Diurnal variation is an additional problem any clean grid has to address somehow. I am optimistic about the potential to shove consumption around within the day-night cycle - particularly if the shift is consistent.

That is, I do not think either a "All our electricity comes from desertec" or "All our electricity comes from  reactors that do not have throttles at all"  grid would face insurmountable problems. Ask people to charge at a given hour every day and that is not going to be a difficult habit to follow  
 But asking people to not go to work today because it has been a quiet week? That is not going to fly. So large variations in daily supply either require storage that can hold weeks of electricity, or the grid has to get bigger until the variations go the frack away.