Although short by comparison with other composites markets, the history
of wind turbine blade manufacturing has been quite varied in terms of
the materials and technologies used. Yet, to this point, there has been
one constant: Utility-scale blades, made from prepreg or via various
forms of resin infusion, have invariably involved thermoset matrices.
Given the unprecedented long-term growth predictions of the market and
the need to bring the cost of wind energy generation in line with other
forms of electric power production, blade manufacturers are already
feeling the pinch of production inefficiencies and taking measures to
accelerate build rates (see “Part I,” by clicking on the title in
"Editor's Picks," in "Learn More" at right). But one aspect of
thermoset molding that has remained inherently resistant to these
efforts is the cure cycle. The need to avoid excessive exotherm, yet
thoroughly crosslink the matrices in these massive, thick-walled molded
parts necessarily imposes a limit on how fast blades can be produced.
Although efforts are ongoing to improve thermoset materials (see "Learn More"), several blade manufacturers are researching the use of
thermoplastics in their place. Because the linear polymer chains in
these inherently tough polymers are melt-processable — they do not
crosslink and, therefore, require no cure cycle — thermoplastic
composites offer the potential for greater production speed.
Quantum change in process to cut blade production time in halfLM Glasfiber announced in May a dramatic change in its direction regarding wind blade manufacturing. The Lunderskov, Denmark-based blade manufacturer has set a goal of cutting total production time by 50 percent through a new blade technology program called Blade King, and has committed to having composite blades using completely new types of material on the market by 2015.
LM’s VP of R&D, Frank V.
Nielsen notes, “The industry needs a quantum leap in materials and
production technology.” According to communications director Steen
Broust Nielsen, by 2020 LM Glasfiber will grow to 50,000 people (from
7,000 employees worldwide now) if it follows the projected industry
growth while maintaining an annual efficiency gain of 2.5 percent. “But
we would much rather be a company of 15,000 people,” he says. “The aim
is for Blade King to help us achieve that.”
While LM Glasfiber reveals few details about
Blade King, Steen Broust Nielsen does say that the company has
definitely laid out a development path with distinct material
candidates, but stressed that extensive testing must be carried out to
prove process viability and that the program is not necessarily
confined to thermoplastic technology. Blade King targets process
improvements based on benchmarks established in LM’s Future Blade and
continuous technology improvement programs. Although it will involve
“high levels of automation” to “significantly increase the efficiency”
of laying the fiber in the mold, Nielsen would not comment on whether
Blade King will make use of fiber placement and/or automated tape
laying (ATL) technology.
Risø evaluated 0˚/90˚ fabric and 0˚ unidirectional laminates made from Twintex glass fiber/PP by the OCV Reinforcements div. of Owens Corning Composite Materials LLC (Toledo, Ohio) and Comfil’s glass fiber/L-PET. L-PET is an amorphous polyethylene terephthalate (PET) that is modified to melt at 160˚C to 180˚C (320˚F to 356˚F) vs. 255˚C/ 491˚F for PET. Vacuum consolidation was chosen as a process because it is relatively simple, consisting of layup, application of vacuum, ramp to process temperature, cooldown and demolding. Because no additional pressure is needed, no autoclave is required. Both types of laminates featured a fiber volume of 40 percent. Samples were processed successfully and used in testing to measure tension/tension fatigue and shear fatigue properties. All material configurations produced samples that withstood roughly 1 million cycles. Twintex 0˚ unidirectional laminates produced the highest fatigue strength (approximately 275 MPa/40 ksi). Shear fatigue properties were measured using short three-point bending test specimens. Here, Comfil 0˚ unidirectional samples provided the highest values, with shear fatigue strengths ranging from 25 to 27 MPa (3.6 to 3.9 ksi) at close to 1 million cycles.
The maximum laminate
thickness at the root-end of a large rotor blade is typically close to
100mm/3.9 inches; thus, processing prove-outs included compacting a
250-mm/9.8-inch high unidirectional glass/PP stack into a 100-mm
laminate, by soaking it at 200˚C/392˚F for 12 hours as part of a
24-hour process cycle (ramp-up, soak, cool-down).
The vacuum pressure (0.1MPa/14.5 psi) used in compaction was not high enough to conform the material to the mold geometry in all locations; however, heat-shrinking using a hot-air blower solved the problem by shrinking the thermoplastic fibers in the commingled fabrics locally, only where needed. Shrinking the polymer fibers actually caused the glass fibers to wrinkle, but they were re-stretched when the material was conformed to the mold. Other compaction issues included the web-to-skin joint, where vacuum bags were unable to apply sufficient consolidation pressure, and the blade section’s trailing edge, where the vacuum bag did not reach. For each of these areas, PMI foam (provided by Evonik Degussa GmbH, Darmstadt, Germany) provided the solution, expanding further (“re-foaming”) when heated to the process temperature, and thus applying the necessary pressure for complete consolidation.
Blade King’s primary thrust is to boost throughput, Nielsen says the
technology also will be able to produce the much longer blades —
80m/262 ft to as much as 100m/328 ft — that LM Glasfiber expects to
supply for offshore turbines, a wind energy market segment that will
play a large role in the company’s growth.
Liquid-moldable PBT targets two-thirds cut in mold timeIn 2004, Gaoth Tec Teo (Galway, Ireland) announced that it had signed agreements with Mitsubishi Heavy Industries (MHI, Nagasaki, Japan) and Cyclics Corp. (Schenectady, N.Y.) to vacuum-infuse thermoplastic composite rotor blades for big utility-scale wind turbines. The means to that end would be Cyclics’ CBT 200 resin. CBT is described by Cyclics as the cyclic form of the engineering thermoplastic polybutylene terephthalate (PBT). During processing, CBT begins as a thermoplastic polyester that is broken down into a cyclic oligomer form. When heated to a specified temperature, the oligomer drops to a water-like viscosity, a significant aid to fiber wetout. When catalyzed and cooled, the oligomer returns to more conventional viscosity and forms a long-chain, high-molecular-weight PBT. Thus, the material offers the properties of a thermoplastic but can be processed like a thermoset.
In the initial phase of the Gaoth Tec Teo/MHI/Cyclics
joint program, a series of 12.6m/41-ft wind blades was to be designed
and manufactured in Ireland and then tested at an MHI facility in
Nagasaki, Japan. One of the program’s main goals was to produce the
world’s first recyclable wind blade.
The MechTool system applies electrically
generated heat directly to the tooling in the location where it is
required and uses high-temperature composite mold materials, capable of
withstanding process temperatures up to 400˚C/752˚F (CBT 200 resin has
a processing range between 190˚C and 240°C (374˚F and 464˚F).
Reactive processing, in-situ polymerization aid liquid-molded thermoplasticsA thermoplastic that processes like a thermoset also is the focus of research at Delft University of Technology (Delft, The Netherlands), sponsored by the Dutch government through the We@Sea consortium. The latter was founded to develop 6,000 MW of offshore wind farms in the North Sea by 2020. Involved in thermoplastic composites development since the 1980s, Delft University has targeted aerospace applications using press-forming and various thermoplastic welding technologies. Working with Ten Cate Advanced Composites (Nijverdal, The Netherlands), Delft has seen its development work succeed in commercial processes, such as the fabrication by Stork Fokker AESP (Hoogeveen, The Netherlands) of stiffening ribs in the J-nose thermoplastic composite leading edge components that are now flying on the Airbus A340 and A380 (see “Learn More”).
As part of the We@Sea technology development initiative, resin infusion of thermoplastic composite wind blades has been explored since 2002. According to Dr. Harald Bersee, part of Delft’s Aerospace Engineering faculty and leader of the thermoplastic composite wind blade research program, “The goal is automation, using the advantages of thermoplastic processing, such as quick cycle time and continuous welding.” Delft research involved thinking conceptually, before dealing practically with materials and processes, and drew on its history in thermoplastic aircraft structures. “We see the wind turbine blade as a kind of airplane wing,” Bersee explains. “The aircraft industry uses hat-stiffened monolithic composites extensively because it is a more efficient structural design,” Bersee says. A quite different approach from the typical foam-cored sandwich construction used in most wind blade shells, “hat-stiffened solid laminate design is too labor intensive for use in traditional thermoset composite wind blades,” Bersee notes. However, it is possible with thermoplastic composites, as demonstrated in the press-molded thermoplastic composite ribs and stiffeners of the A380 J-nose, which are resistance-welded to cured skins made from thermoplastic semi-preg.
For wind blades,
Delft’s approach is to combine traditional wind blade vacuum infusion
processing with a thermoplastic box spar/skin structural design, and
hopefully to remove most of the foam core in the blade. Foam is costly
to machine and absorbs resin; eliminating it cuts material cost and
saves a manufacturing step. Under such a strategy, the blade would be
produced by infusing either longitudinal halves or modular transverse
sections, which then could be continuously welded together using an
automated process. Kok & van Engelen Composite Structures (The
Hague, The Netherlands) has developed a robotic induction welding
system being implemented in cooperation with Stork Fokker, and the
National Research Council (NRC) in Montreal, Quebec, Canada, is
refining a continuous resistance welding process.
Dr. Bersee’s research, however,
identified anionic PA-6 (APA-6) as a potential alternative: APA-6
processes reactively, like a thermoset, and the in-situ polymerization
of the APA-6 matrix around the fibers during infusion greatly improves
APA-6 processing begins using an
in-house designed mixing unit to prepare the caprolactam monomer, an
anionic initiator, and an activator into two separate solutions at
110˚C/230˚F in a nitrogen atmosphere to suppress reaction: mo-nomer and
activator in tank A; monomer, activator and anionic initiator in tank
B. The two solutions are mixed in a 1:1 ratio inside a heated
(110˚C/230°F) buffer vessel, degassed at 0.25 bar/0.15 psi for five
minutes and then infused into a fiber preform that has been preheated
in a hot flat-platen press at 180˚C/356˚F. The caprolactam monomer’s
low viscosity (10 cps, 35 times less viscous than a typical epoxy)
enables rapid infusion of the preform at a pressure of 250 mbar/3.63
psi. Demolding is possible after 60 minutes, producing APA-6 composite
laminates up to 2 cm/0.79-nch thick with fiber content of 50 percent by
To analyze mechanical
properties and fatigue performance, Delft com-pared three different
laminates, using APA-6, PA-6 and epoxy to vacuum infuse identical,
symmetrical 12-ply layups composed of Ten Cate Advanced Composites’ SS
0303 8-harness satin weave E-glass fabric with fiber volume of 49.4±1.8
percent. The epoxy matrix system was Prime 20LV, supplied by SP Systems
(the marine business division of Gurit AG, based in Isle of Wight,
U.K.) and the PA-6 matrix was HPA-6 Akulon nylon film from DSM
Composite Resins AG (Heerlen, The Netherlands). The epoxy cured at room
temperature for 16 hours and then postcured at 65˚C/149˚F for 7 hours.
The PA-6 composite was melt-impregnated in a hot press at 275˚C/527˚F
with a 9˚C/min (48˚F/min) ramp-up and -20˚C/min (-68˚F/min) cool-down.
The APA-6 specimens experienced in-situ polymerization in a mold
preheated (prior to infusion) to 170˚C/338˚. All three composites were
tested for static tension, compression and shear properties in dry, as
molded (DAM) and moist conditions. For DAM properties, the APA-6
composite had the highest tensile strength at 4.2 GPa, followed by the
epoxy composite at 3.3 GPa and the HPA-6 specimens at 3.0 GPa. Tensile
modulus was again highest with APA-6 laminates at 96 MPa, followed by
the PA-6 composites at 84 MPa and epoxy specimens at 66 MPa. APA-6
again performed best in shear and compressive strengths and compressive
modulus, and only slightly lower than the other two systems in shear
modulus. However, APA-6 samples performed worst in wet conditions,
suffering a nearly 50 percent knockdown in shear modulus vs. DAM
values. It is believed this is due to higher void content and a weaker
interface between the fiber and matrix. To solve this problem, Bersee
is currently testing APA-6 composites using a matrix modified by adding
5 percent nanoclay platelets. He believes that the nanoclay technology,
coupled with efforts to decrease voids and increase the crystallinity
and conversion levels of the APA-6 matrix to better capture the higher
values of neat APA-6 polymer, will increase adhesion and improve wet
properties as well as increase long-term resistance to creep and
fatigue. Extensive research on fiber surface treatments and sizings is
also underway to augment the fiber/matrix chemical bond, further
increasing static and fatigue properties.
The Delft program will manufacture a 2m/6.6-ft long demonstrator blade, which will be tested in the University’s wind tunnel by second or third quarter 2009. The team also is investigating alternative heating systems that won’t require heating the entire mold mass to such high temperatures. Already, processing is being upgraded to handle the thicker laminates in follow-on 3m/9.8-ft and 10m/32.8-ft blades. These will be developed in cooperation with and installed on turbines for in-service testing by the Wind Energy Unit of the Energy Research Centre of the Netherlands (ECN, Petten, The Netherlands). Bersee sees these stages as realistic steps toward MW-sized offshore wind turbines equipped with affordable thermoplastic composite blades embedded with structural-health monitoring sensors and camber control devices (i.e., Smart Blade technology) by 2020.
Despite the strong
interest in reinforced thermoplastics, some question their
practicality in today’s — and especially tomorrow’s — increasingly long
blades. In a March 2007 article in New Energy magazine, Lars Weigel,
CEO of rotor blade manufacturer Abeking & Rasmussen Rotec (A&R
in Lemwerder, Germany), sounded a cautionary note with respect to
processing thermoplastic composite rotor blades. “Theoretically, that
is an interesting alternative to GRP in blade production, but I don’t
see the practicality of it yet,” he said. Noting that “from the end of
2009, we will make blades longer than 61m [200 ft] in Bremerhaven,
[Germany],” he expressed skepticism about processing a material that
requires 200˚C/392˚F: “In molds of 40 to 60m [131 to 197 ft] the
thermal expansion is too great.”
Bersee acknowledges that thermal
expansion will be an issue, particularly at greater blade length, but
he points out that it’s an issue others in the aerospace industry have
faced before, and he believes that it can be overcome in the design of