I came across the 2013 paper "The thermodynamics of direct air capture of carbon dioxide" by Klaus Lackner https://www.sciencedirect.com/science/article/abs/pii/S0360544212006901. It describes the theoretical minimum energy needed to separate CO2 from the surrounding air. One interesting takeaway is that it is energetically less costly to "skim" CO2 from the air (capture a small fraction of the CO2 in the air and then moving on to more fresh air). This means that direct air capture is not necessarily much more energy intensive than scrubbing exhaust from smokestacks. In the scrubbing use case, you want to get all the CO2 before adding the exhaust to the atmosphere.




