Sources of Energy | Revision Notes

Sources of Energy | Revision Notes

Sources of Energy

A source of energy provides adequate amount of energy over a long period of time.

Renewable Sources of Energy

  • They are inexhaustible.
  • They are being produced continuously in nature.
  • Example: wood.

Non-Renewable Source of Energy

  • They are exhaustible.
  • They are not produced continuously in nature.
  • Example: Coal

 A good source of energy would be one which would:

  • Enables us to do a large amount of work per unit volume or mass.
  • Is easily accessible.
  • Is easy to store and transport.
  • Is economical.

The materials which can be burnt to produce heat energy are known as fuels. Wood, coal, petrol, kerosene etc. are fuels. Sources of energy can also be categorised as conventional sources of energy and non-conventional sources of energy.

Conventional Sources of Energy

The traditional sources of energy which are familiar to most people are known as conventional sources of energy.

The types of conventional sources of energy are

Fossil Fuel

  • Natural fuel formed deep under the Earth from the remains of living organisms is called fossil fuel.
  • Coal, petroleum and natural gas are fossil fuels.
  • Take millions of years to form.
  • Available in very limited amount.
  • These are non-renewable sources of energy.

Pollution Caused by Fossil Fuels

  • Released oxides of carbon, nitrogen and sulphur (acidic in nature) which causes acid rain that damages trees, plants, reduces fertility of soil.
  • Produces large amount of CO2 in the atmosphere which causes green house effect leading to excessive heating of the earth.

Thermal Power Plant

  • A thermal power plant generates electric power from heat produced by burning fossil fuels, i.e. coal and petroleum.

Hydro Power Plants

  • Hydropower plants utilise the kinetic energy of flowing water to generate electricity.

Advantages of Hydro power plants

  • No environmental pollution.
  • Flowing water is a renewable source of electric energy.
  • Construction of dams prevents flooding of rivers, provide water for irrigation.

Disadvantages of Hydro Power Plants

  • Large areas of agricultural land, a vast variety of flora and fauna, human settlements get submerged in the water of reservoir formed by the dam.
  • Large ecosystems are destroyed.
  • Vegetation that submerged under water rots under anaerobic conditions and produces large amount of methane which is a green house gas.
  • Creates the problems of satisfactory rehabilitation of displaced people.


  • Biomass is the fuel obtained from dead parts of plants and waste material of animals.
  • This fuel does not produce much heat on burning and a large quantity of smoke is given out when it is burnt.
  • Biogas is obtained when cow dung, sewage and various plant materials (such as vegetable waste and residue of crops after harvesting) are decomposed in the absence of oxygen. It is popularly known as gobar gas.

Advantages of Bio gas

  • It is an excellent fuel as it contains upto 75% methane (CH4 ).
  • It burns without smoke.
  • Leaves no residue like ash in wood & coal burning.
  • Slurry left behind is used as excellent manure rich in nitrogen and phosphorus.
  • Safe and efficient method of waste disposal.

Charcoal: When wood is burnt in limited supply of air, then water and other volatile materials gets removed and charcoal is formed.

Charcoal is better fuel than wood because

  • It has higher calorific value than wood.
  • Does not produce smoke while burning.
  • It is a compact fuel, easy to handle and convenient to use.

Wind Energy

  • Air in motion is called wind.
  • Unequal heating of the landmass and water bodies by solar radiations generate air movement and causes wind to blow.
  • It possesses kinetic energy. Thus, it can be used to produce electricity.
  • Windmills are used to generate electricity from wind energy.
  • A windmill is a simple machine with a structure similar to a large fan erected at some height. The rotatory motion of the windmill is utilised to run the turbine of the electric generator, thus producing electricity.
  • The output of a single wind mill is quite small so a number of windmills are erected over a large area called wind energy farm.
  • The minimum wind speed for wind mill to serve as a source of energy is 15-20 km/h.

Advantages of wind energy

  • Eco-friendly.
  • Efficient source of renewable energy.
  • No recurring expenses for production of electricity.

Disadvantages of wind energy

  • Wind energy farms need large area of land.
  • Difficulty in getting regular wind speed of 15-20 KmPH.
  • Initial cost of establishing wind energy farm is very high.
  • High level of maintenance of blades of wind mill.


  • Denmark is called the ‘Country of Winds’. 
  • India is ranked 5 th in harnessing wind energy for the production of electricity.  
  • In India largest wind energy farm has been established near Kanyakumari in Tamil Nadu and it generates 380 MW of electricity.

Non-Conventional Sources of Energy

Sources of energy which are not familiar to most people are known as non-conventional sources of energy.

Reasons for alternate sources of energy

  • The fossil fuel reserves in the earth are limited which may get exhausted soon if we use them at the current rate.
  • Reduce the pressure on fossil fuels making them last for a much longer time.
  • To reduce the pollution level and to save the environment.

The types of non-conventional sources of energy are

Solar Energy

  • The Sun is the most powerful source of radiation energy. It has been radiating energy for the past 5 billion years and will continue to do so for the next 5 billion years.
  • India receives approximately 5000 trillion kWh of solar energy per year.
  • The solar constant is the solar energy reaching unit area at the outer edge of the Earth’s atmosphere exposed perpendicularly to the rays of the Sun at an average distance between the Earth and the sun. Its value is approximately equal to 1.4 kJ per second per m2 or 1.4 kW/m2.
  • A device which either uses solar energy directly as heat or converts it into electricity is called a solar energy device. For example, solar cooker, solar cell, solar water heater etc.

Energy from the Sea

Tidal Energy

  • Tidal energy is the energy derived from the rising and falling tides in the ocean. It is a renewable source of energy.

Wave Energy

  • Sea waves have both kinetic and potential energy as they rise and fall. The energy possessed by these waves is called wave energy and it is a renewable source of energy.

Ocean Thermal Energy

  • 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.
  • The Ocean Thermal Energy Conversion (OTEC) is the process of utilising OTE. The devices used for this purpose are called OTEC power plants.

Geothermal Energy

  • Geothermal energy is the heat energy from hot rocks present inside the Earth.
  • It is a source of energy which does not come directly or indirectly from solar energy.
  • When underground water comes in contact with ‘hot spot’, steam is generated.
  • Steam trapped in rocks is routed through pipes to a turbine and used to generate electricity.

Advantages of Geothermal energy

  • Economical to use geothermal energy.
  • Does not cause any pollution.

Limitations of Geothermal energy

  • Geothermal energy is not available everywhere.
  • Deep drilling in the earth to obtain geothermal energy is very difficult and expensive.

In New Zealand and USA, there are number of power plants based on geothermal energy are operational.

Nuclear Energy

  • The energy obtained from the nucleus of an atom is called nuclear energy.
  • A nuclear reaction in which the particle which initiates the reaction is also produced during the reaction and it carries the reaction further is called a nuclear chain reaction.
  • The sum of the masses of products of a nuclear reaction is somewhat less than the sum of the masses of the reactants. The difference in mass appears as mass defect ( Δ m). It is this mass defect which appears in the form of energy according to Einstein’s mass–energy relation, E = (Δm)c2.

Nuclear Fission

  • ‘Fission’ means split up.
  • The process in which the heavy nucleus of a radioactive atom (such as uranium, plutonium or thorium) split up into smaller nuclei when bombarded with low energy neutrons, is called nuclear fission.
  • A tremendous amount of energy is produced.
  • U-235 is used as a fuel in nuclear reactor in form of uranium rods.

Energy from Nuclear Fission

  • In a nuclear reactor self sustaining chain reaction releases energy at a controlled rate, which is used to produce steam and further generate electricity.

Advantage of Nuclear Energy

  • Production of large amount of useful energy from a very small amount of nuclear fuel.
  • Does not produce green house gases like CO2

Limitations of Nuclear Energy

  • Environmental contamination due to improper nuclear waste storage and its disposal.
  • Risk of accidental leakage of harmful radiations.
  • High cost of installation.
  • Limited availability of nuclear fuel.

Major Nuclear Power Plants

  1. Tarapur (Maharashtra)
  2. Rana Pratap Sagar (Rajasthan)
  3. Kalpakkam (Tamil Nadu)
  4. Narora (U. P.)
  5. Kakrapar (Gujrat)
  6. Kaiga (Karnataka)

Nuclear Fusion

  • When two nuclei of light elements (like hydrogen) combine to form a heavy nucleus (like helium) and tremendous amount of energy is released is called nuclear fusion.
    _{1}^{2}\textrm{H} +_{1}^{2}\textrm{H} \rightarrow _{2}^{3}\textrm{He} + _{0}^{1}\textrm{n}+\text{Energy}
  • Very-very high temperature and pressure is needed for fusion.
  • Hydrogen bomb is based on this phenomenon.
  • Nuclear fusion is the source of energy in the sun and other stars.

Environmental Consequences

Factors to be kept in mind while choosing a source of energy are:

  • The economics of extracting energy from the source
  • The efficiency of the technology available
  • The damage to environment which will be caused by using that source

Some environmental consequences of the increasing energy demands are:

  • Burning fossils causes air pollution
  • Assembly of solar cell causes some environmental damage
  • The cutting down of trees from the forests causing soil erosion and destroys wild life