In the geothermal power plants the hot water or steam from the geothermal reservoir is used to generate electricity. There are three types of geothermal power plants as described below:
1) In first type of geothermal power plants the geothermal reservoir produces hot steam and very little amount of water. Such reservoir is called dry steam reservoir. In these reservoirs you can see the fumes of steam rising above the surface of the earth (see figure below). In such cases the holes are drilled into the earth and steam is siphoned off via the pipes just like the cold drink is siphoned off by straw.
The steam obtained this way is passed through the turbine just like the conventional thermal power plants. However, in conventional power plants you need fuel to generate steam from water, but in this type of geothermal power plant you don’t need any fuel. The steam expands in turbine and rotates its blades. The shaft of the turbine rotates in the generator where electricity is produced. The steam is then passed to the cooling tower where it is cooled off. The largest geothermal power plant of such type is named The Geyser located 90 miles north of San Francisco.
2) The geothermal reservoir where hot water is generated is called as hot water reservoir. Hot water from this reservoir is used in the flash point type of geothermal power plants. Here, when the hot water is brought from the well to the surface of the earth some water flashes to steam, which can be used to rotate the turbine and generate electricity.
3) In many hot reservoir water is available at temperature of about 250-350 degree Fahrenheit and it cannot flash. In such cases binary power plants are used. Here, the hot water from geothermal reservoir is passed through the heat exchanger where it gives of the heat to chemical like isopentane. The chemical gets heated and converted into vapor. The vapors of chemical are used to rotate the turbine and generate electricity.
