Classification of solar collectors⦁ The classification is based on how they collect solar radiation. The non-concentrating type absorbs the radiation when received on the collector surface while the concentrating type first increases the concentration of the radiation per unit area before absorbing it.⦁ Moreover, based on the technique used for concentrating radiation, the concentrating type is further classified into focused and non-targeted types. The focusing type is subdivided into linear and point focusing based on the focusing method.⦁ While unfocused type collectors consume both beam and diffuse radiation, focused type collectors use only beam radiation , because diffuse radiation cannot be concentrated.⦁ The important characteristics of a solar collector are:⦁ Collector efficiency: It is defined as the ratio between the energy actually absorbed by the collector and the energy incident on the collector .⦁ Concentration rate (CR): It is the ratio of the surface area of ​​the opening of the system to the area of ​​the receptor. The opening of the system is the projected area of ​​the collector facing the beam.⦁ Temperature range: It is the temperature range to which the heat transfer fluid is heated by the collector.⦁ Non-concentrated collector or planar collector⦁ In it- here, the surface of a collector for capturing solar radiation is equal to the absorbing plate. Here, no optical system is used to reduce the solar radiation and the concentration ratio is therefore only 1.⦁ The planar collector is the most important component of any solar thermal energy system. It is essentially a heat exchanger that transfers radiant energy from incident sunlight to the sensible heat of a working fluid, liquid or air.⦁ Flat plate collectors are designed to operate in a low temperature range, from room temperature to 60 °C, or the medium temperature range, from room temperature to 100 °C.⦁ Construction details of a simple planar sensor are shown in Fig. 1.2. Construction of a planar sensor Basic elements of a planar sensor:⦁ Transparent cover (one or two sheets) of radiation-transmitting glass or plastic film.⦁ Blackened absorber plate, usually made of copper, aluminum, or steel, usually 1 to 2 mm thick.⦁ Tubes (generally 1 to 2 cm in diameter), passages or channels, in thermal contact with or connected to the absorber plate.⦁ Thermal insulation generally made of fiberglass or Spent polystyrene, usually 5 to 10 cm thick, is provided at the back and sides to minimize heat loss.⦁ Sealed container or case for inserting the above ingredients.Working of flat plate collector : ⦁ When solar radiation hits a specially treated metal absorber plate, it is absorbed and increases its temperature.