The CO2 that is extracted from the water is run through a purification process that uses activated carbon in the form of charred coconut husks, and is then ready to be stored.
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In a scaled up system, it would be fed into geological CO2 storage. Before the water is released, its acidity is restored to normal levels, making it ready to absorb more carbon dioxide from the air.
“This discharged water that now has very low carbon concentrations needs to refill it, so it’s just trying to suck CO2 from anywhere, and it sucks it from the atmosphere,” says Halloran. “A simple analogy is that we’re squeezing out a sponge and putting it back.”
While more tests are needed to understand the full potential of the technology, Halloran admits that it doesn’t “blow direct air capture out the water in terms of the energy costs,” and there are other challenges such as having to remove impurities from the water before releasing it, as well as the potential impact on ecosystems. But, he adds, all carbon capture technologies incur high costs in building plants and infrastructure, and using seawater has one clear advantage: It has a much higher concentration of carbon than air does, “so you should be able to really reduce the capital costs involved in building the plants.”
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Mitigating impacts
One major concern with any system that captures carbon from seawater is the impact of the discharged water on marine ecosystems. Guy Hooper, a PhD researcher at the University of Exeter, who’s working on this issue at the SeaCURE site, says that low-carbon seawater is released in such small quantities that it is unlikely to have any effect on the marine environment, because it dilutes extremely quickly.
However, that doesn’t mean that SeaCURE is automatically safe. “To understand how a scaled-up version of SeaCURE might affect the marine environment, we have been conducting experiments to measure how marine organisms respond to low-carbon seawater,” he adds. “Initial results suggest that some marine organisms, such as plankton and mussels, may be affected when exposed to low-carbon seawater.”
To mitigate potential impacts, the seawater can be “pre-diluted” before releasing it into the marine environment, but Hooper warns that a SeaCURE system should not be deployed near any sensitive marine habitats.
There is rising interest in carbon capture from seawater — also known as Direct Ocean Capture or DOC — and several startups are operating in the field. Among them is Captura, a spin off from the California Institute of Technology that is working on a pilot project in Hawaii, and Amsterdam-based Brineworks, which says that its method is more cost-effective than air carbon capture.
According to Stuart Haszeldine, a professor of Carbon Capture and Storage at the University of Edinburgh, who’s not involved with SeaCURE, although the initiative appears to be more energy efficient than current air capture pilot tests, a full-scale system will require a supply of renewable energy and permanent storage of CO2 by compressing it to become a liquid and then injecting it into porous rocks deep underground.
He says the next challenge is for SeaCURE to scale up and “to operate for longer to prove it can capture millions of tons of CO2 each year.”
But he believes there is huge potential in recapturing carbon from ocean water. “Total carbon in seawater is about 50 times that in the atmosphere, and carbon can be resident in seawater for tens of thousands of years, causing acidification which damages the plankton and coral reef ecosystems. Removing carbon from the ocean is a giant task, but essential if the consequences of climate change are to be controlled,” he says.