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Ahmad HemamiDirect-drive wind turbines using a permanent magnet synchronous generator have been developed after observing the problems with the gearbox. Nonetheless, nothing is 100% perfect. Obviously, during recent decades there have been efforts to advance the technology, in all directions. That includes improving the performance of the gearbox. One advantage in removing the gearbox from the loop is a reduction in the tower top weight, but this is not always possible. In a recent article by van de Kaa et al. (2020), the subject of the comparison of direct-drive turbines and geared turbines has been systematically studied to determine which technology will become the dominant one in the future. This implies that in the future the other technology needs to switch. The final conclusion of this study is that cost of energy and reliability are the two most important factors, and both technologies still have equal chance of success.
By Ahmad Hemami, McGill University, Montreal, Canada
Undoubtedly all the major players in the field have great experience in what they are doing and have confidence in their products. They have gained the experience required to enhance the reliability, as per customer expectations. Among the top players, Vestas has continued with the traditional way of using the gearbox, whereas Siemens Gamesa, Enercon and Goldwind have adopted the other way.
On the other hand, discrepancies exist. First, unexpected things can happen and, second, we do not know what the status in the future will be. The literature on direct-drive wind turbines focuses on: 1) removing the troublesome gearbox, 2) reducing the nacelle weight, 3) increasing reliability by reducing the number of components (it is claimed that there are ca 1,000 pieces in a wind turbine gearbox), and 4) decreasing the downtime (if a gearbox fails, its replacement/repair takes a long time, while if a power converter fails its repair takes a much shorter time). Although no data is readily available, it seems that the failure of power converters is not less frequent than the failure of the gearboxes. There is also mention of the gearbox efficiency (losing 3 to 4% of the generated power in the gearbox). For this, I suppose we have the same loss in the power converters.
Two issues that are noticeable here and need more attention are: 1) the cost of energy, as referred to in the study, and 2) the permanent magnet generators.
1. The cost of energy: how much is it?
If electricity produced by wind is cheaper than electricity generated by oil (or coal, gas, etc.), it is still acceptable. This implies that the cost of energy depends on the cost of oil (for example). How much is the cost of oil, really? For the consumer, it is what the oil companies charge. If there is a war somewhere, they charge more! Which price is correct?
There are a lot of issues here related to price. One issue is embedded in the question ‘Why is almost everything made in China?’ Why did the manufacturers of ‘….’ take the manufacturing to China? The world has become unbalanced. So, what is the price (or cost) of energy?
2. Permanent magnet generator
A permanent magnet generator, compared with the doubly-fed induction generator, does not have brushes and slip rings; thus, it needs less maintenance work, and from this viewpoint it is more suitable for costly repair/maintenance works at sea. But the weight of a permanent magnet synchronous generator (PMSG) is much more than that of an electrically excited synchronous generator (EESG) because permanent magnets must provide the required force for the necessary torque between the rotor and stator. In particular, for a given power, if the rotational speed is low (which is the case in direct-drive wind turbines), the torque is high. The copper current carrying wires of an EESG inevitably consume some energy and generate unwanted heat. These are two drawbacks, despite the advantage of the ability to control the magnetic field. When substituted by permanent magnets in a PMSG, the advantages and disadvantages of the EESG disappear. The price to be paid lies in the increase in both the weight and the cost.
There are a few types of PMSGs, and more research and development can make them better and more efficient. Something that is common to all PMSGs is the category of the magnets used in them. There are rare-earth (RE) and non-rare-earth (NRE) magnets. In a comparison between these two for a 10kW flux reversal generator (FRG), Bharathi et al. (2022) indicate that for a rotational speed of 375rpm, the average efficiency of the generators with RE magnets and NRE magnets is similar, but the torque density of NRE-FRG is only 51% that of RE-FRG. The NRE-FRG design is heavier, with the total active mass being 2.6 times higher than that of the RE-FRG. If these significant differences apply also to larger generators, there is no doubt why the RE magnets are preferred.
To justify the price difference, one thought is that for a direct-drive turbine we pay a higher price upfront, whereas for a geared turbine we pay later. However, the catch is probably more serious. If a country has a monopoly on RE magnets, then the future status of the turbines using them is uncertain. Who knew 10 years ago about COVID-19 and its disruption of almost everything? And who knows what will happen 10 years from now with regard to a monopoly on RE material? Although there is some good news about a possible replacement for RE magnets, this is at an early research stage right now and its future is yet to be seen.
Further Reading
van de Kaa, G., van Ek, M., Kamp, L.M. and Rezaei, J. 2020. Wind turbine technology battles: Gearbox versus direct drive - opening up the black box of technology characteristics. Technological Forecasting and Social Change 153, 119933.
Bharathi, M., Akuru, U.B. and Kumar, M.K. 2022. Comparative design and performance analysis of 10 kW rare-earth and non-rare earth flux reversal wind generators. Energies 15, 636.
Potential rare earth magnet replacement has been discovered. 25 October 2022.
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