Discussion about this post

User's avatar
mike landmeier's avatar

This is a great article in so many ways.  You cannot imagine how many social gatherings I go to with my wife, and no one wants to discuss the thermodynamics of DAC with me! ;)

But this article has a weakness.  This statement needs to be reconsidered.  "Because Gibbs free energy is a state function, it doesn’t matter exactly how you get from mixed to unmixed."

You have analyzed DAC as being one endpoint and one strategy and attribute the results to all endpoints and strategies.  For this article, the endpoint is compressed, pure CO2 sequestered underground and under great pressure.  The energy required to accomplish this is high because the choices are poor.

Now, I understand that this is the leading DAC solution being pursued by the US DOE.  The energy required is actually higher as your analysis has not included losses attributed to transportation.  That captured and compressed CO2 is going into a pipeline and transported to somewhere (Wyoming?) to be sequestered.  There will be energy considerations associated with the friction of transport.

The point that needs to be considered is that different endpoints can have different energy requirements.  And different strategies for an endpoint can influence the necessary energy as well.  For an understanding of how strategy influences energy considerations, review Lackner's analysis of Sherwood's rule, Passive Direct Air Capture and his support for mechanical trees.

The major problem with traditional DAC is that it is a process that concentrates very dilute CO2 into a pure gas stream, compresses and transports that stream and then stores all that CO2 and all that energy in a geological formation.  This process requires a great deal of energy because it is designed by chemical engineers who optimize processes for mass transport and deal with the energy consequences later.

DAC needs to be defined as any process that removes CO2 from the air and renders that CO2 inert to the atmosphere.  This broader definition of DAC will provide a greater variety of solutions to be considered.

All around us, living organisms capture CO2 directly from the air while using much less energy. They don't ever create a pure concentration of CO2.  And they never compress that captured CO2.  Why?  Too much damn energy.

I am not a nature-based CO2 kinda guy.  I am firmly in the engineered CO2 capture/sequester camp.  I see nature providing us with insights as to how to better design engineered CO2 capture and engineered CO2 sequestration solutions.

And the key to all this is to optimize for energy rather than mass transport.  That is a whole 'nuther discussion.

Expand full comment
Matthew C. Nisbet's avatar

Hi Andrew -- I share your skepticism about direct air capture. In 2019 I spent a summer writing a commissioned report reviewing the literature on the many social and political challenges in developing, deploying, and scaling direct air capture. The link below is to a column I wrote for Issues in Science and Technology summarizing my analysis and conclusions. Among the key takeaways related to your analysis is the immense land footprint / land use required to scale direct air capture considering that each plant needs its own power source either in the form of renewables, nuclear, or in the nearer term natural gas. And that you have to build a massive pipeline system to transport the captured CO2 to geographic locations where it is possible to be buried. https://issues.org/sciences-publics-politics-carbon-removal/

Expand full comment
29 more comments...

No posts