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.”
I would still guess lifting water through pipes (specially if the reservoir is uphill in a mountain) is way easier than building something chained at the bottom of the ocean, which would need pulleys and a big heavy weight down there.
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.