Direct air capture technology strips the carbon dioxide out of air anywhere in the world. But how does it work?
One way to think of direct air capture is to imagine it like “space-efficient artificial trees.” Trees, after all, are the original carbon-capturing machines. But even if we were to cover Earth’s landmasses with natural trees, we still wouldn’t be able to suck up enough carbon to mitigate climate change in the future, explains Klaus Lackner, director of the Center for Negative Carbon Emissions at the University of Arizona.
Biological approaches, like planting trees, can offer a start to controlling carbon dioxide. But in the long-term, they may not be able to keep up with human activity. Put simply: “Sixty years from now, you better have another forest the size of all the forests in the world,” Lackner says.
Though planting trees will still be an essential part of the pathway to 1.5 degrees C, direct air capture can play a similar role while using less space. It works like this: Air, sucked in with a fan, is passed through a filter which absorbs the carbon dioxide. Once those filters are full of the gas, the units that house the fans are closed. The whole unit is then heated to between 80 and 100 degrees C, which kickstarts a process called desorption, releasing the gas.
From there, the carbon dioxide can be cooled down, combined with water, and pumped deep underground. Through a process of rapid underground mineralization, it is removed safely and permanently from the air.
Naturally, the whole process requires some energy use. The laws of thermodynamics tell us that we’ll need to use the same amount of energy to release the carbon dioxide captured as we use to collect it.
Generating energy for direct air capture is a “vexing” problem, but there are ways to mitigate the issue. If a carbon capture plant gets energy solely from renewable sources about 10 tons of carbon dioxide are emitted for every 100 tons sequestered. That's a good trade off.
Better yet, that whole process from capture to storage can be done in a small space. That’s the factor that could tip the balance in favour of direct air capture in the future. When compared to trees, we need 400 times less space to remove the same amount of CO2, and that includes the renewable energy we need to create for removal.
