Geothermal Energy

B

Space Heating

One of the most common uses of geothermal water is for heating individual buildings or groups of buildings (district heating). A typical geothermal heating system supplies heat to buildings by pumping water (usually 60° C/140° F or hotter) from a geothermal reservoir. Heat from the geothermal water is transferred through a heat exchanger to city water contained in an adjacent separate piping system. This heated city water is then pumped into the buildings, while the geothermal water is injected back into the reservoir to be reheated so it can be used again.

The world's largest geothermal system for district heating is in Reykjavík, Iceland. Almost all the buildings in that city use geothermal heat. Eighteen geothermal heating districts exist in the United States—the most extensive are located in Boise, Idaho, and San Bernardino, California. Modern district heating systems also warm homes in France, Turkey, Poland, and Hungary. Experts believe that in the western United States more than 270 communities are close enough to geothermal reservoirs for potential development of geothermal district heating.

VI

Geothermal Energy and the Environment

Geothermal energy is a renewable resource: Earth’s heat is continuously radiated from within, and each year rainfall supplies new water to geothermal reservoirs. Production from individual geothermal reservoirs can be sustained for decades and perhaps even centuries.

Compared to other types of power plants, geothermal plants have relatively little effect on the environment. Geothermal power plants have been successfully operated in farm fields, in sensitive desert environments, and in forested recreation areas.

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Hydrogen sulfide gas (H₂S), which can be toxic at very high concentrations, is sometimes present in geothermal reservoirs. However, this gas is removed from geothermal water with antipollution “scrubbing” equipment.

Geothermal reservoirs contain higher concentrations of minerals and chemicals than do aquifers used for drinking water. Therefore, wells drilled into geothermal reservoirs have several layers of pipes, or casing, cemented into the ground nearly the entire length of a well. The pipes and cement prevent the deep geothermal water from mixing with freshwater aquifers.

Geothermal reservoirs contain some carbon dioxide (CO₂) which is released when the hot water turns into steam. Some scientists believe that the buildup of carbon dioxide in the atmosphere—partially the result of burning fossil fuels—is contributing to what may be a gradual increase in global temperatures, because carbon dioxide traps heat from the Sun that is reradiated by Earth. The amount of carbon dioxide released from geothermal power plants, however, ranges from zero to 4 percent of the carbon dioxide released by an equivalent power plant fueled by coal or petroleum.

VII

Outlook for Geothermal Energy

The resource base (estimated total amount of energy) of geothermal energy is larger than the resource bases of coal, petroleum, natural gas, and uranium (uranium is used in the production of nuclear energy) combined.

The discovery and development of additional geothermal reservoirs depends on several factors. These factors include the depth, heat, and water content of a particular reservoir, as well as the permeability (property of having fractures that allow liquids to flow through) of the rock in the reservoir. The depth and heat of a geothermal reservoir cannot be changed, but the amount of water in the reservoir and the permeability of the rock can be increased. Private and government research projects underway in the United States, in Japan, and in Europe are focused on finding ways to increase the permeability and amount of water in certain types of hot rock (so that water can circulate more freely throughout the rock and become better heated).

Engineers are increasing the amount of water at The Geysers steam field in California by pumping treated wastewater from nearby communities down into it. This additional water increases the amount of steam available to produce electricity and provides nearby communities with an environmentally safe method for disposing of their wastewater.

The permeability of hot rock can be increased by hydraulic fracturing—injecting large volumes of water down into a well at a pressure high enough to break the rock. A second well is drilled nearby into the fractured rock. Cold water is pumped down one well, and once the water has heated, it is pumped up through the second well for use in a geothermal plant. This 'hot dry rock' technology is currently being tested in Japan, Germany, France, the United Kingdom, and the United States.

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