- Selective surface
solar thermal collectors, a selective surface is a means of increasing its operation temperature and/or efficiency. The selectivity is defined as the ratioof radiation- absorption("alpha") to radiation-emission ("epsilon"),. 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 simply
copperwith a layer of black cupric oxide. Black chromium("black chrome") nickel-plated copperis another selective surface that is very durable, highly resistant to humidityor oxidizingatmospheres and extreme temperatures while being able to retain its selective properties - but expensive. Another combination consists of steelplated with gold, silicon, and silicon dioxide.
Although ordinary black paint has high absorption, it also has high emissivity, and thus its selectivity is low. So it is not considered to be a true "selective 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.
Solar box cooker
Solar thermal collector
* [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]
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