Intra-farm Wind Speed Variability Observed by Nacelle Anemometers in a Large Inland Wind Farm

Using 11-month, 15-min averaged measurements by nacelle anemometers at 274 wind turbines, we studied the spatial and temporal variability of wind speed in a wind farm over an area of about 20 km by 20 km located in mountainous terrain. The intra-farm variability is compared between winter and summer: During summer the dominant time scale of wind speed fluctuation is the diurnal cycle of 24 h. During winter, however, the dominant scale is 48 h. The spatial variability of wind speed on scales ≤ O(10 km) is closely associated with the temporal variability on scales ≤ O(10 h). Over a time span ≤ 3 hours rapid drop-offs in wind speed occur more often than rapid increases, and the rapid drop-offs are more frequent during summer than winter. The intra-farm spatial variability of wind speed has a diurnal cycle with that reaches its maximum in the nighttime and its minimum around midday. For accurate wind power modeling, it may be of importance to consider the intra-farm variability of wind speed distribution. We found that when energy production exceeds 200 MW, its estimation based on the averaged wind speed is significantly larger than when based on the speeds at individual turbines.