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An extraordinary amount of heat is trapped below Earth's surface, as erupting volcanoes show with their violence. Geothermal energy seeks to use this heat to generate electricity and warm up buildings and roads.
Roughly 8,000 megawatts of geothermal electricity are currently produced around the world, including about 2,800 megawatts in the United States, or less than one-half of 1 percent of the electricity the nation produces. There are three kinds of geothermal plants, which all rely on hot vapors to help drive electric turbines.
First, dry steam plants use steam from underground, which arises when water seeps into the Earth's crust and gets heated up. This is the oldest type of geothermal plant, first used in Larderello, Italy, in 1904 and still effective today. Nowadays, dry steam plants at The Geysers in Northern California represent the largest single generator of geothermal power in the world, according to the U.S. Department of Energy, with a net capacity of roughly 725 megawatts, enough to power 725,000 homes.
Next, flash steam plants draw up super-hot high-pressure water. When depressurized, this water then rapidly vaporizes or "flashes" into steam.
Last, binary cycle plants flows moderately hot geothermal water past another fluid such as isobutane, which boils at temperatures much lower than water. This fluid absorbs the heat, flashing to vapor used to drive turbines. Such moderately hot water is by far the most common resource, and most geothermal plants in the future will be binary cycle, according to the U.S. Department of Energy.
In California, the state with the largest amount of geothermal power online, geothermal energy accounts for roughly 5 percent of its electricity, while Iceland and El Salvador each use geothermal plants to supply roughly a quarter of their electricity, according to the Union of Concerned Scientists.
In addition to geothermal plants, geothermal heat pumps can be used to heat or even cool residential and commercial buildings. By the end of 2005, more than 600,000 geothermal heat pumps were installed in the United States, with new installations happening at a rate of 50,000 to 60,000 a year, according to the Geothermal Energy Association. "That's a lot of usage there, but it's still small compared with the overall U.S. heating and cooling market," said Cliff Chen, senior energy analyst for the Union of Concerned Scientists, a science advocacy group.
The principle of geothermal heat pumps is simple. Nearly everywhere, the upper 10 feet of the Earth's surface maintains a nearly constant temperature between 50 and 60 degrees Fahrenheit (10 and 16 degrees Celsius), making it warmer than the air above it during the winter and cooler in the summer. Geothermal heat pumps use pipes filled with water or a water-antifreeze mixture to absorb or release warmth.
A number of cities also pipe hot spring water under roads and sidewalks to melt snow and ice.
Geothermal energy is very clean. Geothermal fields produce just one-sixth of the carbon dioxide that a relatively clean natural-gas-fueled power plant generates, and very little if any of the nitrogen oxides or sulfur-bearing gases spewed out by coal plants that result in acid rain and smog, according to the U.S. Department of Energy. Binary cycle plants are closed loops, essentially releasing no emissions.
Also, geothermal energy is abundant and constantly available 24 hours a day, unlike wind energy, which depends on fickle breezes, or solar energy, which does not work well in the dark. And it is homegrown, making nations less dependent on foreign oil.
Geothermal energy is also well-proven. "The Geysers unit in California has been operating for more than 40 years," Chen said. "We have a lot of experience with this technology. It's not some pie-in-the-sky technical concept that's decades away from coming to fruition — it's being used right now."
Since geothermal energy comes from underground, geothermal plants also do not take up much space. "This minimizes the visual impact these can have in scenic areas," Chen noted.
Geothermal energy requires a lot of money up front before energy starts getting produced, roughly $2,500 per installed kilowatt in the United States, according to the U.S. Department of Energy. "It also requires relatively long lead times — we're talking three years minimum, often more like five," Chen said. "That's because you need to do some prospecting first, drill tests to look for the best places.
Geothermal plants do rely on hot steam or water from underground, which can deplete over time. To recharge The Geysers, wastewater from the city of Santa Rosa, Calif., is now getting piped into the geothermal field. "Injecting water into geothermal facilities can keep production up if that's the economically feasible thing to do," Chen said. "At The Geysers, it was — at others, it may not make sense. You might have to retire facilities if production declines past a certain level."
The best geothermal reservoirs are located in volcanically or seismically active places, such as the western United States, Alaska and Hawaii. It may be of limited use elsewhere, meaning it might just account for 20,000 to 30,000 megawatts at most in the United States, or roughly 4 percent of its existing capacity. "That's much less than the hundreds of thousands of megawatts or even millions that solar and wind potentially can generate," Chen said.
Still, advances in geothermal energy could make it more widely available. Enhanced geothermal systems, which drill miles down to access hot dry rock or magma, could lead to a 40-fold increase over present geothermal power generating capacity, or more than 100,000 megawatts, according to the U.S. Department of Energy.
However, such enhanced geothermal systems are not without risk. One site triggered four earthquakes in Basel, Switzerland, ranging from 3.1 to 3.4 on the Richter scale. Experts insist that site was not picked correctly, and that quakes should not be a concern in properly chosen locations.
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