There are three components to the geothermal resource base: (1) geothermal heating and cooling, or direct heating and cooling by surface or near-surface geothermal energy; (2) hydrothermal systems involving the production of electricity using hot water or steam accessible within approximately 3 km of Earth's surface; and (3) enhanced geothermal systems (EGS) using hydraulic stimulation to mine the heat stored in low-permeability rocks at depths down to 10 km and use it to generate electricity. Currently, geothermal heating provides approximately 28 GW of energy (mainly for heating and industrial applications). For example, municipalities and smaller communities provide district heating by circulating the hot water from aquifers through a distribution pipeline to the points of use. The barriers to increased penetration of direct geothermal heating and cooling systems are not technical, but with the high initial investment costs and the challenges associated with developing appropriate sites. The resource for direct heating is richest in the western states, and geothermal heat pumps have extended the use of geothermal energy into traditionally nongeothermal areas of the United States, mainly the Midwestern and eastern states. A geothermal heat pump draws heat from the ground, groundwater, or surface water and discharges heat back to those media instead of into the air. The electric heat pump is standard off-the-shelf equipment available for installation in residences and commercial establishments. There are no major technical barriers to greater deployment. The United States currently has 700,000 installed units and the rate of installation is estimated to be 10,000 to 50,000 units per year (NRC, 2009d). One barrier to growth is the lack of sufficient infrastructure (i.e., trained designers and installers) and another is the high initial investment cost compared to conventional space-conditioning equipment.
In terms of electricity generation, hydrothermal systems are mature systems relying on conventional power-generating technologies. Technology is not a major barrier to developing conventional hydrothermal resources, but improvements in drilling and power conversion technologies could result in cost reductions and greater reliability. There is some potential for expanding electricity production from hydrothermal resources and thus providing additional regional electricity generation. For example, a study of known hydrothermal resources in the western states found that 13 GW of electric power capacity exists in identified resources within this region (WGA, 2006). However, in general the potential for major expansion of electricity produced from hydrothermal resources in the United States is relatively small and concentrated in the western states.
Enhanced geothermal systems represent the much larger resource base—the theoretical potential EGS resource below the continental United States is over 130,000 times the total 2005 U.S. energy consumption (MIT, 2006). Though this resource is vast, it exists at great depths and low fluxes. Accessing the stored thermal energy would first require stimulating the hot rock by drilling a well to reach the hot rock, and then using high-pressure water to create a fractured rock region. Drilling injection and production wells into the fractured region would follow next, and the stored heat would then be extracted, using water circulating in the injection well. The heat extraction rate would depend on the site. EGS reservoirs can cool significantly during heat-mining operations, reducing extraction efficiency with time and requiring periodic redrilling, fracturing, and hydraulic stimulation. Even so, the MIT report assumes that the individual reservoirs would only last around 20 to 30 years. Other challenges include a general lack of experience in drilling to depths approaching 10 km, concerns with induced seismicity, the need to enhance heat transfer performance for lower-temperature fluids in power production, and improving reservoir-stimulation techniques so that sufficient connectivity within the fractured rock can be achieved. Further research and demonstration projects will thus be needed before EGS is deployed on large scales.
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