Another seawater hydro storage system?

[AndrewE]

This isn't new, but when I looked up the Japanese sea-water pumped hydro I noticed on the Wikipedia page (https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity) an item about submerged spheres.

In March 2017 the research project StEnSea (Storing Energy at Sea) announced their successful completion of a four-week test of a pumped storage underwater reservoir. In this configuration a hollow sphere submerged and anchored at great depth acts as the lower reservoir, while the upper reservoir is the enclosing body of water. Electricity is created when water is let in via a reversible turbine integrated into the sphere...(snip)...
The challenge of designing salt water pumped storage in this underwater configuration brings a range of advantages:

    No land area is required,
    No mechanical structure other than the electrical cable needs to span the distance of the potential energy difference,
    In the presence of sufficient seabed area multiple reservoirs can scale the storage capacity without limits,
    Should a reservoir collapse, the consequences would be limited apart from the loss of the reservoir itself,
    Evaporation from the upper reservoir has no effect on the energy conversion efficiency,
    Transmission of electricity between the reservoir and the grid can be established from a nearby offshore wind farm limiting transmission loss and obviating the need for onshore cabling permits.

I disagree with the second bullet: it seems to me that this can only work if the sphere is open to the atmosphere, which means a rigid vertical breather pipe (i.e. able to withstand the crushing force at the full depth) the whole distance up to the surface, and able to stay safe in all weathers and not get flooded.  They surely can't have planned to operate it against a vacuum can they?
I don't remember reading about anything like this before, maybe they realised that the need for a breather is the hurdle that can't be jumped.
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[Stig]

Maybe they don't need a breather pipe.  If the sphere were 10m below sea level then letting in water would half fill it as the air would be compressed to 2 atmospheres, the water could then be pumped out until the inside is dry (i.e. empty of water but full of air) again.

At 20m depth you'd get it 2/3 full of water and the air at 3atm etc.

[Ted]

https://regridintegrationindia.org/wp-content/uploads/sites/14/2019/12/9A_4_RE_India19_109_presentation_Ernst_Bernhard.pdf

This looks like the spheres are only ever going to be half-full of water, and half air. Presumably the air is compressed when water is allowed in under sea-pressure and then expands again when the water is pumped out.  The full size spheres are/were planned to be 500-700 m deep.

Claims 75-85% efficiency as well.

[AndrewE]

Thanks for digging that out.  It's an interesting idea...  I would guess that the forseeable failure mode is when the air has eventually all dissolved in the water.  Don't "cushioning" air cylinders on hydraulic ram outputs have valves top and bottom so that you can drain them down?  Maybe their maintenance plan would be to lift the spheres periodically and drain them right down while servicing the turbines.
I wonder whether a diaphragm (as in our pressurised primary solar tube circuits) would do the job instead?
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