⇒ Aside: the »place« problem is that you can’t build solar panels and wind turbines just anywhere, and they need a lot of space. E. g. Germany has now the problem that the wind blows much better in the north, but the industry is more in the south. So, you need a lot more/stronger transmission lines. Same for offshore wind: more wind at sea, but you need a lot of cables.
The more wind and solar you already have, the more the good places are already taken.
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⇒ (But at least we already have transmission tech, it is now just a question of materials and effort.)
So, assume that we have enough wind and solar that we can regularly produce 100% of demand from them. You can imagine peaks just touching the demand line at top demand.
(You could imagine more than that, but that would mean overbuilding, which hurts the economics quite badly while not making the end result much better.)
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⇒ Now the volatile supply line has valleys between the peaks. If you integrate over time and place, the supply line covers about 40% of demand in this situation.
That is /very rough/ and depends on a lot of factors, but my point is the same if it were 30% or 60%: where does the rest come from?
- Transmission: as already mentioned, we know how to transmit electric energy, it’s just material and effort. This smoothes out the »place« dimension.
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⇒ - Storage: obviously, we’d want to smoothen out the time dimension as well. This means adding storage that can meet 100% of demand as well (volatile sources frequently drop to 0), and feeding it with enough additional clean sources that it can fill every expected gap (and gap accumulation).
And here I’d like to repeat my point from before: the best (most effective) storage we have right now is pumped hydro, by far. And pumped hydro is not enough, by far.
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