Gemini was proud to be able to use the latest proven technology for wind turbines: the Siemens 4.0 turbine. This wind turbine has a generating capacity of four megawatts and a rotor diameter of 130 metres. The design was a further advance on the popular SWT 3.6 family, of which more units have been sold and installed worldwide than any other offshore wind turbine.


With its 4-megawatt output, the SWT 4.0-130 wind turbine joins thE well-known 3.6-megawatt turbines on the Siemens G4 platform. It utilises all the key technologies of the proven 3.6-megawatt family, but the advances integrated into the design of the SWT 4.0-130 increase energy capture by up to 15 percent. The nacelle and tower are advanced variants of the 3.6-megawatt wind turbine design.

The workhorse

The rotor blades are manufactured using Siemens’ proven IntegralBlade process, cast as a single piece without the use of adhesive bonding. The new B63 rotor blade, measuring 63 metres in length, is the longest and technologically most advanced blade in the 4-megawatt class. The B63 sweeps an area equivalent almost to the size of two football fields. Thanks to optimised coupling of blade bending and twisting, these aero-elastic blades react more flexibly to high wind loads, absorbing forces in a similar way to the cushioning effect of shock absorbers on cars. This technology enables longer rotor blades to be used, which boosts wind power capture and therefore increases unit performance.



The word nacelle is derived from the Old French nacele, which means a small boat or dinghy, which was in turn derived from the Latin navicella. The term is commonly used in aviation design to refer to a covered housing that holds engines. The construction – like everything else on the turbine – had to resist harsh environmental conditions. Inside, we find numerous technical components that are vital in the production of renewable energy: the transmission system, mechanical brake, generator, yaw system (responsible for the orientation of the wind turbine rotor towards the wind) and the controller. The Siemens SWT-4.0-130 makes use of several key technologies which have been well-proven in offshore applications of the previous market leader, the Siemens 3.6 MW turbine. The nacelle and tower structures are essentially upgraded 3.6 MW variants. Onshore certification testing of a prototype of the 4MW design began in December 2012 at Østerild, Denmark, where Siemens was also testing its SWT 6.0-154 model.

The nacelles



Molenaar came in contact with Gemini when Siemens submitted a proposal for the supply of turbines for the wind park: ‘We were in competition and we studied the options very closely. By upscaling our 3.6-megawatt turbine to a 4.0-megawatt capacity, we were able to reduce the number of turbines from 168 to 150, but still with a total capacity of 600 megawatts. That really suited the then developers of Gemini.’

Passion for wind energy

During the phase when Siemens was selected as a preferred partner, Molenaar helped draw up the service contract to be signed for a period of 15 years. The focus was on Operations & Maintenance and also on the importance of Gemini for employment – particularly in the northern Netherlands. ‘Not many Dutch employees were deployed to build the turbines because they were delivered directly from Denmark. But the maintenance situation was a very different story. At a very early stage, we asked the Noorderpoort College in Delfzijl if they could train young men and women to perform the maintenance for us. At a time when we’re adding 700 megawatts of wind parks every year, the demand for good personnel is increasing all the time.’
The importance of Gemini should not be underestimated, according to Molenaar: ‘‘Gemini is the proof that offshore wind really works. And all the political parties can now see that too. It’s a fantastic source of energy for the coming decades. Gemini has been a real tipping point.’
The energy transition is coming up to speed and Gemini has played a vital role in this process. The CEO of Siemens Wind Power, David Molenaar, has been involved with the project from the early days. Time to look back, but also ahead.



Considering the wide range of countries from which all the parts of the new Siemens 4.0 turbine originate, building a state-of-the-art wind turbine is a real European undertaking. For example, the transformers are manufactured in Vienna (Austria), but the blades come from Aalborg (Denmark).

Siemens supply chain

Finally, the assembly also takes place in Denmark, after which the turbines are shipped out to sea from Esbjerg, on the west coast of the Jutland peninsula in southwest Denmark.