Solar Energy
Introduction
The surface of the earth receives about 10(pow,14) kW from sun in the form of solar energy which is approximately five orders of magnitude greater than that currently being consumed from all resources. It is evident that sun will last for 10(pow,11) years.

There are two obvious obstacles to harnessing solar energy. Firstly it is not constantly available on earth. Thus some form of storage is needed to sustain solar energy through the night and during rainy season. Secondly the solar energy is diffused. Although the total amount of energy is enormous, the collection and conservation of solar energy into useful forms must be carried out over a large area which entails large capital investments.

SOLAR WATER HEATING
By using solar radiation, water or any fluid can be heated by using a solar collector. Such systems can provide hot water for different applications in industries directly or as boiler feed and also in hostels, hotels and canteens.

Flat plate collector:

The absorber plate is metallic. It is usually coated black to absorb more heat energy. Tubes, passages or channels integral with the collector carry water or other working fluid. Insulation should be provided at the back and at the sides to minimize the heat losses. Usually glass wool is used as insulation material. A transparent cover (glass) will be provided at the top to permit the radiation from the sun to the metal plate.

Parabolic or concentrating collector Highly polished metallic surfaces are used as the reflector. The reflector will have a parabolic shape so that the sun rays striking the profile will be reflected on its focal point. If a tube carrying a fluid is kept along the focal line, the fluid will be heated to a very high temperature.

SOLAR THERMAL CONVERSION
Low temperature system

This system uses a flat plate collector, which can heat water up to about 800C only. Water is circulated through the flat plate collector. Heat exchanger is used to vaporize a low boiling fluid (butane) and the butane vapor will run the turbine, butane vapor is condensed and reused.

High temperature system

Solar power tower is employed for high temperature system. A receiver is mounted on the tower which is 100 to 500m high and designed to withstand the weight of the receiver. Here reflecting mirrors are known as heliostat. The heliostats are arranged in such a way that they direct the solar energy on the receiver throughout the day. This is achieved by a microprocessor based tracking system, which turns the heliostats depending on the position of sun. The receiver is designed and positioned in such a way to intercept, absorb and transfer maximum heat to working fluid. The temperature achieved is 1500?C to 1700?C. Steam generated in the receiver runs the turbine, condensed to water and recycled.

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