Heather Johnstone, Senior Editor
According to the Carbon Trust, the UK has one of the best tide and wave energy resources in the world, with official figures indicating that combined they could provide a fifth of the country’s electricity. Similarly, a report by the Sustainable Development Commission last year estimated that Britain’s tidal currents could generate at least five per cent of the nation’s power. Others have put it even higher. Professor Stephen Salter of Edinburgh University, a well respected marine energy expert, has said that the Pentland Firth, the narrow body of water between the north Scottish mainland and the Orkney Islands, could generate up to a quarter of the UK’s electricity, describing it as the “Saudi Arabia of marine energy”.
All this sounds very impressive, but the UK, and the government in particular, has been relatively slow in exploiting this renewable energy resource for power generation. However, things look set to change, and change in a big way, with the recent installation of the world’s first and largest commercial tidal current device.
The Luck of the Irish
In the beautiful surroundings of Northern Ireland’s Strangford Lough, in the early hours of the morning of Wednesday, 2 April 2008, the initial installation phase of tidal current device, called SeaGen, was completed.
SeaGen is located in the 500 m wide Narrows of Strangford Lough, where it will be able to take full advantage of the currents that have been shown to exceed speeds of seven knots.
The 1.2 MW device is currently undergoing a 12-week commissioning period, and will begin commercial operation later in the summer. It will operate for between 18-20 hours a day and generate enough carbon-free electricity to power 1000 homes. Most importantly, it will supply electricity to the local grid. ESB Independent Energy, the retail subsidiary of ESD, Ireland’s national electricity company, has already signed a power purchase agreement to supply the electricity produced by SeaGen to its customers.
No doubt with the successful installation of SeaGen achieved, Dr Peter Fraenkel, the pioneering engineer who came up with the concept and his colleagues at Marine Current Turbines (MCT), the Bristol-based developer of the technology, will breath a collective sigh of relief.
Originally scheduled for installation in 2006, this groundbreaking tidal energy project has been beseiged by delays. The most recent occurring only a month or so ago, when bad weather prevented the Rabiz crane barge transporting SeaGen from the world famous Harland & Wolff dockyard in Belfast, where it had its final assemby, to Strangford Lough. However, such annoyances are now long forgotten.
Twice the Power
SeaGen generates power in a similar way to a wind turbine, except that it harnesses the movement of water rather than air. Thus, it is unsurprising that it is often described as ‘an upside down windmill’.
The device comprises twin axial flow rotors of 16 m in diameter, each driving a generator via a gearbox. The twin power units of each system are mounted on wing-like extensions on either side of a tubular steel monopile, approximately 3 m in diameter, which is set into a hole drilled into the seabed.
SeaGen incorporates two rotors, which capture twice the energy of one rotor but not at twice the cost, thereby making it very cost effective. Furthermore, unlike wind turbines, where it is possible to increase energy capture simply by increasing the size of a single rotor, the depths of water limits rotor size and make it necessary to ‘grow’ the system sideways.
The rotors have full-span pitch control, much like a modern wind turbine, which allows the structural loads to be limited and provides a means to safely stop and park the rotors in full flow. It also enables the rotors to be reversed through 180 degrees for efficient bi-directional flow operation.
Seagen also has a sophisticated rotor design with an efficiency of 45 per cent. It has been fully optimized to achieve the best hydrodynamic shape, with its design methodology adapted from that used for wind turbine design, since the physical principles are analogous with a model based on blade element theory. However, a major difference between tidal stream and wind rotors are the load cases, i.e. much higher forces are generated in water, consequently much more carbon fibre is used in their construction.
Another issue identified at the time of SeaGen’s development was that any submerged system in the water column needed to be able to be positioned underwater and then retrieved occasionally to the surface for maintenance, inspections and ultimately for replacement. A concept was therefore developed in which the monopile support structure would project above the surface such that the rotor and power train could be raised up the monopile, so as to be accessible above the sea.
Moreover the cross-arm carrying the pair of rotors are raised above the surface of the sea using a hydraulic jacking system similar to that commonly used for lifting jack-up barge legs. In essence, a pair of hydraulic jacks walk a lifting rod with an integral rack up or down as required. The cross-arm complete with power trains and rotors weighs approximately 120 tonnes, so substantial lifting capacity is needed to ensure that the system can break away easily when raised from its lowest position.
The ability to lift the rotor is essential to permit safe and reliable maintenance. This is because underwater interventions in a tide race are at best difficult and at worst impossible. When necessary the rotors and power trains can be swapped-out for refurbished units using a motorized barge or Seacat that can be positioned below the raised units to collect them and to deliver a replacement.
Finally, SeaGen’s rotor have two blades, rather than the wind industry standard of three, primarily because by parking the rotors horizontally they need not be lifted so high when raised above the surface. Also, even making allowance for a marginal reduction of efficiency for two blades compared with three, a twin bladed rotor has been found to be more cost effective than a three-bladed one.
Coming from Hardy Stock
SeaGen has a long develoment history dating back to the mid-1970s. However, it wasn’t until almost 20 years later, with the installation of the 300 kW Seaflow system (direct precursor to SeaGen) off Lynemouth in Devon in 2003, that a major step towards the realization of a device that could produce commercial-scale electricity from tidal currents was made.
Seaflow remains unique in being the only marine renewable device (tidal or wave) to function in exposed offshore conditions for an extended period. And most importantly it provided a number of key results that ultimately led to the development of SeaGen
Moving up a Notch
Although MCT has reached a significant landmark in its efforts to exploit the power of tidal currents to produce electricity, the company is not sitting back and basking in the glory. The very oposite, in fact.
In February of this year, MCT and npower renewables, part of the RWE Group, announced a partnership to deliver a 10.5 MW tidal stream project off the coast of Anglesey, north Wales. The project is being taken forward through a newly created development company, called SeaGen Wales, and although subject to successful planning consent and financing, the tidal farm could be commissioned as early as 2011 or 2012.
Artist’s impression of SeaGen in action
MCT also continues to develop its technology, with the potential to scale down the rotors to 8 m diameter or less, or conversely scale them up to a diameter of around 24 m. MCT already has a design option to use 18 m diameter rotors and it is expected that this design could be stretched to permit up to about 24 m rotors to be used.
The company believes that the way forward is to deploy horizontal arrays of smaller or bigger rotors for example six 8 m rotors would sit in 12 m of water and produce 660 kW, while at the other extreme, six 24 m rotors could deliver over 8 MW. In both cases it is to be expected that a six-rotor system will be more cost effective than the equivalent three twin-rotor systems.
The main engineering issue that MCT is now addressing is to find a suitable structure to deploy a row of turbines. The company has one possible configuration, already patented in the UK and with foreign patents pending, which has the advantage of being fully submerged when operational but can still be raised to the surface for maintenance when necessary.
MCT has labeled this as ‘second generation technology’ and it is expected to follow about two to three years behind SeaGen and most importantly exploit the benefits of the experience gained from SeaGen in terms of using tried and tested power trains.
Analysis has indicated that in addition to the greater flexibility of second generation technology in terms of the spectrum of depths that can be addressed, something like a 25-30 per cent reduction in generating unit costs may be achieved compared with SeaGen. Therefore, MCT expects that in the longer term, multi-rotor systems of this kind will take over as the technology of choice.