The level of absorption indicates the amount of short-wave solar radiation being absorbed (i.e., not reflected). As the absorber warms up to a temperature higher than the ambient temperature, it gives off most of the accumulated solar energy in the form of long-wave heat rays. The ratio of absorbed energy to emitted heat is indicated by the degree of emission. In order to reduce energy loss through heat emission, the most efficient absorbers have a selective surface coating. This coating enables the conversion of a high proportion of the solar radiation into heat, simultaneously reducing the emission of heat.
The usual coatings provide a degree of absorption of over 90%. Solar paints, which can be mechanically applied to the absorbers (with either brushes or sprays), are less or not at all selective, as they have a high level of emission. Galvanically applied selective coatings include black chrome, black nickel, and aluminum oxide with nickel. Relatively new is a titanium-nitride-oxide layer, which is applied via steam in a vacuum process. This type of coating stands out not only because of its quite low emission rates, but also because its production is emission-free and energy efficient.
Related category• SOLAR ENERGY AND POWER
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