How Geothermal Energy
Works
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Contents
1. The Geothermal Resource
2. How Geothermal Energy Is Captured
3. The Future of Geothermal Energy
Introduction
Heat from the earth can
be used as an energy source in many ways, from large and complex power stations
to small and relatively simple pumping systems. This heat energy, known as
geothermal energy, can be found almost anywhere-as far away as remote deep
wells in Indonesia and as close as the dirt in our backyards. Many regions of
the world are already tapping geothermal energy as an affordable and
sustainable solution to reducing dependence on fossil fuels, and the global
warming and public health risks that result from their use. For example, more
than 8,900 megawatts (MW) of large, utility-scale geothermal capacity in 24
countries now produce enough electricity to meet the annual needs of nearly 12
million typical U.S. households (GEA 2008a). Geothermal plants produce 25
percent or more of electricity in the Philippines, Iceland, and El Salvador.
The United States has more geothermal capacity than any other country, with
more than 3,000 megawatts in eight states. Eighty percent of this capacity is
in California, where more than 40 geothermal plants provide nearly 5 percent of
the state's electricity.In thousands of homes and buildings
across the United States, geothermal heat pumps also use the steady
temperatures just underground to heat and cool buildings, cleanly and
inexpensively.
The Geothermal Resource
Below the Earth's crust, there is a layer of hot and molten rock called magma.
Heat is continually produced there, mostly from the decay of naturally
radioactive materials such as uranium and potassium. The amount of heat within
10,000 meters (about 33,000 feet) of Earth's surface contains 50,000 times more
energy than all the oil and natural gas resources in the world.
U.S.
geothermal potential. Source: NREL
The areas with the highest underground temperatures are in regions with active
or geologically young volcanoes. These "hot spots" occur at plate
boundaries or at places where the crust is thin enough to let the heat through.
The Pacific Rim, often called the Ring of Fire for its many volcanoes, has many
hot spots, including some in Alaska, California, and Oregon. Nevada has
hundreds of hot spots, covering much of the northern part of the state.
These regions are also seismically active. Earthquakes and magma movement break
up the rock covering, allowing water to circulate. As the water rises to the
surface, natural hot springs and geysers occur, such as Old Faithful at
Yellowstone National Park. The water in these systems can be more than 200°C
(430°F).
Seismically active hotspots are not the only places where geothermal energy can
be found. There is a steady supply of milder heat-useful for direct
heating purposes-at depths of anywhere from 10 to a few hundred feet below the
surface virtually in any location on Earth. Even the ground below your
own backyard or local school has enough heat to control the climate in your
home or other buildings in the community. In addition, there is a vast
amount of heat energy available from dry rock formations very deep below the
surface (4-10 km). Using a set of emerging technologies known as Enhanced
Geothermal Systems (EGS), we may be able to capture this heat for electricity
production on a much larger scale than conventional technologies allow.
If these resources can be tapped, they offer enormous potential for electricity
production capacity. In its first comprehensive assessment in more than 30
years, the U.S. Geological Survey (USGS) estimated that conventional geothermal
sources on private and accessible public lands across 13 western states have
the potential capacity to produce 8,000-73,000 MW, with a mean estimate of
33,000 MW. State and federal policies are likely to
spur developers to tap some of this potential in the next few years. The
Geothermal Energy Association estimates that 132 projects now under development
around the country could provide up to 6,400 megawatts of new capacity. As EGS technologies improve and become
competitive, even more of the largely untapped geothermal resource could be
developed. The USGS study found that hot dry rock resources could provide
another 345,100-727,900 MW of capacity, with a mean estimate of 517,800 MW.
That means that this resource could one day supply nearly all of today's U.S.
electricity needs.
Not only do geothermal resources in the United States offer great potential,
they can also provide continuous baseload electricity. According to the U.S.
National Renewable Energy Laboratory, the capacity factors of geothermal
plants-a measure of the ratio of the actual electricity generated over time
compared to what would be produced if the plant was running nonstop for that
period-are comparable with those of coal and nuclear power. With the combination of both the size of
the resource base and its consistency, geothermal can play an indispensable
role in a cleaner, more sustainable power system.
How Geothermal Energy Is
Captured
Geothermal springs for power plants. The most common current way
of capturing the energy from geothermal sources is to tap into naturally
occurring "hydrothermal convection" systems where cooler water seeps
into Earth's crust, is heated up, and then rises to the surface. When heated
water is forced to the surface, it is a relatively simple matter to capture
that steam and use it to drive electric generators. Geothermal power plants
drill their own holes into the rock to more effectively capture the steam.
There are three designs for geothermal power plants, all of which pull hot
water and steam from the ground, use it, and then return it as warm water to
prolong the life of the heat source. In the simplest design, the steam goes
directly through the turbine, then into a condenser where the steam is
condensed into water. In a second approach, very hot water is depressurized or
"flashed" into steam which can then be used to drive the turbine.
In the third approach, called a binary system, the hot water is passed through
a heat exchanger, where it heats a second liquid-such as isobutane-in a closed
loop. The isobutane boils at a lower temperature than water, so it is more
easily converted into steam to run the turbine. The three systems are shown in
the diagrams below.
Dry Steam Flash Steam Binary Cycle
Click any of the images to see a larger version. Source: NREL
The choice of which
design to use is determined by the resource. If the water comes out of the well
as steam, it can be used directly, as in the first design. If it is hot water
of a high enough temperature, a flash system can be used, otherwise it must go
through a heat exchanger. Since there are more hot water resources than pure
steam or high-temperature water sources, there is more growth potential in the
heat exchanger design.
The largest geothermal system now in operation is a steam-driven plant in an
area called the Geysers, north of San Francisco, California. Despite the name,
there are actually no geysers there, and the heat that is used for energy is
all steam, not hot water. Although the area was known for its hot springs as
far back as the mid-1800s, the first well for power production was drilled in
1924. Deeper wells were drilled in the 1950s, but real development didn't occur
until the 1970s and 1980s. By 1990, 26 power plants had been built, for a
capacity of more than 2,000MW.
The Geysers (Source: NREL)
Because of the rapid
development of the area in the 1980s, and the technology used, the steam
resource has been declining since 1988. Today, owned primarily by California-
utility Calpine and with a net operating capacity of 725 MW, the Geysers
facilities still meets nearly 60 percent of the average electrical demand for
California's North Coast region (from the Golden Gate Bridge north to the
Oregon border). The plants at the Geysers use an evaporative
water-cooling process to create a vacuum that pulls the steam through the
turbine, producing power more efficiently. But this process loses 60 to 80
percent of the steam to the air, without re-injecting it underground. While the
steam pressure may be declining, the rocks underground are still hot. To remedy
the situation, various stakeholders partnered to create the Santa Rosa Geysers
Recharge Project, which involves transporting 11 million gallons per day of
treated wastewater from neighboring communities through a 40-mile pipeline and
injecting it into the ground to provide more steam. The project came online in
2003, and in 2008 provided enough additional electricity for approximately
100,000 homes. The city of Santa Rosa plans to further expand this
program by increasing the amount of wastewater sent to the Geysers to nearly 20
million gallons per day.
One concern with open systems like the Geysers is that they emit some air
pollutants. Hydrogen sulfide-a toxic gas with a highly recognizable
"rotten egg" odor-along with trace amounts of arsenic and minerals,
is released in the steam. In addition, at a power plant at the Salton Sea
reservoir in Southern California, a significant amount of salt builds up in the
pipes and must be removed. While the plant initially started to put the salts
into a landfill, they now re-inject the salt back into a different well. With closed-loop
systems, such as the binary system, there are no emissions; everything brought
to the surface is returned underground.
Direct use of geothermal heat. Geothermal springs can also be
used directly for heating purposes. Hot spring water is used to heat
greenhouses, to dry out fish and de-ice roads, for improving oil recovery, and
to heat fish farms and spas. In Klamath Falls, Oregon, and Boise, Idaho,
geothermal water has been used to heat homes and buildings for more than a
century. On the east coast, the town of Warm Springs, Virginia obtains heat
directly from spring water as well, using springs to heat one of the local
resorts.
In Iceland, virtually every building in the country is heated with hot spring
water. In fact, Iceland gets more than 50 percent of its energy from geothermal
sources. In Reykjavik, for example (population
115,000), hot water is piped in from 25 kilometers away, and residents use it
for heating and for hot tap water.
Ground-source heat pumps. A much more conventional way to tap geothermal energy is by using geothermal heat pumps to provide heat and cooling to buildings. Also called ground-source heat pumps, they take advantage of the constant year-round temperature of about 50°F that is just a few feet below the ground's surface. Either air or antifreeze liquid is pumped through pipes that are buried underground, and re-circulated into the building. In the summer, the liquid moves heat from the building into the ground. In the winter, it does the opposite, providing pre-warmed air and water to the heating system of the building.
Ground-source heat pump (Source: NREL)
In the simplest use of
ground-source heating and cooling, a tube runs from the outside air, under the
ground, and into a house's ventilation system. More complicated, but more effective
systems use compressors and pumps-as in electric air conditioning systems-to
maximize the heat transfer.
In regions with temperature extremes, such as the northern United States in the
winter and the southern United States in the summer, ground-source heat pumps
are the most energy-efficient and environmentally clean heating and cooling
system available. Far more efficient than electric heating and cooling,
these systems can move as much as 3 to 5 times the energy they use in the
process. The U.S. Department of Energy found that heat pumps can save a typical
home hundreds of dollars in energy costs each year, with the system typically
paying for itself in 8 to 12 years. Tax credits and other incentives can reduce
the payback period to 5 years or less.
More than 600,000
ground-source heat pumps supply climate control in U.S. homes and other
buildings, with new installations occurring at a rate of about 60,000 per year. While this is significant, it is
still only a small fraction of the U.S. heating and cooling market, and several
barriers to greater penetration into the market remain. For example, despite
their long-term savings, geothermal heat pumps have higher up-front costs. In addition,
installing them in existing homes and businesses can be difficult, since it
involves digging up areas around a building's structure. Finally, many heating
and cooling installers are just not familiar with the technology.
However, ground-source heat pumps are catching on in some areas. In rural areas
without access to natural gas pipelines, homes must use propane or electricity
for heating and cooling. Heat pumps are much less expensive to operate, and
since buildings are widely spread out, installing underground loops is not an
issue. Underground loops can be easily installed during construction of new
buildings as well, resulting in savings for the life of the building.
Furthermore, recent policy developments are offering strong incentives for homeowners
to install these systems. The 2008 economic stimulus bill, Emergency
Economic Stabilization Act of 2008, includes an eight year extension (through
2016) of the 30 percent investment tax credit, with no upper limit, to all home
installations of EnergyStar certified geothermal heat pumps.
The Future of Geothermal Energy
Geothermal energy has the potential to play a significant role in moving the United States (and other regions of the world) toward a cleaner, more sustainable energy system. It is one of the few renewable energy technologies that-like fossil fuels-can supply continuous, baseload power. The costs for electricity from geothermal facilities are also declining. Some geothermal facilities have realized at least 50 percent reductions in the price of electricity since 1980. A considerable portion of potential geothermal resources will be able produce electricity for as little as 8 cents per kilowatt-hour (including a production tax credit), a cost level competitive with new conventional fossil fuel-fired power plants. There is also a bright future for the direct use of geothermal resources as a heating source for homes and businesses in any location. However, in order to tap into the full potential of geothermal energy, two emerging technologies require further development: Enhanced Geothermal Systems (EGS) and co-production of geothermal electricity in oil and gas wells.
Enhanced Geothermal
Systems. Geothermal heat occurs
everywhere under the surface of the earth, but the conditions that make water
circulate to the surface are found only in less than 10 percent of Earth's land
area. An approach to capturing the heat in dry areas is known as enhanced
geothermal systems (EGS) or "hot dry rock". The hot rock reservoirs,
typically at greater depths below the earth's surface than conventional
sources, are first broken up by pumping high-pressure water through them. The
plants then pump more water through the broken hot rocks, where it heats up,
returns to the surface as steam, and powers turbines to generate electricity.
Finally, the water is returned to the reservoir through injection wells to
complete the circulation loop. Plants that use a closed-loop binary cycle
release no fluids or heat-trapping emissions other than water vapor, which may
be used for cooling.
The Department of Energy, several universities, the geothermal industry, and
venture capital firms (including Google) are collaborating on research and
demonstration projects to harness the potential of hot dry rock. Australia,
France, Germany, and Japan also have R&D programs to make EGS commercially
viable. The DOE hopes to have EGS ready for commercial development by 2015 and
is currently funding several demonstration projects.
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Source: EERE
One cause for careful consideration with EGS is the possibility of induced
seismic activity that might occur from hot dry rock drilling and
development. This risk is similar to that associated with hydraulic
fracturing, an increasingly used method of oil and gas drilling, and with
carbon dioxide capture and storage in deep saline aquifers. Though a
potentially serious concern, the risk of an induced EGS-related seismic event
that can be felt by the surrounding population or that might cause significant
damage currently appears very low when projects are located an appropriate
distance away from major fault lines and properly monitored. Appropriate site
selection, assessment and monitoring of rock fracturing and seismic activity
during and after construction, and open and transparent communication with
local communities are also critical.
Co-production of Geothermal Electricity in Oil and Gas Wells. Oil and gas fields already under production represent another large potential source of geothermal energy. In many existing oil and gas reservoirs, a significant amount of high-temperature water or suitable high-pressure conditions are present, which could allow for the production of electricity and oil or gas at the same time. In some cases, exploiting these resources could even enhance the extraction of the oil and gas itself. An MIT study estimated that the United States has the potential to develop 44,000 MWs of geothermal capacity by 2050 by co-producing electricity, oil, and natural gas at oil and gas fields-primarily in the Southeast and southern Plains states. The study projects that such advanced geothermal systems could supply 10 percent of U.S. baseload electricity by that year, given R&D and deployment over the next 10 years.
These exciting new developments in geothermal will be supported by unprecedented levels of federal R&D funding. Under, the American Recovery and Investment Act of 2009, $400 million of new funding was allocated to the DOE's Geothermal Technologies Program. Of this $90 million is expected to go towards a series of up to 10 demonstration projects to prove the feasibility of EGS technology. Another $50 million will fund up to 20 demonstration projects for other new technologies, including co-production with oil and gas and low temperature geothermal. The remaining funds will go exploration technologies, expanding the deployment of geothermal heat pumps, and
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