Vattenfall and Siemens Gamesa are working on the development of a new 11 MW wind turbine for the unsubsidised offshore wind farm Hollandse Kust Zuid. In due course 140 of the 200m rotor diameter WTGs will be installed. They will be the biggest ever to be installed on this scale. Siemens Gamesa is currently testing a prototype installed onshore at Østerild, Denmark.

At the start of the collaboration between Vattenfall and Siemens, the Siemens direct drive 11 MW unit was selected as the turbine of choice for HKZ. At that time, each turbine was planned to feature a 193 m rotor diameter. A few months ago it was decided that it could be upgraded to a rotor diameter of 200 m, resulting in a 2100 sq m increase in the swept area. The greater swept area means higher output, but crucially at lower wind speeds, which is important for a subsidy-free wind farm such as HKZ.

Although the turbine to be installed in Hollandse Kust Zuid is new and the first of its kind to be deployed offshore, it builds on existing technology, being an upgrade of previous direct drive machines. Siemens has already installed the first prototype of the 6 MW DD turbine in 2011 and began its first serial production in 2014. The latest turbine is a larger version of the 8 MW model, which is currently being installed offshore in Vattenfall’s Danish Kriegers Flak project. It features mostly the same technology, although Siemens has optimised a number of features, particularly in the nacelle.

Chief engineer Rasmus Holm: “For this turbine type, we needed to use carbon blades instead of fibreglass; a more expensive option, but much stiffer and lighter. We also added a hydraulic crane instead of the rail system used on Kriegers Flak for example, to help with the installation and maintenance.” 

“The decision to extend the rotor diameter to 200 m did mean that we had to re-evaluate some of the choices we had made earlier,” said Kevin Metcalfe, Wind Turbine Package manager at Vattenfall. “For example, the detailed design process for the wind farm had already started. But now we had to reconsider whether the foundations could handle the increased size and loads from the turbines, and whether the in-field cables and electrical equipment could deal with increased production per turbine. We also had to have another look at the installation campaign and ensured that the ships transporting and installing the different turbine parts were able to handle the longer blades and towers safely and efficiently.”

The biggest challenge that Siemens Gamesa sees for the coming year is scaling up the production process from a single prototype to serial production of one to two turbines per day. While the towers are almost the same for the 8 MW turbines and production can be scaled up quickly with familiar suppliers, the nacelle contains all kinds of new parts. Kevin Metcalfe commented: “That is why five pre-serial production nacelles are currently being produced by Siemens to ensure that production runs smoothly in the future. This allows us to identify and solve any possible production issues well before the wind turbines need to be delivered.” Next to that, Siemens Gamesa has already started to produce blades for the Direct Drive 11 MW at a reduced pace, to make sure that production process will also run smoothly when it is scaled up.