Less need of Chinese rare-earths with large diameter direct drive

Transcription

1 Rev Less need of Chinese rare-earths with large diameter direct drive China now controls 97 % of the supplies of rare-earth metals such as the neodymium vital for permanent magnets used in direct drive generators. It has recently introduced measures to restrict the resource to its own industry. Although neodymium is not that rare in the earth crust, the Chinese policy will create less supply and increasing prices. Hence it is vital to use the amounts available as economically as possible. Here Staffan Engstrom demonstrates how large diameter direct drive generators such as the NewGen may alleviate the situation. Since some years there is a Chinese policy to gain control of the mining of rare-earth metals, in most applications used in small although vital quantities. By means of domestic resources, buying of foreign mines and a low price policy this now has resulted in control of 97 % of the currently available extraction on earth. Recently China adopted restrictions to exports, thus effectively reserving the resources for its own industry. The rare-earth metal neodymium amounts to almost a third weight-wise of the neodymiumiron-boron alloy used in permanent magnets with outstanding properties. Applications creating need of large and increasing volumes of such magnets are motors for electrical cars and direct drive generators for wind turbines. The 17 rare-earth metals generally appear together in ores found in many areas of the world, such as Inner Mongolia in China, South Africa, California, Estonia and Greenland. Due to China s low-price policy only mines in Estonia and those controlled by China are today in operation. It takes some years to open a new mine. The today closed Mountain Pass mine in California is expected to open already in Even if the currently restricted supply will alleviate in some year s time, extraction costs and increasing demand will increase the prices of high performance magnets. The much-increased use of neodym magnets will probably raise prices also inside China. Thus it is important to use the material in an economical way. Magnet needs decreases with generator diameter In a direct drive generator the function of the otherwise used gearbox is substituted by more electrically and magnetically active material and by larger dimensions of the generator. The active material consists of the copper windings and the iron sheets of the stator, and of the permanent magnets of the rotor (in older designs electric magnetisation is used). Since the amount of active material is inversely proportional to the diameter at the air gap between the rotor and the stator, it is possible to decrease the amount by increasing the diameter. On the other hand, in conventional designs the mechanical structure then gets very heavy. This is due to the long load paths between the central bearings and the air gap, in combination with a need to keep the structure very rigid in order to maintain the air gap at a few millimetres. NewGen minimizes rare-earth use. Rev 1

2 The NewGen-concept was elaborated in order to obtain a more economical generator design. It implies that the generator rotor is supported by steel wheels around the periphery of the stator of the generator. This is a radical way to reduce the load paths and reduces the generator weight to at least half of that of today s typical direct drive PM generators. The reduction is even larger in comparison with generators with electrical excitation (Enercon). In a practical sense it means that railway technology is introduced in generator design, since such is based on steel wheels that roll on a rail of steel. VG-Power in Vasteras, Sweden (ex ABB Generation), normally supplying large hydro generators, designed and built a small (2 m diameter, 160 kw) generator in order to verify the technical solution of the NewGen. It works quite as foreseen. A 3 MW project in cooperation with the wind turbine manufacturer Scanwind in Norway was supported by both the EU and by the Swedish Energy Administration. Vattenfall decided to purchase the first installation. Economical reasons however forced Scanwind to withdraw and the project was not possible to finalize. Magnet use cut to a third In order to explore the potential of the NewGen concept to minimize the need of rare-earth magnets, the designs of NewGen type generators were outlined with either an air gap diameter as in a conventional permanent magnet direct drive generator or with as large an air gap as is possible to realise with the NewGen design. The results are presented in Table 1. It appears that when using the potential of the NewGen concept for a large diameter generator, the amount of magnets may be reduced to as little as a third of that of a small diameter direct drive. The stator iron will be reduced likewise, whereas the amount of copper in the windings is slightly less reduced, due to the influence of the end windings. For this rather extreme generator, the amount of mechanical structure however increases slightly. The net result is a considerable reduction of material, and especially of the expensive active material. For a generator for a certain power level, RPM and air gap diameter, the amount of active material depends on parameters such as type of rectifier, magnet quality and generator efficiency. When scaling this generator for other levels of power, RPM or in this case air gap diameter, the amounts of active material will however vary as in the table. Since the NewGen electrically works as any permanent magnet direct drive generator, these results are generally applicable Table 1. Outline of data for 3 MW direct drive generators of NewGen type. Small diameter Large diameter RPM 12,4 Reduction Wind turbine diam, m 110 Tip speed, m/s 71 Torque, knm 2310 Air gap diam, m 3,9 12,0 Length, m 2,4 0,5 Magnets, kg % Stator iron, kg % Copper, kg % Sum active components, kg % Structure, kg % Total incl misc. 20 % % kg/knm 24,6 17,2 30% 2

3 Other features for cost saving An outline of a wind turbine using the NewGen generator is depicted in Fig. 1. The design implies that the load carrying structure of the inner section of each blade is constituted by a cylinder of welded steel plate, similar to a tower section. The inner end is flanged to the turbine hub and the outer end is connected to the blade pitch bearing and its pitch mechanism. Producing the inner section in steel saves cost although not weight. The torque of the turbine is transferred directly to the outside rotor of the generator, thus saving weight in the inner parts of the structure and of the hub. The outer parts of the turbine blades are conventionally designed in GRP. Putting the pitch bearing at some distance from the rotor axis reduces the moment acting on the bearing and its pitch mechanism, hence also saves cost. The design presents an elegant solution to the problem of producing the turbine blade in sections not too long for transportation. Larger production The large nacelle, which follows with the use of a NewGen generator, may be considered as a drawback, but rather is an asset. The tradition is to end the aerodynamically active part of the turbine blade inwards at 10 % of the blade radius, since the area inside geometrically only accounts to 1 % of the turbine disc, which is considered reasonable to sacrifice. However the losses through this hole may be much larger according to one source as much as 10 %. By letting the active airfoil extend all the way towards the spinner it is possible to utilize also this amount of energy. Among the manufacturers of large wind turbines it is today only Enercon that has introduced this design, which is easier to accomplish if the inner part of the turbine blade is fixed as in the proposed design. Conclusions Chinese export restrictions and increased use will raise prices for high performance permanent magnets in the years coming. By building direct drive generators with large diameters the specific use may be cut to as little as a third. NewGen presents a practical solution for a large diameter direct drive generator. It also offers other benefits regarding wind turbine cost, efficiency and transportation. Staffan Engstrom Further reading S. Engstrom. NewGen. A New type of Direct-Drive Generator. Windtech International January/February 2008 S. Engstrom. The NewGen Generator. How Direct Drive helps in the Transportation of Large Wind Turbines. Windtech International March/April US fears Chinese stronghold on rare metals will tighten. NewsAnalysis. Windpower Monthly. November Biography of the Author Staffan Engstrom, MSc, Mech. Eng., has been working in wind power since 1975, initially at the National Swedish Board for Energy Source Development (NE) and the National Swedish Energy Administration (Statens Energiverk). He has also been active in Nordic Windpower. Today he runs his own consultancy in Agir Konsult AB. He is holder of several patents. 3

4 Affiliation Staffan Engstrom Ägir Konsult AB Drottvagen Lidingo Sweden agir@engstrom.st 4

5 Figure 1. The nacelle of a 3 MW wind turbine with a NewGen generator. 5

6 Figure 2. The principle structure of a NewGen generator. The steel wheels are attached to the stator of the generator (green) and carries the outer rotor of the generator (blue) with its PM magnets (violet). The stator windings are red. Figure 3. The NewGen generator in its test stand. 6

7 . Figure 4. 3 MW wind turbine with control of the outer parts of the turbine blades. 7