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.
- Publication:
-
Journal of Applied Physics
- Pub Date:
- June 1981
- DOI:
- 10.1063/1.329270
- Bibcode:
- 1981JAP....52.4205G