The promise of seabed storage

hat offshore energy storage systems could be used alongside the wind farms and tidal and wave arrays taking shape off our coasts. Photo credit: MIT

With renewable energy increasingly being located at sea, its worth asking what offshore energy storage systems could be used alongside the wind farms and tidal and wave arrays. Photo credit: MIT

The renewable energy storage concept announced by the Massachusetts Institute of Technology (MIT) earlier this year is quite literally a load of balls. Aimed at storing excess power from offshore wind farms, says MIT: “The key to this concept is the placement of huge concrete spheres on the seafloor under the turbines.

“These structures, weighing thousands of tons apiece, could serve both as anchors to moor the floating turbines and as a means of storing the energy they produce.”

The massive airtight balls in MIT’s design would be hollowed out and filled with water, which would be pumped out using spare energy from the wind farm. Then, when more energy is needed, for example to compensate for a drop in wind, water would be allowed back into the sphere, driving a turbine along the way.

MIT calculates a single 25-metre-diameter sphere 400 metres underwater could store 6MW-hours of power. “That means 1,000 such spheres could supply as much power as a nuclear power plant for several hours,” gushes the press release.

Building spheres of concrete

The team proposes building the spheres of concrete three metres thick, which would weigh enough to keep the structure on the seabed even when full of air.

MIT researchers Alexander Slocum and Brian Hodder believe the build-and-deployment cost for the concept would come in at around USD$12m per sphere, which they say equates to $0.06 per kilowatt-hour, a commercially acceptable rate.

So far, so fanciful. But the MIT team is not the only one touting submarine energy storage concepts. Subhydro, a Norwegian developer, is also working on an energy storage system based on concrete tanks located between 400 and 800 metres down on the seabed.

Like MIT, it aims to use excess renewable energy to empty the tanks and then drive a turbine as they fill again, claiming the system has an 80% efficiency rate.

Compressed air energy storage

Meanwhile a team at the University of Windsor, Ontario, has teamed up with a company called Hydrostor for a compressed air energy storage (CAES) system consisting of high-strength polyester bags sunk 50 to 500 metres underwater.

The idea is that these will be filled with air using excess wind power and then, when energy is needed, the water pressure will be used to push the air back out of the balloon, driving a turbine.

The team has already tested a full-sized prototype in Lake Ontario, and is said to be aiming for a commercial system later this year. Toronto Hydro has helped build the demonstration unit and MaRS Cleantech Fund is backing the project.

With so much interest emerging in recent months, the big question regarding these submarine ideas has to be how they stack up to more conventional energy storage technologies.

Fairly established principles

The good news is that they all seem to be based on fairly established pumped hydro or CAES principles, which a recent Stanford University study confirmed are way ahead of batteries in terms of the ratio of energy stored to energy expended.

And placing them underwater seems a good idea on two counts. The first is that if they are close to offshore wind farms, or tidal or wave plants, they can store excess power with minimum transport losses and then deliver it using existing export cabling and infrastructure.

The second is that hiding storage underwater is unlikely to excite the kind of not-in-my-backyard susceptibilities that have dogged onshore wind developments, for example.

There will be a need for environmental assessments, of course, but underwater storage does not a priori appear to carry any more risks for sea life than current marine renewable energy generation technologies. Having said that, there are also likely to be significant engineering challenges with seabed-based energy storage.

Size of the units

The depths involved are one thing: offshore wind is only just getting round to tackling waters beyond around 30 metres in depth, while the storage concepts being proposed go down more than 10 times that distance. Another problem is the size of the units.

The MIT admits that “no existing vessel has the capacity to deploy such a large load” and a large-scale array of its spheres could require as much concrete as the Hoover Dam (although they could use fly ash instead of cement, says the team).

On this latter point, at least, the Hydrostor CAES idea might seem to have an advantage, which is possibly why it appears to be making good progress. And in any case it is probably worth remembering that current plans for marine renewable energy storage include such outlandish concepts as building whole artificial islands.

If such ideas deserve serious attention (and few in the energy business would doubt that they do), then giant submarine balls should too. Perhaps this is one proposal that could sink without trace… yet still leave a lasting impression.

Written by Jason Deign

A version of this article was previously published in Marine Renewable Energy.

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