A new-generation direct-drive permanent magnet generator (PMG) concept that could clear the way for engineers to develop giant 25MW offshore wind turbines is on track to be market-ready by 2022, with prototype trials about to start at the UK’s Offshore Renewable Energy (ORE) Catapult R&D centre.
The PMG, developed by British start-up Greenspur Renewables, uses ferrites – an iron-rich ceramic – for its magnets rather than current go-to rare-earth materials – supply of which is monopolised by China – and is built around a modular architecture, making it scaleable and easy to repair and maintain.
The design is calculated capable of cutting the capital cost of direct-drive generator by a third, shaving almost 5% off the price of a turbine.
“GreenSpur’s new technology addresses major issues facing the next generation of DD [direct-drive] PMGs,” said Greenspur commercial director Andrew Hine.
“The current generation of direct-drive generators used in today’s offshore wind [turbines] rely on rare-earth magnets. Not only is the supply of these dominated by China – and may possibly be restricted as part of the ongoing US-China trade war – there is also competition for magnet supply from other fast-growing industries, including defence, computing and electric vehicles.
“While a mobile phone only uses a tiny amount of rare earths, each large-scale DD-PMG generator may use several tonnes – and commodity prices have now started rising again.”
Replacing high-price rare-earth materials with ferrites would cut the cost of PMG magnets from £40 ($50) a kilogram (kg) to around £1/kg.
And Hine points out that as ferrite is a waste material produced in the steel-making process, the Greespur concept also removes environmentally damaging rare-earth mining from the industrial impact of the PMG design.
The modularity means generators to be can be constructed in stages, with a 12MW offshore turbine created by “stacking” three 4MW units in parallel.
“We have modelled three-stage units up to 17MW, that is 5.7MW per stage. Increasing the number of stages to four would increase the output to 22.7MW,” Hine told Recharge. “We haven’t attempted to determine a maximum upper limit, but we certainly believe that 25MW would be possible.”
Greenspur tested a 75kW version of generator at the ORE Catapult in 2017. The unit moving into trials now, supported by £1.25m grant from government funding body Innovate UK, is single-stage 250kW model, which is part of a four-stage 1MW design.
Tony Quinn, test and validation director at ORE Catapult, said: “Our 1MW drive train test facility is capable of providing the perfect environment for GreenSpur to test and demonstrate its innovative new technology, enabling the scale-up from 75kW to 250kW.”
Hine notes that as well as significantly cutting the price of offshore wind turbine PMGs, the modularity should help reduce turbine downtime and curb operations and maintenance costs.
“As our generator is modular each stage can be set up to operate independently. For example, if we deployed a three-stage 12MW generator, [if] a single stage failed the other two stages could continue to operate independently and deliver 8MW.
“This means that rushing to fix the generator would not be as vital as for existing generators, where a failure would knock out the entire turbine.”
Discussions to use the PMG are “underway” with a number of OEMs, Hine confirmed, but Greenspur was “unable to disclose names at the moment”.