Power from marine currents

P L FraenkelMarine Current Turbines Limited, 2 Amherst Avenue, Ealing, London W13 8NQ, UK

Abstract: This paper describes the rationale and the engineering approach adopted for the development oftechnology for converting the kinetic energy in marine currents for large-scale electricity generation.Although the basic principles involved are relatively straightforward and well understood, being similar tothose of a wind turbine, a practical and cost-effective large-scale system designed to extract the kineticenergy of � owing water has yet to be developed. This paper describes the research and development beingundertaken through an industrial consortium with the aim of achieving this goal for the � rst time, i.e. toachieve the world’s � rst commercially viable systems for delivering power from marine currents.

Keywords: tidal, stream, marine, current, energy, turbine, axial � ow, kinetic energy conversion

NOTATION

A cross-sectional area swept by rotorH overall depth of waterK0 constant proportional to mean spring peak velocity

at siteK1 constant proportional to ratio of mean spring and

neap peak currentsKn ‘spring neap factor’ (e.g. for maximum neap stream

of 60 per cent of spring, Knˆ 0.57)Ks velocity shape factor (0.424 for a sinusoidal � ow)P powerT0 diurnal tidal period (usually 12.4 h)T1 spring neap period (usually 353 h)V free stream velocity local to hub height of turbine

rotorV(Z) velocity at height above seabed ZV(mean) depth-averaged velocity for whole water columnV(peak) maximum spring tide velocityZ height above seabed in the water column

r density (of sea water)

1 RATIONALE FOR USING THIS RESOURCE

There is an emerging market for ‘green’ electricity derivedfrom renewable resources which in the UK and many othercountries includes government-sponsored � scal incentivesfor utilities to acquire an increasing proportion of renewableenergy-based generating plant. In the UK, the RenewableEnergy Obligation will oblige utilities to source at least 10per cent of their electricity from renewable resources by

2010. However, land-based renewable energy technologiesare already facing constraints owing to con� icts over landuse, so an important factor is that the seas offer large openspaces where future new energy technologies could bedeployed on a grand scale, perhaps with considerably lessimpact on either the environment or other human activities.

In fact the oceans represent an energy resource which istheoretically far larger than the entire human race couldpossibly use, although in practice most of this huge resourceis inaccessible. The main potential sources of marine energyare waves, currents and ocean thermal energy; offshore windis of course also a marine resource, although the energy isnot derived from the sea itself. Arguably, unless marinerenewable energy resources are developed and used, it willnot be possible to meet future energy needs without riskingserious damage to the environment either through continu-ing to burn increasing quantities of fossil fuels or throughplacing increasing reliance on nuclear power. This is themain justi� cation for investing in these new and so far little-developed energy solutions. However, marine renewableenergy resources are generally more costly and dif� cult toexploit reliably than the land-based options (which is whyno great effort to develop them has been made untilrecently).

It has been obvious to seafarers for centuries that power-ful sea currents exist in certain locations where the � owstend to be concentrated. Such locations tend to be wherecurrents, which in the open sea move at low speeds of just afew cm=s, are channelled through constraining topography,such as straits between islands, shallows between open seasand around the ends of headlands. These relatively rapidtidal currents typically have peak velocities at spring tide inthe region of 2–3 m=s (4–6 knots) or more. The gross kineticenergy in such � ows is extremely large and it appearsregularly and predictably in perfect tune with the relative

The MS was received on 10 April 2001 and was accepted after revision forpublication on 13 July 2001.

SPECIAL ISSUE PAPER 1

A01801 # IMechE 2002 Proc Instn Mech Engrs Vol 216 Part A: J Power and Energy

motion of the Earth, Moon and Sun. Therefore this is arenewable resource that is relatively potent, yet it is one ofthe few renewable energy resources that can deliver powerpredictably to a time table, a factor which adds to the valueof the output, since from the planning point of view a utilityneeds to be able to deliver power rather than energy, even ifit is energy which generates the revenue. Since the tides areout of phase at different points around the coast, power canoften be available at one installation at times when there isslack tide and no power at another. However, at neap tide theenergy availability is signi� cantly reduced compared withthe springs, and this will be true everywhere. Advancedknowledge of the availability of tidal power will permit autility to seek to capitalize on occasions when good tidal� ows coincide with expected periods of high electricitydemand.

Recent studies [1–3] indicate that marine currents havethe potential to supply a signi� cant fraction of futureelectricity needs and, if successfully developed, the technol-ogy required could form the basis of a major new industry toproduce clean power for the 21st century. The most detailedstudy so far, ‘Marine Currents Energy Extraction: ResourceAssessment’ [2], analysed 106 locations in European terri-torial waters with certain prede� ned characteristics to makethem suitable for energy exploitation, and the aggregatecapacity of this selection of sites amounted to an installedrated capacity of marine current turbines of over 12 000 MW,capable of yielding some 48 TW h of electrical energy perannum. Because of the lack of public domain data on tidal� ows, the total exploitable resource remains uncertain, but itis likely to be signi� cantly larger than this.

In fact, some locations, such as the Pentland Firth(between Scotland and Orkney) and the Alderney Race(between the Channel Islands and France), or the so-calledBig Russell (off Guernsey), have especially intense currentsover a suf� ciently large sea area to offer potential forexploitation on a multigigawatt scale. Examples of otherintense locations include the Severn Estuary (UK’s northDevon coast), the straits between Rathlin Island and North-ern Ireland, the Straits of Messina between Italy and Sicilyand various channels between the Greek islands in theAegean. Other large marine current resources can befound in regions such as South East Asia (e.g. thePhilippines, Indonesia, Japan), Australia and New Zealand,Canada and the south east coast of South Africa.

Therefore, in short, there will be a greatly increasingdemand for clean electricity and the marine current resourcecould make a major contribution to this need with the addedvalue of being able to deliver power predictably.

2 THE TIDAL=MARINE CURRENT RESOURCE

Marine currents are primarily driven by the tides (i.e. tidalcurrents or tidal streams) which occur because of thevariation in level of the surface of the sea caused byinteraction of the gravitational � elds of the Moon and to a

lesser extent the Sun with the Earth. There are alsogeotrophic (or oceanic) currents caused primarily byCoriolis forces acting on the water in the major oceans asa result of the rotation of the Earth. Currents are alsogenerated by density differences in the seas resulting fromsalinity and temperature variations in different sea areas.However, in European waters the main marine currentresource is tidally driven.

2.1 The energy in � owing water

The power available from a stream of water is

P ˆ 12 rAV 3

where r is the density of water, A is the cross-sectional areaof the rotor used to intercept the � ow and V is the freestream velocity of the current. The consequence of this cubelaw relationship is that power and hence energy capture arehighly sensitive to velocity. This is clearly indicated inTable 1 showing power densities for various water velocities(in sea water), compared with the wind and solar resources.

It can be seen that the marine current resource at locationswith currents exceeding 2 m=s is a relatively intense renew-able energy source compared with the better-known alter-natives such as solar and wind. It should be noted that13 m=s is the typical rated velocity, i.e. velocity at whichrated (or maximum) power is achieved for a wind turbine.

2.2 The tidal cycle

The tidal cycle can be approximated by a double sinusoid;one with a period of 12.4 h representing the diurnal tidal ebband � ow cycle, and the other with a period of 353 hrepresenting the fortnightly spring neap period. The follow-ing equation provides a reasonable model for predicting thevelocity V of a tidal current:

V ˆ K0 ‡ K1 cos2ptT1

cos2ptT0

where K0 and K1 are constants determined from the meanspring peak and the ratio between the mean spring peak andthe mean neap peak currents, T1 is the spring neap period

Table 1 Relative power density of marine currents comparedwith wind and solar resources

Energy resource

Marine currents Wind Solar

Velocity(m=s)

1 1.5 2 2.5 3 13 Peak atnoon

Velocity(knots)

1.9 2.9 3.9 4.9 5.8 25.3

Power density(kW=m2)

0.52 1.74 4.12 8.05 13.91 1.37 ¹ 1.0

Proc Instn Mech Engrs Vol 216 Part A: J Power and Energy A01801 # IMechE 2002

2 P L FRAENKEL

(353 h) and T0 is the diurnal tidal period (12.4 h). Generally,in UK waters, the maximum mean spring current velocitywill be approximately twice the maximum mean neap tidevelocity.

An alternative idealization of this is to assume a meanpower level in a cross-section of � ow as follows [4]:

P…mean† ˆ 12 rAKsKnV

3…peak†

where r is the density, A the cross-sectional area of � ow, Ks

a velocity shape factor (0.424 for a sinusoidal � ow), Kn aspring neap factor (e.g. for maximum neap stream of 60 percent of spring, Knˆ 0.57) and V(peak) is the maximum springtide velocity.

The velocity of a tidal current is also generally modi� edto some extent by a combination of other factors such asresidual momentum, global oceanic marine circulation,wind fetch and density variations and superimposed on allthis, at least near the surface, are the rotating velocityvectors caused by passing waves. However, at high-velocitysites of the kind suitable for power generation, when astrong tide is � owing, the tidal current vector generallyexceeds all these other effects by a signi� cant margin.

The current will also vary in velocity as a function of thedepth of � ow. The velocity at a height above the seabed Zapproximately follows a seventh power law as a function ofdepth in the lower half of the � ow [5]

V…Z† ˆ Z

0:32h

1=7

V…mean†; for 0 < Z < 0:5H

and in the upper half

V…Z† ˆ 1:07V…mean†; for 0:5H < Z < H

It can be shown from this that approximately 75 per cent ofthe energy is to be found in the upper 50 per cent of the � ow.

In practice, areas with high � ow velocities will generallybe relatively turbulent and � ow modelling is not as simple asthis and many variations can occur depending both onlocation (and hence seabed bathymetry) as well as on thestate of the tide (� ow patterns and velocity shear changesthrough the tidal cycle) .

2.3 Siting requirements for energy exploitation

The main aim is to � nd locations with fast � owing water, arelatively uniform seabed (to minimize both turbulence andthe loss of velocity near the seabed), suf� cient depth ofwater to allow a large enough turbine to be installed andpreferably such conditions extending over a wide enougharea to permit the installation of a large enough array (or‘farm’) of turbines to make the overall project cost effective.In addition the location needs to be suf� ciently near to ashore-based grid connection to allow the energy produced tobe delivered at reasonably low cost. Finally, it also has to be

where it will not cause serious obstruction to other users ofthe sea.

The depth of water ideally needs to be in excess of about15 m at low tide and probably no more than 40 or 50 m athigh tide. The minimum level will accommodate a rotor ofabout 10 m diameter, probably about the smallest that mightbe considered robust and powerful enough to be cost-effectively deployed in this way at sea, while the upperlimit depends on the type of technology and installationmethod to be used. In practice most fast � owing locations inUK waters are in any case less than 50 m deep since it isoften a reduction in depth that causes the � ow to accelerate.

The velocity characteristic of a tidal stream can best bede� ned by the mean spring peak. Economic analysis of thetechnical concepts being developed by Marine CurrentTurbines Limited suggests that a minimum of approximately2 m=s mean spring peak is needed and ideally 2.5 m=s ormore (i.e. 4–5 knots) in locations with typical bidirectionalsinusoidal tidal � ows. However, a lower peak velocity in theorder of 1.2–1.5 m=s becomes economically viable in loca-tions with continuous or quasi-continuous � ow (e.g. rivers,oceanic or geotrophic currents). This is important since themain justi� cation for applying this technology relates to thecost effectiveness with which it can deliver useful energyand one of the most sensitive parameters in the economicanalysis is the � ow velocity (since, as indicated earlier,energy is related to velocity cubed).

3 CONCEPTS FOR THE EXPLOITATION OFMARINE CURRENTS

3.1 Basic requirements

The end product is delivered energy, which clearly needs tobe obtained at the least possible cost. The energy capturedwill be a function of the swept area of the turbine rotors in a� eld, their average ef� ciency at converting kinetic energyand the load factor (or capacity factor) for the site. Loadfactor may be de� ned as the ratio of the average power ofthe system to the rated power.

It follows from this that the rotor area needs to be as largeas possible to � ll the space, although there are constraintswhich limit this since if the rotor blades pass too close to the� ow boundaries the effects on performance are likely to becounterproductive. Ef� ciency also needs to be as high aspossible with costs as low as possible, two requirements thatare sometimes not easily reconciled and therefore need goodengineering compromises. In the last resort it is cost effec-tiveness that matters, but good cost effectiveness generallyrequires good ef� ciency.

The load factor also needs to be as high as possible; thelower the rated velocity (which is the velocity at which ratedpower is developed) relative to the site mean peak velocity,the higher the resulting load factor. Therefore there is atrade-off between rated velocity (hence rated power) andload factor in order to maximize energy capture. Exactly as

A01801 # IMechE 2002 Proc Instn Mech Engrs Vol 216 Part A: J Power and Energy

POWER FROM MARINE CURRENTS 3

with wind turbines, there is always an optimum ratedvelocity for a given site which will maximize the resultingenergy capture (and hence the return from the investment).

Clearly any outages for technical reasons will severelyreduce the practical load factor, so reliability will be a keyrequirement for any successful offshore renewable energytechnologies, especially bearing in mind that adverseweather may delay remedial measures much longer thanwould apply for an onshore technology. Therefore, apartfrom considerations of energy capture and general cost-effectiveness, any power system operating in the sea needsto be suf� ciently robust and reliable to function for lengthyperiods without human intervention. It is believed that themaintenance interval for marine renewable energy systemsof any kind needs to be � ve years or more, because the costof offshore operations needed to undertake maintenance orreplacement of components is much higher than for anequivalent land-based technology and is of course subjectto suitable ‘weather windows’. Therefore it is worth invest-ing money to achieve long operational lives for components.

A further factor relating to underwater technologies is thatthere is likely to be a build-up of marine growth andcorrosion also raises a range of technical problems.Modern techniques such as cathodic protection may givesome protection but the design needs to be tolerant of theseeffects.

3.2 Types of rotor

The physical principles for exchanging kinetic energybetween � owing water and a rotor are similar to those for

wind. Although there are a large number of devices thathave been promoted for extracting energy from � uid � ows,in practice, if reasonable ef� ciency is to be achieved, it isnecessary to use a mechanism which relies primarily ongenerating torque and hence power from lift forces ratherthan drag. There are only two generic types of kinetic energyconversion rotor that are driven primarily by lift forces andthose are the conventional axial � ow or ‘propeller’ type ofrotor and the cross-� ow or ‘Darrieus’ rotor (see Fig. 1).

The main advantage of the Darrieus rotor is that the shaftpower is taken out perpendicularly to the � ow, which lendsitself to having a drive train either on the seabed or in asurface vessel, while an axial � ow rotor either has to havethe drive train entirely at rotor hub level (i.e. in the centre ofthe � ow) or needs some form of right-angled energytransmission mechanism. However, the Darrieus has severaldisadvantages:

1. It involves considerably more rotor structure per unit ofswept area (which increases costs).

2. It generally does not self-start but needs to be driven upto a speed at which the rotor blades unstall when crossingthe � ow.

3. It could be dif� cult to stop in an emergency situation,because it cannot readily be turned out of the � ow (eitheran axial � ow rotor can be yawed ‘edge on’ to the � ow orthe individual rotor blades can be pitched into a featheredposition).

4. It depends much more than an axial � ow rotor on havinga good surface � nish in order to maintain a high lift:dragratio which is needed to achieve reasonable ef� ciency—this may be dif� cult to maintain under the sea [6].

Fig. 1 Basic rotor concepts

Proc Instn Mech Engrs Vol 216 Part A: J Power and Energy A01801 # IMechE 2002

4 P L FRAENKEL

There are other disadvantagesof the Darrieus, such as greatersensitivity to cavitation, plus, in the case where there are fouror fewer rotor blades, the development of major cyclic lateralforces. These problems individually might be consideredsolvable, but when considered in combination they tend tomake the Darrieus considerably less attractive than it mightappear on � rst consideration. However, this has not stopped anumber of people from proposing their use on the grandscale as tidal current turbines.

Both types of rotor can have � xed blades or variable pitchblades. A complication with variable pitch cross-� ow orDarrieus-type rotors is that the blades need to be cyclicallypitched, i.e. they need to go through a complete pitch rangecycle once per rotor revolution, while variable pitch axial� ow rotors only need adjustment on a time scale related tothe variation in � ow (i.e. normally coinciding with thediurnal tidal cycle).

A further option often suggested is to enclose the rotor ina Venturi-like shroud in order to accelerate the � ow andthereby to gain a higher rotational speed (which reducestorque and hence might simplify the mechanical drive train).This has been tried on a number of occasions with windturbines where generally the structural complexity and costof such a large passive component as a diffuser surroundingthe rotor has made the savings from a smaller and fasterrotor generally seem insuf� cient to compensate for theadded cost of the shroud. In water a further problem isthat (as will be explained in more detail later) the rotor bladetip velocity needs to be limited to around 10–15 m=s toavoid the development of cavitation. Increasing the � owvelocity tends to need the entire rotor to be speeded up inproportion, so a marine current turbine in a Venturi will tendto be more dif� cult to design to avoid serious loss ofperformance due to rotor blade tip cavitation.

3.3 Extractable energy and loads on the rotor

Whatever type of rotor is used, the theoretical upper limit ofef� ciency for extracting energy from a free stream (at least

where the � ow boundaries are relatively far from the rotorperiphery) is 59.3 per cent, which can be derived fromactuator disk theory [7]—a conclusion commonly attributedto Betz.

In practice, a tidal turbine rotor, which generally will belarge in relation to the � ow cross-section (i.e. with its upperand lower edges close to the surface and the seabed) maybene� t from an effect called ‘blockage’. This is where the� ow is constrained by the boundaries so that it cannotdiverge to the extent that it would when decelerated througha rotor in free space, and as a result a greater mass � ow of� uid passes through the rotor and this manifests itself as ahigher ef� ciency than might be expected had the boundariesbeen further away. With wind tunnel tests of wind turbinerotors the effect of � ow boundaries preventing the develop-ment of the same � ow divergence as in free space needs tobe corrected for, or an exaggerated idea of the rotoref� ciency would be gained, but in the submarine turbinecase this may be an exploitable bene� t in which the � owboundaries effectively act as an ef� ciency augmentor byincreasing the mass � ow through the rotor compared withwhat would happen if � ow boundaries were far from therotor. As mentioned before, excessive use of blockage will,however, prove counterproductive.

Although the much greater density of water comparedwith air (by a factor of around 800) yields high energydensities at low velocities, it also yields correspondinglylarge forces on the rotor structure compared with a windturbine. Hence, a submarine current energy converter hasloadings that are quite different from those of an equivalentwind turbine. Figure 2 indicates the salient characteristics ofspeed, power, thrust, load and torque that could apply to anaxial � ow submarine turbine with rotors in the diameterrange 10–24 m.

The dominant steady state loads are bending forces on therotor blades caused by the high lift forces that can begenerated in such a dense medium as water; however,unlike a wind turbine, weight is of secondary importanceand centrifugal loads, which partially balance bending loads

Fig. 2 ‘Typical’ operating conditions for a tidal turbine rotor (ratedvelocity 2 m=s, average ef� ciency30 per cent)

A01801 # IMechE 2002 Proc Instn Mech Engrs Vol 216 Part A: J Power and Energy

POWER FROM MARINE CURRENTS 5

on a wind turbine, are of course negligible at such lowrotational speeds. Note that the torque and thrust � guresrelate not to normal ‘rated’ operating conditions but tooccasional overload conditions corresponding to 4 m=svelocity which could conceivably occur if extreme windfetch � ow coincided in both timing and direction with anextreme spring peak � ow.

Rotor speed has to be limited by the need to avoidsigni� cant cavitation; this is not so much to limit potentialsurface degradation (another worry with cavitation) but toprevent boundary layer separation which has the same effectas stalling, i.e. of dramatically reducing the lift–drag ratioand hence the ef� ciency of the rotor. With an axial � owrotor, cavitation develops at the rotor blade tips in particularat the upper part of the swept path where static pressure isleast; in practice blade tip velocity needs to be limited toaround 12–15 m=s. Figure 3 indicates how cavitation varieswith depth; clearly the greater the static pressure the higherthe velocity that is possible before the overall pressure fallsto the vapour pressure of water at which cavitation can beinitiated. This situation is more critical with a Darrieus typeof rotor where the whole blade rather than just the tipsmoves at much the same relative velocity to the water.

It should be added that the pressure distribution around alifting surface such as the blades of a tidal turbine variessigni� cantly with the pro� le; e.g. a sharp-nosed pro� le willhave a much more ‘peaky’ pressure distribution (a parametercalled the suction coef� cient can be used to describe the‘peakyness’ of the suction pressure distribution; this is theratio of the least pressure—usually occurring at around 0.25

chord—to the average suction pressure). Conversely, a bluntand relatively thick section will have a lower suctioncoef� cient, implying a more uniform negative pressuredistribution across the chord length on the suction side. Itfollows from this that the tidal turbine rotor designer needsto employ a hydrofoil section having as low a suctioncoef� cient as is compatible with maintaining reasonablehydrodynamic ef� ciency, in order to delay the onset ofcavitation. Fortuitously, this also results in a thicker andtherefore a stronger rotor blade, which is an importantadditional bene� t in terms of resisting the high � apwiserotor blade bending forces that are generated.

3.4 Dynamic effects

The dynamic and transient loadings on the system arecomplex and a major part of the planned research anddevelopment programme is to address the need to get abetter understanding of dynamic effects. The designapproach for the � rst systems is to try to use a rigid (i.e.non-compliant) structure wherever possible so that anyresonant frequencies generated are lower than the naturalfrequency of key structural members. The main drivers interms of excitation are the passing frequency of rotor bladesto the pile, vortex shedding from the pile and effects frompassing waves. Passing waves also have implications forcausing transient cavitation at moments when the wavevector reinforces the current vector. Structural problemscaused by waves (e.g. slam on the structure above the

Fig. 3 Velocity needed for cavitation to be initiated as a function of water depth

Proc Instn Mech Engrs Vol 216 Part A: J Power and Energy A01801 # IMechE 2002

6 P L FRAENKEL

surface) have been calculated to be relatively smallcompared with the loadings caused by rotor thrust. Hencereserves of strength to cater for worst wave conditions (say50 year return) are relatively small. Interactions betweenwaves and strong currents have been reported to be severe incertain locations and therefore preferred locations will bechosen, at least for early projects, that are not too exposed tooceanic waves. The site for the planned experimentalprototype off north Devon is relatively sheltered in thatrespect.

The variations in static pressure and velocity across thevertical water column also impose interesting dynamiceffects on the rotor blades to an extent that is not paralleledeither by wind turbines or by conventional low head hydroturbines; the tip of the blade of a typical 15 m diameter rotorwill experience a 1.5 bar cyclic pressure variation as itmoves from the top to the bottom of its circular path, inaddition to potentially large velocity � uctuations.

3.5 Methods for installation

Installing any kind of system in the sea is at best dif� cult(it has to withstand storms, corrosion and a host ofother hazards for a long time). Economic analysis suggeststhat systems of this kind need an overall design life aver-aging 20–30 years to achieve a good return. The overridingtechnical problem with tidal current turbines is the highaxial thrust that needs to be reacted against, which candemand designing for an extreme horizontal force onthe rotor of megawatt sized systems in the order of1000–3000 kN depending on the local maximum streamvelocity and rotor size.

As shown in Fig. 4, there is a choice of mounting a rotorunderneath a � oating vessel which is then moored or ofinstalling it on a structure attached to the seabed. The

� oating vessel could either be on the surface or be heldby tension moorings below the surface. An installationcould be piled into the seabed or be held in place as agravity structure (no doubt with something more positivethan simple friction to prevent movement).

Figure 4 illustrates how a gravity base is only feasible inshallow water while a � oating device is essential in deepwater. The use of a monopile (or a jacketed multiple piledsupport in deeper water) offers an intermediate solution withcost advantages over the other options.

In practice it is almost impossible to carry out extensiveunderwater operations using either divers or remotely oper-ated vehicles (ROVs) in the kinds of locations of interest,owing to the high � ow velocities and limited periods ofslack tide. Therefore Marine Current Turbines Limited hasdeveloped a turbine concept based on using a monopile(similar to the second from left in Fig. 4) installed from ajack-up barge. A monopile is a tubular steel tower insertedinto a hole drilled in the seabed. A jack-up barge can drillthe necessary hole, usually in hard seabed surfaces, by usinga rotary rock drill and install the pile using its on-boardcrane. Marine Current Turbines’ commercial partners,Seacore Limited, are specialists in placing monopiles(having installed a number of wind turbines on monopilesat sea) and they have the capability to drill holes and to placepiles up to 4 m in diameter even in hard rock. Most locationswith high velocity currents tend to have hard or rockyseabeds because scour from the currents tends to removeany soft or loose material.

Although a � oating system appears to be more versatile,since installation or removal of a � oating moored systemshould be relatively quick and hence inexpensive, mooringand anchoring such a turbine reliably in the open sea istechnically dif� cult. There is a large upward force compo-nent on any anchorage in addition to the large horizontalcomponent, unless a catenary mooring with clump weights

Fig. 4 Examples of possible system con� gurations

A01801 # IMechE 2002 Proc Instn Mech Engrs Vol 216 Part A: J Power and Energy

POWER FROM MARINE CURRENTS 7

is used, which demands a lot of horizontal space. Moreover,the thrust force reverses direction with each tide and thisneeds to be accommodated by the mooring system. Thetechnology used by the offshore oil and gas industry formooring semi-submersibles may offer one possible solution.

However, so far as � rst generation systems are concerned,Marine Current Turbines Limited plans to install rows ofturbines mounted on individual monopiles much like thearrays of wind turbines that make up a wind farm. However,because tidal streams are generally bi-directional, theturbines can be installed close together transversely acrossthe stream; wind turbines (which can receive � ow fromvirtually any direction) need to be suf� ciently spaced out toprevent the wake of any turbine seriously interfering withthe � ow through a downstream unit. Therefore it is possibleto group marine turbines much more closely together thanwind turbines, with advantages in terms of installation costsand cable costs. A packing density for tidal stream turbinesof from 50 to 100 MW=km2 seems feasible, which is up to10 times the array density of typical wind farms.

3.6 The drive train and extracting the power

The rotor of a tidal turbine runs at low rotational speeds(10–20 r=min) and generates correspondingly high levels oftorque (200–1000 kN m). Therefore the drive train posessome interesting technical challenges. The most likelysolution in the short term will be to use a similar systemto a wind turbine, namely a gearbox driving a generator. Formost machines this will need at least a two-stage speedincreaser, requiring an epicyclic gearbox to keep the sizeand weight within reasonable limits. There are also possi-bilities for using hydraulic transmission or for developingspecial low speed directly driven alternators (a seeminglypromising trend at present with wind turbines). The powertrain may be encased in an air-� lled nacelle using sealingarrangements for the drive shaft much like those used forship (or submarine) propeller shafts, or alternatively it ispossible to use dedicated sealed gearbox=generator unitsthat can run immersed in water and therefore need noexternal casing. Submersible mechanical and electricalmachinery is becoming relatively commonplace andlarge submersible pumps of similar power levels to thegenerators needed for a tidal turbine are standard commer-cial products. The output is delivered via a marine cable laidacross the seabed to the shore, probably at a voltage of 11 or33 kV.

3.7 The technology and its competitive advantages

There are undoubted dif� culties in developing technology tooperate reliably for long periods under the sea, but this hasbeen successfully achieved in other contexts and there aregood reasons to expect the rewards from developing thistechnology to be more than suf� cient to justify the neces-

sary investment in research and development. This isbecause marine current turbines have a number of advan-tages that should ensure that this technology � nds a majorrole in delivering clean energy in the future. The mainreasons that marine current turbines can be expected to becost competitive are the following:

1. A suitably energetic marine current location can offerfour times the energy intensity of a good wind site andsome 30 times the energy intensity of sunshine in theSahara (see Fig. 5) in terms of energy captured per unitarea of interface; hence marine current turbines needonly be a quarter the swept area of a wind turbine of thesame power (or 1=30 the size of an equivalent solarphotovoltaic array) and smaller size generally produceslower costs.

2. A marine current turbine can deliver energy to a time-table since the tides are accurately predictable: thereforethe end product of predictable power is inherently morevaluable to a utility than randomly generated electricalenergy; wave and to some extent wind energy is predict-able over short time horizons, but tidal currents arepredictable years in advance.

3. Not only are the turbines relatively small (compared withother renewable energy devices) but because of thebidirectional � ow they can be packed much more closelytogether, aligned across the current at less than 50 mintervals; this leads to a compact installation which inturn yields further savings in cabling and installationcosts.

4. Since weight is not a critical factor for a submergedturbine, the construction can be a steel fabrication sothe manufacturing costs per tonne promise to berelatively low.

5. Physical conditions under water tend to be relativelypredictable and calm, so extreme conditions requiremuch less ‘over-engineering’ than would apply forwind- or wave-powered technologies. This also helps tokeep costs down; there is no underwater analogy to anatmospheric hurricane.

Fig. 5 Relative energy capture per unit size from four types ofrenewable resource (per m for waves and per m2 for therest) taking account of typical load factors and likelytechnical ef� ciency of the systems

Proc Instn Mech Engrs Vol 216 Part A: J Power and Energy A01801 # IMechE 2002

8 P L FRAENKEL

Apart from these potential economic advantages, anenvironmental impact study [3] indicated that the technol-ogy should pose no serious threat to marine life or to thelocal environment. The low tip velocity of tidal turbinerotors makes them signi� cantly less of a threat. Also, thelow visual pro� le (in some cases zero visual pro� le) shouldmake it acceptable to planners to grant permission for largeinstallations relatively close onshore.

4 REVIEW OF RECENT DEVELOPMENT WORKSO FAR COMPLETED

The � rst attempt to evaluate a national tidal currentresource, in 1992–3, was the ‘UK tidal stream energyreview’ [1]. This desk study con� rmed that there is alarge tidal current energy resource, capable theoretically ofmeeting some 19 per cent of total UK electricity demand atthat time, of which 20 TW h could be delivered at around10 p=kW h, i.e. not economically under the cautious and(with the bene� t of hindsight) incorrect costing assumptionsapplied.

In 1993–4, a consortium consisting of IT Power, ScottishNuclear and NEL developed a ‘proof of concept’ experi-mental tidal current system. This involved an axial � ow3.5 m diameter rotor suspended below a � oating catamaranpontoon. It successfully developed some 15 kW in 2.25 m=scurrent velocity at Loch Linnhe, Scotland, in 1994 and,although quite small, it remains the largest marine currentturbine so far demonstrated [8].

Under the JOULE 2 energy research programme, DGXIIof the EU supported a technical and resource assessmentof marine current energy in Europe [2]. The technicalstudy (completed in 1996) examined relevant technologyfrom related areas (wind, hydropower, ships and theoffshore oil and gas industry) and found that electricitycost is specially sensitive to the size of machine, economicparameters (lifetime, discount rate), O & M (Operation andMaintenance) costs and the load factor obtainable at aparticular site. It estimated electricity unit costs for ‘� rst-generation systems’ deployed on a small scale at around0.05 ECU=kW h (3.5 p=kW h) for a 3 m=s rated currentunder favourable circumstances (i.e. with high loadfactors). This is encouragingly close to what is neededfor commercial viability.

The Regional and Urban Energy Programme, DGXVIIof the EU, � nanced a feasibility study on supplyingOrkney and Shetland with electricity from tidal streamturbines [3]. Island communities, which frequently havehigher than normal conventional energy costs, are likely tobe the most attractive initial market for electricity fromtidal streams. Actual on-site current measurements wereused in conjunction with a three-dimensional computermodel to produce the tidal stream characteristics for twosites. Consideration of a cluster of eight turbines of

20 m diameter showed a predicted electricity cost ofapproximately 6 p=kW h. Larger clusters would reducecosts further.

Commercial marine current turbine technology alreadyexists, albeit on a very small scale. At least two products aremade in the UK, both consisting of a small propeller-likerotor, about 50 cm in diameter, that can be hung over theside of a yacht or leisure boat so that the passing � ow can beharnessed to keep its batteries charged either when mooredin a tidal � ow or when under sail without the bene� t of anengine.

Therefore the basic concept of utilizing marine currentenergy has been convincingly demonstrated and is wellunderstood. What remains to be proven is that it can beapplied on a large scale at a cost and with a level ofreliability that can allow it to compete successfully withconventional methods of energy generation.

A project was launched in September 1998 with the� nancial support of the JOULE 3 programme of DGXII ofthe European Commission [9]. It involves the developmentof what is expected to be the world’s � rst ‘commercialscale’ marine current turbine, a system rated at 300 kW tobe installed on a monopile, socketed into the seabedin south-western UK coastal waters. The target datefor commissioning the system is summer of 2002.Marine Current Turbines Limited was formed to takeresponsibility for the commercial development of thetechnology [10].

It is expected that this project will lead directly to asecond phase involving the development of a full-scaledemonstration of a cluster of perhaps four or � ve largerturbines each of about 1 MW. This in turn will lead tocommercial exploitation of the technology within a leadtime of 5–10 years.

A recent development is that the DTI, through ETSU,commissioned and published a study by consultants inApril 2001, which among other things revisited the resultsof the 1993 tidal stream review, but which primarily aimedto provide an independent evaluation of the cost andperformance being claimed by advocates of marine currentenergy such as Marine Current Turbines Limited; thisstudy [11] broadly endorsed the results of earlier studiessuch as those of references [2] and [3] and estimated thatelectricity costs from sites with mean spring peak velo-cities of more than 2 m=s will be in the range 4–6 p=kW hand that there is a reasonable chance for costs to fall afurther 30 per cent with economies of scale from largerschemes. It concluded that ‘these � gures are of the rightorder of magnitude to encourage commercial interest in thetechnology’.

Following the conclusions of this report, the DTI under-took to include tidal stream technology within the scope ofits support for renewable energy research and developmentand it has also provided some important and substantial� nancial assistance for the initial part of the research anddevelopment programme that Marine Current TurbinesLimited is undertaking with its industrial partners.

A01801 # IMechE 2002 Proc Instn Mech Engrs Vol 216 Part A: J Power and Energy

POWER FROM MARINE CURRENTS 9

5 MARINE CURRENT TURBINES’ PLANNEDDEVELOPMENT PROGRAMME

5.1 The Marine Current Turbines axial � ow turbine ona monopile concept

5.1.1 Phase 1 of research and development programme:single-rotor 300 kW experimental test rig

Marine Current Turbines Limited in partnership with aconsortium of other companies is developing initially aform of turbine similar to that illustrated in Fig. 6. It consistsof a rotor mounted on a monopile in such a way that theturbine drive train and rotor can be raised above the surfacefor maintenance. The � rst prototype will be similar to theartist’s impressions of Fig. 6 and will be installed, subject togaining the necessary permissions, off Foreland Point, northDevon, during 2002. It will be rated at 300kW (e) and havea single 12 m diameter rotor.

The project involves Marine Current Turbines Limited asthe coordinator and developer, working with SeacoreLimited (which specializes in the installation of piles atsea—having carried out this function for two offshore windfarms already—who will do the installation using one of

their jack-up barges), IT Power Limited (the original devel-opers of the technology carrying out some of the designwork), Bendalls Engineering (precision steel fabricator, whowill be mainly responsible for the steelwork), Corus UKLimited (formerly British Steel—until merged recently withHoogovens of the Netherlands—who are supplying special-ized expertise and materials), ISET (which is a design andresearch department of the University of Kassel inGermany—doing much of the electrical design as well aswork on rotor design in partnership with MCT and ITPower) and Jahnel-Kestermann (a German gearbox manu-facturer specialising in wind turbine and marine gearboxes).Also W. S. Atkins has a key role with technical inputsrelating to the rotor and structural design.

Jahnel-Kestermann are designing a two-stage epicyclicgearbox unit speci� cally for the system which contains themain bearings with an overhung shaft to carry the rotor. Thecasing is a structural component and is also watertight,making it effectively a submersible gearbox which isseawater compatible (Fig. 7).

The generator is in a watertight casing bolted to the backof the gearbox with a disk brake and coupling internallymounted between the gearbox and generator. Cooling is

Fig. 6 How a turbine system may be installed on a monopile so that it can be raised above the surface forservicing and replacement

Proc Instn Mech Engrs Vol 216 Part A: J Power and Energy A01801 # IMechE 2002

10 P L FRAENKEL

effectively provided through the submerged and wettedcasings of the drive train components. The system isdesigned to run at variable speed using electronic powerconditioning to produce a mains quality 50 Hz output.

The original concept envisaged a � xed pitch constantspeed system as is commonly used for wind turbines, but itwas felt to be too risky to rely on stall regulation of such anovel system since failure to stall would potentially causeunacceptably high loads on the drive train and the onlymethods for control would be either to trip the electricalsystem and let the rotor run away or to rely on themechanical brake. Since it is believed for reasons to beelaborated below that rotor pitch control is a necessaryrequirement for the commercial technology, it was decidedto introduce this on the prototype too. The facility forvarying the rotor blade pitch not only permits accuratespeed control but also is expected to signi� cantly improvethe ef� ciency, especially at part load.

The rotor blades are of fabricated steel construction,consisting of a tapered main spar at approximately one-third chord, with ribs skinned with steel plate, much thesame as most ship’s rudders or ship stabilizer hydrofoils.The pro� le for the prototype is a modi� ed NACA 634xx, xxbeing 35 (or 35 per cent thickness:chord ratio) near the rootand tapering to 15 per cent near the tip.

The drive train and rotor are mounted in a chassisattached to a liftable collar that is a loose concentric � t tothe pile. The lifting mechanism for the prototype is areduced-scale version of the hydraulic system commonlyused to lift jack-up barge legs.

It is planned to run the single-rotor experimental proto-type for only about one year before replacing it with asystem that will be intended to be the precursor for thecommercial systems to follow later.

5.1.2 Phase 2 of research and development programme:twin-rotor 600 kW pre-commercial prototype

The primary goal of the technical development programmeis to improve the cost effectiveness of the technology inorder to bring down the cost of the electricity that can beproduced so as to make the technology commerciallycompetitive with other methods of generation as soon aspossible. A major part of the cost consists of the overheadsinvolved in installing a pile at sea and providing a cableconnection to the shore. Therefore by installing twin rotorson a pile as shown in Fig. 8, it is possible to double theenergy produced compared with a single rotor of the samesize (the diameter is generally limited by the depth of water)yet this can be done for signi� cantly less than twice the cost.Depending on various factors, a twin-rotor system willtypically be 30 per cent more cost effective (installed cost)than a single-rotor system of the same rated power, andmoreover it can operate in shallower water owing to thesmaller vertical height of the rotors.

There are several other advantages of a twin-rotorsystem of the kind shown in Fig. 8 compared with asingle-rotor one:

1. The torque reaction of the two rotors and the swirlcomponent in their wakes will tend to cancel eachother out.

2. The wake from a streamlined horizontal wing used tosupport each power train and rotor has less disruptingeffect on the � ow through the rotor than the wake fromthe much larger thickness of pile does for a single rotor.

3. When raised, the ‘wings’ form a working platform formaintenance purposes, making access to the power trainsand rotors much easier, as shown on the right-handpicture of Fig. 8.

Fig. 7 Prototype 300 kW tidal turbine drive train shown installed in the chassis which is in turn attached to acollar concentric with the pile, which can be raised above the surface of the sea

A01801 # IMechE 2002 Proc Instn Mech Engrs Vol 216 Part A: J Power and Energy

POWER FROM MARINE CURRENTS 11

However, despite its undoubted advantages, the twin-rotorsystem introduces a number of potential structural anddynamic problems which are being studied, but it is believedthesecan all be solved.Oneof themain areas for concern is theexcitation modes that the structure might experience fromvarious excitation drivers such as the rotor blade passingfrequency and also from vortex shedding by the pile.

5.1.3 Phase 3 of research and development programme:array of twin-rotor 1 MW pre-commercial prototypes

The twin-rotor ‘phase 2 prototype’ will form the basis forcommercial technology to follow. This will be tested andre� ned further in phase 3 as a small cluster or ‘farm’ of fouror � ve twin-rotor turbines (see Fig. 9), primarily to gainoperational experience of a working array of turbines as wellas to check on interactions and control problems that can beexpected when running multiple units in parallel with eachother.

The phase 3 systems will be uprated to from 0.75 to1 MW (2 6 375 to 2 6 500 kW rotors and drive trains)depending on the local current rated velocity, by a combina-tion of improvements in ef� ciency and slight increase inrotor diameter. Phase 3 will also be partially self-� nancingfrom revenue resulting from sale of electricity.

5.1.4 Commercial projects

After approximately 12 months of satisfactory operation ofthe phase 3 system, it is planned that work will start to

install commercial projects. These will generally involvebatches of systems with an aggregate installed capacity inthe range 20–30 MW, as this scale of installation is neededto obtain favourable economics. It also so happens thatabout 30 systems can generally be installed during theweather window from around March to the end ofSeptember and that 30 MW is a practical power level for acluster of systems having a common electrical ring main andtransformer. Hence it is expected that large projects will bebuilt up from 20–30 MW groups, each taking a jack-upbarge a summer season to install. Each such group will havean independent ring main, probably of 11 kV, and probably atransformer to feed into the 33 kV ‘umbilical’ taking thepower ashore.

It is also possible that a high-voltage d.c. system will beused for interconnection of the variable frequency a.c.generators and to link to a power conditioning systemashore, although such developments will be more likelylater on with larger projects.

5.2 Economics—driving down costs

The development of a commercially successful technologywhich can make a useful contribution to future worldenergy needs is dependent on getting the system costsdown to the level where the cost of generation can competefavourably, � rst with other renewables and ultimately withany other form of power generation. On the basis of existing

Fig. 8 Artist’s impressions of how a twin-rotor turbine system may be installed on a monopile so that it can beraised above the surface of servicing and replacement. Note that a small work boat or tug is adequatefor gaining access, ‘wings’ act as a working platform and rotor blades are set horizontally and to neutralpitch when raised

Proc Instn Mech Engrs Vol 216 Part A: J Power and Energy A01801 # IMechE 2002

12 P L FRAENKEL

technical design options, the plan is to seek to drive downcosts through the development programme as indicated inFig. 10.

Points 1 to 3 on the cost-reduction curve are the systemsto be developed under the previously described phasedresearch and development programme, point 4 is the � rstcommercial project and the improvements thereafter can begained from a combination of improving the technologygenerally, economies of scale and reductions in the marginalcost of projects generated by expanding existing projects (soas to avoid duplicating a large part of the grid connectioncosts).

Although this may seem ambitious, similar cost reduc-tions have been achieved for other technologies such aswind turbines. Installed costs for on-shore wind turbineshave come down in real terms from over £2000=kW in theearly 1980s to around £700=kW today, while offshore windturbines are expected to cost around £1000=kW today.Offshore wind turbines are in some ways the maincompetitor with tidal current turbines as a large-scale

source of clean electricity, but there are many synergiesbetween these technologies, not least being that they bothlend themselves to installation on monopiles and bothdemand signi� cant quantities of submarine cable for inter-connections. Therefore marine current turbines are not inreality an alternative to offshore wind, but rather it can beexpected that success for offshore wind will be helpful fortidal current developments too, since both require similarinvestment in infrastructure and installation plant such asjack-up barges.

6 CONCLUSION

There is no doubt that it is technically feasible to extractenergy from marine currents and that the tidal currentresource is large enough to have the potential to make amajor contribution towards meeting future energy needs.However, the main area of uncertainty relates to the

Fig. 9 Phase 3 of the research and development programme envisages the installation of a small array of � veunits, each of from 2 6 375 to 2 6 500 kW. Note that the second system in the array is shown hereraised for maintenance, with the nearest rotor blade pitched and the far rotor blade furled (neutral pitch)

A01801 # IMechE 2002 Proc Instn Mech Engrs Vol 216 Part A: J Power and Energy

POWER FROM MARINE CURRENTS 13

economics; there is no guarantee that cost effective systemscan be developed although there are good indications thatthe potential to achieve this goal exists, given suitableresearch and development. More to the point is to predicthow long it might take to drive the costs of this method ofpower generation down to the level where a thriving newindustry can develop in order to take it forward on a self-sustaining commercial basis. The analysis suggests that thisshould be possible within 5–10 years, a goal which MarineCurrent Turbines Limited and its partner companies arecommitted to.

REFERENCES

1 Tidal stream energy review, UK DTI. Report ETSUT=05=00155=REP, prepared by Engineering and Power Devel-opment Consultants Limited, Binnie and Partners, Sir Robt.McAlpine and Sons Limited and IT Power Limited for theETSU, UK Department of Energy. Crown Copyright 1993.

2 The exploitation of tidal marine currents (non-nuclear energyJOULE II project results). Report EUR 16683EN, DG Science,Research and Development, The European Commission Of� cefor Of� cial Publications, Luxembourg, 1996.

3 Feasibility study of tidal current power generation for coastalwaters: Orkney and Shetland. Final Report, EU contractXVII=4 1040=92-41, ICIT, March 1995.

4 Cave, P. R. and Evans, E. M. Tidal stream energy systems:economics, technology and costs. In Third InternationalSymposium on Wave, Tidal, OTEC and Small Scale HydroEnergy, Brighton, 1986.

5 Offshore Installations: Guidance on Design, Construction andCerti�cation, 1990 (UK Department of Energy, HMSO,London).

6 Kihoh, S. and Shiono, M. Power generation by tide and tidalcurrent. In Proceedings of Advanced Materials ’93, Vol. V=A,Eco-materials, 1994, pp. 41–44.

7 Duncan, W. J., Thom, A. S. and Young, A. D. The Mechanicsof Fluids, 1960 (Edward Arnold, London).

8 Paish, O. and Fraenkel, P. Tidal stream energy: zero-headhydropower. In International Conference on Hydropower intothe Next Century, Barcelona, 5–8 June 1995.

9 Fraenkel, P. L., Clutterbuck, P., Stjernstrom, B. and Bard, J.‘Sea� ow’: preparing for the world’s � rst pilot project for theexploitation of marine currents at a commercial scale. InProceedings of Third European Wave Energy Conference,Patras, September–October 1998.

10 http://www.marineturbines.com.11 Commercial prospects for tidal stream power. Binnie, Black and

Veatch, Redhill, DTI=ETSU, Harwell, April 2001.

Fig. 10 How Marine Current Turbines Limited intends to develop its technology; what will be needed to maketidal current turbines commercially competitive

Proc Instn Mech Engrs Vol 216 Part A: J Power and Energy A01801 # IMechE 2002

14 P L FRAENKEL