Radiative cooling to low temperatures: General considerations and application to selectively emitting SiO films

Abstract

Radiative cooling occurs because the atmospheric emittance is low in the wavelength interval 8–13 μm particularly if the air is dry. We derive expressions which specify the optical properties demanded for a surface capable of being cooled to low temperatures. The key factor is infrared selectivity with low reflectance in the 8–13 μm ’’window’’ but high reflectance elsewhere. Considering only radiation balance, ideal surfaces of this type can yield temperature differences of ∼50 °C while the cooling power at near-ambient temperatures is ∼100 W/m2. However, nonradiative exchange limits the practically achievable temperature difference. SiO films on Al were investigated as an example of an infrared-selective surface. The infrared optical properties of SiO were determined by a novel and accurate technique. These data were used to compute the spectral radiative properties of Al coated with SiO films of different thicknesses. The spectral selectivity was largest for 1.0-μm-thick films. This kind of surface was produced by evaporation of SiO onto smooth Al. The measured reflectance agreed with computations. Practical tests of radiative cooling were performed using a SiO-coated Al plate placed under transparent polyethylene films in a polystyrene box. An identical panel containing a blackbody radiator was used for comparison. The performance of the panels was tested during clear nights. It was in good qualitative agreement with theoretical expectations.