Selective surfaceFrom Wikipedia, the free encyclopedia

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(alpha) -

to thermal infrared radiation-emission (epsilon).

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...4.000 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..100°C,

covers approximately the wavelengths 4.000 nm...40.000 nm = 4 um...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 - and 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. E.g. low

emissivity surfaces used in window glasses, which reflect thermal radiation and has a hightransmittance factor

(e.g. is transparent) to visible sunlight.

Selective Coating 

an optical coating applied to the surface of an element of a solar energy device to reduce thermal radiation losses. Both transparent and nontransparent selective coatings are used: the former are applied to the surfaces of transparent (insulating) elements, and the latter to radiation-absorbing elements.

Nontransparent selective coatings have a high absorptivity (∼0.95) to radiation in the visible and near-infrared regions of the optical spectrum—that is, in the spectral range of incident solar radiation. Their emissivity—that is, the ratio of the radiation emitted by such a surface to the radiation emitted by a blackbody—is low (~0.05) in the far-infrared region, which is the spectral region of radiation losses. Such losses consist in the thermal radiation of an absorbing surface heated to a temperature of 100° to 300°C.

Transparent selective coatings are characterized by a high transmissivity for solar radiation and a high reflectivity for long-wavelength infrared radiation. Thin layers of metal oxides, a number of semiconductor compounds, and some dyes have selective properties. Selective coatings are applied by elec-trodeposition, vacuum deposition, or painting.

Selective coatings used to increase radiation losses constitute a special group: such coatings absorb solar radiation weakly and have a high emissivity. They are used to protect such structures as gas tanks and oil tanks located in the open air. The coatings reduce the heating of the structures in sunny weather

COLD MIRRORS:

Cold mirrors are much like hot mirrors in that they are used to separate IR from the non-IR. The major difference between the two is that cold mirrors transmit IR bands and reflect one or more non-IR bands. As can be seen in our UV Cold Mirror graph, the coating represented is specially designed to reflect more than 95% of UV rays from 350 to 450 nm while transmitting more than 90% between 550 and 1200 nm at 45 degrees angle of incidence. This means that you can use this coating to split a beam and have the longer bandwidths (550 to 1200nm) transmitted while UV rays (350 to 450 nm) are reflected and sent another route. This can help isolate bands that are needed for a particular application while unneeded or possibly harmful bands can be isolated and removed from the application.

One particularly useful application for this kind of dichroic filter we’ve discussed before is in reference to optical fiber. Optical fiber can be damaged by Ultra-Violet and Infra-Red radiation. When a cold mirror is used in conjunction with a hot mirror you can isolate application specific visible light from harmful UV and IR bandwidths such that the visible is transmitted to the optical fiber to be used and the UV/IR are reflected away from the fiber where they will not damage or harm the application.

Cold mirrors are often used in lighting applications where excess heat is not desired and IR radiation is not helpful. In these applications the visible light is reflected to the application and the IR is transmitted away from the application.