Predicting Hub-Height Wind Speed for Small Wind Turbine Performance Evaluation Using Tower-Mounted Cup Anemometers

Abstract

Industry standards for small wind turbine (SWT) performance evaluation require estimating hub-height wind speed using either a spatially offset meteorological mast or a cup anemometer extending from a lower elevation on the turbine tower. This paper investigates the use of vertical extrapolation to reduce the uncertainty associated with tower-mounted anemometer wind speed measurements. An experimental study has been performed involving a Bergey XL.1 SWT collocated with a meteorological mast. Results indicate that power law extrapolation can significantly reduce the uncertainty of hub-height wind speed predictions, especially if concurrent wind speed measurements are available at multiple elevations. Best practice methods have been provided. To identify the upper limit of anemometer placement, a porous disk wind tunnel test has been performed and compared with three-dimensional wind speed measurements obtained experimentally. To remain outside the rotor's region of influence, it is recommended that the topmost anemometer is positioned one rotor diameter below hub-height.