Potential energy storage has been around for a long time in various forms. At its simplest, potential energy storage means using energy to lift a weight, apply tension to something, or pressurize a gas with the intention of releasing energy at a later time to perform a task. Clock springs are one example. Compressed air is another.
Advanced Rail Energy Storage North America has set up a system that moves 230 tons of stone up a hill when the grid has more electricity than it's using. When there's a shortfall, the train is allowed to roll downhill, and the electric motor in it runs backwards, turning the motion into electricity, and feeding it back to the grid. It's simple, scalable, and environmentally friendly.