Deviations between wind speed data measured with nacelle-mounted anemometers on small wind turbines and anemometers mounted on measuring masts

Abstract

This article focuses on the readings received from two different types of nacelle- mounted anemometers and their comparison with reference measurements on-site. The aim of the article was to evaluate the influence of the wind turbine rotors on the wind data measured with the nacelle-mounted anemometers. The measurements were made during a case study of two existing small wind turbines. The framework conditions in the analysed cases were similar: both analysed anemometers were mounted on small, 10 kW horizontal axis type wind turbines (HAWT-s) with active yaw and pitch control, and although the wind turbines were situated at different locations, the wind conditions of the measurement sites were relatively similar. The comparative wind speed data for the analysis was acquired in both cases from measurement masts that were installed in the proximity of the analysed anemometers on the basis of the standard EN 61400-12-1:2006. The anemometer readings were logged during measuring periods of two months and saved as 10 minute averages. Three anemometers and a wind direction sensor were used for the reference measurements on both sites. It was found that in both cases the operating state of the wind turbines (presumably the rotation of the rotor blades of the turbine) influenced the readings received from the nacelle-mounted anemometers to a statistically significant extent. The 10 minute average wind speeds of the nacelle-mounted anemometers were significantly lower than the means of the data acquired from the measuring mast anemometers. Despite the fact that the means did not coincide the correlations between the reference wind data and the nacelle-mounted anemometer readings were strong. In the analysed cases the readings from the ultrasonic anemometer were more similar to the reference measurements than the readings from the mechanical cup-anemometer.