Low-Wind-Speed Blade Prototype Advances

A new blade design is promising to increase efficiency and reduce the cost of energy for wind turbines at low-wind-speed sites. The most distinctive characteristic of the blade is a gently curved tip, which allows it to respond to turbulent gusts in a manner that lowers fatigue loads on the blade.

Measuring 27.1 meters — almost three meters longer than the baseline it will replace — the blade is made of fiberglass and epoxy resin, and features a curvature toward the trailing edge. The sites targeted for the prototype have annual average wind speeds of 5.8 meters per second — measured at 10-meter height. Named Sweep Twist Adaptive Rotor (STAR), the blade was developed by researchers at Sandia National Laboratories in partnership with Knight & Carver (K&C) of San Diego. “This design allows the blade to twist more than traditional designs, thus relieving some of the effects of gusty turbulent wind on blade life,” says Tom Ashwill, who leads Sandia’s blade research efforts. “This then allows us to grow the blade length for the same rotor, providing for increased energy capture of 5-10 percent and yet retaining the same expected fatigue life.” The first STAR blade was tested in January at Knight & Carver’s fabrication facility in San Diego to determine its bending and twist behavior due to static loads. Natural frequencies were also measured. This data will be compared to design simulations to determine how well the design concept performs. Four additional blades will be fabricated in the first quarter of 2007 — three of which will be flight-tested on a turbine in Iowa. The K&C contract is part of the Low Wind Speed Technology (LWST) project that targets wind sites that are not the strongest but plentiful. In late 2005 the Department of Energy (DOE) and Sandia awarded Knight & Carver the $2 million contract that includes $800,000 in K&C cost share. Because of budget reallocations, this project was the only one of several LWST projects to receive 2007 funding. “The DOE interest and funding are a big step for us,” Ashwill said. “We’ve been pushing for the incorporation of innovative concepts into utility-scale blades for some time now as a way of reaching program goals of lowered cost of energy.” The continued increase in the average size of utility-grade wind turbines may come to an end before all efficiencies are wrung out unless blade weight growth (which is nonlinear) can be reined in, added Ashwill. The challenge is to develop new concepts that reduce the rate of weight growth, such as the swept STAR blade. Other weight-reducing concepts such as carbon spar caps, off-axis carbon fibers that facilitate bend-twist coupling and new “structural” airfoils have been incorporated at a smaller scale in 9-meter-long prototype blade being flight-tested at Sandia’s test site in Bushland, Texas, at the U.S. Department of Agriculture ARS (Agriculture Research Station) facility.
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