[rob74]

@gorgoiler: that's a "classical" pumped storage facility, not the concrete (or stone) piston version proposed here.

According to this press release https://www.stawm.de/energie/strom/speicherung.html, in April last year they were hoping to be able to start building the Weilheim prototype by December. The seal technology was undergoing trials near the Baltic Sea (translating from the press release).

While this sounds interesting, building the "cylinder" and the "piston" to fit exactly at the huge dimensions they are envisioning, and getting the seal to work at these dimensions and in the long term sounds like a pretty big challenge...

[mech1234]

The sealing technology is a rolling diaphragm. Rolling diaphragms do not have any sliding action, and as long as the diaphragm is fastened on both sides and not punctured, the seal remains intact, without close clearances. They are used already on valves and pneumatics/hydraulics applications of various sizes.