GE is always looking at new glass fibers and resin systems that might reduce weight. “We have had a lot of experience with carbon with our aviation business, and carbon is one-third the density of glass, so certainly that becomes attractive. But it’s a lot more expensive,” he says. “So that’s the tradeoff. What is the investment, what is the value added, what are the tradeoffs between different material systems and design processes?” Another important consideration is the availability of a material, which means checking the supply chain to ensure that potential materials will be available in the quantities that GE requires.
GE researchers have leveraged the tools they developed for aviation blades to improve the infusion process for wind turbine blades. “It sounds simplistic, but it’s quite complex because you are trying to model 40- or 50-meter long blades. You are looking at every material aspect, you are tracking the flow of resins, you are trying to optimize it so that you minimize any defects,” Nath adds.
While GE has experience in automated fiber placement and automated tape layup in the aircraft industry, Nath says it is not yet cost effective for wind because it won’t work in the volumes that the industry requires. But additive manufacturing offers possibilities, especially in tooling. The production of a blade tool can take 10 to 12 weeks, but with additive manufacturing that time could be reduced to a week.
Nath believes, however, that better design tools may play the biggest role in turbine blade improvements. “A wind blade is a combination of the aero structure, the actual structural analysis (the mechanical design) and the performance, because the customer at the end of the day is only interested in the annual energy production,” he says. “These are all conflicting. The aero guy develops a shape, but the manufacturing guys say that’s too expensive. So I think design tools that help us optimize all that are going to be another big innovation.”
Exploring New Materials
Almost all turbine wind blades today are made with thermoset resins, but that could soon change.
In 2013, a group of 11 European research and industry partners launched the WALiD project (Wind Blade Using Cost-Effective Advanced Composite Lightweight Design) to investigate using thermoplastic materials in all the areas of a blade. “We developed new material concepts and processing technologies. At the same time, due to the new process technologies, we were able to modify the design of the blade according to the new material properties,” says Florian Rapp, head of team foam technologies for Fraunhofer Institute for Chemical Technology ICT, Polymer Engineering.
During the four-year project, which ended in January, the WALiD partners developed highly durable thermoplastic foams and composites, a thermoplastic coating with high erosion and UV resistance and an automated fiber placement process for lay-up of hybrid fiber tapes. This resulted in a lighter blade with an improved design and an increase in service life, according to the WALiD website.
The project focus was not on manufacturing a whole blade, but on material definition and process qualification. “We made a lot of characterization regarding mechanical performances on coupon and subcomponent levels,” says Rapp.
While there are no working models of WALiD’s rotor blades as of yet, Rapp says many manufacturers have expressed interest in their work. But manufacturing a complete thermoplastic blade will require a massive change in current production methods, which is especially hard to do when producing huge blades. Rapp believes that the technology WALiD developed might be used sooner for smaller blade production.
Researchers at the National Renewable Energy Laboratory partnered with IACMI-The Composites Institute on a thermoplastic wind blade. Here they prepare the 9-meter turbine blade mold for production. Photo Credit: NREL
Meanwhile, researchers at IACMI-The Composites Institute’s National Renewable Energy Laboratory in Denver have manufactured an experimental thermoplastic blade just 9 meters long.
“We can’t go out and build a 70- or 80-meter blade every time we want to innovate with a new material or a new manufacturing process,” says Derek Berry, wind technology area director. Although the laboratory began its research with coupon-level testing, the 9-meter blade is an ideal size to prove that you can scale up to thicker, bigger and more complex parts without going to the large megawatt-size blades, says Berry. “You can get 80 to 90 percent of the way there in understanding how a material will function in the manufacturing process, what type of properties you can get and things like that … . It gives you a huge amount of information on whether you should move forward with that material and that manufacturing process.”
The most aggressive innovation in the experimental blade was the use of a thermoplastic resin system. “Once you form [a thermoset blade] there is no way of going back; it is a chemical process that is irreversible. At the end of that blade life, 20 to 30 years down the road, the only thing you can do with it is put it in a landfill or maybe chop it up and use it for low-grade application,” he explains.
Thermoplastic resin blades, on the other hand, could be recycled. It might even be possible to pull out the fibers and the resin system and reuse them to make new blades or other composite structures.
IACMI’s experimental 9-meter blade was made with Arkema’s Elium® resin system, which has an exotherm in the same range as thermosets. “It is a thermoplastic that works more like a thermoset when it comes to process,” says Berry. That’s significant because it would mean that blade manufacturers would not have to replace their tools and processing pumps.
While researchers started looking at thermoplastics because of their recyclability, it turns out they offer other benefits that may be even more important to the wind industry. For example, the use of thermoplastics would make it possible to thermo weld parts of the blade together. “Right now, we have two blade skins that we glue together with an adhesive,” says Berry. “Down the road, we could possibly get rid of that adhesive, just put the two skins together and then heat up the sections where they’re touching to bond them together. We could have better, more reliable and possibly less costly blades because of that thermal welding potential for thermoplastic resin systems.” Thermoplastic blades might also be easier to repair in the field.
Although IACMI researchers are still investigating whether thermoplastic blade production would be faster than thermoset, thermoplastic does have another advantage because it doesn’t require post-curing in an oven. That would save time, labor and processing time, in addition to capital costs related to oven purchases. Those savings could reduce blade costs as well.
Berry says the next step will be to make a full-size blade component – probably a large root section with over 100 mm thick walls – to test its exotherm.
“This blade has helped us to understand the challenges and learn more, but it’s just at the beginning,” says Berry. “We have several years’ worth of innovation between now and when thermoplastic resins may be used on megawatt-size wind turbine blades.” That work will include more coupon-level testing (for static, fatigue, lifetime, tensile strength, compression, shear and erosion) so that wind blade designers and manufacturers will have a complete database of the composite’s properties.
The goal is to take this promising manufacturing innovation and bridge the gap from research to commercialization, says Berry. “We’re here to work with our partners so that we can give them the base of what they need to make these decisions and to commercialize this technology to spread further into the market,” he says. Advances in blade manufacture could, in turn, improve the cost effectiveness of wind energy and drive a greater reliance on this renewable resource.
ACMA just released the Wind Blade Repair Recertification and has updated its Wind Blade Repair Certification materials. For more information, visit www.compositescertification.org.
