Maryland Energy Administration

Geothermal Energy

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Geothermal Energy's Source

Deep below the earth's surface, natural processes generate heat (some scientists believe from radioactive decay) in varying depths and intensity. The heat contained within the rock and fluid fills fractures and pores that lie within the topmost layer, or crust. This outer layer of cool rock is approximately 15 to 30 miles deep. A layer of semi-molten rock or mantle makes up the next layer, and beneath this is thought to be a core of liquid iron and nickel or magma.

Instead of being one continuous level of rock, scientists believe the crust is made up of separated continental rock plates which "float" and interact with each other on the mantle level. As the plates interact or shift apart, the molten rock or magma beneath it can break through to form a volcano. When the magma does not break through to the surface but instead stops within the mantle layer, it can transfer its heat to underground water reservoirs. This hot water may surface as a hot spring or geyser, or it may lie dormant and undetected beneath thousands of feet of rock.

History and Geography of Geothermal Energy

This enormous energy source has fascinated people for centuries and been an inspiration to harness its power for practical uses. Ten thousand years ago, North American Indians used geothermal hot springs for warmth, cooking, cleansing, and healing benefits from its minerals. In 1847, William Bell Elliot discovered a geothermal steam field north of San Francisco and named it "The Geysers." Over the following decades, The Geysers developed into a renowned spa and resort attracting many famous clients. Today it is the only commercially developed steam field in the United States, beginning electricity production in 1960, and the largest single source of geothermal power in the world.

Geothermal reservoirs lie throughout the United States, but are most numerous in the Western states and in the Gulf Coast areas of Texas and Louisiana. There are areas in central Texas, Arkansas, the Dakotas, and some parts of the East Coast that demonstrate moderate geothermal reservoirs as well. Current research is concentrating on discovery methods for other hidden and deeper deposits, as well as better techniques for more efficient and economical extraction.

Applications

The growing efficiency of geothermal technology is gaining wide acceptance for both commercial and residential applications. A major attraction is that its use for space heating and cooling and water heating requires the concentration of naturally existing heat rather than use of fossil fuels.

Geothermal resource temperatures are classified into low (less than 90 C or 194 F), moderate (90 to 150 C or 194 to 302 F), and high (more than 150 C or 302 F). The temperature determines the application. Low to moderate temperature resources usually are applied to direct use (not requiring a heat pump or power plant), as well as to ground source heat pumps. Some direct use examples are greenhouses, aquaculture, buildings, industrial processes, and resorts. High temperature resources from hydrothermal fluids are used for commercial electric power generation. See the U.S. Department of Energy's Geothermal Electricity Production.

Residential Applications

Geothermal technology is growing in popularity for residential use with rough estimates of 10,000 to 40,000 ground-source heat pumps installed yearly. Geothermal, or ground source heat pumps (GHP), utilize a series of pipes which are buried either vertically or horizontally near the target building. For heating purposes, water or a mixture of water and antifreeze are circulated through the pipes to absorb heat from the surrounding soil which is then concentrated and transferred to the building. In hot weather, the process is reversed with the loop carrying heat from the building to use the ground as a heat sink. Distribution for both processes usually is dispersed via conventional ductwork.

Geothermal technology also provides hot water for residential use with desuperheaters where excess heat from a heat pump's compressor is transferred to the hot water tank. Although hot water generally has not been provided during the spring and fall when a geothermal heat pump is not functioning, manufacturers are now beginning to offer "full demand" systems using separate heat exchangers to provide a household's complete hot water requirements. The system is reported to be as cost-effective as other hot water heating systems.

A Growing Technology

Although geothermal ranks third among renewable energy technologies, following hydroelectricity and biomass, it is believed that eventually it's enormous potential could provide most of our nation's future energy needs. Research and development, economics, and public awareness play important roles in furthering geothermal energy's use. Recently the technology has gained support in communities seeking economical, efficient and environmentally sound heating and cooling systems for their schools. See the GeoExchange Information Center for updates on the Cambridge, Maryland project: www.geoexchange.org/pdf/cs%2D074.pdf.

The U.S. Department of Energy's Geothermal Research Program supports research and development of geothermal technologies through national laboratories, industrial contractors, and universities. The goal is to develop economical, advanced geothermal technology for electric power generation focusing on the categories of environment, drilling, conversion technology, exploration, and reservoir technology. Scientists now believe the U.S. will be the world's top geothermal energy producer by year 2005.

Programs

• Geothermal Heat Pump Grant Program