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.
The entire point of hydraulic hydro is that energy capacity scales by r^4 (dimensions of the piston + height within the hole). It is possible to just carve out the piston head from existing rock with wire saws. Systems with 1.6TWh storage capacity are not impossible.
that does look like a lot of concrete. I suppose ideally it would be a shell filled with the dirt removed to create the hole, even then it would be substantial at 150m in diameter (the smaller one)
You create the rings of concrete around the cylinder while you are carving it, so actually not so much concrete. I am puzzled about the engineering required for the base, but on the paper it looks quite neat.
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.