The modem mechanized civilization cannot proceed a moment without energy. Work is available at the expense of energy. Uninterrupted energy supply is essential for the survival of all living creatures. The demand of energy is increasing day by day with the development of life style. For this increasing demand of energy man is looking for newer sources of energy. We need to have clear idea about the sources of energy to save the living beings and to continue the supply of energy. We know that the sun is the only source of all energy. Moreover, the nuclear energy in the nucleus and the energy from the hot melted substances are considered to be the sources of energy. All the existing energy of the world is directly or indirectly conies from the sun or are produced using the radiation of the sun.

Chemical Energy/Fuel Energy: Man in the ancient time was dependent wholly on the energy of their muscles. Afterwards, they learnt to tame the wild animals and use their energy in different work. They used to carry their goods and cultivate land with the help of animals. The initial stage of civilization was to produce heat energy from burning wood and leaves and mechanical energy from current of water and flow of wind. The economic development of human being begins from the use of machine energy. Industrial revolution and invention of steam engine reduced men’s dependence on muscle energy of human and animals. Man kept operating different machines with the help of steam energy. Fuel is necessary to produce this steam energy. So we consider different kinds of fuels to be the sources of energy. The most common sources of energy are coal, mineral oil and natural gas. Coal, mineral oil and natural gas from underground are used as filel directly or by refining them slightly.

Coal: Coal is best known to us as one of the sources of energy. It is an organic substance. Once upon a time there were numerous plants and trees. Due to different natural calamities and natural consequences the leaves of the trees and their sterns were buried underneath the earth and began to coagulate. As a result of chemical changes, the leaves of the trees and stems were transformed into coal. Combustion of coal gives us heat directly. This is a well known fuel. Many essential substances can be produced from coal apart from using it as a fuel. Some of the coal products are coal gas, tar, benzene, ammonia, toluene etc. Coal is used to cook food and to drive the steam engines. In modern age, the main use of coal is in thermal power plants. The main fuel in a thennal power station is coal. The main problem of a coal fueled power station is that it emits sulphur smog which causes acid rain. Though this acid is very weak, it kills the fishes of pond, cannel and lake, destroys forest and damages the sculptures of stone.

Mineral Oil: Petroleum or mineral oil is one of the principal sources of energy. It is widely used as a fitel in the present world. It is being used right from a rural cottage to the most modern transport system. Petrol, pitch that is used to pave roads, kerosene and chemical fertilizers are all petroleum products. There is nothing like petrol to be used as fuel. On the other hand, many kinds of artificial fabrics can be developed from petroleum. These are terry line, polyester, caslunlion etc. Moreover, various types of cosmetics are produced from petroleum. But it is basically used as a fuel. Petroleum products are used to produce electric and mechanical energy. Petroleum is a Latin word. It is a combination of two minor words: Petro and Olitun. In Latin language `petro’ means stone and ‘olium’ means oil. So, petroleum means oil of stones i.e. oil stored inside stones. In tertiary age i.e. almost five to six awes of years ago the trees and the animals fell buried in the different layers of sedimentary rock on the bottom of the sea. Due to different chemical changes these were transformed into mineral oil. Most of the solid regions of the present world were a part of the bottom region of the sea in prehistoric age.

Natural Gas: Natural gas is a well known source of energy. Specially, the use of natural gas in Bangladesh is wider. Use of natural gas is very high in all advanced countries. It is also used in different industries as fuel. In Bangladesh it is widely used for domestic requirement basically for cooking purposes. It is also used in fertilizer factories. Heat energy is produced by the combustion of gas and electric energy is produced from heat energy in a thermal power station. Natural gas is obtained from underneath the earth. Digging very deep well the gas may be taken out from underneath the earth. Tremendous temperature and pressure inside the earth is the root cause of the creation of the gas. Natural gas is also available in petroleum well. The principal element in a natural gas is the methane gas. These are called fossil energy. The three sources of energy discussed above are reducing very fast due to the men’s increasing demand of energy. The physical condition of the world is such that these sources-coal, mineral oil, natural gas cannot be recreated, these are called nonrenewable energy. So men are looking for alternate sources of energy of which solar energy, energy from water flow, energy from tide and ebb, geothermal energy, wind energy, biomass etc are the main sources. These sources are directly or indirectly dependent on the sun. As long as earth receives sunlight the energy supply from these sources is possible. These are called renewable sources.

Solar Energy: The energy that we obtain from the sun is called Solar Energy. It is known to all that the sun is the source of all energy. The origin of all forms of energy is from the solar energy in one way or the other. For an example, the fossil fuel such as coal, mineral, oil, and natural gas is actually a store of solar energy for a long time.

From ancient times man is using sunlight directly to dry things. At present man is adopting various means to use the solar energy round the clock. Ignition can be done by concentrating sun’s rays- with the help of a convex lens. Solar cooker consists of a metallic bowl on which the solar rays are reflected. The cooker can be used in cooking purposes.