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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. 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).

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

 
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