[don_esteban]

What is the conversion efficiency for electricity + C02 + H20 -> ethanol/hydrocarbons?

Because that is the overall path (for long-term storable chemical energy, i.e. usable for transport or seasonal energy storage in countries where solar is highly seasonal).

[thijson]

There's an alternative path that removes carbon from the cycle:

electricity + H20 + N2 -> NH3 + O2

Ammonia can be liquified and stored similar to Propane, it does attack copper and brass.

It can be burned in an internal combustion engine, it's about half as energy dense as hydrocarbons though.

There's a danger to humans from it though, it requires sprinkler systems if there is ever a leak.

I think that a large part of the energy budget in a plant is harvesting and concentrating CO2 from the air. N2 is a lot more abundant in the air.

There is work currently on using giant sodium batteries in these large container ships. That might be more cost effective than the above longer term.

[don_esteban]

Yeah, concentrating CO2 from atmospheric concentrations is not easy. The benefit is that it actually removes carbon from the atmosphere. Whether it can ever be done on a large scale is a question, though.

Is there any work on doing that, at a low energy cost? (I mean concentrating CO2, not removing it by weathering rocks?)

Yeah, ships are not really weight constrained, unlike airplanes, really cheap sodium batteries should be feasible.