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

The vast majority of the total tonnage used in wind turbine blade manufacturing are glass fiber and thermoset (primarily epoxy and vinyl ester) resins. Let’s take a look at new wind energy resin/fiber material, processing and testing 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.

Let’s start with high-stiffness wind turbine blade carbon-fiber composite technology. With increasing wind generating capacity, the trend by Vestas Wind Energy is moving towards bigger, longer and lighter blades. As blades increased in size, blade designers have substituted stronger, more durable epoxy systems with glass and carbon fiber for the earlier thermoset polyester glass fiber materials.

Prepreg molding with a woven or unidirectional fabric is more costly but offers greater consistency because it already contains the matrix material, typically an epoxy. A matrix resin of unsaturated polyester is easier to process and is less expensive, but epoxy offers stronger mechanical performance — particularly tensile and flexural strength — especially important for blades longer than 26 meters (85 feet). Now, greater use of carbon fiber, despite disadvantages of higher cost/tight supplies, is envisioned due to its higher stiffness and lighter weight than standard E-glass.

Delving further automated components, deposition offers radical improvement in blade quality while shortening process cycle times. Today, building a blade with VARTM (Vacuum Assisted Resin Transfer Molding) is still a relatively time-intensive and somewhat error-prone process.

Workers must first lay out all of the dry components in the mold, positioning fiber in various thicknesses at different places. There is the potential for misalignments and wrinkles that can compromise the integrity of the final composite product. With automated components deposition, the fiber would be placed mechanically and monitored electronically.

The electronic monitoring would allow any necessary corrections to be made prior to the infusion process. This would make for much more predictable and repeatable parts and the increases in quality control would also allow for more complex blade designs. Spanish specialty equipment manufacturer MTorres has developed an automated blade production process that reduces labor, cuts cycle time by 75 percent and produces more consistent blades.

Other suppliers such as GE Wind Energy are promising a new lightweight carbon-fiber blade design to significantly increase turbine efficiency and expedite system manufacture and delivery. The company is working to automate the resin infusion process that could lead to mass production of carbon fiber blades.

Yet, glass fiber competitive companies like LM Glasfiber are focused on optimized design and engineering to allow the use of E-glass in longer blades. Furthermore, PPG has commercialized Hybon 2026 glass fiber roving, which the company says will enable the manufacture of longer, lighter composite wind turbine blades.

Vestas and Gamesa are the principle top-tier firms using carbon fiber, primarily in the structural spar cap (central spine) of longer blades (40 meters and up). Most carbon fiber use is in Europe, and it is applied sparingly with glass due to its relatively higher cost.

Vestas Carbon fiber-reinforced wind turbine blades and tower

Elsewhere, LM Glasfiber, in collaboration with Riso DTU, the national laboratory for sustainable energy at the Technical University of Denmark, and laser technology specialist NKT Photonics, are developing "intelligent blades" that measure wind and adapt to current wind conditions. The technology is expected to improve energy yield by up to 5 percent over the turbine’s 20-year life.

Let's next turn our attention to a low viscosity polyurethane (PUR) for resin infusion processing. The resin infusion process has traditionally used vinyl ester, unsaturated polyester and epoxy materials to form large and high-performance parts.

However, these materials have drawbacks, including limited strength and the presence of styrene, among others. Polyurethane has usually been associated with fast processing, such as reaction injection molding, not resin infusion, where gel times can be an hour or two.

The Baydur RTM (Resin Transfer Molding) polyurethane system developed by Bayer MaterialScience LLC gives processors a better alternative to traditional resin infusion materials. A versatile material, it has potential applications that include wind turbine root rings and blades, as well as lightweight, structural components for the transportation industry.

The low viscosity and long gelling times of this novel PUR system is well suited for molding very large wind blades. The infusion rate is about two times faster than epoxy. The following wind blade root ring features 63 layers of Vectorply biaxial glass fabric infused with Baydur polyurethane.

Bayer MaterialScience Wind turbine blade structural root ring

This novel PUR-based system outperforms epoxy and vinyl ester samples in tensile fatigue, interlaminar fracture toughness and fatigue crack growth. Tensile fatigue is about 10 times better than epoxy, and fracture toughness is about two times higher than epoxy.

The polyurethane also gives off fewer VOCs (volatile organic compounds). The polyurethane has very good coupling to carbon and polymer fibers, allowing broader use of these composites. The formulation also offers faster demold time than epoxy, providing improved process efficiency. Stronger than polyester, it is possible to reduce part wall thickness using the Baydur system producing lighter parts at equivalent strength.

Finally, let’s take a look at wind turbine blade structural health monitoring. Structural health monitoring (SHM) systems integrate a series of sensors into composite laminates to detect damage that might lead to failure of the structure, and then alerts (electronically, in real time) to the type and location of the damage.

Working from a dynamic distributed sensing system manufactured by Luna Technologies and developed at NASA, which uses Rayleigh scatter of unaltered optical fiber, the sensor is designed to obtain dynamic strain data over a continuous object. Luna Technologies specializes in products for fiber-optic testing of components, as well as integrated optics and distributed sensing solutions.

Luna Technologies 29.5-foot wind blade with three molded-in defects

With measurement rates of up to 100 Hz, this technology offers cost-effective fiber sensors and significantly reduced installation time compared to equivalent foil strain gauges. Optical fiber offers several advantages, including a relatively low cost (approximately $10 per meter), lightweight, easy integration into composite structures and the ability to precisely measure temperature and strain at hundreds of points per meter of fiber to accurately detect and locate damage.

In a series of tests on a 29.5-foot wind blade with carbon-fiber spar caps that featured intentional defects cast from resin prior to layup and introduced into the spar cap region, the sensors not only pinpointed developing damage but also accurately predicted which load location would fail first.

Luna Technologies Data from the test show a close correlation between measurements from traditional foil gauges and the optical fiber sensors provided by Luna.