A solar thermal collector collects heat by absorbing sunlight. The term "solar collector" commonly refers to a device for solar hot water heating, but may also refer to large power generating installations such as solar parabolic troughs and solar towers, or to non-water-heating devices such as solar cookers or solar air heaters. Solar thermal collectors are either non-concentrating or concentrating. In non-concentrating collectors, the aperture area (i.e., the area that receives the solar radiation) is roughly the same as the absorber area (i.e., the area absorbing the radiation). A common example of such a system is a metal plate that is painted a dark color to maximize the absorption of sunlight. The energy is then collected by cooling the plate with a working fluid, often water or glycol running in pipes attached to the plate. Concentrating collectors have a much larger aperture than the absorber area. The aperture is typically in the form of a mirror that is focussed on the absorber, which in most cases are the pipes carrying the working fluid. Since the sun moves through the sky during the day, concentrating collectors often require some form of solar tracking system, and are sometimes referred to as "active" collectors for this reason.
Non-concentrating collectors are typically used in residential, industrial and commercial buildings for space heating, while concentrating collectors in concentrated solar power plants generate electricity by heating a heat-transfer fluid to drive a turbine connected to an electrical generator.
Heating water Flat-plate and evacuated-tube solar collectors are mainly used to collect heat for space heating, domestic hot water, or cooling with an absorption chiller. In contrast to solar hot water panels, they use a circulating fluid to displace heat to a separated reservoir. The first solar thermal collector designed for building roofs was patented by William H. Goettl and called the "Solar heat collector and radiator for building roof". Evacuated flat-plate solar collectors are a more recent innovation and can be used for Solar Heat for Industrial Cooling (SHIC) and Solar Air Conditioning (SAC), where temperature in excess of 100 °C (212 °F) are required. These non-concentrating collectors harvest both diffuse and direct light and can make use of steam instead of water as fluid.
Flat plate collectors
Flat-plate collectors are the most common solar thermal technology in Europe. They consist of (1) an enclosure containing (2) a dark-colored absorber plate with fluid circulation passageways, and (3) a transparent cover to allow transmission of solar energy into the enclosure. The sides and back of the enclosure are typically insulated to reduce heat loss to their surroundings. A heat transfer fluid is circulated through the absorber's fluid passageways to remove heat from the solar collector. The circulation fluid in tropical and sub-tropical climates is typically water. In climates where freezing is likely, a heat transfer fluid similar to an automotive antifreeze solution may be used instead of water, or in a mixture with water. If a heat transfer fluid is used, a heat exchanger is typically employed to transfer heat from the solar collector fluid to a hot water storage tank. The most common absorber design consists of copper tubing joined to a high conductivity metal sheet (copper or aluminum). A dark coating is applied to the sun-facing side of the absorber assembly to increase its absorption of solar energy. A common absorber coating is black enamel paint. In higher performance solar collector designs, the transparent cover is tempered soda-lime glass having reduced iron oxide content same as for photovoltaic solar panels. The glass may also have a stippling pattern and one or two anti-reflective coatings to further enhance transparency. The absorber coating is typically a selective coating, where selective stands for having the special optical property to combine high absorption in the visible part of the electromagnetic spectrum coupled to low emittance in the infrared one. This creates a selective surface, which reduces black body energy emission from the absorber and improves performance. Piping can be laser or ultrasound welded to the absorber sheet to reduce damage to the selective coating, which is typically applied prior to joining to large coils in a roll-to-roll process. 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-shaped pipe that maximises temperature but not total energy yield in variable flow systems, used in compact solar domestic hot water only systems (no space heating role); flooded: consisting of two sheets of metal molded to produce a wide circulation zone that improves heat transfer; boundary layer: 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 being accumulated in the circulating liquid. A flat plate collector making use of a honeycomb structure to reduce heat loss also at the glass side too has also been made available commercially. Most flat plate collectors have a life expectancy of over 25 years..
Evacuated tube collectors
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