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 240°C 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.
|
 |
| The Geysers near the city of Santa Rosa in northern California is the world's largest electricity-generating geothermal development. Most of the wells are about 3,000 meters deep and produce nearly pure steam. Pipes carry steam to turbine generators and associated condensers. Vapor plumes from condensers are visible here. Generators range from about 10 to 100 megawatt ratings; many are about 50 megawatts. Several steam wells feed into a single generator.After geothermal development, the land is available for other purposes, such as grazing. (Photograph by Julie Donnelly-Nolan, U.S. Geological Survey.) |
| Geothermal gradients and equivalent heat flow that illustrate differences in the amount of heat escaping from the Earth for broad regions and smaller areas of the United States (Sierra Nevada, Basin and Range physiographic province, Battle Mountain area of Nevada and nearby states, and east of the Rocky Mountains). The heat flow shown for the area east of the Rocky Mountains is equivalent to the average heat flow for continental crust worldwide. Illustration by USGS. |
| Geothermal gradients and equivalent heat flow that illustrate differences in the amount of heat escaping from the Earth for broad regions and smaller areas of the United States (Sierra Nevada, Basin and Range physiographic province, Battle Mountain area of Nevada and nearby states, and east of the Rocky Mountains). The heat flow shown for the area east of the Rocky Mountains is equivalent to the average heat flow for continental crust worldwide. Illustration by USGS. |
| Geothermal Energy in the News |
| 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. |
|
