The system is based on an underwater pumped storage concept that replaces conventional upper and lower reservoirs with a sealed, pressure-resistant hollow sphere deployed on the lakebed or seabed. The surrounding water body serves as the “upper reservoir,” while the interior of the sphere functions as the “lower reservoir.”
Oh now I found time to read the article. I thought they would sink and rise hollowed balls, when they actually keep the balls down and pump water in and out of them.
I wonder if they are heavier than water, so you just need a small anchor to prevent drifting.