The marine energy challenge

February 4, 2014

Marine energy technology has been in development for decades and, in the past 10 years, interest and investment in marine and tidal technologies have increased.

However, marine energy still lags 15-20 years behind offshore wind, says Kevan Stokes, technical director at Aberdeen consultancy Augmentias Maritime and Offshore Engineering

In the early days there were high hopes marine energy would develop quickly. There were plans for a construction hub at Methil, located in Fife, Scotland. Two wave energy devices were to be built there; Fred Olsen’s FoBox and a wave bobber, designed by the University of Manchester. Neither came to fruition.

While the site was recently used to build Aquamarine Power’s Oyster wave energy device, Methil is now mostly used for wind projects, including jackets for German offshore wind projects, and the recently commissioned 150MW Ormonde wind farm in the Irish Sea, Stokes says.

Stokes, speaking at a Royal Institute of Naval Architects talk in Aberdeen, says while offshore wind has developed progressively, based on established onshore turbine technology, marine energy technology has been characterized by divergent concepts and novel technology, in a harsher operating environments, and with more stringent environmental controls. 

Offshore wind

Early offshore wind projects started in 1990, with a 0.2MW device in shallow water, he says. It took some time, and a steady progression, for offshore wind turbines to breach the 2MW barrier and move to deeper waters, by the 2000s.

The early farms had no tides to contend with, hardly any waves, and walk to work access. The turbines were also based on standard onshore turbines.

On early Baltic Sea wind farms, concrete gravity bases were used, due to rocky seabeds and seasonal pack-ice, but again, there was still little tide and low salinity seawater.

“The first real full scale offshore wind farm in open waters was Horns Rev 1,” he says. It had a combined 160MW, offshore Denmark, in 2002, Stokes says.

Lack of marinization was an issue, in the early days, but this was soon addressed, Stokes says.

Most wind farms used monopile foundations, which would be pile-driven into the seabed. As depths increased, there has been a move to pin-piled jacket foundations, such as on the 5MW Beatrice offshore demonstrator in the Moray Firth, and some tripod foundations.

Now floating turbine foundations are being developed, including Norwegian operator Statoil’s Hywind, which is based on a spar concept for 150m water depth. Statoil developed and deployed offshore Norway a 2.3MW prototype, and it is now looking to deploy a small offshore wind farm, using the concept, offshore Peterhead, near Aberdeen.

Other concepts include semisubmersible floating foundations from US-based Principle Power and a Japanese prototype, developed following the Fukushima nuclear incident.

“For deeper waters, floaters may not be commercially available yet, but they are currently feasible and proven technically,” Stokes says.

Not all projects have been as successful. The potential of Wind Power Ltd’s Aerogenerator concept, which was based on a floating vertical axis wind turbine, was being assessed with US$4.57million (£2.8 million) funding from the UK-based Energy Technologies Institute, under a project called Nova. But it has gone quiet, Stokes says.

But, Stokes says, “By and large, to adapt an onshore wind turbine for use offshore, little needs to be done. There are a few issues around structural assessments, such as consideration of resonance, but offshore there is generally a steadier wind regime than onshore. Marinization is required, as salt gets everywhere.”

This has meant the development of offshore wind has been incremental, with little novel technology, little diversity of concepts, and with an ability to draw on oil and gas expertise.

“There very little novel technology involved, very little diversity in concepts and configurations, especially the turbine, they’re almost universal in design,” Stokes says. “Most of the cost is in the turbine, so there is very little technical risk and financial risk, they are very well understood. So they have large firms behind them.”

Marine energy

“It is a very different story for marine energy,” Stokes says. “There is a great diversity of concepts. According to EMEC there are seven types of tidal device, and nine types of wave device, and six different ways to attach them to the sea bed, and nearly all the technology is novel with very little to build on. Most are still prototypes and designed by small firms with limited funding. Wave and tidal devices also have a whole host of problems offshore wind does not have to deal with.”

According to EMEC there are 88 companies designing tidal stream devices, and 156 designing wave energy devices. In the wind industry, there are fewer than 10 serious wind turbine builders, Stokes says.

“So the risks are high, and the rewards are less, and you can only guess what the operating costs are,” he says. “They are going to be high. You cannot just send a man out in a white van to change a fuse when your device is 30m deep. We also have no idea of the longevity of these devices. Nearly everything that has gone into the sea on a test bed has broken (nearly all devices with blades have shed their blades in the water), so there is real uncertainty.

Tidal, or current, energy

All offshore renewable energy devices are also really getting hit these days on environmental issues, far more so than was the case for the early days of oil and gas,” Stokes added.

One of the successes in marine energy has been Bristol-based Marine Current Turbines (MCT), now owned by Siemens. It tested two 600kW twin-blade tidal devices, totaling 1.2MW, in Strangford Lough in Ireland. But, its testing was subject to stringent controls, including not testing at night, due to seal populations in the area. 

Other companies with tidal devices, all of which have had issues with blades shedding in the water, include OpenHydro, based in Dublin, Ireland and owned by France’s DCNS, and New York-based Verdant Power.

Norway’s Hammerfest Strøm is experimenting with wooden blades, for its tidal device. Other more novel devices include Ocean Renewable Power Company’s cross flow turbines, tested in the Bay of Fundy, and Swedish firm Minesto’s underwater flying kite, also tested in Strangford Lough.  

Most of the devices have not been tested in full open waters, Stokes says.

The limitations of tidal, or stream, energy, are that there are few places they can be installed with good enough flow of water, sufficient depth, and where they are not in a shipping lane or obstructing other users’, Stokes says. Ideal areas, such as the Pentland Firth, Scotland, have the current, but due to this the seabed has been swept to rock, and in this case, it is a badly fractured rock, which is “not very nice to try and anchor in to.”

The varying dynamics of the flow when influenced by wave patterns, also causes issues, particularly to the blades.

Wave energy

 Pelamis wave energy converter

Wave energy development in the UK goes back 40 years to Edinburgh University’s Professor Stephen Salter, who designed a device that was eventually called the bobbing duck. But its development has been even more divergent than tidal energy.

Schemes include Voith Hydro Wavegen’s Limpet device, which is attached to the shore and uses the wave stroke to compress air, and turn a Wells turbine. A device installed on Islay, a Scottish island, suffered from rocks gathering at the opening where water should flow in and out, and, when that was cleared, was ingress into the turbine during particularly strong waves, Stokes says.

Edinburgh-based Aquamarine Power’s Oyster, built at Methil, offers an electronics-free solution, using hydraulics to pump water onshore, using the motion of the waves. However, its power generation capabilities are low, at 800kW, compared to the size of the unit. Aquamarine Power has been investigating using composites for the project, but this would increase its cost, Stokes says.

Scotland-based AWS Ocean Energy proposes a device with a multi-cell array of flexible membrane absorbers which covert wave power to pneumatic power through compression of air within each cell.

Another device, the Wavetreader, was proposed to be attached to wind turbine foundations, to generate additional power from the surrounding waves’ motion, eliminating the need for a transformer station and boosting the economics of the turbine. The project has been discontinued.

One of the key issues is maintenance and the ability to remove and reinstall devices, without a dedicated supplier market and only expensive oil and gas resources to rely on.

Pelamis, based in Edinburgh, (pictured right) attempted to address this with a device which can be untethered, replaced with another unit, and taken to shore for repair, without the need to go into the water.

The list could continue. “If you look back at the progress since Salter’s duck, it (wave energy technology development) has gone very slowly, with virtually no convergence. There are also not many wave sites which are close to (power) demand.”

Funding is also a difficulty, limiting engineering resource available to companies.

“It does sound a bit depressing for marine energy extraction,” Stokes says. “But I wouldn’t like you to go away with the idea it is futile. It is just difficult.”

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