Energy Systems and Technologies for the Coming Century ...

produce the magnets. An exception to this is Japan, where there is also a considerablecapacity to process rare earth metals into permanent magnets. An additional possibility isinvestment in a new rare earth mining operation. While this helps the security of supplyissue, it is still likely that all produced materials will need to be sent to China formanufacturing.The alternative is to begin developing wind turbine generators with a reduced, oreliminated, requirement of rare earth metal. While there are several advantages to usingpermanent magnets, it may be possible to deliver high level solutions with somealternative topologies which will be presented in the following section of this paper.In either case, the experience of Toyota is interesting for other manufacturers that arebecoming reliant on rare earth materials. Toyota has taken multiple actions to mitigate itsneed to import rare earth material from China. First of all, Toyota has made a significantinvestment in exploration for rare earth elements in Vietnam. Should this exploration besuccessful, Toyota will have a secure supply, as well as the liberty of processing rareearth metals in whatever location they deem to be the most advantageous. Additionally,Toyota is developing induction motor technology which would maintain customerexpectations for performance, and also eliminate the need for rare earth materials in themotor [7], although lanthanum would still be required if they continue to use NiMHbatteries.3 Alternatives to DDPMThere is no shortage of alternative wind turbine generator topologies. After all, windturbines have been successfully used for several decades, and only recently havepermanent magnet generators become popular. The classic solution is a simple inductiongenerator, connected to the main shaft through a step-up gearbox. This is, however, atopology which operates at a fixed speed. It is possible to operate the wind turbine moreefficiently if the rotational speed can change as the wind speed changes and this lead tothe development of several additional popular solutions, including the doubly-fedinduction generator. Also, the gearboxes used to step-up the generator shaft speed wereobserved to be a significant source of failure. This led to the industry trend toward directdrive,or gearless wind turbines. Enercon has been developing direct-drive wind turbinessince 1992 [15], the longest of any wind turbine manufacturer. They have used, andcontinue to use, electrically-excited synchronous generators resulting in very heavygenerators, which in large scale are not suitable for offshore applications.The increased interest in offshore wind power has demanded a strong focus in windturbine reliability, and this is a big reason that many companies have moved to developdirect-drive wind turbines. With the low shaft speed of a direct-drive generator, thepreviously-used generators would not be sufficient. Therefore the development of ‘multipole’generators commenced. With the reduced prices of rare-earth materials, as well asthe discussed advantages of permanent magnet generators, the direct-drive permanentmagnet wind turbine generator has been adopted by many wind turbine manufacturers asthe industry standard in modern offshore wind turbines. However, it is worth noticingthat one of the major suppliers of offshore wind turbines, namely Vestas, has chosen toretain the gearbox in all their wind turbines in favour of the direct drive solution.Due to the aforementioned issues associated with the rare-earth market, the time is nowto develop a strategy to ensure a dependable supply chain. The decision as to whether ornot the supply chain should include permanent magnets needs to be carefully considered.If not, the generator topologies discussed in this section are ready for a closer look inregards to their viability for use in offshore wind turbines.3.1 Geared SolutionsWhile direct-drive generators have been gaining popularity, there is still plenty ofinterest in continuing to utilize geared wind turbine topologies. One advantage overdirect-drive is the generator size, which can be much smaller when a gearbox is used dueRisø International Energy Conference 2011 Proceedings Page 231