Comment by ChrisFoster

The piston idea seems like a appealing design requiring minimal new technology. The plant is the same as existing pumped hydro plant, the weight can come from the native rock.

Using a rolling membrane for the seal is discussed at

https://heindl-energy.com/technical-concept/engineering-chal...

Naively that does seem plausible but it's a very large sheet of material to manufacture. What are your thoughts on that approach?