Classification of solar collectors
⦁ The classification is based on the way they collect solar radiation. The non-concentrating type absorbs the radiation as it is received on the surface of the collector while the concentrating type first increases the concentration of radiation per unit area before absorbing it.
⦁ Further, based on the technique employed for concentration of radiation, the concentrating type is further categorized into focus and non-focus types. The focus type is further subdivided into line and point focus depending on the focusing method.
⦁ While non-focus type collectors consume both beam and diffuse radiation, the focus type collectors make use of beam radiation only, as diffuse radiation cannot be concentrated.
⦁ The important features of a solar collector are:
⦁ Collector efficiency: It is defined as the ratio of the energy actually absorbed by the collector to the energy incident on the collector.
⦁ Concentration ratio (CR): It is the ratio of the area of the aperture of the system to the area of the receiver. The aperture of the system is the projected area of the collector facing the beam.
⦁ Temperature range: It is the range of temperature to which the heat transport fluid is heated by the collector.
⦁ Non-concentrating collector or flat plate collector
⦁ In it, the area of a collector to catch the solar radiation is equal to the absorber plate. Here no optical system is used to reduce the solar radiation and therefore the concentration ratio is just 1.
⦁ Flat plate collector is the most substantial component of any solar thermal energy system. It is basically a heat exchanger which transfers the radiant energy of the incident sunlight to the sensible heat of a working fluid-liquid or air.
⦁ Flat plate collectors are designed for operation in the low temperature range, from ambient to 60˚C, or the medium temperature range, from ambient to 100˚C.
⦁ The constructional details of a simple flat plate collector are shown in Fig. 1.2.
Construction of flat plate collector
Basic elements of a flat plate collector:
⦁ Transparent cover (one or two sheets) of glass or radiation transmitting plastic film.
⦁ Blackened absorber plate usually of copper, aluminium or steel, typically 1-2mm thick.
⦁ Tubes (typically 1-2 cm DIA.), passages or channels, in thermal contact with the absorber plate or connected to it.
⦁ Thermal insulation usually of fiberglass or expended polystyrene, typically 5-10cm in thickness is provided at the back and sides to minimize the heat losses.
⦁ Tight container or casing to insert the above ingredients.
Working of flat plate collector:
⦁ As solar radiation impinges on specially treated metallic absorber plate, it is absorbed and increases its temperature. This heat is transferred to a heat transfer fluid circulating in the tube below the absorber plate and in intimate...