[Mchl]

Seems to me that if you use compressed air to displace water within the sphere, you could actually store more energy this way... That would require the air to drive electric generators when the sphere is being filled with water though.

[skosch]

Once you start compressing and decompressing air you get a lot of heat loss though, that I would imagine isn't as much of an issue when you're just pumping plain water back and forth