Selective surface

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In solar thermal collectors, a selective surface is a means of increasing it's operation temperature and/or efficiency. The selectivity is defined as the ratio of radiation-absorption (alpha) to radiation-emission (epsilon),. Selective surfaces take advantage of the differing wavelengths of incident solar radiation and the emmissive radiation from the absorbing surface.

Normally, a combination of materials is used. One of the first selective surfaces investigated was simply copper with a layer of black cupric oxide. Black chromium ("black chrome") nickel-plated copper is another selective surface that is very durable, highly resistant to humidity or oxidizing atmospheres and extreme temperatures while being able to retain its selective properties - but expensive. Another combination consists of steel plated with gold, silicon, and silicon dioxide. Ordinary steel or aluminum can also be spray-painted black for a cheaper (but less effective) collector surface.

Typical values for a selective surface might be 0.90 absorption and 0.10 emissivity. but can range from 0.8/0.3 for paints on metal to 0.96/0.05 for commercial surfaces and emissivities as low as 0.02 have been obtained in laboratories.