Wind Power Monthly: Windtech: Concepts to eliminate or reduce PMG use
Rare earth-free axial-flux generator
Another novel concept comes from UK-based GreenSpur Renewables, whose low-cost axial-flux generator concept has no rare-earth elements for its magnets. The company's generator design offers a magnetic field strength that provides the same overall generated energy as that of the 60KN/m2 range found in comparable PMGs containing rare-earth elements, according to GreenSpur director and inventor Hugh-Peter Kelly. Fundamentally, GreenSpur chose a multi-disk axial-flux generator instead of the common radial-flux generator (inner or outer rotor) design applied in most current multi-megawatt-class turbines.
After obtaining a grant for Supply Chain innovation for Offshore Renewable Energy (Score) through OrbisEnergy (UK) and private-investor equity, GreenSpur's design team researched and built a scaled prototype within nine months.
"The project focus was to model the use of cheaper ferrite (iron-based) magnets, and to determine from first principles if it would be possible to design a new topology that could deliver a competitive direct-drive PMG," says Kelly. "This worked out much better than anticipated, and the potential benefits are significant."
Ferrite can be obtained from iron ore and is abundantly available with no supply-chain restrictions or market monopolies, explains Kelly.
Measured by mass, ferrite-based magnets are also roughly 27 times cheaper than neodymium iron boron (NdFeB) magnets. As demand for NdFeB for direct-drive turbines is in the range of 600- 700kg/MW, this represents a considerable Capex share, especially for 6-7MW offshore turbines.
Additionally, overall demand for NdFeB-based magnets is increasing in established as well as new technology applications, such as electric vehicles.
"Future shortage of the 'heavy' rare earth Dysprosium, which is critical in the manufacture of NdFeB magnets to enable them to operate at high temperatures, is often not fully appreciated," says Kelly. "A WindPower Monthly, 31 May 2016 Dysprosium shortage or price hike could become a real problem in the ongoing transition to a 'greener' world economy."
GreenSpur's generator design comprises multiple rotor disks positioned on a central rotating support cylinder shaft, which is not sensitive to air-gap size and is still acceptable with 5mm or more, according to Kelly. These disks are individually spaced with a pre-determined air gap between the opposed inner faces of the ferrite magnets, in turn attached to each disk side except the end disks.
"Concentric rings of specially designed stator coils are sandwiched between, and by simply making efficient use of the rotor and stator areas available, Capex can be reduced and output boosted substantially," he adds.
The goal was to establish the strongest field intensity between facing poles, cutting as many productive turns of stator coils as possible, and field intensities increased as expected when using thicker magnets, but only up to a point. In order to establish the optimum configuration, a mathematician was engaged to model a movable software coil, while Cambridge University provided additional scientific validation with finite element analysis and electromagnetic validation.
The research, aimed at optimising magnetic paths, boosting flux density and reducing flux leakage losses, resulted in unusual and highly innovative electric machine characteristics. Kelly is unable to reveal the full details for IP reasons, but says: "The main breakthrough feature is a novel rearrangement of the rotor (ferrite) magnets. By magnetising and placing them in a new configuration the effect was an average field strength increase across the air gap of 22%, and a power output increase of 48%."
Greater output means higher heat dissipation demand, already a well known challenge in large-scale axial-flux generators. GreenSpur tackled this with a forced air cooling solution. "The principle is that air is forced out from radial orifices, and from there through the air gap to the outer surfaces. This design principle was also built into the demonstration prototype generator," says Kelly.
The potential to further boost output and efficiency from the demonstrator generator level to a commercialised large-scale product range is substantial, according to Kelly. The areas of optimisation include: coil design; coil placement; dimensioning of the disks and their internal spacing; magnet thickness; air gap; and minimising pole-switching cogging effects. "The demonstration turbine model outputs were used for a theoretical exercise and a 7.0-metre concept rotor with three stator coil rings. Results indicated over 1MW output can be achieved with an overall axial length of 200mm, or about 1.4 metres for a 7MW turbine," he concludes.