Dry steam geothermal fields occur when the pressure is not much above atmospheric pressure, and the temperature is high. In this situation, water boils underground and generates steam at temperatures of about 165°C (330°F) and pressures of about 100 psi. The steam taken from such a geothermal well can be used directly to drive a turbine. Such dry steam geothermal plants like these are unique. The ones at Larderello, Italy, and The Geysers, California, use dry steam.
The dry steam fields of The Geysers were discovered in 1847 by a hunter looking for grizzly bear. Seeing the natural steam venting into the air, he later told his friends he thought he had found the “gates of Hell.” A resort was built in the area in the 1860s, featuring therapeutic hot springs. Electric power for the resort was provided in the 1920s by using the steam from several wells to run a steam turbine generator. Thus, the first geothermal power plant in the United States came into existence. Large – scale operations did not begin for another 30 years, when two small private power companies invited Pacific Gas and Electric (PG&E) to come into the area and test the dry steam wells that they had drilled. Engineering and economic conditions seemed right, so in 1960, the first unit of The Geysers went into operation, with an output of 11 MWe. The generating capacity reached 2000 MWe in the 1980s, but in 2010, it was down to less than 1000 MWe, with 18 power plants. Whereas some units have been running out of steam, new initiatives at reinjecting treated waste water from surrounding communities into the wells will extend the life of The Geysers. Nevertheless, The Geysers remains the largest complex of geothermal generating facilities in the world. In 1999, under divestiture, The Geysers was sold by PG&E to FPL Energy and later to Calpine Corporation.
During operation, the dry steam used for the turbines is at a temperature of 165°C (330°F) and a pressure of115 psi. Well depths are between 250 and 1500 m. The steam leaving the turbine goes to a condenser and then to a cooling tower. (The evaporated water from the cooling tower is seen as steam in Figure 18.6.) Some of the water leaving the cooling tower is returned to the condenser to cool the incoming steam, while the rest of the water is reinjected into the ground.