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Early in December, Siemens installed the first prototype of its 4MW SWT-4.0 offshore wind turbine at a Jutland test site for large wind turbines in Østerild, Denmark. The machine represents a second upgrade of the initial 3.6MW geared SWT-3.6-107 platform introduced in 2004 and the current 3.6MW SWT-3.6-120 with enlarged rotor of 2009. Around 500 units of these two offshore models have been installed so far, with Siemens reporting a 1200-turbine order backlog.

The prototype has initially been fitted with a 120-metre diameter rotor. In early 2013 it will be refitted with 63-metre B63 blades that are being developed in house to give the turbine a 130-metre rotor diameter.

The nacelle and tower structures are essentially upgraded 3.6MW variants, and the B63 blades are manufactured using in-house IntegralBlade technology that eliminates shell bonding joints. Siemens retained the proven high-speed drivetrain concept but the SWT-4.0 incorporates a new higher rated gearbox and an optimised induction generator cooling system.

A 302W/m2 specific power rating represents one of the lowest values for current IEC class I offshore products, while the specifications make the turbine potentially well suited for medium and high-wind speed onshore sites too. Siemens chief technology officer Henrik Stiesdal, however, says that onshore market appetite for a machine of this size is hard to predict.

After extensive testing, the SWT-4.0-130 will go on sale in spring 2013, with some pilot projects planned for onshore application and serial production starting in 2015. One large offshore project where the new turbine model might be employed is the Gemini 600MW project off the Dutch coast.

Structural capacity

A key question for many in the wind industry is why Siemens decided for a new 4MW geared turbine, while having developed the more powerful 6MW direct-drive platform. In response, Stiesdal admits that bigger turbines offer obvious benefits especially in terms of savings in foundations and infrastructure costs. But he added: "There are many old offshore permits to consider as well, and the offshore wind industry suffers from a long planning horizon, posing unfortunate restrictions on turbine specifications and dimensions to be employed. Another practical parallel consideration was that our 3.6MW turbine still had considerable structural capacity left, and an evaluation showed that upgrading would involve rather moderate design and overall effort."

The new B63 Siemens blade forms part of a new-generation of aero-elastically tailored slender blades. It was a key enabler in combining improved performance with reduced turbine loading. A third and final reason for the 4MW turbine, Stiesdal stresses, is that development of the 6MW turbine should not be rushed. The maximum ramp-up rate is limited by parallel requirements for demonstration, intermediate product upgrades and optimising, he adds.

Speaking about the SWT-4.0-130's technology features, Stiesdal says that each time a new product is developed, a common idea in engineer’s minds is that all further possibilities have been exhausted already. He adds: "Such perceptions are based upon extrapolation of current knowledge mirrored towards the future but most often prove mistaken because some possible future gains are simply unknown.

"Unexpected and in reality non-predictable innovation potential therefore continues to surprise us. For example, when we developed the B52 blade for the SWT-3.6-107 back in 2003, it fully stretched our technological capabilities. Today’s B63 blade weighs less despite being 11 metres longer and remarkably without having to use carbon. Also, the B75 blade mass is below the value we predicted a decade ago when it would have to be made in carbon."

He further explains that the upgrade follows familiar company lines for the  1MW => 1.3MW and 2MW => 2.3MW platform upgrades. "We have built a substantial record in upgrading machines. The first rule is always to change as little as possible - for a fast time to market and to limit risks of failures and reduced platform reliability."

In a final remark that confirm Siemens' full confidence in its new product, Stiesdal says: "The SWT-4.0-130 offers around 13% yield increase compared with the SWT-3.6-120 at typical 9-10m/s mean wind speed sites, offering faster return on investment and superior CoE."