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.
Efficient radiative cooling is feasible with surfaces which radiate predominantly in the 8–13-μm wavelength range, where the atmosphere emittance is low. The desired reflectance profile was approximated with 1.0 μm of SiO evaporated onto Al. It results from strong lattice absorption in SiO in conjunction with destructive interference. Refrigeration to 40 K below the ambient appears to be possible under suitable climatic conditions.
Transparent and heat reflecting indium tin oxide films were prepared by electron beam evaporation of In2O3+9 mol.% SnO2 onto glass in an oxygen atmosphere of ∼5×10−4 Torr. Visible light absorption less than 2%, thermal infrared reflectance exceeding 90%, and a dc resistivity of ∼3×10−4 Ω cm were obtained for 0.3-μm-thick films deposited on a substrate at 300 °C. Similar properties could be achieved with substrate temperatures as low as 150 °C. By antireflection coating the films with an MgF2 layer, the visible transmittance exceeded the transmittance of the uncoated glass while the infrared reflectance was practically unchanged.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.