The traditional approach to using geothermal energy is to build a plant on a hydrothermal site and use the hot water directly for heating nearby buildings or in steam plants to produce electricity. Iceland provides an example of using geothermal energy as a major energy source, with minimal reliance on coal, oil or natural gas to meet energy demands.
One major limitation to expanding the use of this geothermal heat in the US has been that suitable hydrothermal sites are limited in number and in location. As a result, geothermal energy, while a substantial renewable resource, has yet to be a significant alternative energy contributor.
As reported in a recent release by Energy Pathways, a new approach that uses carbon dioxide as the primary working fluid may increase the efficiency of geothermal energy production by 2X to 10X. This new technology can expand the geographic areas where geothermal energy can be cost-effective
Some results to date were presented at the American Geophysical Union's annual meeting in San Francisco on Dec. 13, 2013. A graphical presentation, Geothermal Energy: Enhancing Our Future, was done by Shannon Gilley of the University of Minnesota and Jeffrey Bielicki, now of Ohio State University (OSU).
This research is based on a process called Carbon Dioxide Plume Geothermal (CPG). The worldwide patent holder of the CPG process is Heat Mining Company, LLC (HMC). The company purchased the rights to the CPG technology from the initial developers at the University of Minnesota. HMC has been able to define the key operating parameters that enable the generation of electricity with a net zero carbon dioxide contribution. There is also an application to use CPG technology in the extraction of oil from depleted fields.
In an interview with Prof. Jeffrey Bielicki of OSU, Bielicki stated that the idea to use carbon dioxide in sedimentary basin geothermal resources was initiated by Prof. Martin Saar and his former PhD student James Randolph at the University of Minnesota. They came up with the idea for CPG while driving to an energy conference. The carbon dioxide is put under high pressure to become a super critical fluid that is much more efficient at heat extraction than water.
While a post-doctoral Research Associate at the University of Minnesota, Bielicki met, and eventually worked with, Saar and Randolph. Bielicki now holds a joint appointment in the Department of Civil, Environmental and Geodetic Engineering and the John Glenn School of Public Affairs at OSU.
Tom Buscheck, a geoscientist at Lawrence Livermore National Laboratory (LLNL) is another member of the team investigating novel ways to use geothermal resources to produce electricity. Members of the team have received funding from the National Science Foundation and Department of Energy. Buscheck added the idea of using nitrogen as another fluid in this geothermal process.
With the increased efficiency of extracting geothermal heat from deep sedimentary basins using fluids other than water, there are many more potential sites west of the Mississippi River. There may also be a few sites in the Eastern US, including sites near White Sulfur Springs, WV and in Georgia.
There are still issues to be addressed in order to bring these new processes into wider usage. For example, carbon dioxide is abundantly produced at coal burning power plants, but it may have to be transported it to a location where it may then be used to extract geothermal heat in order to produce electricity.
Another issue to address is that carbon dioxide mixed with water produces a slight acid. This acid could affect wells and other piping necessary to circulate the fluids to, and within, the power plant.
An issue with all power generation facilities is the ability to meet a fluctuating demand. Commercial power plants adjust the fueling rate to increase or decrease output to meet demand. Renewable energy approaches are dependent upon having their primary source available, e.g. sun for solar, wind for windmills, and adequate water for hydroelectric plants. If there is a day without adequate wind or sunlight, facilities that rely on these renewable, but variable, sources cannot produce electricity.
Under the right conditions, the supercritical carbon dioxide can be stored below ground during lower power demand periods. The heated carbon dioxide could be used to produce electricity later when demand increases. As a result, geothermal energy could be used to reduce fluctuations in output from wind and solar energy fields.
Bielicki summarized the overall impact of implementing this enhanced geothermal technology.
“This new process expands the economically viable geothermal resource base.”
Heat Mining Company, LLC is building a pilot plant project that will document the improvements in energy efficiency and address the issues that need to be resolved to expand geothermal energy production. This development that uses carbon dioxide emissions while generating electricity is a step towards better environmental stewardship.
Geothermal energy production can now have a more significant role in renewable energy for the US. These enhancements will widen the areas of application and improve the overall economics of the process. This is a positive step in the continuing effort to use new technologies to get more energy out of renewable processes that include geothermal, solar, wind and water sources.