Re: Im going to say the quiet bit out loud...
Amplifying this point.
Lets say we had a bunch of, I dunno, windmills producing electricity no-one can use. Then efficiency is no longer the issue, just make something that can be burned. Could be hydrogen, which can then be combined with CO2 from, say, a massive green-tinted wood-burning station. Or it is possible to do it directly from solar, apparently. Seems more likely than giant battery packs.
The resulting fuel can drive turbines, be they in wings, or in a power station. Rolls Royce might have some spare these days.
Efficiency will suck, but that just means we need more windmills. You can buy a shitload of windmills and infrastructure for £50 billion, or whatever a nuke costs these days.
So, m'lud, I suggest this is all just green posturing by airlines and their political cronies desperate to avoid having to do anything before their gold-plated pensions are due.
Maybe Elon could help before he goes off to Mars?