The word geothermal comes from the Greek words geo (earth) and therme (heat).
Geothermal heat occurs everywhere under the surface of the earth, but the conditions that make water circulate to the surface are found only in less than 10 percent of the land area of the earth. An approach to capturing the heat in dry areas is known as "hot dry rock." The rocks are first broken up by pumping high pressure water through them. Water is then pumped from the surface down through the broken hot rocks. After the water heats up, it is brought back to the surface through a second well and used to drive turbines or to provide heat.
Researchers at the Los Alamos National Lab in New Mexico have studied hot dry rock since 1974. The Fenton Hill plant involves a well drilled 11,500 feet into rock at 430 degrees F. Water pumped down the well at 80 degrees returned to the surface at 360 degrees F. The plant has produced as much as 5 megawatts of power, proving the technical feasibility of hot dry rock.
However, a number of barriers must be overcome before hot dry rock can become a commercial source of power. The wells must be quite deep, deeper than for conventional geothermal plants. Also, the flow of heat through dry rock is slow, which means the heat removed through the well will be slow to be renewed. Finally, the most promising sites for hot dry rock are in dry areas of the West, which means that water may be hard to come by.
Ground-source heat pumps. A much more conventional way to tap geothermal energy is by using geothermal heat pumps to provide heat and cooling to buildings. Also called ground-source heat pumps, they take advantage of the constant year-round temperature of about 50 degrees F just 5 to 10 feet underground. Either air or an antifreeze liquid is pumped through pipes that are buried underground, and recirculated into the building. In the summer, the liquid moves heat from the building into the ground. In the winter, it does the opposite, providing prewarmed air and water to the heating system of the building.
In the simplest use of ground-source heating and cooling, a tube runs from the outside air, under the ground, and into a house's ventilation system. More complicated but more effective systems use compressors and pumps, like in electric air conditioning systems, to maximize the heat transfer.
In regions with temperature extremes, such as the northern United States in the winter and the southern United States in the summer, ground-source heat pumps are the most energy-efficient and environmentally clean heating and cooling system available. A study by the Environmental Protection Agency found that they are as much as 72 percent more efficient than electric heating and air conditioning systems. The Department of Energy found that heat pumps can save between $300 and $800 a year in energy costs for a typical home, with the system paying for itself in three to eight years.
About 150,000 ground-source heat pumps had been installed by 1992, with the number growing by about 30,000 each year. While this is significant, it is less than 1 percent of the heating and cooling market. Heat pumps have higher up-front costs. Installing them in existing buildings can be difficult, since it involves digging up the yard around a house (provided it has a yard). Finally, many heating and cooling installers are just not familiar with the technology.
Ground-source heat pumps are catching on in some areas though. In rural areas without access to natural gas pipelines, homes must use propane or electricity for heating and cooling. Heat pumps are much less expensive to operate, and since buildings are widely spread out, installing underground loops is not a problem. Underground loops can be easily installed during construction of new buildings as well, resulting in savings for the life of the building.