This isnt a problem of 100% penetration. It is a problem that shows up at 20% - if you have enough renewables to supply a fifth of all the kwh's you consume, then when high output and low consumption coincide, 100% of your current electricity production is renewable, and all else must be shut down. This is the reason Denmark is as tied into the Norvegian and Swedish grids as it is - during wind production peaks, the bulk of the power is exported, and dams further north conserve waterhead, functioning as a pumpless "pumped storage" facility.

Hmm. Let me see if I can be perfectly clear. The vision most people here have of the future grid is one in which at least 90% of all electricity production is low carbon, and supply and demand is managed via a supergrid, storage and demand managment. This is a fair description, yes?

The problem is that the only part of that vision which is seeing RnD done and concrete poured is the generating hardware. Nobody is seriously investing in storage, nobody is working on the problem of continental scale grid interconnection. And these are not problems that can be solved quickly as they come up, - Utility scale storage means rockworks! -  and the actual consequence of building intermittent generation without the other two legs of the tripod in place is either that most of the grid load will have to be supplied by gas turbines, or that the entire project gets scubbered when people revolt against ever increasing electricity prices. Possibly both.