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Nearly all of California’s geothermal electric power generation comes from areas where eruptions occurred in the past tens of thousands of years

The California Volcano Monitor is a new effort to spotlight the research and the stories behind California’s volcanic regions, with monthly articles written by our scientists in the style of the Yellowstone Caldera Chronicles and HVO’s Volcano Watch. This month’s contribution is by Shaul Hurwitz, a CalVO research hydrologist and specialist on geothermal systems. 

The molten rock (magma) in the Earth’s crust that drives volcanic eruptions can also provide heat for electricity generation. Geothermal energy is derived from heat within the Earth’s crust. The word geothermal comes from the Greek words geo (Earth) and therme (heat). Geothermal energy is a renewable energy source because heat is continuously produced inside the Earth. Hot groundwater is used to generate electricity, to heat buildings (“direct use”), and for bathing. A geothermal resource requires an abundant supply of heat and water and high permeability (a measure of the ability of rocks to allow the flow of a fluid, such as water).

Around the world, geothermal energy is commonly produced in areas with young volcanoes, and such is also the case in California. The USGS California Volcano Observatory aims to advance scientific understanding of volcanic processes and lessen the harmful impacts of volcanic activity in California and the USGS Geothermal Resource Investigations Project is focused on identifying, mapping, and assessing the geothermal resources, and evaluating the impacts of geothermal development. Thus, some USGS scientists research both the hazards posed by California volcanoes and the renewable energy they can provide.

The Geysers Geothermal Field near Clear Lake, California....
The Geysers Geothermal Field near Clear Lake, California. Photo by Julie Donnelly-Nolan.

In some of California’s geothermal plants (as in The Geysers Geothermal Field), energy is produced from steam pumped from deep reservoirs in the ground directly to a turbine, which drives a generator and creates electricity. In other geothermal power plants, energy is produced with binary cycle systems, where geothermal fluid exchanges heat with a "working" or "binary" hydrocarbon fluid with a much lower boiling temperature. Heat from the geothermal fluid causes the working fluid to flash to vapor, which then drives turbines. At most geothermal power plants, any water that is pumped out of the ground is later injected back underground to sustain pressure in the geothermal reservoir.

A short tour of California’s geothermal areas starts with The Geysers, which is the world’s largest geothermal electric power producer. It is located at the southwest margin of the Clear Lake Volcanic Field, approximately 70 miles (120 km) north of San Francisco, and contains a complex of 18 geothermal power plants over an area of about 30 square miles (78 square kilometers). These plants draw steam from more than 350 wells that produced about 20% of California's renewable energy in 2019. Since 1997 the Geysers reservoir has been recharged by injection of treated wastewater, following large pressure drops in the geothermal reservoir, and in 2003 the city of Santa Rosa began delivering treated wastewater to further replenish it.

In the foreground, muddy mounds sit in the midst of pools of bubbling water. Behind, a geothermal power plant vents steam.
A geothermal power plant near the Salton Sea in the Imperial Valley, with the Davis-Schrimpf mud pots (or mud volcanoes) in the foreground. Photo by Shaul Hurwitz in January 2016.

In southern California, the Imperial Valley Geothermal Area consists of eleven active power plants along the southeastern shore of the Salton Sea. The heat supply there is associated with volcanism of the Salton Buttes, where the most recent eruptions formed obsidian lava domes about 1,800 years ago. In the future, the Hell’s Kitchen geothermal project, currently under development, will use geothermal power to extract lithium sustainably from geothermal brine.

On the southwest side of the resurgent dome in Long Valley Caldera, the Casa Diablo geothermal power plant (also called the Mammoth Geothermal Complex) sits just outside of the town of Mammoth. The power plant taps into the caldera's hydrothermal system at a point where the hot water is only a few hundred feet below ground. The complex currently consists of 3 geothermal facilities generating 30 MW of power, but a new plant scheduled to come online in 2022 is expected to double the power generation.

The Coso volcanic field in eastern California has 38 rhyolite domes that formed within the past one million years, with the 17 youngest emplaced between ~ 100 ka and ~ 78 ka. Power plants here link to the southern California power grid and currently generate only a little more than 50% of the peak capacity.

Altogether, California’s active volcanoes sustain the largest geothermal electric generation capacity in the United States. In 2020, there were 40 operating geothermal power plants in the state which provided about 6% of California’s in-state electric generation. For comparison, 48% of California’s electricity was generated by natural gas, 16% by photovoltaic solar panels and 7% by wind turbines. Continued research on the hazards posed by California’s volcanoes, and parallel research on new sources of clean energy, will benefit all residents of California. This includes a future where geothermal power and heat can potentially play a key role in the transition to a renewable, decarbonized energy system.