The Geysers is a complex of 22 geothermal power plants, drawing steam from more than 350 wells, located in the Mayacamas Mountains 72 mi (116 km) north of San Francisco, California. The largest in the world, the Geysers has 1517 MW of active installed capacity with an average production factor of 63 % (955 MW). Calpine Corporation operates and owns 19 of the 22 active plants in the Geysers and is currently the United States’ largest producer of geothermal energy. Two other plants are owned jointly by the Northern California Power Agency and the City of Santa Clara’s municipal Electric Utility (now called Silicon Valley Power). The Bottle Rock Power plant owned by the US Renewables Group has only recently been reopened. Another plant is under development by Ram Power Corp, formerly Western Geopower, with operation set to begin in 2010. Since the activities of one geothermal plant affects those nearby, the consolidation of plant ownership at The Geysers has been beneficial because the plants operate cooperatively instead of in their own short-term interest.
The Geysers geothermal development spans an area of around 30 mi² (78 km²) in Sonoma, Lake and Mendocino counties in California, located in the Mayacamas Mountains. Power from The Geysers provides electricity to Sonoma, Lake, Mendocino, Marin, and Napa counties. It is estimated that the development meets 60 % of the power demand for the coastal region between the Golden Gate Bridge and the Oregon state line.
Steam used at The Geysers is produced from a greywacke sandstone reservoir, that is capped by a heterogeneous mix of low permeability rocks and underlaid by a Felsite intrusion. Gravity and seismic studies suggest that the source of heat for the steam reservoir is a large magma chamber over 4 mi (7 km) beneath the ground, and greater than 8 mi (14 km) in diameter. Unlike most geothermal resources, the Geysers is a dry steam field, which means it mainly produces superheated steam. Because the power plant turbines require a vapor phase input, dry steam resources are generally preferable. Otherwise, a two-phase separator is required between the turbine and the geothermal wells to remove condensation that is produced with the steam. The Geysers complex is now recharged by injecting treated sewage effluent from the City of Santa Rosa and the Lake County sewage treatment plant. This sewage effluent used to be dumped into rivers and streams and is now piped to the geothermal field where it replenishes the steam produced for power generation.
