Great way to repurpose a coal plant. Why start from scratch to build a storage facility?
I guess the answer to that question might be line loss. It probably makes a lot of sense to store the excess close to the point of production. That said, a lot of renewable power is generated far from point of use and a lot of the legacy plants are close, so if you actually have excess power, it makes some sense to incur the line loss when you have surplus, store the energy closer to point of use, and then convert it to electricity and only have a short trip to the city.