This seems like a workable idea but just electric cranes lifting and lowering weights seems like a simpler approach and I think that has already been proposed.
My guess is there are many ways to balance the grid and the biggest is the utilities not wanting pay.
One thing to consider is the America power grid is in poor shape already and utilities are aiming to avoid modernizing. IE, adding storage to the grid would involve the double of cost of the actual balance equipment and the fixing the old equipment that needs fixing anyway. And utilities are looking avoid both cost.
I think when you lift a cubic meter of water by one meter at a depth of 500 meters, it is like lifting an entire column of 500 cubic meters. It's a huge multiplier effect.
There is no air involved. They are pumping water out, creating a vacuum. Water rushing back in generates power. No need to compress air or have a hose to the surface.
Making a big hollow object that can withstand crushing is much cheaper than something that can contain high pressure. Achieving the same pressure difference on the surface wouldn't be possible with concrete alone.
My guess is there are many ways to balance the grid and the biggest is the utilities not wanting pay.
One thing to consider is the America power grid is in poor shape already and utilities are aiming to avoid modernizing. IE, adding storage to the grid would involve the double of cost of the actual balance equipment and the fixing the old equipment that needs fixing anyway. And utilities are looking avoid both cost.