Power Plant Engineering

68 POWER PLANT ENGINEERING

Solar radiation arriving from the sun reaches the earth’s surface as short wave radiation. All of

the energy arriving from the sun is eventually re-radiated into deep space otherwise the temperature of

the earth would be constantly increasing. This heat is radiated away from the earth as long-wave radia-

tion. The art of extracting the power from the solar energy source is based around the principle of

capturing the short wave radiation and preventing it from being reradiated directly to the atmosphere.

Glass and other selective surfaces are used to achieve this. Glass has the ability to allow the passage of

short wave radiation whilst preventing heat from being radiated in the form of long wave radiation. For

storage of this trapped heat, a liquid or solid with a high thermal mass is employed. In a water heating

system this will be the fluid that runs through the collector, whereas in a building the walls will act as

the thermal mass. Pools or lakes are sometimes used for seasonal storage of heat.

2.16.2 Solar Thermal Power Plant

In the solar power plant, solar energy is used to generate electricity. Sunrays are focused using
concave reflectors on to copper tubes filled with water and painted black outside. The water in the
tubes then boils and become steam. This steam is used to drive steam turbine, which in turn causes the
generator to work. A plant using this principle is working on experimental basis in Gurgaon in Haryana.
Its capacity is 500 kilowatt. Another plant of similar type is being constructed in Jodhpur in Rajastan.
Many power plants today use fossil fuels as a heat source to boil water. The steam from the
boiling water rotates a large turbine, which activates a generator that produces electricity. However, a
new generation of power plants, with concentrating solar power systems, uses the sun as a heat source.
There are three main types of concentrating solar power systems: parabolic-trough, dish/engine, and
power tower.
Parabolic-trough systems concentrate the sun’s energy through long rectangular, curved
(U-shaped) mirrors. The mirrors are tilted toward the sun, focusing sunlight on a pipe that runs down
the center of the trough. This heats the oil flowing through the pipe. The hot oil then is used to boil
water in a conventional steam generator to produce electricity.

A dish/engine system uses a mirrored dish (similar to a very large satellite dish). The dish-
shaped surface collects and concentrates the sun's heat onto a receiver, which absorbs the heat and
transfers it to fluid within the engine. The heat causes the fluid to expand against a piston or turbine to
produce mechanical power. The mechanical power is then used to run a generator or alternator to
produce electricity.

A power tower system uses a large field of mirrors to concentrate sunlight onto the top of a
tower, where a receiver sits. This heats molten salt flowing through the receiver. Then, the salt’s heat is
used to generate electricity through a conventional steam generator. Molten salt retains heat efficiently,
so it can be stored for days before being converted into electricity. That means electricity can be pro-
duced on cloudy days or even several hours after sunset.

‘Solar Power Tower’ Power PlantThe first is the 'Solar Power Tower' design which uses
thousands of sun-tracking reflectors or heliostats to direct and concentrate solar radiation onto a boiler
located atop a tower. The temperature in the boiler rises to 500 – 7000°C and the steam raised can be
used to drive a turbine, which in turn drives an electricity producing turbine. There are also called
central Receiver Solar Power Plants.

It can be divided into solar plant and conventional steam power plant. The flow diagram is given
in Fig. 2.12.