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 U.S. will need in the future, probably at competitive prices and with minimal environmental impact, according to a new MIT-led study, The Future of Geothermal Energy, sponsored by the U.S. Department of Energy.
Although geothermal energy is produced commercially today and the U.S. 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,” said panel head Jefferson Tester, the H. P. Meissner Professor of Chemical Engineering at MIT.
The report 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 multiyear federal commitment to demonstrate the concept in the field at commercial scale.
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,” Tester said.
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.
Electricity produced annually by geothermal energy systems now in use in the U.S. 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 U.S.
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.
Panel member Brian Anderson, an assistant professor at West Virginia University, noted that 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.
The panel recommends that:
- More detailed and site-specific assessments of the U.S. geothermal energy resource should be conducted.
- Field trials running three to five 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 should also 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.