Geothermal power plants can provide an extremely reliable base-load capacity 24 h a day. There are three types of commercial geothermal power plants: dry fields steam, flash steam, and binary cycle. Dry steam sites are rare, with only five fields discovered in the world to date. Reservoirs that contain hot, pressurized water are more common. Flash steam power plants use resources that are hotter than 175°C. Before fluids enter the plant, their pressure is reduced until they begin to boil or flash. The steam is used to drive the turbine and the water is injected back into the reservoir.
Binary-cycle plants use geothermal resources with temperatures as low as 85°C. The plants use heat exchangers to transfer the heat of the water to another working fluid that vaporizes at lower temperatures. This vapor drives a turbine to generate power. This type of geothermal plant has superior environmental characteristics compared to others because the hot water from the reservoir, which tends to contain dissolved salts and minerals, is contained within an entirely closed system before it is injected back into the reservoir. Hence, it has practically no emissions. Binary power plants are the fastest growing geothermal generating technology.
Large-scale geothermal power development is currently limited to regions near tectonic plate boundaries, such as the western United States, Central America, Italy, the Philippines-Indonesia-Japan Pacific area, and East Africa. These areas are likely to be the most promising for large development in the near term. If current R&D efforts are successful, however, geothermal potential will expand to other regions.
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