Solar thermal collector
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{{Mergefrom | Solar collector | Talk:Solar thermal collector#Merge proposal |date=June 2008 }}
[[Image:Russiasolar.jpg|right|thumb|Solar Thermal Collector Dish]]
{{portal|Energy}}
A '''solar thermal collector''' is a [[solar collector]] specifically intended to collect [[heat]]: that is, to [[Absorption (optics)|absorb]] [[sunlight]] to provide heat. Although the term may be applied to simple [[solar hot water panel]]s, it is usually used to denote more complex installations. There are various types of thermal collectors, such as solar parabolic, solar trough and solar towers. These type of collectors are generally used in [[solar power plant]]s where solar heat is used to generate [[electricity]] by heating water to produce steam and driving a [[turbine]] connected to the [[electrical generator]].
== Types of collectors==
Flat plate and box-type collectors are typically used in domestic and light industry applications. Parabolic troughs, dishes and towers are used almost exclusively in [[solar power generating station]]s or for research purposes. Conversion efficiency of a solar collector is expressed as eta0 or η0.
===Flat plate===
[[Image:Solar panels, Santorini.jpg|thumb|right|Solar thermal system for water heating - these are deployed on flat roof.]]
A flat plate is the most common type of solar thermal collector, and is usually used as a solar hot water panel to generate [[solar hot water]]. A weatherproofed, insulated box containing a black metal absorber sheet with built in pipes is placed in the path of sunlight. Solar energy heats up water in the pipes causing it to circulate through the system by natural convection. The water is usually passed to a storage tank located above the collector. This passive solar water heating system is generally used in hotels and homes in sunny climates such as those found in southern Europe.
For these purposes, the general practice is to use flat-plate [[solar energy]] or evacuated tube collectors with a fixed orientation (position). The highest efficiency with a fixed flat-plate collector or evacuated tube collector is obtained if it faces toward the [[sun]] and slopes at an angle to the horizon equal to the latitude plus about 10 degrees. Solar collectors fall into two general categories: non-concentrating and concentrating.
In the non-concentrating type, the collector area (i.e. the area that intercepts the solar radiation) is the same as the absorber area (i.e., the area absorbing the radiation).
There are many flat-plate collector designs but generally all consist of (1) a flat-plate absorber, which intercepts and absorbs the solar energy, (2) a transparent cover(s) that allows solar energy to pass through but reduces heat loss from the absorber, (3) a heat-transport fluid (air or water) flowing through tubes to remove heat from the absorber, and (4) a heat insulating backing. One flat plate collector is designed to be evacuated, to prevent heat loss.
The most effective use of collectors is with a sealed heat exchange system, rather than having the potable water flow through the collectors. A mixture of water and propylene glycol (which is used in the food industry) can be used as a heat exchange fluid to protect against freeze damage, up to a temperature that depends on the proportion of propylene glycol in the mixture.
The first accurate model of flat plate solar collectors were developed by Hottel and Whillier in the 1950's.
===Evacuated Tube===
[[Image:Evacuated tube collector.gif|thumb|Evacuated (or vacuum) tubes panel.]]
These collectors have multiple [[Vacuum|evacuated]] [[glass]] tubes which heat up solar absorbers and, ultimately, solar working fluid (water or an [[antifreeze]] mix -- typically [[propylene glycol]]) in order to heat domestic hot water, or for [[hydronic]] space heating. The vacuum within the evacuated tubes reduce conducted heat losses, allowing them to reach considerably higher temperatures than most flat-plate collectors. For this reason they can perform well in colder conditions. The advantage is largely lost in warmer climates, except in those cases where very hot water is desirable, for example commercial process water. The high temperatures that can occur may require special system design to avoid or mitigate overheating conditions. A further advantage this design has over the flat-plate type is that the constant profile of the round tube means that the collector is always perpendicular to the sun's rays and therefore the energy absorbed is approximately constant over the course of a day.
===Pool or Unglazed===
This type of collector is much like a flat-plate collector, except that it has no glazing/transparent cover. It is used extensively for pool heating, as it works quite well when the desired output temperature is near the ambient temperature (that is, when it's warm outside). As the ambient temperature gets cooler, these collectors become extremely ineffective.
===Air===
These collectors heat air directly, almost always for space heating. They are also used for pre-heating make-up air in commercial and industrial [[HVAC]] systems
===Box type===
A common solar cooker is a [[solar box cooker|box type collector]]. It is a metal box open from top, and insulated from sides with an equally sized mirror hinged to it (like a simple box with a mirror attached to the underside of the cover).
===[[Parabolic trough]]===
[[Image:Parabolic trough.svg|250px|thumb|left|Parabolic trough]]
This type of collector is generally used in [[solar power plant]]s. A trough-shaped [[parabolic reflector]] is used to concentrate sunlight on an insulated tube ([[Dewar tube]]) or [[heat pipe]], placed at the [[focal point]], containing [[coolant]] which transfers heat from the collectors to the [[boilers]] in the power station.
===Parabolic dish===
[[Image:Parabolic-dish.jpg|right|thumb|Solar Parabolic dish]]
It is the most powerful type of collector which concentrates sunlight at a single, focal point, via one or more [[parabola|parabolic]] dishes -- arranged in a similar fashion to a [[reflecting telescope]] focuses starlight, or a [[dish antenna]] focuses radio waves. This geometry may be used in [[solar furnace]]s and [[solar power plant]]s.
There are two key phenomenena to understand in order to comprehend the design of a parabolic dish. One is that the shape of a parabola is defined such that incoming rays which are parallel to the dish's axis will be reflected toward the focus, no matter where on the dish they arrive. The second key is that the light rays from the sun arriving at the earth's surface are almost completely parallel. So if dish can be aligned with its axis pointing at the sun, the incoming radiation will almost all be reflected towards the focal point of the dish -- most losses are due to imperfections in the parabolic shape and imperfect reflection.
Losses due to atmosphere between the dish and its focal point are minimal, as the dish is generally designed specifically to be small enough that this factor is insignificant on a clear, sunny day. Compare this though with some other designs, and you will see that this could be an important factor, and if the local weather is hazy, or foggy, it may reduce the efficiency of a parabolic dish significantly.
In some power plant designs, a [[stirling engine]] coupled to a dynamo, is placed at the focus of the dish, which absorbs the heat of the incident solar radiation, and converts it into electricity. See [http://sunenergyworld.blogspot.com/2006/03/knowing-parabolic-concentrators.html Knowing Parabolic Concentrators] and [http://www.energylan.sandia.gov/sunlab/overview.htm#dish Concentrating Solar power overview]
===Power tower===
[[Image:Power Tower.jpg|thumb|Power Tower]]
A power tower is a large tower surrounded by small rotating (tracking) mirrors called [[heliostat]]s. These mirrors align themselves and focus sunlight on the receiver at the top of tower, collected heat is transferred to a power station below.
===Solar pyramids===
Another design is a pyramid shaped structure, which works by drawing in air, heating it with solar energy and moving it through turbines to generate electricity. Solar pyramids have been built in places like [[Australia]]. Currently [[Solar power in India|India]] is building such pyramids.<ref>[http://www.treehugger.com/files/2006/03/solar_pyramids.php Solar Pyramids]</ref>
==Advantages==
* Very high temperatures reached. High temperatures are suitable for electricity generation using conventional methods like [[steam turbine]] or some direct high temperature chemical reaction. (reference needed)
* Good efficiency. By concentrating sunlight current systems can get better efficiency than simple solar cells.
* A larger area can be covered by using relatively inexpensive mirrors rather than using expensive [[solar cell]]s.
* Concentrated light can be redirected to a suitable location via [[optical fiber cable]]. For example illuminating buildings, like here ([http://www.ornl.gov/sci/solar/ Hybrid Solar Lighting]).
==Disadvantages==
* Concentrating systems require dual axis [[solar tracker|sun tracking]] to maintain Sunlight focus at the collector.
* Inability to provide power in [[diffuse reflection|diffused light]] conditions. Solar Cells are able to provide some output even if the sky becomes a little bit cloudy, but power output from concentrating systems drop drastically in cloudy conditions as diffused light cannot be concentrated passively.
== See also ==
*[[Solar hot water]]
*[[Insulated glazing]]
*[[Selective surface]]
*[[Solar collector]]
*[[Solar oven]]
*[[Solar heating]]
*[[Trombe wall]]
*[[Zeolite]]
== External links ==
* [http://www.solarenergy.ch/spf.php?lang=en&fam=1&tab=3 formula for calculating efficiency of solar collector]
==References==
<references />
*[http://solar-rating.org/ratings/ratings.htm The US ratings] of solar thermal collectors, updated regularly -- flat plate, evacuated, air and pool collectors rated
*[http://www.solarenergy.ch/publ/materialien/sorption01.pdf [[Sorption]] Materials for Application in Solar Heat Energy Storage]
*Hottel, H. C. and Whillier, A.: "Evaluation of Flat-Plate Solar Collector Performance," Trans. of the Conference on the Use of Solar Energy - The Scientific Basis, Vol. 2, Tucson, AZ, Oct. 31- Nov. 1, 1955, pp 74-104.
[[Category:Solar thermal]]
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