Solar absorptance and emittance properties of several solar coatings

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Solar absorptance αs and total hemispherical emittance εt,H properties of two potential solar selective coating systems are reported. The first coating system studied is a semiconductor-pigmented paint which consists of a high-temperature silicone binder mixed with small particles of Ge, Si, or PbS. Although most of the paints have αs values above 0.90, all the paints suffer from high εt,H values (∠0.70 to 0.90), which are shown to be due to the high emittance of the silicone binder itself. The second system studied is electroplated films of ’’black nickel’’ and a proprietary ’’black chrome.’’ Both αs and εt,H values have been obtained as a function of the electroplating time and the substrate surface roughness. Increasing the plating time initially increases both αs and εt,H; however, for long plating times, there is no improvement in αs, while εt,H continues to increase. By increasing the surface roughness of nickel-plated substrates, the solar absorptance can be increased while maintaining approximately the same total hemispherical emittance. The coatings show minor decreases in solar absorptance at angles of incidence up to 75° from normal (e.g., about 25% decrease at 75°).