Basics of Geothermal Energy Production
Apr 8, 2009 - Leslies Blodgett & Kara Slack
Geothermal energy is defined as heat from the Earth.
It is a clean, renewable resource that provides energy
in the U.S. and around the world in a variety of applications
and resources. Although areas with telltale signs
like hot springs are more obvious and are often the
first places geothermal resources are used, the heat
of the earth is available everywhere, and we are learning
to use it in a broader diversity of circumstances.
The heat continuously flowing from the Earth's interior,
which travels primarily by conduction, is estimated
to be equivalent to 42 million megawatts (MW) of power,
and is expected to remain so for billions of years
to come, ensuring an inexhaustible supply of energy.
A geothermal system requires heat, permeability,
and water. The heat from the Earth's core continuously
flows outward. Sometimes the heat, as magma, reaches
the surface as lava, but it usually remains below
the Earth's crust, heating nearby rock and water,
sometimes to levels as hot as 700 degrees F. When
water is heated by the earth's heat, hot water or
steam can be trapped in permeable and porous rocks
under a layer of impermeable rock and a geothermal
reservoir can form. This hot geothermal water can
manifest itself on the surface as hot springs or geysers,
but most of it stays deep underground, trapped in
cracks and porous rock. This natural collection of
hot water is called a geothermal reservoir.
To develop electricity from geothermal resources,
wells are drilled into a geothermal reservoir. The
wells bring the geothermal water to the surface, where
its heat energy is converted into electricity at a
geothermal power plant.
There are four commercial types of geothermal power
plants: flash power plants; dry steam power plants;
binary power plants; and flash/binary combined power
Flash Power Plant. Geothermally heated water
under pressure is separated in a surface vessel (called
a steam separator) into steam and hot water. The steam
is delivered to the turbine, and the turbine powers
a generator. The liquid is injected back into the
Dry Steam Power Plant. Steam is produced
directly from the geothermal reservoir to run the
turbines that power the generator, and no separation
is necessary because wells only produce steam.
Binary Power Plant. Recent advances in geothermal
technology have made possible the economic production
of electricity from geothermal resources lower than
150 degrees C (302 degrees F). Known as binary geothermal
plants, the facilities that make this possible reduce
geothermal energy's already low emission rate to zero.
Binary plants typically use an Organic Rankine Cycle
system. The geothermal water (called geothermal fluid)
heats another liquid, such as isobutane or other organic
fluids such as pentafluoropropane, which boils at
a lower temperature than water. The two liquids are
kept completely separate through the use of a heat
exchanger, which transfers the heat energy from the
geothermal water to the working fluid. The secondary
fluid expands into gaseous vapor. The force of the
expanding vapor, like steam, turns the turbines that
power the generators. All of the produced geothermal
water is injected back into the reservoir.
Flash/Binary Combined Cycle. This type of
plant, which uses a combination of flash and binary
technology, has been used effectively to take advantage
of the benefits of both technologies. In this type
of plant, the portion of the geothermal water that
flashes to steam under reduced pressure is first converted
to electricity with a backpressure steam turbine and
the low-pressure steam exiting the backpressure turbine
is condensed in a binary system.
The geothermal power production in the U.S. today
provides enough electricity to meet the electricity
needs of about 2.4 million California households.
In 2007, geothermal was the fourth largest source
of renewable energy in the U.S. Today the U.S. has
about 3,000 MW of geothermal electricity connected
to the grid. Geothermal energy generated 14,885 gigawatt-hours
(GWh) of electricity in 2007, which accounted for
4 percent of renewable energy-based electricity consumption
in the U.S. (including large hydropower).
The U.S. continues to produce more geothermal electricity
than any other country, comprising approximately 30
percent of the world total. In California, the state
with the largest amount of geothermal power on line,
electricity from geothermal resources accounted for
5 percent of the state's electricity generation in
2003 on a per kilowatt-hour basis. Geothermal is the
largest non-hydro renewable energy source in the state,
significantly exceeding the contribution of wind and
As of August 2008, almost 4,000 MW of new geothermal
power plant capacity was under development in the
U.S. (this includes projects in the initial development
phases). Those states with projects currently under
consideration or development are: Alaska, Arizona,
California, Colorado, Florida, Hawaii, Idaho, Nevada,
New Mexico, Oregon, Utah, Washington, and Wyoming.
Combined, these states have approximately 103 projects
in development ranging from initial to advanced stages.
Direct use applications of geothermal energy occur
today in 26 states, almost as many states as produce
coal. New direct use projects are encouraged by the
provisions of the Geothermal Steam Act Amendments
passed by Congress in 2005. There is interest in new
direct use projects in numerous states and on various
Indian reservations within several states.