What
is geothermal energy?
What are the different ways
in which geothermal energy can be used?
What more can you tell me about
geothermal electric power plants?
What is a baseload resource?
What is “availability
factor” and “capacity factor”?
Where can I find more detailed
information about geothermal energy?
What is geothermal energy?
Geothermal energy is defined as heat from the Earth.
It is a clean, renewable resource that provides energy
in the United States and around the world. It is considered
a renewable resource because the heat emanating from
the interior of the Earth is essentially limitless.
The heat continuously flowing from the Earth’s
interior is estimated to be equivalent to 42 million
megawatts of power.(1)
The interior of the Earth is expected to remain extremely
hot for billions of year to come, ensuring an inexhaustible
flow of heat.
What are the different ways
in which geothermal energy can be used?
Geothermal energy can be used for electricity production,
for direct use purposes, and for home heating efficiency
(through geothermal heat pumps).
Geothermal electricity: To develop
electricity from geothermal resources, wells are drilled
into the natural hot water or steam, known as a geothermal
reservoir. The reservoir collects many meters below
the groundwater table. Wells bring the geothermal liquid
to the surface, where it is converted at a power plant
into electricity (see below for more information about
the different types of geothermal electricity production).
Direct Use: Direct use applications
utilize geothermal heat without first converting it
to electricity, such as for space heating and cooling,
food preparation, industrial processes, etc. People
have been taking advantage of direct use applications
for centuries, with documentation of early uses tracing
back to ancient Roman times.
Geothermal Heat Pumps (GHPs): Geothermal
heat pumps are devices that take advantage of the relatively
constant temperature of the Earth's interior, using
it as a source and sink of heat for both heating and
cooling. When cooling, heat is extracted from the space
and dissipated into the Earth; when heating, heat is
extracted from the Earth and pumped into the space.
Geothermal heat pumps can be used anywhere on Earth,
and are considered by the EPA to be one of the most
efficient heating and cooling systems available. For
more information about GHPs, please visit www.geo-exchange.org.
What more can you tell me
about geothermal electric power plants?
There are four widely used types of geothermal power
plants, and three types that are more experimental at
this time.
1) Flash Power Plant: Geothermal steam
is separated in a surface vessel (steam separator) and
delivered to the turbine, and the turbine powers a generator.
2) Dry Steam Power Plant: steam directly
from the geothermal reservoir runs the turbines that
power the generator, and no separation is necessary
because wells only produce steam. The image below is
a more simplified version than the image above.
3) Binary Power Plant: Recent advances
in geothermal technology have made possible the economic
production of electricity from lower temperature geothermal
resources, at 100o C (212o F) to 150o C (302 o F). Known
as binary geothermal plants, these facilities reduce
geothermal energy’s already low emission rate
to near zero. In the binary process, the geothermal
water heats another liquid, such as isobutane, that
boils at a lower temperature than water. The two liquids
are kept completely separate through the use of a heat
exchanger used to transfer the heat energy from the
geothermal water to the “working-fluid."
The secondary fluid vaporizes into gaseous vapor and
(like steam) the force of the expanding vapor turns
the turbines that power the generators.
4) 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 flashed steam 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.
For more information about the above four types of
power plants, access GEA’s Environmental
Guide.
5) Enhanced Geothermal System or Hot Dry Rock
(not commercial): Producing electricity from hot dry
rock requires fracturing hot rocks, pumping water into
and out of the hot rock, and generating electricity.
Research applications of this technology are being pursued
in the US, France, Australia, and elsewhere. They are
not yet economically viable or even near-commercial.
6) Kalina System: A small demonstration
powerplant using the "Kalina" cycle operated
as part of Iceland's Husavik GeoHeat Project. The Kalina
cycle uses an ammonia-water mixed working fluid that
claims higher efficiency. This system is not considered
commercial and reports on the demonstration are not
available.
7) Rankine Cycle System: The U.S.
Department of Energy is proposing to demonstrate a remote
geothermal power system at Chena Hot Springs in Alaska
using the Rankine Cycle. In this system, a compressor/motor
module is expected to be converted into a turbogenerator
by simply reversing the flow direction. This is a demonstration
project, and this system is not considered commercial.
(For more information about the Chena Hot Springs Project,
click here)
What is a baseload resource?
A baseload resource operates most efficiently at a relatively
constant level of generation and is not limited by changes
in weather patterns or other factors. Geothermal relies
on a readily available, constant source of heat for
generation, and is therefore considered a baseload resource.
Other resources such as coal, oil, and natural gas are
also considered baseload resources.
Because some renewable energy sources can only operate
under favorable weather conditions, they are often considered
to be limited in their ability to meet the looming large-scale
power needs of the twenty-first century. Geothermal,
however, has the potential to provide reliable sources
of electricity while still offering significantly lower
emissions levels than fossil fuel sources and avoiding
problems of radioactive waste disposal.
What is “availability
factor” and “capacity factor”?
Availability factor is measured as the number of hours
that a power plant is available to produce power divided
by the total hours in a set time period, usually a yea.
Geothermal’s availability factor is about 95 percent.
This means that geothermal electric-power plants are
available for generation 95 percent of any given time,
based on decades of observations by plant operators.(2)
While availability factor measures a plant’s potential
for use, capacity factor measures the amount of real
time during which a facility is used. To understand
availability and capacity factor, consider the analogy
of a working car. When a car is not in use, but is free
from defects and available to be used, we may speak
of the car’s availability factor. When the car
is actually being driven, we may speak of the car’s
capacity factor. Geothermal’s capacity factor
ranges from 89 to 97 percent, depending upon the type
of geothermal system in place.
Where can I find more detailed information
about geothermal energy?
The Geothermal Energy Association (GEA) has recently
produced several updated, comprehensive documents on
the issues of cost, employment, and the environment,
all of which can be found at the GEA website. The environmental
paper also includes more detailed basic information
about geothermal energy. Click below to access the following
links:
Factors
Affecting Cost of Geothermal Power Development
Geothermal
Industry Employment - Survey Results and Analysis September
2005
A
Guide to Geothermal Energy & The Environment
If you are looking for a current update about
geothermal energy, renewable energy, and global warming
issues in the U.S., the world, and in our nation’s
capitol, take a look at GEA’s latest Update.
For the truths behind common geothermal myths, take
a look at our Mythbusters
section.
For the list of useful links with more information and
resources related to geothermal energy, click here.
(1)Energy and Geosciences
Institute, University of Utah. Prepared by the U.S.
Geothermal Industry for the Renewable Energy Task Force
(1997), Briefing on Geothermal Energy. Washington, D.C.
(2) U.S. DOE. Energy
and Geosciences Institute at University of Utah, (May
2001). Geothermal Energy: Clean Sustainable Energy for
the Benefit of Humanity and the Environment. [Brochure].
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