Flat plate collectors

Flat plate collectors, developed by Hottel and Whillier in the 1950s, are the most common type. They consist of (1) a dark flat-plate absorber of solar energy, (2) a transparent cover that allows solar energy to pass through but reduces heat losses, (3) a heat-transport fluid (air, antifreeze or water) to remove heat from the absorber, and (4) a heat insulating backing. The absorber consists of a thin absorber sheet (of thermally stable polymers, aluminum, steel or copper, to which a matte black or selective coating is applied) often backed by a grid or coil of fluid tubing placed in an insulated casing with a glass or polycarbonate cover. In water heat panels, fluid is usually circulated through tubing to transfer heat from the absorber to an insulated water tank. This may be achieved directly or through a heat exchanger. Most air heat fabricators and some water heat manufacturers have a completely flooded absorber consisting of two sheets of metal which the fluid passes between. Because the heat exchange area is greater they may be marginally more efficient than traditional absorbers.
Sunlight passes through the glazing and strikes the absorber plate, which heats up, changing solar energy into heat energy. The heat is transferred to liquid passing through pipes attached to the absorber plate. Absorber plates are commonly painted with "selective coatings," which absorb and retain heat better than ordinary black paint. Absorber plates are usually made of metal—typically copper or aluminum—because the metal is a good heat conductor. Copper is more expensive, but is a better conductor and less prone to corrosion than aluminum. In locations with average available solar energy, flat plate collectors are sized approximately one-half- to one-square foot per gallon of one-day's hot water use.
There are a number of absorber piping configurations:
  • harp — traditional design with bottom pipe risers and top collection pipe, used in low pressure thermosyphon and pumped systems
  • serpentine — one continuous S that maximizes temperature but not total energy yield in variable flow systems, used in compact solar domestic hot water only systems (no space heating role)
  • completely flooded absorber consisting of two sheets of metal stamped to produce a circulation zone.
  • boundary layer absorber collectors consisting of several layers of transparent and opaque sheets that enable absorption in a boundary layer. Because the solar energy is absorbed in the boundary layer, the heat conversion may be more efficient than for collectors where absorbed heat is conducted through a material before the heat is accumulated in a circulating liquid.
As an alternative to metal collectors, new polymer flat plate collectors are now being produced in Europe. These may be wholly polymer, or they may include metal plates in front of freeze-tolerant water channels made of silicone rubber. Polymers, being flexible and therefore freeze-tolerant, are able to contain plain water instead of antifreeze, so that they may be plumbed directly into existing water tanks instead of needing to use heat exchangers which lower efficiency. By dispensing with a heat exchanger in these flat plate panels, temperatures need not be quite so high for the circulation system to be switched on, so such direct circulation panels, whether polymer or otherwise, can be more efficient, particularly at low light levels.
Some early selectively coated polymer collectors suffered from overheating when insulated, as stagnation temperatures can exceed the melting point of the polymer. For example, the melting point of polypropylene is 160 °C (320 °F), while the stagnation temperature of insulated thermal collectors can exceed 180 °C (356 °F) if control strategies are not used. For this reason polypropylene is not often used in glazed selectively coated solar collectors. Increasingly polymers such as high temperate silicones (which melt at over 250 °C (482 °F)) are being used. Some non polypropylene polymer based glazed solar collectors are matte black coated rather than selectively coated to reduce the stagnation temperature to 150 °C (302 °F) or less.
In areas where freezing is a possibility, freeze-tolerance (the capability to freeze repeatedly without cracking) can be delivered by the use of flexible polymers. Silicone rubber pipes have been used for this purpose in UK since 1999. Conventional metal collectors are vulnerable to damage from freezing, so if they are water filled they must be carefully plumbed so they completely drain down using gravity before freezing is expected, so that they do not crack. Many metal collectors are installed as part of a sealed heat exchanger system. Rather than having the potable water flow directly through the collectors, a mixture of water and antifreeze such as propylene glycol (which is used in the food industry) is used as a heat exchange fluid to protect against freeze damage down to a locally determined risk temperature that depends on the proportion of propylene glycol in the mixture. The use of glycol lowers the water's heat carrying capacity marginally, while the addition of an extra heat exchanger may lower system performance at low light levels.
A pool or unglazed collector is a simple form of flat-plate collector without a transparent cover. Typically polypropylene or EPDM rubber or silicone rubber is used as an absorber. Used for pool heating it can work quite well when the desired output temperature is near the ambient temperature (that is, when it is warm outside). As the ambient temperature gets cooler, these collectors become less effective.
Most flat plate collectors have a life expectancy of over 25 years.
Applications: The main use of this technology is in residential buildings where the demand for hot water has a large impact on energy bills. This generally means a situation with a large family, or a situation in which the hot water demand is excessive due to frequent laundry washing. Commercial applications include laundromats, car washes, military laundry facilities and eating establishments. The technology can also be used for space heating if the building is located off-grid or if utility power is subject to frequent outages. Solar water heating systems are most likely to be cost effective for facilities with water heating systems that are expensive to operate, or with operations such as laundries or kitchens that require large quantities of hot water.
unglazed liquid collectors are commonly used to heat water for swimming pools. Because these collectors need not withstand high temperatures, they can use less expensive materials such as plastic or rubber. They also do not require freeze-proofing because swimming pools are generally used only in warm weather or can be drained easily during cold weather.
While solar collectors are most cost-effective in sunny, temperate areas, they can be cost effective virtually anywhere in the country so should be considered.

Origin :  http://en.wikipedia.org/wiki/Solar_bowl 
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