The promise of seabed storage

hat offshore energy storage systems could be used alongside the wind farms and tidal and wave arrays taking shape off our coasts. Photo credit: MIT

With renewable energy increasingly being located at sea, its worth asking what offshore energy storage systems could be used alongside the wind farms and tidal and wave arrays. Photo credit: MIT

The renewable energy storage concept announced by the Massachusetts Institute of Technology (MIT) earlier this year is quite literally a load of balls. Aimed at storing excess power from offshore wind farms, says MIT: “The key to this concept is the placement of huge concrete spheres on the seafloor under the turbines.
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Doughnuts: just the thing for storage?

Depending on your point of view the plan announced by Belgium earlier this year is either pure James Bond, or straight from The Simpsons. The Bond take: the Belgian government is going to create a private island to help harvest power from the sea. Simpsons: the Belgians are building a giant doughnut to do away with nuclear power plants like Springfield.

Whatever your perspective, though, there is no doubt the Belgian scheme has the potential to change the script for marine renewable energy storage. In essence the idea is simply a new solution for that age-old conundrum of how to add storage to intermittent renewable energy sources such as wind, solar or wave power so they can be used for base-load power generation. But what a solution!

According to Reuters, the concept announced by Belgian North Sea minister Johan Vande Lanotte at a Zeebrugge port presentation will involve building a doughnut-shaped island out of sand three kilometres off the coastal village of Wenduine.

The hole in the middle will be filled with water that will be pumped out whenever there is excess power coming from the country’s offshore wind farms, which are ultimately planned to be able to deliver up to 2,300MW.

In times of need, meanwhile, gates in the walls of the doughnut/island will be opened up and the water flowing into the hole in the middle will be used to drive turbines, delivering hydro power that can be sent to shore.

The government reckons it will take at least five years to build but will help the country shift away from its reliance on nuclear power, which in 2011 accounted for about 57% of the energy consumed. Belgium currently has seven nuclear reactors in two power plants, at Doel and Tihange, generating a total of 5,761MW.

Two of the reactors were shut down last year after the operator, GDF Suez subsidiary Electrabel, found flaws in their reactor casings. Two more are due to be retired in 2015, with the rest following suit by around 2025. “This is a great solution,” a ministerial spokeswoman said in the Reuters report.

“We have a lot of energy from windmills and sometimes it just gets lost because there isn’t enough demand for the electricity.”

She is absolutely right. Although novel in terms of its scale and location, this is essentially a pumped hydro project. And pumped hydro goes with wind energy like Charlie Chaplin with a walking stick. The technique is already used extensively by Iberdrola of Spain to maximise the value of its myriad wind energy farms.

Meanwhile the Spanish island of El Hierro is on track to become 100% self-sufficient using wind energy, thanks to the Gorona del Viento project that uses a double-dam setup powered by onshore Enercon turbines.

On a wider scale, the immense hydro resources across Scandinavia could potentially play an important role in helping to store and redistribute energy from offshore wind farms in Germany and the UK. What does the Belgian proposal add to this picture, in terms of its significance for the marine renewable energy storage market? Three things, at least.

The first is that it would vindicate the idea that intermittent renewable energy, including marine generation from wind, wave and even floating solar, can be more than a match for existing base-load sources if combined with appropriate storage.

After all, let us not forget that the Belgians are not planning to replace gas peakers here, but nuclear power plants, arguably one of the steadiest pillars of base-load power provision.

The second point of note is any power generation scheme that involves shifting water in and out of a closed environment within a marine context would seem to be just crying out for integration with other ocean-based energy sources.

Belgium is clearly intending for the island storage facility to be used with wind power, for example, but could additional efficiencies be gained through clever engineering to trap tidal or even wave energy, too?

Finally, a point that will not escape the attention of policy makers elsewhere is that this is a project that will allow Belgium to produce its own, clean base-load power indefinitely, based on offshore wind. In other words, integrating wind power into its grid will not be a problem. It will not require a massive juggling act involving smart grids or electric vehicle batteries.

Nor will it involve costly and tricky grid connections to neighbours. Best of all, it will probably create thousands of jobs during the construction phase, and hundreds for ongoing operations, equivalent to the building of a new nuclear power plant but without the discomfort of radioactive waste or pressure groups.

Of course, the project, which is still awaiting a green light, is presumably not without its challenges. Building an entire island is no mean feat. One wonders where all that sand is going to come from, for a start. The Belgian government has also already recognised that Elia, the national grid provider, will have to strengthen its connections to the region.

Cost could be an issue, too; a government estimate of ‘about the same as a wind farm’ is difficult to evaluate without further details. Nevertheless, what is certain is that developments off the Belgian coast could be attracting a lot of attention in the next half-decade. Let’s just hope this doughnut doesn’t leave a bad aftertaste.

This article was previously published in Marine Renewable Energy.

Another take on undersea batteries

Little more than a fortnight after a Massachusetts Institute of Technology proposal to store wind energy in a form of underwater pumped hydro comes news of a Norwegian company with a similar idea, according to Gigaom. The system proposed by Subhydro would consist of concrete tanks sited on the seabed at depths of between 400 and 800m.

‘Discharging’ the battery would consist of opening a port to allow seawater to rush into these empty tanks at many times atmospheric pressure, driving a turbine and generating electricity. Recharging is simply a matter of reversing the process, using cheaper non-peak or surplus energy to pump the water out of the containers. The company is claiming an 80% efficiency for the system, which they say is comparable to efficiencies achieved at conventional plants.

New ball game for sea power storage

It’s only a proposal. But it’s an idea that could make offshore wind a lot more dependable and cost-effective, says the Massachusetts Institute of Technology (MIT): storing surplus energy from wind turbines in a concrete sphere under water. The principle is straightforward.

During periods when there is more wind energy than immediate demand, the additional power would be used to pump seawater out of a hollow, submarine sphere. 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. MIT has calculated that a 25m sphere in 400m-deep water could store up to 6MWh of power.

Multiply that figure up and 1,000 turbines with this form of pumped hydro energy storage could rival a nuclear power station, for a matter of hours at least. The big drawback is a minimum 200m depth of water required to make the scheme feasible, especially as off-shore wind, which currently only enjoys popularity in Europe, is currently sited in waters shallower than 30m.

But the ability to provide despatchable energy without the need for large-scale batteries on or offshore is clearly a huge plus for the idea.