Carbon Fiber Boosts Blade Performance

November 17, 2008

6 Min Read
Carbon Fiber Boosts Blade Performance

Demand for wind turbines is now exploding, shaking up materials' technology in the long-moribund industry.

For more than 30 years, turbine blades have been made from glass-reinforced thermoset composites, pretty much the standard technology long used to make boats and some automotive parts — most famously the body panels for the Chevrolet Corvette.

“The recent dramatic growth in the wind industry is just beginning,” says Steven C. Lockard, CEO of TPI Composites of Scottsdale, AZ. There will be eight wind blade manufacturers in the U.S. by the end of this year, up from two three years ago. Twenty-eight new plants have been announced since January 2007.

TPI is opening a new blade manufacturing plant in Newton, IA, the same town devastated by outsourcing in the appliance industry just last year. Many wind plants are being built in states such as Texas, Oklahoma and North Dakota that experience steady and strong prairie breezes. Wind plants are also being constructed in California, Michigan, New York and nine other states.

“The U.S. is on pace to surpass Germany and become the nation with the largest installed base of wind power by the end of 2009,” says Victor Abate, vice president, renewables, GE Energy, which has become one of the largest global players in the field. Others are Suzlon Energy, India; Gamesa, Spain; and Vestas Wind Systems, Denmark. In September, Siemens announced plans to expand its role in the market.

Wind turbines are comprised of thousands of components, most of which are used in a large box called a nacelle that sits on top of the tower and houses the generator that converts mechanical energy to electricity. Huge composite wind blades and steel towers are being built near assembly sites to avoid shipping headaches and costs.

Since 2002, GE has increased the rotor diameter of its 1.5 MW turbine to 82m (269 ft), increasing capacity factor — turbine efficiency — by nine points. A one point increase in capacity factor provides electricity for 150,000 average U.S. homes. Improved reliability has also been a major goal of new designs and materials. In 2002, state-of-the-art turbines were available to produce electricity less than 85 per-cent of the time. “Technology advances in remote monitoring and diagnostics and the utilization of GE reliability monitoring have increased the reliability of our wind turbines by 12 points,” says GE's Abate. A 1 percent increase in reliability provides power to an additional 50,000 homes.

But reliability has become a problem for some wind turbines because the explosion in size and power has at times exceeded the capability of existing materials' systems.

For example, turbine blades split at U.S. wind farms financed by Edison International and John Deere, prompting the supplier — Suzlon International — to recall more than 1,200 blades. Failures were also reported in Suzlon's home market of India. “The machines are not fit to handle the wind,” says Shrenik Baldota, managing director of MSPL Ltd., a Suzlon customer. Two wind turbines made by Vestas collapsed in Denmark. Vivek Kher, Suzlon's vice president of communications, did not respond to an interview request by Design News.

Part of the problem is the supply chain. There is a rush to fill orders from a supply base that is rapidly increasing in size. Producers now have a three-year backlog, up from just six months two years ago. More significant, though, are the engineering problems. Design engineers are trying to ramp up the size of turbines with 30-year-old technology. New patents, however, show significant changes are coming.

“One of the most promising opportunities for significantly reduced cost of energy is through the coordinated development of superior low-cost materials using reliable, high-volume component manufacturing techniques for components used in wind power applications,” says Mansour H. Mohamed, founder and chief scientific officer of 3Tex Inc. of Cary, NC. “Reducing blade weight has a dramatic weight-saving effect throughout the rest of the wind turbine. However, a careful balance must be achieved between reductions in blade weight and the higher costs typically associated with specialized lightweight materials such as carbon composites.”

Carbon fiber-reinforced plastics (CFRP), the material used to make the Boeing 787 Dreamliner, possess about three times the stiffness of glass-reinforced plastics and also have significantly better fatigue properties. E-glass costs around $1 per lb, while standard modulus carbon fiber costs $6 to $20 per lb, depending on type, tow-size and purchase volume.

Benefits of CFRP

According to Mohamed, the use of CFRP offers four specific advantages in wind turbine manufacturing:

  • Potentially thinner and more efficient profiles resulting in higher energy output.

  • Stiffer blades resulting in shorter nacelles.

  • More slender blades resulting in lower extreme loads on towers and nacelles.

  • Lower blade mass resulting in easier to handle production and mounting.

Mohamed's company, 3Tex, is proposing wind turbine components made from woven carbon and glass fibers. A fully automated 3-D weaving process has been developed at North Carolina State University in Raleigh, NC. This process does not involve the building up of layers in the fabric. Rather, 3-D fabric is formed during each weaving cycle.

“There are at least three revolutionary advances contained within this process, including the automated use of multiple weft insertion in a single weaving cycle, the automated method of producing net-shaped forms in various cross-sectional shapes, including 'I,' 'T' and 'P' shapes, as well as core or pile structures, and the ability to include controlled amounts of Z direction fiber, for example up to 1/3 of the total fiber volume,” says Mohamed. A recently awarded patent discloses a 3-D, carbon/glass hybrid woven wind blade spar cap that can be used to strengthen a blade.

Balsa wood cores are also being used to reduce weight in areas of rotor blades where high shear and compression strengths are required. Foam cores are used for similar applications to lower costs.

GE was awarded a U.S. patent last July for an approach that combines the use of carbon fiber and balsa in a new modular approach designed to overcome the problems of large composite blade construction. “The size of a continuous blade is not conducive to high-quality composite material processing and, therefore, structural performance,” says Ron Cairo, an engineer at GE Energy's Global Center of Excellence for Gas Turbine engineering designs in Greenville, SC.

The sheer size of a blade made from fiberglass/epoxy composite material using full blade-length female tooling makes it difficult to eliminate all trapped air during the lamination process and to achieve uniform processing along the entire blade length. “The likelihood of re-work during manufacturing is high because the cost of scrapping is high,” says Cairo. “Blade quality may suffer as a result and the cost of manufacturing is high.”

Cairo's GE innovation has a blade with several modular segments stacked in the shape of the blade and held together with cables. Tensioning cables run through the conduits of the stacked segments and hold the modular segments together. Unidirectional carbon/epoxy caps are used to carry blade bending loads in axial tension and balsa-wood webs are used to carry transverse shear caused by blade bending.

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