  We report 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 (humidity = 75%) under direct
sunlight irradiance (AM1.5). Experimentally, the sample is cooled by a 2.6C
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.
