Frederic Hendrick, offshore vice-president at the firm, says his company's design team assessed several multiple-drive-train solutions for robustness of systems, efficient operation and total cost of energy.
"They looked into many alternative options, including number of rotor bearings, direct drive versus fast-speed geared, hybrid solutions and permanent-magnet or electrical-field-excitation generators," he says. "For direct drive, both frontal and rear generator mounting were evaluated with a focus on top-head mass, service access and easy component exchange."
The final concept consists of a front-mounted, airand water-cooled, direct-drive permanent-magnet generator, 7.5 metres in diameter. It incorporates Alstom Pure Torque, a drive solution that fully separates rotor bending moments and rotor torque. The rotor-bending moments are led directly into the support structure and tower, whereas a separate internal torque shaft feeds the torque to the generator.
The power-electronic converter and transformer are located in the tower foot to reduce top-head mass. The Converteam medium-voltage advanced high-density generator consists of three 120-degree stator sections, with each section feeding power into a separate converter, designed to prevent turbine stoppage.
The rotor blades are slender, yet still long, at 73.5 metres. They were co-developed with LM Wind Power and build on LM's latest GloBlade structural and aerodynamic design principles. Hendrick claims that a favourable blade mass can be achieved without having to integrate carbon fibres into highly stressed structural sections. "We will be the world's first to install a turbine with a 150-metre rotor, which adds about 40% rotor-swept area compared to the largest existing rotors in the 6MW class," he adds.
Alstom's flagship turbine has finally been optimised to International Electrotechnical Commission Class 1B standards for offshore sites with average wind speeds in the 9-9.5m/s range. "The turbine can also be employed at higher wind-speed sites and is expected to yield around 60% capacity factor or over 31GWh at average 10m/s winds. This is at least 15% more compared to achievable yield levels with 6MW turbines featuring standard rotor blades in the 125-metre diameter range," says Hendrick.
For these harsh marine-environment conditions, Alstom is developing an integrated and optimised substructure comprising a four-legged mini-jacket with a tubular steel tower on top.