Nuclear Energy
In recent years, we have much hopes for getting nuclear energy. It is made available through the two processes : (a) nuclear fission, where a nucleus of an element is broken into two nuclei or more and releases sufficient amount of energy, and (b) nuclear fusion, in which case energy is released as a result of joining of two very small nuclei. For getting energy of the first kind, nuclear reactors are set up in the developed and some of developing countries like India. Energy is generated from uranium for the peaceful work. Inspite of development of nuclear waste management technology, still there is fear for the disposal of radio active nuclear waste. Radio active chemicals are long lived, and if entered into human systems it can cause death also. The second method of nuclear energy generation is still in infantary stage. It may take more than 50 years to be developed.

Fossil Fuel Energy
The living plants buried during the carboniferous period (about 330-350 million years ago) have been a source for fossil fuels (coal, oil and gases). Coal is the major reserve, followed by oil and natural gas. It is widely distributed and occurs in high quantity. It may reach its peak of production in another 150 years (McMullan et al, 1976). Oil stands second to coal; its price is increasing day by day due to high cost of extraction and purification. However, during World War II, oil was cheaper than coal. But its price increased gradually with oil-based economics in most of the developed countries.

Since 1973, when oil producing countries in the middle-east decided to reduce the oil production and raised the oil price, there has been very significant effect on the economies of non-oil producing nations.

With the onset of oil crisis, most of the countries became aware of their total dependence on only one form of energy. In fact, it was soon realized that mankind had been living in a petroleum society and that this crisis threatened the life style, as also the national security because most of the defense systems use petroleum as energy. It also become clear that such an abrupt crisis would not have affected any nation if a broad energy policy, involving many sources, had been followed (Khoshoo, 1988).

Non-fossil and Non-nuclear Energy
In addition to energy sources as described above, the star (sun), planet (earth), satellite (moon) and water and wind are the other sources of energy owing to which our existence is possible. Global power potential of some renewable resources is given in Table 19.1.

Energy from tides (due to moon) and geothermal one (hot interior of the earth) have least contribution. Recently, in Gujarat, the Central Electricity Authority at the Kutch Tidal Power Project, Navlakhi, is investigating for the possibility of electricity generation from tidal wave energy, along specific areas of the Indian coastline. It is hoped that 900MW electricity can be generated. Investigations for assessment of tidal power potential in the Gulf of Kutch were conducted by the National Hydroelectric Power Corporation (NHPC) in association with National Institute of Oceanography, Geological Survey of India (G.S.I.) and Central Water and Power Research Station.

Moreover, investigations on geothermal energy potential in some part of Uttar Pradesh are being conducted by Wadia Institute of Himalayan Geology, National Geographical Research Institute and Geographical Survey of India. No details are yet available except at Tapovan and Badrinath. Works on geothermal energy would be fruitful in remote places where both fuel and hydroelectric power would be difficult to supply (Pant and Singh, 1987).

Energy from wind can also be utilized by technological breakthrough. National Aeronautical Limited, Bangalore is engaged in research and development of power generation through wind mills. In many states, wind mills have been set up for irrigation purposes. However, by using wind energy, power generation plants can also be set up. Much work is to be done on utilization of wind energy.

Sun has the most unlimited source of energy i.e. the solar energy. It is due to constant nuclear reactions going on naturally in it. A total of 35 per cent energy is reflected by air molecules, dust, cloud, etc. present in the atmosphere, and through surface reflection as well.

Total energy present on earth surface (3 x 10²⁴J/year) comes from sun at the rate of 173 x 10¹² KW, of which about 0.1 per cent energy per annum is utilized by plant life, which results in net annual production of 2 x 10^{1 [} tonnes of organic matter having energy content of 3 x 10^l2J. Total annual energy use, however, is the order of 3 x 10²⁰J (Hall, 1978 and 1979).

There are three routes of utilization of solar energy: (a) solar thermal route, (b) solar photovoltaic route, and (c) biological route. The first two routes are out of scope of this text, whereas the biological route or photosynthesis is very important as far as renewable source of energy is concerned.