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In Depth: Romax wind drivetrain aims at platform for flexibility

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

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.

Flexibility and speed have been the watchwords in developing Butterfly. Having a template to tailor different transmission-system layouts via a platform open to multiple component and sub-assembly contractors can potentially shorten the time to market for innovative, lower-risk turbine designs. That, in turn, aims to provide closer ties between turbine designers and manufacturers and their technology supply chain.

Initially sized for 6MW machines but scaleable around the same "core layout" down to 3MW and up to 10MW, the concept is designed to aid development of large offshore wind turbines around an unchanging drivetrain architecture.

"For a turbine designer, Butterfly should make it quicker to get a design to market; for a turbine developer, it should mean a more consistent O&M [operations and maintenance] strategy, so when you go from 6MW to 8MW to 10MW, you don't have to tear everything up every time - you assemble it the same way, you maintain it the same way, it's the same fundamental architecture," explains platform team leader Gary Johnstone.

"What has been of the utmost importance to us is that the technology we come up with does not quickly become redundant - it needs to be adaptive, to be around for the long term."

Romax's approach is to look past capital expenditure to the question of the lifetime cost of a transmission system, factoring in "reliability and maintenance cost projections as well as performance". The company, based in Nottingham, central England, has a pedigree in gearboxes for the automotive and aerospace industries, and has developed 29 drivetrain designs for wind turbines.

The Butterfly came out of a vetting of "hundreds of different [drivetrain] layouts", which Romax Technology whittled down to several designs it then ran through in-house cost of energy (CoE) modeling software "to make sure we got the selection right in terms of [operational expenditure] too", says Johnstone.

"We feel, having looked at high-speed, medium-speed and direct-drive architectures, that medium-speed drivetrains will offer the lowest CoE for the next generation of designs," he adds.

Butterfly drivetrains will be 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 5,700-12,600 kilonewton metres.

The Butterfly aims to strike a balance between "complete modularity, with every component mounted on bed plate" and heavy integration - "where the gearbox and generator are almost one machine".

"It is three separate modules [rotor assembly, gearbox and generator] that can be replaced individually, but we have designed it as a complete system to ensure proper load distribution and alignment of components," says Johnstone. Engineers also paid "very close attention" to bearings and lubrication systems, particularly in start-up/shut-down and idling conditions.

"The most modular designs often have problems in how the pieces move and fit together, while the most integrated designs make for attractively lightweight drivetrains but have trade-offs such as maintenance challenges - or the reality that if you have a failure somewhere in the drivetrain you have to replace the whole unit."

Engineered to be "a little bigger and a little heavier than the most integrated designs but without the operational headaches of the more modularised ones", the Butterfly platform is certified for a 25-year design life, with diagnostics covered by Romax's InSight predictive maintenance software. "In the past, condition monitoring might have been something of an afterthought. Now it is central to every design."

Currently in discussions with "quite a number" of potential customers, Romax foresees finishing detailed design of the platform for a 6MW drivetrain in the coming year, with a prototype built in 2014 for bench-testing, and a flagship turbine to follow.