The efficency is said the be 80%, just like any other pumped storage system. It just uses considerably less space than a water reservoir (actually close to none

The efficency is said the be 80%, just like any other pumped storage system. It just uses considerably less space than a water reservoir (actually close to none, as the top of the cylinder remains untouched) and is scalable by adjusting the radius. I guess the main problem is to keep the water confined (200bar of pressure) while keeping the mass movable. Maybe someday they build a small testing facility, so we know for sure.