The first company to develop a credible ferrite-based direct drive permanent magnet generator for wind turbines believes the concept could enter production in as little as three years – depending on where commercialisation takes place
Late 2018/early 2019 will see a UK firm test a new, larger version of the direct drive permanent magnet generator (DD PMG) it has developed as it searches for investors – and a turbine manufacturer – with which to work on the industrialisation of the machine.
Essex firm GreenSpur Renewables hopes to eliminate costly rare-earth magnets from direct drive turbines and is developing a new type of generator that could help reduce the cost of wind energy. It believes its innovative technology is capable of delivering a generator capex reduction of 33% compared to current designs.
Direct drive generators are an increasingly widespread feature of offshore wind turbines as operators shy away from gearboxes, however the generators’ need for rare-earth magnets – which are mainly produced in China – means they are an expensive alternative, with the market effectively subject to a monopoly. This is something OEMS have already recognised, with Siemens Gamesa highlighting the fact that the permanent magnet generator in its latest offshore turbine operates a lower temperature than earlier models, reducing the amount of rare-earth material needed in the new unit.
Rare-earths will be required for a long time to come, however, and with help from the Offshore Renewable Energy Catapult (ORE Catapult) and funding from Innovate UK, GreenSpur Renewables is currently building a 250-kW demonstrator that is rare-earth free that will be tested at the ORE Catapult on its test rig. The 250-kW machine is due to be completed and ready for tests in Q4 2018.
After forming a working relationship, the company tested a scale 75-kW generator at ORE Catapult’s test rig in Blyth in Q4 last year. They then signed an agreement with ORE Catapult and Warwick Manufacturing Group to build the 250-kW direct drive generator, with the support of a £1.25M (US$1.65M) grant award from Innovate UK.
Working with the ORE Catapult, GreenSpur Renewables is hoping to develop a UK-based supply chain and plans to establish a manufacturing operation within three years. Scaling up short-term production in the UK could create hundreds of new jobs. If the technology is deployed at the 2 GW to 3 GW scale it could lead to the creation of over 3,000 new UK jobs, it is claimed.
As Andrew Hine, executive director at the British company told the Global Offshore Wind 2018 conference in Manchester in June, apart from direct drive generators relying on rare-earth magnets produced mainly in one country, there is also competition for magnet supply from other fast-growing industries, including defence, computing and electric vehicles. Added to this, extracting and processing these metals is environmentally damaging.
Mr Hine told the conference GreenSpur Renewables is working on a generator that uses ferrite magnets, which are abundant, considerably less expensive and can be manufactured in the UK, opening up the potential for significant job creation and investment. Apart from not using rare-earth materials, unlike other direct drive machines it is an axial flux machine, rather than a radial-flux generator as used in most direct drive multi-megawatt-class turbines. The rotors carry the generators' permanent magnets while the coils are embedded within the stators, sandwiched by the rotors.
For obvious reasons, the company is unable to go into too much detail about the design of the ferrite-based DD PMG but has confirmed that a novel arrangement for the magnets is one of the key features. Electromagnetic test results on the new concept have been endorsed by Warwick Manufacturing Group, with whom GreenSpur Renewables continues to work, and the generator design has been validated by DNV GL. GreenSpur has also filed seven patents in relation to its design.
When it started work on the new concept in September 2014, GreenSpur was aware there had been a market move towards direct drive permanent magnet generators, partly to stimulate cost reduction.
Conventional wind turbines use a gearbox to convert the slow rotational speed of the rotor blades up to a sufficiently high rpm to drive conventional generators. With the absence of a gearbox, DD PMGs need a large diameter to achieve an adequate translational speed between the generator’s stator and its rotor to generate an adequate electrical power output. This necessitates building large cross-section nacelles to house generator components. A lesser, but relevant consideration is the transportation challenge associated with delivering large and heavy generator components to site. However, Mr Hine and his colleagues were aware that although they have many advantages compared to geared turbines, the market-leading DD PMGs being developed were reliant on using scarce and expensive rare earth magnets.
In 2015 annual production of the rare-earth elements neodymium and dysprosium – used in the manufacture of the rare-earth magnets – was estimated at 35,900 tonnes and 1,902 tonnes respectively. By contrast, ferrite magnets of the type that GreenSpur Renewables is working on are made from iron ore, an abundantly available material. It is estimated that worldwide reserves of iron ore amount to 800Bn tonnes.
High growth forecasts for the global offshore wind market is likely to create strong demand for DD PMGs. To meet rising demand, it is predicted that by 2025 DD PMGs could require 35% of global rare-earth magnet production. This could result in significant price hikes and long supply lead times, particularly if competing with more influential industries, such as defence which also requires additional supplies of rare earth magnetic materials. Although rare-earth magnets are more powerful than their ferrite counterparts, they are 30 times the price by weight of commonly-available ferrites, so magnet costs could be reduced to one tenth of current levels.
Mr Hine told the conference that by using a low-cost and abundantly available material capable of delivering an equivalent output at a comparable weight, GreenSpur hopes to significantly expand the future market for large-scale DD PMGs.
GreenSpur believes that, to some extent, designers of existing DD PMGs have been seduced by the power of neodymium boron iron (NdFeB) magnets, which are three times the strength of standard ferrite magnets. However, as Mr Hine also told the conference, there are problems associated with using rare-earth magnets. Apart from being subject to high demand, they are expensive. In the case of some rare-earth magnets, to ensure they can operate safely at raised temperatures, they must be dosed with dysprosium, which heightens the security of supply risk.
Further practical difficulties relate to the generator assembly processes. NdFeB needs to be handled with extreme care and is vulnerable to corrosion in seawater atmospheres. They can also demagnetise if safe operating temperatures are exceeded (160°C). By contrast, ferrite magnets are inert and can operate at temperatures of up to 460°C.
The GreenSpur team went back to basics and looked at the design of a new ferrite-based DD PMG from first principle and is already confident it can be scaled to multi-MW levels and that a 15-MW unit could be available in five years.
“At the moment we are working towards a 15-MW design,” said Mr Hine, “however, with the right partnerships and manufacturing relationships in place it would be possible to go significantly beyond 15 MW, but to do so would obviously need to be linked to market demand,” and herein lies a choice for the company.
Speaking to OWJ after the conference, Mr Hine explained it has already had informal contact with leading turbine OEMs and wanted to find one with which to commercialise the concept. It also needs to find engineering partners and investors to help take the DD PMG concept to the next stage.
Mr Hine anticipates that European OEMs would probably be more cautious partners. Until such time as rare-earth costs rise significantly, or a supply crunch occurs, they could remain wedded to existing technology. Working with European partners it might take five years to make the ferrite-based DD PMGs available commercially, he said.
That might not be the case if the company were to find a partner in other countries, which is where the ORE Catapult’s relationship with China comes in. In 2017, it signed an agreement with China’s Tus-Wind to work together to advance offshore wind technology.
Collaboration between innovative small businesses and universities is at the heart of the agreement, under which the companies will, among other things, establish a UK-China Technology Growth Accelerator to boost UK SME technology innovation and deployment in the Chinese offshore wind market.
If commercialisation of the axial flux DD PMG were to take place in association with a Chinese turbine company, ferrite generator technology could be available somewhat sooner than would otherwise be the case.
Compact generator could reduce costs
Due to be completed shortly, a project that forms part of the EU Demo Wind series, the Compact High Efficiency Generator (CHEG) aims to advance wind turbine generator technology by reducing the cost of the turbine and increasing the efficiency of the generator, reducing the cost of energy.
In a conventional (non-direct drive) turbine the generator and gearbox are major components and constitute a large part of the cost, complexity – and failure modes – of wind turbines. Unlike a conventional geared turbine, CHEG uses an innovative pseudo direct drive (PDD) generator based on innovative magnetic gear technology developed by Magnomatics. The technology is said to have significant advantages in efficiency and reduced costs of maintenance due to the way it transmits torque through the magnetic gear without physical contact, eliminating the need for lubrication, as required by traditional mechanical gearing.
A presentation at All Energy 2018 in May by consultants BVG Associates' associate director Mike Blanch, highlighted that modelling shows significant advantages from the PDD system compared to other machine topologies, with increased efficiency and cost reduction of 2% or more. The PDD also helps to reduce generator mass and maintenance costs.
PDD technology had already been developed to prototype level for applications such as traction motors for electric vehicles, actuators for aircraft control surfaces and propulsion motors for small marine vessels. The efficiencies and compactness of the machines when compared to best-in-class machines from competing technologies indicate that significant reductions in mass, size and cost could be achieved when using the PDD technology as a generator in wind and tidal generation applications.
Experimental data has confirmed theoretical efficiencies on small PDD machines built under laboratory conditions of more than 95% and similar theoretical efficiencies for larger machines are over 98%; their volume could be some 40% less and mass 35% less than best-in-class permanent magnet machines.
The consortium developing it believes a PDD would be within 2.5% of the cost of the current state-of-the-art technology when manufactured in similar volumes for a similar output turbine.