In conversation with an IGOR (Ignorant Grandstanding Oblivious Rabble-rouser), I pointed out that the idea of using atmospheric carbon dioxide to make fuel isn't entirely new (Nature did it first), but the technologies being proposed aren't realistic, for the reason above.
IGOR countered that these processes could, in his view, be the solution to our energy crisis (do we have one?), because the fuel produced by carbon-capture will provide the energy to keep the process going.
Ahem. Let's think about this, with a diagram:
What reasonable people say is that the energy extracted from the fuel will partially cover the energy needs of the capture and conversion process (that is $x > y$ but not by much); what IGORs say is that $y>x$. But if that were so, we could feed the exhaust from the energy production system into the input for the capture system, and get a perpetual motion machine that generates free energy.
Some of the more reasonable proponents of this carbon-capture and conversion idea suggest that the energy coming in can itself be green energy, like solar, and therefore there's a net "carbon-based" energy coming out of the system. Two points:
First, that's fine, but then why use part of that solar energy to create carbon-based fuels, instead of using the solar energy to replace the carbon-based fuels? Note that any $\mathrm{CO}_2$ that gets turned into fuel will yield another $\mathrm{CO}_2$ after the energy generation (conservation of the carbon), so no advantage there.
Second, the designs proposed look extremely wasteful of energy: capturing $\mathrm{CO}_2$ after it has diffused into the atmosphere is bound to require a lot of energy to flow non-$\mathrm{CO}_2$ gases in the atmosphere through the carbon-capture process. Better to stop $\mathrm{CO}_2$ at the source, if that's what you're after.Of course, as I mentioned, Nature does provide us with a technology to use solar power to capture $\mathrm{CO}_2$ and turn it into fuel:
It also has the advantage of being pretty, giving shade, operating in silence, and bearing fruit. Trees. It's trees. Let's plant more trees. I like trees.
One particularly oblivious IGOR insinuated I was anti-environment because I prefer trees to useless noisy subsidy-harvesting machines.
With friends like that, the environment is doomed.
