The smartest people that I talk to have been mentioning the word Geothermal more and more frequently over the last 4 years or so.
I was lucky enough last week to interview Wilson Ricks, a Princeton PhD candidate who is working with Jesse Jenkins in a very active group that models potential clean energy futures.
Now they have a new paper, lead by Ricks, describing some of the most promising approaches for Geothermal.
Enhanced geothermal systems (EGSs) are human-made or enriched reservoirs within the Earth’s subsurface, from which heat can be extracted to produce geothermal energy. The energy produced by these reservoirs could serve as an alternative source of electricity, helping to mitigate carbon emissions.
So far, EGSs have been primarily viewed as potentially stable and reliable sources of electricity that could operate consistently over time. However, these geothermal reservoirs could potentially also store energy for longer before it is converted into electricity, rather than converting it all at a given point in time.
Researchers at Princeton University’s ZERO Lab and EGS developer Fervo Energy recently carried out a study exploring the potential impact of utilizing EGSs flexibly and storing the energy they generate on long-term ongoing efforts aimed at decarbonizing electricity generation. The findings of their project were published in Nature Energy.
“This paper was a collaboration between my research group at Princeton, the ZERO Lab, and Fervo Energy, an enhanced geothermal systems technology startup,” Wilson Ricks, co-author of the paper, told Tech Xplore. “The project was intended to explore the potential benefits of flexible operations for both EGS as an industry and for the decarbonized electricity grids in which it might participate.”
The idea that EGSs could be operated flexibly has been around for decades and was first demonstrated in the 1990s. Yet climate change and ongoing efforts aimed at decarbonizing power systems have re-awakened interest in this idea, ultimately inspiring Ricks and his collaborators to carry out their study.
“The recent resurgence of EGS development and the increasing need for flexibility and energy storage in electricity systems with large amounts of wind and solar power inspired us to take a more thorough look at the benefits of this approach,” Ricks explained.
“My co-authors and I first performed detailed simulations of EGS reservoirs undergoing flexible operations. We used these to develop an accurate representation of flexible EGS in GenX, the open-source electricity system planning model that the ZERO Lab develops and maintains.”
GenX is an optimization model developed by researchers at Massachusetts Institute of Technology (MIT) and the ZERO lab at Princeton, designed to make predictions about electricity demand, generation and storage. The model works by identifying the least expensive configurations for the electricity system given a pool of available technologies and various physical and policy constraints.
As part of their study, Ricks and his collaborators specifically used GenX to explore how the flexible operation of EGSs could impact the long-run deployment of geothermal power in the Western United States. In addition, they set out to determine whether these flexible operations could reduce the costs of decarbonized electricity systems in the same region.
“We tried to represent the available geothermal resource base in as much detail as possible, mapping availability at various temperatures and depths across the region,” Ricks said. “We also incorporated additional details like local hourly air temperature variability, which has an impact on the power output of some geothermal plants.”
The analyses carried out by this team of researchers suggest that EGSs could be a far more helpful resource for decarbonizing electricity than previously anticipated. The flexible operation of these systems and in-reservoir energy storage was found to greatly boost their contribution to decarbonization efforts, significantly reducing bulk electricity supply costs.
Enhanced geothermal has many similarities with both fracking for gas and oil, and leach mining for uranium. Some of the differences –
– Leach mining for uranium is now the largest source of the metal, and so provides ~five percent of the world’s electricity. Similarly, fracking has boosted US gas output to the point where the country has gone from being a net importer to the world’s largest producer and exporter. Enhanced geothermal is still at the trial stage, with some notable failures in its past (eg in Australia and Germany.)
– Uranium can be mined at the best sites in the world, and the resultant energy used anywhere – Australia, which bans civilian atomic energy (though not for its navy!) exports yellowcake to South Korea, where it is easily the largest source of low-carbon power. Similarly, gas can be sent anywhere on a continent (pipelines permitting), and anywhere in the world if you build an LNG terminal. Geothermal heat has to be used in the immediate area.
– Geothermal is so far low-grade heat. It can be used for electricity, though at a pretty low Carnot efficiency (~10%), or for district heating, greenhouses, etc. It is much less useful for industrial heat, in the region of 600 to over 1000C, for which gas reigns supreme. Current water-cooled reactors are in between. At about 320C, they give reasonable efficiency for power production (~35%), with the waste heat used in district heating and desalination, notably in China. Lead-, gas- and salt-cooled reactors, of which a few are running in China, Russia, and India, promise to give methane more competition, with temperatures approaching 600C so far.
– Fracked gas can be a potent source of atmospheric methane, from leaks along the supply chain, and from improperly capped depleted wells. That’s in addition to the CO2 from burning it, of course – about 60% of coal’s, per watt.
Some geothermal plants can have quite high emissions for a non-fossil power source, though much better than burning trees. The median emissions figure for New Zealand’s geothermal is about four times that for wind, with the worst case about double that. Presumably the only emissions from closed-loop geothermal would be those from drilling, though the fracked variant might be leakier. The largest CO2 emissions from uranium leach mines are from pumping the solution between bores. Using solar or SMRs to power the pumps would greatly diminish that. Entrained CO2 from ore-bearing sandstones is minimal.