Direct Air Capture
Direct air capture systems filter CO2 out of the ambient air similar to plants and trees. The CO2 is filtered out by passing the air over special sorbents and is then liquefied. To remove the CO2 from the atmosphere for the long term, it must either be reused (carbon capture and usage, or CCU) or stored underground (carbon capture and storage, or CCS).
Potential and scale
If intensive effort is put into its further development, direct air capture could in principle remove unlimited amounts of CO2 from the atmosphere. However, as CO2 is contained in air at very low concentration – about 0.04 percent – it would involve filtering enormous quantities of air. That would require huge installations and consume enormous amounts of energy. Estimates of the cost per tonne of liquefied CO2 vary widely. The average is US$600 per tonne of CO2. A large pilot plant recently taken into operation in a project in the Province of British Columbia, Canada, aims to attain a price of around US$100 per tonne. The deciding factor for such plants is that their use only makes sense if they run on renewable energy, since operating them on fossil fuels would emit more CO2 than they could capture. It is not yet clear where the absorbed CO2 could be safely stored for long periods or how it could be reused. Air capture can therefore only work on a large scale if the energy for the installations is generated in a climate-neutral manner, sufficient storage capacity is established for the captured CO2 (CCS) and ideas are found for its subsequent use (CCU).
Application readiness and research needs
The main problem facing the various direct air capture methods in principle is thus energy efficiency. They also depend on the establishment of CCS infrastructure or the development of CCU applications. Currently, three relatively large and several smaller direct air capture pilot plants are in operation. The captured CO2 is either put to use (in greenhouses, for example) or injected into geological formations and thus, permanently removed from the atmosphere.