D.H. Strongheart Sustainable Energy Technologies Dr. Stephen Wust October 26, 2010 Enhanced Geothermal Systems—or Hot, Dry Rock—as a Sustainable Source of Energy in the United States Introduction Geothermal energy 1 production in the United States has advanced at a rapid rate. The geothermal industry currently has 3086.6 MW of total installed capacity in the U.S., with an additional 7057.26 MW of new capacity currently under development (Jennejohn, 2010). The geothermal industry promises to comprise a significant portion of the U.S.’s renewable energy profile, though after about 30 years of exploration, the estimates of total developmental potential have not increased. (USDOE, 2008). This is an indication of the relative scarcity of potential sites appropriate for traditional geothermal energy production. As a recent U.S. Department of Energy report states, “The natural hydrothermal resource is ultimately dependent on the coincidence of substantial amounts of heat, fluids, and permeability in reservoirs, and the present state of knowledge suggests that this coincidence is not commonplace in the earth” (USDOE, 2008). Thus, the potential for traditional geothermal energy sources to meet the U.S. demand for electricity generation is greatly limited by geographic and geologic factors. However, the most abundant type of geothermal energy available, and perhaps the most environmentally sound, remains largely unexplored and unfunded. Enhanced—or Engineered—Geothermal Systems (EGS) involves the creation of an artificial, or controlled, hydrothermal reservoir, and can be implemented virtually anywhere in the world where a moderate amount of geothermal heat energy exists. EGS Technology and Production Potential The first EGS research well was drilled at Fenton Hill in Los Alamos, NM in 1974. A second, and more successful well, was drilled at the same site in 1981. The origins of the “Hot‐Dry Rock Geothermal” concept, as it was then called, is in the research of former Los Alamos National Laboratory (LANL) chemist Bob Potter (USDOE, 2010). Potter’s concept (see figure 1) was based on the notion that naturally‐occurring heat in the earth’s crust could be utilized at virtually any depth, by drilling a pair of wells and injecting them with pressurized water. The fluid pressure is used to open and propagate the fractures from an initial well, which creates artificial permeability in the rock (MIT, 1991). This highly permeable well is connected to a second well, which becomes the “production well”, while the first 1 In this paper, the term “geothermal energy” refers exclusively to hydrothermal energy production, which is the primary source of industrial geothermal generation in the U.S. Direct utilization of geothermal energy for heating, which does not necessitate a heat pump or power plant, is not included.
Transcript
D.H. Strongheart Sustainable Energy Technologies Dr. Stephen Wust October 26, 2010
Enhanced Geothermal Systems—or Hot, Dry Rock—as a Sustainable Source of Energy in the United States
Introduction
Geothermal energy1 production in the United States has advanced at a rapid rate. The geothermal industry currently has 3086.6 MW of total installed capacity in the U.S., with an additional 7057.26 MW of new capacity currently under development (Jennejohn, 2010). The geothermal industry promises to comprise a significant portion of the U.S.’s renewable energy profile, though after about 30 years of exploration, the estimates of total developmental potential have not increased. (USDOE, 2008). This is an indication of the relative scarcity of potential sites appropriate for traditional geothermal energy production. As a recent U.S. Department of Energy report states, “The natural hydrothermal resource is ultimately dependent on the coincidence of substantial amounts of heat, fluids, and permeability in reservoirs, and the present state of knowledge suggests that this coincidence is not commonplace in the earth” (USDOE, 2008). Thus, the potential for traditional geothermal energy sources to meet the U.S. demand for electricity generation is greatly limited by geographic and geologic factors. However, the most abundant type of geothermal energy available, and perhaps the most environmentally sound, remains largely unexplored and unfunded. Enhanced—or Engineered—Geothermal Systems (EGS) involves the creation of an artificial, or controlled, hydrothermal reservoir, and can be implemented virtually anywhere in the world where a moderate amount of geothermal heat energy exists. EGS Technology and Production Potential The first EGS research well was drilled at Fenton Hill in Los Alamos, NM in 1974. A second, and more successful well, was drilled at the same site in 1981. The origins of the “Hot‐Dry Rock Geothermal” concept, as it was then called, is in the research of former Los Alamos National Laboratory (LANL) chemist Bob Potter (USDOE, 2010). Potter’s concept (see figure 1) was based on the notion that naturally‐occurring heat in the earth’s crust could be utilized at virtually any depth, by drilling a pair of wells and injecting them with pressurized water. The fluid pressure is used to open and propagate the fractures from an initial well, which creates artificial permeability in the rock (MIT, 1991). This highly permeable well is connected to a second well, which becomes the “production well”, while the first
1 In this paper, the term “geothermal energy” refers exclusively to hydrothermal energy production, which is the primary source of industrial geothermal generation in the U.S. Direct utilization of geothermal energy for heating, which does not necessitate a heat pump or power plant, is not included.
well is use as the “injection well”. Water is circulated from the surface, gaining heat as it passes through the fractured rock network of the injection well (See figure 2). The heated water returns to the surface through the production well, and passes through an appropriately designed power plant (See figure 3)(MIT, 1991).
In theory, the EGS system envisioned by Potter should operate as a closed‐loop system, with no emissions of Green‐House‐Gasses other than water vapor and only minimal need for additional water input, since the initial influx of pressurized water can be cooled and reintroduced to the well. After decades of research and development work at the Fenton Hill EGS site, there is significant indication that EGS is a viable option for large‐scale energy production in the U.S. and around the world. Similar pilot projects have been inaugurated in England and in California, with similarly optimistic results regarding the energy production potential of EGS.
There is also ample recognition of the versatility of EGS technology. Where geothermal resources are minimal—i.e., less geothermal heat—ESG technology can be used in tandem with existing, fossil fuel‐based electricity production facilities, and can result in a 15% reduction in GHG emissions (MIT, 1991). Existing EGS production plants can easily be scaled‐up, with each additional set of parallel wells enhancing production capacity. The EGS system can even utilize urban sewer treatment plant effluents as pressurized fluid, and can produce potable water from the latter.
Figure 1. Bob Potter’s original proposal for the Hot‐Dry Rock geothermal energy system. Image from U.S. Department of Energy’s History of Geothermal Energy and Research and Development in the United States: Reservoir Engineering. Available from http://www1.eere.energy.gov/library/default.aspx?page=4
Figure 2. From U.S. DOE electronic document, The Basics of Enhanced Geothermal Systems.
Available at http://www1.eere.energy.gov/library/resultssearch.aspx.
Recent reports from MIT (MIT, 2006) and the U.S. Department of Energy
(USDOE, 2008) indicate that, with sufficient investment in EGS over the next 15 years, EGS production plants could easily comprise 100,000MW of competitivelypriced electricity generation in the U.S. by 2050. In fact, 100,000MW is the low end of a conservative estimate, based on 2% recovery rate of geothermal energy (MIT, 2006). While there is not enough data from long‐term EGS operation to make a precise estimate regarding a likely recovery rate, a period of record of several decades from a geothermal field in northern California know as “The Geysers” suggests that even a 10% recovery rate might be conservative (DOE, 2008). Challenges for ESG Electricity Production In order for Enhanced Geothermal System technology to join the ranks of wind and solar energy as a major source of sustainable energy in the U.S., a number of challenges will need to be overcome in the years ahead. Technological Readiness Even though EGS test fields have been in operation for nearly 40 years, they have not been numerous or well‐funded enough to allow for the refinement of EGS technology to be ready for the industrial scale production of energy. The Geothermal Technologies Program of the U.S. Department of Energy aims at “achieving EGS technology readiness by 2015”. (http://www1.eere.energy.gov/geothermal/). Economic Viability A great amount of data regarding EGS power plant operation under a variety of
Figure 3. From The Future of Geothermal Energy, MIT Press, 2006. Available at
different conditions and in a variety of locations is still needed before robust economic forecasts can be made about the economics of EGS energy production. Depending on the geology of a potential EGS site, water losses during reservoir operation may be higher or lower than the median figure of 2% given in a recent study (MIT, 2006), and higher water losses would significantly increase the price of an EGS plant’s operation. Economic forecasts for the future of EGS can become encouraging if the forecast assumes uncertainties such as ample investment and the future introduction of carbon taxes on currently dominant fossil fuel‐based energy production practices. Water Losses and Seismic Constraints In order to achieve steady‐state production of energy, water must be injected into an EGS well at a steady rate and at a minimum pressure. As the pressure of the injective water increases, existing fissures in the rock can be widened, increasing the rate of water loss. If current practices are not refined to minimize water loss, EGS technology could have a negative impact on scarce water resources in the Western U.S., which is precisely where the implementation of EGS technology is best suited. The injection of pressurized water into wells for EGS systems is also known to induce micro seismic activity. While no significant earthquakes have resulted from the operation of EGS systems, it remains an uncertainty in the long‐term viability of EGS technology. Insufficient Data Perhaps the most significant hindrance to the progress of EGS technology is the lack of sufficient data of the long‐term, steady state operation of EGS power plants. If EGS is to gain momentum in the sustainable energy portfolio of the United
States, it will require significant investment in the coming years. Investors, however, are hesitant to commit to EGS without ample evidence that it is a safe and economically viable energy production technology. Because there are only a small handful of EGS test plants in operation in the world today, data accumulates at a very slow rate, and it is thus difficult to draw concrete, broad‐based conclusions about EGS technology. Conclusion On paper, Enhanced Geothermal Systems technology is a very promising possibility for sustainable energy production in the U.S. and around the world. In the literature, one often encounters impressive facts and figures suggesting that EGS could provide a virtually endless supply of energy to the U.S. for centuries to come (DOE, 2008; MIT, 1991). However, in lieu of a sufficiently long period of record of the steady state operation of EGS power plants, and in lieu of a sufficient number of test facilities in a sufficiently varied geographical area, recent studies have been forced to make conservative estimates regarding EGS’s potential contribution to the sustainable energy portfolio in the U.S. It appears certain that, if EGS receives a significant investment from both the private and public sector, it will soon arrive on the scene as a major source for sustainable energy production around the world. References Tester, Jefferson W., Wood, David O., and Ferrari, Nancy A., 1991. Energy and the Environment in the 21st Century. Cambridge, Massachusetts: MIT Press. Brown, D.W. et al. Hot Dry Rock Geothermal Energy—An Emerging Energy Resource with Large Worldwide Potential. Found in: Jennejohn, Dan, 2010. U.S. Geothermal Power Production and Development Update. Geothermal Energy Association. Available from http://www.geo‐energy.org/ (Accessed October 27, 2010). DOE/United States Department of Energy, 2010. A History of Geothermal Energy Research and Development in the United States: Reservoir Engineering 19762006. Available from http://www1.eere.energy.gov/library/default.aspx?page=4 (Accessed October 27, 2010). DOE/ United States Department of Energy, 2008. An Evaluation of Enhanced Geothermal Systems Technology. Available from http://www1.eere.energy.gov/library/resultssearch.aspx?Page=4. (Accessed October 27, 2010). DOE/ United States Department of Energy. The Basics of Enhanced Geothermal Systems. Available from http://www1.eere.energy.gov/library/default.aspx?page=4. (Accessed October 27, 2010). MIT/Massachusetts Institute of Technology, 2006. The Future of Geothermal Energy: Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21st Century. Available from http://geothermal.inel.gov. (Accessed October 27, 2010).
