Interesting concept, but not very scalable. It's basically a reversed dam - when it's full, there's 0m head of water. Then with excess energy, you lower the level inside, storing the energy in the water outside. E.g -2m head. Water then flows in to equalise head, and doing so, regenerates electricity. Adding depth to supercharge pressure differentials is a good idea, although I wonder how they limit the flow rate, or otherwise prevent cavitation shocks each cycle.

Could be useful as a private industrial battery, but a dam would still be better on an infrastructural level.