Direct Drive for Economically Sustainable Wind Energy Figure 3: Superconductor Wire Pictured with Equivalent Copper Cable
Figure 4: Comparative Material Use for Equivalent Current Carrying Requirement
Permanent Magnets – An Interim Measure Looking more closely at some of the latest turbine designs to come to market, wind farm developers will see a common trend quietly emerging that speaks volumes about concerns manufacturers have about the reliability of gearboxes and the overall drive train. After all, why fix something that is not broken? This trend has been to replace traditional high-speed copper generators in favour of lower-speed permanent magnet (PM) generators. The advantage being that slow- turning PM generators enable the use of simpler and more robust gear-boxes. This improves reliability, reducing downtime and increasing FLHs. One example of this trend is the latest 3Mw machine from Vestas. Named the V112 and succeeding the 3Mw V90, the machine has a PM generator in place of the high-speed copper machine used in its (same rated) predecessor. Then there is the 3Mw machine from WinWind that uses a PM generator. There is also the 5Mw Multibrid machine, produced exclusively for offshore deployment, also powered by a PM generator. Clipper Wind’s evolving ‘Quantum’ drive is powered by no less than four PM generators. Although the list continues to grow, PM generators offer a limited solution and cannot be considered as a long-term solution to reliability. This is due to problems surrounding the weight of the generators, their degrading performance, difficulty in handling sizable PMs and the increasingly- restricted supply of rare earth materials from China.
The Electrical Approach
Figure 5: Superconductor Rotor Employing Superconducting Coils Supplied by Zenergy Power
Luckily there is another, more sophisticated approach to the problem of slow-spinning blades and high-speed generators. This is to take an electrical engineering approach to the problem as opposed to the mechanical engineer’s dependence on clunky, unreliable gearboxes. The outcome here is the use of multipole ‘direct-drive’ generators. Put simply, these generators are able to produce electricity while spinning at the same low speed as the wind turbine blades. This is achieved by adding more electrical ‘poles’ or copper coils into the generator. These extra poles enable the machine to generate torque at low speeds and efficiently convert the slow movement of the turbine blades into useable electricity. This eliminates the need for a gearbox and ultimately creates a far simpler design (in mechanical terms) with far fewer parts moving at far slower speeds (see Figure 2).
Image courtesy of Converteam SAS.
This approach has already been put to good use by German turbine manufacturer Enercon, which produces the most reliable but not necessarily the cheapest, direct-drive wind turbines on the market. In view of the economic impact of FLHs, the financial aspects of ‘cheap’ require further consideration. The attraction of the direct-drive approach recently received another significant endorsement when GE acquired the Norwegian direct-drive turbine manufacturer Scanwind as part of its well-publicised bid to improve its presence in the offshore market. More recent industry developments show a spreading of this trend and this year Siemens has followed suit by releasing its first direct-drive wind turbine – the 3Mw SWT-3.0-101. Therefore, it is safe to say that wind turbines powered by direct-drive generators with no gearbox requirements are going to play a major role in improving overall turbine reliability and financial return for wind developers. However, even within this technology there is a limitation. Owing to the additional copper poles used to generate the torque at low speeds, the generators needed to make direct-drive turbines very large and very heavy. Furthermore, the weight of direct-drive generators increases rapidly with increases in
88 MODERN ENERGY REVIEW – VOLUME 2 ISSUE 2
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