Fast-moving material advances in wind energy
By Don Rosato

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Note: This is the second article of a four-part series covering wind energy (1) trends, (2) material advances, (3) process technologies and (4) applications.

INDUSTRY PULSE

What is the most important advanced material trend in wind energy?
  • 1. Mechanical performance of epoxy resin with processing advantage of unsaturated polyester resin
  • 2. New high-modulus, lighter-weight glass fibers
  • 3. Lighter blades from carbon fiber used in structural parts
  • 4. Urethane and vinyl ester resins that incorporate nanotechnology

The vast majority of the total tonnage used in wind turbine blade manufacturing is glass fiber and thermoset (primarily epoxy and vinyl ester) resins. Let's take a look at new wind energy resin and fiber material advances. The increasing size of wind turbine blades poses a big challenge to designers and engineers to design lightweight structures that meet the requirements in terms of stiffness and, predominantly, fatigue.

A new resin (ZW7844 from DSM Composite Resins now commercialized as Daron 90) offers wind turbine blade manufacturers the mechanical performance of epoxy resin with the processing advantages of unsaturated polyester resin. Composites based on traditional unsaturated polyester resins generally show lower fatigue performance compared to epoxy-based composites but have clear processing advantages (low viscosity, fast cure at room temperature, increased processing robustness) compared to epoxy. This could result in a significant reduction of cycle times and therefore increased productivity.

The costs of making the molds and of the raw materials are significantly lower with polyester resins. Polyester resins also give shorter infusion and cycle times due to faster cure at room temperature. On the other hand, the mechanical performances of epoxy resins are higher, especially with regards to resistance to fatigue.

ZW7844 (Daron 90) resin mechanical performance


The new ZW7844 (Daron 90) resin system based on a novel proprietary chemistry exhibits static as well as dynamic mechanical performances comparable to the current epoxy benchmark systems. At the same time, it provides a significant reduction of cycle time as the result of processing and curing behavior similar to unsaturated polyester resin.

The Daron 90 resin is also said to offer:
  • a pot life of more than 90 minutes;
  • manufacture of thick and thin laminates;
  • compatibility with multiple glass fiber types;
  • excellent fatigue performance.
The resin is currently being trialed in preparation for qualification. If successful, the first test blades could be produced this year.

Elsewhere, high-performance fiberglass for longer, lighter wind blades is advancing rapidly. New high-modulus glass has been introduced by 3B — The Fibreglass Company with outstanding mechanical properties, providing significantly greater strength and strain-to-failure than traditional E-glass. Compared to traditional E-Glass, HiPer-tex fiber delivers superior performance:
  • up to 30 percent higher strength
  • Up to 17 percent higher stiffness
  • 30 percent lower CLTE (coefficient of linear thermal expansion)
  • 10 times longer lifetime in fatigue
  • Significantly higher corrosion resistance
  • Significantly higher temperature resistance
HiPer-tex high-performance fiber is the result of a new patented glass formulation, combined with new and more optimized melt fiberizing and sizing technologies.

HiPer-tex glass fiber fatigue performance


In comparison to blades manufactured with traditional E-glass, HiPer-tex W2020 achieves up to 10 percent weight saving for the same blade design and length. Alternatively, the blade length can be extended by up to 6 percent while maintaining the same weight but offering up to 12 percent more energy output.

HiPer-tex W2020 has been specifically engineered for epoxy polymer systems used in resin infusion or prepreg processes. HiPer-tex W3030 glass roving is optimized for the manufacture of wind turbine blades using resin infusion processes with polyester or vinyl ester resins.

Going one step further, carbon fiber is paying off in next-generation wind turbines. As blades grow longer, the idea of converting structural areas of the blade from E-glass to significantly stiffer and lighter carbon fiber begins to make sense, despite the latter's greater upfront cost.

The lighter blades from carbon fiber used in select structural blade parts require less robust turbine and tower components with the cascading cost savings justifying the additional cost of the carbon fiber. The switch to carbon fiber allows increased turbine efficiency through the use of longer blades without adverse addition in blade weight.

Wind turbine blade cross section structural elements


Currently, carbon fiber is used primarily in the spar, or central structural laminate element of wind blades. The higher stiffness and lower density of carbon fiber allows a thinner blade profile while producing stiffer, lighter blades.

The rough rule of thumb for weight reduction is at least 20 percent weight savings when moving from an all-glass blade to one with a carbon fiber-reinforced spar cap. A 100-meter blade made entirely out of glass fiber could weigh up to 50 metric tons. Achieving a 20 percent savings by incorporating carbon fiber represents a weight savings of 10 metric tons per blade or a total of 30 metric tons per rotor, which can make a significant difference.

GE plans to use carbon fiber to form the primary structures of 1,600 next-generation 160-foot blades. Because of the sheer quantity of standard-modulus carbon fiber required for these increasingly large wind blades, blade manufacturers are likely to eclipse aerospace manufacturers in carbon-fiber consumption during the next 10 years. Last year alone, GE Energy consumed about 3,000 metric tons of carbon fiber for turbine blades.

Ongoing resin development will also play a role in large-turbine blade development. Urethane and vinyl ester resins that incorporate nanotechnology are coming into play. Bayer MaterialScience recently introduced a new class of nano-enhanced Baydur polyurethane systems. In comparison to systems based on epoxy and vinyl ester, the nano-enhanced polyurethane offers blade manufacturers low VOC (volatile organic compound) emissions, faster infusion time and greater interlaminar fracture toughness.

Incorporation of a small amount of multiwalled carbon nanotubes improves the fracture of both polyurethane and the epoxy composites by as much as 48 percent, making the addition of carbon nanotubes a viable option to improve the strength of wind turbine blades.

Dr. Donald V. "Don" Rosato serves as president of PlastiSource, Inc. a prototype manufacturing, technology development and marketing advisory firm located in Concord, Mass., and is the author of the Vol 1 & 2 "Plastics Technology Handbook".