Surface roughness for offshore wind energy

Abstract

The Northeastern coast of the U.S. is expected to increase its offshore wind capacity from 30 MW today to 86 GW by 2050. Measurements of wind speeds are available near sea level, but not at hub height, thus extrapolation is often required using the surface roughness, z 0. The focus of this study is to estimate the surface roughness length off the Northeastern coast of the U.S. using field measurements from Nantucket Sound, MA, with three methods: 1) analytical, dependent on friction velocity and atmospheric stability, 2) the Charnock relationship between zo and friction velocity, and 3) a statistical method, based on wind speed observations at three heights. The main results of this paper are: a comparison of the three methods, a comprehensive error analysis of each method, a regional z 0 value of 10–3 m, and a new mathematical interpretation of surface roughness.