16. The radiation from the Sun is in the form of heat (i.e., infrared) radiation, visible light, ultraviolet rays and gamma-rays.
Solar constant is the amount of solar radiation received on a unit area exposed normally to the Sun rays at an average distance between the Sun and the earth. Its value is about 1.4 kW m-2.
Nearly half of solar energy reaching at the outer layer of earth’s atmosphere is absorbed by earth’s atmosphere and the rest reaches the earth’s surface. Most of the ultraviolet rays are absorbed by the earth’s atmosphere and solar radiation reaching earth’s surface is mostly infrared radiation and visible light.
17. Mankind has been utilising solar energy for various purposes e.g., for drying of foodgrains, fish and firewood or drying of clothes, obtaining salt from seawater, etc. Modern scientific devices used to harness solar energy are solar cookers, solar water heaters, solar water pumps and solar cells.
18. On the basis of their working principle solar energy devices are classified into two categories :
(i) The devices like solar cooker, solar water heater etc., in which solar energy is collected in the form of heat and then used.
(ii) The devices like solar cell in which solar energy is directly converted into electricity and then used.
19. For commercial solar energy devices, we must collect energy over a larger area or over a long period of time or both. Moreover, the surface receiving the solar energy should be blackened because a black surface is the best absorber of heat.
20. Glass has a peculiar property for infrared radiation. It transmits smaller wavelength infrared radiation but does not allow infrared radiation of longer wavelengths to pass through it. This property of glass is used in greenhouses, which are designed to keep plants warm (i.e., to protect them from excessive cold) in cold climatic regions. This property of glass is also made use of in solar cookers.
21. A box type solar cooker consists of a box of non-conducting material with its inner walls painted in black. The box is covered with a glass sheet. An adjustable plane mirror is hinged on the top of box so as to reflect sunlight into box. The glass sheet helps in retention of heat inside due to greenhouse effect. When placed in Sun for 2-3 hours temperature inside the solar cooker may go up to 140°C. Such cookers can be used for boiling rice, pulses or vegetables.
22. Solar cells convert solar energy directly into electricity. Modem solar cells have efficiencies up to 25% and use silicon because it is environmentally friendly and is available in abundance. When sunlight falls on a wafer thin sheet of silicon, electricity is produced. A typical solar cell develops a voltage of 0.5 to 1 volt and can produce about 0.7 watts of electricity. Solar panels consisting of a large number of solar cells combined together can deliver enough electricity for practical use.
Although silicon is present in abundance but the technology to obtain silicon in pure form is quite expensive. To connect different solar cells in a solar panel, a conducting material like silver is used, which is again costly. Due to this reason solar cells are costly but gradually their cost is decreasing. The electricity generated by a solar panel during day time is used to charge storage batteries, which can then be used during night time to light the bulbs etc.
23. In spite of high cost and low efficiency, the solar cells are used as the source of energy in artificial satellites, radio or wireless transmission systems, at TV relay stations in remote areas, traffic lights, calculators and many toys. During last few years prices of solar cells has fallen due to advancement in scientific technology and large size solar power plants are being installed all over the world.
24. Oceans are huge reservoirs of energy. The energy possessed by the rising and falling water in tides is known as tidal energy. Winds blowing across the surface of the oceans are converted into waves. Kinetic energy of water that moves along the waves is another form of ocean energy. The difference in temperature between warm surface waters heated by the Sun and colder waters found at ocean depth is yet another form of solar energy and is known as ‘ocean thermal energy. All these forms of ocean energy can be harnessed to produce electricity. However, efficient commercial exploitation of ocean power is very difficult.
25. Nuclear fission is a process in which the nucleus of a heavy atom like uranium, plutonium or thorium, when bombarded with low energy neutrons, is split up into two lighter nuclei along with the release of a large amount of energy. The fission of a uranium nucleus produces 107 times the energy produced by the combustion of an atom of carbon from coal.
In nuclear fission, the sum of the masses of the individual products formed after fission is slightly less than the original mass. This difference (loss) in mass Δm is converted into energy E as per Einstein’s mass-energy relation E = Δm. c2, where c = speed of light in vacuum.
26. In a nuclear reactor, a self-sustained fission chain reaction of the nuclear fuel (uranium or plutonium) releases heat energy at a controlled rate. The heat energy released is used to produce steam. This steam is used to do work for rotating the turbine of an electric generator and thus to generate electricity. In India, nuclear power reactors are located at Tarapur (Maharashtra), Rana Pratap Sagar, Kota (Rajasthan), Narora (UP), Kalpakkam (Tamil Nadu), Kakrapar (Gujarat), Kaiga (Karnataka) etc. At present 3% of total electricity generation of India is produced by nuclear reactors. More nuclear power plants are under various stages of construction and erection.
27. Nuclear power generation has certain disadvantages. Main hazard is storage and disposal of used fuels. There is a risk of leakage of nuclear radiation. High cost of nuclear power plant, high risk of environmental contamination and limited availability of nuclear fuel are some other limiting factors.
28. Nuclear fusion is the process in which lighter nuclei (like deutrons) combine together to form a heavier nucleus (helium) along with the release of a large amount of energy. Nuclear fusion reaction is the source of energy in Sun and stars. For nuclear fusion reactions we need temperatures of millions of degrees and pressure of million of pascals. Scientists are trying to design devices which may generate electrical power based on nuclear fusion technique.
29. A clean fuel is that which produces large amount of energy which can be used efficiently without damaging the environment. CNG is such a clean fuel. Solar cells are also pollution free.