WindTech: Novel 2.8MW ready to scale up

GERMANY" Stephan Jockel, managing director of German engineering consultancy Wind-direct, introduced a novel 2.8MW wd2.8+109 direct-drive turbine at a drivetrain conference in October in Bremen, Germany.

In-house designed… Wind-direct's generator segments are easily replaced
In-house designed… Wind-direct's generator segments are easily replaced
The key features include an in-house designed passive-cooled permanent magnet generator and a stationary shaft (main pin) with two rotor bearings.

The prototype was installed and commissioned in Tamil Nadu, India, in October, for the first licensee, Indian company ReGen Powertech.

The technology was jointly developed with a dedicated ReGen Powertech research and development team and Wind-direct, focusing on the possibilities and requirements for emerging markets. Before founding Wind-direct in 2008 Jockel, gained experience of developing a 2.5MW direct-drive turbine at Vensys Energy.

The smart looking wd2.8-109 turbine for IEC class IIIA features a distinct black-coated inner-rotor ring generator with external stator cooling fins for passing airflow behind the rotor. Passive cooling systems are considered fail-safe due to the elimination of moving parts.

External fins on the spinner outer surface, aimed at enhancing accelerated intermittent airflow behind the rotor, give it an unusual look. "Our in-house developed inner-rotor permanent magnet generator features a patented segmented stator and rotor design," explains Jockel. "Each full circle is subdivided into 12 compact segments. These are manufactured in a standardised industrial process and do not require final machining, which allows favourable manufacturing and assembly costs."

Low cost

Jockel believes that, for the first time, this direct-drive generator will beat any drivetrain solution in terms of cost and efficiency, because the way the company produces the generator segments is especially suited for markets with low labour costs. "An additional feature is that we can exchange defect segments on the turbine without needing a large mobile crane," he adds. "Our stator cooling solution is low-cost, reliable and highly effective as the external cooling fins incorporated in the stator laminates directly cool the coils. The generator rotor magnets are fully encapsulated and impregnated. Active magnet cooling is enabled by two radial fans and a nacelle based air-to-air heat exchanger, a measure aimed at optimising temperature management and reducing magnet de-magnetisation risk."

Two bearings

Instead of a single rotor bearing solution applied in various modern direct-drive turbine models, Wind-direct chose a stationary main shaft with two bearings. "We evaluated several alternative solutions and chose this proven uncomplicated design principle, which is also applied in Enercon turbines," Jockel says. This allows the company to use easily available modest-size grease-lubricated bearings. According to Jockel, the top head mass of the 2.8MW turbine, with its 109-metre rotor is 153 tonnes, in line with competing direct-drive turbines, including some which are fitted with a single rotor bearing. "Seeing the increasing reports on problems with single main bearings, we take this as a confirmation of our final design choice," he says.

Scaled to 10MW

Wind-direct also presented results of an in-house scaling study using a 3MW reference turbine with 100-metre rotor diameter and linearly scaling up to 10MW and 182 metres.

The study required careful analysis in terms of dimensions, manufacture, logistical issues and expected mass properties. The starting point was a wd3.0+100 with a PMG measuring 4.9-metre inner diameter and 52 tonnes total mass. For the 10MW scaling the team retained the tip speed and a constant generator utilisation parameter or "radial airgap force density" of 65 kN/m2. A third system boundary condition was linear scaling of all generator dimensions, including airgap diameter and width.

"Our initial worry was that increasing generator efficiency losses (= heat to be dissipated) could cause cooling issues," Jockel explains. "But specific losses and efficiency losses proved constant. Outcomes for generator active specific mass (kg/kW) (magnets + magnetic steel in generator rotor and stator + copper), also showed favourable. Generator active and structural mass increases sub-linear with rated power."

According to Jockel, the study results indicate that a 10MW PMG would have a 9-metre diameter and 200 tonnes total generator mass, and that Wind-direct's natural cooling system still works. He concludes that innovative generator technology is a must and that if direct drive technology is to become a viable solution at this scale and dimension, that it requires generator component segmentation.

A model upgrade of the wd2.8+125 with enlarged 125-metre rotor diameter is envisaged for 2015, and series production start in 2016/17.

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