The developer of a new type of direct-drive generator believes it could open the way to massive offshore wind turbines
The developer of a new type of direct-drive generator being tested at the Offshore Renewable Energy Catapult believes it could open the way to massive offshore wind turbines far larger than those already on the drawing board.
GreenSpur Renewables has so far modelled the development of ferrite-based offshore wind turbines with direct-drive permanent magnet generators of up to 17 MW, based on three 5.7-MW stages.
In 2018, the company said it was working towards a 15-MW design but believed it was possible to go significantly beyond 15 MW, if the market demanded it.
Now, the company has confirmed to OWJ that increasing the number of stages in the generator would increase the output to 22.7 MW.
However, GreenSpur commercial director Andrew Hine told OWJ, “We haven’t attempted to determine a maximum upper limit, but we certainly believe that 25 MW would be possible.”
The design offers rapid scaling through a unique modular design that enables generators to be constructed in stages.
The company has built the world’s largest ferrite-based direct-drive permanent magnet generator (DD-PMG) capable of reliable multi-MW electricity generation, with no requirement for rare-earth magnets.
Not only is GreenSpur Renewables’ latest DD-PMG the largest of its type in the world – capable of producing 250 kW – its scalable technology also uses a radical new construction method the company believes can solve many of the problems experienced by offshore wind turbines today, not least the need for large quantities of rare earth magnets.
The generator was assembled in the northeast of England and is now at the Offshore Renewable Energy (ORE) Catapult in Blyth, Northumberland for system testing.
“Current generation direct-drive generators used in today’s offshore windfarms 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,” said Mr Hine.
“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.
“Not only does our generator use no rare-earth magnets, our source material – ferrite – is actually a waste product of the steel making process – unlike rare earths, whose mining is environmentally damaging.”
Replacing scarce and expensive rare-earth magnets that GreenSpur estimates can cost £40.00 (US$50) per kg with cheap and abundant ferrite magnets that cost £1.00 per kg (US$1.25) could deliver generator capex savings of around 33%.
The company tested a 75-kW generator at the ORE Catapult in Blyth in 2017. It then signed an agreement with ORE Catapult and Warwick Manufacturing Group to build a single-stage 250-kW direct-drive generator, which is part of a four-stage 1-MW design.
With the support of a £1.25M (US$1.57M) grant from Innovate UK, it will be tested over the next few months at the Blyth test facility.
GreenSpur’s technology has been identified as a potential game-changer by the ORE Catapult. With multiple patents pending, it is hoped to encourage the growth of DD-PMG technology within the global wind turbine market.
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 Renewable Energy 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.
In recent weeks, as its trade war with the US has developed, China has threatened to limit exports of rare-earth elements. The country controls around 80% of global rare-earth production.