Longer, Stronger, Lighter, Cheaper

Improvements in composite turbine blades will power the wind energy industry.

Wind is an increasingly important source of renewable energy. In 2015, wind power installations generated about 435 gigawatts (GWs) of power worldwide, approximately seven percent of all global power, according to the Global Wind Energy Council (GWEC). By 2030, GWEC projects that wind power could supply 2,110 GW, or about 20 percent of global electricity demand. At least some of that growth will depend upon the advances made in the next decade in the composition, design and construction of composite turbine blades. Navigant Research, which specializes in analyzing clean energy markets, says that stakeholders involved in the wind turbine industry are devoting a greater amount of research and development investment to wind blades than to any other component in wind turbines. Blades are key to energy production; the longer the turbine’s blades, the longer the swept area and the more power they generate. Long 73.5-meter blades are producing the power for Deepwater Wind’s new Block Island Wind Farm off the coast of Rhode Island. The five-turbine, 30-megawatt wind farm, the first offshore wind energy installation in the U.S., began feeding electricity into the New England power grid last December. [caption id="attachment_4325" align="alignleft" width="300"] LM Wind Power manufactured the blades for the Block Island Wind Farm in Denmark, then
shipped them to the U.S. for installation in June 2016. Photo Credit: Deepwater Wind[/caption] GE Renewable Energy supplied the turbines. LM Wind Power, subcontractor for the turbine blades, built them at its Denmark plant and then shipped them to Block Island for installation in June 2016. The 73.5-meter blades were the longest in the world when LM Wind Power introduced them in 2012, according to Lene Mi Ran Kristiansen, senior manager, communications and sustainability, global communications. “With this blade, several innovative features were introduced to keep the weight down and ensure a smaller root diameter, but it was based on existing polyester technology.” The company used different materials to create its record-breaking 88.4-meter blade last June. A new hybrid carbon fiber material combined properties from the less-expensive glass fiber with very light but expensive carbon fiber. “With all the blades, there’s a balance to strike between weight and length, cost and performance,” says Kristiansen. The challenges when building these giant blades are very much related to having adequate manufacturing facilities, equipment and logistics solutions to get them from the plant to their destination. That destination is offshore for these big blades, and tools that can cost effectively enable storage on land, transport by boat and crane handling offshore are key. For manufacturing, several processes, such as laying up the raw materials in the mold had to be adjusted and rethought. With the huge size of the mold the operators are no longer able to reach out and manually put material in place in the way they can in smaller molds. In addition, as the vertical surfaces get bigger, the glass tends to slide down. These were just some of the production details that evolve into problems as the mold size increases. It took lots of preparation and continuous dialogue to generate ideas and develop solutions that were the key to overcoming these challenges, says Kristiansen. Reducing Variation Molded Fiber Glass Companies (MFG) has been manufacturing blades, spinners, nose cones and nacelles for wind turbines for more than 25 years. “The units have to get cheaper to install, cheaper to operate and therefore provide more margin for the bottom line customers, the operators and finally the users of electricity,” says Carl LaFrance, senior vice president, quality. That means producing longer blades. “When you double the length of a blade, you quadruple the amount of composites in it, unless you start narrowing down the design margin,” he says. A tighter design requires less variability in processes and materials. To gain more control over those factors, MFG is constantly looking for better materials, finishing technologies, and measurement and inspection methods. [caption id="attachment_4326" align="alignleft" width="750"] Molded Fiber Glass Companies (MFG) manufactures wind turbine blades at its plant in South Dakota. Carl LaFrance, senior vice president, quality, says that “variation is the enemy” when it comes to composite blade production. Photo Credit: MFG[/caption] “We are driving our vendors to change their processes to reduce variation. Variation is the enemy,” LaFrance says. MFG wants to know, for example, exactly how much glass is in a particular volume of laminate. “If you put in more glass than you need, you’re paying too much money. If you put in less than you need, you risk failure.” It’s essential to ensure that the glass goes in at exactly the right orientation or combination of orientations. “Blades are primarily unidirectional, because you want to tailor the strengths and stiffness in a very specific direction from root to tip,” says LaFrance. “If you are off by a degree in how you lay that material down, it can affect performance.” MFG has developed some proprietary processes to help improve accuracy and is beginning to use robots for the production of smaller turbine components. (Blades are too large.) “We’re also doing some work using pultruded components for wind turbine blades, but there are technical issues associated with it that have not been resolved,” says LaFrance. “The challenges in technology are really in the blades because they have to get bigger – much faster than cells or spinners do – and they have to get lighter and less costly. We are providing aerospace quality for a commodity price,” he says. MFG’s customers design most of their blades with E-glass fiberglass, but some request S-glass if they require more stiffness or strength. Although some turbine blade designers are now incorporating carbon fiber into their blades to improve stiffness, using the material causes additional challenges. “It has a much narrower window for manufacture, and it doesn’t like bending at all,” LaFrance adds. Carbon fiber is also conductive, making it more susceptible to lightning strikes, which are the most common source of blade damage. Developing Design Tools GE’s Global Research Lab’s research with composite blades in the aviation industry has informed its work in wind turbines. “We have developed manufacturing processes – an understanding of materials and materials’ behavior and design tools,” says Shridhar Nath, technology leader, composites. [caption id="attachment_4327" align="alignleft" width="350"] Molded Fiber Glass Companies (MFG) now uses a Combi-Lift blade handling unit, which has reduced its blade handling time and cost by an average of 70 percent. The company uses the unit to transfer blades to trucks for transport to the field sites. Photo Credit: MFG[/caption] He notes that longer blades present logistical as well as materials challenges. As blade size increases, transportation costs become a larger factor. GE is investigating whether a two-piece blade would resolve some of those issues, but that would require blade manufacturers to find ways to join them in the field. The manufacturing process for such blades would be slightly different. “For example, where do you break the blade up? It places more emphasis on the design aspect; we have to understand what are the loads, what is the wind regime, what are the mechanical issues or the structural issues that go into those?” says Nath. “Those are the challenges that we are addressing and that we are working toward resolving.” 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. [caption id="attachment_4328" align="alignleft" width="300"] 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[/caption] 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. [divider]Update on the CCT - Wind Blade Repair[/divider] ACMA just released the Wind Blade Repair Recertification and has updated its Wind Blade Repair Certification materials. For more information, visit www.compositescertification.org.