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##### U.S. Geological Survey   Circular 1249

## **Geothermal Energy—Clean Power From the Earth’s   Heat**

### *By* Wendell A. Duffield and John H. Sass

| **Introduction**

Societies in the 21st century require enormous amounts        of energy to drive the machines of commerce and to sustain the        lifestyles that many people have come to expect. Today, most        of this energy is derived from oil, natural gas, and coal, supplemented        by nuclear power.        Local exceptions exist, but oil is by far the most common        source of energy worldwide. Oil resources, however, are nonrenewable        and concentrated in only a few places around the globe,        creating uncertainty in long-term supply for many nations.        At the time of the Middle East oil embargo of the 1970s,        about a third of the United States oil supply was imported,        mostly from that region. An interruption in the flow of this        import disrupted nearly every citizen’s daily life, as well as        the Nation’s economy. In response, the Federal Government        launched substantial programs to accelerate development of        means to increasingly harness “alternative energies”—primarily        biomass, geothermal, solar, and wind. The new emphasis        on simultaneously pursuing development of several sources of        energy recognized the timeless wisdom found in the proverb of        “not putting all eggs in one basket.” This book helps explain         the        role that geothermal resources can play in helping promote such        diversity and in satisfying our Nation’s vast energy needs as we        enter a new millennium.        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        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 the decay of naturally occurring radioactive isotopes—principally        of uranium, thorium, and potassium—that occur in        small amounts in all rocks. This heat then rises to and through        the Earth’s surface, where it escapes into the atmosphere. The        amount of heat that flows annually from the Earth into the atmosphere        is enormous—equivalent to ten times the annual energy        consumption of the United States and more than that needed to        power all nations of the world, if it could be fully harnessed.        Even if only 1 percent of the thermal energy contained        within the uppermost 10 kilometers of our planet could be        tapped, this amount would be 500 times that contained in all oil        and gas resources of the world. How might we benefit from this        vast amount of thermal energy beneath our feet? Where, by what        means, and how much of the Earth’s natural heat can be usefully        harnessed? These are especially important questions to contemplate,        because global population is expected to soon exceed        seven billion and many scientists believe that the world’s fossilfuel        resources may be substantially depleted within this century.        Faced with such prospects, both the public and private sectors are        working toward more fully utilizing the Earth’s abundant thermal        energy and other alternative energy resources.        A skeptic might question the wisdom of devoting much        national effort to geothermal energy development, especially        because many experts think that geothermal heat can contribute        at most about 10 percent to the Nation’s energy supply        using current technologies. However, ongoing advances in        exploration and heat-extraction technologies are improving        our ability to use the resource and may substantially increase        the geothermal contribution to the Nation’s energy supply.        In an attempt to help national planners and average citizens        alike understand the nature and energy potential of geothermal        resources, this book (1) describes the distribution and nature of        geothermal energy, (2) reviews the common types of geothermal        systems that provide useful energy with current technology, (3)        considers potential geothermal resources that might someday be        tapped with developing technologies, and (4) summarizes the        role of earth-science information in assessing and harnessing        geothermal resources wherever they occur worldwide.        The predecessor to this book (Tapping the Earth’s Natural        Heat, U.S. Geological Survey Circular 1125, published in 1994)        summarized the situation in the early 1990s. In an effort to        support national energy planners, this new circular incorporates        more recent advances in geothermal science and technology. |
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Department of the Interior
U.S. Geological Survey
Michael Diggles