... a sphere nine metres in diameter and weighing 400 tonnes will be submerged off the coast of California at a depth of 500 to 600 metres. It will have a storage capacity of 0.4 megawatt hours (400 kWh) ...

i will try a rough calculations : suppose we can have concrete at $100 per ton, then it's a minimum investment of $40,000. Also suppose electricity is stored with a large added value of 10 cents per kilowatt hour, so, for every cycle a rough gain of $40. By these numbers, 1,000 cycles would pay for the concrete ... so, it may look good considering they plan a life of about 50 years for such devices.
On the other hand if competitive battery storage cost only one cents per kilowatt hour (temporary in and out storage) and if concrete and fabrication goes up 10 times to $1,000 per ton then it is not economically viable anymore.

A good calculation of profitability would need to take into account the less than 100% energy efficiency of batteries cycling and of hydraulic energy cycling, ... and so many more parameters which have to be studied.

[–] blarghly@lemmy.world 20 points 1 month ago

Add to this: The chemical process of creating concrete is itself a significant contributor to CO2 emissions. So assuming the goal is to reduce CO2eq, that also needs to be accounted for.