Sources of Energy


1. a good source of energy would be one with

(i) A large amount of work per unit volume or mass;
(ii) easily accessible;
(iii) easy to store and transport; and
(iv) economical.

2. We can classify energy sources in different ways. One classification is based on:-

(i) the reserves (supply) of an energy source [ Conventional and non-conventional sources of energy]

(ii) Another is based on how long we have been using it. [Renewable and non-renewable sources of energy]

3. Renewable sources of energy are those which can be generated by us or which are constantly being generated by natural processes or whose supply is unlimited. Wood is a renewable source of energy although its supply is limited. Nuclear fuels such as uranium are required in very small quantities to generate electricity. So, their reserves will last for a long, long time. Therefore, they are classified as renewable sources of energy.

4. Non-renewable sources of energy are those which were produced in the past by natural processes, whose supply is limited and which we cannot generate ourselves. Coal, natural gas and petroleum are non-renewable sources of energy.

5. Sources of energy that have been in use for centuries are called conventional sources of energy. Conventional sources include wood, coal, petroleum and flowing water. Conventional sources like coal and petroleum are non-renewable, while sources like flowing water are renewable.

6. Sources of energy that we have started using in new ways or only in recent times are called non-conventional or alternate sources of energy. These include energy from the sun, the heat inside the earth (geothermal energy), tides, ocean waves, etc. Nuclear energy is also a nonconventional source. Noon-conventional energy sources are renewable. We have been using wind and biomass (like cow dung) for energy for ages. However, they were not used conventionally to do tasks like electricity generation, which has now been made possible with improvement in technology. In that sense, they can also be called nonconventional sources of energy.


1. Fossil Fuels are Coal and Petroleum.

(i) about 25% of our total energy consumption comes from coal. More than 90% of this coal is used for electricity generation in thermal power plants.

(ii) In most thermal power plants, coal is burnt to heat water for producing steam. The steam falls on the blades of a special kind of wheel called a turbine. A turbine is a device that rotates when steam, water or windfalls on its blades. The turbine turns the shaft of an electric generator to which it is connected. In this way, electricity is produced.

2. The burning of most fossil fuels causes air pollution. The pollutants produced include carbon dioxide, carbon monoxide, oxides of nitrogen, sulphur dioxide and unburnt particles.

(i) The burning of fossil fuels produces carbon dioxide leading to the greenhouse effect and global warming.

(ii) carbon monoxide is produced by incomplete combustion of coal and petrol. Excessive inhalation of this poisonous gas can cause death.

(iii) During the combustion of coal sulphur dioxide and oxides of nitrogen are formed combine with oxygen at the high temperatures inside a running automobile engine to form sulphuric acid and nitric acid. These fall on the earth with rain, which we call acid rain. Acid rain damages soil, water bodies, crops, living tissues and structures like the Taj Mahal.

(iv) The unburnt particles produced during the combustion of fossil fuels are carried by smoke. They affect our lungs, and blacken clothes and buildings.

3. Burning fossil fuels has other disadvantages too. The oxides of carbon, nitrogen and sulphur that are released on burning fossil fuels are acidic oxides. These lead to acid rain which affects our water and soil resources.

4. Hydro Power Plants – Another conventional source of energy was the kinetic energy of flowing water or the potential energy of water at a height. Hydropower plants convert the potential energy of falling water into electricity. A quarter of our energy requirement in India is met by hydropower plants.

5. Advantages of hydroelectricity: –

(i) The source of energy (water) is free and renewable.

(ii) Harnessing the energy of flowing water is a pollution-free process, with no smoke, chemicals, etc., being produced.

(iii) The cost of electricity generation is low as compared to electricity generated from other types of power plants.

(iv) Flowing water is a more reliable source of energy than wind.

(v) Hydroelectricity can be generated on a large scale from a single plant.

(vi) Dams built for hydroelectric plants also help in flood control and irrigation.

(vii) Small hydroelectric stations are one of the best options for generating electricity from renewable sources, as they do not affect the environment much.

Srisailam Dam

6. To produce hydel electricity, high-rise dams are constructed on the river to obstruct the flow of water and thereby collect water in larger reservoirs. The water in the reservoir would be refilled each time it rains (hydropower is a renewable source of energy). But, constructions of big dams have certain problems associated with it.

(i) Large areas of agricultural land and human habitation are to be sacrificed as they get submerged.

(ii) Large eco-systems are destroyed when submerged under the water in dams.

(iii) The vegetation which is submerged rots under anaerobic conditions and gives rise to large amounts of methane which is also a greenhouse gas.

(iv) It creates the problem of satisfactory rehabilitation of displaced people.

Improvements in the Technology for using Conventional Sources of Energy

7. Bio-Mass – When fuels are plant and animal products the source is said to be bio-mass. These fuels do not produce much heat on burning and a lot of smoke is given out when they are burnt. Therefore, technological inputs to improve the efficiency of these fuels are necessary.

(i) Cow-dung, various plant materials like the residue after harvesting the crops, vegetable waste and sewage are decomposed in the absence of oxygen to give bio-gas. Since the starting material is mainly cow-dung, it is popularly known as ‘gobar-gas’.

(ii) Bio-gas is produced in a plant. The plant has a dome-like structure built with bricks. A slurry of cow-dung and water is made in the mixing tank from where it is fed into the digester.

(iii) The digester is a sealed chamber in which there is no oxygen. Anaerobic micro-organisms that do not require oxygen decompose or break down complex compounds of the cow-dung slurry.

(iv) It takes a few days for the decomposition process to be complete and generate gases like methane, carbon dioxide, hydrogen and hydrogen sulphide.

(v) The bio-gas is stored in the gas tank above the digester from which they are drawn through pipes for use.

(vi) Bio-gas is an excellent fuel as it contains up to 75% methane. It burns without smoke, leaves no residue like ash in wood, charcoal and coal burning. Its heating capacity is high. Bio-gas is also used for lighting.

(vii) The slurry left behind is removed periodically and used as excellent manure, rich in nitrogen and phosphorous. The large-scale utilisation of bio-waste and sewage material provides a safe and efficient method of waste-disposal besides supplying energy and manure.

biogas plants

8. Advantages of Biogas Plants

(i) Reduces burden on forests and fossil fuels
(ii) Produces a clean fuel – helps in controlling air pollution
(iii) Provides nutrient-rich (N & P) manure for plants
(iv) Controls water pollution by decomposing sewage, animal dung and human excreta

9. Limitations of Biogas Plants

(i) Initial cost of installation of the plant being high, and is beyond the reach of an average rural Indian

(ii) Number of cattle owned by an average family of farmers is inadequate to feed a biogas plant


10. Wind Energy – Unequal heating of the landmass and water bodies by solar radiation generates air movement and causes winds to blow.

(i) The kinetic energy of the wind can be used to do work. This energy was harnessed by windmills in the past to do mechanical work. Today, wind energy is also used to generate electricity.

(ii) The output of a single windmill is quite small and cannot be used for commercial purposes. Therefore, a number of windmills are erected over a large area, which is known as wind energy farm. The energy output of each windmill in a farm is coupled together to get electricity on a commercial scale.

Wind Energy Farm

11. Advantages of wind energy

(i) The source of energy (wind) is free.

(ii) Harnessing wind energy is a pollution-free process, with no smoke, chemicals, etc., being produced.

(iii) A small wind-electric plant can be set up near a factory to provide pollution-free power for its use.

12. Limitations of wind energy

(i) Wind energy cannot be harnessed at places where wind does not blow regularly. A wind-electric generator works only on winds of at least 15 km/h.

(ii) Wind is not a dependable source of energy because sometimes the air is absolutely still and at other times there are storms.

(iii) It is expensive to set up a wind farm for generating electricity because wind farms need a large area.

(iv) Moreover, since the tower and blades are exposed to the vagaries of nature like rain, Sun, storm and cyclone, they need a high level of maintenance.


13. Solar Energy is radiant light and heat from the Sun.

14. A solar heating device is one which allows collection of a large amount of heat from the sunlight in a given region and restricts the loss of heat to the surrounding in the form of radiation.

15. The following facts help us to design an efficient solar cooker and solar water heaters:

(i) A black body absorbs maximum heat

(ii) Loss of heat by convection can be restricted in a closed system

(iii) Radiation losses can be avoided by reflecting the rays inside the body with the help of reflecting object like a plane mirror

(iv) Conduction can be reduced by using poor conducting materials


16. Advantages of Solar Cooker-

(i) high performance
(ii) no fuel
(iii) eco-friendly
(iv) one-time investment

17. Disadvantages of Solar Cooker-

(i) can’t be used in cloudy weather
(ii) take longer to cook
(iii) need to learn special methods to cook

solar cooker

18. Solar cells are the devices where solar energy is directly converted into electricity. A large number of solar cells are, combined in an arrangement called solar cell panel that can deliver enough electricity for practical use.


19. Advantages of using Solar Cells

(i) They have no moving parts and hence require little maintenance and work quite satisfactorily without any focusing device

(ii) It does not cause any environmental pollution like the fossil fuels and nuclear power

(iii) Solar cells last a long time and have low running costs.

(iv) They can be set up in remote and inaccessible hamlets or very sparsely inhabited areas

20. Disadvantages using Solar Cells

(i) The entire process of manufacture is still very expensive as silver is used for interconnection of these cells in the panel, which is a very expensive metal.

(ii) A practical problem linked with the use of solar cell panels is regarding the storage of electricity storage batteries which give us only direct current. But to operate our devices we need alternating current. Therefore, we need to convert DC to AC before using any appliance and thus it increases the cost of such solar panels as the sources of electricity.

21. Uses of Solar Cells: –

(i) Artificial satellites and in space probes like Mars orbiters
(ii) Wireless transmission systems or TV relay stations in remote locations
(iii) Traffic signals, calculators and in toys


22. Sea Energy can be harnessed through:

(i) Tides
(ii) Sea-waves
(iii) Temperature differences in the water

23. Tidal Energy – Due to the gravitational pull of mainly the moon on the spinning earth, the level of water in the sea rises and falls. This phenomenon is called high and low tides and the difference in sea-levels gives us tidal energy.

(i) Tidal energy is harnessed by constructing a dam across a narrow opening to the sea.

(ii) During high tide, when the level of water in the sea is high, sea-water flows into the reservoir of the barrage and turns the turbines. The turbines then turn the generator shaft to produce electricity.

(iii) During low tide, the sea-water stored in the barrage reservoir flows out into the sea. This flowing water also turns the turbines and generates electricity.

tidal energy2

24. Limitations of Tidal Energy

(i) The tides do not possess enough energy to generate electricity on a large scale

(ii) Only few sites are suitable for building such barrages (or tidal dams)

25. Wave Energy – The waves are generated by strong winds blowing across the sea.

(i) Kinetic energy possessed by huge waves near the seashore can be trapped to generate electricity.

(ii) A wide variety of devices have been developed to trap wave energy for rotation of turbine and production of electricity.

(iii) Wave energy would be a viable proposition only where waves are very strong.

wave energy devices


26. Ocean Thermal Energy – there is always a temperature difference between the water “at the surface of ocean” and “at deeper levels.” At many places in the ocean, this difference in temperatures of water is up to 20oC. The energy available due to the difference in the temperature of water at the surface of the ocean and at deeper levels is called ocean thermal energy(OTC).

(i) These plants can operate if the temperature difference between the water at the surface and water at depths up to 2 km is 293 K (20°C) or more.

(ii) In one type of OTEC [Ocean Thermal Energy Conversion] power plants, the warm surface-water is used to boil a volatile liquid like ammonia. The vapours of the liquid are then used to run the turbine of generator. The cold water from the depth of the ocean is pumped up and condense vapour again to liquid.


27. Advantages of OETC are

(i) A greater advantage of the ocean thermal energy is that it can be used continuously for 24 hours throughout the year.

(ii) Ocean thermal energy is a renewable source of energy and its use does not cause any pollution.

28. Limitation – The installation and maintenance of an OTEC power plant is very high and efficient commercial exploitation is difficult.

29. Geothermal Energy is that heat energy obtained from hot rocks present inside the earth. This heat can be used as a source of energy to produce electricity. When underground water comes in contact with the hot spot, steam is generated. Such outlets are known as hot springs. The steam trapped in rocks is routed through a pipe to a turbine and used to generate electricity.

30. Advantages of Geothermal Energy

(i) Geothermal plants can operate round the clock, unlike those based on solar and tidal energy

(ii) Geothermal energy, is almost pollution-free

(iii) The cost of a geothermal plant is high as compared to a coal-based plant, but operating costs are lower

Geothermal Power
31. Disadvantages of Geothermal Energy

(i) Not all areas are suitable for production of electricity in this way
(ii) It depends on the structure of rocks
(iii) It creates noise pollution
(iv) very few commercially viable sites where such energy can be exploited.

32. There are a number of power plants based on geothermal energy operational in New Zealand and the United States of America.


1. Nuclear Energy – In a process called nuclear fission, the nucleus of a heavy atom (such as uranium, plutonium or thorium), when bombarded with low-energy neutrons, can be split apart into lighter nuclei. When this is done, a tremendous amount of energy is released

2. The released energy can be used to produce steam and further generate electricity.

Nuclear Power Plant

3. Advantages of Nuclear Power –

(i) Nuclear power is a viable option where fossil fuels like coal are not available
(ii) If operated properly, nuclear power plants produce less atmospheric pollution than thermal power plants

(iii) It consumes very little fuel. It can operate for more than a year without needing new fuel elements

(iv) A sizeable amount of fuel (uranium and plutonium) can be reclaimed by processing the spent fuel material in contrast to fuels like coal which cannot be reclaimed

(v) Some radioactive by-products in the process are used in medicine and industry

4. Disadvantages-

(i) The high cost of installation of a nuclear power plant

(ii) high risk of environmental contamination

(iii) limited availability of uranium makes large-scale use of nuclear energy prohibitive

(iv) The disposal of radioactive wastes in the fission process is a major problem. Expensive long-term storage areas have to be built

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