It is one thing to have grid storage for local use, but even more is called for when it comes to regional use.
I personally like Donald's comment that "if you want to build something dirt cheap, then use dirt". I have always had a similar idea when it comes to regional storage of electrical energy for the grid. My idea is to use large insulated storage tanks similiar to oil storage tanks filled with sand, which are used to store electrical energy in the form of heat. Electrical resistance elements are used to convert excess electrical energy into heat, which turns the sand into molten sand as the means of storing electrical energy. These thermal storage units (thermal batteries) would then be used to generate electrical energy at night time.
BTW, there was a technology developed back in the mid 1960s, for turning low grade thermal energy into electrical energy at an efficiency rate close to 70%. It was 70% efficient because it used no moving parts to generate this electricity. It used low temperature / pressure PURE steam (which is usually thrown away as waste heat) to which an electrical charge was added to these steam particles, and then the charged steam flowed uphill into an electrically isolated cold condensation chamber (which generated a relative vacuum) where the steam was condensed into water and the electrical charge was removed. It was basically a variation of the Van de Graff generator, and it was designed to emulate Mother Nature's generation of lightning in thunderclouds. In actual practice either cold ocean water or cold high altitude air would be used to provide the cold sink for this electrical generation process. Just wanted to share. Incidentally, they used ordinary glass as the electrical isolation means for the cold condensation chamber in these early experiments. - Rick Carter