Selective surface
Encyclopedia
In solar thermal collector
s, 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:
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 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 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.
factor (e.g. is transparent
) to visible sunlight.
Solar thermal collector
A solar thermal collector is a solar collector designed to collect heat by absorbing sunlight. The term is applied to solar hot water panels, but may also be used to denote more complex installations such as solar parabolic, solar trough and solar towers or simpler installations such as solar air...
s, a selective surface or selective absorber
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
is a means of increasing its operation temperature and/or efficiency. The selectivity is defined as the ratio
Ratio
In mathematics, a ratio is a relationship between two numbers of the same kind , usually expressed as "a to b" or a:b, sometimes expressed arithmetically as a dimensionless quotient of the two which explicitly indicates how many times the first number contains the second In mathematics, a ratio is...
of solar radiation-absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
(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 copperCopper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
with a layer of black cupric oxide. Black
Black
Black is the color of objects that do not emit or reflect light in any part of the visible spectrum; they absorb all such frequencies of light...
chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
("black chrome") nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
-plated copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
is another selective surface that is very durable, highly resistant to humidity
Humidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
or oxidizing atmospheres and extreme temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
s while being able to retain its selective properties - but expensive. Another combination consists of steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
plated with gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
, silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
, and silicon dioxide
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
.
Although ordinary black paint has high solar absorption, it also has high thermal 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 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 high transmittanceTransmittance
In optics and spectroscopy, transmittance is the fraction of incident light at a specified wavelength that passes through a sample. A related term is absorptance, or absorption factor, which is the fraction of radiation absorbed by a sample at a specified wavelength...
factor (e.g. is transparent
Transparency and translucency
In the field of optics, transparency is the physical property of allowing light to pass through a material; translucency only allows light to pass through diffusely. The opposite property is opacity...
) to visible sunlight.
See also
- Insulated glazingInsulated glazingInsulated glazing also known as double glazing are double or triple glass window panes separated by an air or other gas filled space to reduce heat transfer across a part of the building envelope....
- Solar box cooker
- Solar oven
- Solar powerSolar powerSolar energy, radiant light and heat from the sun, has been harnessed by humans since ancient times using a range of ever-evolving technologies. Solar radiation, along with secondary solar-powered resources such as wind and wave power, hydroelectricity and biomass, account for most of the available...
- Trombe wallTrombe wallA Trombe wall is a sun-facing wall separated from the outdoors by glass and an air space, which absorbs solar energy and releases it selectively towards the interior at night. The essential idea was first explored by Edward S. Morse and patented by him in 1881...