<\/a>1.1. What is geothermal energy?<\/strong><\/p>\n Heat has been radiating from the center of the Earth for some 4.5 billion years.\u00a0 At 6437.4 km (4,000 miles) deep, the center of the Earth hovers around the same temperatures as the sun’s surface, 9932\u00b0F (5,500\u00b0C) (Figure 1).\u00a0 Scientists estimate that 42 million megawatts (MW) of power flow from the Earth\u2019s interior, primarily by conduction<\/p>\n Geothermal energy is a renewable resource. <\/a>\u00a0 One of its biggest advantages is that it is constantly available.\u00a0 The constant flow of heat from the Earth ensures an inexhaustible and essentially limitless supply of energy for billions of years to come.<\/p>\n <\/a> The National Energy Policy Act of 1992 (Sec. 1202) and the Pacific Northwest Electric Power Planning and Conservation Act of 1980 (Sec. 12H, 839a(16), page 84) both define geothermal energy as a renewable resource.<\/em><\/p>\n<\/div>\n<\/div>\n Figure 1: Earth\u2019s Temperatures <\/strong><\/p>\n The uses of geothermal for heat and other purposes were indigenous practices across a variety of world cultures: \u201cThe Maoris in New Zealand and Native Americans used water from hot springs for cooking and medicinal purposes for thousands of years. Ancient Greeks and Romans had geothermal heated spas. The people of Pompeii, living too close to Mount Vesuvius, tapped hot water from the earth to heat their buildings. Romans used geothermal waters for treating eye and skin disease. The Japanese have enjoyed geothermal spas for centuries.\u201d <\/a><\/p>\n <\/a> Nersesian, page 334<\/em><\/p>\n<\/div>\n<\/div>\n A viable geothermal system requires heat, permeability, and water. Developers explore a geothermal reservoir to test its potential for development by drilling and testing temperatures and flow rates.<\/p>\n Rainwater and snowmelt feed underground thermal aquifers (Figure 2).\u00a0 When hot water or steam is trapped in cracks and pores under a layer of impermeable rock, it forms a geothermal reservoir.<\/p>\n Figure 2: The Formation of a Geothermal Reservoir<\/strong><\/p>\n At the Larderello, Italy dry steam field, Prince Piero Ginori Conti first proved the viability of geothermal power plant technology in 1904 (Figure 3). Larderello is still producing today.<\/p>\n <\/a>1.2. What is a baseload power source?\u00a0 What is a dispatchable power source?<\/strong><\/p>\n A baseload power plant produces energy at a constant rate.\u00a0 In addition to geothermal, nuclear and coal-fired plants are also baseload.\u00a0 Because the energy is constant, its power output can remain consistent nearly 24 hours a day, giving geothermal energy a higher capacity factor than solar or wind power, which must wait for the sun to shine or the wind to blow, respectively. <\/a> \u00a0This means a geothermal plant with a smaller capacity than a solar or wind plant can provide more actual, delivered electricity.<\/p>\n <\/a> \u201cCapacity\u201d and \u201ccapacity factor\u201d essentially refer to the distinction between megawatts (MW) and megawatt-hours (MWh).\u00a0 MW is a unit of power or the rate of doing work, whereas MWh is a unit of energy or the amount of work done.\u00a0 One MWh is equal to 1 MW (1 million watts) applied over the period of an hour.\u00a0 In geothermal development, one megawatt is roughly equivalent to the electricity used by 1,000 homes.\u00a0<\/em><\/p>\n<\/div>\n<\/div>\n A geothermal plant can also be engineered to be firm, flexible, or load following, and otherwise support the needs of the grid. <\/a> \u00a0\u00a0Most geothermal plants being built now have adjustable dispatching capabilities. In addition to geothermal, natural gas is dispatchable.\u00a0 \u00a0This means a geothermal plant can meet fluctuating needs, such as those caused by the intermittency of solar and wind power.<\/p>\n <\/a> GEA \u201cThe Values\u201d <\/em><\/p>\n<\/div>\n<\/div>\n <\/a>1.3. How does a conventional geothermal power plant work?<\/strong><\/p>\n After careful exploration and analysis, wells are drilled to bring geothermal energy to the surface, where it is converted into electricity.\u00a0 Figure 4 shows the geothermal installed capacity in the U.S. from 1975 to 2012, separated by technology type.<\/p>\n The USGS has defined moderate-temperature resources as those between 90\u00b0C and 150\u00b0C (194 to 302\u00b0F), and high-temperature geothermal systems as those greater than 150\u00b0C. <\/a><\/p>\n Figures 5-7 depict the three commercial types of conventional geothermal power plants: flash, dry steam, and binary.<\/p>\n In a geothermal flash power plant, high pressure separates steam from water in a \u201csteam separator\u201d (Figure 5) as the water rises and as pressure drops.\u00a0 The steam is delivered to the turbine, and the turbine then powers a generator.\u00a0 The liquid is reinjected into the reservoir.<\/p>\n Under one-third of the installed geothermal capacity in the U.S. is comprised of flash power plants, with the majority in California. <\/a><\/p>\n <\/a> GEA \u201cAnnual\u201d 2012, page 7<\/em><\/p>\n<\/div>\n<\/div>\n Figure 5: Flash Power Plant Diagram, <\/strong>Photo: Dixie Valley, NV, Flash Plant<\/strong><\/p>\n In a geothermal dry steam power plant, steam alone is produced directly from the geothermal reservoir and is used to run the turbines that power the generator (Figure 6).\u00a0 Because there is no water, the steam separator used in a flash plant is not necessary.\u00a0 Dry-steam power plants account for approximately 50% of installed geothermal capacity in the U.S. and are located in California. <\/a><\/p>\n Figure 6: Dry Steam Plant Diagram, Photo: The Geysers, CA, Dry Steam Plant <\/strong><\/p>\n In 1981 at a project in Imperial Valley, California, Ormat Technologies established the technical feasibility of the third conventional type of large-scale commercial geothermal power plant: binary. \u00a0The project was so successful that Ormat repaid its loan to the Department of Energy (DOE) within a year. <\/a>\u00a0 Binary geothermal plants have made it possible to produce electricity from geothermal resources lower than 302\u00b0F (150\u00b0C).\u00a0 This has expanded the U.S. industry\u2019s geographical footprint, especially in the last decade.<\/p>\n Binary plants use an Organic Rankine Cycle system, which uses geothermal water to heat a second liquid that boils at a lower temperature than water, such as isobutane or pentafluoropropane. This is called a working fluid (or \u201cmotive fluid\u201d in Figure 7).\u00a0 A heat exchanger separates the water from the working fluid while transferring the heat energy.\u00a0 When the working fluid vaporizes, the force of the expanding vapor, like steam, turns the turbines that power the generators.\u00a0 The geothermal water is then reinjected in a closed loop, separating it from groundwater sources and lowering emission rates further (see section 5).\u00a0 Most new geothermal plants under development in the U.S. are binary.<\/p>\n Figure 7: Binary Power Plant, Photo: Burdett, NV, Binary Power Plant Hybrid power plants allow for the integration of numerous generating technologies.\u00a0 In Hawai\u2019i, the Puna flash\/binary combined cycle system optimizes both flash and binary geothermal technologies.\u00a0 Geothermal fluid is flashed to a mixture of steam and liquid in a separator.\u00a0 The steam is fed to a turbine as in a flash-steam generator and the separated liquid is fed to a binary cycle generator (Figure 8).<\/p>\n Figure 8<\/strong>: Flash\/Binary Power Plant Diagram, Photo: Puna, Hawaii, Flash\/Binary<\/strong><\/p>\n Another type of hybrid plant is Enel Green Power\u2019s solar-geothermal plant in Stillwater, Nevada.<\/p>\n <\/a>1.4. How do geothermal heat pumps work?<\/strong><\/p>\n Animals burrow underground for warmth in the winter and to escape the heat of the summer.\u00a0 The same basic principle of constant, moderate temperature in the subsurface is applied to geothermal heat pumps (GHPs). <\/a>\u00a0 \u00a0GHPs utilize average ground temperatures between 40\u02daand 70\u02daF. <\/a>\u00a0 In 1948, a professor at Ohio State University developed the first GHP for use at his residence.\u00a0 A groundwater heat pump came into commercial use in Oregon around the same time. <\/a><\/p>\n <\/a> Geothermal Exchange Organization<\/em><\/p>\n<\/div>\n <\/a> U.S. Department of Energy \u201cA History\u201d<\/em><\/p>\n<\/div>\n<\/div>\n Figure 9: Geothermal Heat Pump Diagram<\/strong><\/p>\n GHP heating and cooling systems circulate water or other liquids to pull heat from the Earth through pipes in a continuous loop through a heat pump and conventional duct system.\u00a0\u00a0 For cooling, the process is reversed; the system extracts heat from the building and moves it back into the Earth loop.\u00a0 The loop system can be used almost everywhere in the world at depths below 10 ft to 300 ft.\u00a0 GHPs are used in all 50 states and are over 45% more energy efficient than standard heating and cooling system options. <\/a> <\/a> <\/a>1.5. How do direct use applications work?<\/strong><\/p>\n Geothermal heat is used directly, without a power plant or a heat pump, for applications such as space heating and cooling, food preparation, hot spring bathing and spas (balneology), agriculture, aquaculture, greenhouses, snowmelting, and industrial processes. \u00a0\u00a0Geothermal direct uses are applied at aquifer temperatures between 90\u02daF and 200\u02daF. <\/a><\/p>\n Examples of direct use applications exist all across the U.S. Boise, Idaho\u2019s Capitol Building uses geothermal for direct heating and cooling. \u00a0President Franklin D. Roosevelt frequented Georgia\u2019s healing hot springs and founded the Roosevelt Warm Springs Institute for polio treatment in 1927. And the City of Klamath Falls, Oregon began piping hot spring water to homes as early as 1900. <\/a><\/p>\n In a typical geothermal direct use configuration, geothermal water or steam is accessed and brought to a plate heat exchanger (Figure 10).\u00a0 New direct use projects in numerous states, including some on Indian reservations, are encouraged by the provisions of the Geothermal Steam Act Amendments passed by Congress in 2005 (see section 4).<\/p>\n <\/a> Geothermal Exchange Organization<\/em><\/p>\n<\/div>\n![](\"http:\/\/geo-energy.org\/images\/basics_clip_image002_0005.jpg\")
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<\/p>\n<\/a>Figure 4: Total U.S. Geothermal Installed Capacity by Technology (MW) 1975\u20132012<\/h4>\n
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\nHomeowners who install qualified GHPs are eligible for a 30% federal tax credit through December 31, 2016.\u00a0 They can be buried conveniently on a property such as under a landscaped area, parking lot, or pond, either horizontally or vertically (Figure 9).\u00a0 A GHP system can also direct the heat to a water heater unit for hot water use.<\/p>\n
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