The Wind Shear Exponent: Comparing Measured Against Simulated Values and Analyzing the Phenomena That Affect the Wind Shear

Abstract

When assessing a region for wind energy the wind shear is a key factor to consider because of its profound effect on power density as a function of height. Traditionally, wind shear parameters are derived either from local velocity measurements at two or more heights or from surface roughness characteristics to predict the wind speed at hub height for a particular site. However, recent measurements in a complex terrain (non-mountainous) region indicate that the measured wind shear exponent is significantly higher than the value predicted by land use characteristics and modeled results. Virtual wind shear parameters: alpha and zo, created by the modeled flow fields of the complex terrain of southeastern Ohio’s Appalachian foothills are determined with computational fluid dynamics simulations designed for complex terrain. Then the first year’s measurements from the extra-tall tower in the region provide a direct evaluation of the wind shear parameters: alpha and zo. These values, characteristic of the measurements, are compared against values determined from the local land use characteristics as well as those found by modeling with a computational fluid dynamics wind simulator. It has been found that the measured value of the wind shear exponent is larger, by a factor of 2, than the values currently used in published state wind maps. Phenomena affecting wind shear are also analyzed. Diurnal and changes in reference heights have large effects on the measured wind shear. It is demonstrated that for this site an overall annual average value of the wind shear coefficient is an inaccurate representation of the wind shear because of the range of variability that occurs seasonally. It is also shown that extrapolating from near-surface measurements to hub heights can yield inaccurate predictions of wind speed and, more importantly, wind power.