Selective surface: Difference between revisions
→top: commonly indicated with Greek letters |
m ce, mainly punctuation |
||
| Line 4: | Line 4: | ||
<!-- 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: |
<!-- 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: |
||
* Solar radiation covers approximately the wavelengths 350 nm to 4000 nm; [[UV-A]], [[Visible light|visible]] and [[near infrared]] ([[Near infrared|NIR]] |
* Solar radiation covers approximately the wavelengths 350 nm to 4000 nm; [[UV-A]], [[Visible light|visible]] and [[near infrared]] ([[Near infrared|NIR]], or [[IR-A]] plus [[IR-B]]). |
||
* Thermal 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: [[Mid-infrared|MIR]], [[LWIR]] or [[IR-C]]. |
* Thermal 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: [[Mid-infrared|MIR]], [[LWIR]] or [[IR-C]]. |
||
== Selective materials == |
== Selective materials == |
||
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 [[temperature]]s while being able to retain its selective properties |
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 [[temperature]]s while being able to retain its selective properties, but expensive. Another combination consists of [[steel]] plated with [[gold]], [[silicon]], and [[silicon dioxide]]. |
||
Although ordinary black paint has high solar absorption, it also has high thermal emissivity, and thus it is not a selective surface. |
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 |
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. |
||
== Other selective surfaces == |
== Other selective surfaces == |
||
There exists other selective surfaces that are not normally used on solar thermal collector surfaces |
There exists other selective surfaces that are not normally used on solar thermal collector surfaces, 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== |
==See also== |
||
Revision as of 10:35, 24 January 2018
 |
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 (αsol) to thermal infrared radiation emission (εtherm).
Selective surfaces take advantage of the differing wavelengths of incident solar radiation and the emissive radiation from the absorbing surface:
- Solar radiation covers approximately the wavelengths 350 nm to 4000 nm; UV-A, visible and near infrared (NIR, or IR-A plus IR-B).
- Thermal 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, LWIR or IR-C.
Selective materials
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.
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.
Other selective surfaces
There exists other selective surfaces that are not normally used on solar thermal collector surfaces, such as low emissivity surfaces used in window glasses, which reflect thermal radiation and have high transmittance factors (being transparent) for visible sunlight.