SPECIALISSUE P APER 1

Powerfrom marine currents

P L Fraenkel MarineCurrent T urbinesLimited, 2 AmherstA venue,Ealing, London W13 8NQ, UK

Abstract: Thispaper describes the rationale and the engineering approach adopted for thedevelopment of technologyfor convertingthe kinetic energy in marine currents for large-scaleelectricity generation. Althoughthe basic principles involved are relatively straightforward and well understood, being similar to thoseof a windturbine, a practicaland cost-effective large-scale system designed to extract the kinetic energyof owingwater has yet to be developed.This paper describes the research and development being undertakenthrough an industrial consortium with the aim of achieving this goal for theŽ rsttime, i.e. to achievethe world’ s Žrst commerciallyviable systems for deliveringpower from marinecurrents.

Keywords: tidal,stream, marine, current, energy ,turbine,axial  ow,kineticenergy conversion

NOTATION 2010.However, land-based renewable energy technologies arealready facing constraints owing to con icts over land A cross-sectionalarea swept by rotor use,so an important factor is thatthe seas offer largeopen H overalldepth of water spaceswhere future new energy technologies could be K0 constantproportional to mean spring peak velocity deployedon a grandscale, perhaps with considerably less at site impacton either the environment or other human activities. K1 constantproportional to ratio of mean spring and Infact the oceans represent an energy resource which is neappeak currents theoreticallyfar largerthan the entire human race could Kn ‘springneap factor’ (e.g. for maximumneap stream possiblyuse, although in practice most of thishuge resource of60per cent of spring, K 0.57) isinaccessible. The main potential sources of marineenergy n ˆ Ks velocityshape factor (0.424 for asinusoidal ow) arewaves, currents and ocean thermal energy; offshore wind P power isof course also a marineresource, although the energy is T0 diurnaltidal period (usually 12.4 h) notderived from thesea itself. Arguably ,unlessmarine T1 springneap period (usually 353 h) renewableenergy resources are developed and used, it will V free streamvelocity local to hub height of turbine notbe possibleto meetfuture energy needs without risking rotor seriousdamage to the environment either through continu- V(Z) velocityat height above seabed Z ingto burn increasing quantities of fossil fuels or through V(mean) depth-averagedvelocity for wholewater column placingincreasing reliance on nuclear power .Thisis the V(peak) maximumspring tide velocity mainjustiŽ cation for investingin these new and so far little- Z heightabove seabed in the water column developedenergy solutions. However, marine renewable energyresources are generally more costly and difŽ cult to r density(of seawater) exploitreliably than the land-based options (which is why nogreat effort to develop them has been made until recently). 1RATIONALEFOR USING THIS RESOURCE Ithas been obvious to seafarers for centuriesthat power- fulsea currents exist in certain locations where the  ows Thereis an emerging market for ‘green’electricity derived tendto be concentrated. Such locations tend to be where from renewableresources which in the UK andmany other currents,which in the open sea move at low speeds of just a countriesincludes government-sponsored Ž scalincentives few cm=s,are channelled through constraining topography, for utilitiesto acquire an increasing proportion of renewable suchas straits between islands, shallows between open seas energy-basedgenerating plant. In the UK, theRenewable andaround the ends of headlands. These relatively rapid EnergyObligation will oblige utilities to source at least 10 tidalcurrents typically have peak velocities at spring tide in percent of their electricity from renewableresources by theregion of 2–3 m =s(4–6 knots)or more. The gross kinetic TheMS wasreceived on10April 2001 and was accepted after revision for energyin such  owsis extremely large and it appears publicationon 13July 2001. regularlyand predictably in perfect tune with the relative

A01801 # IMechE 2002 ProcInstn Mech Engrs V ol216 Part A:JPowerand Energy 2 P L FRAENKEL motionof the Earth, Moon and Sun. Therefore this is a lesserextent the Sun with the Earth. There are also renewableresource that is relatively potent, yet it is one of geotrophic(or oceanic)currents caused primarily by thefew renewableenergy resources that can deliver power Coriolisforces acting on the water in the major oceans as predictablyto a timetable, a factorwhich adds to the value aresultof the rotation of the Earth. Currents are also ofthe output, since from theplanning point of viewa utility generatedby density differences in the seas resulting from needsto be ableto deliver power rather than energy ,evenif salinityand temperature variations in different sea areas. itis energy which generates the revenue. Since the tides are However,in European waters the main marine current outof phaseat different points around the coast, power can resourceis tidally driven. oftenbe available at one installation at times when there is slacktide and no powerat another.However,at neaptide the energyavailability is signiŽ cantly reduced compared with 2.1The energy in  owingwater thesprings, and this will be true everywhere. Advanced Thepower available from astreamof wateris knowledgeof the availability of tidal power will permit a utilityto seek to capitalize on occasions when good tidal 1 3 P 2 rAV owscoincide with expected periods of high electricity ˆ demand. where r isthe density of water, A isthe cross-sectional area Recentstudies [ 1–3]indicatethat marine currents have ofthe rotor used to intercept the  owand V is the free thepotential to supply a signiŽcant fraction of future streamvelocity of thecurrent. The consequence of thiscube electricityneeds and, if successfully developed, the technol- lawrelationship is that power and hence energy capture are ogyrequired could form thebasis of amajornew industry to highlysensitive to velocity. This is clearly indicated in produceclean power for the21st century .Themost detailed Table1 showingpower densities for variouswater velocities studyso far, ‘MarineCurrents Energy Extraction: Resource (insea water), comparedwith the wind and solar resources. Assessment’[ 2 ], analysed106 locations in European terri- Itcan be seenthat the marine current resource at locations torialwaters withcertain predeŽ ned characteristics to make withcurrents exceeding 2 m =sisa relativelyintense renew- themsuitable for energyexploitation, and the aggregate ableenergy source compared with the better-known alter- capacityof this selection of sites amounted to an installed nativessuch as solar and wind. It should be noted that ratedcapacity of marine current turbines of over12 000MW , 13 m=sisthe typical rated velocity ,i.e.velocity at which capableof yielding some 48 TW hofelectrical energy per rated(or maximum)power is achieved for awindturbine. annum.Because of thelack of publicdomain data on tidal ows,the total exploitable resource remains uncertain, but it islikely to be signiŽ cantly larger than this. 2.2The tidal cycle Infact, some locations, such as the Pentland Firth Thetidal cycle can be approximated by a doublesinusoid; (betweenScotland and Orkney) and the Alderney Race onewith a periodof 12.4h representingthe diurnal tidal ebb (betweenthe Channel Islands and France), or the so-called and owcycle, and the other with a periodof 353 h BigRussell (off Guernsey),have especially intense currents representingthe fortnightly spring neap period. The follow- overa sufŽciently large sea area to offer potentialfor ingequation provides a reasonablemodel for predictingthe exploitationon a multigigawattscale. Examples of other velocity V ofa tidalcurrent: intenselocations include the Severn Estuary (UK’ s north Devoncoast), the straits between Rathlin Island and North- 2pt 2pt ernIreland, the Straits of Messina between Italy and Sicily V K0 K1 cos cos andvarious channels between the Greek islands in the ˆ ‡ T1 T0 Aegean.Other large marine current resources can be foundin regions such as South East Asia (e.g. the where K0 and K1 areconstants determined from themean Philippines,Indonesia, Japan), Australia and New Zealand, springpeak and the ratio between the mean spring peak and Canadaand the south east coast of South Africa. themean neap peak currents, T1 isthe spring neap period Therefore,in short, there will be a greatlyincreasing demandfor cleanelectricity and the marine current resource Table 1 Relativepower density of marine currents compared couldmake a majorcontribution to thisneed with the added withwind and solar resources valueof beingable to deliver power predictably . Energyresource

Marinecurrents Wind Solar 2 THE TIDAL=MARINE CURRENT RESOURCE Velocity 11.52 2.53 13Peak at (m=s) noon Marinecurrents are primarily driven by the tides (i.e. tidal Velocity 1.92.9 3.9 4.9 5.8 25.3 currentsor tidal streams) whichoccur because of the (knots) variationin level of the surface of the sea caused by Powerdensity 0.521.74 4.12 8.05 13.91 1.37 1.0 (kW=m2) ¹ interactionof the gravitational Ž eldsof the Moon and to a

ProcInstn Mech Engrs V ol216 Part A:JPowerand Energy A01801 # IMechE 2002 POWERFROM MARINE CURRENTS 3

(353 h) and T0 isthe diurnal tidal period (12.4 h). Generally, whereit willnot cause serious obstruction to other users of inUK waters,the maximum mean spring current velocity the sea. willbe approximately twice the maximum mean neap tide Thedepth of waterideally needs to be inexcess of about velocity. 15m atlow tide and probably no more than 40 or 50 mat Analternative idealization of this is to assume a mean hightide. The minimum level will accommodate a rotorof powerlevel in a cross-sectionof  owas follows [ 4]: about10 mdiameter,probably about the smallest that might beconsidered robust and powerful enough to be cost- 1 3 P mean 2 rAKsKnV peak effectivelydeployed in this way at sea, while the upper … † ˆ … † limitdepends on the type of technology and installation methodto be used.In practice most fast  owinglocations in where r isthe density , A thecross-sectional area of ow, Ks UK watersare in any case less than 50 m deepsince it is avelocityshape factor (0.424 for asinusoidal ow), Kn a springneap factor (e.g. for maximumneap stream of 60 per oftena reductionin depth that causes the  owto accelerate. Thevelocity characteristic of a tidalstream can best be centof spring, Kn 0.57) and V(peak) isthe maximum spring tidevelocity . ˆ deŽned by the mean spring peak. Economic analysis of the Thevelocity of atidalcurrent is also generally modiŽ ed technicalconcepts being developed by Marine Current tosome extent by a combinationof other factors such as TurbinesLimited suggests that a minimumof approximately residualmomentum, global oceanic marine circulation, 2 m=smeanspring peak is needed and ideally 2.5 m =s or windfetch and density variations and superimposed on all more(i.e. 4– 5 knots)in locations with typical bidirectional this,at least near the surface, are the rotating velocity sinusoidaltidal  ows.However, a lowerpeak velocity in the vectorscaused by passing waves. However, at high-velocity orderof 1.2–1.5 m =sbecomeseconomically viable in loca- sitesof the kind suitable for powergeneration, when a tionswith continuous or quasi-continuous ow(e.g. rivers, strongtide is  owing,the tidal current vector generally oceanicor geotrophiccurrents). This is important since the exceedsall these other effects by a signiŽcant margin. mainjustiŽ cation for applyingthis technology relates to the Thecurrent will also vary in velocity as afunctionof the costeffectiveness with which it can deliver useful energy depthof  ow.Thevelocity at a heightabove the seabed Z andone of the most sensitive parameters in the economic approximatelyfollows a seventhpower law as afunctionof analysisis the  owvelocity (since, as indicated earlier, depthin the lower half of the ow[ 5] energyis related to velocity cubed).

Z 1=7 V Z V mean ; for 0 < Z < 0:5H … † ˆ 0:32h … † 3CONCEPTS FOR THEEXPLOITATION OF MARINE CURRENTS andin the upper half 3.1Basic requirements

V Z 1:07V mean ; for 0:5H < Z < H … † ˆ … † Theend product is delivered energy ,whichclearly needs to beobtained at the least possible cost. The energy captured Itcan be shownfrom thisthat approximately 75 per cent of willbe afunctionof theswept area of theturbine rotors in a theenergy is tobefoundin the upper 50 percent of the ow. Želd,their average efŽ ciency at converting kinetic energy Inpractice, areas with high  owvelocities will generally andthe load factor (or capacityfactor) for thesite. Load berelatively turbulent and  owmodelling is notas simple as factormay be deŽ ned as the ratio of the average power of thisand many variations can occur depending both on thesystem to the rated power . location(and hence seabed bathymetry) as well as on the Itfollows from thisthat the rotor area needs to be aslarge stateof the tide ( ow patterns and velocity shear changes aspossible to Ž llthe space, although there are constraints throughthe tidal cycle) . whichlimit this since if the rotor blades pass too close to the owboundaries the effects on performance are likely to be 2.3Siting requirements for energy exploitation counterproductive.EfŽ ciency also needs to be as high as possiblewith costs as low as possible, two requirements that Themain aim is toŽ ndlocations with fast  owingwater, a aresometimes not easily reconciled and therefore need good relativelyuniform seabed (to minimize both turbulence and engineeringcompromises. In the last resort it is cost effec- theloss of velocity near the seabed), sufŽ cient depth of tivenessthat matters, but good cost effectiveness generally waterto allow a largeenough turbine to be installed and requiresgood efŽ ciency . preferablysuch conditions extending over a wideenough Theload factor also needs to be as high as possible; the areato permit the installation of a largeenough array (or lowerthe rated velocity (which is thevelocity at which rated ‘farm’) ofturbinesto make the overall project cost effective. poweris developed)relative to the site mean peak velocity, Inaddition the location needs to be sufŽ ciently near to a thehigher the resulting load factor .Thereforethere is a shore-basedgrid connection to allow the energy produced to trade-offbetween rated velocity (hence rated power) and bedelivered at reasonably low cost. Finally ,italso has to be loadfactor in order to maximize energy capture. Exactly as

A01801 # IMechE 2002 ProcInstn Mech Engrs V ol216 Part A:JPowerand Energy 4 P L FRAENKEL withwind turbines, there is always an optimum rated wind.Although there are a largenumber of devices that velocityfor agivensite which will maximize the resulting havebeen promoted for extractingenergy from uid ows, energycapture (and hence the return from theinvestment). inpractice, if reasonable efŽ ciency is to be achieved, it is Clearlyany outages for technicalreasons will severely necessaryto use a mechanismwhich relies primarily on reducethe practical load factor, so reliability will be a key generatingtorque and hence power from liftforces rather requirementfor anysuccessful offshore renewable energy thandrag. There are only two generic types of kinetic energy technologies,especially bearing in mind that adverse conversionrotor that are driven primarily by lift forces and weathermay delay remedial measures much longer than thoseare the conventional axial  owor ‘ propeller’type of wouldapply for anonshore technology .Therefore,apart rotorand the cross- ow or ‘ Darrieus’rotor (see Fig.1). from considerationsof energy capture and general cost- Themain advantage of theDarrieus rotor is that the shaft effectiveness,any power system operating in the sea needs poweris takenout perpendicularly to the  ow,whichlends tobe sufŽ ciently robust and reliable to function for lengthy itselfto having a drivetrain either on the seabed or in a periodswithout human intervention. It is believed that the surfacevessel, while an axial  owrotor either has to have maintenanceinterval for marinerenewable energy systems thedrive train entirely at rotor hub level (i.e. in the centre of ofany kind needs to be Žveyearsor more, because the cost the ow) orneeds some form ofright-angled energy ofoffshore operations needed to undertake maintenance or transmissionmechanism. However ,theDarrieus has several replacementof components is much higher than for an disadvantages: equivalentland-based technology and is of course subject 1.It involves considerably more rotor structure per unit of tosuitable ‘ weatherwindows’ . Thereforeit isworth invest- sweptarea (which increases costs). ingmoney to achievelong operational lives for components. 2.It generally does not self-start but needs to be drivenup Afurtherfactor relating to underwater technologies is that toa speedat which the rotor blades unstall when crossing thereis likely to be a build-upof marine growth and the  ow. corrosionalso raises a rangeof technical problems. 3.It could be difŽ cult to stop in an emergency situation, Moderntechniques such as cathodic protection may give becauseit cannot readily be turned out of the ow(either someprotection but the design needs to be tolerant of these anaxial  owrotor can be yawed‘ edgeon’ to the  owor effects. theindividual rotor blades can be pitchedinto a feathered position). 4.It depends much more than an axial  owrotor on having 3.2Types of rotor agoodsurface Ž nishin order to maintain a highlift:drag Thephysical principles for exchangingkinetic energy ratiowhich is needed to achieve reasonable efŽ ciency— between owingwater and a rotorare similar to those for thismay be difŽ cult to maintain under the sea [ 6].

Fig. 1 Basicrotor concepts

ProcInstn Mech Engrs V ol216 Part A:JPowerand Energy A01801 # IMechE 2002 POWERFROM MARINE CURRENTS 5

Thereare other disadvantages of theDarrieus, such as greater wherethe  owboundaries are relatively far from therotor sensitivityto cavitation, plus, in the case where there are four periphery)is 59.3 per cent, which can be derived from orfewer rotorblades, the development of major cyclic lateral actuatordisk theory [ 7]—aconclusion commonly attributed forces.These problems individually might be considered to Betz. solvable,but when considered in combination they tend to Inpractice, a tidalturbine rotor, which generally will be makethe Darrieus considerably less attractive than it might largein relation to the  owcross-section (i.e. with its upper appearon Žrst consideration.However, this has not stopped a andlower edges close to the surface and the seabed) may numberof people from proposingtheir use on the grand beneŽt from aneffect called ‘ blockage’. Thisis where the scaleas tidal current turbines. owis constrained by the boundaries so that it cannot Bothtypes of rotorcan have Ž xedblades or variablepitch divergeto the extent that it wouldwhen decelerated through blades.A complicationwith variable pitch cross- ow or arotorin free space,and as a resulta greatermass owof Darrieus-typerotors is thatthe blades need to be cyclically uidpasses through the rotor and this manifests itself as a pitched,i.e. they need to go througha completepitch range higherefŽ ciency than might be expected had the boundaries cycleonce per rotor revolution, while variable pitch axial beenfurther away .Withwind tunnel tests of wind turbine owrotors only need adjustment on a timescale related to rotorsthe effect of owboundaries preventing the develop- thevariation in  ow(i.e. normally coinciding with the mentof thesame  owdivergence as in free spaceneeds to diurnaltidal cycle). becorrected for, or an exaggerated idea of the rotor Afurtheroption often suggested is to enclosethe rotor in efŽciency would be gained, but in the submarine turbine aVenturi-likeshroud in order to accelerate the  owand casethis may be an exploitable beneŽ t inwhich the  ow therebyto gain a higherrotational speed (which reduces boundarieseffectively act as an efŽ ciency augmentor by torqueand hence might simplify the mechanical drive train). increasingthe mass owthrough the rotor compared with Thishas been tried on a numberof occasions with wind whatwould happen if  owboundaries were far from the turbineswhere generally the structural complexity and cost rotor.As mentionedbefore, excessive use of blockagewill, ofsuch a largepassive component as a diffusersurrounding however,prove counterproductive. therotor has made the savings from asmallerand faster Althoughthe much greater density of water compared rotorgenerally seem insufŽ cient to compensate for the withair (by a factorof around 800) yields high energy addedcost of the shroud. In water a furtherproblem is densitiesat low velocities, it also yields correspondingly that(as willbe explainedin moredetail later) the rotor blade largeforces on the rotor structure compared with a wind tipvelocity needs to be limited to around 10– 15 m =s to turbine.Hence, a submarinecurrent energy converter has avoidthe development of cavitation. Increasing the  ow loadingsthat are quite different from thoseof anequivalent velocitytends to need the entire rotor to be speeded up in windturbine. Figure 2 indicatesthe salient characteristics of proportion,so a marinecurrent turbine in a Venturiwill tend speed,power, thrust, load and torque that could apply to an tobe more difŽ cult to design to avoid serious loss of axial owsubmarine turbine with rotors in the diameter performancedue to rotor blade tip cavitation. range10– 24 m. Thedominant steady state loads are bending forces on the rotorblades caused by the high lift forces that can be 3.3Extractable energy and loads on the rotor generatedin such a densemedium as water; however, Whatevertype of rotoris used, the theoretical upper limit of unlikea windturbine, weight is of secondary importance efŽciency for extractingenergy from afree stream(at least andcentrifugal loads, which partially balance bending loads

Fig. 2 ‘Typical’operating conditions foratidalturbine rotor (ratedvelocity 2m =s, averageefŽ ciency 30per cent)

A01801 # IMechE 2002 ProcInstn Mech Engrs V ol216 Part A:JPowerand Energy 6 P L FRAENKEL ona windturbine, are of course negligible at such low chord—to theaverage suction pressure). Conversely, a blunt rotationalspeeds. Note that the torque and thrust Ž gures andrelatively thick section will have a lowersuction relatenot to normal ‘ rated’operating conditions but to coefŽcient, implying a moreuniform negative pressure occasionaloverload conditions corresponding to 4 m =s distributionacross the chord length on the suction side. It velocitywhich could conceivably occur if extreme wind followsfrom thisthat the tidal turbine rotor designer needs fetch owcoincided in both timing and direction with an toemploy a hydrofoilsection having as low a suction extremespring peak  ow. coefŽcient as is compatible with maintaining reasonable Rotorspeed has to be limited by the need to avoid hydrodynamicefŽ ciency ,inorder to delay the onset of signiŽcant cavitation; this is not so much to limit potential cavitation.Fortuitously, this also results in a thickerand surfacedegradation (another worry withcavitation) but to thereforea strongerrotor blade, which is an important preventboundary layer separation which has the same effect additionalbeneŽ t interms of resisting the high  apwise asstalling, i.e. of dramatically reducing the lift– drag ratio rotorblade bending forces that are generated. andhence the efŽ ciency of the rotor. With an axial  ow rotor,cavitation develops at the rotor blade tips in particular atthe upper part of the swept path where static pressure is 3.4Dynamic effects least;in practice blade tip velocity needs to be limited to around12– 15 m =s.Figure 3 indicateshow cavitation varies Thedynamic and transient loadings on the system are withdepth; clearly the greater the static pressure the higher complexand a majorpart of the planned research and thevelocity that is possible before the overall pressure falls developmentprogramme is to address the need to get a tothe vapour pressure of water at which cavitation can be betterunderstanding of dynamic effects. The design initiated.This situation is more critical with a Darrieustype approachfor theŽ rst systemsis to try to use a rigid(i.e. ofrotor where the whole blade rather than just the tips non-compliant)structure wherever possible so that any movesat much the same relative velocity to the water . resonantfrequencies generated are lower than the natural Itshould be addedthat the pressure distribution around a frequencyof key structural members. The main drivers in liftingsurface such as the blades of a tidalturbine varies termsof excitation are the passing frequency of rotorblades signiŽcantly with the proŽ le; e.g. a sharp-nosedproŽ le will tothe pile, vortex shedding from thepile and effects from havea muchmore ‘ peaky’pressure distribution (a parameter passingwaves. Passing waves also have implications for calledthe suction coefŽ cient can be used to describe the causingtransient cavitation at moments when the wave ‘peakyness’of the suction pressure distribution; this is the vectorreinforces the current vector .Structuralproblems ratioof theleast pressure— usually occurring at around 0.25 causedby waves (e.g. slam on the structure above the

Fig. 3 Velocity needed for cavitation to be initiated asa function of waterdepth

ProcInstn Mech Engrs V ol216 Part A:JPowerand Energy A01801 # IMechE 2002 POWERFROM MARINE CURRENTS 7 surface) havebeen calculated to be relatively small oatingvessel could either be on the surface or be held comparedwith the loadings caused by rotor thrust. Hence bytension moorings below the surface. An installation reservesof strength to cater for worstwave conditions (say couldbe piled into the seabed or be held in place as a 50year return) are relatively small. Interactions between gravitystructure (no doubt with something more positive wavesand strong currents have been reported to be severein thansimple friction to prevent movement). certainlocations and therefore preferred locations will be Figure4 illustrateshow a gravitybase is only feasible in chosen,at least for earlyprojects, that are not too exposed to shallowwater while a oatingdevice is essential in deep oceanicwaves. The site for theplanned experimental water.The use of a monopile(or ajacketedmultiple piled prototypeoff northDevon is relatively sheltered in that supportin deeper water) offers anintermediate solution with respect. costadvantages over the other options. Thevariations in static pressure and velocity across the Inpractice it is almost impossible to carry out extensive verticalwater column also impose interesting dynamic underwateroperations using either divers or remotelyoper- effectson therotor blades to an extent that is not paralleled atedvehicles (RO Vs) inthe kinds of locations of interest, eitherby wind turbines or by conventional low head hydro owingto the high  owvelocities and limited periods of turbines;the tip of theblade of atypical15 mdiameterrotor slacktide. Therefore Marine Current T urbinesLimited has willexperience a 1.5barcyclic pressure variation as it developeda turbineconcept based on using a monopile movesfrom thetop to the bottom of its circular path, in (similarto the second from leftin Fig. 4) installed from a additionto potentially large velocity  uctuations. jack-upbarge. A monopileis a tubularsteel tower inserted intoa holedrilled in the seabed. A jack-upbarge can drill thenecessary hole, usually in hardseabed surfaces, by using arotaryrock drill and install the pile using its on-board 3.5Methods forinstallation crane.Marine Current T urbines’commercial partners, Installingany kind of system in the sea is at best difŽ cult SeacoreLimited, are specialists in placing monopiles (ithas to withstand storms, corrosion and a hostof (havinginstalled a numberof wind turbines on monopiles otherhazards for alongtime). Economic analysis suggests atsea) andthey have the capability to drill holes and to place thatsystems of this kind need an overall design life aver- pilesup to4 mindiameter even in hard rock. Most locations aging20– 30 years to achieve a goodreturn. The overriding withhigh velocity currents tend to have hard or rocky technicalproblem with tidal current turbines is the high seabedsbecause scour from thecurrents tends to remove axialthrust that needs to be reacted against, which can anysoft or loose material. demanddesigning for anextreme horizontal force on Althougha oatingsystem appears to be more versatile, therotor of megawatt sized systems in the order of sinceinstallation or removal of a oatingmoored system 1000–3000 kN depending on the local maximum stream shouldbe relatively quick and hence inexpensive, mooring velocityand rotor size. andanchoring such a turbinereliably in the open sea is As shownin Fig. 4, there is achoiceof mountinga rotor technicallydifŽ cult. There is a largeupward force compo- underneatha oatingvessel which is then moored or of nenton any anchorage in addition to the large horizontal installingit on a structureattached to the seabed. The component,unless a catenarymooring with clump weights

Fig. 4 Examples of possible systemconŽ gurations

A01801 # IMechE 2002 ProcInstn Mech Engrs V ol216 Part A:JPowerand Energy 8 P L FRAENKEL isused, which demands a lotof horizontalspace. Moreover, saryinvestment in research and development. This is thethrust force reverses direction with each tide and this becausemarine current turbines have a numberof advan- needsto be accommodated by the mooring system. The tagesthat should ensure that this technology Ž ndsa major technologyused by the offshore oil and gas industry for rolein delivering clean energy in the future. The main mooringsemi-submersibles may offer onepossible solution. reasonsthat marine current turbines can be expected to be However,so far asŽ rst generationsystems are concerned, costcompetitive are the following: MarineCurrent T urbinesLimited plans to install rows of 1.A suitablyenergetic marine current location can offer turbinesmounted on individual monopiles much like the fourtimes the energy intensity of a goodwind site and arraysof wind turbines that make up awindfarm. However, some30 times the energy intensity of sunshine in the becausetidal streams are generally bi-directional, the Sahara(see Fig.5) interms of energy captured per unit turbinescan be installed close together transversely across areaof interface; hence marine current turbines need thestream; wind turbines (which can receive  owfrom onlybe aquarterthe swept area of a windturbine of the virtuallyany direction) need to be sufŽciently spaced out to samepower (or 1 =30the size of an equivalent solar preventthe wake of any turbine seriously interfering with photovoltaicarray) andsmaller size generally produces the owthrough a downstreamunit. Therefore it is possible lowercosts. togroup marine turbines much more closely together than 2.A marinecurrent turbine can deliver energy to a time- windturbines, with advantages in terms of installationcosts tablesince the tides are accurately predictable: therefore andcable costs. A packingdensity for tidalstream turbines theend product of predictable power is inherently more offrom 50to 100 MW =km2 seemsfeasible, which is up to valuableto a utilitythan randomly generated electrical 10times the array density of typicalwind farms. energy;wave and to some extent wind energy is predict- ableover short time horizons, but tidal currents are predictableyears in advance. 3.6The drive train and extracting the power 3.Not only are the turbines relatively small (compared with otherrenewable energy devices) but because of the Therotor of a tidalturbine runs at low rotational speeds bidirectional owthey can be packed much more closely (10–20 r=min)and generates correspondingly high levels of together,aligned across the current at less than 50 m torque(200– 1000 kN m). Thereforethe drive train poses intervals;this leads to a compactinstallation which in someinteresting technical challenges. The most likely turnyields further savings in cabling and installation solutionin the short term will be to use a similarsystem costs. toa windturbine, namely a gearboxdriving a generator.For 4.Since weight is not a criticalfactor for asubmerged mostmachines this will need at least a two-stagespeed turbine,the construction can be a steelfabrication so increaser,requiring an epicyclic gearbox to keep the size themanufacturing costs per tonne promise to be andweight within reasonable limits. There are also possi- relativelylow . bilitiesfor usinghydraulic transmission or for developing 5.Physical conditions under water tend to be relatively speciallow speed directly driven alternators (a seemingly predictableand calm, so extreme conditions require promisingtrend at present with wind turbines). The power muchless ‘ over-engineering’than would apply for trainmay be encased in an air-Ž lled nacelle using sealing wind-or wave-powered technologies. This also helps to arrangementsfor thedrive shaft much like those used for keepcosts down; there is no underwater analogy to an ship(or submarine)propeller shafts, or alternatively it is atmospherichurricane. possibleto use dedicated sealed gearbox =generatorunits thatcan run immersed in water and therefore need no externalcasing. Submersible mechanical and electrical machineryis becoming relatively commonplace and largesubmersible pumps of similar power levels to the generatorsneeded for atidalturbine are standard commer- cialproducts. The output is delivered via a marinecable laid acrossthe seabed to theshore, probably at a voltageof 11or 33 kV.

3.7The technology and its competitiveadvantages Thereare undoubted difŽ culties in developingtechnology to operatereliably for longperiods under the sea, but this has Fig. 5 Relativeenergy capture per unit sizefrom four types of beensuccessfully achieved in other contexts and there are renewableresource (per mfor wavesand per m 2 for the goodreasons to expect the rewards from developingthis rest)taking account of typical load factorsand likely technologyto be more than sufŽ cient to justify the neces- technical efŽciency of the systems

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Apartfrom thesepotential economic advantages, an 20m diametershowed a predictedelectricity cost of environmentalimpact study [ 3]indicatedthat the technol- approximately6 p =kWh.Larger clusters would reduce ogyshould pose no serious threat to marine life or to the costsfurther . localenvironment. The low tip velocity of tidal turbine Commercialmarine current turbine technology already rotorsmakes them signiŽ cantly less of a threat.Also, the exists,albeit on averysmall scale. At leasttwo products are lowvisual proŽ le (in some cases zero visual proŽ le) should madein the UK, bothconsisting of a smallpropeller-like makeit acceptable to plannersto grant permission for large rotor,about 50 cmin diameter, that can be hung over the installationsrelatively close onshore. sideof ayachtor leisure boat so that the passing  owcan be harnessedto keep its batteries charged either when moored ina tidal owor when under sail without the beneŽ t ofan engine. Thereforethe basic concept of utilizing marine current 4REVIEW OFRECENT DEVELOPMENTWORK energyhas been convincingly demonstrated and is well SOFARCOMPLETED understood.What remains to be proven is that it can be appliedon a largescale at a costand with a levelof TheŽ rst attemptto evaluate a nationaltidal current reliabilitythat can allow it to compete successfully with resource,in 1992– 3, was the‘ UKtidalstream energy conventionalmethods of energy generation. review’ [1]. Thisdesk study conŽ rmed that there is a Aprojectwas launchedin September 1998 with the largetidal current energy resource, capable theoretically of Žnancialsupport of theJOULE 3programmeof DGXII of meetingsome 19 percent of totalUK electricitydemand at theEuropean Commission [ 9].Itinvolves the development thattime, of which 20 TWhcouldbe delivered at around ofwhat is expected to be the world’ s Žrst ‘commercial 10 p=kWh, i.e. not economically under the cautious and scale’marine current turbine, a systemrated at 300 kWto (withthe beneŽ t ofhindsight) incorrect costing assumptions beinstalled on a monopile,socketed into the seabed applied. insouth-western UK coastalwaters. The target date In1993– 4, a consortiumconsisting of IT Power,Scottish for commissioningthe system is summer of 2002. Nuclearand NEL developeda ‘proofof concept’ experi- MarineCurrent T urbinesLimited was formedto take mentaltidal current system. This involved an axial  ow responsibilityfor thecommercial development of the 3.5m diameterrotor suspended below a oatingcatamaran technology[ 10]. pontoon.It successfully developed some 15 kWin2.25 m =s Itis expected that this project will lead directly to a currentvelocity at Loch Linnhe, Scotland, in 1994 and, secondphase involving the development of a full-scale althoughquite small, it remains the largest marine current demonstrationof a clusterof perhaps four or Ž velarger turbineso far demonstrated[ 8]. turbineseach of about 1 MW.Thisin turn will lead to Underthe JOULE 2energyresearch programme, DGXII commercialexploitation of the technology within a lead ofthe EU supporteda technicaland resource assessment timeof 5–10 years. ofmarine current energy in Europe [ 2]. Thetechnical Arecentdevelopment is that the DTI, throughETSU, study(completed in 1996) examined relevant technology commissionedand published a studyby consultants in from relatedareas (wind, hydropower, ships and the April2001, which among other things revisited the results offshoreoil and gas industry) and found that electricity ofthe 1993 tidal stream review ,butwhich primarily aimed costis specially sensitive to the size of machine,economic toprovide an independent evaluation of the cost and parameters(lifetime, discount rate), O &M(Operationand performancebeing claimed by advocates of marine current Maintenance)costs and the load factor obtainable at a energysuch as Marine Current T urbinesLimited; this particularsite. It estimated electricity unit costs for ‘Žrst- study [11]broadlyendorsed the results of earlier studies generationsystems’ deployed on a smallscale at around suchas those of references [ 2] and [3]andestimated that 0.05 ECU=kW h (3.5 p=kW h) for a 3 m=sratedcurrent electricitycosts from siteswith mean spring peak velo- underfavourable circumstances (i.e. with high load citiesof more than 2 m =swillbe in the range 4– 6 p =kW h factors).This is encouragingly close to what is needed andthat there is a reasonablechance for coststo fall a for commercialviability. further30 per cent with economies of scale from larger TheRegional and Urban Energy Programme, DGXVII schemes.It concluded that ‘ theseŽ guresare of the right ofthe EU ,Žnanceda feasibilitystudy on supplying orderof magnitudeto encouragecommercial interest in the Orkneyand Shetland with electricity from tidalstream technology’. turbines [3].Islandcommunities, which frequently have Followingthe conclusions of this report, the DTI under- higherthan normal conventional energy costs, are likely to tookto include tidal stream technology within the scope of bethe most attractive initial market for electricityfrom itssupport for renewableenergy research and development tidalstreams. Actual on-site current measurements were andit has also provided some important and substantial usedin conjunction with a three-dimensionalcomputer Žnancialassistance for theinitial part of the research and modelto produce the tidal stream characteristics for two developmentprogramme that Marine Current T urbines sites.Consideration of a clusterof eight turbines of Limitedis undertaking with its industrial partners.

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5MARINE CURRENT TURBINES’PLANNED theirjack-up barges), IT Power Limited(the original devel- DEVELOPMENTPROGRAMME opersof the technology carrying out some of the design work),Bendalls Engineering (precision steel fabricator, who 5.1The Marine Current Turbinesaxial  owturbine on willbe mainly responsible for thesteelwork), Corus UK amonopileconcept Limited(formerly British Steel— until merged recently with 5.1.1Phase 1 ofresearch and development programme: Hoogovensof theNetherlands— who are supplying special- single-rotor300 kWexperimental test rig izedexpertise and materials), ISET (whichis a designand researchdepartment of the University of Kassel in MarineCurrent T urbinesLimited in partnership with a Germany—doing much of the electrical design as well as consortiumof other companies is developing initially a workon rotor design in partnership with MCT andIT form ofturbinesimilar to that illustrated in Fig.6. It consists Power) andJahnel-Kestermann (a Germangearbox manu- ofa rotormounted on a monopilein such a waythat the facturerspecialising in wind turbine and marine gearboxes). turbinedrive train and rotor can be raisedabove the surface AlsoW .S.Atkinshas a keyrole with technical inputs for maintenance.The Ž rst prototypewill be similar to the relatingto the rotor and structural design. artist’s impressionsof Fig.6 andwill be installed,subject to Jahnel-Kestermannare designing a two-stageepicyclic gainingthe necessary permissions, off ForelandPoint, north gearboxunit speciŽ cally for thesystem which contains the Devon,during 2002. It will be ratedat 300 kW(e) andhave mainbearings with an overhung shaft to carry the rotor .The asingle12 mdiameterrotor . casingis a structuralcomponent and is also watertight, Theproject involves Marine Current T urbinesLimited as makingit effectively a submersiblegearbox which is thecoordinator and developer, working with Seacore seawatercompatible (Fig. 7). Limited(which specializes in the installation of piles at Thegenerator is in a watertightcasing bolted to the back sea—having carried out this function for twooffshore wind ofthe gearbox with a diskbrake and coupling internally farms already—who will do the installation using one of mountedbetween the gearbox and generator .Coolingis

Fig. 6 How aturbine systemmay be installed on amonopile sothat it canbe raisedabove the surfacefor servicing and replacement

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Fig. 7 Prototype 300 kW tidal turbine drive train shown installed in the chassiswhich isin turn attached to a collar concentric with the pile, which canbe raisedabove the surfaceof the sea

effectivelyprovided through the submerged and wetted 5.1.2Phase 2 ofresearch and development programme: casingsof the drive train components. The system is twin-rotor600 kW pre-commercial prototype designedto run at variable speed using electronic power Theprimary goal of the technical development programme conditioningto produce a mainsquality 50 Hz output. isto improve the cost effectiveness of the technology in Theoriginal concept envisaged a Žxedpitch constant orderto bring down the cost of the electricity that can be speedsystem as iscommonly used for windturbines, but it producedso as to make the technology commercially was feltto be too risky to rely on stall regulation of sucha competitivewith other methods of generation as soon as novelsystem since failure to stall would potentially cause possible.A majorpart of thecost consists of the overheads unacceptablyhigh loads on the drive train and the only involvedin installing a pileat sea and providing a cable methodsfor controlwould be either to trip the electrical connectionto the shore. Therefore by installing twin rotors systemand let the rotor run away or to rely on the ona pileas shown in Fig. 8, it is possible to double the mechanicalbrake. Since it is believed for reasonsto be energyproduced compared with a singlerotor of the same elaboratedbelow that rotor pitch control is a necessary size(the diameter is generallylimited by thedepth of water) requirementfor thecommercial technology, it was decided yetthis can be done for signiŽcantly less than twice the cost. tointroduce this on the prototype too. The facility for Dependingon various factors, a twin-rotorsystem will varyingthe rotor blade pitch not only permits accurate typicallybe 30 per cent more cost effective (installed cost) speedcontrol but also is expected to signiŽ cantly improve thana single-rotorsystem of the same rated power, and theefŽ ciency ,especiallyat part load. moreoverit can operate in shallower water owing to the Therotor blades are of fabricated steel construction, smallervertical height of the rotors. consistingof a taperedmain spar at approximately one- Thereare several other advantages of a twin-rotor thirdchord, with ribs skinned with steel plate, much the systemof the kind shown in Fig. 8 comparedwith a sameas most ship’ s ruddersor ship stabilizer hydrofoils. single-rotorone: TheproŽ le for theprototype is amodiŽed NACA 634xx,xx being35 (or 35percent thickness :chordratio) near the root 1.The torque reaction of the two rotors and the swirl andtapering to 15 per cent near the tip. componentin their wakes will tend to cancel each Thedrive train and rotor are mounted in a chassis other out. attachedto a liftablecollar that is a looseconcentric Ž tto 2.The wake from astreamlinedhorizontal wing used to thepile. The lifting mechanism for theprototype is a supporteach power train and rotor has less disrupting reduced-scaleversion of the hydraulic system commonly effecton the  owthrough the rotor than the wake from usedto lift jack-up barge legs. themuch larger thickness of piledoes for asinglerotor. Itis planned to run the single-rotor experimental proto- 3.When raised, the ‘ wings’form aworkingplatform for typefor onlyabout one year before replacing it with a maintenancepurposes, making access to the power trains systemthat will be intended to be the precursor for the androtors much easier, as shown on the right-hand commercialsystems to follow later . pictureof Fig. 8.

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Fig. 8 Artist’s impressions of how atwin-rotor turbine systemmay be installed on amonopile so that it canbe raisedabove the surfaceof servicing and replacement.Note that asmallwork boat or tug is adequate for gaining access,‘ wings’act as a working platform and rotor blades areset horizontally and to neutral pitch when raised

However,despite its undoubted advantages, the twin-rotor installcommercial projects. These will generally involve systemintroduces a numberof potential structural and batchesof systems with an aggregate installed capacity in dynamicproblems which are being studied, but it is believed therange 20– 30 MW,asthis scale of installation is needed thesecan all be solved.One of themain areas for concernis the toobtain favourable economics. It also so happens that excitationmodes that the structure might experience from about30 systems can generally be installed during the variousexcitation drivers such as the rotor blade passing weatherwindow from aroundMarch to the end of frequencyand also from vortexshedding by the pile. Septemberand that 30 MWisa practicalpower level for a clusterof systemshaving a commonelectrical ring main and 5.1.3Phase 3 ofresearch and development programme: transformer.Hence it isexpectedthat large projects will be arrayof twin-rotor1 MWpre-commercialprototypes builtup from 20–30 MWgroups,each taking a jack-up bargea summerseason to install. Each such group will have Thetwin-rotor ‘ phase2 prototype’will form thebasis for anindependent ring main, probably of 11kV,andprobably a commercialtechnology to follow .Thiswill be tested and transformerto feed into the 33 kV‘ umbilical’taking the reŽned further in phase 3 asa smallcluster or ‘ farm’of four powerashore. orŽ vetwin-rotor turbines (see Fig.9), primarily to gain Itis also possible that a high-voltaged.c. system will be operationalexperience of aworkingarray of turbinesas well usedfor interconnectionof the variable frequency a.c. asto checkon interactionsand control problems that can be generatorsand to link to a powerconditioning system expectedwhen running multiple units in parallel with each ashore,although such developments will be more likely other. lateron with larger projects. Thephase 3 systemswill be uprated to from 0.75to 1 MW (2 6 375 to 2 6 500kWrotors and drive trains) dependingon thelocal current rated velocity ,bya combina- tionof improvements in efŽ ciency and slight increase in 5.2Economics— driving down costs rotordiameter .Phase3 willalso be partially self-Ž nancing Thedevelopment of a commerciallysuccessful technology from revenueresulting from saleof electricity. whichcan make a usefulcontribution to future world 5.1.4Commercial projects energyneeds is dependent on getting the system costs downto the level where the cost of generationcan compete After approximately12 months of satisfactory operation of favourably,Ž rstwith other renewables and ultimately with thephase 3 system,it is planned that work will start to anyother form ofpowergeneration. On the basis of existing

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Fig. 9 Phase 3of the researchand development programme envisages the installation of asmallarray of Žve units, eachof from 2 6 375 to 2 6 500 kW.Note that the second systemin the arrayis shown here raisedfor maintenance, with the nearestrotor blade pitched and the farrotor blade furled (neutral pitch)

technicaldesign options, the plan is to seek to drive down sourceof clean electricity, but there are many synergies coststhrough the development programme as indicated in betweenthese technologies, not least being that they both Fig. 10. lendthemselves to installation on monopiles and both Points1 to3 onthe cost-reduction curve are the systems demandsigniŽ cant quantities of submarine cable for inter- tobe developed under the previously described phased connections.Therefore marine current turbines are not in researchand development programme, point 4 isthe Ž rst realityan alternative to offshore wind, but rather it can be commercialproject and the improvements thereafter can be expectedthat success for offshorewind will be helpful for gainedfrom acombinationof improving the technology tidalcurrent developments too, since both require similar generally,economiesof scaleand reductions in the marginal investmentin infrastructure and installation plant such as costof projects generated by expanding existing projects (so jack-upbarges. asto avoid duplicating a largepart of the grid connection costs). Althoughthis may seem ambitious, similar cost reduc- tionshave been achieved for othertechnologies such as 6CONCLUSION windturbines. Installed costs for on-shorewind turbines havecome down in real terms from over£ 2000 =kW in the Thereis no doubt that it is technically feasible to extract early1980s to around £ 700 =kWtoday ,whileoffshore wind energyfrom marinecurrents and that the tidal current turbinesare expected to cost around £ 1000 =kW today. resourceis large enough to have the potential to make a Offshore windturbines are in some ways the main majorcontribution towards meeting future energy needs. competitorwith tidal current turbines as a large-scale However,the main area of uncertainty relates to the

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Fig. 10 How Marine Current Turbines Limited intends to develop its technology; what will be needed to make tidal current turbines commerciallycompetitive

economics;there is noguaranteethat cost effective systems 3 Feasibilitystudy of tidal current power generation for coastal canbe developed although there are good indications that waters:Orkney and Shetland. Final Report, EU contract thepotential to achieve this goal exists, given suitable XVII=4 1040=92-41,ICIT ,March1995. researchand development. More to the point is to predict 4 Cave, P. R. and Evans, E. M. Tidalstream energy systems: howlong it might take to drive the costs of thismethod of economics,technology and costs. In Third International powergeneration down to the level where a thrivingnew Symposiumon Wave,Tidal, OTEC andSmall Scale Hydro Energy industrycan develop in order to take it forward ona self- ,Brighton,1986. 5 OffshoreInstallations: Guidance on Design,Construction and sustainingcommercial basis. The analysis suggests that this CertiŽcation ,1990(UK Departmentof Energy ,HMSO, shouldbe possiblewithin 5– 10 years, a goalwhich Marine London). CurrentT urbinesLimited and its partner companies are 6 Kihoh, S. and Shiono, M. Powergeneration by tide and tidal committedto. current.In Proceedings of AdvancedMaterials ’ 93 , Vol. V=A, Eco-materials ,1994,pp. 41– 44. 7Duncan, W.J.,Thom, A.S. and Young, A. D. TheMechanics of Fluids,1960(Edward Arnold, London). REFERENCES 8 Paish, O. and Fraenkel,P . Tidalstream energy: zero-head hydropower.In International Conference on Hydropowerinto 1 Tidalstream energy review ,UKDTI. ReportETSU theNext Century ,Barcelona,5– 8 June1995. T=05=00155=REP,preparedby Engineering and Power Devel- 9Fraenkel,P .L.,Clutterbuck, P .,Stjernstrom,B. and Bard, J. opmentConsultants Limited, Binnie and Partners, Sir Robt. ‘Seaow’ : preparingfor the world’ s Žrstpilot project for the McAlpineand Sons Limited and IT PowerLimited for the exploitationof marine currents at a commercialscale. In ETSU,UKDepartmentof Energy.CrownCopyright 1993. Proceedingsof Third European Wave Energy Conference, 2 Theexploitation of tidal marine currents (non-nuclear energy Patras,September– October 1998. JOULE IIprojectresults). Report EUR 16683EN,DG Science, 10 http://www.marineturbines.com. Researchand Development, The European Commission OfŽ ce 11 Commercialprospects for tidal stream power. Binnie, Black and forOfŽ cial Publications, Luxembourg, 1996. Veatch,Redhill, DTI =ETSU, Harwell,April 2001.

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