If in the end part of the solution were synthetic fuel it wouldn't surprise me. H2 from water electrolysis, C from biomass, and the required energy coming from PVs/other renewables, because li-ion batteries fall short for planes and ships: these need the good old high density liquid (hydrocarbon) fuels.
[...]
Just leaves nucelar.....
But when the oil runs out Fischer–Tropsch synthetic fuels might be the solution, because we need planes and ships and planes can't fly with nuclear.
The big idea is to have way too much capacity so that even at minimum production "no wind or sun over the whole of western Europe mythical day", the reliance on co generation or batteries is kept to a minimum.
All the other days, 364 of them, the wind would produce hydrogen with the excess (otherwise lost - aka nearly free) energy and convert that to methane. Once we have methane, longer hydrocarbons are possible. But methane, of itself, is already a easy to manage fuel.
The same method could be used to keep Nuclear plants running at optimal power levels, as they don't like low loads...
But the problem is getting to methane, as the Sabatier method is a bit iffy, but serious efforts are going into this:
https://www.technologyreview.com/s/510066/audi-to-make-fuel-using-solar-power/ for example.
But, to be honest, I can't even pretend to have dug into it.
Could be the next big thing or "Solar Roadways II, the CH4 connection" for all I know...
That mythical day where there was no sun for solar and no wind to spin wind-turbines was 2 weeks. Both Great Brittan and Germany were affected. Both burned more coal and purchased nuclear produced electricity from France. If you look you will see a spike in Germany's burning of coal last year as a result of the renewables not producing.
As for Audi - They need to look at the Hydrogen fuel video to see why this isn't going to work. The amount of electricity involved in electrolysis of water into H-H O and storage is enormous. But they are adding a twist by using methane. So maybe........ But highly unlikely this would work. Watch the following vidoe.
The new fields solve that by going further north, further out, covering a wider area in more locations and using higher turbines.
Right now, the present capacity can be caught out, but that will be more and more unlikely.
Flat days happen, we had one going round the Cape Fisterra two days back, and that is extremely rare!
(But it was sunny and I had an after work tea just below the bridge with only the vastness of a flat Atlantic, lovely).
As a side note, from a tech perspective, I am far more a fan boy of PV in vast open spaces (Central Spain, Morroco, Algerian Sahara) and modern nuclear than offshore wind on the long term.
(PS, sorry, cannot watch Youtube, satellite internet... Doh!)