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Geothermal energy has been used throughout the world for thousands of years. Hot springs were used by the both the Romans and the Native Americans. In 1904, the first geothermal power plant was built in Larderello, Italy. This plant powered five light bulbs, demonstrating that geothermal power was possible. In 1960, the Geysers in California were developed to generate electricity. Another wave of geothermal development occurred in the 1980s, fueled by high electricity costs, and expensive fossil fuels. With today’s concern about energy, there is a renewed interest in geothermal power.
Generating Electricity
Geothermal energy is literally “heat from the ground.” The deeper one goes beneath the earth’s surface, the hotter it becomes. When water from the earth’s surface seeps into the earth, it absorbs the heat of the ground around it. This forms a geothermal reservoir.
While you could theoretically use geothermal energy to produce electricity from any point on the earth’s surface, in most places it is economically impractical. To be hot enough for electricity generation, the temperature should be at least 150C (300F). For most of the world, this means drilling down several kilometers, which would be prohibitively expensive. However, in volcanic areas, hot zones can be very close to the earth’s surface.
In addition to being close to the surface, the region must have a favourable geology to allow water to seep into the reservoir. Exploration is not an exact science, and finding suitable locations is the subject of much research. Typically, these regions are in areas where tectonic plates are moving apart from each other. On a local scale, a good geothermal prospect often has hot springs nearby, or areas where steam is emitted from the surface.
Once a suitable location is found, the technology to convert geothermal heat into electricity is well established. There are two types of technology used, depending on the type of geothermal source. A flash system uses steam to drive a turbine and generate electricity. A binary system relies on a heat exchanger, and vaporization of a secondary fluid to drive a turbine.
If the reservoir source is very hot (200C+) flash technology is used. When water is under very high pressure, the boiling temperature rises. When pumped to the surface where the pressure is lower, the superheated water boils and turns into steam, which is used to drive a turbine.
For lower temperature geothermal sources, there is not enough energy in the steam to effectively drive a turbine. For these lower temperature resources, binary systems are used to extract the energy and convert it to electricity. A binary system uses a secondary fluid that boils at a lower temperature. The process is similar to running a refrigerator in reverse. The geothermal water is run through a heat exchanger with the secondary fluid. The secondary fluid vaporizes when heated, and this vapor drives the turbine. The vapor condenses and the secondary fluid is circulated back into the heat exchanger, repeating the process.
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