Editing Selective surface

{{Short description|Ordinary surfaces re-radiate heat they absorb. Selective ones re-radiate only a little.}}
{{More citations needed|date=July 2014}}
[[Image:Col·lectors solars de tub de buit.JPG|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]]s, a '''selective surface''' or '''selective [[Absorption (electromagnetic radiation)|absorber]]''' is a means of increasing its operation temperature and/or efficiency. The selectivity is defined as the [[ratio]] of [[solar radiation]] [[absorption (electromagnetic radiation)|absorption]] (α<sub>sol</sub>) to [[thermal infrared radiation]] [[emission (electromagnetic radiation)|emission]] (ε<sub>therm</sub>).

<!-- Materials that exhibit this combination of characteristics do not, however, exist in nature. -->Selective surfaces take advantage of the differing wavelengths of incident solar radiation and the emissive radiation from the absorbing surface:<ref>{{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}}</ref>
* Solar radiation covers approximately the wavelengths 350&nbsp;nm to 4000&nbsp;nm; [[UV-A]], [[Visible light|visible]] and [[near infrared]]&nbsp;([[Near infrared|NIR]], or [[IR-A]] plus [[IR-B]]).
* Thermal [[infrared radiation]], from materials with temperatures approximately in the interval -40 to 100&nbsp;°C, covers approximately the wavelengths 4000&nbsp;nm to 40,000&nbsp;nm =&nbsp;4&nbsp;um to 40&nbsp;um; The thermal infrared radiation interval being named or covered by: [[Mid-infrared|MIR]], [[LWIR]] or [[IR-C]].

== Materials ==
Normally, a combination of materials is used.  One of the first selective surfaces investigated was a semiconductor-metal tandem<ref name=":0">{{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}}</ref> – simply [[copper]] with a layer of black [[cupric oxide]]. Silicon on metal is also another example.<ref name=":0" /> A different design has ceramic–metal composites ([[cermet]]s) on metal substrates.<ref name=":0" /><ref>{{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}}</ref> [[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 [[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.<ref>{{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}}</ref><ref name=":0" /><ref>[https://supergreensolutions.com Renewable Clean Energy], 23 January 2023</ref>

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.<ref name=":0" />

== Other applications ==
Selective surfaces are used for other applications than solar thermal collectors, such as [[low emissivity]] surfaces used in [[Window#Glazing and filling|window]] glasses, which reflect thermal radiation and have high [[transmittance]] factors (being [[Transparency and translucency|transparent]]) for [[visible light|visible sunlight]].

==See also==
*[[Insulated glazing]]
*[[Solar cooker]]
*[[Solar power]]
*[[Trombe wall]]

==References==
{{reflist}}

==External links==
*[https://web.archive.org/web/20110616122534/http://www.bcas.net/Publication/EnergyNewsletter/Document/2003-5.PDF Selective Surface for efficient Solar Thermal Conversion]
*[http://www.redrok.com/concept.htm#absorptivity Table of absorption and emissivity for various materials]

[[Category:Solar thermal energy]]
[[Category:Optical filters]]