1. Energy of a system is its capability to do work. We need energy in all walks of life. We cannot think of any activity, which does not require energy. For almost all forms of energy available to us on the earth, the ultimate source of energy is the Sun.
2. A source of energy is that which is capable of providing an adequate amount of useful energy. A good source of energy must be convenient to use and easy to transport and store. Moreover, a good source of energy should be capable of delivering desired quantity of energy at a steady and economical rate over a long period of time.
3. Fuels e.g., firewood, cow-dung cakes, charcoal, coal, cooking gas, petrol, kerosene oil, diesel etc., are important sources of energy and we commonly use them. Electrical energy is a convenient form of energy being used for variety of purposes.
4. Renewable sources of energy are those natural resources which are inexhaustible, that is which can be replenished as we use them and which can be used to produce energy again and again. The solar energy, energy harnessed from flowing water, wind, tides, ocean waves and bio-gas are some examples of renewable sources of energy. Nuclear energy and geothermal energy are also expected to be available for a long time to come.
5. Non-renewable sources of energy are those natural resources which are exhaustible and cannot be replenished once they have been consumed. Fossil fuels, e.g., coal, oil, petroleum, natural gas are some of the non-renewable sources of energy. Although trees can be grown again but it takes more than 15 years for a tree to mature. Due to this reason, firewood obtained by cutting trees is considered a non-renewable source of energy.
6. Fossil fuels are non-renewable source of energy. Burning of coal and petroleum causes air pollution. Oxides of carbon, nitrogen and sulphur released on burning fossil fules are acidic oxides. It leads to acid rain which adversely affects our water and soil resources.
7. Pollution caused by burning fossil fuels can be reduced by increasing the efficiency of the combustion process and using special techniques to reduce the escape of harmful gases and ashes into the surroundings. Fossil fuels can be used to generate electric power, which can be used as a clean source of energy.
8. In thermal power plants, fossil fuels are burnt to produce heat energy, which is then converted into electrical energy. The transmission of electricity is more efficient than that of fossil fuels. Due to this reason, large thermal power plants are set up near coal or oil fields.
9. Energy of flowing water in rivers and streams is utilised to produce electricity in hydroelectric power plants. High rise dams are constructed to collect water in large artificial lakes by obstructing the river flow. The stored water possesses potential energy.
Water from the top of dam is made to fall through pipes over the turbine blades located at the bottom of dam and make the turbine rotate. Moving turbine rotates the armature of electric generators so as to produce electricity.
10. Big dams can be constructed only at a limited number of places in hilly terrain and involve submergence of large areas of agricultural land and human habitation. Large eco-systems are destroyed. Submerged vegetation rots under anaerobic conditions and produces methane which is a greenhouse gas. It also creates problem of satisfactory rehabilitation of displaced people. So, we must consider these problems while planning to set up a new hydroelectric power plant.
11. Bio-mass i.e., firewood, cow-dung cakes etc., produce heat when burnt. In traditional chulhas, only 8-10 % energy of the fuel gets actually used and the rest is wasted. Moreover, it produces lot of smoke and is a health hazard.
When wood is burnt in a limited supply of oxygen, water and volatile material present in it gets removed and charcoal is left behind. Charcoal is a better fuel than wood because of its bums without flames, is comparatively smokeless and has a higher heat generation efficiency.
12. During the decay of bio-mass e.g., animal dung, sewage, crop residue, vegetable waste etc., in the absence of oxygen biogas is produced. Biogas is a mixture of methane, carbon dioxide, hydrogen and hydrogen sulphide. Bio-gas may contain up to 75% methane. It is an excellent fuel and burns without smoke. Hence, biogas can be used as a fuel in industries and at homes. Biogas is also used to produce electricity.
13. Biogas plants are installed to produce bio-gas from decay of bio-mass. Basically, a biogas plant is a digester, in which bio-mass slurry is allowed to undergo anaerobic decomposition. The biogas produced is stored in a cylinder and can be used conveniently. The waste slurry remained as a residue in a biogas plant is a good source of nitrogen and phosphorus and can be used as manure.
14. Wind is a solar-related source of energy. Wind energy is the kinetic energy possessed by moving air. A windmill is used to convert wind energy into electrical energy.
The blades of a windmill are designed to create a pressure difference between its different regions when wind strikes them. The pressure difference produces a turning effect to make the blades rotate. The rotatory motion of windmill is utilised to rotate the turbine of an electric generator. A number of windmills coupled together can generate electricity on a large scale. Such an arrangement is known as ‘Wind energy farm”. For a windmill to be functional minimum wind velocity should be about 15 km h-1.
There are many limitations while harnessing wind energy. Wind energy farms require large land area. The initial cost of establishing a wind farm is quite high. A high level of maintenance is needed since the tower and blades of windmill are exposed to Sun, rain, storm, cyclone etc.
15. The Sun is radiating energy at a rate of about 4.5 x 1023 kW for the last 5 billion years and is expected to radiate energy almost at the same rate for about 5 billion years to come. Sun radiates its energy in all directions in space and earth directly receives only a small fraction of total energy radiated by the Sun.