Close up - Mecal's plan for a 12MW offshore turbine

EUROPE: At EWEA 2013, Dutch engineering consultancy Mecal will present a study on a 12MW offshore concept turbine. Windpower Monthly spoke exclusively with chief engineer design Sebastian Schmitz.

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The turbine’s primary product-market focus is the North Sea. Schmitz said: "Building IEC class I wind farms with turbines of this size and overall project complexity requires experienced project developers, suppliers and installation firms for limiting risk on things to go wrong. These combined demands make Europe the primary market for this turbine, whereas only two years ago China was considered the place to be."

6MW starting point

Mecal’s starting point was a 6MW offshore turbine with 155-metre rotor diameter and 98-meter hub height. In-house experience, engineering know-how and a comprehensive database combined with wind turbine scaling trends resulted in 12MW with 200-metre rotor diameter and 120-metre hub height.

Schmitz said there were a number of challenges involved in this. He explained: "A factor two leap in size requires setting high-level targets. This is for ensuring that the right questions are asked regarding methods, materials and main constraints, and for allocating key factors that could hinder reaching these objectives.

"Simultaneously, conservatism and clinging to actual technology status are the main dangers that could cost money later. On the other hand, self-restraint to a maximum of two main technology innovations in a new product developments has proven to be key in curbing project risk."

As part of the overall effort Mecal engineers made graphs with actual nacelle and rotor masses for a range of different 1.5 – 6.5MW turbine models. The plotted lines show the relationship between power rating and mass increment, and extrapolation to 12MW indicates about 404T rotor mass and 645T nacelle mass (= 1,050 tonnes head mass).

Such massive figures prevent cost-effective turbine design said Schmitz, adding that in practice mass increment with size does not follow typical engineering scaling rules: "We were, for instance, able to simulate dominant turbine loads rather accurately due to the availability of advanced modelling and simulation tools. Other contributing factors include the use of high-strength materials, a continuously expanding know-how base and general learning curve progress."

Reducing the cost of energy

One project target was reducing lifecycle-based cost of energy (CoE) by 40%. Figures from 2010 indicate that the turbine and tower account for around 28% of CoE total, and operations and maintenance (O&M) for 20.5%, plus 10.4% attached to logistics and installation, and 13.3% for the support structure.

With the turbine as main study focus, Mecal reviewed three drivetrain solutions — high-speed, direct drive and an intermediate ‘hybrid’ — for their individual benefits and challenges. The generator was in all three cases treated as a black box. For direct drive an outcome was a generator with about 12-metre diameter and 350-tonne mass. The second option was a conventional non-integrated high-speed system with separate main shaft and generator.

Upscaling would result in a big and heavy nacelle further characterised by many separate components and systems. As winner Mecal selected a semi-integrated medium-speed drivetrain with a single rotor bearing and ‘tube-shaped’ flanged gearbox and generator assembly. The drivetrain assembly fits inside a compact cast main carrier, while the nacelle externally features a helicopter platform with active rear cooler.

Schmitz explained: "This compact medium-speed lightweight overall design offers superior transport logistics and installation benefits and the lowest support structure costs. It further allows a high degree of pre-assembly onshore thanks to a modular ‘building blocks’ design. From bankability points of view, medium-speed being a proven concept offers an optimal balance between the most feasible and mature design aspects."

Main challenges

The three single main challenges for further developing the concept turbine into a commercial product include the 96-metre rotor blades and a rotor bearing with 6-metre diameter. Schmitz said: "Both components are currently not available, while the huge bending moment at the tower base represents another key challenge. An overall challenge is ensuring that solving a technical problem in one area does not automatically shift somewhere else, but this is an issue we are used to tackle through our multi-disciplinary integrated design approach."

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