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Orkney tidal power: A hotbed on the seabed

The 20,000 people on the rugged islands of Orkney in Scotland's extreme north are vastly outnumbered by sheep. But out to sea, beyond the wind-scoured landscape and dramatic cliffs, is the world's leading tidal energy research center.

The European Marine Energy Centre (EMEC) at the Fall of Warness, to the west of the island of Eday, offers five test berths at depths ranging from 25 metres to 50 metres in an area 2 km across and 4 km in length.

The site was chosen for its high velocity marine currents which reach almost 4 metres per second at spring tides. In each of the five test beds, 11 kV subsea cables extend to the centre of the tidal stream. Developers are responsible for installing their devices, connecting to the designated cable and removing their devices when testing is complete.

World's first tidal current turbine

In 2003, Norwegian tidal energy technology firm Hammerfest Strom installed a 300 kW test turbine, the world's first tidal current turbine, in Kvalsund, Norway, within the Arctic Circle near Hammerfest, Europe's most northern city. Operating at a depth of 50 metres, the turbine – developed in conjunction with ABB, Rolls-Royce, Skelmer Skanska, SINTEF Energy Research, Statoil and Venturos – has been generating electricity for the grid since 2004, when it was permanently connected to the national grid, supplying 700 MWh per year.

The submerged structure weighs 120 tonnes and has gravity footings of 200 tonnes. The three-bladed turbine is made of fibreglass-reinforced plastic and measures 10 metres from hub to tip. By rotating the propeller blades around their own axis at slack water when the current turns the turbine is ready for the reversing current (pitch control), keeping the nacelle fixed.

Each propeller is coupled to a generator, from which the electricity generated is fed to shore via a subsea cable. The turbine was designed for a three-year test period, but was operated for approximately four years without maintenance, according to Hammerfest Strom.

The HS1000 upgrade

Hammerfest Strøm will be installing a 1 MW prototype, the HS1000, at the EMEC site this summer. The company, which was set up in 1997, plans to test two turbines a season to make sure that it has calculated the effects of the waves correctly. 

After testing, the plan is to install ten HS1000 tidal turbines by 2013 in Argyll Sound, between the Inner Hebridean islands of Islay and Jura off Scotland’s west coast. Spanish utility Iberdrola's subsidiary ScottishPower Renewables, a shareholder in Hammerfest Strom, has already submitted a planning application to develop this array of HS1000 turbines.

An impressive list of companies is backing Hammerfest Strom to succeed. In addition to ScottishPower Renewables, which holds an 11 percent stake, Norway's state-owned oil and gas giant Statoil holds a 22 percent share.

The major shareholder, however, is Austrian hydropower solutions provider Andritz Hydro, which in August 2010 acquired a 33 percent stake in Hammerfest Strom, including its Scotland-based subsidiary Hammerfest Strom UK.

Last August, the Norwegian firm awarded contracts to construct the first of its HS1000 tidal power turbines. Fife-based Burntisland Fabrications was awarded the £2 million ($3.1 million) contract to build the sub-structure for the 25-metre high HS1000 tidal turbine, which is to be deployed at the EMEC site, at its factory at Arnish in Stornoway on the Isle of Lewis. 

The HS1000 is essentially an upscaled version of the same technology, says Stein Atle Andersen, managing director of Hammerfest Strøm. “With the experience of the 300 kW version, we have designed the HS1000, which is based on the same technologies. But there are, of course, improvements and upscaling,” he said.

Similar in basic design to competing tidal stream turbines, like Atlantis Resources' AK1000, the HS1000 is essentially a wind turbine fastened to the seabed. While the Atlantis machine has two turbines, one for upstream and one for downstream, the HS1000 is a single rotor turbine which operates on a significantly variable pitch.

The diameter of the HS1000 turbine totals 23 metres. The HS1000 is a gravity-based turbine, which requires a great deal of ballast in addition to the nacelle and the sub-structure. The sub-structure of the nacelle for the HS1000 weighs about 300 tonnes, not including the ballast. The amount of ballast required depends on the tidal flow of the site, says Andersen.

The HS1000 is specifically designed for the EMEC test site in Orkney, which means it is designed for a current of more than 4 metres per second plus wave influence, i.e. the additional flow from the turbulence of waves.

Costs and subsidies

Producing electricity with the original 300 kW turbine in Norway is estimated to cost from $0.043 to $0.05 per kWh, three times that of typical hydropower generated in Norway. Under the current UK Renewables Obligation subsidy scheme, tidal power projects are eligible for three Renewable Obligation Certificates (ROCs) in Scotland and two in England, but the industry is pushing for this to be raised to five ROCs.

Andersen said: "There is an ongoing discussion, but the industry and tidal project developers would welcome a move in that direction. In the long term there will be a competition between marine energy technology like wave, tidal and offshore wind. I cannot see any reason why all three shouldn’t be at more or less the same cost level in the future."

Other than Scotland, Andersen says Hammerfest Strom is exploring Ireland, Australia, New Zealand and the Americas as target markets for the HS1000 tidal stream turbine.

Hammerfest Strom is looking particularly closely at harnessing the tides of Canada's Bay of Fundy, on the northeast end of the Gulf of Maine between the Canadian eastern provinces of New Brunswick and Nova Scotia. Last year, Hammerfest Strøm executives met officials from the Nova Scotia provincial government and the Fundy Ocean Research Center for Energy to look at the medium to long-term possibilities in the region.

Tidal Generation Limited

Tidal Generation Limited (TGL), formed in Bristol in 2005 and since December 2009 a wholly owned subsidiary of Rolls-Royce, has developed a 500 kW tidal stream test turbine, which was installed and connected to the UK national grid in September 2010. As with all other test turbines at the Orkney site, TGL's unit is connected via a single meter point at its test bay to the EMEC substation.

The TGL 500 kW turbine consists of a three-bladed, upstream pitch controlled rotor with a relatively standard drivetrain and power electronics. The turbine unit itself is buoyant to allow it to be retrieved to shore for maintenance within a single slack water period, i.e. 30 minutes. The device operates in depths of between 35 metres and 100 metres and has a foundation that TGL describes as a lightweight steel tripod, installed using patented fast drilling techniques.

Ease of deployment and retrieval enables all maintenance to be carried out onshore, significantly reducing the need for costly marine operations, while allowing a spare machine to continue generating, says TGL. The one-off demonstrator turbine was built at the Rolls-Royce Marine facility in Dunfermline, on the eastern side of Scotland.

"Our strategy was to build a 500 kW demonstrator at EMEC. It is now generating power at around the 400 kW level at present. We expect to operate at full power as time moves on with various tests and, once certain criteria have been met, we will build a 1 MW version later this year,” said a spokesman for TGL.

"We are ahead of the pack in some respects in terms of generating electricity. We think the market space is interesting but it depends on a lot of factors as to how we take the business forward. We don’t yet know what our final value proposition will be, but we do not plan on developing tidal power arrays ourselves – we will be an equipment supplier to array developers."

Scotrenewables 250 kW prototye

In July 2010, Orkney-based Scotrenewables, founded in 2002, appointed world-renowned Belfast shipbuilder Harland and Wolff to manufacture a 250 kW prototype floating tidal turbine, the SR250. The 30-metre long device’s two 8-metre diameter rotors deliver a very competitive power to weight ratio, says Scotrenewables. The prototype is being manufactured in Belfast, Northern Ireland, and is scheduled to be deployed at the EMEC test facility in March 2011.

By concentrating not only on conversion technology but – more importantly – on making its tidal turbine easy to install and maintain, Scotrenewables has joined a small group of developers, says Harland and Wolff.

"Our input was to assist their engineers in developing the design to make it production friendly. This makes it cheaper to manufacture and enhances quality," says Harland and Wolff engineering manager Fred Black.

Scotrenewables aims to develop a full-scale 1 MW prototype. The one-fifth scale, 250 kW model, will maximize Scotrenewables’ chances of success in commercially developing its tidal turbine, as a 1 MW prototype was too big a leap, says Trevor Walls, lead electrical engineer.

OpenHydro tidal turbine

In May 2008, Irish company OpenHydro became the first tidal energy company to complete the connection of a tidal turbine to the UK's national grid.

The 250 kW Open-Centre Turbine, which was installed at the EMEC site off Orkney in 2006, underwent 18 months of testing prior to connection to the grid.

The Open-Centre Turbine is a large underwater turbine resembling a jet engine affixed to the sea floor. The device is designed to be mounted onto the seabed at a depth that prevents the device from interfering with shipping and leaves no part of the structure visible from the surface.

The single 250 kW turbine is powerful enough to supply electricity to 100 homes but it paves the way for a significant tidal power development in the Channel Islands off the northern French coast.
OpenHydro plans to deploy 1 MW Open-Centre turbines and aims to be in a position to build an array of the devices off Alderney, where the local renewables developer Alderney Renewable Energy holds the right to export power both to France and the UK, where the company secured the right to supply 2000 MW of capacity into the UK’s national grid in 2010.

In March 2010, OpenHydro was awarded exclusive rights by the UK's Crown Estate to develop a 200 MW tidal energy farm at Cantick Head in the Pentland Firth and Orkney Waters. OpenHydro and SSE Renewables have formed a joint venture called Cantick Head Tidal Development Limited to develop the site.

Atlantis Resources' AK1000 turbine

Also installed at the EMEC site in Orkney is Atlantis Resources’ AK1000, which it describes as the largest and most powerful tidal power turbine ever built. AK series tidal turbines are commercial-scale horizontal axis turbines designed for open ocean deployments in harsh environments.

AK turbines feature a unique twin rotor set with fixed pitch blades, which eliminates the requirement for subsea nacelle rotation to improve operational reliability.

Dispatching 1 MW at a water velocity of 2.65 metres per second, the AK1000 turbine has an 18-metre diameter rotor, weighs around 1300 tonnes and has a height of 22.5 metres.

The AK1000 nacelle is fabricated by Soil Marine Dynamics in Newcastle, England, while the gravity base structure and system assembly is completed by Isleburn Engineering of Invergordon on Scotland’s northeast coast.

Steel for the turbines is provided by Corus' Scunthorpe facility. Soil Marine Dynamics and Isleburn Engineering are yet to be in a position to mass produce the AK1000, said Atlantis. But Atlantis is actively exploring the possibility of manufacturing the AK1000 turbine in-house. 

"Going from a single, one-off build to a commercial unit that is part of a factory system will bring a lot of cost savings. Production engineering will drive the cost down," a spokesman for Atlantis told Power Engineering International. Atlantis has developed a range of tidal stream turbines and is yet to ascertain the optimum size for mass production.

"The AK1000 is fully scaleable, but the problem with scale is that it has to fit the resource available. You can’t just make a 5 MW tidal turbine with 35-metre diameter rotors, because it won’t fit the available resource. Unlike a wind turbine, tidal stream turbines have to be specifically designed for the seabed surface resource available."

MeyGen project

The AK1000 turbine has been earmarked for deployment in the Pentland Firth by MeyGen, a joint venture between International Power, Morgan Stanley and Atlantis. A 400 MW tidal array is also planned for the Inner Sound between the Island of Stroma and the mainland of Caithness.

The project will be built in six phases by 2020, the first consisting of 15 MW. MeyGen is seeking planning consent in the first half of 2012, with the first turbines entering the water either in 2012 or 2013.

The EMEC projects will be supported by a purpose-built service and supply base at Lyness on the island of Hoy, which is expected to be operational this March.

FenderCare Marine Solutions, which is installing the facility, says the base will offer essential marine services including deployment and mooring of wave and tidal devices and operational through-life support, as well as specialist fendering and sub-sea tooling requirements.

While several test turbines are connected to national grids around the world, no commercial turbines – i.e. sold at a profit – are yet in operation. That is set to change if the pace of development in the Scottish islands of Orkney continues apace.

This article was reprinted with permission from Power Engineering as part of the PennWell Corporation Renewable Energy World Network and may not be reproduced without express written permission from the publisher.

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