Analysis and Tests of Pultruded Blades for Wind Turbine Rotors

NOTICE This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. FOREWORD The National Renewable Energy Laboratory's (NREL's) National Wind Technology Center is supporting the efforts of its industry partners to develop advanced, utility-scale wind turbines. Part of the research being conducted focuses on innovative components and subsystems that eventually may be incorporated into these advanced turbines. PS Enterprises, Inc. (PSE) chose to investigate a flexible, downwind, free-yaw, five-blade rotor system employing pultruded blades. Studies conducted by PSE showed that, for a given rotor solidity, increasing the number of blades reduced the rotor weight. And from previous experience with both helicopter and wind turbine rotors, it was known that the pultrusion process resulted in blades having a very low cost-per-unit-weight. Indeed, pultruded blades were employed on wind turbines by StormMaster, Windtech, Dynergy, and Bergey Windpower. However, in some cases, problems were reported with yaw instability and occasional tower strikes. Furthermore, because pultruded blades are constrained to constant cross sections, without taper or twist, they are known to suffer a degradation in aerodynamic performance. So the challenge of the PSE project was to design and test a dynamically-and structurally-stable rotor that demonstrated the anticipated weight and cost savings while maintaining reasonable aerodynamic performance. PSE assembled a diverse group of consultants from around the United States to work on the project. The expertise of the project participants included aerodynamics, mechanical design, structural dynamics and testing. They worked closely with NREL to accomplish design reviews, modal tests, blade structural tests and field tests. This approach had the effect of adding logistics challenges to the acknowledged technical difficulties. It can be said with virtual certainty that engineering projects of this nature always encounter …