Power Law Extrapolation of Wind Measurements for Predicting Wind Energy Production

Abstract

This study investigates the level of uncertainty that would be expected if anemometer data from a short tower (less than 40 meters) was used to predict wind speeds and power production at typical utility-scale wind turbine hub-heights. Data from five tall towers was used to predict wind speeds at levels above 70 m based on anemometer data from levels below 40 meters. 1/7 power law, two level power law fit, and hybrids of these methods were applied. Predicted wind speeds were compared to the measured wind speeds at the higher levels to assess the level of error in the predictions. Accuracy of predicting upper level winds varied considerably between sites. Predicting this accuracy at a site without upper level wind measurements or prior knowledge of the upper level wind climate is very difficult, and significant uncertainty in the predicted results must be accepted.