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Published on April 18th, 2011 | by Susan Kraemer


Energy Bags Under the Sea to be Tested to Store Off-Shore Wind

April 18th, 2011 by  

An innovative new way to store wind power has been invented by researchers at the the University of Nottingham. Literally in inflatable bags under the ocean. A university spinoff, NIMROD Energy Ltd, has been launched by Professor Seamus Garvey, based on the research. Next month a prototype is to be tested in seawater.

E.ON, a leading renewable energy company in Europe, provided a grant to the university researchers to develop the undersea Energy Bags™  in 2008.

Like pumped storage –  that relies on the relentless forces of gravity to make energy when a reservoir of water is released downhill – these huge inflatable undersea bags would leverage the relentless power of compression found deep under the sea.

When excess wind power is generated, because there is no use for it on the grid – for example, at night – the energy generated would be used to pump compressed air into the bags, expanding them. Then, when needed, the natural force of the sea itself would be allowed to squeeze the compressed air out of the energy storage bags to run turbines to make electricity on demand.

Garvey believes it is possible to store energy at a cost well below $16/kWh – less than 20% of the cost for pumped hydro energy, the cheapest competing technology, and at 90% efficiency. He envisions that storage to match a third of wind power capacity will be needed by 2020.

“If you have 1MW of integrated compressed air system (including large energy stores) for every 3MW of conventional generation, then the whole set of offshore wind equipment starts to look like a very versatile power generating system which can adjust its output to match demand — notwithstanding what the wind is doing.”

According to Powermag, Garvey says that storing the equivalent of 2 GW for four days will require 7 million cubic meters of air storage. “The optimal dimensions for energy bags are around 20 meters in diameter and each has a volume (when full) of about 4,000 cubic meters,” he says. “For 7 million cubic meters, we would need 1,750 of these bags. The seabed area covered by these would be less than one square kilometer and the total surface area of bag material would be 2.2 million square meters.”

Like the best energy storage technologies, the process is simple in concept. However, it requires a new, much larger fleet of offshore wind turbines to become economic.

And that is just what is now beginning to happen around Scotland and the UK, as turbine manufacturers continue to develop ever-larger turbine sizes (try 20 megawatt wind turbines! Twenty times the size of many turbines in the US.) and ever larger wind farms off the coasts of Europe, and a gigantic North Sea grid to connect them all.

Susan Kraemer@Twitter

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About the Author

writes at CleanTechnica, CSP-Today and Renewable Energy World.  She has also been published at Wind Energy Update, Solar Plaza, Earthtechling PV-Insider , and GreenProphet, Ecoseed, NRDC OnEarth, MatterNetwork, Celsius, EnergyNow, and Scientific American. As a former serial entrepreneur in product design, Susan brings an innovator's perspective on inventing a carbon-constrained civilization: If necessity is the mother of invention, solving climate change is the mother of all necessities! As a lover of history and sci-fi, she enjoys chronicling the strange future we are creating in these interesting times.    Follow Susan on Twitter @dotcommodity.

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  •  Cool, the world need more of this technologies. but now… not in “the future”


  • Kerry Swift

    Susan – how are they being anchored? That is a huge issue.

  • Anumakonda Jagadeesh

    Great Innovation to store offshore wind energy through bags under the sea. I am glad more and more approaches to harness wind energy are coming up.

    Dr.A.Jagadeesh Nellore(AP),India
    Wind Energy Expert
    E-mail: anumakonda.jagadeesh@gmail.com

  • Pumping air to depth takes huge energy… an alternative. Send the power to the bottom in electric cables. Use the electricity to split water into hydrogen and oxygen. You now have two very valuable resources at industrial working pressure.

  • Anonymous

    This storage technique would seem to have an advantage over CAES in a salt dome or other underground location. As the stored air is used there will be no decrease in pressure as the ocean above will fill in the now-vacated space.

    This also suggests onshore storage possibilities. Drill a modest-sized well very deep and then excavate a large underground area. Install air bags in the excavated area and then flood the area above the storage bags. Going deep underground also means storing air at hotter temperatures.

    Even using abandoned mines (which are often naturally flooded) might provide storage opportunities. They probably already transmission lines running to them.

  • Ben

    Susan, this sentence is quite clunky:

    “these huge inflatable undersea bags would also use the relentless power of compression found deep under the sea, to make energy as the sea is allowed to compress the bags air forced under the sea.”

    • Susan Kraemer

      Tell me about it! Many of my sentences are clunky…

      • Susan Kraemer

        I went back and de goofed it a bit, hope I wrangled some sense out of it.
        I was really trying to compare it to the simple natural force of gravity used in releasing the stored energy in pumped storage.

      • Anonymous

        I feel a need to clarify…

        The reason that I clicked on ‘Liked’ is because Susan responded back and in such a nice way. I appreciate it when people who write articles engage with commentators.

        I’ve got no opinion on Susan’s sentences. I just like the good info she and the other authors bring to the site.

  • Nikodean1

    Are you sure it is $16/lWh? Or 16 cents?

    $16 sounds very extreme. I don’t think it can be that expensive.

    • Susan Kraemer

      Yes, I tend to think it is $0.16 per KWh too, but I wasn’t able to confirm with the editor of Power Mag, who I am quoting Prof Garvey as saying. Elsewhere – and a few years ago, Garvey said $20 per MWh, which is even more evidence that it would not be so high.

      • Anonymous

        Let me copy something over from a GMT article…

        “The Ionex Energy Storage System is a 1-megawatt-hour unit capable of producing 1 megawatt or 2 megawatts of continuous AC power from a 40-foot shipping container weighing 35,000 kilograms. The container can be mounted on a concrete pad or on a wheeled trailer.


        We think two-hour systems will be about $1,200 per kilowatt.” But he stressed that “we really need to see a fully installed system under $300 to $400 per kilowatt-hour.””


        And from another GMT piece about Duke Energy contracting with Xtreme Power to provide a 36MWh storage system for its 153-megawatt wind farm in Notrees, Texas…

        … he has confidence that Xtreme could one day get its prices down to $500 a kilowatt hour, or less than flow batteries and sodiums, but Xtreme is not near that price now. ”

        $16/kWh would be an extreme breakthrough in storage.

        Remember, that’s the cost of the facility. It’s not what storage would add to the cost of a delivered kWh of electricity. The facility would get used over and over and over….

        • Susan Kraemer

          That makes sense. So its just a one time cost to build it, not for every kwh stored. Thanks.

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