word geothermal comes from the Greek words geo (earth) and therme
(heat), and means the heat of the earth. Earth's interior heat originated from
its fiery consolidation from dust and gas over 4 billion years ago and is
continually regenerated from the decay of radioactive elements that occur in all
HEAT AND VOLCANIC REGIONS
is almost 6,500 kilometers (4,000 miles) from the surface to the center of the
Earth, and the deeper you go, the hotter it gets. The outer layer, the crust, is
three to 35 miles thick and insulates us from the hot interior.
the surface down through the crust the normal temperature gradient (the increase
of temperature with the increase of depth) in the Earth’s crust is 17 - 30°C
per kilometer of depth (50-87°F per mile). Below the crust is the mantle, made
of highly viscous, partially molten rock with temperatures between 650 and 1,250°C
(1,200-2,280°F). At Earth's core, which consists of a liquid outer core and a
solid inner core, temperatures may reach 4,000-7,000°C (7,200 to 12,600°F).
heat always moves from hotter regions to colder regions, the Earth’s heat
flows from its interior toward the surface. This outward flow of heat from
Earth’s interior drives convective motion in the mantle rock which in turn
drives plate tectonics -- the "drift" of Earth's crustal plates that
occurs at 1 to 5 cm per year (about the rate our fingernails grow). Where plates
move apart, magma rises up into the rift, forming new crust. Where plates
collide, one plate is generally forced (subducted) beneath the other. As a
subducted plate slides slowly downward into regions of ever-increasing heat, it
can reach conditions of pressure, temperature and water content that cause
melting, forming magma. Plumes of magma ascend by buoyancy and force themselves
up into (intrude) the crust, bringing up vast quantities of heat.
magma reaches the surface it can build volcanoes. But most magma stays well
below ground, creating huge subterranean regions of hot rock sometimes
underlying areas as large as an entire mountain range. Cooling can take from
5,000 to more than 1 million years. These shallow regions of relatively elevated
crustal heat have high temperature gradients.
the best known of these volcanic regions are in the countries that border the
Pacific Ocean -- the geologically active area known as the Ring of Fire -- where
the oceanic plates are being subducted under the continental plates. Other
volcanic chains form along mid-ocean or continental rift zones (where plates
move apart) -- in places such as Iceland and Kenya, or over hot spots (magma
plumes continuously ascending from deep in the mantle) such as the Hawaiian
Islands and Yellowstone.
OF GEOTHERMAL RESERVOIRS
some regions with high temperature gradients, there are deep subterranean faults
and cracks that allow rainwater and snowmelt to seep underground -- sometimes
for miles. There the water is heated by the hot rock and circulates back up to
the surface, to appear as hot springs, mud pots, geysers, or fumaroles.
the ascending hot water meets an impermeable rock layer, however, the water is
trapped underground where it fills the pores and cracks comprising 2 to 5% of
the volume of the surrounding rock, forming a geothermal reservoir. Much hotter
than surface hot springs, geothermal reservoirs can reach temperatures of more
than 350°C (700°F), and are powerful sources of energy.
geothermal reservoirs are close enough to the surface, we can reach them by
drilling wells, sometimes over two miles deep. Scientists and engineers use
geological, electrical, magnetic, geochemical and seismic surveys to help locate
the reservoirs. Then, after an exploration well confirms a reservoir discovery,
production wells are drilled. Hot water and steam shoot up the wells naturally
(or are pumped to the surface) where -- at temperatures between around 120-370°C
(250-700°F) -- they are used to generate electricity in geothermal power
plants. Shallower reservoirs of lower temperature -- 21-149°C (70-300°F)
-- are used directly in health spas, greenhouses, fish farms, and industry and
in space heating systems for homes, schools and offices.
ELECTRICITY: GEOTHERMAL POWER PLANTS
geothermal power plants, we use the natural hot water and steam from the earth
to turn turbine generators to produce electricity. Unlike fossil fuel power
plants, no fuel is burned. Geothermal power plants give off water vapor, but
have no smoky emissions. (See Environmental Aspects, below.)
Steam Plants. Most
geothermal power plants operating today are "flashed steam" power
plants. Hot water from production wells is passed through one or two separators
where, released from the pressure of the deep reservoir, part of it flashes
(explosively boils) to steam. The force of the steam is used to spin the turbine
generator. To conserve the water and maintain reservoir pressure, the geothermal
water and condensed steam are directed down an injection well back into
the periphery of the reservoir, to be reheated and recycled.
Steam Plants. A few
geothermal reservoirs produce mostly steam and very little water. Here, the
steam shoots directly through a rock-catcher and into the turbine. The first
geothermal power plant was a dry steam plant, built at Larderello in Tuscany,
Italy in 1904. The power plants at the Larderello dry steam field were destroyed
during World War II, but have since been rebuilt and expanded. That field is
still producing electricity today. The Geysers dry steam reservoir in northern
California has been producing electricity since 1960. It is the largest known
dry steam field in the world and, after 40 years, still produces enough
electricity to supply a city the size of San Francisco.
Power Plants. In a
binary power plant, the geothermal water is passed through one side of a heat
exchanger, where it's heat is transferred to a second (binary) liquid,
called a working fluid, in an adjacent separate pipe loop. The working fluid
boils to vapor which, like steam, powers the turbine generator. It is then
condensed back to a liquid and used over and over again. The geothermal water
passes only through the heat exchanger and is immediately recycled back into the
binary power plants are generally more expensive to build than steam-driven
plants, they have several advantages: 1) The working fluid (usually isobutane or
isopentane) boils and flashes to a vapor at a lower temperature than does water,
so we can generate electricity from reservoirs with lower temperatures. This
increases the number of geothermal reservoirs in the world with
electricity-generating potential. 2) The binary system uses the reservoir water
more efficiently. Since the hot water travels through an entirely closed system
it results in less heat loss and almost no water loss. 3) Binary power plants
have virtually no emissions.
Power Plants. In some
power plants, flash and binary processes are combined. An example of such a
hybrid system is in Hawaii, where a hybrid plant provides about 25% of the
electricity used on the Big island.
2850 megawatts of geothermal generation capacity is available from power plants
in the western United States. Geothermal energy generates about 2% of the
electricity in Utah, 6% of the electricity in California and almost 10% of the
electricity in northern Nevada. The electrical energy generated in the
U.S. from geothermal resources is more than twice that from solar and wind
USES OF GEOTHERMAL WATER
reservoirs of lower temperature -- 21-149°C (70-300°F) -- are used
directly in health spas, greenhouses, fish farms, and industry and in space
heating systems for homes, schools and offices
is only during the last century that we have used geothermal energy to produce
electricity. But using geothermal water to make our lives more comfortable is
not new: people have used it since the dawn of mankind. Wherever geothermal
water is available, people find creative ways to use its heat.
Spring Bathing and Spas (Balneology)
centuries, peoples of China, Iceland, Japan, New Zealand, North America and
other areas have used hot springs for cooking and bathing. The Romans used
geothermal water to treat eye and skin disease and, at Pompeii, to heat
buildings. Medieval wars were even fought over lands with hot springs. Today, as
long ago, people still bathe in geothermal waters.
Europe, natural hot springs have been very popular health attractions. The
first known "health spa" was established in 1326 in Belgium. (One
resort was named "Espa" which means "fountain." The English
word "spa" came from this name.) All over Eurasia today, health spas
are still very popular. Russia, for example, has 3,500 spas.
is considered the world’s leader in balneology. The Japanese tradition of
social bathing dates back to ancient Buddhist rituals. Beppu, Japan, has 4,000
hot springs and bathing facilities that attract 12 million tourists a year.
Other countries with major spas and hot springs include New Zealand, Mexico and
the United States.
resources are used worldwide to boost agricultural production. Water from
geothermal reservoirs is used to warm greenhouses to help grow flowers,
vegetables and other crops. For hundreds of years, Tuscany in Central Italy has
produced vegetables in the winter from fields heated by natural steam. In
Hungary, thermal waters provide 80% of the energy demand of vegetable farmers,
making Hungary the world’s geothermal greenhouse leader. Dozens of geothermal
greenhouses can also be found in Iceland and in the western United States.
aquaculture, the "farming" of water-dwelling creatures, uses natural
warm water to speed the growth of fish, shellfish, reptiles and amphibians. This
kind of direct use is increasing in popularity. In China, for example,
geothermal aquaculture is growing so fast that fish farms cover almost 2 million
square meters (500 acres). In Japan, aqua farms grow eels and alligators. In the
U.S. aquafarmers in Idaho, Utah, Oregon and California grow catfish, trout,
alligators, and tilapia -- as well as tropical fish for pet shops. And
Icelanders hope to raise as many as two and a half million abalone a year.
heat from geothermal water is used worldwide for industrial purposes. Some of
these uses include drying fish, fruits, vegetables and timber products, washing
wool, dying cloth, manufacturing paper and pasteurizing milk. Geothermally
heated water can be piped under sidewalks and roads to keep them from icing over
in freezing weather. Thermal waters are also used to help extract gold and
silver from ore and even for refrigeration and ice-making.
oldest and most common use of geothermal water, apart from hot spring bathing,
is to heat individual buildings, and sometimes entire commercial and residential
geothermal district heating system supplies heat by pumping geothermal water --
usually 60° C (140°F) or hotter -- from one or more wells drilled into a
geothermal reservoir. The geothermal water is passed through a heat exchanger
which transfers the heat to water in separate pipes that is pumped to the
buildings. After passing through the heat exchanger, the geothermal water is
injected back into the reservoir where it can reheat and be used again.
the Paris basin in France, historic records show that geothermal water from
shallow wells was used to heat buildings over six centuries ago. An increasing
number of residential districts there are being heated with geothermal water as
drilling of new wells progresses.
first district heating system in the United States dates back to 1893, and still
serves part of Boise, Idaho. In the western United States there are over two
hundred and seventy communities that are close enough to geothermal reservoirs
for potential implementation of geothermal district heating. Eighteen such
systems are already in use in the U.S. -- the most extensive in Boise, Idaho and
San Bernardino, California.
it is a clean, economical method of heating buildings, geothermal district
heating is becoming more popular in many places. Besides France and the U.S.,
modern district heating systems now warm homes in Iceland, Turkey, Poland and
Hungary. The world's largest geothermal district heating system is in Reykjavik,
Iceland, where almost all the buildings use geothermal heat. The air around
Reykjavik was once very polluted by emissions from reliance on fossil fuels.
Since it started using geothermal energy, Reykjavik has become one of the
cleanest cities in the world.
geothermal technology that helps keep indoor temperatures comfortable by using
Earth's heat is the geo-exchange system, or geothermal heat pump. Geothermal
heat pumps do not use geothermal reservoirs, so they can be used almost
everywhere in the world -- in areas with normal as well as high temperature
gradients. By pumping fluid through loops of pipe buried underground next to a
building, these systems take advantage of the relatively constant temperature 7
- 13°C (45 - 55°F) of the Earth right beneath our feet to transfer heat into
buildings in winter and out of them in summer.
heat pumps reduce electricity use 30-60% compared with traditional heating and
cooling systems, because the electricity which powers them is used only to move
heat, not to produce it. There are about 300,000 heat pump installations in the
U.S.; Switzerland and several other countries are implementing heat pump
programs. The U.S. Environmental Protection Agency rates geothermal heat pumps
among the most efficient of heating and cooling technologies.
heat is continuously radiated from within, and each year rainfall and snowmelt
supply new water to geothermal reservoirs. Production from individual geothermal
fields can be sustained for decades and perhaps centuries. . The U.S. Department
of Energy classifies geothermal energy as renewable.
we use renewable geothermal energy for direct use or for producing electricity,
we conserve exhaustible and more polluting resources like fossil fuels and
uranium (nuclear energy). Installed geothermal electricity generation capacity
around the world is equivalent to the output of about 10 nuclear plants.
direct uses of geothermal water avoids the combustion of fossil fuels equivalent
to burning of 830 million gallons of oil or 4.4 million tons of coal per year.
Worldwide electrical production from geothermal reservoirs avoids the combustion
of 5.4 billion gallons of oil or 28.3 million tons of coal.
OF THE ENVIRONMENT
all sources of energy, developers and consumers must work to protect the
environment. The challenges differ with the type of energy resource, and the
differences give geothermal energy certain advantages. Geothermal direct use
facilities have minimal or no negative impacts on the environment. Geothermal
power plants are relatively easy on the environment. They are successfully
operated in the middle of crops, in sensitive desert environments and in
forested recreation areas.
of the Air and Atmosphere.
Hydrogen sulphide gas (H2S) sometimes occurs in geothermal
reservoirs. H2S has a distinctive rotten egg smell that can be
detected by the most sensitive sensors (our noses) at very low concentrations (a
few parts per billion). It is subject to regulatory controls for worker safety
because it can be toxic at high concentrations. Equipment for scrubbing H2S
from geothermal steam removes 99% of this gas.
dioxide (a major climate change gas) occurs naturally in geothermal steam but
geothermal plants release amounts less than 4% of that released by fossil fuel
plants. And there are no emissions at all when closed-cycle (binary) technology
Geothermal water contains higher concentrations of dissolved minerals than water
from cold groundwater aquifers. In geothermal wells, pipe or casing (usually
several layers ) is cemented into the ground to prevent the mixing of geothermal
water with other groundwater.
highly-mineralized geothermal water needs to be stored at the surface, such as
during well testing, it is kept in lined, impermeable sumps. After use,
the geothermal water is either evaporated or injected back to its deep
reservoir, again through sealed piping.
Protection. No power
plant or drill rig is as lovely as a natural landscape, so smaller is better. A
geothermal plant sits right on top of its fuel source: no additional land is
needed such as for mining coal or for transporting oil or gas. When geothermal
power plants and drill rigs are located in scenic areas, mitigation measures are
implemented to reduce intrusion on the visual landscape. Some geothermal power
plants use special air cooling technology which eliminates even the plumes of
water vapor from cooling towers and reduces a plant profile to as little as 24
feet in height.
observing federal and state regulations, geothermal developers avoid
interference with geysers and hot springs in areas set aside for their scenic
beauty. Development in National Parks such as Yellowstone is specifically
the 1970's the geothermal industry, with the assistance of government research
funding, has overcome many technical drilling and power plant problems.
Improvements in treatment of geothermal water have overcome early problems of
corrosion and scaling of pipes. Methods have been developed to remove silica
from high-silica reservoirs. In some plants silica is being put to use making
concrete, and H2S is converted to sulphur and sold. At power plants n
the Imperial Valley of California, a facility is being constructed to extract
zinc from the geothermal water for commercial sale.
a result of government-assisted research and industry experience, the cost of
generating geothermal power has decreased by 25% over the past two decades.
Research is currently underway to further improve exploration, drilling,
reservoir, power plant and environmental technologies. Enhancing the
recoverability of Earth’s heat is an important area of ongoing research.
energy is accessible if there is sufficient heat, permeability, and water in a
system, and if the system is not too deep. The available heat cannot be
increased, but the permeability and water content can be enhanced. Private and
government research projects in the United States, Japan and in Europe are
improving the accessibility of geothermal energy by developing new technology to
increase the permeability of the rocks and to supplement the water in hot,
water-deficient rocks. Engineers estimate that by the year 2020, man-made
geothermal reservoirs could be supplying 5 to 10% of the world’s electricity.
Reservoir Water. One
unique example of enhancing reservoir water is at The Geysers steam field in
California, where treated wastewater from nearby communities is being piped to
the steamfield and injected into the reservoir to be heated. This increases the
amount of steam available to produce electricity. With this enhancement,
reservoir life is increased while providing nearby cities with an
environmentally safe method of wastewater disposal.
Permeability can be created in hot rocks by hydraulic fracturing -- injecting
large volumes of water into a well at a pressure high enough to break the rocks.
The artificial fracture system is mapped by seismic methods as it forms, and a
second well is drilled to intersect the fracture system. Cold water can then be
pumped down one well and hot water taken from the second well for use in a
geothermal plant. This "hot dry rock" technology is being tested in
Japan, Germany, France, England and the U.S.
FUTURE FOR GEOTHERMAL ENERGY
outlook for geothermal energy use depends on at least three factors: the demand
for energy in general; the inventory of available geothermal resources;
and the competitive position of geothermal among other energy sources.
Demand for energy will
continue to grow. Economies are expanding, populations are increasing (over 2
billion people still do not have electricity), and energy-intensive technologies
are spreading. All mean greater demand for energy. At the same time, there is
growing global recognition of the environmental impacts of energy production and
use from fossil fuel and nuclear resources. Public polls repeatedly show that
most people prefer a policy of support for renewable energy.
Inventory of accessible
geothermal energy is sizable. Using current technology geothermal energy from
already-identified reservoirs can contribute as much as 10% of the United States
energy supply. And with more exploration, the inventory can become larger. The
entire world resource base of geothermal energy has been calculated in
government surveys to be larger than the resource bases of coal, oil, gas and
uranium combined. The geothermal resource base becomes more available as methods
and technologies for accessing it are improved through research and experience.
depends primarily on cost:
Shorter and Longer Term. Production of fossil fuels (oil, natural
gas and coal) are a relative bargain in the short term. Like many renewable
resources, geothermal resources need relatively high initial investments to
access the heat, hot water and steam. But the geothermal "fuel" cost
is predictable and stable. Fossil fuel supplies will increase in cost as
reserves are exhausted. Fossil fuel supplies can be interrupted
political disputes abroad. Renewable geothermal energy is a better long term
Direct and Indirect. The monetary price we pay to our natural gas
and electricity suppliers, and at the gas pump, is our direct cost for the
energy we use. But the use of energy also has indirect or external costs
that are imposed on society. Examples are the huge costs of global climate
change; the health effects from ground level pollution of the air; future
effects of pollution of water and land; military expenditures to protect
petroleum sources and supply routes; and costs of safely storing radioactive
waste for generations. Geothermal energy can already compete with the direct
costs of conventional fuels in some locations and is a clean, indigenous,
renewable resource without hidden external costs. Public polls reveal that
customers are willing to pay a little more for energy from renewable resources
such as geothermal energy
Domestic and Importing. Investment in the use of domestic, indigenous, renewable
energy resources like geothermal energy provides jobs, expands the regional and
national economies, and avoids the export of money to import fuels.
demand is increasing rapidly worldwide. Some energy and environmental experts
predict that the growth of electricity production and direct uses of geothermal
energy will be revitalized by international commitments to reduce carbon dioxide
emissions to avert global climate change and by the opening of markets to
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