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Pump and Valves Insulation Closed Cycle System High Conductivity Material (Grout) Bore Holes Heat Pipe Heat Exchanger T<190oF Insulation T>190oF Heat Reservoir Development of a closed-loop system that employs a single well to bring heat rather than geothermal fluid to the surface could provide cost-effective access to the massive hot dry rock resource base. reservoir’s characteristics were fully under- stood, numerous developers, operating hundreds of production wells and venting the steam to the atmosphere, watched as steam and electrical output declined steadily through the 1980s, turning some of their investments sour. A source of water to recharge the Geysers was desperately needed, and several owners contracted with local municipalities to pump treated waste- water to the site for down-hole injection. With years of careful management, they have nearly stabilized output. Similarly, at the Coso field in southeastern California, natural water recharge from the Sierra Nevada could not keep pace with the extraction, forcing operators to seek new sources of water for injection—a difficult proposition in an arid region. Developers are currently building a water storage sys- tem to provide makeup water during sea- sonal or sustained droughts. Enhanced Geothermal Systems: The Grand Prize In street parlance, development of enhanced geothermal systems is “the big enchilada.” If successful, it could increase geothermal power production to unprece- dented levels and expand its footprint to regions well beyond seismically active zones. Experimental work is under way in the United States, France, Germany, Aus- tralia, and other countries. A Massachu- setts Institute of Technology study in 2007 indicated that as much as 50 gigawatts of new U.S. geothermal capacity is feasible by 2050 with accelerated research and development. In the United States, both public funding and private venture capital are aggressively pursuing this option. Such a renewable energy source, with the scale and firm-power equivalent of hydro, could help reshape the power-planning horizon, and long-term economics appear compel- ling. For example, the Northern Califor- nia Power Agency, which invested in the Geysers in the 1980s and just paid off the debt on its investment, has begun offering its members geothermal power at $20 per megawatt-hour. Several developers believe they will be able to create a self-contained, highly per- meable reservoir thousands of feet under- ground by carefully fracturing rock through “stimulation” and monitoring progress with fluid tracers, seismic moni- tors, and other devices to help them see what they are doing. The trick is to pro- duce a highly fractured, permeable void in the middle of solid hot rock that will hold water in place while producers extract the heat. Significant fracturing is the key to increasing the surface area of the exposed rock, and effective water containment can minimize the need for recharging. “Enhanced geothermal systems are a big part of the future,” said Coleman. “The technology is conceptually very simple, but how you apply it in different rock for- mations is incredibly complex.” In theory, two boreholes are drilled, and the solid rock between them is fractured to allow the passage of fluid. Water is injected into one hole, and steam or hot water extracted from the second hole. This is easier said than done. It is not the idea, but the art and science of fracturing and increasing fluid conductivity between holes, that poses the greatest challenges. Part of EPRI’s work is to outline any real or per- ceived risks that might be involved in the process. One possibility is to use supercritical carbon dioxide (CO2) rather than water for hydraulic stimulation and fracturing. Supercritical fluids have the unique ability to diffuse through solids like a gas while retaining the properties of a liquid. Studies suggest that injecting supercritical CO2 to create an artificial reservoir and serve as a hydrothermal fluid might yield heat extraction rates 50% greater than those achievable with water. In addition to reducing costs and improving productiv- ity, this approach could afford an option for sequestering CO2 captured from fossil power plants. EPRI’s program in geothermal energy is working to enhance the effectiveness of proven technology; to bring emerging technology, such as low-temperature binary geothermal, into mainstream use while lowering costs; and to assist the pio- neering efforts in enhanced geothermal systems––all of which could reshape the entire landscape of geothermal energy over the next 20–30 years. All three avenues of development have a single objective: to expand the geographic footprint of cost- effective geothermal power generation, making these resources more widely acces- sible to power producers and consumers. This article was written by Brent Barker. Background information was provided by Travis Coleman, tcoleman@epri.com, 650.855.2009. Travis Coleman manages geothermal energy projects in EPRI’s renewable generation group. Prior to joining EPRI in 2009, he spent eight years at the Public Service Company of New Mexico on projects involving system modeling, resource planning, renewable energy resource assessment, and renewable energy generation development. Coleman holds a B.S. in mechanical engineering from the University of New Mexico and also completed graduate work in computational fluid mechanics. SUMMER 2011 17PDF Image | Expanding the footprint of Geothermal Energy
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