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Geothermal Energy

What is Geothermal Energy?

Thermal springs can be harnessed for heat and energy.

Geothermal resources are simply exploitable concentrations of the earth’s natural heat (thermal energy). The earth is a bountiful source of thermal energy, continuously producing heat at depth, primarily by a small amount of radioactive decay that occurs naturally in small amounts in all rocks. For centuries, people have enjoyed the benefits of geothermal energy available at hot springs, but it is only through technological advances made during the 20th century that we can tap this energy source in the subsurface and use it in a variety of ways, including the generation of electricity.

Geothermal energy is tapped by means of a liquid carrier—generally the water in the pores and fractures of rocks—that either naturally reaches the surface at hot springs, or can readily be brought to the surface through drilled wells. Heated waters of natural hot springs or geysers like Old Faithful at Yellowstone National Park are all natural products of geothermal energy.

This underutilized heat and power resource is renewable, domestic and clean. Through proper management, the rate of energy extraction can be balanced with a reservoir's natural heat recharge rate to create a virtually inexhaustible heat source. Geothermal resources can be harnessed locally for power production without importing fuel. Modern closed-loop geothermal power plants emit no greenhouse gasses. Geothermal power plants consume less water on average over the lifetime energy output than most conventional generation technologies.

Geothermal Uses

The geothermal resource potential map.

Almost any form of geothermal energy can be used in some capacity. At the low end of the spectrum (requiring no heat), geothermal energy can help heat and cool a single residence through “geoexchange.”  Heat pump systems are already in use at more than 350,000 buildings in the United States.

Toward the high end of the spectrum (heat above 392° F (200° C)), steam from a single large-volume, high-temperature well can be harnessed to generate electricity sufficient to serve a city of 1 million people or more. Read more about geothermal electricity generation.

In the middle of the spectrum, naturally warmed water (up to 302° F (150° C)) has a number of direct-use applications. These wells are used in greenhouses, hot baths, onion dehydration, laundries, and even hotel space heating. The capital of Iceland is almost entirely heated with geothermal water. People living in Klamath Falls, Oregon, and Boise, Idaho, have used geothermal water to heat homes and offices for nearly a century.

Arizona’s Geothermal Development

The Clifton area boasts some of Arizona’s highest geothermal potential.Direct-use aquacultural application in a Gila Bend shrimp farm.

Arizona exhibits geothermal potential in direct use application, boasting over 1,250 discrete thermal wells and springs. The two highest temperature springs in the state are Clifton and Gillard, both in the Clifton-Morenci area of southeastern Arizona. The water temperature at these springs ranges from 158–180° F.

Geothermal electrical power plants have not been developed in Arizona, but several power plants are currently in operation just west of Yuma, Arizona, in the Imperial Valley of southeastern California. Although some high temperature geothermal resources exist southeast of Phoenix near the now-retired Williams Air Force Base, they have never been deemed economically feasible.

Arizona focuses on direct-use applications: heating systems, farming, and aquaculture. We lead the nation in the aquacultural use of geothermal fluids to extend the growing season of agricultural crops. At least six fish hatchery and one algae biofuel operations throughout Arizona use geothermal waters to keep an ideal temperature for year-round growth. Federal DOE funding brought geothermal heating to a 7.5 acre tomato greenhouse complex near Willcox in southeast Arizona.

The Arizona Geological Survey (AZGS) first explored Arizona’s geothermal possibilities in the late 1970s. Although geothermal potential was considered high, by the mid-1980s lack of funding mechanisms squelched further exploration. AZGS is now encouraging industry to renew exploratory efforts by making geothermal resource data available online, including well and temperature data from than 2,400 oil and gas geophysical logs. Start-up costs can be higher for geothermal power generation than for similar solar or wind systems, so national investment is needed to promote large scale operations.

Use the Earth​ to C​ool Your Home With a Geoexchange System

A system of thermal energy known as “geoexchange” does not rely on thermal springs, but on shallow low temperature systems in ordinary rock and soil. The temperature underneath the ground remains approximately 50° F at all times, morning and night, winter and summer. Geoexchange uses this stable underground temperature to regulate air temperature inside a building, acting as a heat source in the winter and a heat sink in the summer.

Geoexchange processes use ground source heat pumps to circulate a working fluid, usually water or antifreeze, through buried pipes. Whilst circulating underground, the temperature of the working fluid approaches that of the surrounding earth. The working fluid is then pumped back to the surface, where it is used to absorb or release heat through a heat exchange system.

In places with extreme temperatures, ground source heat pumps are the most energy-efficient and environmentally clean heating and cooling system available. Geoexchange can be used almost anywhere, as long as there is a significant difference between the temperature of the air and the temperature in the earth.  It can altogether replace conventional air conditioning and heating systems in a variety of locations, for one-third to one-fifth the energy cost. According to the Department of Energy, heat pumps can save a home hundreds of dollars in energy costs each year, with the system typically paying for itself in 8 to 12 years. The Emergency Economic Stabilization Act of 2008 includes an eight year extension (through 2016) of the 30 percent investment tax credit, with no upper limit, to all home installations of EnergyStar certified geothermal heat pumps.

Tucson Geoexchange

Geoexchange uses a geothermal heat pump.

Geoexchange uses a geothermal heat pump.Geoexchange uses a geothermal heat pump.

Geoexchange uses a geothermal heat pump.

In May 2012, a Tucson building leased by the GSA installed a ground source heat pump expected to provide about 42 tons of cooling and sufficient heat to do without a boiler. (A fluid cooler stands by to provide backup during peak summer loads.) One-inch pipes circulate air through 32 cores drilled to 350 feet. Other tenants have the option to drill cores for additional geoexchange systems.

Tucson site information provided by Blaine Miller (AZ Gov).

Information from Wendell A. Duffield and John H. Sass , 2003. Geothermal Energy—Clean Power From the Earth’s Heat, U.S. Geological Survey Circular 1249

U.S. DOE Energy Efficiency and Renewable Energy (EERE)