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Comment by blueflow | Hacker News Reader

Instead of supplying any energy to these facilities, one should reduce the production of some coal plant by the same amount of wattage. That would have a better carbon balance.

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These are completely independent variables. You can energize DAC facilities and replace coal power plants simultaneously. Whether you do, or don't do, one of these things has no effect on whether you do the other.

I'm not sure why you are downvoted, this is correct on the thermodynamics. It will always take more energy to remove a ton of CO2 than the amount energy you get from a coal or gas power plant producing a ton of CO2.

So to run a DAC plant from a grid that has a decent proportion of electricity generation from fossil fuels, you are making it worse overall (producing more carbon to power the plant than it is removing), and conversely, if you are running a DAC plant from 100% clean energy, you'd get better bang for your buck if you used that clean energy to displace fossil fuel use (because every kilowatt hour of fossil fuel energy you can replace with clean energy prevents a larger amount of carbon being emitted than the DAC plant could remove with the same amount of energy).

From a thermodynamic point of view would only make sense to run a DAC plant somewhere that already had a 100% clean grid, and only when it happened to be producing a surplus of clean energy.

How are you going to decide who doesn’t get any power after you reduce the output of the coal plant?

The point is the carbon capture plant shouldn't get any power. For sure there is no point in running direct air capture from carbon-emitting power sources. (It may, but very unlikely, be worth running it from some fossil fuel station with carbon sequestration of its own, but these in effect don't really exist outside of a handful of not particularly successful test projects. Though direct air capture is kind of in the same position at the moment, so it's all basically just experimenting with the technology at the moment.)

This might make sense if there were a single big but scarce global bucket of instantaneously available energy. But that's not how it works. The availability of energy is incredibly skewed across location and time.

Indeed. I wasn't insinuating it never made sense to power something like this today, just that it only makes sense when there is excess renewables available.

Yes but there are excess renewables available, in many places at many times. So it's already the case that it makes sense to power something like this, in many places at many times.

if you burn coal, and use _all_ that energy to capture co2, does it even balance out?

No. You are better off not doing the whole thing.

Any coal plant - carbon capture plant combination that captures more than it emits would easily be a perpetuum mobile.

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