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Sweep-Twist Design Offers Higher Output at Lower Load

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.

Navigating a maze of engineering challenges such as these can lead to interesting design directions. At the US Department of Energy’s (DOE) Wind Energy Research Program at Sandia National Laboratories, one result has been the development of a sweep-twist adaptive rotor (STAR).

This curved blade was proposed in earlier theoretical research and has been garnering increasing interest for use in utility-scale applications. The new configuration is seen as a way to reduce operating loads on ever-lengthening blades. If successfully commercialised, the outcome could be larger, lighter, less expensive and more productive wind turbines.

In 2004, Knight and Carver (K&C) Wind Group, a San Diego, California-based wind turbine blade manufacturer, was awarded a DOE contract to develop STAR. Partnering with Sandia, K&C was responsible for design, fabrication, testing and evaluation of a sweep-twist prototype. The firm began by assembling a team of specialised companies and academic institutions, one of which was Seattle-based NSE Composites, brought on board to perform the finite element modelling (FEM) required for the new design.

Sweep-twist adaptive rotor (STAR) prototypes were installed on a Zond 750 kW turbine at the TerraGen wind site in Tehachapi, California. Of note are the aft sweep of the tip and the trailing edge curvature. (MD Zuteck)

‘NSE had done a lot of analyses over the years on composite aircraft and helicopter aerostructures for companies such as Boeing,’ says DM Hoyt, one of the company’s founders. ‘Plus, we were already troubleshooting another blade problem for K&C.’

Hoyt and his partners at NSE had been using Abaqus from SIMULIA, the Dassault Systèmes application for realistic simulation, as their finite element analysis (FEA) tool. ‘Simulation has been a great asset for both our aerospace and wind energy work,’ says Hoyt. ‘It enables us to explore new ideas and look at the performance of multiple designs and materials while minimising expensive testing.’

Sweep-Twist Blade Basics

Rather than a traditional linear profile, a sweep-twist blade has a distinctive gently curving tip (or ‘sweep’) with curvature towards the trailing edge. Theoretically, this shape allows the blade to respond to turbulent wind gusts through a process of controlled twisting and bending. As the blade twists, it sheds loads that would normally be translated as material stresses to the root (or base) of the structure. In nature, a similar sweep can be seen in the wing shape of birds that migrate long distances and in the characteristic profile of cetacean tails and dorsal fins.

Figure 1. Potential buckling near the blade root (NSE Composites)

The engineering upside of twist-coupling is the ability to create longer wind blades while avoiding the higher loads that typically accompany increased length. Reducing loading enables a lighter blade design with lower raw material costs and helps lessen fatigue stresses on the rotating machinery. In early calculations, the STAR design promised a decrease in fatigue loads of 20% using a 3° tip twist. But as the design progressed, longer blades that capture more energy with no increase in load were pursued.

Beyond the potential advantages of altering the traditional length-weight-cost relationship, twist-coupling is seen as a financially attractive solution for tapping low windspeed sites (defined as having an average velocity of 5.8 metres per second at a height of 10 metres). These sites – in contrast to the high wind speed locations that have been the focus of wind-mining to date – are more abundant and typically closer to major power-load centres. If the cost benefit proves favourable, development of low-wind locations could, for example, increase potential domestic wind farm area in the US by a factor of twenty.

Understanding Turbine Behaviour – without the Wind

‘Over the years wind blades have become more and more high tech. The industry is pushing the limits of design and materials,’ says Hoyt. ‘As that happens, engineers need to tighten up the loose legacy tolerances and manufacturing controls that originated in boat-building technology and adopt the more rigorous analyses that we have always done for complex aerospace structures.’

 

Figure 2. Stress/strain results for the STAR blade prototypes. Note the compression stresses on the upper, or low-pressure, side of the airfoil as the blade bends (NSE Composites)

Of particular use in wind blade analyses with FEA, notes Hoyt, is Abaqus’ ability to handle composite properties and control material orientation. It can calculate blade-tip deflection (to avoid ‘tower strike’) and accurately predict both torsional response (including twist angle, which is key to load-shedding) and shear-compression buckling stability (associated with sweep-twist) of composite sandwich structures.

An additional capability key to wind blade analysis is the extraction of accurate equivalent beam properties directly from a solid 3D FEM. These bending and twisting definitions are used in wind-blade-specific dynamics codes to predict the overall performance of the turbine.

‘During the preliminary design phase, the type and amount of input data is often limited,’ says Hoyt. ‘In the wind projects we’ve been involved with to date, there hasn’t been a high-fidelity CAD model available to use as a basis for the FEM.’ So at the start of the STAR analysis, the NSE team only had the blade’s basic geometric parameters – the planform shape, the airfoils, and the chord lengths – to work with. The desire for high-fidelity FEA at a design stage when only the basic parameters of the blade have been defined led to the development of NSE’s bladeMesher software, which is able to create a solid 3D mesh of the blade from the partial data.

Figure 3. Cross-section of the prototype sweep-twist blade airfoil showing major composite construction features (NSE Composites)

‘Our software splines the geometry defined at several locations on the outer mould layer (OML) of the blade and combines it with the composite material thicknesses specified at each location to generate a mesh with the true thickness details,’ says Hoyt. ‘This solid mesh and material definition is then imported into Abaqus to perform a detailed finite element analysis.

‘We have found that a solid FEM has many advantages over shell element FEMs, which have traditionally been used for blade analysis. These benefits include a more accurate prediction of twisting behaviour and the ability to analyse stresses in the adhesive joints between structural elements,’ adds Hoyt.

As the design of the blade progressed, the team explored new airfoils and made adjustments to the sweep geometry to hone in on the optimal amount of twisting.

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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.

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