In solar thermal collectors, a selective surface or selective absorber is a means of increasing its operation temperature and/or efficiency. The selectivity is defined as the ratio of solar radiation absorption (?sol) to thermal infrared radiation emission (?therm).
Selective surfaces take advantage of the differing wavelengths of incident solar radiation and the emissive radiation from the absorbing surface:
Normally, a combination of materials is used. One of the first selective surfaces investigated was a semiconductor-metal tandem - simply copper with a layer of black cupric oxide. Silicon on metal is also another example. A different design has ceramic-metal composites (cermets) on metal substrates.Black chromium ("black chrome") and nickel-plated anodized aluminum 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. One of the more popular designs - a multi-layer broadband solar absorber - consists of a metal substrate coated with multiple layers of metal and dielectric materials. While those have to be vacuum-deposited, they have been widely adopted due to their suitability for vacuum tubes.
Although ordinary black paint has high solar absorption, it also has high thermal emissivity, and thus it is not a selective surface.
Typical values for a selective surface might be 0.90 solar absorption and 0.10 thermal emissivity, but can range from 0.8/0.3 for paints on metal to 0.96/0.05 for commercial surfaces. Thermal emissivities as low as 0.02 have been obtained in laboratories.
Selective surfaces are used for other applications than solar thermal collectors, such as low emissivity surfaces used in window glasses, which reflect thermal radiation and have high transmittance factors (being transparent) for visible sunlight.