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Comment by acyou | Hacker News Reader

In general, increasing the concentration of any product in a closed system requires net energy input across the system boundary. Larger increases in concentration need more energy input than smaller increases of concentration.

Concentrating atmospheric CO2 involves increasing the concentration from approx. 400 ppm, to a higher target concentration. That's 0.04%. There is no way around the energy input requirement within the basic, universally applicable and virtually undisputed laws of thermodynamics. You can make your process as efficient as possible, but there is a minimum energy requirement (theoretical limit) that you can calculate on a per unit basis for atmospheric CO2 capture that is very, very, very high.

For that reason, industry captures CO2 at point source (you start from a higher concentration). Unless you have zero access to point sources, point source capture will always be more "Environmentally Friendly" than atmospheric carbon capture.

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It's curious that you didn't actually include the calculation of what the minimum energy requirement; instead, you just assert without evidence that it is "very, very, very high".

In fact this theoretical minimum is not very high; as https://en.wikipedia.org/wiki/Direct_air_capture#Environment... explains, it is only 250 kWh/tonne CO₂, or 900 kJ/kg in SI units. To remove the ≈60 Gt/year of anthropogenic CO₂ currently being emitted and get us to carbon-neutral with direct air capture would consequently require a theoretical minimum of 1.7 terawatts, which is only about 10% of current world marketed energy consumption, and presumably about 5% of world marketed energy consumption 10 years from now. Kicking climate change into reverse would require a bit more than that, maybe double. Depending on the sorbent system, this energy can be solar thermal; it does not have to be electrical.

Existing direct air capture systems like Climeworks's do not closely approach the theoretical minimum. Do you know how much energy they require?

Point source capture is of course much cheaper but it cannot get us to net negative CO₂ emissions.

> Point source capture is of course much cheaper but it cannot get us to net negative CO₂ emissions.

But it certainly can: by capturing on a point source that burns biological fuel. Without the burning, all the CO2 emitted (or captured) would have cycled back to the air a short time later anyways. And if you mainly do it to have a cheap carbon source to make some intermittent energy source you already have storable/transportable (e.g. creating aircraft fuel), just about any low grade biomass will do. Grass. Leaves. Dried algae. Paper too dirty for recycling (or rather: any paper - does paper recycling even make any sense in an economy that also burns wood for energy?). When you start looking at incineration not as disposal and/or energy source, but as a source of concentrated CO2, almost anything turns into a useful resource. All that stuff contains carbon, and it's all going back into the atmosphere one way or another.

Oh, I guess you're right.

>Unless you have zero access to point sources, point source capture will always be more "Environmentally Friendly" than atmospheric carbon capture.

Ideally we'll get to a point where every CO2 point source either stops being a point source, already has a mechanism for capturing the emissions, or can't economically accommodate carbon capture for some other reason (airplanes, probably).

At that point we might still have too much CO2 in the atmosphere. We'd have to figure out some way to get it out, whether that's by manufacturing hydrocarbons, planting trees, something else, or all of the above.

Electro-swing absorption works efficiently at ambient levels:

https://verdox.com/technology

Point source capture is pointless: We need the capacity to reduce the atmospheric CO2 concentration. Capturing every molecule of CO2 emitted at its source wouldn’t solve global warming.

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