I agree but it is likely that the amount of concrete used to build those spheres is limited compared to say building a highway or a city. This is to be normalized by the amount of used energy storage / traveled roadway / inhabited building by year and by person for the comparison to make any sense.
If the spheres stays in operation for 50+ year, that's a lot of renewable power enabled by this technology.
On those scales equipment could be more of an issue. I shudder to think of the cost of repairing a turbine (or pump), submerged in water at the depth of several hundred meters. This is a little bit equivalent to running several small-scale hydro electric plants under water (where everything gets more complicated and expensive). Maybe the engineering is straight-forward. 50 years without maintenance is impossible.
Was thinking along the same lines with the underwater datacenter Microsoft has been toying around with.
Would you even have to put technology down there? I have absolutely no idea how good or bad humanity is at making very long pneumatic connections, but I would intuitively expect that the pumps and turbines involved would work on air, above the surface.
Is that good or bad? The amount of infinitesimal sea level rise that determines capacity would be the same either way.
One advantage of remote pneumatic pumping would be that power (W) could be scaled independently from capacity (Wh): you either add more spheres or you build bigger pipes/pumps/turbines. Conventional pumped storage installations frequently add more throughput to existing, unchanged reservoirs.
Any specific reasons why you think it should be very expensive? When the water is pumped out of the sphere, I think that the energy needed to bring it up should be very low. With a maintenance boat built purposefully to haul these things up, do the maintenance on the surface, and then let them sink again, I wouldn't expect the costs to be very high.
Volume of a sphere enclosure with radius 15.5 meters and a hollow center sphere 15 meter radius is around 1100 cubic meters. Specific weight of concrete is 2.4 g/cm^3. This monstrosity will weight between 2500 and 3000 tons. It will gain buoyancy when filled with air but the need for maintenance might be due to not being able to move water into and out of the sphere in which case you can add 10^5 more tons to what you need to lift out of the water (in which case I think making repairs at depth might look favorable).
It's probably doable. But it's going to cost a pretty penny.
> This monstrosity will weight between 2500 and 3000 tons
Even if filled with water, you should subtract 1100 tons to the actual weigh as long as the thing is in the water, and I guess that doing repairs with the thing just underwater below/near your ship would be much easier than doing them 700m below the sea. Unless it was possible to do everything with a remotely controlled diving robot, that is :)
If the spheres stays in operation for 50+ year, that's a lot of renewable power enabled by this technology.