Anger became intrigued by natural fibers several years ago during a surfing vacation to the Cornish coast of England. While there he visited the Eden Project in Cornwall. It features two large enclosures with connected domes housing thousands of plant species as well as bio-based applications. A surfboard made of natural fibers caught Anger’s attention and planted the seed later germinated by the SWE: Why not use natural fibers for wind blades?
To make the project a reality, Anger and fellow research assistant Martin Hofsaess, Dipl.-Ing., contacted Composites Evolution about testing and using the company’s bio-based fiber reinforcements. The SWE research team tested various fibers with different resins to validate their performance characteristics. They ultimately selected Composites Evolution’s Biotex Flax high-performance reinforcement yarns and fabrics, which are based on natural flax fiber.
The main shell of each blade was constructed using Biotex Flax 2X2 Twill fabric (400 grams per square meter) to build the form and resist torque. Unidirectional 275 gsm yarn was selected for the belt and root to resist bending and centripetal forces. Students also added flax plain weave to the outer layer to resist small impacts and provide good surface quality and flax hopsack weave to strengthen the root of the blade and resist bending forces and torque.
The 1.25-meter-long blades were built in two halves using a wet lay-up process and partial vacuum infusion. The halves were then hand laminated and joined using Momentive’s RIM 135 and 285 epoxy resins. Anger says these resins were chosen because they fully cure without being tempered, are certified for wind blades in Germany and are commonly used at the university’s Institute for Aircraft Design.
The team conducted several tests on material strips and later actual blades, including Eigen frequency (resonance frequency), bending, tensile and pressure tests. “In comparison to standard serial blades, the natural blades are a bit heavier and a bit softer against bending forces,” says Anger. Still, researchers validated that wind blades for small turbines can indeed be built using natural fiber fabrics.
The natural fiber wind blades replaced traditional FRP ones on a rooftop turbine on campus last fall. The University of Stuttgart is currently preparing the turbine and measurement system for long-term blade testing and power curve testing. To date, there is no commercial interest, but that doesn’t deter the researchers. “After all, this is a student project,” says Anger. “It will grow with the students and their successors.”