an optical coating applied to the surface of an element of a solar energy device to reduce thermal radiation losses. Both transparent and nontransparent selective coatings are used: the former are applied to the surfaces of transparent (insulating) elements, and the latter to radiation-absorbing elements.
Nontransparent selective coatings have a high absorptivity (∼0.95) to radiation in the visible and near-infrared regions of the optical spectrum—that is, in the spectral range of incident solar radiation. Their emissivity—that is, the ratio of the radiation emitted by such a surface to the radiation emitted by a blackbody—is low (~0.05) in the far-infrared region, which is the spectral region of radiation losses. Such losses consist in the thermal radiation of an absorbing surface heated to a temperature of 100° to 300°C.
Transparent selective coatings are characterized by a high transmissivity for solar radiation and a high reflectivity for long-wavelength infrared radiation. Thin layers of metal oxides, a number of semiconductor compounds, and some dyes have selective properties. Selective coatings are applied by elec-trodeposition, vacuum deposition, or painting.
Selective coatings used to increase radiation losses constitute a special group: such coatings absorb solar radiation weakly and have a high emissivity. They are used to protect such structures as gas tanks and oil tanks located in the open air. The coatings reduce the heating of the structures in sunny weather.
REFERENCESSheklein, A. V., and N. B. Rekant. “Nekotorye ekspluatatsionnye kha-rakteristiki selektivnoi prozrachnoi izoliatsii.” GeUotekhnika, 1971, no. 3.
Koltun, M. M. “Selektivnye poverkhnosti i pokrytiia v geliotekhnike.” GeUotekhnika, 1971, no. 5.
Duffie, J. A., and W. A. Beckman. Solar Energy Thermal Processes. New York, 1974.
A. V. SHEKLEIN