Works in theory! Practice? That's something else
60-70% efficient.... so could compete with hydro, but not have hydro's restrictions on location. I particularly like the concept that adding more storage (MWH) is just adding more tanks and it has a far smaller footprint than hydro.
According to this 2017 paper, capex is ~35% that of Lithium and about half of a peaker plant, and 70% hydro.