Power from Marine Currents

SPECIAL ISSUEP 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 the rst time,i.e. to achievethe world’ s rst 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 mainjusti 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 dif 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 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 themain 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 signi 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 signicant 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- It canbe seenthat the marine current resource at locations torialwaters withcertain prede 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 signi 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 sufciently 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 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.

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