Researchers Develop Lightweight Composite Wind Turbine Blades

Fraunhofer researchers have partnered with industry experts to develop highly durable thermoplastic foams and composites that make the blades lighter and recyclable

Offshore wind turbines are becoming ever larger, and the transportation, installation, disassembly and disposal of their gigantic rotor blades are presenting operators with new challenges.

The trend toward ever larger offshore wind farms continues with some rotor blades measuring up to 80 metres in length with rotor diameters of over 160 metres. Since the length of the blades is limited by their weight, it is essential to develop lightweight systems with high material strength.

The lower weight makes the wind turbines easier to assemble and disassemble, and also improves their stability at sea. In the EU’s WALiD (Wind Blade Using Cost-Effective Advanced Lightweight Design) project, scientists at the Fraunhofer Institute for Chemical Technology ICT in Pfinztal are working closely with ten industry and research partners on the lightweight design of rotor blades. By improving the design and materials used, they hope to reduce the weight of the blades and thus increase their service life.

These days, rotor blades for wind turbines are largely made by hand from thermosetting resin systems. These, however, don’t permit melting, and they aren’t suitable for material recycling. At best, granulated thermoset plastic waste is recycled as filler in simple applications.

Florian Rapp, the project coordinator at Fraunhofer ICT said;

In the WALiD project, we’re pursuing a completely new blade design. We’re switching the material class and using thermoplastics in rotor blades for the first time. These are meltable plastics that we can process efficiently in automated production facilities.

For the outer shell of the rotor blade, as well as for segments of the inner supporting structure, the project partners use sandwich materials made from thermoplastic foams and fibre-reinforced plastics. In general, carbon-fibre-reinforced thermoplastics are used for the areas of the rotor blade that bear the greatest load, while glass fibres reinforce the less stressed areas. For the sandwich core, Rapp and his team are developing thermoplastic foams that are bonded with cover layers made of fibre-reinforced thermoplastics in sandwich design. This combination improves the mechanical strength, efficiency, durability and longevity of the rotor blade.

The ICT foams provide better properties than existing material systems, thus enabling completely new applications – for instance in the automotive, aviation and shipping industries. In vehicles, manufacturers have been using foam materials in visors and seating, for example, but not for load-bearing structures.

The current foams have some limitations, for instance with regard to temperature stability, so they can’t be installed as insulation near the engine. Meltable plastic foams, by contrast, are temperature stable and therefore suitable as insulation material in areas close to the engine. They can permanently withstand higher temperatures than, for example, expanded polystyrene foam (EPS) or expanded polypropylene (EPP). Their enhanced mechanical properties also make them conceivable for use in door modules or as stiffening elements in the sandwich composite.

Yet another advantage is that thermoplastic foams are more easily available than renewable sandwich core materials such as balsa wood. These innovative materials are manufactured in the institute’s own foam extrusion plant in Pfinztal.

The process involves melting the plastic granules, mix a blowing agent into the polymer melt and foam the material. The foamed, stabilised particles and semi-finished products can then be shaped and cut as desired. In the area of foamed polymers, the ICT foam technologies research group covers the entire thermoplastic foams production chain, from material development and manufacture of extrusion-foamed particles and semi-finished products to process media and finished components.

The researchers will be presenting a miniature wind turbine made from the new foams and composites at the K 2016 trade fair in Düsseldorf from October 19 to 26.

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