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Enhanced geothermal systems are well suited to store excess renewable power as heat
Geothermal systems carry warmth from Earth''s interior up to the surface for heating or electricity. But geothermal power plants are expensive to build, and will get even less economically viable as wind and solar power get cheaper and more plentiful. However, even as wind and solar grow, so does the need to store electricity from those temperamental sources.
A new proposal could solve those issues and bolster all three renewable technologies. The idea is simple—use advanced geothermal reservoirs to store excess wind and solar power in the form of hot water or steam, and bring up that heat when wind and solar aren''t available, to turn turbines for electricity.
"It would allow next-generation geothermal plants to break from the traditional baseload operating paradigm and earn much greater value as suppliers of wind and solar," says Wilson Ricks, a graduate student in mechanical and aerospace engineering at Princeton University.
Ricks, his Ph.D. advisor Jesse Jenkins, and Jack Norbeck, cofounder and chief technology officer of Houston-based advanced geothermal developer Fervo Energy, ran extensive simulations of such geothermal reservoir energy storage to see if the technical components of the system as well as the economics actually work out. They found that the systems could indeed store electricity over a range of time scales, from a few hours up to many days, as efficiently as lithium-ion batteries. Plus, says Ricks, "the storage capacity effectively comes free of charge with construction of a geothermal reservoir."
Their results apply only to enhanced geothermal plants, like the ones Fervo and other companies such as Cambridge, Mass.–based Quaise Energy and Seattle-based AltaRock Energy are developing.
Conventional geothermal systems drill wells into naturally occurring hydrothermal reservoirs. But these pockets of hot water deep underground do not exist everywhere. In the United States, for instance, they are mostly located in the west.
Enhanced geothermal systems (EGS) get around this geographical limitation by creating artificial reservoirs. Developers create fractures in hot, dry rock formations by drilling into or melting the rock, and then injecting water into the fissures. Production wells bring the heated water up for producing electricity. "For scales necessary to contribute to national or global electricity decarbonization, we need to be able to extract geothermal heat outside of conventional formations," Ricks says.
Fervo Energy raised US $138 million in venture capital funding in August to advance its technology. The company uses innovations from the oil and gas industry, such as horizontal drilling and distributed fiber-optic sensing, to create underground reservoirs. The company plans to use the new funds to complete two pilot projects, including one with Google in Nevada.
Once these EGS systems are in place, they would be ideal for storing energy as well as producing electricity. Excess wind or solar energy could be used to inject water into the artificial reservoirs, where it would accumulate and build up pressure. The production wells could then be opened up when electricity is needed.
"EGS reservoirs are created in rock formations that are naturally impermeable; everything outside the artificial reservoir is sealed off," says Ricks. "It''s very similar to a hydropower reservoir, where you choose when to have water go through the dam and generate electricity."
Depending on the geology and traits of the rocks, Ricks and his colleagues'' simulations found that the systems could store energy with up to 90 percent efficiency over one cycle. That''s comparable with lithium-ion and pumped hydro storage, he says. The cost, meanwhile, would be minimal compared to other energy storage technologies. It would require larger facilities on the surface, but the storage space would be effectively free, since the EGS reservoirs are being built for electricity anyway.
In January, the team received $4.5 million in funding from the Advanced Research Projects Agency–Energy (ARPA-E) to demonstrate a full-scale test of geothermal reservoir energy storage in the field. The detailed findings of their modeling study appear in a paper published recently in the journal Applied Energy.
This article appears in the December 2022 print issue as "Hot Rocks Best Batteries for Energy Storage."
Prachi Patel is a freelance journalist based in Pittsburgh. She writes about energy, biotechnology, materials science, nanotechnology, and computing.
Good article, would be happy to see a longer one with more details & numbers.
A related topic is home heating "geothermal" which seems to simply use local underground as heat sink/source as it is much more stable than the environment. A little hard to get details, would make a great Spectrum article.
Very interesting article!
By leveraging the inherent energy storage properties of an emerging technology known as enhanced geothermal, the research team found that flexible geothermal power combined with cost declines in drilling technology could lead to over 100 gigawatts'' worth of geothermal projects in the western U.S. — a capacity greater than that of the existing U.S. nuclear fleet. Such an innovation would transform geothermal energy from its niche status on the grid today into a major component of a decarbonized future. The researchers published their analysis January 15 in Nature Energy.
"People generally think of geothermal as this always-on, baseload energy source, but we''ve shown that there''s a lot of extra value to be had in operating these plants in a different way," said research leader Jesse Jenkins, assistant professor of mechanical and aerospace engineering and the Andlinger Center for Energy and the Environment.
Since the early 20th century, people have been harnessing the earth''s heat to produce electricity, but these conventional geothermal power plants require a specific set of conditions: hot, permeable rocks close to the earth''s surface, and some sort of fluid to transport heat up from underground. These requirements have limited traditional geothermal energy to only a handful of favorable locations in the western United States and Hawaii — places with naturally occurring geysers, volcanoes, and hot springs. Consequently, geothermal plants generated only 0.4% of the total electricity in the U.S. in 2022.
Yet advances in drilling and hydraulic fracturing technologies have unlocked the enhanced geothermal approach, which removes the need for permeable rocks and greatly expands access to the heat that already exists far beneath our feet.
In the approach, engineers dig two or more boreholes into impermeable rock that extend thousands of feet below the earth''s surface, then drill similar distances horizontally, and subsequently create fracture networks in the horizontal sections to connect those holes deep underground. Afterward, they inject a fluid into one borehole, which heats up as it travels through the fractures. That heated fluid can be pulled up through the other wells and subsequently used to generate electricity.
"Enhanced geothermal is much less geographically dependent than conventional geothermal, which is really only possible in a small number of ideal spots," said Wilson Ricks, first author of the study and a graduate student in mechanical and aerospace engineering. "With enhanced geothermal, you can open up wide swathes of the country — wherever you can dig down and find hot rocks close to the earth''s surface."
Efforts to develop and scale enhanced geothermal energy have historically focused on lowering the costs associated with drilling and other equipment. The U.S. Department of Energy unveiled an Enhanced Geothermal Shot in 2022, for example, which calls for a 90% cost reduction of enhanced geothermal technologies by 2035.
But in their research, the team found an alternate avenue to make geothermal power more market-competitive: flexibility.
By operating enhanced geothermal power plants flexibly — using the inherent energy storage capabilities offered by enhanced geothermal reservoirs to generate more or less energy as needed — the team found that the value of geothermal energy dramatically increased because it could complement and compensate for intermittent energy sources such as solar and wind.
"Wind and solar are now the cheapest sources of clean energy, but their downfall is their variability and weather dependency," said Jenkins. "You can earn a lot of additional revenue if you can find a way to shift your clean energy generation to times without wind and solar."
Fortunately for enhanced geothermal, it can do just that.
As Ricks explained, the impermeable rock required for enhanced geothermal also functions as a self-contained underground reservoir for heated fluid that acts like a giant energy storage system.
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