Selective Coating

Two characteristics indicate the Efficiency of the Absorber installed in a solar Collector:

- a high Absorptivity rate, meaning the amount of solar radiation transformed to heat;

- a low emissivity rate, meaning the relative amount of heat loss by heat emission (long-wave radiaton).

In order to reduce losses due to heat emission, highly efficient absorbers are provided with selective coatings. Such coatings make it possible to absorb and transform a large part of the short-wave Solar Radiation into heat, and, simultaneously, to reduce the emission of the long-wave heat radiation from the Absorber itself significantly.

The "classical" selective coatings black chrome and black nickel, produced by galvanic processes, achieve maximum absorptivity rates between 88 and 96 % and emissivity rates around 10 to 15 %.

During the nineties of the 20th century new coating technologies were developed by different companies in Germany, using vacuum sputtering to apply titaniumoxinitrid layers (TINOX) and ceramical coatings. These dark metallic-blue coatings achieve an absorptivity around 95% and very low emissivity rates around 3 to 5 %. Another new coating is "black crystal", developed at the same time in the USA, using a multiphase nano crystallisation process. The solar absorptance values of black crystal runs from 92% to 98%, the thermal emittance from 8% to 25% (depending on different factors). Every of this new coating technologies greatly reduces the exposure of environmental pollution during production and the consumation of water in the production process, compared to black nickel and black chrome.

With some of these coating processes it's possible to use custom colors like gold or green for selective coatings, slightly reducing the Efficiency.

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