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On the Cutting Edge: Next Generation Materials in Turbine Blade Construction

As machines get ever larger and rotor diameters grow to match, wind turbine blade materials are evolving, with new designs, materials and manufacturing processes. Strength and lightness are the goals of materials scientists working in wind.

It's a hard job being a rotor blade these days. They must be longer and stiffer to extract as much power from the wind as possible, but lighter per unit length, maintenance-free and damage-resistant. Meeting these goals at the lowest possible cost means researching and employing the very latest materials in blade construction.

Stopping longer, slimmer turbine blades hitting towers when deflected by wind loads is probably today's biggest challenge. It requires stiffer composites which must be lighter too: blade mass scales as the cube of the turbine radius and, as blades get longer and heavier, buckling in compression (the blade's tendency to collapse under its own weight as it stands vertically) becomes the major fatigue failure mode instead of flexural fatigue.

Three or more times as strong and stiff as glass, carbon fibre is the alternative to the commonly used glass reinforced plastic (GRP). It helps designers build longer blades with a thinner spar section, while retaining the necessary stiffness. A thinner spar also avoids the problems associated with resin wetting in thick lay-up sections of GRP.

According to a study by SGL Rotec, a carbon-based design for a 53 metre-long blade should be about 20% lighter than the GRP equivalent and will have looser design constraints in areas like flutter. Lighter blades reduce root loadings as well as those on the rest of the structure, in turn reducing weight and cost. SGL's study envisaged a benefit of €100,000 over the lifetime of a 3 MW turbine.

The industry discussion over replacing glass with carbon fibre has been simmering for years. Vestas and NEG Micon, which merged in 2004, were the first to use carbon and today Vestas and Gamesa (which also share a common heritage) along with SGL Rotec and DeWind are among the minority of manufacturers which publically admit to using carbon. Other manufacturers, such as REpower and LM, have flirted with carbon but subsequently discontinued its use.

The 'cost of stiffness' is the main factor here: carbon's cost (maybe 20 times higher than E-glass), consistency and security of supply have hampered its widespread adoption. How well a manufacturer's own resin system and manufacturing process suits carbon is another factor.

GRP's development life is far from over though. LM Wind Power's new 73.5 metre blade for Alstom's Haliade 150-6 MW wind turbine uses pure glass-fibre technology, while Sandia National Labs came up with a 100 metre glass-fibre design last year.

Developing stronger fibres and adding more glass are two ways to increase GRP performance, though composites with glass content much more than 55% by volume are more susceptible to fatigue. This is a current area of research at Fraunhofer IWES while improved E-glass fibres have recently appeared that offer significant performance increases at various price points.

With an urgent requirement for very large turbine blades and prices continuing to drop (despite rising demand from other sectors like automotive), it looks like carbon fibre is poised for a breakthrough. Lux Research predicts wind energy will take over from aerospace as the leading user of advanced composites and will account for nearly 60% of the market by 2020, up from today's 35%.

As the use of carbon rises, how best to mix carbon and glass to form hybrid composites that optimise cost and performance is becoming important. Carbon is about four times as stiff as glass fibre and deforms less under tension, but it performs worse in compression.

What Else is on Offer?

Beyond glass and carbon, little is heard of other fibre types. Basalt seems a likely contender, with higher strength and stiffness than E-glass. It got some attention in wind some six years ago when Ahlstrom worked with Kamenny Vek to produce some biaxial basalt fabrics for testing in wind turbine blade laminates. Professor Paul Hogg, vice principal for Research and Enterprise, Royal Holloway, University of London, says the variation in performance from supposedly similar fibres could be a factor in basalt's lack of popularity.

'There are a lot of other high-modulus fibres available like boron, which has amazing strength and stiffness,' says Dr Arno van Wingerde, head of the Rotor Blade Competence Centre at Fraunhofer IWES. 'But then they told me what it costs - about €5000 per kilo. That's OK for an F-16 maybe, but not a turbine blade.'

The way in which carbon or glass fibres are woven together - their 'fibre architecture' - has a big effect on the strength, stiffness and fatigue performance of the finished composite. For example, conventionally woven mat - where the warp runs over and under the weft - results in built-in defects in the finished composite while not all fibres will take up the load directly along their axis, so putting more of the load through the weaker matrix polymer.

Non-crimp and unidirectional fabrics, whose filaments can be directly aligned with the greatest loads, appeared long ago to tackle these problems. But laying up multiple fabric sections into shapes like spar caps or roots is a complex and time-consuming process that can introduce further weaknesses: wrinkles, dry zones, and poor fibre alignment. Poorly laid 'wrinkled carbon' caused a 7 metre section of a prototype Vestas V112 rotor to snap off during trials at the company's testing centre in Lem, Denmark in 2010.

'Composites are two dimensional, so you have to fold a fabric to make a T-section,' says Hogg. 'Inevitably there will be an area with no fibres, just resin, and that's very susceptible to splitting. Delamination within the structure is the long-term durability challenge.'

By making it easier to form joints and eliminating built-in defects, 3D fabrics improve fatigue performance, increase strength and aid manufacturing. Using single-fibre-thick layers of 2D fabrics, 3D fabrics are built up using a variety of stitching, weaving, braiding and tufting methods that introduce a third, through-the-layers 'Z-fibre' reinforcement. This vertical stitch or tuft helps prevent delamination, while varying its depth means the fabric can easily take on the shape of the desired joint and make sure fibres take up the load rather than the matrix. Suppliers such as 3Tex and Techniweave offer complete pre-woven structures such as blade roots and spars.

Better fibre architecture should also help to increase the volume fraction of fibres in a composite without fatally decreasing fatigue performance - an obstacle when building very large GRP blades. 'The fibres interfere with each other and we are working to overcome that,' says van Wingerde. 'You need to keep the fibres completely straight, parallel and separated from each other. It's a big job and there are lots of hurdles.' IWES is intending to test a glass blade of more than 80 metres this summer.

Most manufacturers make their blade halves separately and bond them together during assembly, so the adhesives used are critical to the performance. 3M's W1101, a two-part epoxy paste specifically developed for this application, offers reduced process times and better durability and crack resistance. According to 3M, it cures two to four hours faster and has 200% higher peel strength and 75% higher fracture energy compared to the 'industry-standard epoxy adhesive'.

Blade core materials are also evolving from end-grain balsa, SAN and PVC foams towards thermoplastics like PET. Easily recycled, PET can easily be cut and formed into complex shapes or melted and bonded to other parts. But, PET requires a slightly higher density to match the mechanical strength and stiffness of SAN and PVC, and a substantially higher density to match balsa. 'The industry wants to use PET because it's cheaper and you can remelt it to recycle, but the characteristics are not there yet,' says Professor Povl Brøndsted of the Department of Wind Energy, Risø DTU.

Blade materials continue to evolve hand in hand with manufacturing: better performance is no use if it adds too much complexity and cost to production. With the industry moving away from traditional manual processes, new materials must be compatible with automated techniques like robotic lay-up or top-coat spraying. Recycling is becoming more important, too, something that current blade materials struggle with.

The bottom line is always cost per kW/h. Any extra power produced must be set against turbine manufacturing, installation and maintenance costs - and any material that doesn't help reduce a turbine's lifetime cost of energy must be ruthlessly discarded.

James Lawson is a freelance journalist focusing on the energy sector.

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