Except recouping the CO2-expenditure of the concrete piston will take a long time. A simpler solution is to use the gravitational potential energy of the water directly. Pumped hydro, as this is known, is super simple if you have a mountain and maybe even an existing hydro-power plant to take the energy back out.

I have never checked this, but a Swiss friend of mine once explained the Swiss energy system as being centered around pumped hydro using free surplus power from the French nuclear power plants.