Everything that happens in the world is the expression of the flow of energy in one of its forms. Most people use the word energy for input to their bodies or to the machines and thus think about crude fuels and electric power. Coal, natural gas, oil and nuclear energy using breeder reactor are net energy yielder and are primary sources of energy. Secondary sources are like solar energy, wind energy, water energy etc. Solar energy can be used through plants, solar cells and solar heaters.
Solar tower is another emerging technology.
Solar drying and solar heating are economical applications when passive methods are used. Because of the dilute nature of solar energy it is difficult to classify the source as a primary one. Better sources are wind, tide, wave and hydroelectric applications. Geothermal and ocean thermal are the other sources, which may well prove worthwhile. It may be necessary in future to develop the secondary sources like solar, wind etc.