Power Plant Engineering

 
Beam
Radiation
Evacuated
Glass Tube
Absorber
Tube
Parabolic
mirror
Fig. 2.13. Distributed (Parabolic) Solar Collector.
 
Fig. 2.14. Distributed (Parabolic) Trough Solar Power Plant.

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70
POWER PLANT ENGINEERING
point. Water/steam working fluid can also be used. The tubes have evacuated glass enclosure to reduce
the losses. The concentration ratio is between 40 and 100. The maximum oil temperature is limited to
400°C as oil degrades above this temperature. Alternately steam at 550°C can be directly generated in
the absorber tube.
These are commercially under operation. Fig. 2.14. shows a flow diagram of parabolic trough
solar power plant. The working fluid is heated in collectors and collected in hot storage tank (2). The
hot thermo-oil is used in boiler (5) to raise steam for the steam power plant. The boiler also is provided
with a back-up unit (6) fired with natural gas. The cooled oil is stored in tank (3) and pumped (4) back
to collector (1). Solar thermal power plants with a generating capacity of 80 MW are functioning in the
USA.
Solar Chimney Power Plant. The air stream is heated by solar radiation absorbed by the ground
and covered by a transparent cover. The hot air flow through or chimney which gives the air a certain
velocity due to pressure drop caused by the chimney effect. The hot air flows through an air turbine to
generate power.
Warm Air
Chimney
Air Turbine
So
lar
Ra
dia
tio
ns
Cold Air
Generator
Cover
Fig. 2.15 Chimney Solar Power Plant.
2.16.3 SOLAR ENERGY STORAGE
It is well known that human beings have been using solar energy for different uses, from ancient
days. Find examples of these uses and add to the list given below.
1. To get salt from sea water.
2. To dry wet clothes
3. To dry firewood
4. To dry cereals
5. To dry fish
6. To dry leather
We now use several appliances which work using solar energy. Appliances like solar cooker and
solar heater absorb solar radiations and convert it into heat.
Then what about a solar cell? Solar energy is converted into electrical energy and it is directly
used or stored in a battery.

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NON-CONVENTIONAL ENERGY RESOURCES AND UTILISATION
71
There are eight possible pathways for conversion of solar radiation to useful energy. Solar ther-
mal conversion method converts radiation to heat using solar flat collectors. Solar thermo chemical
conversion method converts radiation to heat and produce steam then to kinetic energy using a pump or
turbine. Solar thermal electric conversion method converts radiation to steam and to kinetic and electri-
cal energy through a turbine and generator to electrical energy. The above route through a further
electrolysis process gives chemical energy (H
2
fuel). A high temperature catalytic conversion process
produces chemical energy (H
2
fuel) directly. Photovoltaic conversion of solar radiation gives direct
electrical energy. Photosynthesis process produces chemical energy directly from radiation. Chemical
energy (H
2
fuel) is directly produced from solar radiation using the electricity produced by the photo-
voltaic method. A few of these methods are dealt in detail further.
Commercial and industrial buildings may use the same solar technologies photovoltaic, passive
heating, day lighting, and water heating that are used for residential buildings. These nonresidential
buildings can also use solar energy technologies that would be impractical for a home. These technolo-
gies include ventilation air preheating, solar process heating and solar cooling.
Many large buildings need ventilated air to maintain indoor air quality. In cold climates, heating
this air can use large amounts of energy. A solar ventilation system can preheat the air, saving both
energy and money. This type of system typically uses a transpired collector, which consists of a thin,
black metal panel mounted on a south-facing wall to absorb the sun’s heat. Air passes through the many
small holes in the panel. A space behind the perforated wall allows the air streams from the holes to mix
together. The heated air is then sucked out from the top of the space into the ventilation system.
Solar process heating systems are designed to provide large quantities of hot water or space
heating for nonresidential buildings. A typical system includes solar collectors that work along with a
pump, a heat exchanger, and/or one or more large storage tanks. The two main types of solar collectors
used an evacuated tube collector and a parabolic trough collector can operate at high temperatures with
high efficiency. An evacuated-tube collector is a shallow box full of many glass, double-walled tubes
and reflectors to heat the fluid inside the tubes. A vacuum between the two walls insulates the inner
tube, holding in the heat. Parabolic troughs are long, rectangular, curved (U-shaped) mirrors tilted to
focus sunlight on a tube, which runs down the center of the trough. This heats the fluid within the tube.
The heat from a solar collector can also be used to cool a building. It may seem impossible to
use heat to cool a building, but it makes more sense if you just think of the solar heat as an energy
source. Your familiar home air conditioner uses an energy source, electricity, to create cool air. Solar
absorption coolers use a similar approach, combined with some very complex chemistry tricks, to
create cool air from solar energy. Solar energy can also be used with evaporative coolers (also called
“swamp coolers”) to extend their usefulness to more humid climates, using another chemistry trick
called desiccant cooling.