I saw a talk from MIT professor who came up with a similar idea[0]. Using spheres seemed like a pretty good idea on paper, but when they started doing the cost analysis they found that it would be cheaper to use cylindrical pipe.
Even though one uses more concrete, it's cheaper because we already have the infrastructure in place to produce and lay long concrete pipes.
In both cases he also recommended using such energy storage systems as anchors to hold offshore wind turbines in place, because the concrete is more than heavy enough to do so.
The reinforcement cage in concrete weight coating used on subsea pipelines is relatively ordinary steel. The steel is protected from corrosion by the concrete itself and a system of sacrificial anodes that are easy to survey and replace. (I help design pipelines)
> The steel is protected from corrosion by the concrete itself
I guess that the steel is protected by something else because, as the pre-1970 builders ignored and as it dawned on everyone else 20 years ago, concrete is not waterproof.
Would a cylinder be strong enough? The Germans are talking about operating at a depth of 700m, which is more than twice the maximum depth (300m) of the example analyzed in the paper you cited.
These pipelines have some inner pressure. The idea is here however that a pressure differential is used to drive a turbine. The greater the differential the more energy. The engineering challenges are not really comparable.
The US Navy's DSRV's you referenced do not have cylindrical pressure vessels. The cylindrical external hull encloses 3 linked spheres that are the actual pressure vessel. BTW, the DSRV's were retired several years ago.
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.
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
They're testing a concept here that's basically under-water balloons to store potential energy: http://hydrostor.ca
It's a pretty small-scale experiment, but everything has to start somewhere. The Deep Lake Cooling Project was prototyped at some point and that's well into production now, providing inexpensive cold water for chillers through the downtown core: http://www.canadianconsultingengineer.com/features/toronto-w...
The Hydrostor project is roughly located in the same area as the Deep Lake intake pipe comes ashore.
You are referring to the threat of trawling nets yes? So the idea presented in the paper is to use these things as anchors for wind turbines, it would not be a very good idea to go trawling through an offshore wind farm. Of course in either case, it is more likely that such structures will damage the trawling net rather than being damaged by the trawling net.
Even though one uses more concrete, it's cheaper because we already have the infrastructure in place to produce and lay long concrete pipes.
In both cases he also recommended using such energy storage systems as anchors to hold offshore wind turbines in place, because the concrete is more than heavy enough to do so.
[0]https://www.naefrontiers.org/File.aspx?id=48498