Great article! I love that engineers are working on gravity storage systems for intermittent energy sources! It triggers many questions in my mind; for instance, if pumping water during the day and draining it through turbines overnight, what volume of water would need to be moved for one facility? Concrete weighs about 2.5 times the same volume of water, so if they can mechanically move enough concrete blocks to make it worthwhile, they would have to pump 2.5 times as much water. That's a lot, but seems like not so much it couldn't be contained in a covered, closed-loop system to prevent evaporation. Then they could be built anywhere. The presumed benefit over moving concrete blocks would be the simplicity of pumping water, but I'm sure there are other considerations.