1. Geothermal energy: is heat contained beneath the Earth's crust, and brought to the surface in the form of steam or hot water. The five main sources of this geothermal reservoir are dry, super-heated steam from steam fields below the Earth's surface; mixed hot water, wet steam, etc., from geysers, etc.; dry rocks (into which cold water is pumped to create steam); pressurized water fields of hot water and natural gas beneath ocean beds; and magma (molten rock in or near volcanoes and 5 to 30 miles below the Earth's crust). Most Iceland buildings are heated by geothermal energy, and a few communities in the United States, such as Boise, Idaho, use geothermal home heating. Electric power production, industrial processing, space heating, etc., are fed from geothermal sources. The California Geysers project is the world's largest geothermal electric generating complex with 200 steam wells that provide some 1,300 megawatts of power. The first geothermal power station was built in 1904 at Larderello, Italy.
2. Solar radiation: utilization depends on the weather, number of cloudy days, and the ability to store energy for night use. The process of collecting and storing is difficult and expensive. A solar thermal facility (LUZ International Solar Thermal Plant), in the Mojave Desert, produces 274 megawatts and is used to supplement power needs of the Los Angeles utilities companies. Japan has 4 million solar panels on roofs and 2/3 of the houses in Israel have them; 90% of Cyprus homes do as well. Solar photo voltaic cells can generate electric current when exposed to the sun. Virtually every spacecraft and satellite since 1958 utilizes this kind of resource.
3. Tidal and wave energy: contain enormous amounts of energy to be harnessed. The first tidal powered mill was built in England in 1100; another in Woodbridge, England, built in 1170, has functioned for over 800 years. The Rance River Power Station in France, in operation since 1966, was the first large tidal electric generator plant, producing 160 megawatts. A tidal station works like a hydropower dam, with its turbines spinning as the tide flows through them. Unfortunately, the tidal period of 13 ½ hours causes problems of integrating the peak use with the peak generation ability. Ocean wave energy can also be made to drive electrical generators. (The Handy Science Answer Book, compiled by the Science and Technology department of the Carnegie Library of Pittsburgh)