Find answers to frequently asked questions about the California Energy Commission’s Geothermal Grant and Loan program and learn more about the use of geothermal energy in California.
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Geothermal energy is energy that is continually created from naturally occurring heat from within the earth. This energy is used for many purposes including generating electricity.1
Geothermal resources are reservoirs of hot salty water (brine) or steam that naturally exist at varying temperatures and depths below the earth's surface. Geothermal wells, ranging from a few feet to several miles deep, can be drilled into underground reservoirs to tap steam or very hot brine that can be brought to the surface for use in a variety of applications, including electricity generation and heating and cooling. In the United States (U.S.), most geothermal reservoirs are in the western states.2
Additional introduction to geothermal energy is available via this video-YouTube created by the U.S. Department of Energy. It discusses the following benefits of geothermal energy:
- Renewable—Geothermal energy is a renewable energy heat source found under the surface of the earth. The heat flowing from Earth’s interior is continually replenished by the decay of naturally occurring elements and will remain available for billions of years.
- Clean—Geothermal life cycle GHG emissions are as low as solar, wind, and hydropower.3
- Consistent and Available—Geothermal power plants produce electricity consistently and can run essentially 24 hours per day/7 days per week, regardless of weather conditions.
- Domestic—U.S. geothermal resources can be harnessed for power production without importing fuel.
- Small footprint—Geothermal power plants are compact. They use less land per gigawatt-hour of energy produced than comparable-capacity coal, wind, and solar photovoltaic (PV) power plants.
Geothermal energy also has the potential to provide flexible (or dispatchable) power. Flexible operation can be achieved through advanced well flow control, power system configuration, and adoption of surface energy storage systems. Flexibility in power generation is needed to maintain supply–demand balance in the electrical grid.
1 Paz, Silvia (Chair), Ryan E. Kelley (Vice Chair), Steve Castaneda, Rod Colwell, Roderic Dolega, Miranda Flores, James C. Hanks, Arthur Lopez, Luis Olmedo, Alice Reynolds, Frank Ruiz, Manfred Scott, Tom Soto, Jonathan Weisgall. 2022. Report of the Blue Ribbon Commission on Lithium Extraction in California. California Energy Commission. Publication Number: CEC-300-2022-009-F.
2 U.S. Department of Energy Staff. n.d. Geothermal Technologies Office. U.S. Department of Energy.
3 Geothermal Technologies Office. n.d. Geothermal Energy Fact Sheet. U.S. Department of Energy.
Geothermal power plants draw brine from underground reservoirs to the surface to produce steam. This steam then drives turbines that generate electricity. There are three main types of geothermal power plant technologies: dry steam, flash steam, and binary cycle. The type of heat conversion is part of the power plant design and generally depends on the brine (steam or water) and its temperature:
Dry Steam Power Plant
Dry steam plants use hydrothermal fluids that are already mostly steam, which is a relatively rare natural occurrence. The steam is drawn directly to a turbine, which drives a generator that produces electricity. After the steam condenses, it is frequently reinjected back into the reservoir. Dry steam power plant systems are the oldest type of geothermal energy technology, first used in Lardarello, Italy, in 1904. Steam technology is still relevant today and is currently in use in northern California at the Geysers, the world's largest single source of geothermal power with 850 Megawatt (MW) of installed capacity.4
Flash Steam Power Plant
Flash steam plants are the most common type of geothermal power plants in operation today. Fluids at temperatures greater than 182°C/360°F, are pumped from deep underground, then travel under high pressures to a low-pressure tank at the earth’s surface. The change in pressure causes some of the fluid to rapidly transform, or “flash,” into vapor. The vapor then drives a turbine, which drives a generator to produce electricity. If any liquid remains in the low-pressure tank, it can be “flashed” again in a second tank to extract even more energy.5 Flash-steam power plants are the most common types of geothermal power plants.
Binary-Cycle Power Plant
Binary-cycle geothermal power plants can use lower temperature geothermal resources, making them an important technology for deploying geothermal electricity production in more locations. Binary-cycle geothermal power plants differ from dry steam and flash steam systems in that the geothermal reservoir fluids never come into contact with the power plant’s turbine units. Low-temperature (below 182°C/360°F) geothermal fluids pass through a heat exchanger with a secondary, or "binary," fluid. This binary fluid has a much lower boiling point than water, and the modest heat from the geothermal fluid causes it to flash to vapor, which then drives the turbines, spins the generators, and creates electricity.6 Some geothermal fields in Nevada are using binary geothermal power plants.7
Please visit CEC’s webpage on Data on Renewable Energy Markets and Resources and click on the Geothermal Energy menu for more information regarding current geothermal power plants in California.
4 United States Department of Energy staff. n.d. Electricity Generation U.S. Department of Energy.
5 Ibd.
6 Ibd.
7 Ayling, Bridget. 2020. 35 Years of Geothermal Power Generation in Nevada, USA.. A Review of Field Development, Generation and Production Histories. Stanford University.
A naturally occurring traditional geothermal system requires specific geological conditions, with three key elements to generate electricity: heat, salty fluid (brine), and permeability (when water can move freely through the underground rock). Such areas are typically associated with volcanoes and fumaroles (holes in the earth where volcanic gases are released), hot springs, and geysers.8
In many areas, however, the underground rock is hot but there is not enough natural permeability or fluids present. In those cases, an enhanced geothermal system (EGS) can be used to create a human-made reservoir to tap the heat for geothermal energy.9
In an EGS, fluid is injected deep underground under carefully controlled conditions, which cause pre-existing fractures to re-open and grow, creating permeability. Increased permeability allows fluid to circulate throughout the now-fractured hot rock, and the fluid becomes hot as it circulates. Then, operators pump the hot fluid up to the surface, where it generates electricity for the grid10, using one of the geothermal energy technologies discussed in the previous section above.
EGS technology is still under research and testing, but it has the potential to facilitate geothermal development beyond traditional known geothermal resource regions, thereby extending geothermal energy production nationwide. EGS advances are being demonstrated worldwide today, with a recent example being the 3.5 MW geothermal power plant located in northern Nevada and co-developed by Google and Fervo Energy.
8 Department of Energy Staff. n.d. Enhanced Geothermal Systems. Department of Energy.
9 Ibid.
10 Ibid.
According to the U.S Geological Survey, lands shall be classified as a KGRA when "the prospects for extraction of geothermal steam or associated geothermal resources from an area are good enough to warrant expenditures of money for that purpose.”11 A KGRA is influenced by such geologic factors as the pattern of temperature gradient, structure, stratigraphy, porosity, conductivity, permeability, heat source, and rate of recharge of fluids. They are areas where temperatures are higher than normal and where rock has many small holes that liquid and air can pass through, which is known as porous and permeable. The following KGRA’s characteristics were determined by the U.S Geological Survey:
- Low-temperature geothermal reservoirs are less than 90°C.
- Moderate-temperature geothermal resources are 90° to 150° C.
- High-temperature geothermal resources are greater than 150°C.12
11 Godwin, L.H., L.B. Haigler, R.L. Pioux, D.E. White, L.J.P. Muffler, R.G. Wayland. n.d. Classification of public lands valuable for geothermal steam and associated geothermal resources. U.S. Department of the Interior.
12 Williams, Colin, Marshall Reed, Robert Mariner. 2008. A review of methods applied by the U.S. geological survey in the assessment of identified geothermal resources. U.S. Geological Survey.
Geothermal energy is a source of renewable energy in California. The Geysers, the world’s largest geothermal field, is located across Sonoma, Lake, and Mendocino counties. Other major geothermal locations include the Salton Sea area in Imperial County, the Coso Hot Springs area in Inyo County, and the Mammoth Lakes area in Mono County.13 Due to these large geothermal fields, California has almost 60 percent of identified geothermal resources in the United States, followed by Nevada with 15.4 percent and Alaska with 7.5 percent. California also has 38 percent of the unidentified geothermal resources in the country.14 Figure 1 shows the main KGRA’s in California highlighted in orange with the geothermal power plants installed in these KGRA’s identified in green.
Figure 1: California Map of Known Geothermal Resource Areas and Geothermal Power Plants
Source: CEC Staff
13 California Energy Commission Staff. 2022. Known Geothermal Resource Areas. California Energy Commission.
14 Williams, Collin, Marshall Reed, Robert Mariner, Jacob DeAngelo, Peter Galanis. 2008. Assessment of moderate – and high-temperature geothermal resources of the United States. U.S. Geological Survey.
Geothermal energy resources consist of the following main applications:
- Direct use
- Natural hot springs
- Heat pumps
- Heating and cooling
- Electricity production
- Obtaining critical minerals from geothermal brine15 16, such as
- Lithium
- Magnesium
- Zinc
15 Ventura, Susanna, Srinivas Bhamidi, Marc Hornbostel, Anoop Nagar. 2020. Selective Recovery of Lithium from Geothermal Brines. P.5 California Energy Commission. Publication Number: CEC-500-2020-020.
16 Department of Energy Staff. n.d. What Are Critical Materials and Critical Minerals? Department of Energy.
California’s goal to reduce its carbon footprint utilizes clean, reliable, and renewable energy, such as geothermal energy to support its grid. In 2022, geothermal energy represented 2,693 MW of installed in-state generation capacity.17 18 The U.S. Geological Survey estimates that untapped geothermal resources in the United States could supply the equivalent of 10 percent of today’s energy needs, with 60 precent of the total power generated along the Northern California coast coming from geothermal energy.
A key mandate advancing the use of renewable energy has been the Renewables Portfolio Standard (RPS), which requires California load-serving entities (LSEs) to increase their procurement of eligible renewable energy resources (solar, wind, geothermal, biomass, and small hydroelectric) to 33 percent of retail sales by 2020 and 60 percent of retail sales by 2030. In 2021, the RPS-Eligible renewable energy represented 37.2 percent of California retail sales, where RPS-eligible geothermal contributed with 13,826 GWh energy – about 15.5 percent of total estimated 2021 RPS-eligible electric generation.19
17 Nyberg, Michael. 2023. In-state electric generation by fuel type (GWh). California Energy Commission.
18 California Energy Commission Staff. 2020. Estimated annual RPS-certified renewable energy. California Energy Commission.
19 California Energy Commission Staff 2020. RPS-Certified Renewables. California Energy Commission.
The California Energy Commission’s (CEC) Geothermal Grant and Loan Program promotes the development of California’s geothermal resources and supports innovative technologies by offering grant and loan opportunities and other resources to participating parties. Since the inception of the Geothermal Grant and Loan Program in 1980, it has awarded 158 projects and provided $60,358,059 funds throughout California.
Mission of the Geothermal Grant and Loan Program:
- Mitigate any adverse impacts caused by geothermal power development and lithium extraction from geothermal brine.
- Help local jurisdictions offset the costs of providing public services necessitated by geothermal development and related activities.
- Provide funds to local jurisdictions and private entities to support the development and maintenance of California’s geothermal resources and lithium recovery in support of the State’s energy and climate goals (see Geothermal Grant and Loan Program: Frequently Asked Questions about Lithium Recovery from Geothermal Brine in California).
To obtain more information on grant funding and loan opportunities and updates on geothermal energy, we encourage you to subscribe to the geothermal and Electric Program Investment Charge (EPIC program) listservs. Information about subscribing can be found at the links below on the right column – where it reads “subscribe”