Paying for 20 denser material is usually more expensive than just "buying" 20 times as much material. Gravity storage will always be a scaling problem because you would rather have 1000 times the material than 20 time the density. Doing something as nonsensical as using a mine shaft whose size is fixed is not going to work out.
The crane system could work out in theory if you make the crane arms 200m or longer. The covered surface area grows much faster than the length of the crane arm. In practice it's an engineering nightmare.
The piston idea has the benefit that you just use a wire saw and cut out a circle. The primary challenge is building what is basically the largest seal on the planet. The pressure itself is not a problem but the sheer size is.
Compared to hydrogen, reflow batteries, compressed air, heat storage, etc gravity energy storage is extremely questionable unless it's pumped hydro.
The crane system could work out in theory if you make the crane arms 200m or longer. The covered surface area grows much faster than the length of the crane arm. In practice it's an engineering nightmare.
The piston idea has the benefit that you just use a wire saw and cut out a circle. The primary challenge is building what is basically the largest seal on the planet. The pressure itself is not a problem but the sheer size is.
Compared to hydrogen, reflow batteries, compressed air, heat storage, etc gravity energy storage is extremely questionable unless it's pumped hydro.