The use of porcelain enamelled absorbers in flat plate collectors features longevity thanks to the durability and thermal stability of the enamel finish. The porcelain enamel can be made spectral selective by coating with doped tin oxide or indium oxide. The application procedure involves an enamelling step followed by a pyrosol process with tin or indium compounds. The optical properties of tin oxide coated enamel yield values of 0.90–0.92 absorptance and 0.13–0.18 hemispherical emittance. The temperature dependence of the emittance is very small. The thermal stability has been proved up to 400°C in air.

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