Close up - Alstom Haliade 150 - 6MW offshore turbine

Armed with a 6MW direct-drive turbine and budget for two manufacturing centres, Alstom has big plans for the offshore wind sector. Windpower Monthly technical writer Eize de Vries spoke with Alstom Wind's vice president for Offshore Frédéric Hendrick about key challenges, technology and other choices aimed at becoming a leading offshore wind-industry player.

Alstom's 6MW direct drive turbine will be targeted at Round 3

The turbine is at the centre of Alstom's strategy. Hendrick explained that its Barcelona-based design team started reviewing multiple-drive train alternative solutions on the basis of system robustness, efficient operation and total costs of energy. He explained: "They looked into many alternative options, including number of rotor bearings, direct-drive versus fast-speed geared and hybrid solutions and whether to apply generators with permanent magnets or electrical field excitation.

"For direct drive both frontal and rear generator mounting were valuated with a focus on top-head mass, service access and easy component exchange."

Pure torque

The final turbine concept consists of a front-mounted, air and water-cooled, direct-drive permanent-magnet generator with a 7.5-metre diameter. Similar to Alstom's geared 1.67MW and 3MW models, the 6MW turbine incorporates a drive solution that fully separates rotor bending moments and rotor torque. Called Alstom Pure Torque, the main operating principle is that rotor-bending moments are led directly into the support structure and tower, whereas a separate internal torque shaft feeds 'pure torque' to the generator.

The power-electronic converter and transformer are both located in the tower foot, aimed at keeping top-head mass as low as possible. Electrically, the Converteam medium-voltage Advanced High Density generator consists of three 120-degree stator sections, whereby each section feeds power into a separate power-electronic converter.

Hendrick said: "In the event of partial generator and/or converter failure, the turbine can continue to function with two, or even one, active generator segment. Our operational strategy is that turbines should not stop. We further decided on an inner-rotor generator as a safe choice as the turbine concept already incorporates several major technology innovations."

Slender blade

One innovative solution is the slender 73.5-metre rotor blade characterised by a modest width compared to the blade length. The product is a co-development with LM Wind Power and it builds further on LM's latest GloBlade structural design and aerodynamic-design principles.

Hendrick said a favourable blade mass could be achieved without having to integrate carbon fibres in highly stressed structural sections, but denied releasing a figure.

"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. The advanced GloBlade design thereby offers the benefits of a much larger rotor size while keeping turbine loads within a comparable range.

"Haliade's modest 90.8 metres per second rated tip-speed value is explained by the fact that we are cautious of potential erosion-related rotor-blade damages that require expensive offshore remedying actions."

Integrated support structure

The prototype is currently being assembled in a facility near Saint Nazaire and will be installed at the Le Carnet site, both on the Atlantic-western coast of France.

Alstom aims at establishing two identical permanent satellite facilities for blade manufacturing, generator manufacture and nacelle assembly at an industrial park in western France and the UK's east coast.

Alstom's flagship turbine has finally been optimised for North Sea IEC WC 1B offshore sites and 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," said Hendrick.

For these harsh marine-environment conditions, Alstom is developing an integrated and optimised substructure comprising a four-legged so-called mini-jacket with a tubular steel tower put on top. The first prototype substructure detail-design and manufacturing job is subcontracted to specialist firms STX and OWEC.