Stream of developments21 March 2006
Peter Fraenkel explains how Marine Current Turbines has taken a major step forward in the development of tidal stream energy conversion generation systems with the successful testing of its Seaflow technology and the introduction of the SeaGen commercial prototype
UK-based Marine Current Turbines (MCT) is developing new technology for the large-scale generation of clean electricity from the seas using submarine turbines. An experimental test system, called ‘Seaflow’, was successfully installed off Lynmouth in Devon in May 2003 and continues to deliver vital data. This is the world's first offshore tidal turbine and is the first step in developing this entirely new clean method of power generation. It is also a significant example of a ‘wet renewable’ technology that has achieved its rated power and remained operational in an offshore exposed location for almost three years.
As a result of Seaflow’s success, MCT believes that the characteristics of the marine current energy resource give this technology the potential for generating power at a commercially competitive cost within the next few years and eventually on a large scale. Tidal turbines also have minimal environmental impact and the energy they deliver will be as accurately predictable as the movements of the tides – unlike the weather-dependent renewables such as wind, wave and solar energy, or even hydro at times of drought. Energy delivered to a timetable is inherently more easily applicable and hence more valuable than randomly generated electricity.
MCT has initiated a programme of tidal turbine development which started with a five year R&D and demonstration phase, which then resulted in the successful testing of Seaflow and is intended to lead to commercial manufacture. The commercial prototype known as ‘Seagen’ is presently under development. These twin rotor turbines rated at approximately 1MW incorporate a patented system for raising the rotors and power train above the surface of the sea for ease of maintenance. The commercial prototype is under construction and due for installation in Strangford Narrows, Northern Ireland, in late summer 2006. The first commercial array is due for completion in 2008.
Tidal current energy conversion: offshore hydro power
There are a number of factors common with hydro power; indeed this new technology could well be described as ‘offshore hydro power’. The tide races needed to drive a tidal turbine are relatively few and far-between and are much like rivers in the sea, with fast flowing water passing through relatively slow-moving or stationary water. They occur at pinch-points where tidal flows get dramatically accelerated by being driven through a narrowing gap around a headland or between an island and the mainland, or where the sea becomes shallower so that the cross-section for the currents decreases. So just as hydro power can only readily be implemented at points along a river where the natural topography lends itself to creating a head of water sufficient for power generation, offshore hydro depends on the underwater topography creating the necessary velocity head.
Tidal stream or marine current energy involves placing a rotor so that it is immersed in a flowing current, which drives it; effectively it is like zero-head hydro. By immersing a turbine in the sea there is no need for accompanying civil works other than a relatively simple and low cost support structure. As a result the relatively large turbine costs are compensated for by the small civil costs. The faster the current the smaller and less costly can be the turbine to achieve a given power rating and vice-versa. Tidal flows follow a sinusoidal velocity pattern and they reverse direction every six and a quarter hours. Moreover, although the currents and hence the output are intermittent, they have the advantage of being predictable – tidal turbine installation will not deliver energy at slack tide and will deliver much less during Neap tides than at Spring tides, but the power output and times of operation can be calculated in advance for the entire life of the system.
The power generated is also completely ‘green’ and therefore qualifies for all the benefits associated with delivering pollution-free energy, such as Renewable Obligation Certificates (ROCs) in the UK.
The Seaflow project
The Seaflow project involved designing, building and then installing a 300kW (rated) single rotor turbine approximately 3.3km northeast of Lynmouth, Devon in a tidal flow which peaks at about 2.7m/sec at Springs. The project started in 2000 with part-financing from the European Commission; later the UK DTI also agreed to support the project. The State Aid component amounted to nearly 60% of the costs and MCT and its partners found the 40% balance. The gross cost of Seaflow was UK£3.5M (US$6.07M).
The turbine uses a conventional axial flow rotor, 11m in diameter, with full span pitch control. A patented feature is that the rotor blades can be pitched through more than 180º (using electrical servos) and in this way they can address a bi-directional flow with full efficiency.
A key element of MCT’s designs is that they have to be accessible and readily maintained at low cost. Therefore Seaflow’s rotor and power-train assembly is mounted on a collar or sleeve fitted around a monopile support structure; the entire sleeve can be raised up the pile above water level using a hydraulic lifting arrangement. This patented arrangement enables the rotor and power train to be positioned out of the water so that it can readily be inspected and maintained. The whole concept is designed to avoid any form of underwater intervention as divers or ROVs are not capable of operating in a tide-race and slack tide periods are too short to allow useful intervention. A small on-board folding crane is provided which has been tested and certified for carrying personnel in a man-basket; hence it is possible to reach any part of the rotor and power train and even to remove sizeable components that may need overhaul or replacement. The vehicle of choice for access to Seaflow (and its successors) is a Rigid Inflatable Boat (RIB) manned by staff in survival suits; this reaches the site quickly (at 25 knots) and costs very little. In three years of operation MCT has managed to maintain the system using nothing larger than a RIB or a 12m workboat, so O&M costs have been kept very low for an offshore project.
Because Seaflow was never intended to be used for more than a few years, and it is sub-economic for power generation, it was decided not to connect it to the grid since over 3km of marine cable would have been an expensive added cost that would be hard to justify. Therefore a grid simulator was developed incorporating a fan-cooled 300kW resistive dump load, and the power output from the turbine is simply dumped as heat. Since there was no grid connection Seaflow also needed a stand-alone power supply both to provide power for maintenance crews and also to provide the electrical power needed for starting (there are numerous parasitic loads including a hydraulic pump, excitation for the generator, power for the control PCs and the fan for the dump load) and also to keep the navigation lights working at night. This stand alone power system uses a 15kVA diesel generating set, with a large battery bank which can also be charged by the tidal turbine. It is in effect a large UPS. Paradoxically the main source of technical problems was the backup power supply which future projects, to be grid-connected, fortunately will not need.
Seaflow remains in operational condition at the time of writing nearly three years after its installation. It has performed slightly better than the design expectations, with an average rotor efficiency in the region of 40 to 45% (the design model predicted 37%) and it has also exceeded its 300kW rated power by a small margin on occasions when a strong enough tide is available. These may seem low efficiency levels by hydro standards, but the physical principles of kinetic energy conversion are more akin to a wind turbine – where the maximum theoretical efficiency is only 60% – because you clearly cannot get 100% as that implies stopping the flow of water completely (in which case it could not flow away). Seaflow regularly runs at around 200kW and although initially it had numerous teething problems and bugs and was always manually operated with a crew on board, in the last year or so most of the bugs have been dealt with and it has been regularly operated by remote control over the internet (it has a wireless link to shore and a PC located high up on the cliffs in Lynton relays data over the telephone network between Seaflow and MCT’s offices in Bristol).
Seagen is the successor to Seaflow: it is a nominal 1MW prototype for the commercial technology to follow. Work on Seagen started in 2004 and by early 2006 the design was complete and the system under construction. MCT obtained official consent in December 2005 to install the Seagen prototype in Strangford Narrows, Northern Ireland, which has an extremely energetic tide race with peak velocities exceeding 7knots during Springs. Installation is scheduled for late summer 2006. The project cost is expected to be some UK£8.5M (US$14.7M) including the development and design costs, and the UK Department of Trade and Industry is financing 50% of this with MCT’s investors financing the rest.
Strangford is a sheltered location, surrounded by hills and facing south east towards the Irish sea, which will be very important in terms of facilitating the planned test programme. A major difficulty with Seaflow at Lynmouth has been the exposure of the site to Atlantic storms which frequently made it difficult to get access to the turbine in winter conditions when the wave height precluded safely landing staff onto the test rig to carry out maintenance, repairs or planned experimental work. Strangford is so well protected from adverse weather that MCT expects 365 day access through the year. Placing the technology in exposed open-sea sites will be less of a problem in future when it has been thoroughly debugged and developed to the stage where it can be applied to commercial projects, as then the same level of frequent access will not be required.
Another feature of the planned test programme at Strangford is that there will be an intensive environmental monitoring programme to clarify whether or not there are any significant environmental issues. MCT, on the basis of having financed two major environmental impact studies by independent experts and also on the basis of the Lynmouth experience, does not expect any significant environmental impact from the technology, but people obviously worry about issues such as whether the rotors could be a hazard to fish or marine mammals. In reality, cavitation limits the rotor blade tip velocity to 10 or 12m/sec (around 20 to 25 knots) giving Seagen a rotor speed of up to 10rpm – most experts on marine mammals consider that seals, porpoises or other such animals are well enough equipped to sense the presence of the moving rotor and agile enough to take evasive action in the unlikely event that they choose to swim through the centre of a tide race. It has to be remembered that most of the wildlife that frequents areas with strong currents is agile and predatory and has evolved with a good capability to avoid colliding with rocks or other obstructions. It is hoped that the monitoring programme at Strangford, which is a very environmentally sensitive location, will in fact give a first class opportunity to assess any threat and also to develop and prove appropriate mitigating measures if indeed any are needed.
Seagen differs from Seaflow in having twin rotors mounted on wing-like extensions either side of the single supporting monopile. This has the major advantage of keeping the rotors clear of the pile wake when operating in a flow where they are on the downstream side; the wing-like cross arm is streamlined and designed to have a particularly thin wake which is not expected to cause any significant problem for a rotor-blade cutting through it. The other advantage of twin rotors is that they clearly collect exactly twice as much energy as a single rotor of the same size, but at less than twice the overall cost. As a result the twin rotor Seagen-type system is significantly more cost-effective than a system having a single rotor. With wind turbines cost optimisation works differently in that a wind turbine can be scaled up simply by adding a larger rotor (although the current generation of 3 to 5MW wind-turbines may be reaching the limit) but wind turbines have the whole atmosphere above them with no height limit – tidal turbines on the other hand need to operate below the ceiling of the surface of the sea, which limits rotor size. The other limit to rotor size is torque; water kinetic energy conversion involves slower moving rotors than for either wind or hydro turbines, so for a given power level the torque requirement for a gearbox is exceptionally high. At present MCT does not believe individual turbine rotors can be cost-effective for ratings much larger than 1MW per rotor simply because gearboxes capable of handling torque levels much above 1MNm would be excessively heavy and costly.
Seagen, like Seaflow, uses twin-bladed full span pitch control rotors (with carbon and glass fibre composite rotor blades). The reason for twin bladed rotors (when wind turbines commonly use three bladed rotors) is firstly that the rotor can be parked horizontally yet completely out of the water when raised (a three bladed rotor would need a taller monopile to allow it to clear the surface when raised) and secondly a twin bladed rotor is more cost effective than a three-bladed one. The Seagen prototype designed for Strangford Narrows will have twin 16m diameter rotors and will be rated at 600kW per rotor, yielding a peak power of 1.2MW. It will be located only 350m from the grid and will connect via a directionally-drilled sub-seabed cable. A load factor of approximately 45% can be expected (i.e. an output of about 4730MWh/year).
Seagen (and Seaflow) use gearboxes to drive a generator at a nominal 1000rpm. The gearboxes on Seagen are an advanced design with two planetary stages and are unusually light and compact for their power rating. The power-train (rotor-gearbox-generator) is designed to function immersed in the sea with the only dynamic seal being at the gearbox input shaft. Immersion results in effective passive cooling and therefore no cooling system as such is needed.
The control system, which is driven by an industrial PC, is designed to permit variable speed operation. It seeks to set the blade pitch to maximise power for all current velocities below the rated velocity and then it seeks to hold the power at the rated power level for all higher velocities. There is no velocity high enough to require the plant to be shut-down (as with a wind turbine in extreme wind conditions). The system can run at speeds well away from the nominal speed. The generators are subsea brushless induction generators (originally developed as induction motors to drive sub-sea pumps for the offshore oil and gas industry) and they feed into an electronic frequency converter. A transformer delivers the output at 11kV. The electrical control systems, power conditioning equipment, transformer, safety equipment and so on is mostly housed within the top part of the monopile in a climatically controlled environment. The pile surface below sea level is largely used for cooling, via a heat exchanger fitted inside the pile, in order to dispose of the significant amount of heat generated by the electrical system when running at rated power.
The system carries essential safety features including automatic fire-extinguishers, marine navigation lights, life-rafts and safety equipment for personnel, etc.
Seagen will be thoroughly tested and developed as the basis for commercial technology to follow. As a prototype it has features that will not be used in future systems and it is also probably significantly over-engineered. The entire design process has been vetted by DNV (Det Norsk Veritas), the leading ship classification society, with a view to developing a design standard for this kind of technology. Therefore the test programme will be used to validate the design assumptions as well as to permit a value engineering review to take cost out. MCT hopes and expects significant scope for cost-reduction in the commercial technology.
MCT’s Business Plan envisages raising money to finance a sizeable batch of commercial Seagen type turbines, perhaps 20 or 30 units, to be deployed in several arrays both in UK waters and as overseas demonstration projects. MCT is already investigating possible sites both in UK waters as well as in North America and other parts of the world.
While the Seagen system is being developed into a reliable product that can be mass-produced, work is also in hand to develop what MCT calls ‘Second Generation’ systems. This is because Seagen, being monopile mounted, depends on jackup barges for installation. The operational envelope for the stability of a jackup barge limits the depth of water that smaller (and hence more affordable) jackups can stand in to about 50m maximum, yet there are many sites with deeper water. Hence MCT has ideas for larger turbines based on Seagen technology but capable of deployment in much deeper water; these ideas are already subject to patents and patent applications. Similarly, for economic reasons, the Seagen concept cannot be scaled down to stand in much less than 20m depth of water, yet some rivers and tide races offer opportunities for power generation in depths of 10 to 20m. The solution to this requirement is a form of Seagen with a greater number of smaller rotors deployed horizontally across the flow. The second generation technology presently at an early stage of development will address this requirement too. In the long term MCT expects to have the definitive technology for cost-effective water current kinetic energy conversion.
Marine Current Turbines is an independent UK private limited company dedicated to the development of water current kinetic energy conversion systems. It has a number of commercial strategic partners and investor/share holders including: EDF Energy, Guernsey Electricity, Seacore, Bendalls Engineering and BankInvest. At the time of writing MCT is planning a flotation on the UK stock market primarily to raise finance for the deployment of commercial technology.
For further information contact: Marine Current Turbines Ltd, tel: +44 (0)117 979 1888. Email [email protected] www.marineturbines.com