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The Geysers is the world's largest geothermal field, containing a complex of 22 geothermal power plants, drawing steam from more than 350 wells, located in the Mayacamas Mountains approximately 72 miles (116 km) north of San Francisco, California.
For about 12,000 years, Indians built steambaths at the Geysers and used the steam and hot water for healing purposes and cooking. When Euro/Americans first entered the area, six Indian tribes inhabited the area around the Geysers, three bands of Pomo people, two bands of Wappo people, and the Lake Miwok people. According to Stephen Powers in 1871, "Wappo invalids were accustomed to wallow in the hot, steaming mud and pools, receiving benefit therefrom into their bodies." The Wappo also collected sulphur which they called te'ke and a Wappo village, named tekena'ntsonoma (teke sulphur + nan well containing water + tso ground + no'ma village) was located about 12 miles (19 km) southeast of Cloverdale and on the present-day Sulphur Creek.
The Geysers were first seen by Euro/Americans and named in 1847 during John Fremont's survey of the Sierra Mountains and the Great Basin by William Bell Elliot who called the area "The Geysers," although the geothermal features he discovered were not technically geysers, but fumaroles. Between 1848 and 1854, The Geysers was developed into a spa named The Geysers Resort Hotel, which attracted tourists including Ulysses S. Grant, Theodore Roosevelt and Mark Twain. The resort declined in popularity in the mid 1880s, and rebranded itself to appeal to lower-income people. In 1938, the main building was destroyed in a landslide although the bar/restaurant, small cabins and the swimming pool stayed open, despite another fire in March 1957, until about 1979. Unocal Corporation dismantled the remains of the resort in 1980.
The Geysers is the world's largest geothermal field spanning an area of around 30 square miles (78 km2) in Sonoma, Lake and Mendocino counties in California. 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. Unlike most geothermal resources, the Geysers is a dry steam field which mainly produces superheated steam.
Steam used at The Geysers is produced from a greywacke sandstone reservoir, capped by a heterogeneous mix of low permeability rocks and underlaid by a silicic intrusion. Gravity and seismic studies suggest that the source of heat for the steam reservoir is a large magma chamber over 4 miles (6.4 km) beneath the surface, and greater than 8 miles (13 km) in diameter.
The first geothermal wells drilled in Geyser Canyon were the first in the Western Hemisphere. The first power plant at the Geysers was privately developed by the owner of The Geysers Resort and opened in 1921, producing 250 kilowatts of energy to light the resort. In 1960, Pacific Gas and Electric began operation of their 11-megawatt plant at the Geysers. The original turbine lasted for more than 30 years and produced 11 MW net power.
By 1999 the steam to power extraction had begun to deplete the Geysers steam field and production began to drop. However, since October 16, 1997, the Geysers steam field has been recharged by injection of treated sewage effluent, producing approximately 77 megawatts of capacity in 2004. The effluent is piped up to 50 miles (80 km) from its source at the Lake County Sanitation waste water treatment plants and added to the Geysers steam field via geothermal injection. In 2004, 85% of the effluent produced by four waste-water treatment plants serving 10 Lake County communities was diverted to the Geysers steam field. Injecting treated water into the Geysers field increases the amount of power that can be generated.
The injection of wastewater to the Geysers protects local waterways and Clear Lake by diverting effluent which used to be put into surface waters, and has produced electricity without releasing greenhouse gases into the atmosphere.
According to the Lawrence Berkeley National Laboratory Earth Sciences division, seismicity was very low prior to the use of the Geyser steam field for geothermal energy, although this may have been the result of low seismic coverage of the area. Before 1969, there were no earthquakes above magnitude 2 recorded by the United States Geological Survey (USGS) in an approximately 70 square miles (180 km2) area around the Geysers. Studies have shown that injecting water into the Geysers field produces earthquakes from magnitude 0.5 to 3.0, although a 4.6 occurred in 1973 and magnitude four events increased thereafter. Even with increasing injection rates over time, the rate of magnitude 3 earthquakes has remained relatively unchanged since the 1980s, although the amount of earthquakes has increased significantly. A magnitude 4.5 earthquake struck near the Geysers on January 12, 2014. Despite the increases in the number of earthquakes and the fears of local residents, it is unlikely that a large earthquake will occur at the Geysers since there is no large earthquake fault or fracture nearby.
In 2005, abatement equipment was installed at two of the Geysers plants to reduce the amount of mercury released by the waste vapor even though the amount released was below the legal limit for such releases. The Geysers Air Monitoring Programs (GAMP) has shown limited releases of arsenic, but again below a significant level.
Power plants at the Geysers are of the dry steam power plant type, where the steam directly powers the generator. In general, the Geysers has 1517 MW of active installed capacity with an average production factor of 63% (955 MW).
Of nearly two dozen active plants in the Geysers in 2013, Calpine Corporation operated 19 plants in 2004 but only 15 in 2013. of nearly two dozen active plants in the Geysers. 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 U.S. Renewables Group, was reopened in 2007. In July 2009, AltaRock Energy planned to drill more than 2 miles (3.2 km) down to create an "enhanced geothermal" project which was abandoned when federal agencies asked for review. Another plant was under development by Ram Power Corporation, formerly Western Geopower, in 2010, but after Ram Power lost both its CEO and CFO in 2013; it was seeking a buyer for its Geysers property.
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