As an alternative to fossil fuels, geothermal energy resources may be developed where conditions warrant. This technology requires proximity to resources and the infrastructure to ship energy to market. Unlike wind and hydropower, however, availability of geothermal energy does not depend on weather or precipitation.

Volcanic activity heats rock, imbuing it with thermal energy. Where such activity is recent or ongoing, naturally occurring water moving through hot rock may produce natural hydrothermal systems that need only a little engineering to generate electricity. Often there is enough energy for major commercial development. Other locales may have hot rock but little water. The technology needed to produce energy commercially from these systems has not yet been developed. If the cost of producing energy from other sources gets high enough, this technology may be pursued.

Heat flows from hot to cold. As a result of radioactive decay, as well as some residual heat from Earth's formation, the interior of the planet is hot compared to outer space, so heat flows upward from depth through the surface of the Earth. By measuring how rapidly temperature increases with depth and determining the thermal conductivity of rocks, scientists can determine the rate of Earth's heat flow: The planet's average heat flow is about 87 milliwatts per square meter, so the total heat loss from Earth is about 44 x 1012 watts. To put this in perspective, total human energy consumption is about 15 x 1012 watts, and total solar energy reaching the Earth is about 240 watts per square meter, or 12.65 x 1015 watts.

Heat flow varies with location, and in some places it is high enough to generate usable power. In Italy, geothermal resources have been used since 1913, when a 250-kilowatt power station was constructed. Iceland produces over one-quarter of its electricity and over 80 percent of its home heating and hot water from geothermal resources. The Geysers geothermal field in Northern California produced 2 x 109 watts at its peak in 1987, with output generally declining since then but leveling off at about 8.5 x 108 watts. In these locations, naturally occurring water migrating through hot rocks turns to steam in sufficient volumes to power a steam turbine and to heat homes. Such conditions are uncommon and are even less commonly situated in locations near significant human populations. Unlike some energy sources, however, geothermal energy at the locations where it exists is available at any time.

There are many areas with high geologic heat flow but no naturally occurring steam. Efforts to develop these hot dry rock (HDR) resources began in 1970 at the Los Alamos National Laboratory. The project involved drilling a 4-kilometer-deep well at Fenton Hill, New Mexico, then injecting water under pressure to produce cracks in the hot rock at that depth. The cracks were expected to be penny shaped, and another well was drilled down to where the top of the penny was expected to be. Many technological challenges were successfully met; however, the vagaries of fracture propagation within a complex geological terrain were not adequately anticipated. After thirty years of work by some of the country's top geologists and engineers and over $180 million spent, the site was closed and the wells were cemented in. Many lessons were learned, including the realization that huge investments of human and financial capital may not be sufficient to harness a "free" source of energy.

Even where heat flow is not particularly high, geothermal systems can heat and cool buildings. These low-temperature systems pump fluids through the subsurface, absorbing heat in the winter and dispensing it in the summer. This is possible because the Sun and atmosphere are responsible for heating the surface of the Earth, and at a depth of a few meters the temperature is close to the median surface temperature for a given location. Electricity is needed to operate the fluid pumps and the heat pumps, but a well-designed system can be considerably more efficient than one based on fossil fuels or electricity alone.

Huge quantities of energy move through the Earth's systems, providing tempting possibilities for environmentally sound, sustainable energy production. Geothermal energy is one such avenue. In places where high heat flow and naturally occurring water coexist, this resource has often already been developed. In places where there is high heat flow but no water, the risks inherent in trying to put the thermal energy to use are great, as the investments required are enormous and there is no certainty of success.

Low-temperature geothermal systems for space heating and cooling present the same problems on a smaller scale. If there were a good chance that the investment costs of such systems would be paid back quickly in fuel savings, they would be extremely popular. Design improvements, manufacturing efficiencies, and economies of scale will bring down the cost of these systems, while resource depletion and environmental costs will increase the cost of fossil fuel systems. At some point, in some areas, the former may become more economical than the latter.

Bibliography:

1) Decker, Robert W., and Barbara Decker. Volcanoes. New York: W. H. Freeman, 2005.

2) Duffield, W. A., J. H. Sass, and M. L. Sorey. Tapping the Earth's Natural Heat. Washington, D.C.: U.S. Geological Survey, 1994.

3) Pahl, Greg. The Citizen-Powered Energy Handbook: Community Solutions to a Global Crisis. White River Junction, Vt.: Chelsea Green, 2007.

4) Tester, J., et al. "The Future of Geothermal Energy: Impact of Enhanced Geothermal Systems (EGS) on the United States in the Twenty-first Century." Final Report to the U.S. Department of Energy Geothermal Technologies Program. Cambridge, Mass.: MIT Press, 2006.

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