Wood has been the most common source of energy since the advent of human beings on the earth. Population being thin everywhere it seemed on everlasting source. Increase in population meant occupation of land for residential houses. That is how forests started depleting and disappearing too. Ti was not simply a loss in the potential source of energy but created environmental problem too. Since 18th century coal started replacing wood and was known as black gold. But the scientists knew that as the industries grew and the population increased coal reserves may last only for a hundred or two hundred years. The discovery of petrol of course proved a boon. But as the late Shah of Iran put it the stocks may last only by the first quarter of the 21st century.

Non Conventional source of Energy Kudankulam Power Plant
Kudankulam Power Plant Tamilnadu

Nuclear or atomic energy started replacing these conventional sources of energy after the Second World War. Number of plants increased in the developed countries to an extend that by the eighties of the 20th century this too was considered as conventional source of energy. It is based on the principle of fission of a heavy atom of uranium or allied metals like plutonium that releases unlimited energy. It has two limitations. The stock of uranium is limited. Moreover the fission produces dangerously radioactive waste. Dumping of the waste is a problem.

Another big source of energy, specially in metropolitan cities is the garbage and refuse. Big tanks at okhla in New Delhi receive human excreta through drain from a particular area. It is recycled in a way to produce gas that is supplied to the nearby area for cooking. It can also be used for lighting the household. The left over after processing provide rich natural fertilizer. The city garbage in Bombay is supplied to a firm that processes it in a way ultimately changing the garbage into small coal ingots that are used in place of coal in all manufacturing concerns. There are a number of such plants in Bombay. All these unconventional sources of energy are free from environmental hazards and are economical too.

The search for still better sources of energy has been continuing.