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A New Carbon Removal Startup Is Powered by Sunlight and Seawater

Photo Courtesy Steven Van Elk

(Bloomberg) —

A Seattle startup aims to remove millions of tons of carbon dioxide from the atmosphere with a process that avoids the astronomical energy demands of other carbon capture approaches. The technology? Sunlight and seawater. 

The company, Banyu Carbon, says it has developed an ocean carbon dioxide removal (CDR) system that relies on a synthetic molecule that, when exposed to light, changes shape and becomes acidic. When it interacts with seawater, the resulting process scrubs CO2. Though the company is currently working at the gram scale, its technology holds the promise of a low-cost way to limit global warming.

“The main problem we’re trying to solve is that carbon removal takes a ton of energy,” says Banyu co-founder and Chief Executive Officer Alex Gagnon, a chemical oceanography professor who spun the startup out of the University of Washington in 2022 with colleague Julian Sachs. (A marine organic chemist and Banyu’s chief technology officer, Sachs previously conducted fieldwork in the Pacific, and “Banyu” is an Indonesian word for “seawater.”) 

The startup’s workhorse is a unique molecule, dubbed a “reversible photoacid.” After being exposed to sunlight, the photoacid releases acidifying protons that are temporarily transferred to seawater pumped into a tank, where it transforms dissolved CO2 in the water into a gas that can be safely stored. The decarbonized seawater is returned to the ocean where it draws down CO2 from the atmosphere. Once back in the dark, the photoacid resumes its original shape for reuse.

Since blue-spectrum light waves are the catalyst for the process, Banyu mostly uses power to operate seawater pumps. That’s in contrast to many other ocean CDR technologies, which rely on electrochemical processes that are much more energy-intensive to entrap dissolved CO2 in seawater in a limestone-like material that would sink to the ocean floor.

Read more: The World’s Next Big Carbon Capture Challenge? Figuring Out How to Use It

Banyu has raised a $6.5 million seed round from investors that include the Grantham Foundation for the Protection of the EnvironmentPropeller, United Airlines Ventures, Carbon Removal Partners and ReGen Ventures. Grantham led a previous round of nearly $2 million for Banyu. 

The startup is among a slew of companies capitalizing on investor and policymaker interest in removing gigatons — that is, billions of tons — of CO2 from the atmosphere. Doing so will be almost certainly necessary to keep global temperature rise to 1.5C, the aspirational target set by the 2015 Paris Agreement.

So far, Banyu has removed grams of CO2 from water in the laboratory but plans to establish a photoacid pilot project this year at a site in Washington state that aims to remove tens of kilograms of carbon from seawater. Next up is a commercial demonstration project to fulfill a contract to extract 360 metric tons of CO2 by the end of 2026 for Frontier, a fund led by payments company Stripe to speed the development of carbon removal technologies. 

Frontier has agreed to pay Banyu $1,387 per ton of carbon removed by the demonstration project, which Gagnon says will likely be built along the Gulf of Mexico. In its application to Frontier, Banyu projected that at the scale of removing hundreds of millions of tons of CO2 annually, the cost could drop to $60 a ton. That estimate includes CO2 transportation and storage in a geologic formation. Banyu said the extracted CO2 could also be used in industrial processes, such as making sustainable aviation fuel

Like other carbon removal startups, Banyu plans to sell carbon credits. However, CDR companies face significant financial and engineering hurdles in scaling their technologies to have a meaningful and cost-effective impact on global CO2 levels. Those focused on the ocean face additional scrutiny over the consequences for marine ecosystems, such as the potential to alter seawater chemistry and interfere with the lifecycles of organisms. 

Researchers have also been working to remove carbon from the atmosphere on land. One such method is direct air capture (DAC). Since there’s less than a gram of CO2 per cubic meter of the atmosphere, a DAC facility must power “fan farms” and other energy-intensive processes to suck in huge volumes of air and separate the gas. (Some startups are working on processes that consume less energy.)

Read more: A Startup Battles Big Oil for the $1 Trillion Future of Carbon Cleanup

Seawater, on the other hand, contains more than 100 times the concentration of CO2 per cubic meter as air. Even so, most ocean CDR processes would need massive investments in renewable energy at scale. 

For instance, scientists who founded Los Angeles-based ocean CDR startup Equatic estimated in a 2021 paper that removing 10 gigatons of CO2 annually from seawater using Equatic’s electrochemical process would require a $1.4 trillion buildout of solar power stations. Equatic said last year that subsequent efficiency improvements meant that it would need 2 megawatt-hours of electricity to decarbonize 1 metric ton of seawater. (The average US home consumes slightly less than 1 megawatt-hour a month, according to the US Energy Information Administration.)

Sunlight, though, is free, and Banyu projects it’ll need well under 1 megawatt-hour of electricity to remove a ton of CO2. The company expects that not only will its system consume 90% less energy than electrochemical ocean CDR — and 30% less than DAC — it’ll generate surplus electricity. That’s because Banyu plans to embed photovoltaic cells in its system that could produce excess electricity not used for decarbonization.

“Banyu is the only solution that we’ve seen that can actually be energy positive,” says Grantham Director Cyril Yee, noting the foundation has invested in numerous CDR startups. “There’ll be engineering challenges for sure, but the basic science works, the chemistry works.”

Energy demands could be cut further if a facility is located at a power station or a desalinization plant that already pumps and dumps large volumes of seawater. 

Banyu’s low-power photoacid system “sounds magical and if they can work out the technical details, it seems like this could be a good way to go,” says University of Hawaii oceanographer David Ho.

“These are really smart people at Banyu who are working on this and they have an understanding of chemical oceanography,” adds Ho, who co-founded a nonprofit called [C]Worthy to develop protocols to verify ocean carbon dioxide removal and who has been critical of energy-intensive methods. 

He says one difficult challenge for Banyu and other ocean CDR companies that [C]Worthy is working with is how to verify the amount of CO2 reabsorbed by decarbonized seawater. That number can fluctuate, depending on such factors as water circulation and location. 

Margaret Leinen, the director of the Scripps Institution of Oceanography at the University of California at San Diego, notes that the most effective places to decarbonize seawater aren’t necessarily the most feasible, such as the treacherous Southern Ocean surrounding Antarctica. “There’s a lot of air-sea CO2 exchange in the Southern Ocean,” she says, “but that’s not a nice environment to try to do something” like ocean CDR. 

Read more: The Three Biggest Letters in Carbon Removal Are MRV

Photoacids can be synthesized from inexpensive materials but their utility had been limited as they remained acidic for only seconds after being struck by light. But thanks to recent improvements in photoacid chemistry, they can now stay acidic for minutes or hours, according to Gagnon and Sachs. In lab tests, they said Banyu’s photoacid removed 94% of the carbon dioxide in flowing water. The photoacid had a half-life of 10 days and cycled between neutral and acidic states 14,000 times before its effectiveness dropped 50%. If Banyu can extend photoacid life to 30 days, it would take 0.44 kilograms, or just under a pound, of the material to remove a ton of CO2. 

The toxicity of Banyu’s photoacid remains unknown but the company says the molecule is kept separate from seawater and has a structure similar to common red dyes. Gagnon and Sachs say the environmental impact on the ocean would likely be positive as the decarbonized seawater would be less acidic, which would benefit marine life. 

Leinen says little research has been done on the potential impacts of ocean CDR on marine ecosystems, given the proprietary nature of nascent technologies and the small scale of deployments so far. “I’m not in any way opposed to all of these companies but I have great concerns we not do something that is going to have a huge negative impact on ecosystems,” she says. 

To contact the author of this story:
Todd Woody in San Francisco at

© 2024 Bloomberg L.P.


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