A previous version of this article misstated the amount of carbon dioxide U.S. geothermal plants could capture. It is 12.8 million tons, not 12.8 billion tons. The article has been corrected.
The problem with direct air capture, however, has been that it takes energy — a lot of energy. Carbon dioxide only makes up 0.04 percent of ambient air, making the process of its extraction chemically and energy intensive. According to the U.N. Intergovernmental Panel on Climate Change, by 2100 the world needs to remove between 100 and 1,000 billion tons of carbon dioxide from the air to meet its most ambitious climate goals — or between 10 and 100 times China’s annual emissions. But if the energy powering that comes from fossil fuels, direct air capture starts to look less like a time machine than an accelerator: a way to emit even more CO2.
Now, however, a company is working to combine direct air capture with a relatively untapped source of energy: Heat from Earth’s crust. Fervo Energy, a geothermal company headquartered in Houston, announced on Thursday that it will design and engineer the first purpose-built geothermal and direct air capture plant. With the help of a grant from the Chan Zuckerberg Initiative, the company hopes to have a pilot facility online in 3 to 5 years.
If it works, it will be a way to produce carbon-free electricity, while reducing CO2 in the atmosphere at the same time. In short, a win-win for the climate.
“You have to have your energy from a carbon-free source” for direct air capture to make sense, said Timothy Latimer, the CEO of Fervo Energy. “Geothermal is a great match.”
The reason is heat from Earth’s core. The center of the planet, despite Jules Verne’s imaginings, is a molten core with temperatures reaching up to 11,000 degrees Fahrenheit. (According to the Energy Department, just 0.2 percent of Earth’s heat content could supply enough power for all of humanity for 2 million years — if only we could access it.)
Geothermal wells don’t, of course, get anywhere close to Earth’s core, but a geothermal well drilled just 1 to 2 miles into hot rocks below the surface can reach temperatures of up to 1,000 degrees. Water is pumped into the well, heated and returned to the surface, where it can be converted into steam and electricity.
Even after generating electricity, most geothermal plants have a lot of waste heat — often clocking in around 212 degrees. And conveniently, that happens to be the exact temperature needed to pull carbon dioxide out of an air filter and bury it underground.
Hélène Pilorgé, a research associate at the University of Pennsylvania who studies carbon dioxide removal, says that one of the main ways to pull CO2 out of the air is known as the “solid sorbent” method. Big fans draw air into a box with an air filter; the air filter is then heated to around 212 degrees to remove the CO2 for burial. That high temperature “fits well with the energy that geothermal can provide,” Pilorgé said.
Other renewables, like solar and wind, aren’t natural fits. Solar and wind can produce electricity, but they don’t produce high heat easily. (This is also why it’s hard to make things like steel without fossil fuels.) The energy needed for direct air capture, Pilorgé says, is about 80 percent heat and only 20 percent electricity.
According to one study co-authored by Pilorgé, if air capture were combined with all of the geothermal plants currently in the United States, the country could suck up around 12.8 million tons of carbon dioxide every year.
One company already uses geothermal to power its direct air capture: Climeworks, which has one carbon capture plant in Iceland and is building another. Iceland, with its hundreds of volcanoes and hot springs, is already a hot spot for geothermal — 85 percent of homes are currently heated by Earth itself. Climeworks built its carbon capture machinery on top of the already existing Hellisheidi geothermal plant.
By contrast, Latimer says, Fervo Energy will be able to experiment with how to build geothermal energy in a way tailor-made for capturing CO2. “It’s a totally unexplored place,” he said. “What would you do differently in the design of a geothermal power plant if you knew you were pairing it with a direct air capture facility?”
The company plans to design and engineer the plant at one of its demonstration sites — possibly in northwestern Nevada, central Utah, or another spot yet to be announced. It will likely be in the Western United States, where geothermal heat is more easily accessible.
There are still many details to work out. It’s not enough to simply take carbon dioxide out of the air — ideally, it also needs to be stored deep underground, often in porous stones filled with salty water. Pilorgé says that one of the challenges will be placing geothermal plants in areas where such aquifers are readily available.
One potential criticism of the project is that it focuses on drawing carbon out of the air instead of preventing it from getting there in the first place. After all, geothermal energy could play a key role in shifting the electricity grid over to renewables. Unlike wind and solar, a geothermal plant can be on all of the time, producing electricity even when the wind isn’t blowing or the sun isn’t shining.
Latimer says that there is a possibility that a single geothermal plant could have multiple uses. It could produce electricity when needed and suck CO2 out of the atmosphere when wind and solar are dominating the grid. But right now, he added, it’s hard to link geothermal to the electricity grid thanks to long waits to get connected.
There is also much more funding available for direct air capture than there is for geothermal alone. The Energy Department is offering up to $74 million for demonstration projects of new geothermal technologies but a whopping $3.5 billion to establish regional hubs for direct air capture. Geothermal has often been called the “forgotten renewable” — useful, but not as sexy or appealing as solar or wind.
“What we have here is a really compelling way to produce round-the-clock carbon-free electricity and heat,” Latimer said. “The question is what society prioritizes and what policy incentives are put in place.”
Still, if the combination of geothermal and direct air capture works, there will be a kind of poetic symmetry to it. The burning of fossil fuels — pieces of plants and animals crushed under high temperatures and pressure in Earth’s crust for millions of years — has sent planet-warming gases spewing into the atmosphere. It’s only fitting that high temperatures from the crust can pull it back out again.