Does Rotor-Equivalent Wind Speed Differ from Hub-Height Wind Speed? Observations from Complex Terrain during WFIP2

The rotor-equivalent wind speed (REWS) quantifies the momentum entering a wind turbine rotor disk by averaging the weighted wind speed over the turbine’s rotor disk. The REWS theoretically offers a more accurate wind speed estimate for power production than simply using the hub-height wind speed (HHWS). In regions of complex terrain, where complicated wind profiles frequently occur, the REWS could provide a more accurate wind resource assessment, but the cost of deploying measurements that span the turbine rotor must also be considered.

Here, we assess the observed differences between REWS and the traditional hub-height wind speed (HHWS) using profiling lidars data from five sites across the Columbia River Basin in Oregon during the second Wind Forecast Improvement Project (WFIP2) spanning more than 18 months of data. We calculate REWS for a theoretical turbine with a hub height of 80 m and rotor diameter of 40 m. We calculate REWS with four approaches, including REWS, REWS considering changes of wind direction, REWS considering turbulence, and REWS considering wind direction and turbulence.

Overall, the agreement between REWS and HHWS is generally excellent (R² greater than or equal to 0.99, Figure 1). We do not find statistically-significant differences between the methods for calculating the REWS. While these results suggest that even in this complex terrain, the use of REWS is not strictly necessary, we do find that some cases of strong disagreement between the REWS and the HHWS do occur. These outliers (Figure 2) are clustered near lower wind speeds (dark blue in Figure 2). For some locations, these outliers tend to occur in the early morning hours when wind veer is expected to be the strongest. Specific meteorological events such as cold pools may influence these outliers.

Figure 1: Comparison of 80-m wind speed to 40-120m rotor-equivalent wind speed for Gordon Ridge CU Windcube v2 lidar (Nov 2015-Mar 2017)

Figure 2: Time trace of occurrences of differences between hub-height wind speed and rotor-equivalent wind speed in excess of 10% of hub-height wind speed. Dots are colored by hub-height wind speed.