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Home » Geothermal » Geothermal Energy as a Natural Resource

Geothermal Energy as a Natural Resource

Clean Power from the Earth's Internal Heat

Republished from United States Geological Survey Circular 1249 by Wendell A. Duffield and John H. Sass.

Where is Geothermal Energy Found?

Geothermal energy is present everywhere beneath the Earth's surface, although the highest temperature, and thus the most desirable, resources are concentrated in regions of active or geologically young volcanoes. Though the resource is thermal energy rather than a physical substance such as gold or coal, many aspects of geothermal energy are analogous to characteristics of mineral and fossil-fuel resources. Geothermal energy also has some unique, desirable attributes.

Geothermal Gradient and Heat Flow

Measurements made in drill holes, mines, and other excavations demonstrate that temperature increases downward within the Earth. The rate at which the temperature increases (temperature gradient or geothermal gradient) is proportional to the rate at which heat is escaping to the surface through the Earth's crust (heat flow).

Thus, zones of higher-than-average heat flow are the most likely places for encountering high temperatures at shallow depth, perhaps shallow enough to favor exploitation of geothermal energy. The average rate at which heat escapes through the Earth's crust accounts for a prodigious amount each year, but local heat flow can vary widely from region to region.

Plate Tectonics and Zones of High Heat Flow

Large quantities of heat that are economically extractable tend to be concentrated 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 dozen or so slabs of rigid rock (called plates) that form the Earth's lithosphere, which is composed of the Earth's crust and the uppermost, solid part of the underlying denser, hotter layer (the mantle).

According to the now widely accepted theory of plate tectonics, these large, rigid lithospheric plates move relative to one another, at average rates of several centimeters per year, above hotter, mobile mantle material (the asthenosphere).

High heat flow also is associated with the Earth's "hot spots" (also called melting anomalies or thermal plumes), whose origins are somehow related to the narrowly focused upward flow of extremely hot mantle material from very deep within the Earth. Hot spots can occur at plate boundaries (for example, beneath Iceland) or in plate interiors thou-sands of kilometers from the nearest boundary (for example, the Hawaiian hot spot in the middle of the Pacific Plate).

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 favor-able target areas for high concentrations of the Earth's heat at relatively shallow depths.

Heat Flow, Volcanoes and Earthquakes

Zones of high heat flow near plate boundaries are also where most volcanic eruptions and earthquakes occur. The magma that feeds volcanoes originates in the mantle, and considerable heat accompanies the rising magma as it intrudes into volcanoes. Much of this intruding magma remains in the crust, beneath volcanoes, and constitutes an intense, high-temperature geothermal heat source for periods of thousands to millions of years, depending on the depth, volume, and frequency of intrusion.

In addition, frequent earthquakes-produced as the tectonic plates grind against each other-fracture rocks, thus allowing water to circulate at depth and to transport heat toward the Earth's surface. Together, the rise of magma from depth and the circulation of hot water (hydrothermal convection) maintain the high heat flow that is prevalent along plate boundaries.

Target Areas for Geothermal Development

Accordingly, the plate-boundary zones and hot spot regions are prime target areas for the discovery and development of high-temperature hydrothermal-convection systems capable of producing steam that can drive turbines to gener-ate electricity. Even though such zones constitute less than 10 percent of the Earth's surface, their potential to affect the world energy mix and related political and socioeconomic consequences is substantial, mainly because these zones include many developing nations.

Geothermal Energy in the Ring of Fire

An excellent example is the boundary zone rimming the Pacific Plate -- called the "Ring of Fire" because of its abundance of active volcanoes -- that contains many high-temperature hydrothermal-convection systems. For the developing countries within this zone, the occurrence of an indigenous energy source, such as geothermal, could substantially bolster their national economies by reducing or eliminating the need to import hydrocarbon fuels for 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.

Comparison with Other Natural Resources

Geothermal resources are similar to many mineral and energy resources. A mineral deposit is generally evaluated in terms of the quality or purity (grade) of the ore and the amount of this ore (size or tonnage) that can be mined profitably. Such grade-and-size criteria also can be applied to the evaluation of geothermal energy potential.

Grade would be roughly analogous to temperature, and size would correspond to the volume of heat-containing material that can be tapped. For mineral and geothermal deposits alike, concentrations of the natural resource should be significantly higher than average (the background level) for the Earth's crust and must be at depths accessible by present-day extraction technologies before commercial development is feasible.

Developing Geothermal Energy

However, geothermal resources differ in important ways from many other natural resources. For example, the exploitation of metallic minerals generally involves digging, crushing, and processing huge amounts of rock to recover a relatively small amount of a particular element. In contrast, 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. The extraction of geothermal energy is accomplished without the large-scale movement of rock involved in mining operations, such as construction of mine shafts and tunnels, open pits, and waste heaps.

"Low Grade" Geothermal Can Heat Homes

Geothermal energy has another important advantage. It is usable over a very wide spectrum of temperature and volume, whereas the benefits of other natural resources can be reaped only if a deposit exceeds some minimum size and (or) grade for profitable exploitation or efficiency of operation. For example, at the low end of the spectrum, geothermal energy can help heat and cool a single residence.

To do so requires only the burial of piping a few meters underground, where the temperature fluctuates little with the changing seasons. Then, by circulating water or some other fluid through this piping using a geothermal heat pump, thermal energy is extracted from the ground during the coldest times of the year and deposited in the ground during the hottest times. Together, the heat pump and the Earth's thermal energy form a small, effective, and commercially viable heating and cooling system. Heat pump systems are already in use in hundreds of thousands of buildings in the United States.

"High Grade" Geothermal Can Produce Electricity

Toward the high end of the spectrum, a single large-volume, high-temperature deposit of geothermal energy can be harnessed to generate electricity sufficient to serve a city of 1 million people or more. For example, at The Geysers in northern California, fractures in rocks beneath a large area are filled with steam of about 240C at depths that can easily be reached using present-day drilling technology.

This steam is produced through wells, piped directly to conventional turbine generators, and used to generate electricity. 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. At current rates of per capita consumption in the United States, 1 megawatt is sufficient to supply a community with a population of 1,000.

The Geothermal Challenge

Between these relatively extreme examples are geothermal resources that encompass a broad spectrum of grade (temperature) and tonnage (volume). The challenge, for governmental agencies and the private sector alike, is to assess the amount and distribution of these resources, to work toward new and inventive ways to use this form of energy, and to incorporate geothermal into an appropriate energy mix for the Nation and the world.

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Residential geothermal heating and cooling system. During summer ground temperatures are lower than building temperatures and the heat pump extracts heat from the building and transfers it to the ground. In the winter the heat pump extracts heat from the ground and transfers it to the building. USGS illustrations.

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