Close up - the Nordex N150/6000 offshore turbine

Windpower Monthly technical writer Eize de Vries spoke with Nordex chief technical officer Eberhard Voss about the technology and the challenges developing the manufacturer's latest flagship product, the N150/6000.

The N150/6000 is Nordex's first offshore specific machine
The N150/6000 is Nordex's first offshore specific machine

Update 20 April 2012 - Nordex decided to cancel its offshore project following a failure in negotiations to bring in a development partner.

Despite its plans to launch one of the next generation offshore turbines, Nordex is a relative newcomer to the offshore sector. So far its only two marine-specific machines operational are an adapted 2.3MW N90/2300 and a 2.5MW N90/2500 prototypes, which were installed in 2003 and 2006 respectively.

From Nordex’s point of view, the N150 is notable as earmarking its first switch to direct drive. The company says this can result in higher reliability and thus potentially better availability due to significantly fewer components. The turbine also has 55T/MW top head mass, comparing favourably with 70–90T/MW quoted for current ‘relatively heavy and expensive’ second-generation 5MW turbines.

Besides an approximately 300-tonne top head mass for the nacelle and rotor, another distinct N150/6000 design feature is a permanent magnet type liquid-cooled generator located behind the tower. Explaining the drive system layout preferences, Voss said the main considerations, besides lowest cost of energy, were serviceability and easy main components exchange.

Separate drive shafts

Speaking about the design, Voss said: "The rotor and generator are fitted to separate rotating shafts, each with their own bearing sets. These two shafts in turn are interconnected by a hollow torque shaft, which results in a substantial mass saving compared to a rear mounted generator solution but with a single long drive shaft."

He further argues that the turbine’s outer rotor generator design, with the rotating part turning around the stationary part, offers higher torque and superior efficiency compared to inner rotor equivalents. Nordex has long-time experience with liquid-cooled generators, dating back to the 1.3MW N60 turbines of the 1990’s.

Talking about the blades, Voss said Nordex has succeeded retaining a 10.5-tonne mass for three successor blade generations, each time five metres longer to match a corresponding ten-metre rotor diameter increase.

"For the N80, N90, and N100 blades glass fibre reinforced epoxy composite material is applied," he says. "We incorporated carbon fibres for the first time in the about 57-metre slender blades for the N117/2400 and will do this again in the much longer N150/6000 rotor blades. Carbon fibres as a key characteristic enable a combination of high stiffness and slender blade design."

Failure risk

Some competitors have introduced cyclic pitching in their latest large-diameter turbine models, a technology whereby the blade angle is continuously adjusted during each rotor revolution as a load reducing control and optimising measure. However, Voss is not convinced about overall benefits associated with this.

He says: "Rotor blade pitch systems typically operate with 0.1 – 0.2 degrees per second, which is to slow for adequately meeting advanced loads control needs. In addition, frequent pitching actions put high strain upon the pitch system and this could result into accelerated wear and increased failure risk."

During a press conference for the turbines launch, Nordex presented discounted revenues for two fictive offshore wind farm comprising 70 x 6MW turbines, one using a standardised wind turbine model with 125-metre rotor diameter and the other featuring the N150/6000. Both operate at 10m/s average wind speed and the operational lifetime is 25 years. With a calculated gross annual output of 26.5 GWh/a versus 31 GWh/a, the additional revenue for the N150/6000 wind farm was calculated at € 322 million, a 17% increase.

Fast-track entry

Nordex plans to install a first N150/6000 onshore prototype in 2012, followed by an offshore prototype in 2013. Its offshore strategy is built on two main pillars besides the new turbine. Firstly, it launched its offshore division ‘Nordex Offshore GmbH‘ in 2010. Secondly, for a fast-track offshore wind market entry, a 40% share in the major German wind farm ‘Arcadis Ost 1’ project was acquired. Nordex plans to deliver up to seventy turbines for the offshore project in 2014/15.

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