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.
A Hot Idea for Power
Extensive development of the warm-water systems in direct use applications can significantly improve the energy balance of a nation. For example, the use of geothermal water for space heating and other direct-use applications in Iceland substantially benefits the economy of that nation. Great potential exists for additional direct use of geothermal energy in the Western United States.
The U.S. leads the world in geothermal capacity, but its potential is just beginning to be tapped. Since the earth’s heat is permanently present, waiting to be harnessed, this form of renewable energy is highly cost effective—once a system is installed. The U.S. Department of Energy (DOE) initiative, “GeoPowering the West,” is exploring geothermal opportunities to answer the rising costs of energy.
The Philippines, Indonesia, and several countries in Central America already benefit greatly from geothermally generated electricity; additional projects are underway and planned. Of course, the use of geothermal energy already contributes to the economies of industrialized nations along the circum-Pacific Ring of Fire, such as the United States, Japan, New Zealand, and Mexico.
Who Can Use Geothermal Energy?
Geothermal energy is present everywhere beneath the earth’s surface, although the highest temperature, and thus the most desirable, resources are concentrated near active or geologically young volcanoes. Large quantities of heat that are economically extractable tend to be found in places where hot or even molten rock (magma) exists at relatively shallow depths in the earth’s outermost layer (the crust). Such “hot” zones generally are near the boundaries of the tectonic plates that form the earth’s crust.
Most volcanic eruptions and earthquakes occur near plate boundaries. Frequent earthquakes produce fractures in bedrock, thus allowing water to circulate at depth and to transport heat toward the earth’s surface. Regions of stretched and fault-broken rocks (rift valleys) within plates, like those in East Africa and along the Rio Grande River in Colorado and New Mexico, also are favorable target areas for high concentrations of the earth’s heat at relatively shallow depths.
Electrical-Grade Systems: Power Generation
High temperature hydrothermal systems of about 392° F or more (200°C) are able to produce steam at pressures sufficient to drive turbine generators. These systems are developed in porous rock saturated with water, which partly boils to steam when it rises up in production wells. This steam rotates a turbine generator to produce electricity. Prime examples of hot-water systems are the geothermal fields at Coso and the Geysers in California. At both Coso and the Geysers, fractures in rocks beneath a large area are filled with steam at depths that can easily be reached using today’s drilling methods. With a generating capacity of about 1,000 megawatts electric, the Geysers is presently the largest group of geothermally powered electrical plants in the world.
Moderate temperature hydrothermal systems are unable to produce steam at high enough pressure to directly drive a turbine generator. They are, however, hot enough to produce a high-pressure vapor through heat transfer to a second “working” fluid, which in turn drives a turbine generator. The power-generation technique that transfers the geothermal heat to another fluid (for example, isobutane), whose boiling temperature is lower than that of water, is called a binary-cycle, or simply, a binary system. A binary system that produces geothermal electricity is in operation near Mammoth Lakes, east of the Sierra Nevada in central California.