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Geothermal energy is a term for the heat of the earth, usually from produced natural steam; heat recovered from circulated water; or direct heat-to-energy conversion (research is ongoing in this field).
Heat from the earth
The geothermal energy of the Earth's crust originates from the original formation of the planet (20%) and from radioactive decay of materials (80%). The geothermal gradient—the difference in temperature between the core and the surface of the planet—drives a continuous conduction of thermal energy in the form of heat from the core to the surface. Earth's internal heat is thermal energy generated from radioactive decay and continual heat loss from Earth's formation. Temperatures at the core–mantle boundary can exceed 4000 °C (7,200 °F). The high temperature and pressure in Earth's interior cause some rock to melt and solid mantle to behave plastically, resulting in portions of mantle, which is lighter than the surrounding rock, to convect upward. In the crust, rock and water is heated, sometimes up to 370 °C (700 °F).
Geothermal energy produced in hot springs has been used for bathing since Paleolithic times and for space heating since ancient Roman times, but today it is best known for electricity generation. Worldwide, 11,700 megawatts (MW) of geothermal power is online in 2013. An additional 28 gigawatts of direct geothermal heating capacity is installed for district heating, space heating, spas, industrial processes, desalination and agricultural applications in 2010.
Geothermal power is cost effective, reliable, sustainable, and environmentally friendly, but historically it has been limited to areas near tectonic plate boundaries. Recent technological advances have dramatically expanded the range and size of viable resources, especially for applications like home heating, opening a potential for widespread exploitation. Geothermal wells release greenhouse gases trapped deep within the earth, but the emissions are lower per energy unit than those of fossil fuels, meaning geothermal power has the potential to help mitigate global warming if widely used to replace fossil fuels.
Theoretically, the Earth's geothermal resources are more than adequate to supply the world’s energy needs, but only a small fraction may be profitably exploited. Drilling and exploration for deep resources is very expensive. Forecasts for the future of geothermal power depend on assumptions about technology, energy prices, subsidies, and interest rates. Pilot programs like Eugene Water & Electric Board’s (EWEB) customer opt-in Green Power Program show that customers would be willing to pay a little more for a renewable energy source like geothermal. But as a result of government assisted research and industry experience, the cost of generating geothermal power has decreased by 25% over the past two decades. In 2001, geothermal energy cost between two and 10 US cents per kWh.
In the 20th century, demand for electricity led to the consideration of geothermal power as a generating source. On July 4, 1904, Prince Piero Ginori Conti tested the first geothermal power generator at the same Larderello dry steam field where geothermal acid extraction began. It successfully lit four light bulbs. Later, in 1911, the world's first commercial geothermal power plant was built there. It was the world's only industrial producer of geothermal electricity until New Zealand built a plant in 1958. In 2012, it produced 594 megawatts. Lord Kelvin invented the heat pump in 1852and Heinrich patented the idea of using it to draw heat from the ground in 1912, but it was not until the late 1940s that the geothermal heat pump was successfully implemented. It’s thought that the first was Robert C. Webber's homemade 2.2 kW direct-exchange system, but sources disagree as to the exact timeline of his invention. J. Donald Kroeker designed the first commercial geothermal heat pump to heat the Commonwealth Building in Portland, Oregon, and demonstrated it in 1946. Professor Carl Nielsen of Ohio State University built the first residential open loop version in his home in 1948. The technology became popular in Sweden as a result of the 1973 oil crisis, and has been growing slowly in worldwide acceptance since then. The 1979 development of polybutylene pipe improved the heat pump’s economic viability.
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