| |
INTRODUCTION
The 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 rocks.
EARTH’S
HEAT AND VOLCANIC REGIONS
It 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.
From 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).
Since 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.
Where 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.
Perhaps 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.
FORMATION
OF GEOTHERMAL RESERVOIRS
In 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.
If 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.
See
also:
Geothermal Slide Show - Slides
1-13
ACCESSING GEOTHERMAL ENERGY
If 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.
See
also:
Geothermal Slide Show - Slides
14-35
GENERATING
ELECTRICITY: GEOTHERMAL POWER PLANTS
In 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.)
See
also:
Geothermal
Slide Show - Slides
45-47 Steam Power Plants
Flashed 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.
Dry 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.
Binary 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 reservoir.
Although 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.
See
also:
Geothermal Slide Show - Slides
36-64
Hybrid 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.
Geothermal Power Production
Worldwide
As of 1999 8,217 megawatts
of electricity were being produced from some 250 geothermal
power plants running day and night in 22 countries around the world.
These plants provide reliable base-load power for well over 60 million
people, mostly in developing countries.
|
Producing Country
|
Megawatts
in 1999
|
|
United States
|
2,850
|
|
Philippines
|
1,848
|
|
Italy
|
768.5
|
|
Mexico
|
743
|
|
Indonesia
|
589.5
|
|
Japan
|
530
|
|
New Zealand
|
345
|
|
Costa Rica
|
120
|
|
Iceland
|
140
|
|
El Salvador
|
105
|
|
Nicaragua
|
70
|
|
Kenya
|
45
|
|
China
|
32
|
|
Turkey
|
21
|
|
Russia
|
11
|
|
Portugal (Azores)
|
11
|
|
Guatemala
|
5
|
|
France (Guadeloupe)
|
4
|
|
Taiwan
|
3
|
|
Thailand
|
0.3
|
|
Zambia
|
0.2
|
|
Total
|
8,217
MW
|
About 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 combined.
See
also:
Geothermal Slide Show - Slides
65-70
DIRECT
(NON-ELECTRICAL) USES OF GEOTHERMAL WATER
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
It 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.
Hot Spring Bathing and
Spas (Balneology)
For 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.
In 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.
Japan 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.
Agriculture
Geothermal 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
Geothermal 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.
Industry
The 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.
Heating/District Heating
The oldest and most common
use of geothermal water, apart from hot spring bathing, is to heat
individual buildings, and sometimes entire commercial and residential
districts.
A 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.
In 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.
The 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.
Because 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.
See
also:
Geothermal Slide Show - Slides
71-100
Geothermal Heat Pumps
Another 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.
Geothermal 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.
See also:
Geothermal Slide Show - Slides 101-106
Direct Use Developments
Worldwide
Geothermal direct use applications
provide about 10,000 thermal megawatts (MW-th) of energy in about
35 countries. (In an additional 40 countries there are hot springs
used for bathing, but facilities for commercial use have not been
developed.) In the U.S. alone, there are some 18 district heating
systems, 38 greenhouse complexes, 28 fish farms, 12 industrial plants,
and 218 spas that use geothermal waters to provide heat.
|
Thermal
Megawatts in 1998
|
|
European Union
Countries
|
1,031.4
|
|
Austria
|
21.1
|
|
Belgium
|
3.9
|
|
Denmark
|
.1
|
|
France
|
309
|
|
Germany
|
307
|
|
Greece
|
22.6
|
|
Ireland
|
.7
|
|
Italy
|
314
|
|
Portugal
|
.8
|
|
Sweden
|
47
|
|
United Kingdom
|
2
|
|
|
|
Other European
Countries
|
3,614
|
|
Bosnia and Herzegovina
|
33
|
|
Bulgaria
|
94.5
|
|
Croatia
|
11
|
|
Czech Republic
|
2
|
|
Georgia
|
245
|
|
Iceland
|
1,443
|
|
Israel
|
42
|
|
Hungary
|
750
|
|
Macedonia
|
75
|
|
Poland
|
44
|
|
Romania
|
137
|
|
Russia
|
210
|
|
Serbia
|
86
|
|
Slovakia
|
75
|
|
Slovenia
|
37
|
|
Switzerland
|
190
|
|
Turkey
|
160
|
|
Ukraine
|
12
|
|
|
|
TOTAL EUROPE
|
4,645
|
|
|
|
America
|
1,908
|
|
Canada
|
3
|
|
USA
|
1,905
|
|
|
|
Asia
|
3,075
|
|
China
|
1,914
|
|
|
|
Oceana
|
5
|
|
New Zealand
|
5
|
|
|
|
Africa
|
71
|
|
Algeria
|
1
|
|
Tunisia
|
70
|
|
|
|
TOTAL WORLD
|
9,704
|
From: Valgardur
Stefansson and Ingvar B. Fridleifsson, Geothermal Energy European
and Worldwide Perspective, 1998
Nearly every country has
some areas underlain by low- and/or moderate- temperature geothermal
waters. Expansion of direct uses of lower-temperature geothermal water
can contribute to meeting the developing world’s energy needs.
See
also:
Geothermal Slide Show
- Slides 71 ???
RENEWABILITY
AND SUSTAINABILITY
Earth’s 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.
CONSERVATION
OF RESOURCES
When 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.
Worldwide, 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.
PROTECTION
OF THE ENVIRONMENT
With 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.
Protection 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.
Carbon 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 is used.
See
also:
Geothermal Slide Show - Slides
45-47
Protection of Groundwater.
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.
When 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.
Visual 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.
By 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
prohibited.
See
also:
Geothermal
Slide Show - Slides 109-120
IMPROVING
GEOTHERMAL TECHNOLOGY
Since 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.
As 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.
Enhanced Geothermal
Systems
Geothermal 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.
Enhancing 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.
See also:
Geothermal
Slide Show - Slide 108
The Competitive Position
depends primarily on cost:
Costs: 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 investment.
Costs: 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
Costs: 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.
Energy 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 competition.
AUTHORS,
REVIEWERS, AND REFERENCES
Authors
Marilyn L.Nemzer, Executive
Director, Geothermal Education Office, Tiburon, CA
Anna K. Carter, Principal,
Geothermal Support Services, Santa Rosa, CA
Kenneth P. Nemzer, Attorney,
former Chair, Geothermal Resources Ass’n,, Tiburon, CA
Contributors
John W. Lund, Ph.D., President-Elect
Geothermal Resources Council, Director GeoHeat Center, Oregon
Institute of Technology, Klamath Falls, OR
Phillip Michael Wright,
Ph.D., President, International Geothermal Association, Energy and
Geosciences Institute, Salt Lake City, UT
Reviewers
Mark Dellinger, Lake County
Special Districts, Lakeport, CA
Joe LaFleur, Geologist
Extraordinaire, Springfield, OR
Marcelo Lippmann, Lawrence
Berkeley Labs, Berkeley, CA
Jim Lovekin, GeothermEx,
Inc., Richmond, CA
Phil Messer, Bibb &
Associates, Inc., Pasadena, CA
Marshall Reed, U.S. Department
of Energy, Washington, DC
Joel Renner, Idaho National
Engineering and Environmental Laboratory, Idaho Falls, ID
Dan Schochet, Ormat International,
Inc., Sparks, NV
Primary References
Duffield, Wendell A., John
H. Sass, and Michael L. Sorey, 1994, Tapping the Earth’s Natural
Heat: U.S. Geological Survey Circular 1125 (reprints available
from Geothermal Education Office)
Fridleifsson, Ingvar, 1998,
"Geothermal Direct Use Around the World": Geothermal
Resources Council Bulletin, V. 27, No. 8, Nov. 1998
Lund, John W., 1998, Direct
Utilization of Geothermal Resources: Technical Paper, GeoHeat
Center, Oregon Institute of Technology, Klamath Falls, Oregon
Wright, Phillip Michael,
1998, ‘"The earth gives up its heat! Geothermal energy --
a clean, sustainable resource": Renewable Energy World,
Vol. 1, No. 3, Nov. 1998
|
|
