Powering up: UK hills could be used as energy 'batteries'
Smaller scale pumped hydro which should be cheaper and have far more potential sites than traditional pumped hydro. Potential competitor to Tesla battery storage. It will loose out to batteries in being slower to react, and to traditional pumped hydro (where available) due to smaller capacity. Civil engineering and planning delays probably means that this can't be deployed quickly.
I first heard about this idea when I was living in Europe, in 2007. It was all talk, no action back then. Like nuclear powerplants, the NIMBY factor might well be a huge obstacle.