David RuffnerI 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.
I looked up the above paper because I am curious about how direct air capture (DAC) used with renewable energy could create synthetic e-fuels. "E-fuels could extend the reach of wind and solar electricity to potentially all end-use
sectors" according to this paper "Potential and risks of hydrogen-based e-
fuels in climate change mitigation" https://publications.pik-potsdam.de/rest/items/item_25599_6/component/file_26999/content . It walks through the energy efficiency of using e-fuels versus direct electrification.
Generally direct electrification is better, but for high intensity applications it will make sense to use synthetic fuels. Currently e-fuels are much more expensive than fossil fuels, so it is important to support e-fuel development now, while making sure that it is not a distraction from direct electrification.