Monday, August 6, 2012

Wind Turbine Blades: Carbon Fiber vs Glass

From its inception, the wind energy industry has had to fight to compete with other forms of electric power generation. Wind energy producers not only face that battle, but also wage war against each other for a competitive share in the wind market. Both battles boil down to a need to improve the economics of wind energy through increased energy capture. This has prompted a well-documented growth spurt in the size of turbines and rotor blades for land-based and offshore systems (see “Wind turbine blades: Big and getting bigger,” under “Editor’s Picks,” at top right). Offshore turbines are moving quickly from 3 MW to next-generation turbines rated at 5 MW and larger, on which blade lengths for both on- and offshore systems regularly exceed 45m/148 ft. As blades grow longer, the idea of converting structural areas of the blade from E-glass to significantly stiffer and lighter carbon fiber begins to make sense, despite the latter’s greater upfront cost.

Carbon fiber already has proven to be an enabling technology for turbine manufacturers Vestas Wind Systems A/S (Aarhus, Denmark) and Gamesa Technology Corp. (Zamudio, Vizcaya, Spain). Both companies embraced carbon fiber years ago, using it in select structural parts of their blades and taking advantage of the lighter weight blades throughout the turbine system. Lighter blades require less robust turbine and tower components, so the cascading cost savings justify the additional cost of carbon. “Vestas and Gamesa designed their turbines around the use of carbon fiber and, by virtue of that, the whole system cost is less than a system with an all glass-fiber blade,” confirms Dr. Philip L. Schell, executive VP of wind energy at carbon fiber manufacturer Zoltek Corp. (St. Louis, Mo.).

And that is before the increase in turbine efficiency that additional length enables. For example, the switch to carbon fiber enabled Vestas, initially, to add 5m/16 ft in blade length without any additional weight gain. The Vestas V112-3MW turbine is designed for low- and medium-wind areas and sports three 54.6m/179-ft blades. These blades have the same width as the company’s 44m/144-ft blades, but they sweep an area that is 55 percent larger. The result is considerably higher energy output.

More recently, GE Energy (Greenville, S.C.) joined the fray, specifying carbon fiber in its next-generation wind blades, including the 48.7m/160-ft blades for its 1.6-100 turbine. Yet, speaking at CompositeWorld’s 2011 Carbon Fiber conference in Washington D.C., Nirav Patel, senior lead engineer of GE Energy-Manufacturing Technology, issued a warning that carbon fiber cost and supply concerns could be showstoppers to further use of carbon fiber in GE applications. Patel also called for increased automation and improvements in manufacturing processes (see “What is carbon fiber’s place in wind energy systems?” under “Editor’s Picks”).

Notably, it may be GE’s use of carbon fiber to increase blade length on its 1.6-MW system that will ultimately push more wind energy companies to embrace carbon fiber. “GE’s decision to put a 100m [328-ft] diameter rotor on a 1.6-MW turbine has captured the attention of a lot of companies in the industry,” says Dr. Kyle Wetzel, president, Wetzel Engineering (Lawrence, Kan.), which designs wind turbines and rotors. “As a result, we are being approached by a number of companies that want to similarly upsize their machines.”
A carbon backbone

Retrofitting existing turbine designs with longer blades that incorporate carbon has become a shortcut to marketability. “It’s always best to do a system-level design — treating the rotor, the turbine, and the tower as one system — but the reality is that the energy market is so competitive and everyone is so worried about what their competitors are doing, that they often don’t have time to do a system-level design,” explains Wetzel. “So, if a company decides to go to an extremely large blade on an existing system, then carbon fiber becomes an enabling technology by allowing for increased blade length without increased weight.”

Currently, carbon fiber is used primarily in the spar, or structural element, of wind blades longer than 45m/148 ft, both for land-based and offshore systems. The higher stiffness and lower density of CF allows a thinner blade profile while producing stiffer, lighter blades. The rough rule of thumb for weight reduction, offers Schell, is at least 20 percent weight savings when moving from an all-glass blade to one with a carbon fiber-reinforced spar cap. Offshore wind systems — where the smallest turbines are rated at 3 MW — will especially benefit from the characteristics of carbon.

“Assuming that offshore continues in its positive direction and costs remain under control, I wouldn’t be surprised to see 8-MW to 10-MW turbines with 80m to 100m [263-ft to 328-ft] long blades in the next three to five years,” says Schell.

“A 100m blade made entirely out of glass fiber could weigh up to 50 metric tonnes [110,231 lb],” he notes. “When you consider achieving a 20 to 30 percent weight savings by incorporating carbon fiber, that’s a weight savings of 15 metric tonnes [33,069 lb]. Multiply that by three and it can make a significant difference,” Schell stresses.

“In a 100m blade, the weights get so high that we are starting to investigate using carbon in the skins of the blade for added weight reduction,” says Wetzel. In a conventional, land-based blade design, however, the spar cap is the only area where Wetzel would recommend CF, but Schell notes that one company is using a hybrid glass/carbon reinforcement in the root section of the blade.

“In some very specialized blades, we’ve incorporated carbon in the trailing edge in an effort to tune some of the natural frequencies of the blade,” says Wetzel. “And carbon could come into play in aeroelastic tailoring,” he adds, noting that the idea is to build a small amount of twistability into the load response of a blade with asymmetric fiber layup in the blade skin to shape the power curve and reduce loads. “It’s a concept that’s been around for about 10 years, but I think it’s going to soon find its way into some commercial wind blades — very large blades.”

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