What is Geothermal 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.
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
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.