Updated by Leslie Blodgett GEA 2014
Geothermal energy—the heat of the Earth—is a clean, renewable resource that provides energy in the U.S. and around the world. The U.S. has been using commercial, large-scale geothermal power plants at deep resource temperatures (between 200˚F and 700˚F) since the 1960s. Geothermal energy development and production is a thriving international market.
1.1. What is geothermal energy?
Heat has been radiating from the center of the Earth for some 4.5 billion years. At 6437.4 km (4,000 miles) deep, the center of the Earth hovers around the same temperatures as the sun's surface, 9932°F (5,500°C) (Figure 1). Scientists estimate that 42 million megawatts (MW) of power flow from the Earth’s interior, primarily by conduction
Geothermal energy is a renewable resource.
One of its biggest advantages is that it is constantly available. The constant flow of heat from the Earth ensures an inexhaustible and essentially limitless supply of energy for billions of years to come.
The National Energy Policy Act of 1992 (Sec. 1202) and the Pacific Northwest Electric Power Planning and Conservation Act of 1980 (Sec. 12H, 839a(16), page 84) both define geothermal energy as a renewable resource.
Figure 1: Earth’s Temperatures
The uses of geothermal for heat and other purposes were indigenous practices across a variety of world cultures: “The Maoris in New Zealand and Native Americans used water from hot springs for cooking and medicinal purposes for thousands of years. Ancient Greeks and Romans had geothermal heated spas. The people of Pompeii, living too close to Mount Vesuvius, tapped hot water from the earth to heat their buildings. Romans used geothermal waters for treating eye and skin disease. The Japanese have enjoyed geothermal spas for centuries.”
A viable geothermal system requires heat, permeability, and water. Developers explore a geothermal reservoir to test its potential for development by drilling and testing temperatures and flow rates.
Rainwater and snowmelt feed underground thermal aquifers (Figure 2). When hot water or steam is trapped in cracks and pores under a layer of impermeable rock, it forms a geothermal reservoir.
Figure 2: The Formation of a Geothermal Reservoir
At the Larderello, Italy dry steam field, Prince Piero Ginori Conti first proved the viability of geothermal power plant technology in 1904 (Figure 3). Larderello is still producing today.
1.2. What is a baseload power source? What is a dispatchable power source?
A baseload power plant produces energy at a constant rate. In addition to geothermal, nuclear and coal-fired plants are also baseload. Because the energy is constant, its power output can remain consistent nearly 24 hours a day, giving geothermal energy a higher capacity factor than solar or wind power, which must wait for the sun to shine or the wind to blow, respectively. This means a geothermal plant with a smaller capacity than a solar or wind plant can provide more actual, delivered electricity.
“Capacity” and “capacity factor” essentially refer to the distinction between megawatts (MW) and megawatt-hours (MWh). MW is a unit of power or the rate of doing work, whereas MWh is a unit of energy or the amount of work done. One MWh is equal to 1 MW (1 million watts) applied over the period of an hour. In geothermal development, one megawatt is roughly equivalent to the electricity used by 1,000 homes.
A geothermal plant can also be engineered to be firm, flexible, or load following, and otherwise support the needs of the grid. Most geothermal plants being built now have adjustable dispatching capabilities. In addition to geothermal, natural gas is dispatchable. This means a geothermal plant can meet fluctuating needs, such as those caused by the intermittency of solar and wind power.
1.3. How does a conventional geothermal power plant work?
After careful exploration and analysis, wells are drilled to bring geothermal energy to the surface, where it is converted into electricity. Figure 4 shows the geothermal installed capacity in the U.S. from 1975 to 2012, separated by technology type.
The USGS has defined moderate-temperature resources as those between 90°C and 150°C (194 to 302°F), and high-temperature geothermal systems as those greater than 150°C.
Figures 5-7 depict the three commercial types of conventional geothermal power plants: flash, dry steam, and binary.
In a geothermal flash power plant, high pressure separates steam from water in a “steam separator” (Figure 5) as the water rises and as pressure drops. The steam is delivered to the turbine, and the turbine then powers a generator. The liquid is reinjected into the reservoir.
Under one-third of the installed geothermal capacity in the U.S. is comprised of flash power plants, with the majority in California.
GEA “Annual” 2012, page 7
Figure 5: Flash Power Plant Diagram, Photo: Dixie Valley, NV, Flash Plant
In a geothermal dry steam power plant, steam alone is produced directly from the geothermal reservoir and is used to run the turbines that power the generator (Figure 6). Because there is no water, the steam separator used in a flash plant is not necessary. Dry-steam power plants account for approximately 50% of installed geothermal capacity in the U.S. and are located in California.
Figure 6: Dry Steam Plant Diagram, Photo: The Geysers, CA, Dry Steam Plant
In 1981 at a project in Imperial Valley, California, Ormat Technologies established the technical feasibility of the third conventional type of large-scale commercial geothermal power plant: binary. The project was so successful that Ormat repaid its loan to the Department of Energy (DOE) within a year. Binary geothermal plants have made it possible to produce electricity from geothermal resources lower than 302°F (150°C). This has expanded the U.S. industry’s geographical footprint, especially in the last decade.
Binary plants use an Organic Rankine Cycle system, which uses geothermal water to heat a second liquid that boils at a lower temperature than water, such as isobutane or pentafluoropropane. This is called a working fluid (or “motive fluid” in Figure 7). A heat exchanger separates the water from the working fluid while transferring the heat energy. When the working fluid vaporizes, the force of the expanding vapor, like steam, turns the turbines that power the generators. The geothermal water is then reinjected in a closed loop, separating it from groundwater sources and lowering emission rates further (see section 5). Most new geothermal plants under development in the U.S. are binary.
GEA “Annual” 2012, page 7
Figure 7: Binary Power Plant, Photo: Burdett, NV, Binary Power Plant
Hybrid power plants allow for the integration of numerous generating technologies. In Hawai’i, the Puna flash/binary combined cycle system optimizes both flash and binary geothermal technologies. Geothermal fluid is flashed to a mixture of steam and liquid in a separator. The steam is fed to a turbine as in a flash-steam generator and the separated liquid is fed to a binary cycle generator (Figure 8).
Figure 8: Flash/Binary Power Plant Diagram, Photo: Puna, Hawaii, Flash/Binary
Another type of hybrid plant is Enel Green Power’s solar-geothermal plant in Stillwater, Nevada.
1.4. How do geothermal heat pumps work?
Animals burrow underground for warmth in the winter and to escape the heat of the summer. The same basic principle of constant, moderate temperature in the subsurface is applied to geothermal heat pumps (GHPs). GHPs utilize average ground temperatures between 40˚and 70˚F. In 1948, a professor at Ohio State University developed the first GHP for use at his residence. A groundwater heat pump came into commercial use in Oregon around the same time.
Also called a geoexchange system or Ground Source Heat Pump.
Geothermal Exchange Organization
U.S. Department of Energy “A History”
Figure 9: Geothermal Heat Pump Diagram
GHP heating and cooling systems circulate water or other liquids to pull heat from the Earth through pipes in a continuous loop through a heat pump and conventional duct system. For cooling, the process is reversed; the system extracts heat from the building and moves it back into the Earth loop. The loop system can be used almost everywhere in the world at depths below 10 ft to 300 ft. GHPs are used in all 50 states and are over 45% more energy efficient than standard heating and cooling system options.
Homeowners who install qualified GHPs are eligible for a 30% federal tax credit through December 31, 2016. They can be buried conveniently on a property such as under a landscaped area, parking lot, or pond, either horizontally or vertically (Figure 9). A GHP system can also direct the heat to a water heater unit for hot water use.
U.S. Environmental Protection Agency “Heat Pumps”
1.5. How do direct use applications work?
Geothermal heat is used directly, without a power plant or a heat pump, for applications such as space heating and cooling, food preparation, hot spring bathing and spas (balneology), agriculture, aquaculture, greenhouses, snowmelting, and industrial processes. Geothermal direct uses are applied at aquifer temperatures between 90˚F and 200˚F.
Examples of direct use applications exist all across the U.S. Boise, Idaho’s Capitol Building uses geothermal for direct heating and cooling. President Franklin D. Roosevelt frequented Georgia’s healing hot springs and founded the Roosevelt Warm Springs Institute for polio treatment in 1927. And the City of Klamath Falls, Oregon began piping hot spring water to homes as early as 1900.
In a typical geothermal direct use configuration, geothermal water or steam is accessed and brought to a plate heat exchanger (Figure 10). New direct use projects in numerous states, including some on Indian reservations, are encouraged by the provisions of the Geothermal Steam Act Amendments passed by Congress in 2005 (see section 4).
Geothermal Exchange Organization
Figure 10: Direct Use Geothermal Heating System Configuration
Even more information . . .
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