Classifying Rotor-Layer Winds for Offshore Wind Resource and Available Power Assessment

Prior to offshore wind farm construction, an accurate estimate of preconstruction energy yield is required to optimize wind farm layout and justify the project’s economic viability. Unfortunately, uncertainties exist during this stage due in-part to limited measurements to characterize the offshore wind resource and related uncertainties predicting a turbine’s available power. To better understand these preconstruction energy yield uncertainties, Doppler wind lidar and other met-ocean measurements were collected offshore within and near Maryland’s Wind Energy Area from July-August 2013. Given the diversity of vertical wind speed profile (VWP) observations, VWPs are classified based on the goodness-of-fit to several mathematical expressions. Results demonstrate on average VWP type variability is related to the magnitude of hub-height (100m) wind speed and wind direction (i.e. offshore fetch), as power law, logarithmic-like, VWPs occur during slightly weaker, northeasterly flow, while more unexpected VWPs are associated with stronger, southwesterly flow, from land to sea. In addition, compared to power-law VWP classes, unexpected VWPs types demonstrate slightly warmer air and SSTs, as well as stable surface conditions. Classifying VWPs also provides a useful tool for relating preconstruction offshore wind resource and turbine available power uncertainties. Using an NREL 5MW offshore reference turbine’s power curve, the sensitivity of available turbine power estimates is investigated using buoy extrapolated surface to hub-height (100m) wind, lidar measured hub-height wind, and several Rotor Equivalent Wind (REW) techniques. On average, traditional hub-height wind power yields the highest available power estimate, approximately 9-70 percent greater than other techniques. Further, unexpected VWPs demonstrate the greatest variability in critical superimposed meteorological controls known to impact turbine performance, thus yield greatest deviation from hub-height power and uncertainty between available power estimates. To elucidate the most accurate approach for estimating turbine available power and quantify the impact of VWP type variability and atmospheric stability on actual turbine performance in the Mid-Atlantic, results from the VERTical Enhanced mixing (VERTEX) measurement campaign, in which Doppler wind lidar and other atmospheric data are collected in-front of a 2 MW coastal wind turbine, are presented.