Study shows geothermal could be an ideal energy storage technology

The best place to store energy for the electrical grid?  you could stand on it

An injection well at the Blue Mountain geothermal plant. Credit: Dennis Schroeder/NREL

For parts of the US, the best place to store massive amounts of power for the power grid might be right under our feet.

Geothermal energy, which is based on hot rock deep below the earth’s surface, has long been used as a source of heating and electricity generation. But recent advances in drilling technology have opened up new opportunities to deploy geothermal energy widely. He spurred researchers at Princeton University to demonstrate in a journal article Applied Energy that geothermal can also serve as an ideal technology for energy storage. Additionally, geothermal energy can supplement wind and solar power, providing energy when the sun is not shining or the wind dies down.

“In the western United States, where there is a lot of geothermal potential, this could be the missing piece of the puzzle to get to a carbon-free electricity system along with lots of wind and solar power and shorter-lasting batteries and demand flexibility.” said Jesse Jenkins, the project’s principal investigator and assistant professor of mechanical and aerospace engineering and the Andlinger Center for Energy and the Environment.

Geothermal is an ancient technology and has been used for heating for centuries. Boise, Idaho heats much of downtown with geothermal energy. In modern times, geothermal has expanded into the energy industry, powering heat pumps and supplying electrical power to the grid. The advantages of renewable energy technology include its constant generation, relatively low maintenance, and zero carbon production.

But for grid-scale electricity, geothermal remains a niche player. That’s because the technology requires specific locations. Primarily, engineers need hot geologic regions fairly close to the surface, fissured rock formations to serve as radiators, and access to fluids to move heat to the surface. (Here’s an overview of geothermal energy.) That is changing rapidly as engineers are developing new technologies with a view to vastly expanding geothermal electricity generation.

The key innovation harnesses technologies from the oil and gas industry, including directional drilling and hydraulic stimulation, to create artificial fracture systems wherever hot, impermeable rock can be found. If successful, companies commercializing these new techniques could unlock a clean, renewable resource capable of eventually supplying hundreds of gigawatts of power in the United States alone.

“That ability to get away from these very specific places where you have all the right things in the right place, to anywhere where you have hot enough rock accessible without drilling too deep, means enhanced geothermal can open up a much broader resource base.” .” Jenkins said.

It turns out that these novel techniques have another hidden advantage that has been overlooked until now. The water that circulates through the artificial fracturing system is contained within impermeable rock, meaning it can’t seep out, and that makes these geothermal reservoirs a great way to store large amounts of energy when demand is low and then release it when demand is high. . Storing energy and shifting production to the most valuable times increases geothermal profitability and acts as a perfect complement to climate-dependent variable renewable systems such as wind and solar.

“We run reservoir simulations to evaluate the systems we’re designing,” said Jack Norbeck, co-founder and CTO of Fervo Energy, a Houston-based development company pioneering these advanced geothermal technologies. The simulations showed that their geothermal systems could work to provide constant power, or base load, but also to store and exchange energy efficiently for later use. “We can operate them in both baseload and flex modes, which is a big step forward for geothermal technology.”

In 2020, Fervo engineers were confident that their system would work. But they wanted to know about systems economics and how to optimally integrate the technology into the power grid. Seeking answers, Fervo reached out to Jenkins, head of Princeton’s ZERO Lab.

“That’s exactly the kind of questions we love to see,” Jenkins said. “These are practical questions that will guide real-world decision-making, investment and innovation, but they haven’t been answered in the academic literature yet. So that’s the perfect project for us, something that is an open question in research where the answer matters today, immediately, for the decisions real people are making about how to allocate their time, money, and innovation efforts.”

Norbeck, CTO of Fervo, provided technical support for the study. He said the core of the idea was to combine thermal energy from underground rocks with mechanical energy from overlying rock layers. Fervo engineers use horizontal drilling techniques to create a series of injection and production wells that are connected to each other by many small channels in the rock, forming an underground reservoir some 10,000 feet below ground where water can be heated. . Rather than immediately using heated water to drive turbines for electricity, technicians direct the heated, pressurized water into the reservoir’s network of canals. Fluid builds up in the reservoir and flexes the rock, and that pressure can later be released to drive hot fluid to the surface to drive turbines to generate electricity.

The researchers showed that this system can be used to store and send electricity over a wide range of durations, from a few hours to many days at a time, setting it apart from most other storage technologies. “The efficiency depends on the geology and other characteristics of the rock,” Norbeck said. But overall, “it turns out that this form of energy storage turns out to be one of the cheapest forms of long-term energy storage.”

Wilson Ricks, a Ph.D. a candidate in mechanical and aerospace engineering and a ZERO Lab researcher, led the research and said the results of the study exceeded what he had initially expected.

“The idea seemed simple and elegant to me: You have this system, it has these inherent properties and maybe we can exploit them to store energy…almost like icing on a cake,” said Ricks, the paper’s lead author. “It turned out to be unequivocally more valuable in almost all contexts, and indeed a huge potential asset.”

The paper, The Value of Reservoir Energy Storage for Flexible Geothermal Power Delivery, was published in Applied Energy.

The initial document analyzed the impact of a plant, the first of its kind. But as the technology is deployed at scale, it may change the price of electricity or the dynamics of the market, so now the team is using long-term power capacity planning models to examine the long-term equilibrium outcome. term and the impact on the markets. The results of the first study helped Fervo demonstrate the added value of this novel storage method and win a very competitive grant from the Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E). The latest project is a joint effort of Fervo, Princeton’s ZERO Laboratory, Lawrence Berkeley National Laboratory, and Rice University and will include a field demonstration and real-world data collection on the performance of the artificial fracture network and the energy storage in the reservoir.

“This is the kind of thing that we find really exciting, where you can answer this kind of open-ended question with our power system modeling tools, which then leads directly to further investment and innovation and hopefully accelerates the adoption of technologies.” impacts that can help us address climate change,” Jenkins said.

The institute demonstrates the first small-scale pumped thermal energy storage system, the first of its kind

More information:
Wilson Ricks et al, The Value of Reservoir Energy Storage for Flexible Geothermal Power Dispatch, Applied Energy (2022). DOI: 10.1016/j.apenergy.2022.118807

Provided by Princeton University

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