Direct Air Capture of CO2

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Readers letters

DAC is not currently an economically viable approach to mitigating climate change

1. I have recently received a copy of the IMechE's position statement on Direct Air Capture (DAV) of CO2. I must say that I am surprised that, in an effort to spearhead innovation, the Institution appears to have left behind some of the principles that those of us working in the field take for granted. I would therefore welcome the opportunity to acquaint your readership of these principles which may assist them in their understanding and aid them in any debate that they might have with better-informed colleagues.

Direct Air Capture may have a role to play eventually in countering emissions from some decentralised sources of CO2 such as from buildings and vehicles (ships, planes) that prove expensive to reduce by other means, but there is other relatively “low hanging fruit” to be picked before addressing these sources. Examples of this:

energy constraint (do we need to use it in the first place?)
energy efficiency (using less of it to do the same job)
use of lower CO2 energy sources (e.g. Power stations, wind and wave)
CO2 capture from large point source emitters (e.g. Power stations, steelworks, refineries) and storage

Until capture from these sources has been implemented on a global scale, DAC is never going to be cost-effective alternative.

2. High CO2-emitting energy sources are not viable options for providing the power for DAC systems because their CO2 emissions may well exceed the CO2 captured. Compressing CO2 to conditions suitable for long-term geological storage is an energy-intensive process: the sensible place to start is with CO2 at pressure, then CO2 at atmospheric pressure and lastly that under vacuum conditions, because compressor power increases exponentially with the pressure differential.

3. The storage part of CO2 capture and storage (CCS) would have to be both inexpensive and feasible at a grand scale for any additional CO2 captured by DAC to be economically viable. The current mechanisms to incentivise this are inadequate by several orders of magnitude.

4. Economic models of global CO2 emissions stretching over very long periods of time, particularly those that feature “overshoot trajectories” and rely on DAC should be viewed with extreme caution, given the large uncertainties associated with technology roll-out and global uptake.

I would conclude that DAC is not currently an economically viable approach to mitigating climate change. At best DAC is a dream, and at worst it is a distraction of funds and engineering effort from more promising technologies. With a consequential delay in their implementation.