17-11-2012, 03:28 PM
Geothermal— The Energy Under Our Feet
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Introduction
The United States possesses vast underground stores of heat whose full potential has yet to be realized. The Earth’s interior reaches temperatures greater than 4,000°C (>7,200°F), and this geothermal energy flows continuously to the surface. The energy content of domestic geothermal resources to a depth of 3 km (~2 mile) is estimated to be 3 million quads, equivalent to a 30,000-year supply of energy at our current rate for the United States! While the entire resource base cannot be recovered, the recovery of even a very small percentage of this heat would make a large difference to the nation’s energy supplies. New low-temperature electric generation technology may greatly expand the geothermal resources that can be developed economically today.
Geothermal resources could meet a substantial portion of the nation’s energy needs in the 21st century. In fact, when including geothermal heat pumps (GHPs), geothermal energy is used in all 50 U.S. states today. The U.S. Department of Energy’s (DOE) Geothermal Technologies Program seeks to make geothermal energy the nation’s environmentally preferred baseload energy alternative. The Program’s mission is to work in partnership with U.S. industry to establish geothermal energy as an economically competitive contributor to the nation’s energy supply.
The purpose of the workshop was to re-examine domestic geothermal resource estimates. There were two guiding questions: what is the total potential accessible resource in the United States, and, given favorable circumstances and using existing practices with improved technology, and with institutional issues solved, how much of the resource is developable by 2015, 2025, and 2050? Resource types include hydrothermal, deep geothermal systems, co-produced, geopressured, direct use, and GHPs.
Resource Definitions
The total resource base in the United States, both renewable and non-renewable, is very large, with an energy content of over 657,000 billion barrels of oil equivalent (BBOE), or nearly 50,000 times the annual current rate of national energy consumption. Figure 1 shows graphically what the total resource base looks like, and in descending order, the values for the estimated accessible geothermal resource and estimated developable resource.
Description of Geothermal Resource Categories
Hydrothermal
A hydrothermal system is defined as a subterranean geothermal reservoir that transfers heat energy upward by vertical circulation of fluids driven by differences in fluid density that correspond to differences in temperature (see Figure 2). Hydrothermal systems can be classified into two types—vapor-dominated and hot water—depending on whether the fluid is steam or liquid water, respectively.
Most high-temperature geothermal resources occur where magma (molten rock) has penetrated the upper crust of the Earth. The magma heats the surrounding rock, and when the rock is permeable enough to allow the circulation of water, the resulting hot water or steam is referred to as a hydrothermal resource. Such resources are used today for the commercial production of geothermal power. They benefit from continuous recharge of energy as heat flows into the reservoir from greater depths.
Deep Geothermal Systems
Deep geothermal systems (a.k.a. enhanced geothermal systems or EGS) are defined as engineered reservoirs that have been created to extract heat from economically unproductive geothermal resources. The deep geothermal/EGS concept is to extract heat by creating a subsurface fracture system to which water can be added through injection wells. The water is heated by contact with the rock and returns to the surface through production wells, just as in naturally occurring hydrothermal systems.
Hydrofracturing and stimulation techniques are used widely in the oil and gas industry to extend production, and can be used to greatly extend and expand use of geothermal resources. Figure 3 gives a graphic idea of the domestic scope of geothermal resources at just 6 kilometers (3.7 miles), a nominal drilling depth in the oil and gas industry.
Geothermal Heat Pumps
Geothermal heat pumps (GHPs) use the Earth’s huge energy storage capability to heat and cool buildings, and to provide hot water. GHPs use conventional vapor compression (refrigerant-based) heat pumps to extract the low-grade heat from the Earth for space heating. In summer, the process reverses and the Earth becomes a heat sink while providing space cooling (see Figure 5). GHPs are used in all 50 U.S. states today, with great potential for near-term market growth and savings.
Geopressured Resources
The geopressured resource consists of deeply buried reservoirs of hot brine, under abnormally high pressure, that contain dissolved methane. Geopressured brine reservoirs with pressures approaching lithostatic load are known to occur both onshore and offshore beneath the Gulf of Mexico coast, along the Pacific west coast, in Appalachia, and in deep sedimentary basins elsewhere in the United States.
The resource contains three forms of energy: methane, heat, and hydraulic pressure. In the past, DOE conducted research on geopressured reservoirs in the northern Gulf of Mexico sedimentary basin, and operated a 1-megawatt (MW) power plant using the heat and methane from the resource (i.e., Pleasant Bayou, TX, 1989 – 1990).
Co-Produced Geothermal Fluids
Sometimes referred to as the ‘produced water cut’ or ‘produced water’ from oil and gas wells, co-produced geothermal fluids are hot and are often found in waterflood fields in a number of U.S oil and gas production regions (See Table 1). This water is typically considered a nuisance to the oil and gas industry (and industry is accountable for proper disposal), but could be used to produce electricity for internal use or sale to the grid.
Like geopressured resources, co-produced geothermal resources can deliver near-term energy savings, diminish greenhouse gas emissions, and extend the economical use of an oil or gas field. New low-temperature electric generation technology may greatly expand the geothermal resources that can be developed economically today.