Wind turbine blades' basic physics and economics are relatively simple. For one, their power output is roughly proportional to the square of blade length. This relationship pushes designers to create increasingly longer blades for harvesting additional energy. Secondly, as blades get longer, weight increases – by about the cube of the length – which raises raw material costs. This correlation sends designers in search of weight-efficient geometries that are strong and rigid enough to weather the increased loading inherent in longer blades.

To determine whether the sweep-twist geometry would shed loads as predicted, two wind scenarios were applied to the model: an operating load and an extreme-wind conditions case – 50-year gusts at 156 miles/hour (251 km/hour). The analysis was used to predict the blade deflection and twist, perform detailed stress calculations, and investigate potential shear buckling due to the increased twist inherent in the design (see Figure 1: page 5). For normal loads, there was excellent agreement with the tip deflection results from the section analysis. For extreme wind cases, strain value comparisons were also good. In further detailed studies, the engineering team found that their design met critical buckling limits at more than five times those of extreme wind conditions – a large margin of safety driven by the fact that stiffness to limit deflection, rather than ultimate material strength, was the key structural criterion (see Figure 2).

 

Figure 4. The unidirectional composite fabrics follow the curvature of the blade (NSE Composites)

Static physical testing of prototypes – for shear strain, blade deflection, and twist angle – followed FEA. The static test verified twist response under operating load conditions. Using fatigue testing, the K&C team was able to validate a 20-year lifespan for the new design.

The prototypes were also field-tested, generating extensive data (as well as power) for several months in Tehachapi, California – site of the TerraGen commercial wind facility and also one of the largest wind generation areas in the country.

A Snapshot of STAR’s Composite Makeup

At the STAR project’s outset, total rotor diameter for the prototypes was set at 56 metres. Like most commercial blades, the STARs were to be fabricated using fibreglass and epoxy. Composite design included a stressed-shell approach in which the top and bottom shells are connected by a single shear web, rather than the more typical double web construction (see Figure 3). As the design went through numerous iterations over the course of the project, specific composite materials were carefully chosen for each blade feature.

In the sweep-twist blade design version that NSE analysed with Abaqus FEA, unidirectional roving was called out for the blade’s spar caps, and double-biased fabric covering a balsa core was the choice for the shear webs and shell panels. For the skin, a mix of dual-biased and spanwise-oriented (along the length of the blade) unidirectional fabric was selected during preliminary design tests for its ability to follow the curvature of the blade during fabrication (see Figure 4). This combination increased the overall stiffness-to-weight ratio of the blade while improving torsional response – a key factor in the twisting and load-shedding capability of the design. The maximum curvature at which fabrication and layup would become problematic from a material standpoint was also investigated.

A Simple Twist with Major Industry Implications

With simulation and testing complete, the sweep-twist design’s promised load-shedding response resulted in what Hoyt characterises as a ‘very impressive’ 12% power output boost over similarly-rated turbines now in operation – without load increase.

‘I think it’s the future of blade design,’ says Hoyt. ‘People are actively pursuing it.’ GE and Siemens, two of the biggest players in the industry, are currently developing swept blades. A start-up, Zimitar – founded by researcher Mike Zuteck, who first conceived the sweep-twist design – won a US$4 million contract with the DOE to pursue the technology for bigger offshore wind turbine installations.

However, some questions remain regarding scaling of the technology to rotor diameters of 50-60 metres typically seen offshore.

At the fifth Sandia Blade Workshop (in spring 2012), engineers discussed possible downsides. Will torsional loading due to sweep introduce other problems like new modes of aeroelastic flutter? Or will it produce too much fatigue stress in the adhesive joints? There may also be issues on the manufacturing and transport side stemming from the harder-to-handle curved shape, says Hoyt. ‘But I wouldn’t be surprised if in 10 years a lot of production wind blades had some sweep in them.’

With savings in design time, reduced testing and potentially increased power output, this new twist on traditional design could be a big win for wind.