Late May 2011 Siemens Wind Power installed, commissioned and put into operation the prototype of a new 6MW direct drive offshore turbine with 120-metre rotor diameter. It stands at a test site south of Høvsøre in Western Jutland, Denmark.
With around an 600-turbine offshore track record and a 1100-order backlog, Siemens is an offshore market leader. The bulk of new orders are for the popular 3.6MW workhorse, (the latest version is the SWT-3.6-120 model version with enlarged 120-metre rotor). What is remarkable is that with the new-generation 58.5-metre lightweight slender B58 blades, the rotor swept area could grow by 26% without significantly increasing turbine loads.
Outside the cylindrical-shape nacelle looks like a bigger expanded sister version of the 3MW SWT-3.0-101 direct drive model introduced in 2009 and now a series product. A prominent offshore feature is a helicopter-hoisting platform integrated into the nacelle rear, whereas the outer-rotor generator is located in the nacelle front.
Explaining on key technology features Stiesdal said that the 6.0MW turbine builds upon the SWT-3.0-101 concept. He adds: "The water-cooled permanent magnet generator is based upon the same design principles but is longer, has more poles and the outer diameter is 6.5m. The prototype rotor blade and rotor hub are similar to those of an SWT-3.6-120.
"Main component reuse combines benefits of proven rotor technology and a measured power curve up to 3.6MW output level. The first 6.0MW series will keep the 120-metre rotor, but later we will introduce a significantly larger rotor as a key instrument to substantially drive down offshore costs of energy further."
Stiesdal declined to offer specific information on the top head mass or THM – nacelle plus rotor, but extrapolating from the low weight of the 3.0MW DD wind turbine THM of the Siemens 6.0MW may be approximately 300 tonnes, subdivided into about 200T for the nacelle and 100T for the rotor. If this THM value turns out to be correct, it is indeed favourable compared to other state-of-the-art geared 6 – 6.5MW class wind turbines in the market, typically having a 400 – 450 tonne THM range. A low THM lessens the mass and cost of tower and support structures.
Cast main carrier
Inside the nacelle, a cast main carrier forms the central structural element of the SWT-6.0-120. In front the generator and single rotor bearing are bolted on, and in the horizontal plane multiple yaw motors for directing the turbine rotor towards the prevailing wind direction. Similar to the smaller 3MW model, easy internal hub access is enabled via the hollow generator and rotor bearing inner parts.
The turbine further incorporates two individual power converters located above the tower, and a medium-voltage transformer accommodated into an explosion-free enclosed space at lower level inside the nacelle behind the tower. The spacious nacelle rear section provides ample room for service activities.
Compared with the SWT-3.6-120’s 225 tonne THM, the SWT-6.0-120 mass increment may be as low as 75T, despite a 67% higher power rating. It should be further noted that in SWT-3.6-120 turbines only the rectifier that converts generator AC-power into DC-power is located inside the nacelle. The grid-side DC-AC inverter and medium-voltage transformer are both located in the tower foot. For a 6MW power rating this would have normally added about 30T if these power-electronic components were put inside the nacelle, making the modest 6.0MW turbine mass increase even more impressive.
From geared to direct drive
Talking about Siemens’ technology switch from geared turbines to a new direct drive concept, Stiesdal said that one of the most interesting wind industry phenomena has been variations in wind technology choice.
"In the late 1990’s 600kW and 750kW stall turbine concepts were mainstream and one could believe that future wind turbines would basically all look the same," he says. "However, after 2000 several new concepts like Multibrid were introduced, and today we experience a battle between different concepts. Over time some technologies may turn out to be real winners. The only firm conclusion so far we can draw is that the apparent wind technology consolidations of the late 1990s proved not viable in the long run."
He finally stresses that big strides made in the fields of aeroelastic modelling and structural design have enabled new generations larger wind turbines to be made both lighter and more cheaply. Stiesdal says: "The biggest overall challenge is to outmanoeuvre the negative effects linked to up-scaling, and this is where over thirty years of wind industry experience does count."