Ferrofluid-gaps for direct-drive wind turbine generators

Abstract

The commercial offering of offshore wind turbines has generally converged upon three-blade horizontal-axis designs with variable speed generators and pitch-regulated blades. Arguably, the most significant distinguishing feature is whether the powertrain has a direct-drive or geared configuration.

In a direct-drive powertrain, there is no gearbox that steps up the rotational speed and steps down the torque from the turbine rotor for input into the generator. This low-speed and high-power operation necessitates the production of immense electromagnetic torque: on the order of 20 MNm for a 15 MW wind turbine. As a result, direct-drive wind turbine generators are large, heavy, and require a substantial amount of rare earth permanent magnet material in their construction: on the order of 20 tonnes for a 15 MW wind turbine. Rare earth permanent magnets are expensive and environmentally damaging to procure, designers typically attempt to make the air-gap clearance of direct-drive wind turbine generators as small as practicably possible so that the air-gap magnetic reluctance is minimised: reducing the amount of permanent magnet material required.

This thesis investigates an unconventional approach to reducing the permanent magnet material requirements of direct-drive wind turbine generators in which the air-gap region is flooded with magnetically permeable ferrofluid such that it becomes a ‘ferrofluid-gap’.

The initial chapters of this thesis examine the design of multi-megawatt scale direct-drive wind turbine generators. A detailed design procedure is specified which is supported by an advanced thermal modelling methodology. A review of air-gap dimensioning is provided, and insights are presented that improve understanding in this area.

Subsequent chapters focus on the ferrofluid-gap concept. First, it is explained how the magnetic and mechanical behaviour of ferrofluid-gaps can be modelled according to theoretical predictions. Prior work regarding the ferrofluid-gap concept is reviewed and the opportunities for further research are highlighted.

To validate the theoretical models of ferrofluid-gap magnetic and mechanical behaviour, a novel ferrofluid-gap test rig was designed, fabricated, and tested. The test rig dimensions were scaled such that the fluid dynamic conditions expected in multi-megawatt scale direct-drive wind turbine generators could be achieved at a laboratory scale. The resulting experimental data provides unprecedented validation of the theoretically predicted magnetic and mechanical behaviour of ferrofluid-gaps in radial-flux electrical machines.

Drawing upon the analytical models, design tools, and experimental results developed throughout this work, the feasibility of ferrofluid-gaps for direct-drive wind turbine generators is assessed. It is found that, in principle, ferrofluid-gaps can enable a reduction in permanent magnet material (~10%) without unacceptably compromising efficiency due to drag losses. It is also demonstrated that a ferrofluid-gap has the potential to enhance thermal performance. However, it can be concluded that the potential benefits of a ferrofluid-gap are outweighed by the costs and difficulties associated with their implementation in multi-megawatt scale direct-drive wind turbine generators.