In traditional pumped storage, you use the pressure of water pumped up some elevation to generate electricity by spinning a turbine as it falls back down. In this instance, you use the pressure of the bottom of the ocean and pump the water out, so that when it is allowed to flow back in, it spins a turbine and you generate electricity.
It's necessary to the design for them to be underwater.
Sorry to nitpick, but the concept works just as well above water. It also has the benefit of not requiring the pressure vessel.
Building a swimming pool of the same volume 700m up the side of a mountain, pumping water up and using it to drive a turbine on the way down is the same concept (and would give the same energy storage capacity [Pi * 4/3 * (30/2)^3 * 700 * 1000 * 9.81 / 3600 / 1e6] = ~27 MWh (assuming 100% efficiency).
I would imagine the benefit of doing this in the ocean is that the plumbing is simpler (don't need to lay the pipes) and you don't need to find a 700m mountain and available land for the pool, pipes and base station.
In the undersea version, what you're really doing when you pump the water out of the vessel is lifting the entire ocean level by a minuscule amount and then generating power when you allow it to fall back down.
Right, you're just describing traditional pumped storage. I suppose I should clarify because I didn't actually say what the advantage was, just the ways that the two designs are similar, but different.
In short, the advantage is that it exploits the same concept but in a very different way that avoids the pitfalls of traditional pumped storage.
The (dis)advantages:
- Safety. Dam failure is pretty catastrophic (see Vajont Dam disaster.) Although it's yet to be seen what failure of this device would look like, it's likely to be much less threat to human life. It would probably kill or deafen nearby marine life.
- Ecological concerns: a dam flooding a mountain valley destroys possibly unique ecosystems, and causes uncontrolled greenhouse gas emission as all the vegetation now underwater rots
- Land availability: land is at a premium, but seabed is abundant
Also, mountains for pumped storage are not always near population centers, and are often not near sources of renewable production that need the pumped storage. In that case, you have to transmit power from its point of generation to its point of storage to its point of use. In this scenario, with these spheres potentially being used as anchor points, the transmission distance between generation and storage is ~700m, and then you just need to transmit to your nearest coastal city of choice.
It's necessary to the design for them to be underwater.