There's a local company which has developed exactly this concept: https://heindl-energy.com/technical-concept https://heindl-energy.com/technical-concept Effic

There's a local company which has developed exactly this concept: https://heindl-energy.com/technical-concept https://heindl-energy.com/technical-concept Efficiency of around 80% (comparable to pumped hydro), capacity of 8GWh for a 250m diameter piston.