An interview with Stephen Nolet, PE, senior director of innovation and technology, TPI Composites
TPI Composites is a global, independent manufacturer of composite wind blades for the onshore and offshore wind market. The company is headquartered in Arizona and operates factories in the United States, China, Mexico, Turkey and India. TPI has worked with a number of major players in the industry and has a specific area of insight into the future of wind manufacturing. We talked with Stephen Nolet, PE, senior director of innovation and technology at TPI Composites, about the latest developments in wind blades.
What are today’s wind turbine blades typically made of?
All utility-scale wind turbine blades are manufactured using one form or another of reinforced polymer composites. The most widely use material system is a combination of E-glass (or boron-free E-CR glass) and liquid epoxy that is cured in shell-molds that define the aerodynamic surface of the blade. However, a large number of blades have also been put into service using other liquid resins including polyester and vinyl-ester. But in fact, many blades by the largest OEMs have also incorporated carbon fiber for the spar caps that make up the “backbone” (bending element) of the blade. Carbon provides almost five-times the axial stiffness per kilogram of weight compared with fiberglass and can enable longer blades without increased rotor loads on the drive train and generator.
How have the different composites evolved over time?
As the location with wind rich resources are developed, we see movement toward regions with lower wind speeds and lower net rotor energy (i.e., < 250 W/m2). This has driven rotor designs that are larger and larger for a given generator nameplate capacity. As blade length increases, we see more use of carbon fiber for spar caps. Given the cost of carbon fiber and the cost of conversion to textiles for resin infusion, a new trend has evolved in the use of “pultruded” plates that are laminated to convert the lower cost carbon roving form directly into unidirectional spar caps where translation of properties yields the highest specific stiffness at lowest possible cost. This is innovative and very effective.
Other emerging trends include the investigation of different resins (to replace epoxy and polyester). The emergence of 2K urethanes can take advantage of room temperature (or near room temperature) curing combined with ultra-low mixed viscosities to reduce cycle time and reduce tooling cost. The inherent “toughness” (i.e., high strain to failure) of the urethane resins may yield higher fatigue strength and improve long term blade reliability. There’s still much work to do here. The development of liquid molding resins (such as the Arkema Elium resin) that can be infused in reinforcements and cured at room temperature to yield a high molecular weight thermoplastic acrylic will pave the way for blade disposal solutions where the blade can be ground up and compounded with virgin thermoplastic materials to create long fiber reinforced pellets for injection molding of a whole array of consumer and automotive products.
What are the benefits of composite materials?
The primary benefits of any material selection has to be measured in terms of performance and cost. The use of glass and carbon fiber reinforced composite materials provide both advantages over traditional engineering materials (i.e., steel and aluminum). With similar specific stiffness and significantly better specific strength, glass fiber composites result in lighter weight blades compared to metallic designs. The rich palette of materials and ever increasing properties of both glass and carbon fiber has enabled a continued trend of blade length extension to achiever ever higher turbine capacity factors.
Indeed, there are many markets and applications where composite materials do not compete well with engineering metals because of significant differences in the fundamental material and labor costs. However, with rotor blades, composite materials can leverage their extreme scale to neutralize labor content and their inherent advantage for lower cost of tooling (i.e., mold cost) compared to progress stamping dies or forming tools (for hydroforming of aluminum, for example) required to build an equivalent metal design. While a composite wind turbine blade is molded in and assembled from as little as a dozen subassemblies, an equivalent metallic design would require as much as an order of magnitude higher number of parts with the requisite set of jigs and assembly fixtures required to rivet or weld the blade into a unitary construction. For extremely high volumes, this might favor metallic design in terms of bill of materials and labor, but volumes of any given blade design has been limited in the extreme to maybe 5,000 to 15,000 units and the design is replaced by new (longer) products and new planforms. The cost of re-tooling metal designs would be cost prohibitive.
How are composites influencing new wind turbine sizes?
Composite materials enable the use of higher specific property (i.e., strength/density and stiffness/density) materials to reduce blade weight even as rotor diameters continue to push higher and higher. The mechanisms of damage accumulation (i.e., fatigue) are far slower in bonded composite assemblies compared to equivalent riveted aluminum construction. The combination of higher reliability with longer inspection intervals favor composite designs and encourage this trend of rotor growth for both on-shore and off-shore installation. Growing capacity factors drive down LCOE keeping wind in its position as the lower cost form of electrical energy generation.