Thursday, November 27 2014
Deliberations over the best drivetrain architecture for supersize offshore wind turbines are moving on — to generators.
Generators are where wind energy — captured by the blades, coiled through the drivetrain — becomes power. Here, the double-fed induction generators (DFIGs) that have been fundamental to the modern turbine’s development are pitted against permanent-magnet generators (PMGs), whose high-efficiency, light-maintenance regimes have turned many in favour of the technology.
The subject was debated by a panel of experts at Recharge’s Technology roundtable discussion at WindEnergy Hamburg in September.
“When I came into wind power, I found an industry using a generator technology [DFIGs] that had been obsolete in the automotive business for 15 years,” said Jukka-Pekka Mäkinen, chief executive of Finnish PMG maker The Switch, leading off the discussions.
For the generator, this comes down to two things: revolutions per minute and torque. “So a PMG is better for efficiency, reliability — all extremely important for machines in hostile offshore environments — though the DFIG would win on cost,” Andersen explained. “It’s a trade-off. But the PMG wins overall, on the total LCoE, because it has the optimal balance on capex, opex and unit production.”
Paul Jordan, global market sector head for clean energy and power generation at Ricardo, which had a hand in the early design work on the transmission system for Samsung’s 7MW S-171-7.0, takes a holistic view that weighs the “many recent advances in gearboxes and DFIGs” against the “many strengths” of direct-drive and PMGs.
Schröter pointed out that DNV GL’s survey of those transmission system architectures in use worldwide shows the industry remains reluctant to back one technology over another. “There is no clear trend yet.” Behind the “technological beauty” of a given drivetrain concept, he said, politics looms large for any design that opts for a PMG, which relies on a supply of rare-earth materials, reserves of which are concentrated in a few countries, principally China.
Villanueva-Monzón said the supply chain is the second key factor behind a turbine’s “unit power” in deciding on its drivetrain topology. “We are not a multinational and we are not Chinese — we don’t have the buying power or the domestic resource. We have had to be careful in how we go to the market.”
Andersen concurred: “The dynamics of supply-chain cost, particularly the fluctuations in the rare-earth markets, have a great impact on the technology.”
Schröter was more circumspect, framing cost reduction in a matrix of “technology and its industrial ecosystem, politics, joint ventures” into which “everything feeds”, so that DFIGs and PMGs will both find homes in turbines designed for different locales. “We believe that DFIG and PMG systems will be with the industry for some time, and for good reasons: each has their disadvantages, but nothing that can’t be overcome.”
For the biggest turbines now up and running — and those still on the drawing board — PMGs are clearly in the ascendancy. The question now is what drivetrain architecture these generators will be married with: direct-drive, or medium-speed, one- or two-stage geared.
Hendrick noted: “PMG as a choice is more straightforward. Medium-speed or direct-drive is trickier. We have all seen these consultants’ presentations on the subject. Ultimately — because you really can make numbers say whatever you want — it comes down to this: What do you believe in, especially if you are projecting into the future?”
Mäkinen was more partisan, seeing a path for a new, high-speed class of PMG transmission system being pioneered by “forward-looking” Chinese turbine makers with 5MW models of this type already up and running.
“The Chinese are more progressive. As the average size of turbine gets bigger and more powerful, the industry will have to have a more open mind about the direction the technology takes,” he said. “Even high-speed PMGs could one day become the more obvious choice.”