SPECIAL ISSUE PAPER 159 Marine current turbines: pioneering the development of marine kinetic energy converters P L Fraenkel Marine Current Turbines Ltd, The Court, The Green, Stoke Gifford, Bristol BS34 8PD, UK email: [email protected]

The manuscript was received on 21 March 2006 and was accepted after revision for publication on 17 November 2006.

DOI: 10.1243/09576509JPE307

Abstract: This paper gives the rationale and background to an already advanced research and development (R&D) programme aimed at developing technology for the commercial exploita- tion of kinetic energy from marine currents. This is followed by a brief overview of the charac- teristics of the tidal stream resource, the technical principles by which it may be exploited, and the key technical challenges that need to be overcome. The paper includes a description of the pioneering ‘Seaflow’ Project involving the installation and testing, since May 2003, of a prototype 300 kW tidal turbine 3 km off Lynmouth. The next stage of Marine Current Turbines Ltd’s R&D programme is then described: this involves the development of a 1 MW twin axial-flow rotor system, called ‘Seagen’ which is planned for instal- lation early in 2007. The installation and testing of ‘Seagen’ will mark a landmark stage in the R&D programme because it will form the basis for the commercial technology to follow. A brief outline of future plans beyond ‘Seagen’ is also given.

Keywords: , marine power, , energy converters

1 INTRODUCTION The other, less talked-about key driver is that of ‘peak oil’. It is becoming the accepted wisdom that Marine renewable energy resources, such as tidal in the near future for the first time a point will be current kinetic energy conversion, are technically reached at which world oil production is no longer difficult and potentially costly to develop, so until capable of keeping up with growth of world oil recently there was no incentive to do so. However, demand. Depletion of resources characterized by the growing realization that the unsustainable use the well-known Hubbert ‘peak oil’ curve will force of fossil fuels is rapidly reaching the stage where this to happen [3]. The result of demand exceeding dramatic changes will be forced on us by factors supply, as experienced briefly in 1973, will be a dra- beyond our control has led governments to start to matic rise in energy costs. seek ways to address this problem, such as encoura- Tidal stream (and ocean current) technology is one ging the development of new renewable energy of the most recent forms of renewable energy to be technologies. developed. It has only been considered worthy of It is worth reminding ourselves of some of the key official support by the UK government since 2001, developments which act as drivers for this recent but today it is a key part of the Department of development. Atmospheric carbon dioxide concen- Trade & Industry’s (DTI’s) R&D programme having trations are higher now than at any time in the last real potential to make a significant contribution to 500 000 years and have clearly departed from the the UK’s (and for that matter the rest of the natural cycle. CO2 concentrations in the atmosphere world’s) Kyoto targets. have risen as much in the last 150 years as in the pre- The work so far completed by the author’s com- vious 20 000 years [1]. This effect is commonly con- pany, Marine Current Turbines Ltd. (MCT), has sidered to be likely to cause significant global successfully confirmed the (expected) technical warming which may in turn cause catastrophic feasibility of collecting energy from tidal currents environmental problems [2]. and the work described in this article is intended to

JPE307 # IMechE 2007 Proc. IMechE Vol. 221 Part A: J. Power and Energy 160 P L Fraenkel develop a commercial form of the technology that is highly sensitive to the velocity. Doubling the vel- is expected to demonstrate potential economic ocity results in eight times the energy density and feasibility. of course halving the velocity results in one eighth Figure 1 illustrates Seaflow, the world’s first tidal- of the energy. For example, the energy density in a current-powered experimental turbine to function flow of seawater of 2 m/s is 4.1 kW/m2, but an in a truly offshore location. At the time of writing increase to 3 m/s represents an energy density of this, 300 kW system has been operational for more 13.9 kW/m2. Clearly it is crucial to find locations than three years and has exceeded its key design with the highest possible flow velocities if energy is goals, as will be discussed in detail later in this article. to be extracted as cost-effectively as possible. It should also be noted that the currents tend to run in phase with the rise and fall of the tides, with 2 TIDAL STREAM TECHNOLOGY: STRENGTHS slack tide at high and low tide being the point at AND WEAKNESSES which the currents reverse direction and come to a brief halt, and the maximum velocity being when the tide is at about the mean level. The ebb and 2.1 The tidal stream resource flood of the tides runs on an approximately 12.4 h The tidal resource is driven by the relative motion of period diurnal cycle and superimposed on this is the the gravitational fields of the moon, the sun, and the 334 h spring-neap cycle, where the relative positions earth. The fluctuations in local gravity resulting from of the sun and moon either reinforce their gravita- these movements cause the rise and fall of sea level, tional pull (springs) or are at right angles (neaps). which in turn causes massive flows of water. Most of So far as the UK is concerned, various studies [4–8] these flows are far too slow to present enough energy have been carried out to determine locations suitable density for effective energy recovery, however in cer- for tidal stream energy generation, and although the tain places where there are ‘pinch points’ due to flow UK tidal stream data base is fairly limited at this being constrained by land and seabed topography, stage, there is probably no other country with more the currents can be accelerated to much higher detailed information available. Resource data velocities. In such locations with peak velocities world-wide is sparse but work is in hand to try to exceeding about 2–3 m/s, the energy density remedy this. becomes great enough to warrant deployment of The most recent resource study by Black and appropriate technology. Veatch [8] gives an estimated UK extractable resource Because the kinetic energy in fluid flow is pro- of 22 TWh (electrical energy output per annum) for portional to velocity cubed, the energy availability tidal stream using a modified and probably more

Fig. 1 Seaflow, the world’s first offshore tidal current turbine, rated at 300 kW and installed in May 2003. The 11 m diameter rotor shown raised for maintenance on left and lowered for power generation on right

Proc. IMechE Vol. 221 Part A: J. Power and Energy JPE307 # IMechE 2007 Marine current turbines 161 accurate methodology. In short, depending on energy capture per turbine to such an extent assumptions and methodology, the estimates so far that it will reduce the overall return on invest- completed suggest that something in the order of ment. Therefore, any project can only have a lim- 5–10 per cent of the UK’s electricity supply (at pre- ited effect on natural flow processes and will not sent demand levels) could ultimately be met from cause any localized effect outside the natural tidal stream projects. spectrum of velocities. At this stage, the lack of information means it can 2. Threat of impact on marine wildlife from turbine only be guessed at the gross size of other tidal stream rotors. The speed of underwater turbine rotors is resources. It seems reasonable to assume that there generally low compared with wind turbines or is a world-wide extractable tidal current energy with ship or boat propellers because of a need to resource at least of the order of 100 TWh and possible avoid cavitation (typical tip velocities will be significantly greater. However, even if the resource below 12 m/s). Also a tidal turbine rotor at a proves to be limited to 100 TWh, this would require good site absorbs about 4 kW/m2 of swept area an installed capacity exceeding 25 GW which is from the current, whereas typical ship propellers clearly a multi-billion pound market. Moreover, the release over 100 kW/m2 of swept area into the UK in such a case would have the largest tidal water column; one is a gentle process and the stream resource which suggests the potential for other is violent. Therefore, tidal turbines are developing a UK-based industry to address not just much less likely to be a threat to marine wildlife a significant home market but also the potential than ship propellers. Underwater noise is also lim- world market. ited due to the low speed of operation and the need to minimize cavitation. 3. Conflicts with other users of the sea. Tidal turbines 2.2 Environmental issues can only be applied in locations with unusually high-current velocities, most often close to rocky The most important advantage of tidal or marine coasts, which tend to be hazardous for navigation current systems is that and hence are generally avoided by commercial although they depend on a fluctuating resource ship traffic. Arguably tidal turbines fitted with (with no energy available around slack tide), their navigation aids will provide a fixed reference for output is predictable, and therefore this technology mariners, which may be an aid rather than a hin- offers the possibility of dispatchable power, which drance to navigation. However, exclusion zones is inherently more valuable than the much more for fishing may be needed around turbine farms, randomly generated output of wind, wave, and which does have the benefit of protecting fish solar devices. stocks and the seabed in the area concerned. The ‘raison d’eˆtre’ for this new technology is that it 4. Pollution. Tidal turbines if developed and applied produces energy without pollution and thereby can on a large scale can substitute for fossil fuel gener- substitute for carbon-emitting power generation as ation and thereby diminish atmospheric pol- a means to mitigate atmospheric pollution. It is lution. Lubricating oil or other potential believed that tidal turbine technology generally has pollutants are present in small quantities, but the potential to be used with minimal environmental they are so well contained that they are most unli- impact. Some of the key environmental issues which kely to escape. Only relatively small amounts of are frequently raised are worth summarizing. anti-fouling paints (compared with ships) of the most environmentally acceptable kind (copper, 1. Effect on flow and sediment transfer. Tidal stream glass, or PTFE based) need to be used. Decommis- energy exploitation is unlikely to have any signifi- sioning is relatively rapid and straightforward and cant effect on natural processes. This is because ought to leave conditions almost exactly as they analysis has indicated that extracting more than were before the project. 10–15 per cent of the tidal stream energy from a 5. Energy return on energy invested (ERoEI). for a specific location is about the sensible limit [9]. tidal turbine looks like being better than for The reason for this is that if excessive numbers most energy technologies. This has not yet been of turbines are installed, such that flow velocities investigated rigorously, but the ERoEI for wind are reduced by more than a few per cent, the turbines has been found to be between 4 and 6 excess turbines will be counterproductive by months (depending on the wind regime and the effectively diminishing the performance of the technology) [10]. Since the weight of material rest of the project. Therefore, the tidal stream and the level of energy capture of the kind of resource is unusual in being self-regulating as an tidal turbines under development by MCT are energy supply, because growing a project similar to those parameters for wind turbines, beyond a sensible limit will reduce the overall the ERoEI seems likely to be of the same order.

JPE307 # IMechE 2007 Proc. IMechE Vol. 221 Part A: J. Power and Energy 162 P L Fraenkel

Having summarized the advantages of tidal turbine was initiated by IT Power. At that time, Seaflow technology, the main ‘down-side’ compared with gained the support of the European Commission’s wave or wind technologies is that the siting require- Joule Programme and an industrial consortium was ments for tidal turbines are specialized and relatively formed to implement it, which included Marine Cur- rare. In practice, locations are needed with mean rent Turbines that had been set up as a vehicle to spring peak tidal currents faster than about 4– exploit the technology. The earlier experience with 5 knots (2–2.5 m/s) or the energy density will be the 15 kW proof-of-concept system had indicated inadequate to allow an economically viable project. that the main difficulties of designing and developing In short, the resource is limited to certain locations a viable water current turbine for use at sea relate to with unusually strong currents, but even so it is a the practical details of building, installing, and oper- non-trivial resource in terms of commercial potential. ating something large enough to survive offshore conditions. Therefore, it was judged that testing models, doing other land-based studies or even pla- 3 TIDAL STREAM TECHNOLOGY: BACKGROUND cing smallish devices into the sea, would not solve AND STATE OF THE ART the most challenging problems. Seaflow was, there- fore, designed as the first ‘full-size’ tidal turbine, an As long ago as 1976, the Intermediate Technology experimental system to test all the real problems of Development Group (ITDG), the grand-parent com- developing viable offshore technology. Key issues pany of MCT, was seeking to use renewable energy to such as survivability, techniques for installation and help people in remote areas of the world improve access, control, impact on the local environment, their self-sufficiency. The author was working for etc. were all to be addressed. Seaflow will be dis- ITDG at that time and suggested that an ‘underwater cussed further in the context of MCT’s R&D pro- windmill’, driven by river currents could be used for gramme later in this article. pumping irrigation water out of many fast-flowing By 2001, the DTI officially included ‘tidal stream’ rivers which traverse otherwise arid regions [11]. as being eligible for support from the government’s Therefore, a river current turbine has been developed Renewable Energy R&D programme following com- for pumping irrigation water out of the Nile near pletion of an independent consultant’s study on the Juba in Southern Sudan. This had a 3 m-diameter potential commercial viability of the technology Darrieus-type of rotor driving a pump and was [13] which gave a positive evaluation to MCT’s mounted under a pontoon. It proved capable of techno-economic model. Following this, the DTI pumping 50 cubic metres of water per day through also agreed to cofinance the Seaflow Project. a head of 7 m and ran for nearly two years during Marine Current Turbines became an independent the early 1980s. The Sudanese civil war unfortunately entity from October 2000 and has developed a put a stop to the project. business plan for the commercial development of Falling oil prices through the 1980s prompted a tidal current power generation systems. The ‘Seaflow decline in official support in applying renewable Project’ represented the first phase of this programme. energy, but by the mid 1990s interest began to The tidal current ‘band-wagon’ really started roll- develop in the large-scale use of renewables, largely ing in the UK following the DTI’s declaration of sup- as a response to the perceived threat of global warm- port in 2001, and quite soon a number of new players ing which was highlighted as the Kyoto process got came on the scene. The government has spent in the under way. To this end, by 1994, MCT’s’ parent com- order of £10 million on tidal stream technology pany, IT Power, (which the author founded and also development by 2005. Norway is the only other worked for), in partnership with Scottish Nuclear (as country where significant spending has taken place was), and The National Engineering Laboratory, in this sector in support of a E11 million tidal turbine developed and demonstrated a 15 kW axial-flow project led by Hammerfest Strøm, but a number of tidal turbine system, at the Corran Narrows on small projects have also been initiated in the USA Loch Linnhe [12]. This was intended as a proof-of- and , where interest is growing. Therefore, concept project to lead to larger-scale developments. in conclusion, it can be seen that most tidal stream This project was effectively the starting point for pre- technology projects are relatively recent, mostly sent activity; it proved the concept to be viable but it post-2000, compared with R&D on wave and wind also highlighted numerous technical challenges which goes back to the late 1970s. including the difficulty of reliably mooring floating tidal turbines. Unfortunately, the demise of Scottish Nuclear as an independent entity brought that initial 4 MCT’S 300 KW ‘SEAFLOW’ PROJECT work to an end. In 1998, the ‘Seaflow Project’, to develop the The Seaflow turbine resembles in principle an under- world’s first full-scale (300 kW) offshore tidal turbine water wind turbine, with a single 11 m-diameter

Proc. IMechE Vol. 221 Part A: J. Power and Energy JPE307 # IMechE 2007 Marine current turbines 163 rotor, with full-span pitch control. It was installed in technologies, there have been teething troubles, but a mean depth of seawater of 25 m approximately nothing that could not be repaired by a crew of two 1.1 km off the nearest landfall at the Foreland Point gaining access from a small boat. This has demon- lighthouse below Exmoor in North in May strated the feasibility of developing technology to 2003. It has been shown capable of exceeding its survive in unforgiving conditions exposed to the 300 kW rated power under favourable flow con- incoming Atlantic storms. Indeed after the departure ditions (having delivered some 310 kW) and the of Seacore’s jack-up barge ‘Deep Diver’ on 31 May rotor efficiency also exceeded the design target of 2003, nothing larger than a small workboat has 37 per cent (typically achieving around 45 per cent – been needed to service the system and keep it work- see Fig. 2). The system is not grid-connected but ing, confirming the potential for servicing this tech- dumps its power into fan-cooled resistance heaters nology at low cost. ‘Offshore projects’ and ‘low cost’ capable of absorbing the maximum power. are phrases that rarely appear in the same sentence. A key patented feature of MCT’s technology is that The axial-flow rotor uses electrically driven servos the turbine rotor and power-train are mounted on a built into the hub to permit full-span pitch control steel tubular pile set in a hole drilled in the seabed with provision for turning the blades 1808 to achieve and tall enough always to project above the surface reverse pitch so as to allow efficient operation with of the sea (Fig. 1). The entire rotor and power both the ebb and flood tides. In practice, the single system can be physically raised, using hydraulic rotor Seaflow system is only generally operated uni- rams, so as to slide up the pile to a position above directionally with the ebb tide, which arrives so that the surface to facilitate maintenance or repairs the rotor is upstream of the pile, because operation from a small boat. We believe this is a vital require- on the flood tide involves running the rotor through ment as the use of divers or any other form of under- the pile wake. The pitch control system can also be water intervention is virtually impossible in locations used to limit the power as a means for achieving a with such strong currents. specific power rating, although Seaflow is designed Seaflow is the first sea-powered renewable energy to handle the maximum power ever available at the system world-wide to have been installed in exposed location selected which is marginally over 300 kW. open-sea conditions, and it has survived through The rotor design was developed using a computer three winters with regular Force 8 conditions with model modified from a wind turbine rotor design no significant technical failures. As with all new model and based on blade element theory. It uses an

Fig. 2 Typical power versus current speed at hub height for two test runs, one at neaps (light grey points) and one at springs (dark grey points), showing underlying calculated curves for

three values of CP (based on sampling rate of 2 Hz)

JPE307 # IMechE 2007 Proc. IMechE Vol. 221 Part A: J. Power and Energy 164 P L Fraenkel uncambered NACA 44 series foil and the design effi- An issue unique to axial-flow tidal turbines is the ciency was predicted at 37 per cent (i.e. Cp–power static pressure variation experienced by a rotor coefficient) by the design model in ‘free-stream’ blade as it rotates through the water column. This conditions. Water current turbines have a significantly is enough to induce large forces in the rotor blade different load spectrum compared with wind turbines, skins akin to ‘breathing’ as the blades move from in that centrifugal forces and gravity forces which are the top of the swept circle to the bottom. With significant for wind turbines are relatively unimpor- Seaflow, this problem was dealt with by flooding tant for water current turbines; in this case, the domi- the rotor blades to achieve internal and external nant loads are caused by lift forces which tend to pressure equalization. cause ‘flap-wise’ bending of the rotor blades (i.e. in The gearbox follows typical wind turbine practice the axial direction). Because of the relatively high in that it uses a planetary firststage and has spur mass flow and low velocities, these forces are signifi- intermediate and high-speed stages. The gear ratio cantly higher for tidal turbines than for wind turbines. is 57 : 1 so that a rated input speed of 17.4 r/min pro- For example, a 1 MW wind turbine with typically a duces a nominal 1000 r/min output. However, the 60 m-diameter rotor has an axial thrust on the rotor power-train runs immersed and has water-tight cas- at rated velocity of around 400 kN but a 1 MW tidal ings, which gives excellent passive cooling and has turbine with a rotor of about 20 m diameter would no need for supplementary heat exchangers. The experience a thrust of 1000 kN or more at rated generator is flange mounted on the gearbox with a power. The implication of this is that the rotor spring-loaded hydraulically released brake in a blades see extremely high bending forces which are sealed housing between gearbox and generator. The also dynamic (i.e. fatigue is a major issue) due to tur- input shaft to the gearbox has face seals and the gear- bulence, velocity shear, vortex shedding, and the box is pressurized with compressed air fed from the effect of any passing waves. Therefore, fatigue is a above water housing to approximately the seawater dominant design driver. A key part of the Seaflow static pressure at operating depth. The gearbox test programme was to establish the true magnitudes manufacturer, Jahnell–Kestermann GmbH (from and frequencies of the various key load cases, in par- Bochum, Germany) is well known for both wind ticular the fatigue inducing variances. turbine and marine gearboxes and this unique An example showing a scattergram of spot read- submersible gearbox uses know-how gained from ings of current velocity versus shaft power is given gearboxes for wind turbines and for in Fig. 2. It can be seen that there is a lot of scatter lar- submerged dredger bucket drives. The gearbox gely because at that time the control system was not output and generator are offset from the main well optimized so rotor blade settings sometimes input shaft allowing cables for the rotor pitch mech- happened to be such that a high efficiency was anism servos and instrumentation to be run through achieved and sometimes they were some way from the centre of the main shaft to a multiple electrical being correct. However, even with more effective slip-ring unit mounted at the back of the gearbox; control, there is still significant scatter due to the this allows interconnection from the above-sea- fluctuations resulting from turbulence and other level control system and low-voltage power supplies effects. Wind turbine rotor performance measure- to the rotating components. ments tend to display scatter in a similar manner. The generator is an induction machine designed for What is encouraging is that under the most favour- use as a subsea motor for seabed pumping equipment able conditions, Cp values exceeding 40 per cent are used by the oil and gas industry. Because the system consistently achieved. The enhancement in runs at variable speed, typically nominal þ or 225 performance over what was predicted by modelling per cent, it has electronic power-conditioning equip- is largely due to blockage effects (i.e. it is not in a ment and is controlled by a frequency converter. free-stream). Interconnection of subsea power and instrumenta- The design philosophy for Seaflow was necessarily tion components to the above-sea control equipment cautious because any serious component failure was implemented where possible using submarine could have killed the project, so structural integrity cables, in many cases with underwater mateable was paramount. As a result, the rotor, which had connections. originally been planned as a steel fabrication, was Because Seaflow is over 3.3 km from the nearest finally made from composite materials, using a practical grid connection and is only intended as a carbon fibre reinforced mainspar with glass fibre relatively short lived experimental test bed, it was reinforced ribs and external skinning to achieve an decided at an early stage not to grid connect it, but adequate fatigue life from a much lighter rotor. The to provide a dump load capable of absorbing full rotor blade detail design and manufacture was by rated power, since the high cost of such a long grid Aviation Enterprises Limited, a company with cut- connection seemed to be unjustifiable for a com- ting edge capability in this field. ponent that would not add significant value to the

Proc. IMechE Vol. 221 Part A: J. Power and Energy JPE307 # IMechE 2007 Marine current turbines 165 project. The dump load and backup power supplies The engineering team was pleased that the system needed in lieu of the grid were estimated to be has been reliable enough that all necessary repairs much less costly than a lengthy marine cable. This and maintenance functions have successfully been did add significantly to the design work-load, as the carried out using no more than the on-board equip- dump load and auxiliary power supplies had to be ment for over three years. Visits have always been developed, sourced, and tested. In the event, fan- carried out from a small RIB with twin outboards, cooled air heaters situated in the housing above the which is about the least possible cost for intervention water were used to absorb power. A 15 kVA diesel for any off-shore project; an essential requirement if generating set with a bank of batteries provides the low-cost power is eventually to be produced. necessary power for maintenance functions (e.g. Installation of Seaflow was carried out from a jack- powering a small on board crane, power tools, etc.), up barge equipped with rock drilling equipment. The navigation lights, and not least the parasitic loads procedure, which was implemented in May 2003, to start the tidal turbine. Parasitic loads include the was to position the jack-up on site during slack tide electronics, control PC, fan for the dump load, in the neap period and drop its legs so that it could hydraulic pump to release the brake, etc. A small be jacked out of the water. Once standing on its solar photovoltaic panel and separate storage battery legs with an adequate air gap beneath its hull, the provide backup for the navigation light as it was not jack-up forms a stable working platform, although always possible to run either the tidal turbine or the there was a lot of analysis needed in this case of diesel generating set largely because the exposed determining the limits of its operating envelope location precluded visits by the engineering team from the point of view of adverse combinations of during winter months. Safe access, gained by jump- current, water depth, waves, and wind. ing from a rigid inflatable boat (RIB) onto the A rotary rock drill is driven from an extension on the access ladder is only possible with wave heights of back of the jack-up and a steel sleeve is allowed to ,50 cm, which are rare in winter. follow the drill into the hole to prevent debris being The structure is supported on a tubular steel swept back in by the currents. Seawater is used to monopile, 2.1 m in outside diameter and 52 m long, lubricate the drill and transport the spoil so no alien welded from 6 m ‘cans’. The wall thickness varies materials were introduced into the water column. to suit the load distribution. The pile penetrates Once the socket has reached the required depth 18 m into the seabed and stands in a mean depth (which is a function of the foundation design of water of about 24 m. requirements which are driven by issues such as A ‘collar’ fabricated mainly from steel rectangular the pile natural frequency after installation), the sections surrounds the pile and is free to slide up sleeve is left protruding about 1 m above the seabed and down vertically (Fig. 1). The collar carries the to prevent any debris entering the hole. The pile power-train and rotor and can be raised by pulling was sealed at both ends and floated to site, being it up using a tubular strut that can be ratcheted up positioned by tugs; the jack-up crane then picked or down by a pair of hydraulic rams and removable up one end of the pile and in a delicate operation locking pins that engage with a perforated rack run- timed for slack tide, lifted it vertically and presented ning the length of the strut. This hydraulic lifting it into the sleeved socket. The pile was then held ver- mechanism is similar to that used for raising and tically by the jack-up’s pile hydraulic handling lowering the legs of a jack-up barge of the kind system and slowly sunk into the socket by filling it used by MCT’s partner Seacore for installation. with water. Once in position concrete grout was A6m 3m 3 m box-like housing consisting of pumped through grout pipes provided in the pile a frame with external cladding is bolted on top of so that it would emerge from the base of the pile the pile and carries the dump load, hydraulic lifting and well up into the annular space between the mechanism, the control PC and the electronic pile and the sleeved socket. After the grout had set frequency converter and transformer, diesel gene- the water in the pile could safely by pumped out. rating set, batteries, emergency equipment with The jack-up then backed off and repositioned itself just enough room for two operators to monitor the about 10 m from the pile so that its crane could be system and to maintain, repair, and when necessary used to assemble the collar, rotor, and power-train make modifications. Significant emergency equip- and then the pile top housing onto the pile. All ment is needed, notably navigation lights, a fog these items were brought to site on the jack-up’s horn with fog sensor, life-raft, and fire extinguishing deck, having been partially assembled and pretested system. There is also a small hydraulic folding crane on shore. mounted on a strong point on top of the housing Seaflow was operated for the first time on 31 May with a man-basket to permit virtually all mainten- 2003, the day the jack-up departed the site. It has ance functions to be completed in the absence of a been under test since then and will probably be servicing vessel. decommissioned during the summer of 2007 by

JPE307 # IMechE 2007 Proc. IMechE Vol. 221 Part A: J. Power and Energy 166 P L Fraenkel which time it has been expected to have gained all the useful data that can be gleaned from it. MCT owns the project and is responsible for the design, but various other participants included Sea- core Limited (a leading offshore engineering com- pany), IT Power (a renewable energy consultancy and former parent company), Bendalls Engineering (a steel fabricator from Carlisle), Corus UK (part of the Anglo-Dutch steel company–formerly British Steel) and also German partners in the form of ISET (a Renewable Energy R&D company attached to Kassel University), and Jahnell–Kestermann (a major manufacturer of gearboxes). The total project cost was approximately £3.5 million of which 60 per cent was subsidized by the UK government, the EC, and the German government and 40 per cent came from MCT and the partners. A wealth of useful data has flowed from the Seaflow test programme to inform the development of commercial technology to follow. It has also been suc- cessful in confirming that various key conceptual ideas actually work effectively in practice, including Fig. 3 Artist’s impression of Seagen 1 MW tidal turbine the fundamental concept, the axial-flow rotor, the marinized (submerged) power-train, the use of a surface-breaking monopile and structure, together with low-cost intervention for maintenance from Therefore, the total rated power per installed unit is small boats. Most importantly, experience has shown up to 1200 kW(e) (depending on siting conditions). the system to be harmless to the local environment, or The reasons for the twin rotor configuration are pri- at least no obviously harmful effects have been marily that this permits bidirectional operation with observed so far, and environmental checks are the rotors clear of the pile wake when the rotors are ongoing. downstream of the pile; 1808 rotor blade pitch control MCT has had to cope with a significant number of allows efficient operation when the current reverses. ‘bugs’, as is to be expected with any new technology Also, two rotors clearly deliver twice as much energy of this complexity, but fortunately none of these as one would, but at less than twice the cost, so have been ‘show stoppers’ and as a result, the system enhanced cost-effectiveness is another reason. has functioned better after two years use than it did Essentially, Seagen produces three times the power initially and can be operated automatically or by of Seaflow at around twice the cost, giving a signifi- remote control using an internet connection. Most cant improvement in cost-effectiveness. Seagen will importantly, the team has learnt a number of require- be installed in 2007 and will be grid connected. ments for future technology which should make com- Seagen is a £10 million project, and it involves some missioning and early stage reliability much easier and of the same partners as Seaflow. It is also supported less costly for the planned future project phases. bynewshareholdersofMCTandstrategicpartners, EDF Energy (the UK subsidiary of one of the largest utilities in the world–Electricite´ de France), by Guern- 5 MCT’S 1 MW ‘SEAGEN’ PROJECT sey Electricity (the Channel Island utility which hap- pens to have strong currents around its coast), and The next stage in MCT’s R&D programme is to by BankInvest (a Danish specialist investment bank develop and build the prototype for the commercial focusing on innovative and clean-energy technol- technology to follow a system, which has been ogies). The UK government, through the DTI, is named as Seagen. While Seaflow proved technical again supporting MCT’s R&D, having committed to feasibility, Seagen is needed to prove the economic provide a grant worth £4.3 million. and commercial feasibility. At the time of final editing (October 2006), manu- The Seagen system has its rotors mounted at the facture of Seagen is virtually complete and dry-test- outer ends of a pair of streamlined wing-like arms ing of systems has started. It is planned that Seagen projecting either side of the supporting pile (Fig. 3). will be installed as early in 2007 as possible and it Each rotor is 16 m in diameter and drives a 600 kW will be immediately followed by work to develop an power-train consisting of a gearbox and generator. array of similar systems to be installed in an open

Proc. IMechE Vol. 221 Part A: J. Power and Energy JPE307 # IMechE 2007 Marine current turbines 167 sea location, where economies of scale will yield a combination of turbulence, lack of uniformity in further improvement in cost-effectiveness. the flow, velocity shear, and passing waves and The goal of MCT’s business plan is to have a tech- these act through the rotor axes giving rise to huge nology that can be deployed in commercial power yawing moments, where the cross-arm and collar projects by 2007 to 2008 and which will rapidly interface with the pile. Finding a robust mechanism become cost-competitive with offshore wind projects. to handle these loads proved to be a significant chal- It is also planned to initiate demonstration pro- lenge, but it is believed that an excellent solution to jects in North American waters in parallel to the this problem is obtained. first UK array (which will enable economies of scale As with Seaflow, the power system is variable in procurement of the turbines at the same time as speed, variable voltage, and variable frequency with those for the planned array) and MCT is actively a control system able to vary both the frequency con- seeking US and/or Canadian strategic partners to verter’s parameters and the rotor blade pitch angles head such a program. The first North American pro- in order to optimize performance. The control strat- ject may then be ‘rolled out’ into a larger project soon egy is to achieve a successful start-up by initiating after, once its efficacy is demonstrated. rotation without generation and then under the cor- rect conditions starting the generation process at a current velocity of about 0.7 m/s. The system then 5.1 Seagen: technical details seeks to maximize the power until the current Seagen has inherited most of the successful features speed reaches a level where rated power is achieved, from Seaflow, but it also differs in quite a number of which will typically be at about 2–2.3 m/s depending respects. It still has full-span pitch control with on the local site conditions. For higher current- carbon/glass fibre composite rotor blades. The velocities, the pitch-control mechanism sheds power power-train is again submersible, with a pair of pla- by reducing the angle of attack of the blades to main- netary gearboxes driving induction generators, tain as close to rated power as possible. When the although in this case a UK design from Orbital2 man- tidal stream velocity starts to reduce, the control ufactured by Wikov in the Czech Republic. The main system maintains rated power as long as the velocity support structure is again a rolled-steel monopile, is high enough and thereafter, as the speed ramps although this time 3 m in diameter. down, it maintains maximum power for the con- A major difference however is the so-called ‘cross- ditions, until the velocity falls below 0.7 m/s, at arm’ structure, such as a pair of wings, to carry the which time the system cuts out and the rotor is power trains either side of the pile, far enough apart parked with its blades ‘feathered’. When the tide for the rotors not to cut into the pile wake. The turns (slack tide) and the flow direction reverses, cross-arm wings have some dihedral primarily to the control system pitches the rotor blades 1808 help raise the power trains higher out of the water ready for operation in the reverse direction once for a given collar lift. The dihedral also ensures that the velocity again exceeds the cut-in speed. the rotor blades cut the cross-arm wakes in a scissor- Other design features of interest are that the collar like manner so that only part of a rotor blade is in and cross-arm, carrying the pair of rotors and their the wake at any moment. The cross-arm wing section power trains, can be lifted by a pair of hydraulically is elliptical and designed to minimize the wake activated vertical struts driven by rams, situated in thickness; some CFD analysis was carried out for the above water housing. This housing also provides MCT by QinetiQ at Haslar to optimize the cross-arm a control centre where the control PCs are located geometry. and where ancillary equipment such as a hydraulic The rotors and power trains are held by three- power pack, a small air compressor, safety equip- point mountings under the side wings and designed ment, and a dehumidifier are located. Transformers, so that when raised above the water, a flat-top barge to deliver power into a marine cable at 11 kV, and may be positioned underneath and the power-train power-conditioning equipment are located in the and rotor can be lowered as a complete unit onto top of the pile on three levels and the interior the barge before being replaced by reversing this water-cooled pile surface is used to provide cooling, process. with a fan to circulate the internal air and the afore- Many aspects of this technology have been mentioned dehumidifier to take out condensation. patented or are pending patents, and Seagen is a The interior of the pile and housing are sealed to registered design. The mechanism for locking the minimize ingress of moisture and sea salt. cross-arm in place when the system is operational It is planned to carry through an extremely is the subject of the most recent patent application. thorough testing programme where possible prior This is because the extreme load cases considered to installation to ensure bought-in items are to in the design process have to allow for dynamic specification (this was not always the case with Sea- asymmetrical loads which can be generated by a flow). The turbine will then be commissioned and

JPE307 # IMechE 2007 Proc. IMechE Vol. 221 Part A: J. Power and Energy 168 P L Fraenkel

Fig. 4 Artist’s impression of arrayed Seagen turbines showing one raised for maintenance tested firstly to obtain performance data and then to has the potential to contribute to solving this pro- try various control strategies, before seeking to blem and it has been hoped and expected it to be obtain operational reliability of the highest possible commercially viable well within the next 5 years order. (Fig. 4). A continuous environmental monitoring prog- It is hoped that commercial feasibility will be effec- ramme will be run in parallel with the main pro- tively demonstrated through the Seagen project. The gramme, to confirm that the system is not causing key to arriving at this result is to gain the operational significant adverse environmental impacts, and that experience needed to develop the reliability of the if any such impacts are detected, steps can rapidly systems. The prototype will inevitably be over- be taken to identify any such problem and develop engineered as a means to minimize risks of failure, effective mitigation measures. A large number of so there will be a pressing need to value-engineer potential environmental issues are being investi- the system in order to get costs down and to ensure gated, including possible interactions with marine the resulting product can reliably deliver electricity mammals and fish, effects on benthos (seabed life), from the seas for several decades with minimal underwater noise, size of wake, etc. It is believed environmental impact. that this technology has the potential to generate MCT plans to commission several hundred mega- electricity with minimal adverse environmental watts of turbines by 2012. The potential thereafter impact, which these days is an important selling runs to many gigawatts of capacity even for this point, and therefore it is important to establish first-generation technology which is limited to that this is a correct assumption and also to identify water depths in the range of 20 m to about 50 m. any adverse effects and find methods for mitigating However, the company has already made patent them. applications and started research into more radical second-generation systems that are expected to be functional in much deeper currents, up to 100 m or 5.2 Commercial tidal stream technology: more ( 300 ft) with significantly larger rotors, the future  higher rated power levels, and correspondingly In the face of the developing Global Warming and greater economies of scale. Peak Oil crises, there is an urgent need to prove In conclusion, MCT believes it is well on track to and bring on stream new clean-energy techno- delivering commercial tidal stream technology with logies, such as tidal turbines. It can be anticipated the potential to supply electricity on a large-scale, that security of supply will increasingly preoccupy at low cost and without pollution. It is believed that the governments of industrialized countries all of the concepts under development by MCT will which need to find huge new clean-energy sup- become one of the primary techniques for extracting plies. The technology under development by MCT energy from the seas.

Proc. IMechE Vol. 221 Part A: J. Power and Energy JPE307 # IMechE 2007 Marine current turbines 169

REFERENCES (Tecnomare, ENEL, IT Power, Ponte di Archimede, University of Patras) 1995. 1 Etheridge, D. M., Steele, L. P., Langenfelds, R. L., and 6 Feasibility study of tidal current power generation for coastal waters: Orkney and Shetland. Final report, EU Francey, R. J. Historical CO2 records from the Law Dome DE08, DE08-2, and DSS ice cores. Division of contract XVII/4 1040/92-41, ICIT, March 1995. Atmospheric Research, CSIRO, Aspendale, Victoria, 7 The potential for the use of marine current energy in Australia and J.-M. Barnola Laboratoire de Glaciologie , Peter L Fraenkel–Marine Current et Ge´ophysique de l’Environnement, Saint Martin Turbines Ltd, Adrian Bell–Kirk McClure Morton Ltd, d’He`res-Cedex, France V.I. Morgan Antarctic CRC and Prof. Trevor Whittaker–Queens University Belfast & Australian Antarctic Division, Hobart, Tasmania, Les Lugg–Seacore Ltd, published by the Department Australia, 1998, available from http://cdiac.esd.ornl.gov/ of Enterprise, Trade and Investment, Belfast, NI, 2003, trends/co2/lawdome.html and http://www.gci.org.uk/ available from http://www.detini.gov.uk/cgi-bin/ contconv/cchist.html moreutil?utilid¼41&site¼5&util¼2&fold¼&parent¼ 8 Black and Veatch Consulting Ltd. Tidal stream energy 2 Climate impact of quadrupling atmospheric CO2:an overview of GFDL climate model results, Geophysical resource assessment. Technical report, Carbon Trust, Fluid Dynamics Laboratory at NOAA (National Oceanic London, September 2004. and Atmospheric Administration of the US Department 9 Bryden, I. G. Extracting energy from tidal flows, of Commerce), 2004, available from http://www. Hydraulic Aspects of Renewable Energy, 19th March gfdl.noaa.gov/tk/climate_dynamics/climate_impact_ 2004, Glasgow. webpage.html 10 The energy balance of modern windturbines.Wind- 3 King Hubbert, M. Nuclear energy and the fossil fuels, power Note No. 16, December 1997, available fromhttp:// 1956 (American Institute, Texas), available www.windpower.org/media(444,1033)/The_energy_ from http://www.hubbertpeak.com/hubbert/1956/ balance_of_modern_wind_turbines%2C_1997.pdf 1956.pdf 11 Fraenkel, P. Water pumping devices, 1986 (Intermedi- 4 Tidal stream energy review. Report no. ETSU T/05/ ate Technology Publications, London), revised 1997. 00155/REP, UK DTI, Prepared by Engineering and 12 Fraenkel, P. L., Macnaughton, D., Paish, O., and Power Development Consultants Ltd, Binnie & Part- Hunter, R. Tidal stream turbine development. In Pro- ners, Sir Robt. McAlpine & Sons Ltd and IT Power Ltd ceeding of Conference on Clean Energy 2000, Institute for the ETSU, UK Deptartment of Energy, Crown Copy- of Electrical Engineers, London, 17–19 November 1993. right 1993. 13 Commercial prospects for tidal stream power. Binnie, 5 Marine currents energy extraction: resource assessment. Black and Veatch, and IT Power Ltd, Redhill. DTI/ Final report, EU-JOULE contract JOU2-CT93-0355 ETSU, Harwell, April 2001.

JPE307 # IMechE 2007 Proc. IMechE Vol. 221 Part A: J. Power and Energy