Wind Turbine Blade Spar Cap: Can the Pultrusion Process Reach its Ultimate?

 

 

Carbon Fiber Finds its Place in the ROI Equation

 

As blade lengths continue to increase to capture more wind energy, carbon fiber reinforcement of the spar caps has become an effective way to reduce the overall weight of the blade length and increase blade stiffness to prevent sudden gusts of wind from collapsing the tower.

 

 

Pultrusion on Spar Maximizes Prepreg Performance

 

Epsilon Composites (Gaillen en Medoc, France) has built a high-precision, fiber-placed, state-of-the-art pultrusion line designed to maximize the volumetric fiber content and compressive performance of carbon fiber-epoxy laminates.

 

The 300-mm-wide, 3-mm-thick laminate rolls, trademarked by the company as Carboglulam, were unveiled at the 2017 JEC show and reportedly have significantly higher compressive strength than laminates made from widely used carbon fiber prepreg.

 

 

Pultrusion wins the day

 

Zoltek’s (St. Louis, Mo., U.S.) process for pultruding carbon fiber panels begins with feeding PX 35 carbon fiber tows from a carbon fiber creel into a pultrusion die, where the formed fibers are infused with a thermoset resin—vinyl ester, epoxy, and polyurethane are all candidates, depending on the turbine blade manufacturer customer.

 

 

Thickness and Width Range

 

Zoltek’s pultruded sheets are available in thicknesses of 3-6 mm and widths ranging from 75 mm to 300 mm. Sheets are produced with a porosity of less than 1% and can also be specified with a fiber volume content of 58-70%.

 

 

Pre-processing to Control Variables

 

Before stacking the material on the spar cap mold, a layer of release paper (used in the pultrusion process) is removed to create a clean activated surface that enhances impregnation and lamination. Unlike fabrics or prepregs, the properties of pultruded sheets are fixed before final infusion, which provides wind blade manufacturers with a variable to work with.

 

Pultruded carbon fiber reinforced spar caps were introduced to the market about five years ago as reinforcements for wind turbine rotor blades, a byproduct of the growing development and technological maturity of the global wind energy industry. When the first large-scale commercial wind farms came online more than two decades ago, they consisted of turbines rated at 1 megawatt (MW) or less with glass fiber reinforced blades, with blade lengths of 10 to 15 meters (m).

 

From an engineering and generator perspective, the rationale for longer blades and larger turbines is simple. According to Betz’s law, the power output a turbine extracts from kinetic wind energy is proportional to the product of the wind cubed and the effective area swept by the turbine blades—π(r/2), where r is the rotor diameter), with a maximum achievable conversion efficiency of 59.3%.

 

In general, the longer the blades, the higher the efficiency. That said, Steve Nolet, president of TPI Composites (Scottsdale, Ariz., U.S.), adds that BEM (blade element momentum) theory also assumes a secondary contribution based on its governing performance, namely tip speed ratio, or the ratio of inlet air velocity to rotor blade tip speed.

 

As blades get longer, deflection and stiffness characteristics become more important in turbine blade design and performance, says Philip Schell, executive vice president of carbon fiber at Zoltek Corp. (St. Louis, Mo., U.S.).

 

“The spar cap needs to be able to handle tensile and compressive loads at a certain predicted level,” Schell says, noting that in addition to being about 30% lighter than glass by volume, carbon fiber has about three times the tensile strength of glass and 1.5 times the compressive strength—data that can vary slightly depending on the types of carbon fiber and glass being compared.

 

While weight is reduced, improvements in compressive strain and reduced axial fatigue do help extend blade life, says TPI’s Nolet. The use of carbon fiber composite spar caps is primarily driven by the need to “limit the deflection of these very large upwind rotors, which must avoid tower strikes during operation and in random wind and uncertain load environments.”

 

Pultruded Plates for Spars

 

Zoltek, a wholly owned subsidiary of Toray Carbon Fiber (Tokyo, Japan) since 2014, supplies more than 12,000 tons of carbon fiber material to the wind energy market each year, and this year added new capacity at its Guadalajara, Mexico, facility—in part to meet demand for new projects in the wind energy market. Its product range includes PX35 carbon filaments and PX 35 unidirectional fabrics, as well as PX35 prepregs, but most importantly for this discussion is PX35 pultruded plates.

 

According to Schell, Zoltek entered the pultrusion market about four years ago at the request of one of its customers. Today, it is one of the world’s largest producers of carbon fiber-reinforced pultruded plates, according to Schell.

 

“They are now getting a lot of interest in pultruded carbon fiber spar caps,” Schell reports, noting that Zoltek is working on development projects with many of the world’s largest wind turbine manufacturers.

 

“One aspect of the carbon fiber spar cap that is attractive is that it opens the door to designing blades that are not only lighter, but thinner, with improved aerodynamic performance,” he says.

 

This property of the material, when fully optimized, means that over time, increases in material cost can be offset by greater annual energy production (AEP) and levelized cost of electricity (LCOE).

 

Zoltek produces the pultruded panels at its U.S. manufacturing facility and at facilities in Hungary and Japan. The process requires feeding PX35 carbon fiber tows into a pultrusion die and typically infusing a thermoset resin (vinylester, epoxy or polyurethane) to produce panels with a thickness of 3-6mm and a width of 75-300mm.

 

UD laminates have low void content (<1%), a density of approximately 1.55g/cc, and can be specified with a volume fiber content of 58% to 70%. Release paper can be applied to both sides of the panel during processing and removed before stacking in the spar cap tool, providing a clean activated surface for infusion and lamination. Unlike fabrics or prepregs before infusion, the properties of the panel are already fixed, which is attractive to blade designers, certifiers and manufacturers because it eliminates most of the potential changes in performance at the final stage.

 

Maximizing Compressive Performance

 

The last point is the most important because the mechanical properties of parts made from UD materials are fiber-dominated – That is, when they are loaded in tension and compression, as they are in a rotating turbine blade, the number and type of fibers determine the performance (as opposed to resin-dominated multiaxial laminates). For carbon fiber, the tensile properties are higher than the compressive properties, which means that from a design perspective, compressive strength is the limiting property in spar cap structures.

 

In carbon fiber or glass fiber structures, any increase in compressive strength that can be achieved in pure material properties gives engineers greater design flexibility.

 

In unidirectional fiber tapes, laminate imperfections such as voids or misaligned fibers have exaggerated, detrimental effects on mechanical properties and can even further reduce the modest advantage of carbon fiber over glass fiber in compressive strength.

 

Epsilon Composite (Gaillen en Romain Coullette, commercial director at Epsilon (Medoc, France), said it made sense to use pultrusion to make the spar cap laminates because pultrusion is arguably one of the most stable, repeatable and cost-competitive composite production processes.

 

In 2013, Epsilon launched its trademarked pultrusion process, Carboglulam, to produce stackable carbon fiber laminates for making spar caps. Since then, the company has introduced a “next generation” version of the composite with improved compressive properties, benchmarked against fabrics and prepregs commonly used in structural turbine applications—for spar caps for turbines expected to operate in long-duration, high-speed wind conditions.

 

The latest formulation has up to 70% fiber content by volume compared to the initial version, which had a fiber content of about 60%.

 

Epsilon also designed a pultrusion system that independently controls and aligns each fiber to ensure accurate fiber orientation on the 0° axis, eliminating potential microbending. Finally, the fibers, which have improved fiber/resin adhesion and higher interlaminar shear strength (>75MPa), are infused with a toughened epoxy resin system (as opposed to the vinyl ester in the first formulation).

 

Coullette says another advantage of the laminate is that it can also be produced in rolls of various widths and thicknesses with release paper on both sides, an improved modulus (168GPa) that facilitates the design of thinner spar caps, especially in the flange area. This, in turn, can allow for a reduction in overall blade thickness.

 

Coullette says Epsilon is working on several testing and development projects with customers who currently use glass or carbon fiber prepreg to produce wind turbine spar caps and are evaluating the performance and cost characteristics of pultruded carbon fiber laminates as a potential replacement material. Epsilon’s pultrusion production facility has an annual production capacity of 1,500 tons, according to Coullette.

 

Schell estimates that about 25% of wind turbines worldwide are now being built with carbon fiber spar caps. While that number is trending upward, it also highlights that the majority of turbines are still constructed entirely of glass fiber composites. Is there a cut-off point in terms of turbine size, at what engineering importance carbon fiber will be favored over glass fiber? “Large fiberglass spar caps can be built by adding more material to achieve the desired mechanical properties,” he observes, “but at some point, it becomes so heavy that it makes sense to use carbon.”

 

Schell cites a 2012 study by Sandia National Laboratories (Albuquerque, NM, USA) that compared the cost and performance of a 100m all-glass reinforced “baseline” turbine blade designed for a 13.2MW turbine with an identical spar cap made from unidirectional carbon fiber laminates instead.

 

The analysis predicted that the carbon fiber spar would save 28% of mass (31,861 kg) compared with the all-glass version. The weight savings come from the fact that the design of large blades is driven by fatigue life and panel bending resistance. The reduced mass not only reduces the gravitational load on the blade—thereby improving its fatigue performance—but it also facilitates the design of a spar cap that is 63% thinner than the baseline glass version, reducing the amount of webbing and reinforcement required along the blade’s trailing edge. Then there are the benefits of improved aerodynamics mentioned above.

 

Schell says that if all cost/performance trade-offs are considered, as in an analysis of the type Sandia did, it is entirely possible to replace glass fiber with carbon fiber in the manufacture of spar caps for turbine blades 55 meters or longer.

 

Schel believes that achieving this in practice depends on being able to reduce the cost of pultruded carbon fiber laminates while improving performance, especially compressive strength and modulus.

 

“It’s not possible to get carbon fiber in blades by hype,” Nolet says. “It has to drive lower LCOE.” But he expresses confidence that carbon fiber can and will do that because it is part of a larger system. “Weight reduction in the rotor reduces aerodynamic and inertial loads,” he says, noting that there are often overlooked knock-on benefits. If the blades can be lighter, the size of the blade drive train can be reduced, as well, as can the weight and cost.

 

Each component—input shaft, gearbox, pitch bearing—can be reduced in cost, “ultimately reducing tower and foundation mass,” he notes. “These reductions can and likely will offset the cost of carbon fiber with other material choices,” he argues. As such, they will have a positive impact on economics and ROI (return on investment).