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Wednesday, 1 July 2015

SOURCES OF ENERGY


                                       SOURCES OF ENERGY
Main topics with explanation  
 Characteristics of a good fuel:
(iv) High calorific value
(v) Less smoke
(vi) Less residue after burning
(vii) Easy availability
(viii) Inexpensive
(ix) Easy to store and transport
Fossil fuels: were formed millions of years ago, when plants and animal remains got buried under the earth and were subjected to high temperature and pressure conditions. E.g.: Coal, Petroleum, etc. These fossil fuels are non renewable sources of energy and cause environmental problems due to pollution.

 Thermal power plants:
(i) Use coal, petroleum and natural gas to produce thermal electricity.
(ii) Electricity transmission is very efficient.
(iii) The steam produced by burning the fossil fuels runs the turbine to produce electricity

Hydro power plant:
(i) It is the most conventional renewable energy source obtained from water falling from a great height.
(ii) It is clean & non polluting source of energy.
(iii) Dams are constructed to collect water flowing in high altitude rivers. The stored water has a lot of potential energy.
(iv) When water is allowed to fall from a height, potential energy changes to kinetic energy, which rotates the turbines to produce electricity.

Disadvantages of Hydro power plant:
(i) Highly expensive to construct.
(ii) Dams cannot be constructed on all river sites.
(iii) Large areas o human habitation and agricultural fields get submerged.
(iv) People face social and environmental problems.

 Non conventional sources:
(1) Bio mass:
o It is the source of the conventionally used fuels that are used in our country. E.g.: Cow dung cakes, fire-wood, coal, charcoal
o Bio gas: It is a mixture of gases produced during decomposition of bio mass in the absence of Oxygen. (Anaerobic Respiration).
Methane is the major component of bio gas.
o Bio gas plants: Animal dung, sewage, crop residues, vegetable wastes, poultry droppings, etc. are used to produce Bio gas in Bio gas plants.

o2) Wind energy:
o It can be converted into mechanical and electrical energy.
o Kinetic energy of the wind is used in running of wind mills, which are used to lift water, grind grains, etc.
o Wind mill-
o Advantages: (i) Eco friendly (ii) Renewable
o Disadvantages: (i) Wind speed not uniform always.
(ii) Needs a large area to erect series of wind mills.
(iii) Big amount of investment is needed.
(iv) Out put is less as compared to investment

(3) Solar energy:
o Solar radiations can be converted electricity through solar cells (photo-voltaic cells).
o Photo-voltaic cells convert solar radiations directly into electricity through silicon solar cells.
o Solar cells arrange on a large flat sheets form a solar panel.
o Solar cookers are painted black from outside and a large glass plate to trap solar radiations by green house effect.

o Advantages of Solar cookers:
(i) Eco friendly
(ii) Renewable
(iii) Used in rural areas.
(iv) Retains all the nutrients in food due to slow cooking.

o Disadvantages of solar cooker:
(i) Silicon cells are expensive.
(ii) Solar radiations are not uniform over earth‘s surface.
(iii) Cannot be used at night or on cloudy days.
(iv) Cannot be used to make chapattis for frying as these require a temperature of 1400C or more. (Maximum temperature of 1000C only can be achieved in a solar cooker)
o Other solar devices- Solar water heater, Solar furnace

(4) Geo thermal energy:
(i) Energy harnessed from the heat of the sun is called Geo thermal energy.
(ii) Magma is formed when this heat melts the rocks. The molten rocks and hot gases are called magma
(iii) The magma gets collected at some depths below the earth‘s surfaces. These places are called ‗Hot spots‖
(iv) When underground water comes in contact these hot spots, it changes into steam, which can be used to generate electricity.

o Advantages of Geo thermal energy:
(i) Renewable
(ii) Inexpensive

o Disadvantages of Geo thermal energy:
(i) Only few sites available for harnessing energy.
(ii) Expensive

(5) Nuclear energy:
(i) Energy released when some changes take place in the nucleus of the atom of a substance, is called Nuclear energy.
(ii) It is used for heat generation, fuel for marine vessels.

o Advantages of Nuclear energy:
(i) Alternative source of energy due to depletion of fossil fuels.
(ii) From a small amount of fuel, a large amount of energy is released.
o Disadvantages of Nuclear energy:
(i) Risk of nuclear waste leakage
(ii) High cost of setting up of nuclear plant
(iii) Pollution of environment.

(6) Energy from the sea-
(A) Tidal energy:
Locations in India –Gulf of Kutch, Gujrat & W. Bengal
(i) Depends upon harnessing the rise and fall of sea level due to tidal action.
(ii) Dams are constructed across a narrow part of sea and turbine converts tidal energy into electrical energy.

Disadvantages: Uniform tidal action is not seen

(B) Wave energy:
(i) Kinetic energy of the waves of sea are used to rotate turbines..
(ii) These turbines generate electrical energy

 Important diagrams-
1. Hydro power plant
2. Bio gas plant
3. A wind mill
4. A solar cooker


HOTS QUESTIONS (SOLVED)

1. Name the materials used for making solar cells.
A. Silicon, Germanium and Selenium

2. What fraction of solar energy reaches the earth‘s surface?
A. 47%

3. Name the process that produces a large amount of energy in the sun.
A. Nuclear fusion

4. Why is bio-gas called a clean fuel?
A. Because it- 
(i) leaves no ash 
(ii) does not cause pollution 
(iii) does not produce any poisonous gas.

5. Why Hydrogen cannot be used as a domestic fuel?
A. Hydrogen cannot be used as a domestic fuel, in spite of its high calorific value, because it is explosive in nature and difficult to store & transport.

6. What are OTEC power plants? How do they operate?
A. (i) It is Ocean Thermal Energy Conversion.
(ii) These plants can operate if the temperature difference between water at the surface and water at depths up to 2 km is 293K (200C) or more.
(iii) The warm surface water is used to boil a volatile liquid like Ammonia. The vapour of the liquid is used to turn the turbine.
(iv) The cold water from the depth is pumped up and condensed water again changes into vapour.

7. Though wood is renewable source of energy, but the use of wood as a fuel is not wise decision. Explain.
A. Wood is obtained by cutting trees. A newly plant sapling usually takes about 10-15 years to grow and mature into a tree. Cutting of trees on a large scale cases depletion of forests leading to disturbance of balance in nature. The balance between Oxygen & Carbon dioxide is also disturbed. Therefore the use of wood as a fuel is not wise decision

8. Explain, why a sheet of glass is used in solar heating devices?
A. Glass has a property that allows the infra red rays of short wavelength from the sun to get in the device but does not allow infra red rays of long wavelength to leave the solar heating device.
Therefore, heat is trapped inside the heating device. A kind of green house effect is produced, which helps to increase the temperature inside the heating device.

9. Explain, why is a reflector used in a solar cooker?
A. The reflector is used to increase the area over which the solar energy is collected so that 77iuimore and more heat rays of the sun enter the solar cooker. It increases the efficiency of solar cooker.

HOTS QUESTIONS (UNSOLVED)
1. What is the use of black painted surface in solar heating devises.
2. Why are bio gas plants considered to be boon to the farmers? Give reason.
3. Hydroelectricity generated at a dam may be considered another form of solar energy. Why?
4. How is the slurry left over after the generation of bio-gas in bio-gas plant used?
5. Why is charcoal considered to be a better fuel than wood?
6. Why a solar cooker cannot be used for frying or making chapattis?
7. In parabolic reflector type coolers, even temperature up to 1800C- 2000C can be attained. How?
8. Modern chulahs are more efficient than traditional chulahs. Why?
9. How is hydro energy converted into electrical energy?
10. Explain, why only a part of the solar energy that strikes the upper regions of atmosphere reaches the surface of the earth?

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