Figure shows a solar power plant with a low temperature solar engine using heated water from flat plate solar collector and Butane as the working fluid. This was developed to lift water for irrigation purposes.
1. Solar collectors
a) Flat plate collector
Figure: Flat plate collector
In a flat plate collector (figure), the radiation energy of the sun falls on a flat surface coated with black paint having high absorbing capacity. It is placed facing the general direction of the sun. The materials used for the plate may be copper, steel aluminium. The thickness of the plate is 1 to 2 mm. Tubing of copper is provided in thermal contact with the plate.
Heat is transferred from the absorbed plate to water which is circulated in the copper tubes through the flat plate collection.
Thermal insulation is provided behind the absorber plate to prevent heat losses from the rear surface. Insulating material is generally fibre glass or mineral wool. The front cover is made up of glass and it is transparent to the incoming solar radiations.
b) Cylindrical parabolic concentrator collector
Concentrator collectors (figure) are of reflecting type utilizing mirrors. The reflecting surface may be parabolic mirror. The solar energy falling on the collector surface is reflected and focused along a line where the absorber tube is located. As large quantity of energy falling on the collector surface is collected over a small surface, the temperature of the absorber fluid is very much higher than in flat plate collector.
While flat place collectors may be used to heat water upto 80°C (low temperature), the concentrating type of collectors are designed to heat water to medium and high temperature ranges.
c) Butane boiler
The water heated in flat plate solar collector to 80°C is used for boiling butane at high pressure in the butane boiler. Boiling point of butane is about 50°C.
d) Turbine
The butane vapour generated at high pressure in the boiler is used to run the vapour turbine which drives the electrical generator.
The vapour coming out of the turbine at low pressure is condensed in a condenser using water. The condensed liquid butane is fed back to the butane boiler using feed pump.
Tower concept for power generation
The tower concept consists of an array of plane mirrors or heliostats which are individually controlled to reflect radiations from the sun into a boiler mounted on a 500 metres high tower. Steam in generated in the boiler, which may attain a temperature upto 2000°K. Electricity is generated by passing steam through the turbine coupled to a generator.
Figure: Tower concept for power generation.
Advantages, disadvantages and application of Solar Energy
Advantages
1. Sun is essentially an infinite source of energy. Therefore solar energy is a very large inexhaustible and renewable source of energy and is freely available all over the world.
2. It is environmentally very clean and is hence pollution-free.
3. It is a dependable energy source without new requirements of a highly technical and specialized nature for its wide spread utilization.
4. It is the best alternative for the rapid depletion of fossil fuels.
Disadvantages
1. It is available in a dilute and is at low potential. The intensity of solar energy on a sunny day in India is about 1.1 kW/square meter area. Hence very large collecting areas are required.
2. Also the dilute and diffused nature of the solar energy needs large land area for the power plant for instance, about 30 square kilometers area is required for a solar power station to replace a nuclear plant on a 1 square kilometer site. Hence capital cost is more for the solar plant.
3. Solar energy is not available at night or during cloudy or rainy days.
Applications of Solar Energy:
Applications of solar energy enjoying most success today are:
1. Solar engines for pumping.
2. Solar water heaters.
3. Solar cookers.
4. Solar driers.
5. Solar furnaces.
6. Photo-voltaic conversion (solar cells)
7. Solar power generation.
