other than to store excess offshore wind power why would anyone look at this technology ? onshore pumped hydro is currently buildable technology ... looks like they are grant fishing ...
build more nat gas power plants and you can spin up power production in minutes to meet peak power needs with NO storage losses ...

You can exploit potential energy anywhere. Why in the world would you want to have to build something on the bottom of the sea?

The harder the shell material and the more perfect the sphere the less shell to volume you need, but you will then need extra ballast or a stronger anchor to keep it down. However having the container buoyant is not all bad having it float to the surface for maintenance would be convenient.

a buoyant chamber is an excellent idea Slowburn :) only just buoyant and just tether her off to the seabed. the pumping losses with this sort of idea must be rather impressive though :(

Pumping losses are completely irrelevant if your power source is excess night time wind power. This allows you to save wind power that might otherwise be lost and sell at times of greatest demand and price, sure you have some pumping loss but the utility gain is far greater.

Someone already has ideas to solve lots of these problems. Look for "Seamus Garvey" and "Energy bag" on youtube. He is working on this since before 2009:
www.youtube.com/watch?v=NgvLvZHzJrE
youtu.be/ktup6CAvfGo

re; Jeffrey J Carlson
Some generating systems when running at maximum efficiency are efficient enough to pay for the storage loss and still be more efficient than the Natural Gas plants that spin up in minutes. Then there is government mandated randomly intermittent power generation. (wind and solar)
re; Craig Jennings
The hydroelectric plant is simpler and more efficient. Plus your tether is going to wear out rapidly.
re; Adruna
I like pneumatic energy storage but Hydraulic is more efficient and the tank can sit on the bottom under its own weight the airbags have to be tethered. Also the storage vessels are compatible with storing compressed gas at equivalent pressure. If the gas is C02 the energy storage is impressive but pumping heat in will be necessary.

Heavy concrete is a bit over twice as dense as water, so as long as your volume of air is about the same as the volume of concrete, the chamber will sink and can thus sit nicely on the bottom without tethers. That does impose a volume/cost limit, but I can see concrete chambers holding an enormous amount of pressure, so J/m^3 could be pretty high.
Pumped storage requires that you have somewhere to pump the water to - if you're somewhere flat and coastal with lots of wind power, like say the Netherlands, this seems like an appropriate solution.
Coupling the storage with generation, i.e. storage tanks near turbines should minimise transmission loss.

@Slowburn... You said about hydraulic storage with the last MIT story...
The only reason there is a return of energy is because of the lower density of air... As most hydraulic fluids are mostly water, there will be no return of energy using an under water system such as this.... Remember that the pressure is only there because of a tall column of water above the seabed, if a fluid as dense as water (or nearly) is used, the net pressure at the surface is zero... And this sort of system can only be used by venting it to the atmosphere (or any handy vacuum).. Air just happens to be the most abundant gas to use.
Also the others, this can be used for storing energy in flat countries or like most places, those without adequate mountains near the coast....
Transmission and pumping losses in pumped hydro are significant, in Australia for instance the only viable pumped hydro systems are in the Snowies and Tasmania, sending the power from Sydney to the Snowies for storage and back again, loses something like 30% of the power..... but then burning it off in resistor banks loses all of it... Sending the power from Perth would probably result in a net loss.... BUT Perth could store energy at the bottom of the ocean... (remember there are no mountains on the west coast (Ok, they may call some mountains, but not really... that's just to make the WA residents feel they aren't without a landscape. (haters, just trolling, don't respond.) (Oh maybe I meant Bahrain) oops to many words...

How do you mitigate against damage to sea floor and close-to-sea floor fauna and flora? I mean obviously you can have filters to stop foreign objects from being sucked in, but it isn't going to do sea life any favours being mashed up against a sieve every time there is a peak in energy demand.