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Solar flat plate collectors
The first accurate model of flat plate solar collectors were developed by Hottel and Whillier in the 1950s.
Flat plate collectors are mainly deployed on flat roofs, swimming pool installations or in situataions where they are more aesthetically appropriate. A flat plate collector 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 (thermosyphon). 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, antifreeze 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.
Some types of collectors are specifically designed to contain potable water. In areas where freezing is a possibility, metal collectors must be carefully plumbed so they completely drain down using gravity before freezing can happen so that they do not crack. Other recently developed collectors are freeze tolerant instead. These can freeze without damage because their water channels are typically made of food grade polymers such as silicone rubber, which is flexible and remains stable at temperatures of 200C.
Other collectors are part of a sealed heat exchange system, rather than having the potable water flow directly 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 use of glycol lowers the water's heat carrying capacity only marginally, while the addition of an extra heat exchanger may lower system performance at low light levels.
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