What do you mean by primary and secondary energy sources ? Mention the difference between them with suitable examples.

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Q. What do you mean by primary and secondary energy sources ? Mention the difference between them with suitable examples. (R.G.P.V., Dec. 2014)

Ans. Primary Energy Sources - The energy sources, which provide a net supply of energy are called the primary energy sources. Some of the examples of primary energy sources are coal, oil, uranium, etc. The energy required to obtain these fuels is much less than that can be produced by their combustion or nuclear reaction. These energy sources have higher yield ratio. The yield ratio is defined as the energy fed back by the material to the energy received from the environment.

Some major primary sources are described below-

(i) Coal-Coal is mainly composed of carbon, it also contains hydrogen, oxygen and varying amounts of nitrogen, sulphur and other elements in small quantities. It is developed from decomposition of vegetable matters, trees, plants which fall and buried under the layers of silt. Below the surface of earth they get converted into the coal under heavy pressure, heat and movement of earth crust.

Coal has been the primary energy source since the advent of industrialization. It is available in abundant quantity and will suffice for the next 200 years. Coal on combustion produces various poisonous gases, that is why world is now switching over to other technologies for energy production.
(ii) Oil :- Sedimentary rocks containing plants, animals, which are about 10 to 20 crore years old are the sources of mineral oil. Mineral oil is very unevenly distributed over the earth. Almost 40% of the energy needs of the world are fulfilled by oil.

With present stock of 250000 million tonnes of oil, it would be suffice only for 100 years, so an alternative have to be find out.

(iii) Natural Gas - Natural gas is a mixture of lighter hydrocarbons, mainly methane (CH4) and small amount of nitrogen and other impurities such as hydrogen sulphide.

Earlier it was less used, due to its high transportation cost and low thermal efficiency. But now, its utilization rate has been raised upto 90%. It is mainly used for cooking in houses, and as a raw material in petrochemical industry. (iv) Nuclear Power - Energy released during a nuclear reaction in accordance with the mass-energy equation is called nuclear energy. According to Einstein mass energy equation,

E = mc2

where E Energy obtained by conversion of matter into energy
c = Speed of light
m = Lost mass which is converted into energy called binding energy.

Binding energy is the energy which keeps the nucleus of every atom from flying apart. This binding energy when released slowly and under control produces heat which can be used to drive steam-driven electric generators to produce power. A small quantity of radioactive material can produce an enormous amount of energy. Atoms in matter has high concentration of binding energy. Controlled fission of heavier unstable atoms such as U235, Th232 and Pu239 liberates a large amount of heat energy. For example, complete fission of 1 kg of U235, gives energy which can be obtained by burning 4500 tonnes of coal or 2200 tonnes of oil.

Secondary Energy Sources- The energy sources, which do not produce net energy are called secondary sources. Their energy yield ratio is very much less. Solar energy, wind energy, water energy, etc., are the examples of secondary sources.

(i) Solar Energy-Solar energy is the energy produced and radiated by the sun, more specifically, the sun's energy that reaches to the earth. As sun is expected to radiate for a few billion years, it can be regarded as the most important inexhaustible source.

(ii) Wind Energy-Due to virtue of motion, wind possesses kinetic energy, which is called as wind energy. The wind energy can be utilized to run wind mill which in turn drive a generator and produce electricity. Wind mills can also be used for water pumping.

(iii) Water Energy - Water because of its potential energy is the cheapest, neat and clean source of energy. Water energy is developed by allowing water to fall under the force of gravity from height. This potential energy of water is converted into mechanical energy by using turbines, and these turbines are used to generate electricity.

(iv) Geothermal Energy - Geothermal energy is the heat of the earth' and may be defined as naturally occurring thermal energy found within the rocks in earth's crust and fluids held within these formations. Sources of this thermal energy are magnetic or radioactive decay processes occurring within the interior of the earth.

(v) Magneto Hydro Dynamic (MHD) - MHD is a method of direct conversion of kinetic energy of flow of a fluid into the electrical energy. In this method a conducting fluid is allowed to flow in a magnetic and electric field. The fluid may be gas at elevated temperature or liquid metal like sodium or potassium. An MHD generator converts the heat energy of a fuel directly into electrical energy without a conventional electric generator.