SOLAR ENERGY,How does a solar power plant work?
SOLAR
ENERGY
The sun is
the source of the vast majority of the energy we use on earth. Most of the
energy we use has undergone various transformations before it is finally
utilized, but it is also possible to source of solar energy as it arrives on
the earth's surface. There are many applications for the dries of solar thermal
energy, space heating and cooling, water heating, crop drying and solar
cooking. It is technology, which is well understood and widely used in many
countries throughout the world. Mo solar thermal technologies have been in
existence in one form or another for centuries and have a well established
manufacturing base in most sun-rich developed countries.
The most
common use for solar thermal technology is for domestic water heating. Hundreds
of thousands of domestic hot water systems are in use throughout the world,
especially in areas such as the Mediterranean and Australia where there is high
solar insulation (the total energy per unit area received from the sun). As
world oil prices vary, it is a technology, which is rapidly gaining acceptance
as an energy saving measure in both domestic and commercial water heating applications. Present tic water heaters are usually only found among st wealthier sections of
the community in developing countries. Other technologies exist which take
advantage of the free energy provided by the sun. Water heating technologies
are usually referred to as active solar technologies, whereas other
technologies, such as space heating or cooling, which passively absorb the
energy of the sun and have no moving components, are referred to as passive
solar technologies. More sophisticated solar technologies exist for providing
power for electricity generation. We will look at these briefly later in this
fact sheet.
Sun is the
source of many forms of energy available to us. Do you know how energy is
obtained from the sun? The most abundant element in sun is hydrogen. It is in a
plasma state. This hydrogen at high temperature, high pressure and high density
undergoes nuclear fusion and hence releases an enormous amount of energy
spectrum.
Out of these X-rays, gamma rays
and most of ultraviolet rays do not pass through the earth's where. But heat
energy and light energy are the main radiations that reach the earth. This
energy atoms is the basis for the existence of life on earth.
Sun is a sphere of intensely hot gaseous matter with a diameter of 1.39e9 m and 1.5e11 m away earth. Sun has an effective black body temperature of 5762 K and has a temperature of 8e6 K to 40e6 K. The sun is a continuous fusion reactor in which hydrogen (4 protons) combines to form helium (one He nucleus).
The mass of
the He nucleus is less than that of the four protons, mass having been Lost in
the reaction and converted to energy. The energy received from the sun on a
unit area perpendicular to the direction of propagation of radiation outside
atmosphere is called solar constant, and has a value 1353 Wm. This radiation
when received on the earth has a typical value of 1100 Wm TM and is variable.
The wavelength range is 0.29 to 2.5 micro meters. This energy is typically
converted into usual energy form through natural and man-made processes.
Natural processes include wind and biomass. Man-made processes include
conversion into heat and electricity.
SOLAR
RADIATIONS
Radiation
from sun on entering the earth's atmosphere gets scattered by the atmospheric
gas molecules and dust particles and received on earth from all directions and
is called diffuse radiation. The portion of radiation received on earth from
sun without change in original quality is called beam or direct radiation.
The earth
revolves about the sun in an approximately circular path, with the sun located
slightly off center of the circle. The earth's axis of rotation is tilted 23.5
degrees with respect to its pane of revolution about the sun, the position of
the earth relative to the sun's rays at the time of winter solstice when the
North Pole is inclined 23.5 degree away from the sun. All points on the earth's
surface north of 66.5 N latitude are in total darkness while all regions within
23.5 degree of the South Pole receive continuous sunlight. At the time of the
summer solstice, the situation is reversed. At the time of the two equinoxes,
both poles are equidistant from the sun and all points on the earth's surface
have 12 hours of daylight and 12 hours of darkness. The sun's ray passing
through the center of the earth lies in the equatorial plane at the time of
equinoxes. From vernal equinox to autumnal equinox, the rays lie north of the
equatorial plane. From autumnal equinox to vernal equinox, the rays lie south
of the equatorial plane. The average direction of the sun's rays for the entire
year lies in the equatorial plane. Accordingly to intercept maximum amount of
solar energy over the whole year, a solar collector in the northern hemisphere
should be tilted and face due south.
The Nature
and Availability of Solar Radiation. Solar radiation arrives on the surface of
the earth at a maximum power density of approximately 1 kilowatt per metre
squared (kWm. The actual usable radiation component varies depending on
geographical location, cloud cover, hours of sunlight each day, etc. In
reality, the solar flux density (same as power density) varies between 250 and
2500 kilowatt hours per metre squared per year (kWhm per year). As might be
expected the total solar radiation is highest at the equator, especially in
sunny, desert areas. Solar radiation arrives at the earth's outer atmosphere in
the form of a direct beam. This light is then partially scattered by cloud,
smog, dust or other atmospheric phenomenon. We therefore receive solar
radiation either as direct radiation or scattered or diffuse radiation, the
ratio depending on the atmospheric conditions. Both direct and diffuse
components of radiation are useful, the only distinction between the two being
that diffuse radiation cannot be concentrated for use.
Solar
radiation arriving from the sun reaches the earth's surface as short wave
radiation. Allie the energy arriving from the sun is eventually re-radiated into
deep space otherwise the temperature the earth would be constantly increasing.
This heat is radiated away from the earth as long-wave radiation. The art of
extracting the power from the solar energy source is based around the principle
capturing the short wave radiation and preventing it from being re radiated directly to the atmosphere Glass and other selective surfaces are used to
achieve this. Glass has the ability to allow the passage short wave radiation
whilst preventing heat from being radiated in the form of long wave radiation.
To 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 the thermal mass. Pools or
lakes are sometimes used for seasonal storage of heat.
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 boi 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 dishshaped 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 Plant The
first is the 'Solar Power Tower' design which uses thousands of sun-tracking
reflectors or helio stats 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. A heliostat field
consists of a large number of flat mirrors of 25 to 150 m2 area which reflects
the beam radiations onto a central receiver mounted on a tower. Each mirror is
tracked on two axis. The absorber surface temperature may be 400 to 1000°C. The
concentration ratio (total mirror area divided by receiver area) may be 1500.
Steam, air or liquid metal may be used as working fluid. Steam is raised for
the conventional steam power plant.
Distributed
(Parabolic) Collector System' Power Plant.
The second type is the distributed collector
system. It is also called solar farm power plant as a number of solar modules
consisting of parabolic trough solar collectors are interconnected. This system
uses a series of specially designed "Trough' collectors which have an
absorber tube running along their length. Large arrays of these collectors are
coupled to provide high temperature water for driving a steam turbine. Such
power stations can produce many megawatts (mW) of electricity, but are confined
to areas where there is ample solar insulation.
Every module consists of a
collector as shown in Figs. 2.13 and 2.14. It is rotated about one axis by a
sun tracking mechanism. Thermo-oil is mostly used as heating fluid as it has
very high boiling
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 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
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.
There are eight possible pathways
for conversion of solar radiation to useful energy. Solar thermal 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 electrical energy through
a turbine and generator to electrical energy. The above route through a further
electrolysis process gives chemical energy (H, fuel). A high temperate produces
chemical energy (H, fuel) directly. Photovoltaic conversion of solar radiation
gives direct electrical energy. Photosynthesis process produces chemical energy
directly from radiation. Chemical energy (H, fuel) is directly produced from
solar radiation using the electricity produced by the photovoltaic 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
impr 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.
SPACE
HEATING
In colder
areas of the world (including high altitude areas within the tropics) space
heating is often required during the winter months. Vast quantities of energy
can be used to achieve this. If buildings are carefully designed to take full
advantage of the solar insolation which they receive then much of the heating
requirement can be met by solar gain alone. By incorporating certain simple
design principles a new dwelling can be made to be fuel efficient and
comfortable for habitation. The bulk of these technologies are architecture
based and passive in nature. The use of building materials with a high thermal
mass (which stores heat), good insulation and large glazed areas can increase a
buildings capacity to capture and store heat from the sun. Many technologies
exist to assist with diurnal heating needs but seasonal storage is more
difficult and costly.
For
passive solar design to be effective certain guidelines should be followed:
1. A
building should have large areas of glazing facing the sun to maximise solar gain
2.
Features should be included to regulate heat intake to prevent the building
from overheating
3. A
building should be of sufficient mass to allow heat storage for the required
period
4. Contain
features which promote the even distribution of heat throughout the building.
One
example of a simple passive space heating technology is the Trombe wall. A
massive black painted wall has a double glazed skin to prevent captured heat
from escaping. The wall is vented to allow the warm air to enter the room at
high level and cool air to enter the cavity between the wall and the glazing.
Heat stored during the wall during the day is radiated into the room during the
night. This type of technology is useful in areas where the nights are cold but
the days are warm and sunny.
SPACE
COOLING
The
majority of the worlds developing countries, however, lie within the tropics
and have little need of space heating. There is a demand, however, for space
cooling. The majority of the worlds warm-climate cultures have again developed
traditional, simple, elegant techniques for cooling their dwellings, often
using effects promoted by passive solar phenomenon. There are many methods for
minimising heat gain. These include siting a building in shade or near water,
using vegetation or landscaping to direct wind into the building, good town
planning to optimise the prevailing wind and available shade. Buildings can be
designed for a given climate domed roofs and thermally massive structures in
hot arid climates, shuttered and shaded windows to prevent heat gain, open
structure bamboo housing in warm, humid areas. In some countries dwellings are
constructed underground and take advantage of the relatively low and stable
temperature of the surrounding ground. There are as many options as there are
people.
RECENT
DEVELOPMENTS IN SOLAR POWER PLANTS
Solar
Thermal Applications.The applications include water heating for
domestic, commercial and industrial use, space heating and drying, solar
distillation, solar cooling through absorption & adsorption cycles, solar
water pumping and solar power generation.
Solar Photovoltaic Photovoltaic (PV) or solar cells refers to the creation of
voltage from light. A solar cell is a converter; it changes the light energy
into electrical energy. A cell does not store any energy, so when the source of
light (typically the sun) is removed, there is no electrical current from the
cell. If electricity is needed in the night, a battery must be included in the
circuit. There are many materials that can be used to make solar cells, but the
most common is the element silicon. A typical solar cell is 3-6 inches in
diameter and are now available in various shapes like circular, square, etc.
The conversion processes occurs instantly whenever there is light falling on
the surface of a cell. And the output of the cell is proportional to the input
light.
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