A solar thermal collector collects heat by absorbing sunlight. A collector is a device for capturing solar radiation. Solar radiation is energy in the form of electromagnetic radiation from the infrared (long) to the ultraviolet (short) wavelengths. The quantity of solar energy striking the Earth's surface averages about 1,000 watts per square meter under clear skies, depending upon weather conditions, location and orientation.
The term "solar collector" commonly refers to solar hot water panels, but may refer to installations such as solar parabolic troughs and solar towers; or basic installations such as solar air heaters. Solar power plants usually use the more complex collectors to generate electricity by heating a fluid to drive a turbine connected to an electrical generator.[1] Simple collectors are typically used in residential and commercial buildings for space heating.
Heat collectors
Solar collectors are either non-concentrating or 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). In these types the whole solar panel absorbs light. Concentrating collectors have a bigger interceptor than absorber.[2]
Flat-plate and evacuated-tube solar collectors are used to collect heat for space heating, domestic hot water or cooling with an absorption chiller.
Flat plate collectors
Flat plate thermal system for water heating deployed on a flat roof.
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, (2) a transparent cover that 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.[3] 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. (See: Copper in solar water heaters). 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. Absorber piping configurations include:
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);
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 energy is absorbed in the boundary layer, 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.[citation needed]
Polymer flat plate collectors are an alternative to metal collectors and 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 are flexible and therefore freeze-tolerant and can employ plain water instead of antifreeze, so that they may be plumbed directly into existing water tanks instead of needing heat exchangers that lower efficiency. By dispensing with a heat exchanger, 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 polymer's melting point.[4][5] 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 achieved 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 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 potable water flow directly through the collectors, a mixture of water and antifreeze such as propylene glycol is used. A heat exchange fluid protects 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.
The term "solar collector" commonly refers to solar hot water panels, but may refer to installations such as solar parabolic troughs and solar towers; or basic installations such as solar air heaters. Solar power plants usually use the more complex collectors to generate electricity by heating a fluid to drive a turbine connected to an electrical generator.[1] Simple collectors are typically used in residential and commercial buildings for space heating.
Heat collectors
Solar collectors are either non-concentrating or 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). In these types the whole solar panel absorbs light. Concentrating collectors have a bigger interceptor than absorber.[2]
Flat-plate and evacuated-tube solar collectors are used to collect heat for space heating, domestic hot water or cooling with an absorption chiller.
Flat plate collectors
Flat plate thermal system for water heating deployed on a flat roof.
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, (2) a transparent cover that 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.[3] 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. (See: Copper in solar water heaters). 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. Absorber piping configurations include:
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);
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 energy is absorbed in the boundary layer, 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.[citation needed]
Polymer flat plate collectors are an alternative to metal collectors and 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 are flexible and therefore freeze-tolerant and can employ plain water instead of antifreeze, so that they may be plumbed directly into existing water tanks instead of needing heat exchangers that lower efficiency. By dispensing with a heat exchanger, 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 polymer's melting point.[4][5] 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 achieved 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 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 potable water flow directly through the collectors, a mixture of water and antifreeze such as propylene glycol is used. A heat exchange fluid protects 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.
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