humidity ~70%) under direct sunlight irradiance (AM1.5). Experimentally, the sample is cooled by a 2.6 °C temperature reduction below the ambient air temperature in the sunlight. This performance shows little variance at night. The AAO approach proposed here may provide a promising way to produce low-cost and efficient radiative cooler in large scales for feasible energy conservation.