Unlike GHPs, geothermal plants produce electricity for consumption on a vast scale. They also are available only from relatively few sources deep underground , those with extremely hot water, at least 212 °F (100 °C) (Geothermal 101). Given current technology the range of usable heat is limited; "If the steam is much colder than 150 °C [302 °F], it will start to condense into water before it can be used to turn a turbine. On the other hand, steam hotter than 400 °C [752 °F], although richer in energy, is harder to find and to handle" (Economist).
Rather, a closed loop system recycles the hot water-it rises in the form of steam, powers the electrical turbines, then returns into the earth as a liquid. The cycle is virtually endless: "Once the water is returned to the geothermal reservoir, it is reheated by the Earth's hot rocks and can be used" repeatedly (Geothermal 101). Like such renewables as wind, geothermal, once installed, provides virtually free energy over an open-ended timespan. Unlike most renewables, geothermal has the advantage of being available at every time of day every season of the year. It is thus able to provide baseline power making it a reliable form of energy that can replace coal fired plants. Indeed geothermal has "average availabilities of 90% or higher, compared to about 75% for coal plants" (US DOE Geothermal).
There are three basic types of geothermal systems: Dry steam, Flash, and Binary. Dry steam, first used in Italy in 1904, employs hot steam directly into turbines that generate electricity. The Geysers, located in California, is the only such plant in the United States. Flash plants start with water from between 300 and 700 °F (148 and 371 °C) brought up through a well (Watson). This hot water is "flashed," or rapidly vaporized, at low pressure to become steam (US DOE Hydrothermal), spinning a turbine; the water then returns through an injection well to be reused. Binary plants use pressurized water passed through a heat exchanger, transferred to another liquid with a much lower boiling point-a working fluid-in an adjacent pipe. The working fluid becomes steam, powers a turbine, and is recycled back. This method is more expensive than others but works with lower temperatures.
Chena Hot Springs in Alaska is a unique example of a Binary plant. It employs water at 109 °F (43 °C) up to 165 °F (74 °C), the coldest ever for geothermal electricity. It does so by using "spring water to heat up R134a, a fluid hitherto employed mainly as a refrigerant. Since R134a has a relatively low boiling point, the water is hot enough to convert it into a gas" (Economist). Icy water then cools the fluid to be reused. The Chena Hot Springs plant is relatively low yield, but was estimated to save some 150,000 gallons of diesel fuel, or $540 million, in 2008 (Bryson). The plant was built for only $4 million using air-conditioner parts (Bryson), an innovative reconfiguring of existing technology, rather than building whole systems from scratch.
Like other renewable energy geothermal is clean, with emissions consisting largely of steam. Compared to fossil fuel, both in extraction and emission, impacts are tiny; "Unlike coal and natural gas, geothermal incurs no 'hidden costs' such as land degradation, high air emissions, forced extinction and destruction of animals and plants, and health impacts to humans" (Geothermal 101). Air pollution is virtually nonexistent: "Emissions of nitrous oxide, hydrogen sulfide, sulfur dioxide, particulate matter, and carbon dioxide are extremely low, especially when compared to fossil fuel emissions" (Geothermal 101). One recent anomaly was reported in Reykjavik, Iceland, where a geothermal plant was accused of releasing soot composed of hydrogen sulphide, darkening silverware and possibly harming local moss (Veal). However, officials at the geothermal plant, along with visiting geothermal experts, dispute this (Blodgett).
One other side-effect of geothermal energy is the possibility of triggering seismic activity. Some geothermal fields have triggered microearthquakes, too small to be felt by humans (Jennejohn, Blodgett & Gawell). Residents near The Geysers geothermal field have reported increased tremors, although distinguishing these from already-occurring natural activity is difficult (Ibid).
Currently, commercial scale geothermal plants must be located above an area that includes a combination of heat and water sufficient to produce steam. However vast heat resources exist underneath the earth, lacking the water needed for today's geothermal plants. If these could produce electricity, geothermal energy's reach would be greatly expanded. Enhanced Geothermal Systems (EGS) is the name for a method of doing just this.EGS relies on pumping liquid down to a sufficiently hot depth by drilling in two places perhaps a quarter mile apart, generating a set of fractures. Cold fluid is pumped down one well, resulting in steam at the other well (Dorreen). As with other forms of geothermal electricity, the steam then turns a turbine that powers a generator.
No commercial scale EGS currently exists, although, according to an MIT report, these could be working "in a 10- to 15-year period if we step up and do the engineering work right now" (Dorreen). One danger, is that "injecting fluid into hot, dry rocks occasionally triggers earthquakes. An EGS project in Basel, Switzerland, was suspended in 2006 after it triggered a 3.4-magnitude quake" (Smith et al). However, an Australian study reports the danger of earthquakes from EGS as low, and claims that it can be reduced with proper management (Jennejohn, Blodgett & Gawell). As the technology advances, EGS has tremendous potential to generate clean electricity around the world.
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Special thanks to the Geothermal Energy Association for their help with this Discovery Guide