Many experts believe that growth willhit the buffers for rotors soonerthan for the turbines themselves.

However, LM Glasfiber, the Danish bladespecialist responsible for the 126m rotor onRE Power's new 5M system, is clear that,even this enormous system does not evenapproach the limit. LM's marketing manager,Steen Broust Nielsen, would not be surprisedto see 10MW turbines within a decade andthe first 80m blades in half that time. He isreluctant, though, to predict an ultimate sizedue to long experience of an ever recedinglimit. He recalls that when 9m blades werestate of the art back in the 1980s, cold waterwas poured on the idea that blades could oneday reach the mind-boggling length of 15m!In terms of what will drive size escalation,however, he is more confident.

"The main impetus will come from theoffshore sector," he says. "In an environ-ment where installation and cabling canaccount for 40 - 50% of project costs, it'simportant to limit the number of turbinesproducing a given amount of power."

RE Power project leader Peter Quellagrees - adding that he entered the industrywhen producers were turning out mainly100kW turbines.

"In view of what has happened since," hesays, "I would never suggest any particularlimit. I'm happy just to take things step bystep. We will certainly see bigger yet, espe-cially offshore."

Quell backs his assertion by declaringhow comfortable he feels with the perform-ance so far of his company's 5M turbine. Ithas been operating autonomously since 2ndFebruary, having first delivered its full ratedpower earlier in the year. It has, he says,inspired confidence when operating instrong, gusty winds. As Quell explains,

"Very large rotors have an averaging effecton the wind speed, and this produces steadierconditions for the blades to operate in. The5M has run in winds right up to its cut-outspeed of 25 m/s ( 90 kph) without problems."

Big challengesExperts have feared that gusts and turbulencewould be an Achilles heel for ultra-large sys-tems, but Peter Quell's observations may easethose concerns. He is convinced, for instance,that load-shedding pitch control mechanismscan be made to react fast enough to preventdamage. The variable speed feature of the REPower turbine family also helps, he says,enabling gusts to be absorbed and convertedto electrical power. Quell argues that a biggerinfluence in holding back size will be weight,which will accentuate both structural andlogistical difficulties.

"Weight will be a major constraint, espe-cially for rotors," he asserts, adding, "That'scertainly likely in the near term, though asstructures become more predominantly car-bon for the larger units, material price willprobably have the greater influence."

As rotor size and weight grow, blades andfinal drives are subject to increased cyclicstresses since loads constantly alternatebetween gravitationally-induced compressionand tension as each blade revolves throughthe top then the bottom of its circular path. RE Power is therefore pleasedthat LMGlasfiber has managed to produce a61.5m blade that weighs under 18 tonnes,enabling the three blades between them toaccount for less than half the total weight ofthe rotor. The Danish company has achievedthis with only a modicum of carbonfibre, thestrong but expensive composite that manycommentators think will be essential to makeblades of 60m plus sufficiently light and stiff.As its name suggests, LM Glasfiber is rootedin glass and prefers to use this material wher-ever possible because it is 'about one tenththe cost of carbon'.

Blade materialsGiven that LM is only now beginning toreach the apparent limits of glass, selectivecarbon augmentation should permitprogress into considerably larger blade sizes.Other manufacturers, though, have beenprepared to adopt carbon more fully fortheir large blade structures. Interviewed acouple of years ago when Nordex Rotor hada near-60m giant on the stocks for a 5MWturbine, manufacturing manager DietmarKnunz put the case for this composite suc-cinctly, saying,

"In these sizes stiffness becomes a leadingdriver. As carbon fibre offers about threetimes the stiffness of glass, we can use lessmaterial, producing slimmer blades whichresist tip deflection under high loads and arenot too heavy."

Knunz was convinced that blades in the60m plus category outrun the capability ofglass reinforced plastics and thought that,above a certain size, the glass/carbon cost dif-ferential would become less significant due toless material having to be used. Nordex planswere altered as a result of financial difficultiesand the company currently produces noblades larger than 45m. According to a

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Giant wind turbines

Economics are pushing the size of wind turbines upwards.Having fewer, larger turbines reduces the number of expensivetowers, foundations, erection operations and power collectioncables that are needed. This is more crucial in the offshore sector,where the largest turbines can therefore be expected. Today wesee turbines of 3 - 3.5MW with rotors of 80 - 90m diameter coming into the market, while recently a 5MW unit from REPower, with a 126m rotor, started operating experimentally inGermany. Talk is of turbines at least double this power eventually. But is such talk realistic and are there factors likely tolimit wind turbine growth? George Marsh reports.

WIND TURBINESHow Big Can They Get?

www.re-focus.net March/April 2005 reFOCUS 23

Prototype of the 5MW Multibrid M5000 offshore wind turbine erected in December2004. Photo courtesy: Multibrid

spokesman, however, it hopes to expand inthis area again following a financial restruc-turing and is provisionally targeting a 4.5MWturbine, though not before 2007.

Vestas Wind Systems is equally happy toadopt carbon for large rotors. Explains devel-opment manager Lars Budz,

"Weight is one of the most important fac-tors when it comes to improving efficiency andcompetitiveness. We have actually designed aturbine that, despite an increase in nominaloutput of 50% over our V80-2.0MW model,weighs approximately the same."

Using carbonfibre for the rotor of Vestas'new V90-3.0MW model, currently its largestin production, enabled engineers to developa 44 metre blade that weighs no more thanthe 39m blade used on the V80-2.0MW(about 6.5 tonnes). Carbon's stiff, lowweight characteristics have resulted in a slim-mer, more aerodynamic blade incorporatinga lower volume of material than its predeces-sor. On the subject of a limit Budz declares,

"We at Vestas believe that turbines should notbecome bigger just for the sake of it. Customersare looking for the best combination of price,weight and efficiency to keep costs per kWh aslow as possible. Larger turbines are a possibilityprovided they can offer further improvementsin cost per kilowatt hour generated."

Nevertheless, as a result of a design studycarried out under EU funding, Vestas has

established a technical and economic platformfor the future production of a 5MW windturbine. A prototype for a 4.2MW machine,with a 110m rotor, was produced by NEGMicon, subsequently part of Vestas, and this isbeing scaled up to a V120-4.5MW machinedue to become available next year. Other car-bon adherents reportedly preparing to step upsignificantly in size include GE Energy withplans to uprate its 3.6MW Offshore to over4MW, and Gamesa Eolica aiming for 4MWplus from 2007.

Use of carbon, a key enabler for larger size,may however be restricted by supply chaindifficulties and price. Series production ofultrablades will require huge volumes ofmaterial - a demand for which carbon sup-pliers may be insufficiently prepared.According to wind energy specialist AdrianWilliams at materials supplier SP Systems,

"It's a strategic issue. Some of the volumesbeing talked about would challenge the sup-ply chain as it exists at present."

SP already supplies substantial quantitiesof carbon prepreg material for use in largerotors such as the G87 from Spain's GamesaEolica, a company likely to step up next intothe 100m plus rotor size with 50m plusblades. Though encouraged to see industrial-grade carbon starting to come onto the mar-ket at about two thirds the price of existingfibres, Williams cautions that rapid escala-tion in demand could outstrip the ability ofsuppliers to deliver it. Any prolonged sup-ply/demand instability could inhibit rotorgrowth by restricting the ability of construc-tors to transition to carbon.

Material supply issues aside, Williams isconvinced that, structurally, a 10MW turbineis perfectly feasible and sees any limit as morelikely to result from logistical considerations.

"There will be problems transporting anderecting multi-megawatt systems," he pre-dicts, adding that the reliability of produc-tion processes for ultra large blades is anoth-er potential issue. Improved infusionprocesses and 'semi-preg' materials such asSP's SPRINT and HexFIT from HexcelComposites could help address the latter.

Enercon GmbH may sidestep carbon issuesfor a while since, like LM Glasfiber, it remainsloyal to fibreglass. The German company hasutilised predominantly glass/epoxy through-out its range, including in its latest 'ultrablade', developed for the 114m diameter rotor

of a 4.5MW system it has installed nearEmden in north Germany. For a while, untilRE Power erected its 5M turbine, this proto-type was the 'largest wind turbine worldwide'.Each of its 52m glassfibre/epoxy blades sweepsan area of 10,000sq m and weighs 20t. Theengineer responsible for its development,Klaus Schultes, says that using material moreintelligently creates greater stiffness and theability to withstand greater stresses.

ReliabilityHowever, Enercon takes the view that systemsize is more likely to be limited by reliabilityconsiderations than by anything else and that,in this regard, the generator and its associatedmechanical elements will be just as significantas the wind rotor. Gearboxes, in particular, area known source of failure, with consequencesthat can rise exponentially with size.Attending and repairing or replacing a gear-box at a wind farm miles offshore, say, wouldbe expensive at the best of times - the more sowhen the box is relatively massive.Accordingly, Enercon believes that gearless,direct drive, variable speed technology is theway forward for large turbines, and its4.5MW E-112 model reflects this. The windrotor hub is attached directly to the ring gen-erator, which is carried on an axle having justtwo roller bearings, thus avoiding the multiplebearings and gears of competing products.

Having fewer rotating components, thecompany argues, enhances operating eco-nomics by reducing the number of wearingparts; minimising mechanical stress, frictionlosses and need for oil changes; cutting downon maintenance and increasing service life.Advances in power electronics have madeviable the use of the necessary low-speed syn-chronous generators rather than more con-ventional asynchronous types. Although theoutput voltage and frequency vary with thespeed of the wind rotor, an appropriate DClink and inverter can deliver stable power tothe grid despite the speed variations.

A drawback, critics assert, is the largefrontal area of the nacelle needed to housethe big segmented ring generator, and theconsequent obstruction of wind flowthrough the inner part of the wind rotor.Enercon counters that it has compensatedthis with aerodynamic improvements, espe-cially around the rotor root. Overall effec-tiveness cannot be doubted: during tests a

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Giant wind turbines

REpower's 5MW turbine with LM Glasfiber’s61.5m blade. Photo Courtesy: RE Power.

Giant wind turbines

prototype E-112 is said to have exceeded6MW output for a considerable period andis rumoured to have peaked at 7.8MW.

Some experts believe that permanent mag-net generators can save weight, particularly indirect-drive designs, thus enabling systems tobe made larger for the same weight. AdrianWilson, electrical technology specialist at theUK's New and Renewable Energy Centre(NaREC), says that this approach is a focus ofcurrent research aimed at weight reduction.Wilson argues that present designs will notscale to the 10MW level, much less the 20 to30MW systems that the future may require.Dispensing with the gearbox, whose masstends to rise in proportion to the cube of thevolume of air the wind turbine captures, isone way to break this impasse. NaREC isinvestigating various configurations for thenecessary large-diameter generator, includinga bicycle wheel-like structure in which thespokes support the generator pole pairs.

A permanent magnet generator is featuredin another proposed system that represents a

halfway house between geared and directdrive options. This is the novel Multibrid(multi/hybrid) M5000, championed byengineering consultancy AerodynEnergiessysteme. A prototype of this 5MWsystem, with its 17 tonne, 56.5m glass andcarbon/epoxy rotor blades, was installed nearBremerhaven late last year. Each blade madeup an entire road transporter load when therotor was transported to the erection site. Atower head mass (THM) of 310t makes theM5000 easily the lightest plant in the 5MWcategory. (The 5M's, THM is about 400twhile that for the E-112 is thought by indus-try sources to be about 500t). Lowering theTHM reduces the investment necessary intowers and foundations as well as cuttingerection costs. Alternatively, larger machinescan be produced for the same weight.

New owner of the Multibrid technology,Prokon Nord Energiesysteme, plans to estab-lish a manufacturing, installation and offshoreservice support facility in the Bremerhavenarea. Among the challenges presented by the

system's size is finding an organisation ablereliably to produce the complex 25-40 tonnecastings needed for the machine frame, rotorhub and other items. Other critical compo-nents include large-diameter geared rings forthe gearbox and bearings.

Companies that have not taken the variablespeed route have nevertheless refined theirelectromechanical designs. Vestas, forinstance, reconfigured its nacelle and gearboxin its aim to reduce weight and cut mainte-nance visits from two per year to one. Othermeasures include automatic self lubrication,and self monitoring. Indeed, built-in moni-toring systems will be a key enabler for futuremega-turbines. Embedding optical fibres intothe laminate and adding sensors results in sys-tems like LM Glasfiber's BladeMonitoring orequivalents from other manufacturers. Thesemonitor parameters such as load, temperatureand number of cycles as well as blade struc-tural integrity. Registering events such aslightning strikes or extreme loads contributesto guidance over service requirements.

Finishing work at Vestas’ blade factory, and inset, Vestas Systems has introduced a bigger 'Blade Runner' transport vessel to convey larger blades.

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Giant wind turbines

Companies like SKF Condition Monitoringexpand the scope to the mechanical compo-nents, especially gears and bearings.

Associated rapid-reaction monitors detectthe onset of high winds and gusts so thatpitch adjustment mechanisms can respondby feathering the blades out of the wind.Engineers we have consulted suggest that theincreased mass and inertia of ultra-largeblades need not prevent these load sheddingsystems from acting fast enough. In Japan,Mitsubishi has incorporated its proprietary'smart yaw' system in its largest system, theMWT92/2.4, as a survival feature againsttyphoons. This complements independentpitch control, enabling the blades to 'weath-er cock' into high winds automatically.

LogisticsMany industry insiders see the sheer logisticaldifficulties of moving, transporting and erect-ing multi-megawatt systems becoming a sig-nificant bar to size. Rotors especially are diffi-cult because of the lengths and ungainliness ofthe blades. As Enercon's Klaus Schultes putsit, with classic understatement,

"Transporting the blades will be interesting,particularly for inland sites." He adds, "It's clearthat, when transporting E-112 components,inland shipping will play a significant role."

Vestas Systems would agree, having last yearhad to introduce a bigger 'Blade Runner'transport vessel to convey larger blades from its

UK facility at Newport, Isle of Wight, toSouthampton for transfer to seagoing ships.Large rotors place a premium on having size-able waterside manufacturing facilities andcorresponding marine transport.

Wind turbine installation specialistA2SEA A/S is preparing to meet the demandfor larger offshore installations by adding athird, larger, vessel to its current fleet of two'crane ships with legs'. Sized for turbines of5MW plus and rotors in the 120 plus metresclass, the new ship is due to enter service intime for the 'offshore season', mid next year.Like the two existing 93m by 22m vessels, itwill deploy legs to the sea bed and a large lat-tice crane (450 tonne-capable on the presentcraft) for lifting turbines onto towers. Thecompany is currently bidding to install4.2MW turbines with 110m rotors for anunnamed manufacturer.

Director of sales and marketing MartinHuss tells Refocus that increasing size narrowsthe weather windows under which erectingthe system on site is feasible. A limiting windspeed of 10m/s for an 80m rotor, for instance,is likely to drop to 6 or 7m/s for the largestblades now appearing. Long waits for suitableconditions can threaten economic viability byinflating installation costs.

The largest installations, on or offshore,may stretch lifting technology. Crane manu-facturer Liebherr produced a special modelto install the RE Power 5M. The prototype

worked well and was able to raise the entiresystem in a single lift. Future larger systems,with higher hub heights, may have to be lift-ed in a two-step procedure, the nacelle goingup first followed by the rotor. Most multi-megawatt turbines will be equipped with aninboard crane to assist with installation.However, analysts such as Adam Westwoodof consultants Douglas Westwood Ltdbelieves that an installation bottleneck mightarise because there are too few ships of suffi-cient size to deploy systems of 5MW plus(see Refocus May/June 2004).

Large systems, especially the blades, arelikely to outrun the capability of road andrail transporters to carry them. This couldmake it necessary to produce blades in sec-tions, subsequently conveyed to windfarmsites for assembly prior to erection. REPower's Peter Quell is clear that this wouldbe an unpopular option.

"We would not want this," he declares. "Itwould be too complex and section buildwould make systems heavier. We intend totransfer complete blades by ship from ourfacilities to the port nearest the final destina-tion, use special lorries to take them to the siteand new-generation cranes to erect them."

For a while at least, manufacturers maystrive to keep blade size below limits at whichthey can be transported whole.

RadicalOverall, it seems likely that no single factor willhalt size escalation, but that limits will beimposed by a combination of factors. Turbinesmight simply become too large and unwieldyto be viable economically. In this case lateralthinking may be required. A few experts believethat one way to circumvent the 'disproportion-ately high blade mass and excruciatingly slowrpm' of proposed mega turbines is to revert tosmaller rotors – but have several of them.

Selsam Innovations, for instance, has USpatents on an offshore system that would usemultiple rotors to 'improve the power curve'and escape the need for expensive, heavy drivetrains and gearboxes. Selsam is also targetingon-site infrastructure costs with a floating off-shore version. While these concepts may ornot prove to be part of the ultimate solution,Selsam has at least shown commendable readi-ness to 'think outside the box'. Other radicalthought may be needed if the commercial 20- 30MW wind turbine is ever to be achieved.

A2SEA A/S is preparing tomeet the demand for largeroffshore installations byadding a third, larger, vesselto its current fleet of two'crane ships with legs’. Photocourtesy: A2SEA A/S