Introduction
Many small wind turbines use furling, whereby the rotor either tilts and/or yaws out of the wind to protect itself from overspeed during very high winds. In the past, most small wind turbine designers used trial and error approaches that often used variable geometry test platforms to vary furling offsets. Although recent advances in understanding furling have resulted in furling being incorporated into wind turbine aeroelastic simulation models, to date there has been a limited amount of test data available for validation of small furling wind turbine models [1]. Acquiring good test data for a small furling wind turbine was a recommended action of the National Wind Technology Center (NWTC) Furling Workshop held in July of 2000 [2] and has been mentioned in previous papers on furling [3]. The Small Wind Research Turbine (SWRT) project was initiated to provide reliable test data for model validation of furling wind turbines and to help understand small wind turbine loads. The measurements of thrust and furling are of particular importance to the model validation effort and are unique to this test.
The SWRT is a Bergey Excel 10-kilowatt (kW) turbine that was modified in several ways, including addition of a shaft-bending, torque, and thrust sensor in line with the shaft; modification of the nose cone and nacelle to allow for data acquisition system (DAS) components; and customization of the tower adapter with load cells on the top of each tower leg. The turbine rotor axis is offset from the yaw axis, and in high winds, the turbine furls horizontally out of the wind during rotor thrust and aerodynamic moments.
The FAST (Fatigue, Aerodynamics, Structures, and Turbulence) code, which was developed and is distributed by the National Renewable Energy Laboratory’s (NREL’s) NWTC, is the primary aeroelastic simulator used by the U.S. wind industry to model small wind turbines. The recent upgrades to FAST allow users to simulate lateral offset and skew angle of the rotor shaft from the yaw axis, rotor-furling and tail-furling degrees-of-freedom (DOFs), up- and down-furl stops, and tail inertia and tail fin aerodynamic effects [4,5]. Because the location and orientation of the furling DOFs are user-specified, the simulator is flexible enough to model virtually any furling wind turbine configuration. FAST’s built-in features allow for the determination of full-system modes, including furling, of an operating or stationary turbine. These enhancements to the FAST code were validated using the data sets from the SWRT test described herein [6].
This report will familiarize the user with the scope of the SWRT test and support the use of these data. In addition to describing all the testing details and results, the report presents an analysis of the test data and compares the SWRT test data to simulation results from the FAST aeroelastic simulation model