Thin nanoporous anodic alumina film on aluminium for passive radiative cooling

We demonstrate a simple, low-cost, and passive radiative cooler based on a monolithic design consisting of thin nanoporous anodic alumina (NAA) films grown on aluminium sheets. The NAA/Al structure maintains a high broadband reflectivity close to 98 $$\%$$ within the solar spectrum (0.4–2.2 $$\mu $$ m) and simultaneously exhibits a high average emissivity of 88 $$\%$$ within the atmospheric infrared (IR) transmission window of 8–13  $$\mu $$ m with the peak IR emission approaching 99 $$\%$$ at a wavelength of 10 $$\mu $$ m. Optical modelling of the system using optical parameters of the materials confirms that the high solar reflectance arises due to the transparent nature of NAA and high reflectivity of bottom Al, while the large thermal IR emissivity arises from the interference effects of the NAA film and the high absorption of IR light due to phonon resonances in alumina at wavelength larger than 10 $$\mu $$ m. Further, we estimate the average cooling power of NAA/Al to be about 136 W  $$\hbox {m}^{-2}$$ at ambient temperature even after including the contribution to heat input from external non-radiative processes. This robust and lightweight NAA/Al can be projected as an excellent alternative to optical solar reflectors used in spacecraft for thermal heat management and rooftop cooling green technologies.