Clean Power

Published on April 30th, 2013 | by Guest Contributor


MIT’s Innovative Floating Wind Energy Storage Technology

April 30th, 2013 by  

Following up on an innovative floating wind energy storage technology we covered some time back, here’s a post from MIT itself on details and news about this technology.

wind turbines offshoreBy 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 wind 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.

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 seawater 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-deep water 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 nuclear plant. 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 solar arrays on shore, or from base-load power plants, which operate most efficiently at steady levels. This could potentially reduce reliance on peak-power plants, 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 seafloor 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.

The work was supported by a grant from the MIT Energy Initiative.

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  • marsuvious Grammen

    Actually, it’s a *lot* different from pumped hydro storage. In pumped hydro, you’re storing the potential energy associated with lifting water against gravity (which is weak – not much energy per lb. of water). In this scheme, you’re storing the potential energy associated with pushing energy against the *pressure* of tons of water above the concrete spheres. At 750 meters deep – water pressure of about 75 atmospheres – every pound of water pumped stores the same potential energy as 75 lbs of water pumped in PHS. Less water pumped, less friction, per kWh of energy stored – it’s a win.

  • globi

    Since there’s already variable demand, the grid already has flexible power plants to deal with that variability (be it demand or offshore wind power). Besides, there’s a tremendous hot water and heat energy demand and heat energy can be produced with electricity instead of fossil fuels and can be stored cheaply -> ideal to store surplus wind power.

    Offshore storage would only be sensible, if storing offshore wind power is cheaper than building a larger cable to the shore. I highly doubt this to be the case.

  • dwj

    This is no different to pumped storage hydro, except that it would be much more expensive to build and operate. Think about how tiny a 25 m diameter sphere is compared with a hydro reservoir. With the same “head” of 750 m, the hydro dam would have the same energy per cubic metre.
    Dumb idea, just build the pumped storage hydro.

    • Dave2020

      Easier said than done. To use the jargon – PHS is ‘geographically constrained’! If you don’t have huge reservoirs with an altitude difference, you’re stymied.

      But I agree. It’s not a very good idea, because it would have relatively high running costs. (just like PHS) They both suffer from ’round-trip’ losses, and entail high investment costs.

      If energy storage before-generator was integral to offshore wind (and wave) the ’round-trip’ cost would be zero, and the capital expenditure greatly reduced.

  • Today I would like to be deconstructive: Sounds like a nutty idea!

  • Bob_Wallace

    When you compress air a goodly part of the incoming energy gets turned into heat and with most CAES systems this heat is lost, lowering system efficiency.

    Does the same happen when you compress water?

    • Dave2020

      Water is incompressible, and there’s no compressed air in the MIT design.

      It utilizes the ‘head’ of water above, which is why it has to be at depth.

      • tom lakosh

        You certainly do compress air to match the head of water and
        the only advantage over CAES is the smaller water turbine used for power
        generation. You’d be much better off using suction pilings or rock bolts for
        anchors, (more reliable and cheaper), and then use the turbine tower and floating
        spar to compress air or as a hydraulic accumulator, (CAES or Mitsubishi
        hydraulic wind pump/generator). The generator, run on compressed air or
        hydraulic motor, could then be located at sea level but wiring and ladder/elevator
        to the nacelle would have to be in a separate pressure tube that could be
        internal for compressed air or external to the tower for hydraulics.

        • Dave2020

          “. . . pumping seawater from a 30-meter-diameter hollow sphere.”

          That creates a vacuum. If it were filled with compressed air “to match the head of water” you’d have a very low flow through the turbine!

          The walls are 3m thick because of the huge pressure differential, necessary to get a good energy density. (6MWh in quite a small volume) The depth is essential for this, but it’s a serious drawback in every other respect.

          I agree, its use as one of the 3 or 4 anchors required is immaterial.

          Five years ago I toyed with the idea of hydraulic accumulators, but that volume of hydraulic fluid would cost an arm and a leg.

          I think a closed circuit of fresh water will be the best option and a variable displacement water pump could be used to control the speed of a VAWT, so that mechanical and aerodynamic braking are eliminated. VAWTs are more easily designed for low-cost mass production and ALL the heavy gear is at sea level, making them perfect for floating wind – easier and cheaper to keep stable. There’d be cost savings on installation and O&M too.

          What you describe is energy storage before-generator, which what I have long advocated. (see my profile for the rest)

  • JustSaying

    The only real tie to off shore wind is the anchor. Go back to last year and you see a post here on using bags to hold air underwater as a energy storage approach. Ok, you can also say that you have already run the cable out to the site, which is a saving. But a depth of 750 meters (~.5 miles) that is really deep. And likely a “few” years out before wind turbines are installed at that depth. But when/if it happen I bet you are more likely to see simpler anchors for the turbines, and then one or two large storage tank “attached” to the stepping center of the turbine field. That lets you scale the two component separately to maximize profit. And of course there are places with very deep water near to shore, there you might build the storage without the turbines.

  • Sounds like a well thought-out, scalable solution for off-shore wind turbines.

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