Combining carbon-free energy with permanent storage of CO2 creates a more cost-effective self-sustaining loop.

A research team at Ohio State University (OSU) has proposed the combination of carbon capture with geothermal energy in a cheap, novel method that could make capturing carbon dioxide from the air a viable option. Their system recycles some of the captured CO2 to transport geothermal energy in a closed loop that can make large-scale direct air capture cheaper and more efficient.

Reported in the journal Environmental Research Letters, Martina Leveni and Jeffrey Bielicki have proposed a near-self-sustaining process of carbon capture that would justify the construction of dozens more atmospheric siphons, usually a prohibitive expense slowing the technology's adoption in the United States.

Unlike traditional forms of carbon removal, which involve capturing carbon dioxide at the point of emissions, direct air capture draws carbon dioxide out of the general atmosphere. (Using today's means, pulling emissions from a smokestack also reduces a power plant's efficiency.)

"Carbon removal technologies are especially helpful in mitigating climate change because we can capture types of emissions that would be hard to cap in other ways," said Martina Leveni, lead author of the study and a postdoctoral scholar in civil, environmental, and geodetic engineering. "So, we thought, could we combine technologies that could be beneficial to one another to meet this goal more efficiently?"

Most high-tech carbon capture methods are still in the early stages of development, with direct air technologies using giant fans to blow air over special chemicals that soak up carbon dioxide – methods that require substantial investment, compete for land use, and are energy-intensive to operate.

Scrubbing the air

In the foothills just outside Reykjavík, Iceland sits Orca, one of the earliest examples of direct air capture. This still-running product of Swiss decarbonization company Climeworks, which is powered by waste heat from the geothermal power plant next door, draws air through a chemical filter that extracts CO2.

It pumps the captured CO2 more than 700 meters underground to combine with basalt rock, transforming it into a solid mineral – sequestering a modest 50 tons of carbon dioxide a year and sports little to no carbon footprint beyond the materials used in its construction.

This synergy between green technologies can go a long way to driving financial and carbon costs down, which is the inspiration behind Martina Leveni and Jeffrey Bielicki's investigations toward finding a way carbon capture and geothermal energy technologies could benefit each other.

Called Direct Air CO2 Capture with CO2 Utilization and Storage (DACCUS), the OSU team's proposal would store carbon dioxide captured from air in deep saline aquifers.

The naturally-occurring heat could be used directly, or converted into electricity, to further power the process and continuously produce renewable energy for DACCUS systems. The captured CO2 would be isolated in the same geologic formations, and part of it can be circulated to extract the geothermal heat. This circulation brings the heat to the surface, where it can either be used directly or converted to electricity to power the system.

To demonstrate the self-sustaining potential, the duo presented a case study of a system deployed in the U.S. Gulf, an area well-known for having ample geothermal resources where DACCUS could be used to great success. The researchers estimated up to 25 of these hybrid geothermal-direct air capture systems could be deployed in just one of the 27 geologic formations in the Gulf by 2050.

"The Gulf Coast also has the right geology to safely put carbon dioxide underground and a decent enough heat flux that its geothermal energy could be sufficient to use," said Bielicki. "These characteristics are very favorable."

Growing interest

Direct air capture plants have already been commissioned around the world. According to the International Energy Agency, at least 130 facilities are in various planning and development stages.

For a self-sustaining system to work, it may first need some priming with about five years of CO2 captured from concentrated sources such as smokestacks. After that, the facility would be able to produce enough renewable energy to start extracting the greenhouse gas from the air.

"New technologies can enable one another, and in integrating them, we can tackle climate change," said Leveni in a press release. "There's a lot of work to be done to take into account technological readiness and the policies needed to make that research happen."

The U.S. is leading the race on policy support for direct air capture with a number of policies and programs, including the 45Q tax credit and the California Low Carbon Fuels Standard. The Inflation Reduction Act (IRA) announced important new funding in 2022, expanding the 45Q tax credit up to $180 per ton of CO2 permanently stored.

The Infrastructure Investment and Jobs Act (signed into law in November 2021) includes $3.5 billion in funding to establish four large-scale direct air capture hubs with related transport and storage infrastructure.

Around 35 such projects have been announced since the IRA, including project Bison (aiming to capture five metric tons of CO2/year by 2030) and 30 plants in King Ranch, Texas (each up to 1 metric ton of CO2/year each) with up to 15 plants pushing to be operational by 2030.