Geothermal heating has been used since the time of the Roman Empire as a way of heating buildings and spas by using sources of hot water and steam that exist near the Earth’s surface. The ultimate source of geothermal energy is believed to be from the radioactive decay that occurs deep within the Earth's crust. Some areas, including substantial portions of many western U.S. states, are underlain by relatively shallow geothermal resources. Similar conditions exist in Iceland, parts of Japan, and other geothermal hot spots around the world. These resources can be classified as low temperature (less than 90°C or 194°F), moderate temperature (90°C - 150°C or 194 - 302°F), and high temperature (greater than 150°C or 302°F). Where such geothermal resources are available, it is possible to distribute hot water or steam to multiple buildings. This technique, long practiced throughout the world in locations such as Reykjavik, Iceland, Boise, Idaho, and Klamath Falls, Oregon, is known as geothermal district heating.
United States geothermal reservoir temperatures
Geothermal Heat Pumps
In recent years, the term geothermal heating has frequently been used to refer to the heating and cooling that can be achieved through the use of a geothermal heat pump. This technique is generally for residential use. It involves a refrigerant liquid being pumped through pipes in the ground, thus heating the liquid. This liquid then is brought back into the house, and the heat exchanged. The same technique is used to cool the house.
Geothermal heat pumps take advantage of the natural constant temperature of the Earth. During winter when the ground temperature is warmer than the air above it, geothermal heat pumps use the Earth's soil (or groundwater) to recover the Earth's heat. In contrast, an air-source heat pump will remove heat from the cold outside air and thus requires more energy.
In the summer months, geothermal heat pumps deliver heat to the same relatively cool soil (or groundwater) rather than delivering it to the hot outside air. As a result, the heat is pumped over a smaller temperature difference with a geothermal heat pump, which leads to higher efficiency and lower energy use.
Hot Dry Rock
The conditions necessary for water to circulate to the Earth's surface are present in less than 10 percent of Earth's land area. Therefore, geothermal heating is not always a feasible option. Hot dry rock is a method of capturing the heat embedded in these dry areas. First, high-pressure water is pumped down a borehole and through the rocks to break them apart. Water is then pumped from the surface through the ground and the broken hot rocks. This causes the water temperature to rise. The hot water is returned to the surface through a second well and is used to drive turbines for electricity or to provide heat. The water is then returned to the ground and the process repeated.
Fenton Hill facility
There was an experimental facility built in 1974 in Fenton Hill, New Mexico that studied hot dry rock. A well was drilled 11,500 feet into rock at 430 Â°F to demonstrate the feasibility of hot dry rock technology. The water returned to the surface at 360 Â°F after being pumped down the well at 80 Â°F. The plant produced as much as five megawatts of power, proving that energy from hot dry rock can be extracted for practical applications.
Geothermal energy is a type of renewable energy that encourages conservation of natural resources. According to the U.S. Environmental Protection Agency, geo-exchange systems save homeowners 30-70 percent in heating costs, and 20-50 percent in cooling costs, compared to conventional systems. Geo-exchange systems also save money because they require much less maintenance. In addition to being highly reliable they are built to last for decades and can add considerably the resale value of a structure.
Some utilities, such as Kansas City Power and Light, offer special, lower winter rates for geothermal customers, offering even more savings.
Future of geothermal heating
Geothermal energy is one of the few renewable energy technologies that, like fossil fuels, can supply continuous, base load power. In the past, electricity costs from geothermal facilities were noticeably high. Over the past few decades, these costs have steadily declined. Some geothermal facilities have noticed a 50 percent reduction in the price of electricity over the past 25 years. New facilities can produce electricity for between 4.5 and 7.3 cents per kilowatt-hour, making it competitive with new conventional fossil fuel-fired power plants. The future for the direct use of geothermal resources is becoming more attractive as prices continue to drop. It is predicted that geothermal heating will continue to become a more popular option for homes and businesses seeking long term cost and energy savings.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Geothermal heating."