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Offshore renewable energy storage

An Energy Bag has been developed for undersea compressed air energy storage (CAES) of offshore wind, wave and tidal power.

By Renewable Energy Focus staff

Canadian Thin Red Line Aerospace has developed the Energy Bag, which will see a prototype anchored to the seabed off the coast of Scotland this summer as part of a renewable energy research project led by Professor Seamus Garvey of the University of Nottingham, UK. The project is being supported by E.ON.

Offshore renewable energy devices such as wind turbines fill the balloon-like underwater bags with compressed air that can later drive electrical generators on demand.

The technology, which is particularly suited to countries with relatively deep water near the coast, can be anchored at a depth of around 600 m where the pressure of the ocean takes on the role of high performance pressure vessel.

The pressure at this depth ensures high energy storage density, constant pressure and compatibility with existing high efficiency turbine technology.

Red Thin Line has performed concept development of volumes to 6000 m3.

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Comments

Kari Williamson, Assistant Editor said

06 May 2011
Maxim de Jong kindly responded again:

The bags would be anchored with ballast. Like everything else, it ultimately boils down to economics. The feasibility of using ballast is strongly supported by Thin Red Line’s unique architecture which was briefly described in my last response. While the topic of the ballast itself does not fall under my immediate engineering purview, project leader Professor Garvey at the University of Nottingham has “run the numbers” rather exhaustively. I am certain more details will be made available when this first structure is deployed.
Maxim de Jong

salmo said

06 May 2011
Dear Kari,

Thank you for your follow-up.

Even though they seem to be able to take care of the buoyancy load on the structure by a mesh of fiber tendons, it will still be a rather difficult engineering task to anchor the tendons in the seabed...
First of all they will have to do that at a depth of 600m, but which might be covered by actuel deep sea drilling technology and devices.
But as far as I know, the seabed is very soft and you would have to drill either lots of deep holes or only several, but then even deeper ones. Could this be done at a reasonable cost, ie. what will be the actual ROI figures? (Deep sea technology is extremely expensive!)

Of course it will be - technologically spoken- easier to anchor 150 bags of 40 m3 displacement (=only 400kN buoyancy force per bag) than one of 6000m3...but the costs will be very high for any of these options.

I'd like to see this work (because I had this idea also a long time ago), but I'm sceptical on the actual possibility to anchor it at that depth, the necessary investment and the cost of maintaining it over a longer period of time.

Best regards,
Salmo

Kari Williamson, Assistant Editor said

06 May 2011
This is the response from Thin Red Line's Maxim de Jong:

The ocean provides the structural role of high performance pressure vessel, i.e. there is no pressure differential across the bag membrane. The bag is thereby relegated to a flexible, balloon like structure needing only to restrain the buoyant air bubble contained within—rather than a massive, thick-walled pressure tank of exceptional cost and complexity. As you indicate, the ballast anchoring of the buoyancy load is a significant engineering feat in itself—but remember that it is almost incidental compared to the structure which would be required to contain the pressure load if the ocean did not provide this function. To control its buoyancy load, a 6000 cubic meter bag would sport an array of 10 cm diameter Spectra fiber tendons, each with a tensile strength of 800 metric tons. Designed by Thin Red Line’s Maxim de Jong, the prototype energy bag in the picture displaces 40 tons of seawater, and is to be anchored to the seabed by its array of Vectran® fibre tendons capable of restraining a total load of 250 tons—yet the entire systems weighs only 75 kilograms (165 pounds). The design is based on Thin Red Line’s inflatable space architecture currently being investigated in several NASA programs. Thin Red Line is known for their ultra-high performance fabric structures, having notably developed and manufactured the pressure restraining hulls of the Bigelow Aerospace Genesis 1 and 2 satellites launched in 2006 and 2007, the first spacecraft on orbit successfully incorporating large volume, high-stress inflatable architecture.
Maxim de Jong

Hope that answers your question!

Best regards,

Kari

salmo said

05 May 2011
Dear Sirs,

how exactly do they want to keep these tanks down on the sea ground?
At 6000 m3 volume, this would create a force of around 60MN directed upwards...
Thanks for any further clarification on a flexible structure that would withstand these conditions.

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