Solar energy
Scientists are researching to place solar
farm over the ocean. They think that, since this energy is cheap
and does not pollute the air, therefore, it would be
efficient if they
could make solar farm at sea.
In some
areas people use sun’s energy directly or indirectly for lots of
things. They use it to heat their houses, or generate
electricity. By direct energy, we mean that solar energy is
converted directly to electricity. But in indirect collecting
energy, we have to use water to produce steam in order to turn a
generator on.
There
are two problems associated with using solar energy. The first
is unsteady manner of sun’s radiation. Second is the need for
large area to collect solar energy.
In
U.S.A most of the solar energy generators were located in states
of Arizona, Nevada, Texas, California .In 2002 there were about
15 solar generators in the U.S.A. Some other plants are under
construction.
Photovoltaic
ENERGY
To
produce energy, sunlight should be turned into the electricity.
To convert the sunlight, there is a device which has many cells
made of silicon alloys. This device, which is non-mechanical, is
called photovoltaic cells. Sunlight is consisted of photons.
When photons hit photovoltaic cell they may be absorbed b6y the
cell or reflected or passed through. Only the absorbed photons
produce electricity. These cells are specially treated in
manufacturing process, so as to absorb free electrons. There for
free electrons move to the cells surface. Movement of the
electrons to the surface would make imbalance of charge between
the front and backside of the cell and this would create voltage
potential like a battery. Each cell can vary in size from about
1 cm to 10cm. One cell has the ability to produce 1-2 watts.
To
generate enough power to run an electric device we need many
cells to be connected together as to make a packed module. Many
modules should be connected together to form the array size big
enough to generate electricity. Although the technology to make
photovoltaic cells has been approved since its invention in 1945
by Bell Telephone Company, but the technology’s efficiency was
only 10 percent.
In
1950’s these cells were used in U.S.A. satellites. Now we use
these cells in small calculators, wrist watches or even in
devices to pump water or in generators to produce electricity.
Although
the modular characteristic of photovoltaic cells allow us to
move these cells from one sight to another location, but the
climate conditions restrict us to use them in all places.
The
advantages of these devices are that they have no adverse effect
on environment, and they have no harmful wastes. These devices
can be used in small- scaled generators to produce energy in
remote areas. To be economically feasible the system needs
electric deregulation by the
government. Improvement in the technology could also make it
reliable to distribute the electricity generated by this
system.
SOLAR THEREMAL
HEAT
Now-a-days people mostly use solar energy to heat buildings or
water for domestic use. In U.S.A. they place solar energy
collectors on top of the houses with fixed position with faces
approximately south and angle about 15 degree plus the latitude.
There
are two types of solar collectors: non-concentrating which its
collecting area equal to absorber area, and non-concentrating
which its area is much greater than absorber area. For space
heating, flat- plate collectors of the non-concentrating type
are sufficient.
Generally there are four types of solar collectors.
1- Flat
– plate which absorbs the energy.
2-
Transparent cover which lets the solar energy to pass through.
3- A
heat – transport fluid, which flows through tubes.
4-Wall
insulation.
There
are two systems, which a house could be heated. One is a system
that uses no mechanical device, in which the air is passed
through a solar heat surface. In the second system, the air is
circulated through fluid, which is heated by solar radiation. In
this system, mechanical devices like fans and pumps are used.
SOLAR THERMAL POWER PLANT
In a
solar power plant sunlight’s ray turns water into the steam.
Then the steam generated goes through a turbine, which rotates
very fast and produces electricity. In a conventional power
plant the energy, which rotates the turbine, comes from the
steam generated by fossil fuels. At present, there are three
power systems that are used in power plants. They are:
1-
Parabolic Trough
2- Solar
Dish
3- Solar
Power Tower
Now we
discuss this system accordingly.
1-The
parabolic trough system is the most advanced system of solar
thermal electric generating machine .It is located in Mojave
Desert in California. Almost 99 percent of solar energy is
generated in this facility.
In this
system, a parabolic shaped reflector focuses sunlight on a
linear receiver. These parabolic collectors, because of their
shapes, collect the sun’s radiation 30 - 100 times more than its
normal intensity and then reflect on the pipes located along the
focal line of the collector. The heat generated in this system
may pass 400 degrees Celsius.
The
system largely is consisted from parabolic trough collectors.
The heat generated in the collector transfers to the fluid
circulated in the pipes. Then the fluid is used to generate high
– pressured steam. The steam then turns the turbine to generate
electricity. After the completion of the process, the cooled
liquid is transferred to the solar field. These power plants are
perfectly designed to work with other fuels during period of low
solar energy.
2- Solar
dish, works on two axes, therefore it concentrates more energy
at focal point of dish, because it always moves at the direction
of the sun. In this system fluid temperature exceeds 750 degree
Celsius. So, it generates more steam than the previous system.
In this
system the energy which runs on steam, compresses the fluid when
is cold, heating up the compressed fluid. After the expansion of
the fluid, it turns on a piston to produce enough power to
generate electricity.
3-Solar
Tower. This type of technology is in the early stage of
development. Therefore some numbers of test facilities are being
constructed in different countries. In this system there are
hundreds of flat sun – tracking mirrors, which collect the
sunlight’s energy and reflect it onto a central receiver tower.
The
energy concentrated could reach up to 1500 times that of the
energy coming from the sun. For the future this could be an
economical way to generate energy. Now the world largest solar
tower is located in California, which has bean on operation from
1982 up to 1988. It was called Solar One. Solar Two started its
operation in 1996.
