Enormous quantities of cooling water are needed, and sources are commonly river or lake water. Sometimes, particularly in drier parts of the country, groundwater, or even trades sewage effluent, is used. Harnessing the energy of running water is the oldest method of generating electric power. About 15 percent of the generated by hydroelectric plants. In future, substantial amounts of electricity may be generated by tapping renewable sources of energy such as the sun, wind, tides or geothermal energy.
Geothermal energy appears as stream from deep beneath the earth's surface.
Hydrogeologists assist in locating areas where use of the geothermal energy may be feasible, and then locate and help design well fields to extract the heated water.