# Selective surface
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{{Short description|Surface with certain thermo-radiative properties}}
{{More citations needed|date=July 2014}}
right|thumb|Solar thermal collector system based on evacuated glass tubes. Sunlight is absorbed by a special material at the center of each tube that has a selective surface. The surface absorbs sunlight nearly completely, and emits very little of the solar heat as thermal radiation. Ordinary black surfaces are also efficient absorbers, but they emit thermal radiation copiously.
In [solar thermal collector](/source/solar_thermal_collector)s, a '''selective surface''' or '''selective [absorber](/source/Absorption_(electromagnetic_radiation))''' is a means of increasing its operation temperature and/or efficiency. The selectivity is defined as the [ratio](/source/ratio) of [solar radiation](/source/solar_radiation) [absorption](/source/absorption_(electromagnetic_radiation)) (αsol) to [thermal infrared radiation](/source/thermal_infrared_radiation) [emission](/source/emission_(electromagnetic_radiation)) (εtherm).
Selective surfaces take advantage of the differing wavelengths of incident solar radiation and the emissive radiation from the absorbing surface:[{{Cite web|last=|first=|date=|title=Absorbing Surfaces|url=https://www.impact-absorbing-surfaces.co.uk/|archive-url=|archive-date=|access-date=2020-11-26|website=www.impact-absorbing-surfaces.co.uk}}]
* Solar radiation covers approximately the wavelengths 350 nm to 4000 nm; [UV-A](/source/UV-A), [visible](/source/Visible_light) and [near infrared](/source/near_infrared) ([NIR](/source/Near_infrared), or [IR-A](/source/IR-A) plus [IR-B](/source/IR-B)).
* Thermal [infrared radiation](/source/infrared_radiation), from materials with temperatures approximately in the interval -40 to 100 °C, covers approximately the wavelengths 4000 nm to 40,000 nm = 4 um to 40 um; The thermal infrared radiation interval being named or covered by: [MIR](/source/Mid-infrared), [LWIR](/source/LWIR) or [IR-C](/source/IR-C).
== Materials ==
Normally, a combination of materials is used. One of the first selective surfaces investigated was a semiconductor-metal tandem[{{Cite web|url=https://www.nrel.gov/docs/fy02osti/31267.pdf|title=Review of Mid- to High-Temperature Solar Selective Absorber Materials|last=Kennedy|first=Cheryl|date=2002|website=NREL|accessdate=21 February 2018}}] – simply [copper](/source/copper) with a layer of black [cupric oxide](/source/cupric_oxide). Silicon on metal is also another example.[{{Cite journal|last1=Tesfamichael|first1=Tuquabo|last2=Wäckelgård|first2=Ewa|date=1999-07-01|title=Angular solar absorptance of absorbers used in solar thermal collectors|url=https://www.osapublishing.org/abstract.cfm?uri=ao-38-19-4189|journal=Applied Optics|language=EN|volume=38|issue=19|pages=4189–4197|doi=10.1364/AO.38.004189|pmid=18323901 |issn=2155-3165|bibcode=1999ApOpt..38.4189T|url-access=subscription}}] [Black](/source/Black) [chromium](/source/chromium) ("black chrome") and [nickel](/source/nickel)-plated anodized aluminum is another selective surface that is very durable, highly resistant to [humidity](/source/humidity) or [oxidizing](/source/oxidizing) atmospheres and extreme [temperature](/source/temperature)s, 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.[{{Cite web|url=http://www.almecogroup.com/en/pagina/53-tinox-energy-cu|title=TiNOX energy|last=|first=|date=2018|website=Alemco|accessdate=21 February 2018}}][[https://supergreensolutions.com Renewable Clean Energy], 23 January 2023]
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.