Heat Mining a Key Energy Resource
June 2, 2008
A comprehensive new study of the potential for geothermal energy within the United States has found that mining the huge amounts of heat that reside as stored thermal energy in the Earth’s hard rock crust could supply a substantial portion of the electricity the United States will need in the future, probably at competitive prices and with minimal environmental impact.
An 18-member panel led by the Massachusetts Institute of Technology (MIT) prepared the study, titled “The Future of Geothermal Energy.” Sponsored by the U.S. Department of Energy, it is the first study in some 30 years to take a new look at geothermal, an energy resource that has been largely ignored.
The goal of the study was to assess the feasibility, potential environmental impacts and economic viability of using enhanced geothermal system (EGS) technology to greatly increase the fraction of the U.S. geothermal resource that could be recovered commercially. Although geothermal energy is produced commercially today and the United States is the world’s biggest producer, existing U.S. plants have focused on the high-grade geothermal systems primarily located in isolated regions of the west. This new study takes a more ambitious look at this resource and evaluates its potential for much larger-scale deployment.
“We’ve determined that heat mining can be economical in the short term, based on a global analysis of existing geothermal systems, an assessment of the total U.S. resource and continuing improvements in deep-drilling and reservoir stimulation technology,” says panel head Jefferson Tester. “EGS technology already has been proven to work in the few areas where underground heat has been successfully extracted. And further technological improvements can be expected.”
The expert panel offers a number of recommendations to develop geothermal as a major electricity supplier for the nation. These include more detailed and site-specific assessments of the U.S. geothermal resource and a multi-year federal commitment to demonstrate the concept in the field at commercial scale.
The new assessment of geothermal energy by energy experts, geologists, drilling specialists and others is important for several key reasons. First, fossil fuels – coal, oil and natural gas – are increasingly expensive and consumed in ever-increasing amounts. Second, oil and gas imports from foreign sources raise concerns over long-term energy security. Third, burning fossil fuels dumps carbon dioxide and other pollutants into the atmosphere. Finally, heat mining has the potential to supply a significant amount of the country’s electricity currently being generated by conventional fossil fuel, hydroelectric and nuclear plants.
The study shows that drilling several wells to reach hot rock and connecting them to a fractured rock region that has been stimulated to let water flow through it creates a heat exchanger that can produce large amounts of hot water or steam to run electric generators at the surface. Unlike conventional fossil-fuel power plants that burn coal, natural gas or oil, no fuel would be required. And unlike wind and solar systems, a geothermal plant works night and day, offering a non-interruptible source of electric power. Geothermal resources are available nationwide, although the highest-grade sites are in western states, where hot rocks are closer to the surface, requiring less drilling and, thus, lowering costs. The panel also evaluated the environmental impacts of geothermal development, concluding that these are markedly lower than conventional fossil fuel and nuclear power plants. “This environmental advantage is due to low emissions and the small overall footprint of the entire geothermal system, which results because energy capture and extraction is contained entirely underground, and the surface equipment needed for conversion to electricity is relatively compact,” notes Tester.
The report also notes that meeting water requirements for geothermal plants may be an issue, particularly in arid regions. Further, the potential for seismic risk needs to be carefully monitored and managed.
According to panel member Nafi Toksöz, professor of geophysics at MIT, “Geothermal energy could play an important role in our national energy picture as a non-carbon-based energy source. It’s a very large resource, and has the potential to be a significant contributor to the energy needs of this country.” Toksöz adds that the electricity produced annually by geothermal energy systems now in use in the United States at sites in California, Hawaii, Utah and Nevada is comparable to that produced by solar and wind power combined. And the potential is far greater still, since hot rocks below the surface are available in most parts of the United States.
Even in the most promising areas, however, drilling must reach depths of 5,000 feet or more in the west, and much deeper in the eastern United States. “Still, the possibility of drilling into these rocks, fracturing them and pumping water in to produce steam has already been shown to be feasible,” Toksöz says.
The drilling and reservoir technologies used to mine heat have many similarities to those used for extracting oil and gas. As a result, the geothermal industry today is well connected technically to two industry giants in the energy arena, oil and gas producers and electric power generators. With increasing demand for technology advances to produce oil and gas more effectively and to generate electricity with minimal carbon and other emissions, an opportunity exists to accelerate the development of EGS by increased investments by these two industries.
Government-funded research into geothermal was very active in the 1970s and early 1980s. As oil prices declined in the mid-1980s, enthusiasm for alternative energy sources waned, and funding for research on renewable energy and energy efficiency (including geothermal) was greatly reduced, making it difficult for geothermal technology to advance. “Now that energy concerns have resurfaced, an opportunity exists to pursue the EGS option aggressively to meet long-term national needs,” Tester observes.
Tester and colleagues emphasize that federally funded engineering research and development still must be done to lower risks and encourage investment by early adopters. Of particular importance is to demonstrate that EGS technology is scalable and transferable to sites in different geologic settings.
The panel’s recommendations:
- More detailed and site-specific assessments of the U.S. geothermal energy resource should be conducted.
- Field trials running 3 years to 5 years at several sites should be done to demonstrate commercial-scale engineered geothermal systems.
- The shallow, extra-hot, high-grade deposits in the west should be explored and tested first.
- Other geothermal resources, such as co-produced hot water associated with oil and gas production and geopressured resources, also should be pursued as short-term options.
- On a longer time-scale, deeper, lower-grade geothermal deposits should be explored and tested.
- Local and national policies should be enacted that encourage geothermal development.
- A multiyear research program exploring subsurface science and geothermal drilling and energy conversion should be started, backed by constant analysis of results.