Temperatures from the surface to the center of the Earth warm by 17° to 30°C for every kilometer in depth, reaching 5100°C in the inner core (nearly the temperature of the sun).  The energy that heats Earth’s interior derives from several sources. First is the decay of radioactive isotopes, especially 238uranium, 235uranium, 232thorium, and 40potassium. Second, because the Earth first formed regardless of density, heavy metals (like iron, nickel, and copper) are continually sinking towards the center of the Earth, displacing lighter elements (like aluminum, sulfur, and silicon), creating friction and heat. Third, the Earth’s interior still retains some of the energy from its formation 4.5 billion years ago.
Earth’s crust varies in thickness from about 5 km to 75 km. Locations with thinner crust, such as at boundaries of tectonic plates, fault lines, and volcanoes, receive more heat from the interior and are promising sites for geothermal power. For example, California, which lies at the intersect between the Pacific and North American tectonic plates, has the world’s highest number geothermal power plants which supply 7% of the state’s electricity.
A geothermal power plant pumps water down a deep injection well. This water flows through fractures in the fiery brimstone and heats up until it escapes up a second borehole, the production well, as steam or superheated water (depending on temperature and pressure). If the production well outflow is steam, it may directly drive a turbine in the power plant (dry steam type). If the outflow is super-heated water, a release of pressure generates steam that may drive a turbine (flash-steam type). Alternatively, superheated water from the production well may pass through a heat exchanger that vaporizes a fluid with a lower boiling point such as butane, which then drives a turbine and returns to the heat exchanger (closed-loop binary type). A condenser and cooling tower lower the temperature of the water before it returns back down the injection well. Boreholes for the wells are lined with metal or concrete near the top to prevent water from penetrating through porous rock. A makeup tank replenishes the water lost during its passage through the system.
A geothermal power plant works by pumping water down a deep injection well. The water heats up and escapes into the production well as steam or super-heated water.
Today most commercial geothermal wells are shallower than 3 km.  The petroleum industry is developing more economical methods for drilling oil or gas wells 6 km to 10 km deep.  Transfer of this technology should soon reduce the cost of constructing deeper, hotter wells.
Geothermal electric power plants have many advantages. They are economical; in California, geothermal plants cost less than most other types. They have the potential for expansion; geothermal plants could provide about one-fourth of the electricity in the United States by 2030 at a reasonable price. They provide power day and night, in any weather. They discharge much lower amounts of CO2, nitrogen or sulfur oxides, and particulates than fossil fuel power plants.
Disadvantages of geothermal power are several-fold. Sites suitable for geothermal power are often far from population centers; for instance, the East Coast of the United States has limited possibilities for geothermal power. Injection of water into wells could compromise the stability of the surrounding geological formations, although based on substantial evidence collected so far, the probability of inducing a damaging seismic event is low. Overexploitation of a geothermal site may cause it to cool to the point where electricity generation is uneconomical; if left alone, however, these places will recover their lost heat as it transfers from lower depths.
 Alfe, D., M. J. Gillan, and G. D. Price (2007) Temperature and composition of the Earth's core. Contemporary Physics 48:63-80 doi:Doi 10.1080/00107510701529653.
 Bertani, R. (2005) World geothermal power generation in the period 2001-2005. Geothermics 34:651-690.
 MIT Study Group (2006) The Future of Geothermal Energy, Massachusetts Institute of Technology, Cambridge, MA, http://www1.eere.energy.gov/geothermal/pdfs/future_geo_energy.pdf.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
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