Perhaps an efficient solution would be for every large scale renewable energy farm to have a buffer between itself and the power grid, perhaps only a small fraction of the systems total capacity. Add a flywheel to every wind turbine, a capacitor to every photovoltiac cell, etc. But thermal solar plants are good because they already have a huge buffer from the thermal mass of the working fluid. I think  only after theres ample excess capacity will the issue be conpletely resolved. Or else we could just take all the old steam turbines from decomissioned coal plants and feed solar produced steam into them, using them as flywheels. Probably be more efficient to use electric motors though.