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 refer to large power generating installations such as solar parabolic troughs and solar towers or non water heating devices such as solar cooker, 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. Due to the movement of the sun during the day, concentrating collectors often require some form of solar tracking system, and are sometimes referred to "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. Solar thermal collectors 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 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 an (1) enclosure containing (2) a dark-colored absorber plate with fluid circulation passageways, and (3) a transparent cover