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Solar thermal energy has a number of attractive features, which make it a very desirable energy source for India. Ample sunshine throughout the year ensures uninterrupted energy supply. In India, sunshine varies from 2300 to 3200 hours per year and the annual global radiation is 4?5 kWh/m2 -day, fairly spread over 80% of the country.

Solar thermal technologies
Solar thermal technologies can be used for both, supplying thermal energy as well as for generating electricity. Applications of solar thermal technologies include

solar water and space heating ,
solar process heating for industrial applications ,
solar drying ,
solar refrigeration and air conditioning ,
solar cooking ,
solar passive architecture ,
solar water desalination and water purification , and
solar thermal power generation.

The heart of a solar thermal system is a `solar collector'. As the name implies, it's main function is to collect solar thermal energy and transfer it to the fluid to be heated. There are four different types of solar collectors.
1. Flat plate collector
The FPC (flat plate collector) is the simplest form to transform solar energy into heat. FPC consists of
a selectively coated metallic tube (riser) and plate (fin) arrangement, called an absorber;
top glass cover, and
housing with back and side insulation.

The black plate or fin absorbs solar radiation and transfers it to the water (or any other fluid) flowing in the tubes or risers. The risers are connected to a common header in the collector. The absorber plate is insulated on the backside, and the top is covered with glass to reduce heat loss.

The absorbers are selectively coated so as to minimize heat losses due to emittance. These collectors are suitable for applications that require a maximum temperature of about 85 ?C.

2. Evacuated tube collector
The efficiency of FPCs is low at temperatures above 80 ?C -85 ?C, mainly because of excessive heat loss. One way of reducing these heat loss is to evacuate the space between absorber and glass cover. In ETC(Evacuated Tube Collector), the absorber is housed in an evacuated cylindrical glass tube. As there is no medium between the absorber and cover, the heat loss is minimized.

There are two ways in which heat can be extracted from ETC: by circulating thermic fluid directly through the tubes or by using heat pipes that transfer thermal energy to the fluid flowing in the header.

3. CPC collector (stationary concentrator)
CPC(Compound Parabolic Concentrator) reduces the heat loss of a solar collector by reducing the area of absorber with respect to the collecting area, since the heat loss is proportional to the absorber area, and not to the collecting (aperture) area.

This concentration can be obtained using reflectors that force the radiation incident within a certain angle into the collector aperture in direction to the absorber after one or more reflections. The wide acceptance angle of these collectors allows them to collect both diffuse and beam radiation like a flat plate collector. It varies from parabolic trough/dish concentrators that require beam radiation and continuous tracking.

4. Parabolic trough concentrator
The parabolic trough concentrator is essentially a trough lined with reflective material.

The trough focuses the sunrays on a pipe located along its focal line. A heat-transfer fluid, typically high temperature oil, is circulated through pipes, and the heated fluid is then pumped to a central power block where it exchanges its heat to generate steam. Number of such modules can be interconnected to deliver the desired load.

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