you can build big cement spheres under water, pump the water out, then let it flow back in through turbines.
i ran some numbers.. it should only take hundreds of "cycles" to "pay" for the C02 that came out of the limestone turned into cement.
anyhow there is some stuff written about 10 meter thick spheres 100 meters in diameter, about 700 meters under water. i suppose the thickness, diameter, depth, are all relative and scaling up or down is only a small detail. anyhow the idea is interesting if it doesn't require anything too expensive.
but, pumps and salt water (you could use fresh water, store it on land) could make for an interesting project.