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Basics

1.1. What is geothermal energy?

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. 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, 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. (1)

Figure 1: Earth’s Temperatures

1.2. How does a conventional geothermal reservoir work?

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°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.

Figure 2: The Formation of a Geothermal Reservoir

1.3. What are the different ways in which geothermal energy can be used?

Geothermal energy can be used for electricity production, for commercial, industrial, and residential direct heating purposes, and for efficient home heating and cooling through geothermal heat pumps. For a video presentation on the different ways to use geothermal energy, visit http://geothermal.marin.org/video/vid_pt5.html.

  • Geothermal Electricity: 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 (see below for more information about the different types of geothermal electricity production).
  • Heating Uses: Geothermal heat is used directly, without involving a power plant or a heat pump, for a variety of applications such as space heating and cooling, food preparation, hot spring bathing and spas (balneology), agriculture, aquaculture, greenhouses, and industrial processes. Uses for heating and bathing are traced back to ancient Roman times. (2) Currently, geothermal is used for direct heating purposes at sites across the United States. U.S. installed capacity of direct use systems totals 470 MW or enough to heat 40,000 average-sized houses, according to the GeoHeat Center Web site, http://geoheat.oit.edu/.

    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. The first known "health spa" was established in 1326 in Belgium at natural hot springs. And for hundreds of years, Tuscany in Central Italy has produced vegetables in the winter from fields heated by natural steam. (See the Geothermal Education Office Web site, http://geothermal.marin.org/).

    A few examples of geothermal direct use applications today are at the Idaho Capitol Building in Boise http://idptv.state.id.us/buildingbig/buildings/idcapital.html, Burgett Geothermal Greenhouses in Cotton City, New Mexico http://geoheat.oit.edu/directuse/all/dug0144.htm, and Roosevelt Warm Springs Institute for Rehab in Warm Springs, Georgia http://www.rooseveltrehab.org/index.php

Figure 3: Typical Direct Use Geothermal Heating System Configuration

  • Geothermal Heat Pumps (GHPs): Geothermal heat pumps take advantage of the Earth’s relatively constant temperature at depths of about 10 ft to 300 ft. GHPs can be used almost everywhere in the world, as they do not share the requirements of fractured rock and water as are needed for a conventional geothermal reservoir. GHPs circulate water or other liquids through pipes buried in a continuous loop, either horizontally or vertically, under a landscaped area, parking lot, or any number of areas around the building. The Environmental Protection Agency considers them to be one of the most efficient heating and cooling systems available.

    Animals burrow underground for warmth in the winter and to escape the heat of the summer. The same idea is applied to GHPs, which provide both heating and cooling solutions. To supply heat, the system pulls heat from the Earth through the loop and distributes it through a conventional duct system. For cooling, the process is reversed; the system extracts heat from the building and moves it back into the earth loop. It can also direct the heat to a hot water tank, providing another advantage — free hot water. GHPs reduce electricity use 30–60% compared with traditional heating and cooling systems, because the electricity which powers them is used only to collect, concentrate, and deliver heat, not to produce it.

    For more information about GHPs, please visit www.geoexchange.org and http://www.igshpa.okstate.edu.

Figure 4: Geothermal Heat Pump Diagram




1.4. How does a geothermal power plant work?

There are four commercial types of geothermal power plants: a. flash power plants, b. dry steam power plants, c. binary power plants, and d. flash/binary combined power plants.

a. Flash Power Plant: Geothermally heated water under pressure is separated in a surface vessel (called a steam separator) into steam and hot water (called “brine” in the accompanying image). The steam is delivered to the turbine, and the turbine powers a generator. The liquid is injected back into the reservoir.

Figure 5: Flash Power Plant Diagram

Figure 6: Dixie Valley, NV, Flash Plant

Dixie Valley, NV, Flash Plant

b. 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. The image below is a more simplified version of the process.

Figure 7: The Geysers, CA, Dry Steam Plant

Figure 8: Dry Steam Plant Diagram

http://www.geo-energy.org/images/dry%20steam.jpg

c. Binary Power Plant: Recent advances in geothermal technology have made possible the economic production of electricity from geothermal resources lower than 150°C (302°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” in the accompanying image) 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.

Figure 9: Binary Power Plant

Figure 10: Burdett, NV, Binary Power Plant



d. 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 which “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.

Figure 11: Flash/Binary Power Plant Diagram

Figure 12: Puna, HI, Flash/Binary

For more information about the above four types of power plants, access GEA’s Environmental Guide or Surface Technology Report.

In addition to different power plant technologies in use today, additional applications and technologies continue to emerge. The following are some commonly discussed as areas of future development:

  • Enhanced Geothermal Systems (EGS): Although the deeper crust and interior of the Earth is universally hot, it lacks two of the three ingredients required for a naturally occurring geothermal reservoir: water and interconnected open volume for water movement. Producing electricity from this naturally occurring hot, but relatively dry rock requires enhancing the potential reservoir by fracturing, pumping water into and out of the hot rock, and directing the hot water to a geothermal power plant. Research applications of this technology are being pursued in the U.S., France, Australia, and elsewhere. (3) EGS is also sometimes referred to as Hot Dry Rock. See further discussion of EGS in section 3.2.
  • Mixed Working Fluid/ Kalina System: As of January 2009 the Kalina System was being used at two power plants. The first is a small demonstration power plant operated as part of Iceland's Husavik GeoHeat Project. The second plant to use the Kalina System is in Germany at the Unterhaching Power Station. The Kalina cycle uses an ammonia-water mixed working fluid for high efficiency. The Kalina cycle is only one of the possible mixed working fluid approaches to possibly achieving greater heat transfer efficiency and/or lower temperature production of power. (4)

Figure 13: Kalina Power Plant in Husavik, Iceland

  • Distributed Generation: Geothermal applications can be sized and constructed at geographically remote sites in order to meet on-site electricity demands. Examples of remote geothermal power systems are at Chena Hot Springs in Alaska and at the Rocky Mountain Oil and Gas Testing Center (RMOTC) in Wyoming. In the first, the unit powers a remote resort, in the second the power supplies electricity to operate an oil field. For more information about the Chena Hot Springs Project, visit http://www.geo-energy.org/plantdetails.aspx?id=46x. For more information about the RMOTC project, visit http://www.rmotc.doe.gov/.
  • Supercritical Cycles : Supercritical fluids are at a temperature and pressure that can diffuse through solids. A supercritical fluid such as carbon dioxide can be pumped into an underground formation to fracture the rock, thus creating a reservoir for geothermal energy production and heat transport. The supercritical fluid used to form the reservoir can heat up and expand, and is then pumped out of the reservoir to transfer the heat to a surface power plant or other application. An example of work in this area is the Iceland Deep Drilling Project, and for more information on this effort visit http://www.iddp.is.

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