Murray Grande.  Northern California Power Agency (NCPA), presented a program on the  Geysers geothermal electrical generation facility. He reviewed the history of the region as well as the chronology of  its development.
Murry Grande, a representative of NCPA, presented a program on geothermal electrical generation  taking place at The GVeysers geothermal field in northern Sonoma County. The Geysers is a unique geologic formation including a magma intrusion that is close to the surface. There is a covering of granitic material overlying this magma pocket that in turn is covered with metamorphic rocks that are able to hold water. Cracks in these  metamorphic rocks allow steam to be generated which is contained in vast underground chambers. The steam is under pressure and when brought to the surface is used to power turbines. These in turn produce electricity that is then fed to the grid. The Geyser is the largest dry steam geothermal field in the world and has the potential of generating 2.2 GW of electricity, although they are currently producing around 870 MW.
Murray gave a brief history of the region including its use as a tourist destination in the late 1800s and early 20th century. Electricity generation began in the 1930s. The early developers of the field were PG&E  as well as Unocal. CalPine purchased the power plants, and NCPA was able to purchase,deve;op and maintain 2 of the 21 operating plants.
The geology that makes the geysers unique includes a source of heat, permeable rocks, and availability of underground water. At the present time, the water, which was being depleted at a rapid rate, has been restored with effluent recovery from both Lake County and the city of Santa Rosa. The current cost of producing geothermal energy, which has by far the lowest associated emission of carbon dioxide of all powergenerating facilities other than wind and solar has made it uneconomical for the geysers facilities to function at full capacity. Despite this, they produce a significant amount of energy approximately 850 mW that helps supply the needs of at least 18 different communities throughout the state of California.
To facilitate the pumping of water from the treatment plants in Lake County, solar arrays have been constructed, which supply virtually all of the electrical energy required for pumping the water over a 26 mile course from Lake County to the Geysers. This is a very efficient use of green energy, to further an additional green energy project. In addition, because the steam is of lower pressure now than it was 15 or 20 years ago, the turbines in the generators have been redesigned to function more efficiently with low-pressure steam which helps to reduce the cost of producing power at this facility.
Steam produced at the geysers is thoroughly scrubbed, removing nearly all of the hydrogen sulfide and mercury before it is released into the atmosphere in the cooling towers. As a consequence, the air quality in Lake County is some of the best in the state, as they are downwind from this facility.
 More information is available at the NCPA website,