Conventional electric plants are fairly simple devices when all is said and done. Most use coal, natural gas or oil to heat water, the water creates steam, and the steam turns a huge electric turbine. The nice thing about geothermal energy is that it is easily adapted to this type of electrical generation. The advantage is that there is no need to use fuel to heat the water to create steam, the water is already heated, sometimes to steam level, by geothermal processes. All you need do is add the turbine and you are off on running. For this reason, many areas of the country which have access to geothermal sources have been quick to tap those sources to create electricity.
There are three geothermal power plant technologies being used to convert hydrothermal fluids to electricity. The conversion technologies are dry steam, flash, and binary cycle. The type of conversion used depends on the state of the fluid (whether steam or water) and its temperature. Dry steam power plants systems were the first type of geothermal power generation plants built. They use the steam from the geothermal reservoir as it comes from wells, and route it directly through turbine/generator units to produce electricity. Flash steam plants are the most common type of geothermal power generation plants in operation today. They use water at temperatures greater than 360°F (182°C) that is pumped under high pressure to the generation equipment at the surface. Binary cycle geothermal power generation plants differ from Dry Steam and Flash Steam systems in that the water or steam from the geothermal reservoir never comes in contact with the turbine/generator units.
Dry Steam Power Plants
Steam plants usually rely on tapped geothermal pools where the water is already under pressure and generating steam. In these types of plants a production well has been drilled which taps the steam from an underground source. The steam goes directly to a turbine, which drives a generator that produces electricity. The steam eliminates the need to burn fossil fuels to run the turbine. (Also eliminating the need to transport and store fuels!) Once the steam has passed through the turbine it condenses and is pumped back into the earth via an injection well, thus creating a closed loop. One of the real advantages of this type of approach is that these types of systems emit very little excess steam and gasses and very little in the way of hydrocarbons into the atmosphere.
This is the oldest type of geothermal power plant. It was first used at Lardarello in Italy in 1904, and is still very effective. Steam technology is used today at The Geysers in northern California, the world's largest single source of geothermal power. These plants emit only excess steam and very minor amounts of gases.
Flash Steam Power Plants
Many underground geothermal sources do not have sufficient heat or pressure to use steam directly. However, where the hydrothermal fluids are above 360°F (182°C) another process can be used called flash vaporization. In this type of process fluid is sprayed into a tank held at a much lower pressure than the fluid, causing some of the fluid to rapidly vaporize, or "flash." The vapor then drives a turbine, which drives a generator. If any liquid remains in the tank, it can be flashed again in a second tank to extract even more energy.
Binary-Cycle Power Plants
Most geothermal areas contain moderate-temperature water (below 400°F). With these types of sources the best approach to extracting the energy is to use a heat exchanger. In this type of approach geothermal fluid and a secondary (hence, "binary") fluid with a much lower boiling point than water pass through a heat exchanger. Heat from the geothermal fluid causes the secondary fluid to flash to vapor, which then drives the turbines. Because this is a closed-loop system, virtually nothing is emitted to the atmosphere. Moderate-temperature water is by far the more common geothermal resource, and most geothermal power plants in the future will be binary-cycle plants.
The Future of Geothermal Electricity
Steam and hot water reservoirs are just a small part of the geothermal resource. The Earth's magma and hot dry rock will provide cheap, clean, and almost unlimited energy as soon as we develop the technology to use them. In the meantime, because they're so abundant, moderate-temperature sites running binary-cycle power plants will be the most common electricity producers. Before geothermal electricity can be considered a key element of the U.S. energy infrastructure, it must become cost-competitive with traditional forms of energy. Geothermal companies are currently working to achieve of 3-5 cents per kilowatt-hour which would be very competitive with natural gas and coal plants. This will be even be more the case if these conventional coal and gas plants are taxed for the economic impact of their carbon emissions. Bills to do this are currently being considered by Congress and are likely in the future.
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