Clean Power

Published on May 21st, 2013 | by James Ayre


Energy Storage On The Bottom Of The Ocean — New Pumped Hydroelectric Power Storage Design

May 21st, 2013 by  

Storing electricity at the bottom of the ocean? The idea may sound strange at first, but the new concept from the German engineer Rainer Schramm could be very effective — with an efficiency of around 80%, comparable to conventional energy storage systems. The energy storage system makes use of the pressure differential between the seafloor and the ocean surface.

It’s not a new idea, either, with Lockheed Martin making use of temperature differences between ocean levels to create energy. 

Image Credit: Illustration by Knut Gangåssæter/Doghouse

Image Credit: Illustration by Knut Gangåssæter/Doghouse

“Imagine opening a hatch in a submarine under water. The water will flow into the submarine with enormous force. It is precisely this energy potential we want to utilize,” says Rainer Schramm, inventor and founder of the company Subhydro AS. “Many people have launched the idea of storing energy by exploiting the pressure at the seabed, but we are the first in the world to apply a specific patent-pending technology to make this possible.”

In order to do that, Schramm has joined forces with SINTEF, with the aim of developing the concept. “SINTEF has experts in the fields of energy generation, materials technology and not least offshore and deep-water technology, which means we have all the expertise we need in one place.”

“A pumped storage power plant is a hydroelectric plant which can be ‘charged’ up again by pumping the water back to the upper reservoir once it has passed through a turbine. This type of power plant is used as a ‘battery’, when connected to the power grid.”

In the new design, the pumped storage power plant turbine will be integrated with a storage tank located on the seabed at a depth of around 400-800 meters. The way that it works is: the turbine is equipped with a valve, and whenever the valve is opened water flows in and turns the turbine. This turbine then powers a generator that produces the electricity. And the number of these tanks is completely flexible, as many tanks as you choose. To word it differently, it’s the quantity of tanks that determines how long the system can generate electricity.

“When the water tanks are full, the water must be removed from the tanks,” Schramm explains, which is done by reversing the turbine, essentially turning it into a pump, and allowing it to then function as a battery. If you want to charge the ‘battery’, simply pump the water out.

While it takes slightly more energy to pump the water tanks empty, the efficiency is very comparable to conventional onshore systems — around 80% round-trip, according to Schramm.

One of the primary advantages of the system is its flexibility of scale, it can be easily tailored to user requirements. A typically-sized system will be able to produce around 300 megawatts, for a time period of about 7-8 hours — enough to power about 200,000 British households.

“We envisage that this type of storage plant will function well in conjunction with, for example, wind farms. At strong wind conditions, excess electricity is sent subsea to pump water out of the storage tanks. In periods with little wind, energy can be obtained from this underwater plant instead. The same applies to solar generation: the pumped storage power station can contribute to constant electricity production at night time when there is no sunshine to run a solar power plant,” says Schramm.

The flexibility extends not just to the turbine and tank sizes but also to the depth that the system is installed at — at greater depths the pressure difference between the ocean surface and the seafloor is higher, which means more energy is stored in an individual tank.”

“This is part of the reason why we want to try out the technology in Norway,” says Rainer Schramm. In Germany, where Schramm is from, the sea is rather shallow, too much so to allow for system profitability. Other regions are much more favorable, featuring greater sea depth closer to shore, such as: the coastal areas of Western Europe — Italy, Portugal and Spain — and of course much of North and South America.

There are a couple of design challenges remaining for SINTEF to unravel though. the primary of which is the development of “a type of concrete which can be used to cast the water tanks which are placed on the seabed.”

“The challenge is to find the optimal balance between strength and cost. If we achieve the goal of creating a concrete which will withstand at least 5 times as high loading as ordinary concrete, we can reduce the wall thickness by 75 per cent. This is a critical factor. We need to reach production and installation costs which make storage of energy economical in relation to the price of electrical energy,” explains Tor Arne Martius-Hammer, at SINTEF Building and Infrastructure, an expert with regards to concrete.

One possible solution is the use of concrete that is reinforced with thin steel fibers rather than the steel rebar that is typically used. “This will result in a significant simplification of the production process. Concrete is in existence at present which can be used, but our job is to develop a cheaper alternative,” says Martius-Hammer.

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About the Author

's background is predominantly in geopolitics and history, but he has an obsessive interest in pretty much everything. After an early life spent in the Imperial Free City of Dortmund, James followed the river Ruhr to Cofbuokheim, where he attended the University of Astnide. And where he also briefly considered entering the coal mining business. He currently writes for a living, on a broad variety of subjects, ranging from science, to politics, to military history, to renewable energy. You can follow his work on Google+.

  • nic

    Damn I had this concept 5 years ago

  • guest

    “It’s not a new idea, either, with Lockheed Martin making use of temperature differences between ocean levels to ->create energy. <-"

    ever heared of the first law of thermodynamics?

  • dwj

    Why do people keep reinventing this failed idea? It is essentially the same as a normal pumped hydro scheme except that it requires reinforced concrete pressure vessels instead of simple dams to hold the water and it is installed in a hostile and inaccessible location.

    Even if you dislike dams, it would be much cheaper to find a 400m high escarpment and dig out upper and lower reservoirs. Each cubic metre of material removed represents roughly one kWh of storage. The storage is just two holes in the ground of the same volume as the reinforced concrete pressure vessel proposed here. The scheme proposed in the article still requires a 400m long pressurised tube which equates to the penstock of a normal hydro scheme.

    • Fernando

      This scheme does *not* require a 400m long pressurized tube. Air flow is at surface pressures. And above all, it is certainly *not* easy or feasible to find “400m high escarpments”. Most of what is feasible with damns is essencially built already in developed countries. Dams, in the medium term, are useful above all to regulate water supply and flow, energy storage being a clear 2º benefit. And they all fill up with sediment, over the decades. And they have serious problems re wildlife and ecosystems. And you cannot scale them according to will or necessity, nor locate them where you want.

      • Bob_Wallace

        There are lots of places in the US where one can find abrupt changes in elevation, including abandoned mines. Lots of places to build “closed loop” pump-up.

        We’ve got around 80,000 existing dams. We use about 2,500 for power generation. Extrapolating from a study of dams on federal lands at least 10% of those remaining 77,500 dams should be usable for pump-up. That is, they have sufficient head and are sufficiently close to existing transmission lines.

        Recently a Swiss pump-up system being considered reported it needed a five cent peak to off-peak price differential to be profitable. That’s about as good an estimate as I’ve got for new pump-up.

        My guess is large scale batteries are likely to get around that price and possibly cheaper. Since batteries are so much easier to site and can be located close to point of use they are likely to win out over pump-up.

        • RobS

          The one potential niche for this is its deployment in conjunction with offshore wind farms. The economics in such a situation are helped by the fact that it allows wind power to be stored and sold at times of highest market value, the power distribution infrastructure is already in place for the wind farm and the equipment and expertise for sub-sea construction is already required.

          • James Wimberley

            No, because of the depth factor, Wind farms need shallow seas, like the North Sea; this scheme needs deep water. It would go with floating wind farms, which are still highly experimental.
            However, undersea storage would go well with solar power in North Africa and Spain.

      • dwj

        How do you get air at surface pressure to a tank 400 metres below sea level without the walls of your tube collapsing?

        Your comments on dams are equally incorrect. Reservoirs do not even need to be on water courses for pumped hydro. You can place them virtually anywhere there is a sufficient elevation change.

        • Fernando

          The proposed idea does include air flow at atmospheric pressures. Of course the pipe needs to resist surrounding water pressure. This is not especially challenging.

          As for my comments on dams, they are indeed correct. Nowhere did i state that they need to be on water courses. But the thing is that, without massive and unrealistic terrain modeling, those areas of the earth’s surface that are of adequate shape for holding large water volumes are precisely on river courses. Remember we are not talking about small, but huge volumes of water. And just look at any map, anywhere on earth, trying to find adequate locations, and you will soon realize river courses are indeed the only feasible option. Realize also that imagining building a reservoir on top of a mountain and another below, is not an economically feasible option, given the dimensions involved.

          • Bob_Wallace

            The reservoir volume for pump-up needs to be in days, not months worth of stored electricity.

            Do you think it cheaper to build an underwater structure or excavate a hole in the ground?

  • Andrew

    It’s far too costly to build a structure then to deploy that structure on the ocean floor, its vulnerability to Ocean currents make it uneconomical, high maintenance alone sound alarm bells.

  • JustSaying

    The turbine should have a long life (many cycles), do they beat the EOS system.
    “The Eos Aurora battery is projected to cost $1,000/kW or $160/kWh. The cycle life is 10,000 full cycles (30 year life). And the storage system has a 75% round-trip efficiency. As such, the LCOE is very competitive.”

    Calls for better efficiency 80%, so it again comes down to cost to built/maintain. Time will tell, my guess is we will likely end up with several types of storage.

  • Fernando

    This is an interesting concept. Multiple viable ideas for battery technology seem really to be the best approach for this problem. Constraints relate to distance to the water line, both from inland (consumption/production) and offshore (battery), sea floor depth, and the cost of deploying, installing and maintaining such a system. Knowing the concrete industry, and the material’s characteristics, i would not overly rely on concrete strength greatly deviating from what is currently available. On the plus side, zero impact on land use, water front connection (where most of the demand lies), and good potential for scalability, do seem interesting. This one really needs funding and time for develpment, it would appear. So great to witness human ingenuity at work.

  • Ross

    There was a recent post on MIT early trials of undersea pumped storage technology to be used at considerable depths similar to the ones mentioned in this article. That was mentioned in combination with floating wind.

  • Scotland

    Interesting. This technology is well established in submarines so technical risk should be low for the overall concept (subs routinely operate by filling and emptying their tanks of water/air). Sounds like they are trying to reduce the cost of the vessels/tanks to make it economically viable since the initial cost of submarine systems is not as cost sensitive (and sub systems are probably overbuilt in any case).

    This would be a perfect match for offshore wind farms. Sounds like the design is scalable – can be as large or as small as is needed for the wind farm or as the site requires.

    • James Hilden-Minton

      I wonder if retired submarines could be used as pressure chambers.

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