New wind blade design for less windy sites

Proof load testing the STAR blade using 55-gallon drums

Proof load testing the STAR blade using 55-gallon drums

A new wind turbine blade design for low-speed wind sites promises to increase by 20-fold the available land area that can be economically developed for wind energy in the US, according to its developers Sandia National Laboratories and yacht & blade specialist Knight & Carver.

Named “STAR” for Sweep Twist Adaptive Rotor, the blade is the first of its kind produced at utility-scale. Its most distinctive characteristic is a gently curved tip, termed “sweep,” which unlike the vast majority of blades in current use, is specially designed for low-wind-speed regions, such as the Midwest US. The sites targeted by this effort have annual average wind speeds of 5.8m/s, measured at 10m height. Such sites are abundant worldwide – for example, Hong Kong’s only utility-scale wind turbine operates at an average wind speed of 5.5m/s, which is proving less than ideal. The Hong Kong site however would be a good candidate for the STAR blade.

Sized at 27.1m – almost 3m longer than the baseline it will replace – the blade improves energy capture at lower wind speeds. Instead of the traditional linear shape, the blade features a curvature toward the trailing edge, which allows the blade to respond to turbulent gusts in a manner that lowers fatigue loads on the blade. It is made of fiberglass and epoxy resin.

“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%t 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.

Ashwill says the continued increase in the average size of utility-scale wind turbines may come to an end before all efficiencies are wrung out unless blade weight growth (which is nonlinear) can be reined in. 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 fibres that facilitate bend-twist coupling, and new “structural” airfoils have been incorporated at a smaller scale in 9m prototype blade being flight-tested at Sandia’s test site in Bushland, Texas, at the US Department of Agriculture’s Agriculture Research Station facility.

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