FTA: “some tens of thousands of gigawatt hours of storage, of a form that could store the energy for six or more months and then discharge over the course of months. Such a thing has not yet been invented, and it is not clear that it can ever be invented.”
Well, it does exist, it is called the Sun.
If only we had a useable form of that stored energy, held underground.
Some Finns developed one, using sand.