A concentrator uses lenses or mirror to focus large area of sunlight into a smaller area.
- Concentrator maybe used to produce temperature in excess of 300 F for efficient electrical power generator, for industrial and agricultural drying and for applications where high temperature is needed.
- They increase output of PV cells.
- One method to concentrate sun’s ray is through lens. Eg: Parabolic
- To achieve high concentration, reflector must be steered to remain directed towards the sun and heat exchanger must be steered to remain located at its focus.
- Concentration Ratio ( C): Solar radiation intensity at hot spot/ Unconcentrated direct solar radiation ( qs/qi )
- For perfect parabolic, ratio is found to be function of parabolic rim angle ( Q) and angular diameter of sun. (c= 4/a2 *sin2Q )
- Parabola must be oriented with its axis towards sun. One method is to rotate parabolic concentrator, tracking sun’s motion across sky and move it about 2 axes to account for seasonal as well as daily motion.
- An alternate is to use auxiliary mirror to track sun and reflect it to parabolic mirror. Surface of plane mirror must be greater than parabolic. (But it increases losses and cost).
- For Solar concentrator maximum temperature cannot exceed sun. In theory, ideal parabolic of 100 % reflectivity, temperature of black body would be 10000 F. But due to certain factors it is reduced, (Eg: reflectivity of glass).
- Small parabolic concentrator is capable of producing same temperature as large concentrator, difference being only amount of heat and size of hot spot. Concentrator uses toroiadal, flat or spherical components are usually cheaper than parabolic.
- One type of solar concentrating collector uses parabolic cylinder reflector to concentrate sunlight onto a pipe with quartz or pyrex envelope. Pipe can be coated with a selective coating to retard infrared emission and transparent tube surrounding pipe can be evacuated to reduce convective heat losses. Reflector is steered during the day to keep sunlight focused on collector. This type is known as parabolic trough collector. It cannot produce as high temperature as parabolic reflectors but more than flat plate.