Abstract Body

Introduction: In the drive to meet net-zero targets EU based wind turbine manufacturers such as Siemens Gamesa, Vestas and GE Energy are building turbines with ratings in excess of 15 MW. In China Mingyang has built a 22 MW demonstrator, pushing the EU manufacturers to follow suit. A low speed permanent magnet synchronous generator (PMSG), either direct drive or coupled to a 2-stage gearbox, is used. Due to low speed these generators are very heavy. According to the IEA 15 MW Standard Design, the PMSG used weighs in the region of 372 tons, with the active iron weighing 182 tons [1]. It is very challenging to transport and install such heavy generators as part of the nacelle structure at the top of a tower in an offshore wind turbine. For floating offshore wind turbines such a nacelle mass is challenging for maintaining stability during installation and operation, adding significantly to the cost. The active iron can be reduced by using air-cored copper windings, but this reduces the airgap magnetic field requiring more permanent magnet material, which is more expensive than iron. Edinburgh has demonstrated such a machine, the so-called CGEN modular generator [2], which has been successfully applied to small wind, wave and tidal systems. In [3] the authors have shown how the axial flux multi-stage CGEN air-cored concept has evolved into an HTS machine with all the active iron being eliminated. Each permanent magnet is replaced by a superconducting coil forming an array of air-cored coils as shown in Figure 1. Zhang et al showed that if such an array of coils replaced the permanent magnet structure in the CGEN machine the power per volume could increase by a factor of 4, whilst retaining the conventional air-cored stationary copper winding. However, it was a conceptual design that involved only preliminary numerical modelling analysis. As a follow up, a ten-year project SuperMachine funded by the Royal Academy of Engineering Chair in Emerging Technologies program was awarded to Mueller for development of this technology. In this paper the authors will describe the engineering design, build and test of a prototype Halbach HTS array module, in order to verify the concept.

Method – The main objective of the proof of concept is to show that the coil array is able to produce a magnetic field distribution similar to the permanent magnet module of a CGEN pole pair module, and to verify the design and modelling techniques used.  The sizing of the proof of concept module is based on the volume of the cryostat available within the Edinburgh HTS lab. A 2-stage module is therefore proposed so that it can be accommodated within the cryostat, with dimensions shown in Figure 2. AMSC Type 8502, 12 mm, YBCO tape will be used to wind the coils, 8 in total. Testing is undertaken in 2 stages: (i) Test in a LN2 bath; and (ii) Test in the Edinburgh cryostat. The first test allows rapid testing at lower current levels to verify the models, and to check that all coils are working. In the second test we will design a conduction cooling system for the cryostat to allow testing at lower temperatures, 30 K, so that the array can be tested at higher currents. The conduction cooling design will be used to design a custom modular cryostat for building a complete machine in a future project. The coils and cooling system have been designed, and manufacture is taking place. Results of the experimental work will be presented at the conference.

Results – In order to design the Halbach array module an understanding of the electromagnetic performance is required to calculate the coil forces acting so that the module structural support can be built. COMSOL multiphysics was used to evaluate the electromagnetic, mechanical, and thermal characteristics of the demonstrator The magnetic field distribution is shown in Figure 3(a) and the resulting forces acting on the stator coils is presented in Figure 3(b). With these forces a structural support has been designed as shown in Figure 4, in which the HTS coils can be moved to investigate the magnetic field distribution for different coil positions. The coils and cooling system have been designed, and manufacture is taking place. Results of the experimental work will be presented at the conference to prove the air cored HTS Halbach array concept, providing confidence to design complete machines using such modules as the basis.

References

[1] Definition of the IEA Wind 15-Megawatt Offshore Reference Wind Turbine, IEA Wind TCP Task 37, March 2020

[2] A. McDonald et al. “1MW Multi-stage air cored permanent magnet generator for wind turbines”,  6th  IET International Conference on Power Electronics, Machines & Drives, Bristol, April 2012.

[3] H. Zhang et al, "High temperature superconducting Halbach Array topology for air-cored electrical machines," J. Phys.: Conf. Ser., vol. 1559, 012140, 2020

Acknowledgment

This work has been funded by the Royal Academy of Engineering Chair in Emerging Technologies project awarded to Prof. Mueller.