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IN DEPTH: Hybrid wind drivetrains

Traditional wind turbine gearboxes are notorious for going wrong. Gearless direct-drives are heavy and expensive.

Neither is seen as the perfect solution for the next generation of long-bladed machines being designed for installation in stormy, salt-laden winds as far as 300km from shore.

What the industry needs — according to several drivetrain designers — is a hybrid of the two.

By grafting together the best aspects of geared and direct-drive technologies, hybrid medium-speed drivetrains promise to be smaller, lighter, more reliable and more efficient than their pure-play counterparts in the 3-10MW class.

And not only would these hybrids be cheaper, but they would ratchet up output while cutting installation and maintenance costs — and so shrink the overall cost of energy (CoE).

“We believe medium-speed will be the winning offshore technology in the longer term,” says Mikael Laine, chief executive of Finnish wind turbine drivetrain maker Moventas. “Our FusionDrive will be the flagship of our future offering. All the high-speed [gearbox] components that the industry knows are prone to failure have been taken out for this design, and this will mean the highest availability, which in turn means high annual production output and minimum total life-cycle cost.”

Moventas’ FusionDrive, hatched through a strategic partnership with compatriot permanent-magnet generator (PMG) developer The Switch, tips the scales at a featherweight 34 tonnes — less than half the weight of pure direct-drive systems. The compact 3MW model shortens the drivetrain of conventional geared concepts by half, to three metres, by using a two-stage, planetary gearbox — the reliable low- and medium-speed stages — while its diameter of 2.3 metres is less than half the direct-drive’s average six-metre girth. And it handles torque of 3,000kNm while maintaining a 96% operating efficiency.

“Medium-speed systems are very much in discussion now as a reality, not just a theory, because the removal of the high-speed shaft leads — as several designers, including us, are proving — to so many big advantages on output and reliability, and so to CoE,” says Laine.

Matched against a turbine with a geared, double-fed induction generator, Moventas calculates the FusionDrive could trim an onshore machine’s CoE by at least 6% through higher yields, lower capital fabrication costs and improved serviceability — with CoE savings estimated to be higher for offshore models.

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Arto Lahtela, Moventas’ senior vice-president of wind gears, adds: “We expect there will be room for many different drivetrain designs in the future: high-speed for onshore, direct-drive for some offshore; but we expect the medium-speed will win the lion’s share offshore.”

Tests of the FusionDrive system at Moventas’ plant in Jyväskylä, central Finland, were exhaustive. Following a lengthy programme of individual component checks, the complete 3MW drivetrain was put through a battery of operational trials, including long stretches in a cold-climate chamber chilled to -40°C, before the thermostat was cranked up to a sweltering 60°C.

Factory validation of the drivetrain was wrapped up earlier this year, clearing the way for delivery of two units for installation in a pair of DSME-DeWind D12 prototypes, which are to be erected in 2014 for final field tests either in Korea or Europe as part of a “beacon project” by the Korean turbine maker.

Laine notes that the 3MW FusionDrive has the “strength for all onshore wind classes” in machines with rotor diameters ranging from 100-140 metres and outputs of 3-4.4MW — to make the most of the industry’s current expansion into low-wind-speed projects.

For the burgeoning 6MW-plus offshore market, 7MW and 8MW models of the FusionDrive are ready to come off the drawing board, with Moventas in “detailed discussions with several OEMs” for a first order. 

“The FusionDrive was designed as a platform that can very easily be scaled up — all the features are standardised and scaleable. This is the philosophy behind it,” says Laine.

The 7MW FusionDrive will keep to the streamlined ethos of its smaller stablemate, with an overall weight of 85-95 tonnes, length of 4-4.5 metres and diameter of 2.8 metres. It will be able to cope with a gut-wrenching 7,000kNm of torque while pumping out up to 7.7MW of power, again with an efficiency of 96%.

Moventas has made a virtue of necessity when it comes to drivetrains’ serviceability — a thornier issue offshore than onshore because of the cost of mobilising engineers to locations far from land. By using a modular design where gears, generator, shaft, bearings, rotor and other parts can be dismantled “in nacelle” for repair and maintenance by two men without an external crane, the FusionDrive saves an estimated 15% on operating expenditure (opex) over an onshore turbine’s lifetime. This figure is higher offshore, where opex accounts for a quarter of a wind farm’s total running costs.

“Onshore, replacing a drivetrain can cost up to €300,000 [$414,000],” states Lahtela. “Offshore, it is a much larger multiple of this — we have to use jack-up vessels. In-nacelle maintenance is central to bringing down the CoE of offshore wind.”

Laine adds: “We would expect to see a 7MW or 8MW FusionDrive installed in an offshore turbine in the next couple of years.

“Growth, in the next ten to 15 years, will be coming mainly from the offshore market. But it will require innovation, especially in core technologies, to make offshore cost-efficient, to lower the CoE.”

Winergy

Moventas is not alone in its belief in a hybridised future for drivetrains. Germany’s Winergy has two of its 3MW HybridDrive systems out in the field. One is being tested near Rostock, northeast Germany, housed in a turbine prototype built by now-defunct OEM Fuhrländer and developer Wind2Energy, with another put into service for Iran’s Mapna group on a wind farm in Ukraine.

Winergy, a free-wheeling subsidiary of industrial giant Siemens, has brought to market a mid-range hybrid transmission system, based around a two-stage medium-speed gearbox integrated with a PMG.

“The original idea came from our industrial drives business, where we were designing gearbox and motor as one product or in one housing for 4MW cement mills,” states Winergy head of strategy Felix Henseler. “In these systems we saw very good results in terms of efficiencies and weight in a compact system, and this led us to design compact geared systems for wind turbines to reduce top-head mass as a competitor to heavy direct-drives.

“From this, it was a short step to make a system that combined a shortened gearbox with a generator.”

The prototype HybridDrive, 3.5 metres long and weighing 34 tonnes, is fashioned to be “flexibly integrated in any existing nacelle configuration”. Freed-up space can be used to bolt in converters and transformers, in order to cut low-voltage cable losses, or instead allow for a smaller, more compact, lower-cost nacelle, with the electronics package fitted in the foot of the tower.

Modular componentry means lightweight individual elements can be quickly removed or installed, using an onboard crane.

“We put a lot of requirements for serviceability into the HybridDrive — one product but based on modularised components that would be easily disassembled and reassembled,” says Henseler.

Testing was typically punishing. The HybridDrive prototype was put through its paces in a 600-hour accelerated lifetime test, including a 200-hour spell under 180% loads and torque of up to 2,500kNm. It came through these exertions with flying colours, showing 96.5% efficiency for the combined system.

Winergy has a target of rolling out a further ten 3MW HybridDrives in 2014, though it has no firm orders yet. Plans are to scale up the design for turbines up to 7.5MW “in the coming years”.

“We are developing the HybridDrive without distinguishing between onshore and offshore, low wind speed and high wind speed — we have set one standard, with small alterations to the lubrication system or nacelle covers, for instance,” says Henseler.

“We expected the system to reach around 95% efficiency and on the test bench it was over 96%. It would be hard to improve on this in the field without some sort of perpetual-motion machine.”

Romax

UK drivetrain designer Romax, meanwhile, is building momentum on commercialising its medium-speed Butterfly platform concept for wind turbines of up to 10MW, with the company closing in on installing a first 3MW unit for prototype trials.

The Butterfly is a module-based platform that can mix and match components to build tailored transmission systems with a range of features, including two styles of main-bearing arrangement, any number of generator types and multiple mainframe configurations.

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The platform promises to knit together rotor-shaft assembly, semi-integrated gearbox and generator — to shorten the drivetrain and eliminate poor alignment, bearings, couplings, and braking and lubrication systems.

Current work on the design, initially sized for 6MW machines but scaleable up or down, is focused on the electromechanical interactions between the gearbox and the generator — critical given the deeper integration of the two systems in a hybrid.

“Traditional design methods generally treat the gearbox and generator as separate items bolted together but we strongly advocate the system-modelling approach, where the interactive behaviour of the two elements is looked at together in a single environment,” says Butterfly platform leader Gary Johnstone.

“This feeds into the concept’s scaleability. [With an integrated architecture] you avoid the problems presented by most drivetrains, where as they get bigger and more powerful, the manufacturing changes, the assembly changes, the operation and maintenance changes, meaning retraining, new equipment and so on, and this throws up all sorts of variables.”

Butterfly drivetrains are based on a three-module set-up — incorporating rotor assembly, gearbox and generator — in a two-stage planetary system within a unit ranging from 6.3-11 metres long, 3.5-4.5 metres in diameter, and weighing 90-190 tonnes, with rated input torque of 5,700-12,600kNm.

“The real beauty of medium-speed is increased reliability and efficiency with high output: you’ve got a simpler gearbox and got pure [load] paths and better alignment between gearbox and generator,” says Johnstone.

Preparations for  running a 6MW prototype on the new 15MW test rig at the New and Renewable Energy Centre in Blyth, northeast England, are ongoing, he adds, with Romax ready to build a first unit for accelerated lifetime tests “as soon as a first customer is secured, which looks like being before March 2014”.

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