About: part 2 of a multi-part article looking at the development of offshore wind turbines as companies look to different approaches to improve reliability.
In part 2: the drive for fewer components
A leading weapon in the armoury of any reliability engineer is part count reduction. Quite simply, the fewer components there are, the lower is the chance of something going wrong.
For wind turbines, this philosophy meets its ultimate expression in the deletion from drivetrain designs of the gearbox. This cuts part count not just by one, the gearbox, but by the number of parts that the gearbox and its associated systems (cooling, lubrication, etc.) consist of. Direct connection of the wind rotor to the generator also deletes a number of shafts and bearings present in ‘standard’ systems.
In conventional onshore turbines, the gearbox is needed to convert the slow (typically 10-20 rpm) rotation of the rotor to a hundred times this or more (depending on the number of pole pairs) for a standard double-fed induction generator.
If, however, a different kind of generator is used, one in which many poles rotate around the stator rather than the standard few, this generator can work at a much lower rpm and, in the ultimate, can be driven directly by the wind rotor. The need for a step-up gearbox is therefore avoided.
This direct drive solution is attracting a growing following and turbine manufacturers Multibrid, Siemens, Alstom, Enercon, MTOI, GE Energy and Nordex have all developed or are currently working on machines with this topology.
But there is no consensus that it is the best solution. Detractors argue that the generator rotor, because multiple magnetic poles must rotate past the stator windings at significant speed to develop the required current, has a much larger diameter than that in a conventional drive – typically some 5.5 m in a 3.6 MW machine for example. This in turn requires a nacelle that is more voluminous and therefore has greater windage. Proponents counter this by pointing to the simplification of the nacelle system and its shorter length, as well as the reliability advantage of having no gearbox.
Henrik Stiesdal, Chief Technology Officer of Siemens Energy's wind power business unit is clear about the advantages: “We have developed our SWT-6.0-120 wind turbine specifically for the offshore projects of the future. Our direct drive technology offers a smart, straightforward design that minimises the number of moving parts. We expect that this 6 MW machine will set new standards for performance, robustness and maintenance optimised for the harsh conditions offshore.”
Further simplification can be achieved by using permanent magnets in the generator rotor rather than the wire-wound electromagnetic poles used in traditional generators. The absence of copper windings reduces weight and cost, while permanent magnet generators (PMGs) are in principle more efficient because there is no requirement for electrical power to create the magnetic field.
A disadvantage of the PMG solution, though, is that the power converter needed to condition the output power to the parameters required for grid connection, must manage all of the generator's power output rather than just part of it as with a conventional system. Sizeable cabinets for the power converter electronics and transformer are needed. These may be located in the nacelle or in the support tower.
Another issue is the use of magnets with the rare-earth metal neodymium. Though attractive for its high magnetic flux, this material is expensive to extract because the process is energy intensive, and its supply is concentrated mainly in China. There are therefore concerns over pricing and on-going availability. (Note that use of PMGs is not limited to direct drive turbines and is a feature of some conventionally geared and hybrid machines also.)
Consensus about which technology to use is lacking. Enercon, for example, has avoided the downside of PMGs by utilising electromagnetic pole shoes in direct drive machines. Siemens, on the other hand, has opted for a synchronous generator equipped with permanent magnets in its 6 MW offshore turbine, having previously trialled this approach in two 3.6 MW prototypes.
These reportedly proved superior to conventional counterparts in terms of power, vibration, temperature, noise and reliability. Prototypes for the new 6 MW turbine are now installed and operating, while 3 MW direct drive machines using similar technology have been selected for a number of wind farms.
Alstom is using PMGs engineered by power conversion specialist Converteam in its 6 MW Haliade offshore turbine. Converteam, now majority owned by GE and re-branded Power Conversion, is supplying its advanced high density direct drive PMG, which it claims is lighter and more compact than earlier generation systems.
Meanwhile, GE Energy's new 4 MW machine, announced in March 2011 at the EWEA's offshore wind conference in Amsterdam, has been designed specifically for the offshore sector. This turbine is based on direct drive PMG technology first developed by Norwegian company ScanWind, which was acquired by GE in 2009.
The ScanWind solution is said to result in high availability (for which reliability is a necessary element) and to ease low wind start-ups. A prototype of the 4 MW machine is now operating in southern Sweden.
Part 3 out soon. Further reading - Generation Innovation for wind turbines
About George Marsh: Engineering roles in high-vacuum physics, electronics, flight testing and radar led George Marsh, via technology PR, to technology journalism. He is a regular contributor to Renewable Energy Focus.