Materials and design methods look for the 100-m blade

Craig Collier
President
Collier Research Corp.
hypersizer.com
Tom Ashwill
Technical Leader
Wind Energy Technology Department,
Sandia National Laboratories
windpower.sandia.gov/

Commercial wind-turbine blades have historically relied on fiberglass as a primary material. In 2010, a Sandia National Laboratories’ report estimated annual U.S. industry usage for utility-grade turbine blades at over 70,000 metric tons. As designers build bigger blades in an effort to boost power production and cost efficiency, material systems are evolving to account for the increasing weight and additional gravitational stresses. Engineers are now looking to high-performance composites for greater strength and lighter weight at competitive prices. But consider that a typical 1.5-MW blade is 33 to 40-m long, weighs up to eight tons, and can have composite layups as thick as 4-in. at the root. Now you begin to grasp the engineering challenge inherent in designing an efficient, cost-effective composite blade.

Since the early 2000s, Sandia’s Wind Energy Technology Department has been conducting prototype projects to develop and evaluate a variety of innovations for wind blades, including new material systems, more efficient structural architectures, load alleviation methods, and thicker airfoils for increased structural performance. A program currently underway at the government lab explores the design of a 100-m blade (potentially for a 13.2-MW turbine)  targeted for offshore use and asks the difficult design and material-system questions that accompany increasing blade length.

To help answer some of the questions, Sandia will be working with Virginia-based Collier Research Corp., to apply its composite analysis and optimization software to large-blade-prototype designs. The software, HyperSizer, a NASA technology-transfer spinoff, has been used extensively by the space agency (in the ARES V launch vehicle and Composite Crew Module) and in aircraft to structurally size complex composite and metallic designs. The software complements finite-element analysis (FEA), working in a feedback loop with commercial codes to search for solutions that minimize weight, while maximizing strength and manufacturability—all issues critical to wind-turbine design.

Asking material questions
In place of fiberglass, or glass fiber-reinforced polymer, blade designers are turning to carbon fiber-reinforced polymer for its superior weight-to-strength characteristics. Carbon fiber is already used extensively in the aerospace industry—in the Boeing 787, Airbus 350, Bombardier LearJet 85, and Goodrich engines—where higher strength, lower weight, and greater fuel efficiency are design goals.

The question of when and where to substitute carbon for fiberglass in a wind blade is not simple. For one thing, even though carbon fiber is significantly stronger and stiffer than fiberglass, it is much more expensive. Also, an extensive library of glass and carbon fabrics and tapes with varied fiber orientation, strength, and rigidity, as well as a host of sandwich cores and hybrid laminates with diverse properties, makes materials decisions even more difficult for designers. Tremendous variation in internal loads along the length of a wind blade further amplifies the complexity of the material system design.

To help unravel design uncertainties, Sandia’s past prototype projects focused on the use of carbon fiber to control the loading scenarios of increasingly bigger blades. The CX-100 (carbon experimental) contained a full-length carbon spar cap – at the time a relatively new concept. The TX-100 (twist-bend experimental) used both a fiberglass spar cap that ran only half the length of the blade and unidirectional carbon fibers in the skins to passively shed aerodynamic loads through twist-bend coupling. The third prototype, the BSDS (blade system design studies), also used a full-length carbon spar cap, but experimented with airfoil shapes and dimensions of the root. The current 100-m blade study focuses on designing an all-fiberglass composite blade that can withstand international certification loads including operational, fatigue, and buckling, as well as manufacturability considerations.

While Sandia’s research is advancing blade technology and seeding industry innovation, there are still many gaps in knowledge and practice. Design areas ripe for innovation and optimization include material type, material placement, internal architecture (number of shear webs, spar cap thickness, and more), and airfoil planform. Where materials are concerned, because loads vary over a wind blade’s root, spar, shear webs, and free-flowing surface, it is difficult for a designer to know what shape to make a laminate zone, where to stop one zone and start another, or how to determine an optimum thickness of layups in different zones. It is also difficult (almost impossible) to manually calculate how to handle transitions between zones and where to position many individual ply drops and adds in a single blade. Resin and layup process variables introduce even more complexity and signal a need for additional design tools.

Magnifying material answers
A material design model typically starts by mapping rectangular-shaped sections for the laminate zones, based on accumulated knowledge and rules of thumb. But the reality of buckling, bending, twisting, deflection, and aerodynamic loading is anything but regular. Software such as HyperSizer helps. Using blade-loading results from FEA, the software maps the laminate zones to more accurately represent the blade physics and then calculates a ply stacking sequence for each zone.

To accomplish this, FEA is first run to determine internal loads and deflections in the blade. Those loads are then imported into HyperSizer, which performs tradeoff studies, surveys thousands (or sometimes even millions) of candidate laminates, and exports the new material properties. Then the FEA model is rerun

As part of the analysis, the software performs a sizing optimization, failure-and-fatigue investigations, and weight-trade studies. It also calculates margins of safety (factors of safety) and best configurations for transition zones. Surveying designs in a ply-by-ply and even finite-element-by-element manner, the software leads users to customized laminate solutions early in a design process, using a wide variety of composite materials.

A typical analysis and optimization takes about four hours, while eliminating offline spreadsheets and manual calculations. The software can also exchange laminate specifications with CATIA and FiberSIM.

Wind’s material future
There is currently no “best design” configuration for wind turbines. The engineering community is still searching for the right combinations of structural innovation and complementary material solutions.

But when Sandia’s prototype blade research first started in 2002, engineers didn’t even know if they could mix carbon fiber with glass fibers because their strength properties differed by a factor of three. Now they know a combination of advanced materials including carbon fiber, hybrid laminates, and sandwich cores of all material types can play important roles in blade design. Along the way they have accumulated more than 10,000 fatigue-test results for about 150 different composites, all of which can be downloaded into the software’s material database

Analysts at CompositeWorld’s 2009 Carbon Fiber Conference agreed with Sandia researchers’ findings about the value of new materials. They predicted that by 2014 wind blades will be consuming 35,000 to 50,000 metric tons of carbon fiber annually.

As wind technology matures, engineers are learning how to build longer, stronger, and lighter blades using the latest high-performance composites. Advanced analysis tools, such as HyperSizer, will accelerate that learning curve. The software’s track record in the aerospace industry has been weight reduction averaging 20%. Test cases on wind blades are yielding similar results.

WPE

National Labs and OEMs Have Lots of Ideas, Suggestions for Suppliers

The recent Wind Power Explained conference, presented at the Design and Manufacture Expo in Chicago, presented a full day of discussions to attendees on topics from the Department of Energy, NREL, Sandia Labs, GE Energy, Northern Wind, and Clipper Windpower. Here’s a sampling:

“The DOE recognizes the challenge in making wind energy a reliable and integrated source,” said Ron L. Harris, from the agency’s Office of Energy Efficiency. Harris told how the DOE wants to ensure that the supply chain for wind technology is sufficient to increase market demand and consistent with a goal of supplying 20% of the U.S. electricity needs by 2030. “The agency also wants to maximize opportunities for domestic manufactures of wind-energy equipment by facilitating supply chain development.” In the Q&A session, Harris mentioned that developers of small wind turbines should submit ideas and products to the U.S. Army because when deployed in remote regions, they would like to take power sources other than gas-powered generators, and small turbines might work well.

Sandy Butterfield, Chief Engineer, National Wind Technology Center, discussed his experience with gearboxes. “They were a source of reliability problems and maintenance costs in early designs,” he said. “Fatigue loads were the driving loads. Since then, development of standards, such as IEC 61400-22 have helped improve their design so they are more reliable. Focus now is shifted to bearings.” Regarding turbine size, Butterfield points to the problems of shipping, such as bridges too low for large turbine blades and nacelles, that will keep land based turbines to about the maximum where they are now. “Off shore, however, the designs would have no restrictions so turbines could grow to 10 MW and larger,” he said.

Paul Veers, Technical Staff, Wind Energy Technology Department, Sandia National Labs, discussed how to get more power out of the wind. An early wind engineer Albert Betz observed that about 59% was a theoretical maximum. Recent designs have gotten bigger and taller. But a larger rotor increases in cost with the cube of the length increase, and taller towers cost go up with height to the fourth power. “Hence, the only way to win this cost battle is to build rotors that are smarter and components that are lighter,” said Veers. Then he showed several ideas for doing so.

Lawrence D. Willey, GE Infrastructure, Wind, said the U.S. is likely to see a doubling of energy needs by 2030. Good news is that wind is now cost-competitive with other fuels. “The company has developed software for engineers to guide their design work with regard to cost. The software includes influence coefficients that determine costs.

What’s more, engineers have lots of ideas, and a value analysis is one way to weigh them. “It’s a way to tell we are working on the right thing for customers and ourselves,” he says.

The beginning is the sweet spot and when to put in all the options. “You can’t start early enough when it comes to designing to cost,” he added. Even for subcomponents, this makes sense. Money spent at a project’s start is money spent wisely. When manufacturing begins, its too late for cost savings.

Taylor Robinson, VP of Global Supply Chain for Northern Power Systems, discussed how the design of company turbines makes for an efficient supply chain. “We want a supply chain close to the company and working quickly. The concepts here go back to six sigma and lean.”

“Quality will be essential from suppliers, and they must deliver on time and at the agreed cost,” he said. Components for his firm’s 100 kW turbine come from Europe and U.S. suppliers. One turbine a day is their production target. “Another goal is to get suppliers delivering more complete subsystems rather than a few components. With the right partner, it is possible to have more control over a design from a supplier than if the component were made in your own factory,” he said. Robinson added that he will work on long-term agreements with suppliers, but the arrangements have to be win-win agreements.

Derek Ptech, Director of Engineering for the 2.5 MW turbines at Clipper Windpower, said value analyses come from designing to cost. One trend in the industry is certifying the technology by third parties, such as Germanischer Lloyd.

One industry driver is that energy consumption will be up by 50% in 2020 along with a population growth of 20%. “The more steady feed-in laws in Europe have resulted in steady growth there. Likewise, because Texas has an RES, it has a greater install capacity than other states, so a national RES would be a boost to the entire industry.”

Ptech added that increased reliability is one governing ideal for the company turbines because some components have no backup. “For instance, pitch mechanisms in the hub for each blade work independent of each other and so have batteries to run them in case conditions take the turbine off grid. Dramatic events define design conditions,” he said. Another example is that the Clipper has four generators so that one, two, or three of them can be removed and the turbine still produces power.

Full day wind session at Design & Manufacturing Midwest

In collaboration with Canon Communications, Windpower Engineering (www.windpowerengineering.com) and Design World (www.designworldonline.com) will be primary media sponsors for a one-day session titled, “Wind Power Explained – Design and Component Integration.” The session will be colocated at the Design and Manufacturing Midwest show and scheduled for Wednesday, September 23, 2009 at Donald E. Stephens Convention Center, Rosemont, Illinois. Scheduled speakers will be from the Department of Energy, Sandia National Labs, National Renewable Energy Laboratory, GE Energy, Clipper Wind Power, and Northern Power.

Blade sensors could let turbines adapt faster

Researchers have developed a technique that lets sensors monitor forces exerted on wind turbine blades, a step toward improving their efficiency by letting them adjust to rapidly changing wind conditions. The research by engineers at Purdue University and Sandia National Laboratories is part of an effort to develop a smarter wind turbine. “The goal is to feed information from sensors into an active control system that precisely adjusts components to improve efficiency,” said Purdue doctoral student Jonathan White, who is leading the research with Douglas Adams, a professor of mechanical engineering and director of Purdue’s Center for Systems Integrity.

The system also could help improve wind-turbine reliability by providing critical real-time information to the control system to prevent catastrophic wind-turbine damage from high winds.

The team embedded uniaxial and triaxial accelerometers inside a wind turbine blade as it was built. The sensors measure acceleration in different directions, necessary information to accurately characterize a blade’s bending and twisting and small vibrations near the tip that eventually cause fatigue and possible failure.

The sensors also measure two types of acceleration. One type, dynamic acceleration, comes from gusting winds, while the other, static acceleration, results from gravity and steady background winds. It is essential to accurately measure both forms to estimate forces exerted on the blades. The blade is being tested on a research wind turbine at the U.S. Department of Agriculture lab in Bushland, Texas.

Such sensors could be instrumental in future turbine blades with “control surfaces” and simple flaps like those on an airplane’s wings to change blade aerodynamics. Because these flaps would be changed in real time to respond to changing winds, constant sensor data would be critical.

“The industry is most interested in identifying loads exerted on turbine blades and predicting fatigue, and this work is a step toward accomplishing that,” says White.

“It is useful to control the blade pitch to optimize energy capture by reducing forces on the components in the wind turbine during excessively high winds, or increase loads in low winds. This should also help improve reliability. Turbine towers can be 200 feet tall and more, making it expensive to service and repair damaged components,” says White.

The research is funded by the U.S. Department of Energy through Sandia National Laboratories. Sandia is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin Co., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.