A new role for geothermal power

Solar power’s big problem, especially now that it is cheap, is its lack of reliability. I use reliability in a ‘it’s there when you need it’ sense. We know how to deal with variable demand for electricity but not variable supply. Specifically, we know electricity demand slumps midday and rises in the afternoon, peaking sometime in the evening. Consumer behavior drives this. Since electricity supply and demand must always remain in balance, dispatchable forms of electricity generation like hydropower and natural gas combine cycle turbines ramp up in the afternoon to match peak evening demand.

Solar’s supply curve does not match the demand curve well. Obviously, solar energy peaks in the middle of the day and declines in the afternoon, approaching zero around the time that electricity demand traditionally peaks. Batteries, of course, can shift this supply curve, but their high cost and difficulty to scale limit the vast potential of solar energy. However, one solution to the solar supply curve problem, though still unproven, brims with potential: geothermal energy.

The setting for geothermal

“How absurd,” you might think. “I know something about geothermal. You need really hot water, like at Yellowstone. There are few places in the United States or the world where steam or hot water flows out of the ground to power a turbine.” “What an astute critique,” I respond. I was recently in Hot Springs, North Carolina, a rare site in the eastern US with a natural hot spring and therefore, any geothermal power potential. No US geothermal plants exist east of a tiny one in New Mexico. So I agree that geothermal resources are quite limited unless you live in Nevada, Japan, or Iceland. Therefore, geothermal will never play much of a role on the power grid like solar will. “How, then, could geothermal power address solar’s inconsistency problem?” you ask. Let’s discuss.

The answer: presently expensive, yet unproven, enhanced geothermal power. Enhanced geothermal exploits the fact that anywhere on the planet, if you drill down far enough, rocks get hot. Deep inside Earth, radioactive decay constantly releases a virtually limitless quantity of heat. Even if steam or hot water does not flow out of the ground on its own accord, if you pump water down far enough to access Earth’s internal heat, get it to flow through rock fissures to absorb heat, then pump the water out, you can create effectively an artificial hot spring many places, in theory. The hot water produced could drive a turbine and generate electricity.

Similar precision drilling and fracking technology to that used by the oil and gas industry to make natural gas extraction possible in impermeable shale deposits could allow development of artificial geothermal reservoirs within similar impermeable subsurface rocks. Still, creating a good subsurface reservoir is the biggest challenge and source of uncertainty for enhanced geothermal. Any geothermal plant will have two wells, one pumping cold water down and one pumping hot water up. In conventional geothermal systems, water flows between these two wells easily because the underground rock is a permeable reservoir. For an enhanced geothermal plant to work, you must create a sufficiently large and directed network of fractures to link the two wells – artificial permeability. Alternatively, you could run a continuous loop of pipe underground, but this extracts heat from the ground inefficiently.

The benefit of building and holding pressure underground: shifting supply

If this well connectivity hurdle can be overcome, then the confined nature of an enhanced geothermal reservoir presents enhanced opportunity. If you continue pumping water into a confined reservoir and shut in the production well – stop pumping water out – pressure builds in the reservoir. Then, when you open the production well again, the excess pressure produces excess energy generation that can persist for several hours. Conventional geothermal is already a dispatchable resource – it easily ramps up or down – but enhanced geothermal better recaptures the energy production deferred during a low production period. This is because a conventional geothermal reservoir has higher permeability so does not hold pressure as well.

Finally, we come to the answer to the original question: how can geothermal energy complement solar energy? With a lot of solar on the grid, other sources of electricity supply are not needed mid-day. At this time, geothermal plants shut in their production wells, and the enhanced ones let pressure build. In the afternoon, when solar ramps down and electricity demand ramps up, geothermal plants rapidly ramp up production, and the enhanced ones generate excess power because of the accumulated pressure. The value-add of enhanced geothermal is this ability to shift power production from mid-day to later afternoon. Thus, the enhanced geothermal plant could play two simulaneous roles on the future grid. It both generates additional electricity supply, which will be needed as more buildings, industry, and vehicles electrify, and it functionally serves as short-term energy storage because of the unique ability to shut in production and recover that energy.

If enhanced geothermal played only the first role, its high cost would likely prove prohibitive for widespread adoption. (Largely because of cost, even conventional geothermal has fairly recent and limited adoption in the US.) But batteries are expensive. Because its second role means enhanced geothermal could displace some need to add lithium ion batteries to the grid, its role in a future, solar-heavy grid could become large and indispensable.

Optimism and outlook

One thought I have is that the desert Southwestern region of California, Nevada, Arizona, New Mexico, Southern Idaho, Eastern Oregon. and vicinity would play a critical role on this theoretical future grid with a lot of solar and enhanced geothermal. These states have a triumvirate of potential. They have open land, abundant solar energy, and a rich subsurface heat source. These are by far the best states for high-grade subsurface heat. Because they also happen to be sunny, these states could potentially generate an enormous quantity of cheap and carbon-free electricity. Were the technological and economic feasibility issues for enhanced geothermal to be overcome, the next bottleneck would be our national grid’s need for more long-distance transmission and interconnection infrastructure.

Of course, a solar and enhanced geothermal dominated grid is not currently feasible because enhanced geothermal is unproven commercially. I based this optimistic outlook on exploratory studies and modeling simulations (papers linked within this summary) from Fervo, an energy company, and Princeton’s ZERO lab, which focuses on the economic modeling of a decarbonized grid. This information is also presented well on David Roberts’ Volts podcast by the Princeton papers’ lead author, Wilson Ricks. In 2021, Fervo received a US department of energy grant for a field demonstration. I am excited to see the results. I hope they bolster, technologically and economically, the potential for a future society powered by the most sustainable sources of energy we have on planet Earth: the sun and internal heat from radioactive decay.