NSE Composites used Abaqus FEA to validate a Sandia-funded sweep-twist design that captures 12 percent more energy
The basic physics and economics of wind turbine blades 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 kilowatts. Secondly, as blades get longer, weight increases—by approximately the cube of the length—leading to higher 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 United States Department of Energy’s (DOE) Wind Energy Research Program at Sandia National Laboratories, the result has been the development of a sweep-twist adaptive rotor (STAR). This innovative curved blade was proposed in earlier theoretical research and had 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 commercialized, the outcome would be larger, lighter, less-expensive, and more productive wind turbines.
In 2004, Knight and Carver (K&C) Wind Group, a San Diego-based wind 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. They began by assembling a team of specialized companies and academic institutions, one of which was Seattle-based NSE Composites, who were brought on board to perform the finite element modeling (FEM) of the new design.
“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 NSE’s founders. “Plus, we were already troubleshooting another blade problem for K&C and wanted to diversify our customer base to include more renewable energy, so the fit was a good one.”
Hoyt and his partners at NSE have been using Abaqus from SIMULIA, the Dassault Systèmes application for realistic simulation, as their finite element analysis (FEA) tool for years. As their projects moved toward larger and more complex models, the software’s ongoing developments in simulating composites, crack generation, and fracture kept pace.
“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 minimizing 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 (see Figure 1). Theoretically, this planform 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 the characteristic profile of whale tails and dorsal fins.