3.1. What is the official
government estimate of potential geothermal electric
resources in the U.S.?
The heat of the Earth is considered limitless; its use is only limited by technology and the associated costs. Technology development and further studies are expected to show even greater potential, but here we have cited the first part of a new assessment released in September 2008 by the U.S. Geological Survey (USGS). (1) The report focuses on 13 western states and breaks the geothermal estimate into three categories:
- Identified Geothermal Systems: The resource is either liquid or vapor dominated and has moderate to high temperature. The resource is either producing (the reservoir is currently generating electric power), confirmed (the reservoir has been evaluated with a successful commercial flow test of a production well), or potential (there are reliable estimates of temperature and volume for the reservoir but no successful well tests to date).
- Undiscovered Geothermal Resources: Geothermal resources were assessed for the same 13 states in which the identified resources are located. The assessment was based on mapping potential via regression analysis.
- Enhanced Geothermal Systems (EGS): Resource probability in regions characterized by high temperatures but low permeability and lack of water in rock formations.
The assessment estimates power generation potential as follows:
- Identified Geothermal Systems: 3,675 MWe (95% probability) to 16,457 MWe (5% probability)
- Undiscovered Geothermal Systems: 7,917 MWe (95% probability) to 73,286 MWe (5% probability)
- EGS: 345,100 MWe (95% probability) to 727,900 MWe (5% probability).
The USGS assessment evaluates geothermal resources in the states of Alaska, Arizona, California, Colorado, Hawaii, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming. The assessment identified 241 moderate-temperature (90 to 150°C; 194 to 302°F) and high-temperature (greater than 150°C) geothermal systems located on private and public lands in these states. Geothermal systems located on public lands closed to development, such as national parks, were not included in the assessment. Electric-power generation potential was also determined for several low-temperature (less than 90°C) systems in Alaska for which local conditions make electric power generation feasible.
Although the assessment only accounted for large-scale geothermal power production, the USGS is also in the process of updating information about direct use, small power, oil and gas co-production and geopressured resources and the potential energy contribution of those portions of the geothermal resource base are not included in the estimates above.
The USGS assessment is the first new national geothermal resource assessment since 1979, when USGS released its last geothermal resource estimate, Circular 790. A new component of the 2008 assessment is the inclusion of production potential of EGS techniques. For more information on the USGS assessment, please visit http://www.usgs.gov/newsroom/article.asp?ID=2027&from=rss_home.
In 2006, Massachusetts Institute of Technology (MIT) prepared an analysis of the future geothermal potential in the U.S. The report estimated that geothermal systems could produce 100 GWe in the next 50 years with a reasonable investment in R&D. The report, The Future of Geothermal Energy, is available at http://geothermal.inel.gov/publications/future_of_geothermal_energy.pdfhttp://www1.eere.energy.gov/geothermal/future_geothermal.html
3.2. Are there other examples of how geothermal resources are utilized?
- Distributed generation: Distributed generation facilities such as those at Chena Hot Springs in Alaska, the Burgett greenhouse in New Mexico, and the Oregon Institute of Technology are examples of small-scale electricity produced to cover the electricity needs of each facility. Energy not being used by the facility is sold back to the grid.
Figure 15: Chena Hot Springs, AK, Gains Distributed Generation Begins in 2006
- Geopressured resources: Geopressured resources are deep reservoirs of high-pressured hot water that contain dissolved methane. The Department of Energy built a demonstration plant in Texas which produced electricity from geopressured resources, pictured below. Preliminary testing (Phase 0) of Well No. 2 took place during 1979, reservoir limits testing during 1980 (Phase I), and long-term testing (Phase 11) was conducted during 1981–1983. The plant was dismantled after being deemed a success.( 2)
Figure 16: Geopressured Demonstration Plant in Texas
- Co-production geothermal fluids: Usable geothermal fluids are often found in oil and gas production fields. The Southern Methodist University Geothermal Energy Program has identified thousands of megawatts of potential energy production from hot water being co-produced with oil and gas. There are presently two geothermal co-production demonstrations underway supported by the U.S. DOE, at the Rocky Mountain Oil Test Center in Wyoming and the Jay oil field in Florida. ( 3)
- Enhanced Geothermal Systems (EGS): EGS involves developing tools and techniques that will allow geothermal production by artificially creating permeability in hot rock and introducing water (or another working fluid) to extract the heat. While reaching the full potential of EGS may take a decade or more to realize, there are many aspects of EGS that are already being applied. In California at The Geysers field—the oldest geothermal field in the U.S. and the largest geothermal venture in the world—operators have expanded the capacity of wells by injecting millions of gallons of reclaimed wastewater into the geothermal reservoir. Some experts call the Geysers wastewater project the first large-scale EGS project. There are several EGS projects that are already, or will soon, produce power:
- Soultz project, in France, a 1.5-MW EGS plant already in operation
- Landau project, in Germany, a 2.5-MW operational plant
- Paralana, in Australia, a 7–30-MW plant in drilling stages
- Cooper Basin, in Australia, a 1-MW showcase plant will be operational in 2008 and a 250–500-MW plant in drilling stages, expected to have the first 50 MW EGS plant operating as early as 2011–2012
- Desert Peak, in the U.S. ( Nevada), in planning stages, the expansion of an existing natural geothermal field
In October of 2008, the U.S. Department of Energy selected four new cooperative projects with the U.S. geothermal industry for EGS systems demonstrations in the U.S. which it hopes will lead to technology readiness by 2015. For more information on the DOE effort visit: http://www1.eere.energy.gov/geothermal/enhanced_geothermal_systems.html. Also, the International Partnership for Geothermal Technology provides information about efforts to developed advanced technologies for EGS and related areas. You can visit their web site at: http://internationalgeothermal.org.
Figure 17: Soultz, France, 1.5-MW EGS Power Plant
3.3. How much energy is geothermal electricity capable of supplying to the U.S?
In 2006 the National Renewable Energy Laboratory (NREL) released a report, Geothermal—The Energy Under Our Feet, which estimates domestic geothermal resources. The report estimates that 26,000 MW of geothermal power could be developed by 2015, with direct use and heat pumps contributing another 20,000 MW of thermal energy. The report suggests that by 2025 more than 100,000 MW of geothermal power could be in production, with direct use and heat pumps adding another 70,000 MW of thermal energy. (4)
As the report concludes, “these estimates show the enormous potential of the U.S. geothermal resource.” For power production, the report includes specific estimates of the potential for identified resources, deep geothermal co-produced fluids and geopressured resources, and EGS. In addition, the report examines the potential for geothermal direct use and geothermal heat pumps.
The report does not include hidden or undiscovered geothermal systems, which the USGS report estimates have substantial energy potential. Nor does the report specifically examine small power systems (distributed generation).
For more information on the NREL report, please visit http://www1.eere.energy.gov/geothermal/pdfs/40665.pdf.
3.4. Where are geothermal resources located?
Geothermal energy – the heat of the earth – is located essentially everywhere. The depth at which rocks may reach 100°C may differ from place to place, but there is hot rock beneath our feet everywhere in the U.S. and around the world.
That explains the great promise of EGS technology. As we learn to commercialize EGS plants, geothermal power will be even more widely available.
Below is a map showing the temperature beneath the U.S. at a depth of 6 km. As it shows, the temperature is above boiling at that depth nearly everywhere in the U.S.
Figure 18: United States Heat Flow Map
Most traditional hydrothermal systems being used for power production in the U.S. are located in the western states, where the geology favors natural geothermal reservoirs being formed at shallower depths. Below is a map showing hydrothermal areas in the West with the dots marking identified geothermal reservoirs.
Figure 19: Hydrothermal Areas in the Western United States
In addition to conventional hydrothermal systems, there is also a great interest in producing geothermal power from hot water that is produced by many oil and gas wells. These are known as co-produced geothermal resources. The map below shows some of the areas identified with such co-production potential by the researchers at Southern Methodist University, along with their estimate of the near-term power potential from these sites.
Figure 20: SMU Estimated Co-Produced Geothermal Potential
Geopressured resources are known to be located in several areas of the U.S., with the most significant of these located in Texas, Louisiana, and the Gulf of Mexico. See section 3.2. for more information on geopressured resources. The map below shows U.S. geopressured basins.
Figure 21: Geopressured Basins in the United States
Note: On the map above, the major oil-producing basins (of all types) in the U.S. are highlighted. The gray stippling indicates the parts of those basins where geopressured strata have been encountered.
Today, U.S. geothermal resources are being used in more than 30 states from New York to Hawaii. The map below from the Geo-Heat Center of the Oregon Institute of Technology shows areas where geothermal energy is being used for power, greenhouses, commercial building heating, and other purposes.
Figure 22: Location of Geothermal Projects and Resources
And finally, geothermal heat pumps can be used nearly everywhere in the U.S. The map below shows where geothermal heat pumps were installed in 2006. The states in darker green have the higher number of installations.
Figure 23: Geothermal Heat Pump Installations in 2006
3.5. How much electricity can geothermal supply worldwide?
There has not been a significant new analysis of international geothermal potential comparable to either the USGS report or the NREL report discussed above. A 1999 study that used fairly conservative assumptions about the resource base and technology concluded that geothermal resources using existing technology have the potential to support between 35,448 and 72,392 MW of worldwide electrical generation capacity. Using enhanced technology (defined as the technology expected to be available by 2009), the geothermal resources could support between 65,576 and 138,131 MW of electrical generation capacity. Assuming a 90% availability factor, which is well within the range experienced by geothermal power plants, this electric capacity could produce as much as 1,089 billion kWh of electricity annually. The estimates produced for world energy potential by this study did not assess the limits of geothermal resource base, nor the potential for new development with significantly different technologies, such as engineered geothermal systems. (5) See also section 2.4.
An estimate of world geothermal resources made by the Energy and Geoscience Institute for the President's Council of Advisors on Science and Technology stated the following for different geologic regimes. (6)
World Continental Geothermal Resources:
Geologic Regime: Joules (J) bbl oil equivalent
Magmatic Systems 15 × 10 24 J 2,400,000 × 10 9 bbl
Crustal Heat 490 × 10 24 J 79,000,000 × 10 9 bbl
Thermal Aquifers 810 × 10 18 J 130 × 10 9 bbl
Geopressured Basins 2.5 × 10 24 J 410,000 × 10 9 bbl
Total Oil Reserves (for comparison) 5,300 × 10 9 bbl*
* National Academy of Sciences, 1990: includes crude oil, heavy oil, tar sands, and oil shale
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