Wind power—even without the wind

Apr 26, 2013 by David L. Chandler

Offshore wind could provide abundant electricity—but as with solar energy, this power supply can be intermittent and unpredictable. But a new approach from researchers at MIT could mitigate that problem, allowing the electricity generated by floating wind farms to be stored and then used, on demand, whenever it's needed.

The key to this concept is the placement of huge concrete spheres on the seafloor under the . 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.

Whenever the wind turbines produce more power than is needed, that power would be diverted to drive a pump attached to the underwater structure, pumping from a 30-meter-diameter hollow sphere. (For comparison, the tank's diameter is about that of MIT's Great Dome, or of the dome atop the U.S. Capitol.) Later, when power is needed, water would be allowed to flow back into the sphere through a turbine attached to a generator, and the resulting electricity sent back to shore.

One such 25-meter sphere in 400-meter- could store up to 6 megawatt-hours of power, the MIT researchers have calculated; that means that 1,000 such spheres could supply as much power as a nuclear plant for several hours—enough to make them a reliable source of power. The 1,000 wind turbines that the spheres could anchor could, on average, replace a conventional on-shore coal or . What's more, unlike nuclear or coal-fired plants, which take hours to ramp up, this energy source could be made available within minutes, and then taken offline just as quickly.

The system would be grid-connected, so the spheres could also be used to store energy from other sources, including on shore, or from base-load power plants, which operate most efficiently at steady levels. This could potentially reduce reliance on peak-, which typically operate less efficiently.

The concept is detailed in a paper published in IEEE Transactions and co-authored by Alexander Slocum, the Pappalardo Professor of Mechanical Engineering at MIT; Brian Hodder, a researcher at the MIT Energy Initiative; and three MIT alumni and a former high school student who worked on the project.

The weight of the concrete in the spheres' 3-meter-thick walls would be sufficient to keep the structures on the even when empty, they say. The spheres could be cast on land and then towed out to sea on a specially built barge. (No existing vessel has the capacity to deploy such a large load.)

Preliminary estimates indicate that one such sphere could be built and deployed at a cost of about $12 million, Hodder says, with costs gradually coming down with experience. This could yield an estimated storage cost of about 6 cents per kilowatt-hour—a level considered viable by the utility industry. Hundreds of spheres could be deployed as part of a far-offshore installation of hundreds of floating wind turbines, the researchers say.

Such offshore floating wind turbines have been proposed by Paul Sclavounos, a professor of mechanical engineering and naval architecture at MIT, among others; this storage system would dovetail well with his concept, Hodder says.

In combination, floating turbines and undersea storage spheres could provide reliable, on-demand power, except during extended calm periods. Meanwhile, a siting many miles offshore would provide the benefit of stronger winds than most onshore sites, while also operating out of sight of the mainland. "It provides a lot of flexibility in siting," Hodder says. The team calculated that the optimal depth for the spheres would be about 750 meters, though as costs are reduced over time they could become cost-effective in shallower water.

Jim Eyer, a senior analyst with energy consulting firm E & I Consulting of Oakland, Calif., who was not involved in this research, says the concept "addresses some important challenges associated with wind generation in general, especially the temporal mismatch between production and demand, and generation variability, especially rapid output variations that lead to excessive 'ramping' of dispatchable generation." While he calls the idea "somewhat novel and potentially significant," he adds, "Obviously we'll need a proof-of-concept pilot to take the next development step."

Slocum and some of his students built a 30-inch-diameter prototype in 2011, which functioned well through charging and discharging cycles, demonstrating the feasibility of the idea.

The team hopes to extend its testing to a 3-meter sphere, and then scale up to a 10-meter version to be tested in an undersea environment, if funding becomes available. MIT has filed for a patent on the system.

The researchers estimate that an offshore wind farm paired with such storage spheres would use an amount of concrete comparable to that used to build the Hoover Dam—but would also supply a comparable amount of power.

While cement production is a major source of carbon-dioxide emissions, the team calculated that the concrete for these spheres could be made, in part, using large quantities of fly ash from existing coal plants—material that would otherwise be a waste product—instead of cement. The researchers calculate that over the course of a decade of construction and deployment, the spheres could use much of the fly ash produced by U.S. coal plants, and create enough capacity to supply one-third of U.S. electricity needs.

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User comments : 18

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2.5 / 5 (4) Apr 26, 2013
The concrete spheres aren't forever. One must ask, how are you going to rebuild them after 50-60 years? Where do you get the cement then?
2.8 / 5 (4) Apr 26, 2013
My concern would be sea life pulled in during power generating stages that might not get pumped out and would slowly fill the sphere.
1.9 / 5 (9) Apr 26, 2013
ahh green tech, expensive, impractical, environmentally disastrous and taxpayer funded
2.2 / 5 (5) Apr 26, 2013
3.8 / 5 (4) Apr 26, 2013
I don't think that the spheres would tend to fill with muck as a little clever engineering could be use to flush them out.

One thing that intrigues me is the air supply for the spheres: air must be pumped down into them from the surface, and there would be energy losses do to compressive heating.
3 / 5 (3) Apr 26, 2013
It just came to me that my concern for the adiabatic energy loss is unfounded; all one needs is a snorkel - a simple pipe to the surface from the top of the sphere.
1.6 / 5 (5) Apr 26, 2013
But What is the use of SUCH A TALL STEM when there is NO wind Blowing? It should descend down and smaller auxiliary blades over it should be stirred up by the sea waves!
2.4 / 5 (5) Apr 26, 2013
Of course, suitable nets should prevent damage to fauna by the blades...which again becomes a sophisticated issue.
1 / 5 (4) Apr 26, 2013
Patented Idea. Check out SubGenStor from 2009 WO2009111861A1.
2 / 5 (5) Apr 26, 2013
hemitite: you only need to fill the sphere with air once. Once sunk down the air will compress into a tiny volume as the sphere fills with water, and the system works by pumping the water out to lower the pressure inside, so that the volume of air would expand and fill the sphere again.

Point being that the insides of the concrete sphere must remain at a lower or equal pressure relative to its surroundings because concrete cannot withstand a pull. It has to be under compression. If there was positive pressure inside, if you pumped air in, it would crack, so instead you draw a vacuum by pumping water out, and then recover the energy by letting the water get sucked back in.

You never need to add more air, because the seawater contains dissolved gasses, and suddenly introducing it to a lower pressure will actually make it bubble like club soda. If anything, you may need to remove air from the sphere.
3 / 5 (2) Apr 26, 2013
Your may well be right, but the paper this article refers to has it work your way according to fig. 1 and my way according to the text!

"Tension legs can be in the form
of steel tubing which offers a protected conduit for cables;
in addition, the legs can serve as snorkels so 1-atm pressure
could always be present in the sphere which can enable
simpler pumps to be used by eliminating cavitation as a

Go figure.
1.8 / 5 (6) Apr 26, 2013
The over time the oceans c9ntinuousotion destroys all things. Even the coastline. Its called erosion. Ocean based power generating and storage systems are all future junk that the ocean will eventually rust into microparticles. building in the ocean is a dead end save for oil platforms mining and other temporary high value energy denseextraction.
3.7 / 5 (6) Apr 26, 2013
Everything becomes junk over time, either on land or in sea.... the only question is if the investment both monetary and environmental (carbonIN vs carbonOUT) pays off....
2 / 5 (4) Apr 26, 2013
It's just a matter of money. The value of a big energy storage system times the years it can stand minus the cost to deploy and mantain it. If the result is positive it will be eventually built, otherwise it will not. Surely in some hundred years it will be completely destroyed, but who cares?
5 / 5 (1) Apr 27, 2013
Concrete continues to harden for around 150 years, so gets stronger as time passes. There are Roman concrete structures built in the seawater in the Mediterranean, as part of port construction, etc, which still exist after 2,000 years.
Yes, concrete will erode over time but the reinforcement metal decay is a bigger issue in saltwater. However, if bridge foundations built in the 1930s, before the introduction of epoxy coatings, are still safe to use, more modern construction techniques should easily give a 2 or 3 century working life.
Once the dome has failed, it can be opened up and find a new life as a wildlife refuge.
not rated yet Apr 27, 2013
Cement manufacture is an appalling producer of CO2 and user of energy. Concrete production releases about 380kg of CO2 per cubic metre, and these spheres will each contain approx 10,000 cubic metres of concrete (if my calculator is right), so that's 3800 tonnes of CO2 per sphere - it'll have to be very green to offset that lot...I'm sure they can come up with a better plan - send the electricity ashore and pump water uphill to a reservoir for a conventional stored hydro plant? Much cheaper and easier to build
5 / 5 (1) Apr 27, 2013
it'll have to be very green to offset that lot.

If you had read the article you would have noticed that they have addressed that.

Please read before posting. Thank you.
not rated yet Apr 29, 2013
Hydro pumped storage is THE 'killer app' in energy storage, so the dual-sphere concept is a fine idea. Sea water, however, is NOT. No problem: just fill one sphere half full (or so; not enough so that it stops being bouyant) with fresh water before towing it out to sea. Sea muck will NOT get in because this dual-sphere approach is a closed system. The fact that it ALSO anchors the offshore wind turbine is a PLUS. Only downside I see is doing maintenance on the underwater pump-gen equipment ... which will be at the BOTTOM...near the sea floor.

These things DO require periodic maintenance, which means you also have to build an attached undersea oasis with breathable air, temporary living quarters, turbine-generator maintenance and repair HALL, waste facilities,etc, etc) Accessible by submarine only, which means you'll need to add one submarine - with it's O&M costs - to your projected project cost! $12M is a tad cheap.

BUT HEY! Good job thinking outside the (underwater) box!

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