How wind speed shear and directional veer affect the power production of a megawatt-scale operational wind turbine

Abstract

Abstract. Most megawatt-scale wind turbines align themselves into the wind as defined by the wind speed at or near the center of the rotor (hub height). However, both wind speed and wind direction can change with height across the area swept by the turbine blades. A turbine aligned to hub-height winds might experience suboptimal or superoptimal power production, depending on the changes in the vertical profile of wind, or shear. Using observed winds and power production over 6 months at a site in the high plains of North America, we quantify the sensitivity of a wind turbine's power production to wind speed shear and directional veer as well as atmospheric stability. We measure shear using metrics such as α (the log-law wind shear exponent), βbulk (a measure of bulk rotor-disk-layer veer), βtotal (a measure of total rotor-disk-layer veer) and rotor-equivalent wind speed (REWS), a measure of actual momentum encountered by the turbine by accounting for shear). We also consider the REWS with the inclusion of directional veer, REWSθ, although statistically significant differences in power production do not occur between REWS and REWSθ at our site. When REWS differs from the hub-height wind speed (as measured either by the lidar or a transfer function-corrected nacelle anemometer), the turbine power generation also differs from the mean power curve in a statistically significant way. This change in power can be more than 70 kW, or up to 5 % of the rated power for a single 1.5-MW utility-scale turbine. Over a theoretical 100-turbine wind farm, these changes could lead to instantaneous power prediction gains or losses equivalent to the addition or loss of multiple utility-scale turbines. At this site, REWS is the most useful metric for segregating the turbine's power curve into high and low cases of power production when compared to the other shear or stability metrics. Therefore, REWS enables improved forecasts of power production.

Investigation of Marine Wind Veer Characteristics Using Wind Lidar Measurements

“…In contrast, Eriksson et al [50] examined the effect of wind veer and the Coriolis force on an idealized farm-to-farm interaction case, in which the estimated power production was found to increase with the inclusion of the wind veer effect. Murphy et al [51] concluded that the influence of wind veer on turbine power production is strongly tied with wind speed, in which a large wind veer angle generally results in power gains at a wind speed exceeding 10 m/s. Below this threshold value, the impact of wind veer appears to be somewhat negligible.…”

Validating CFD Predictions of Flow over an Escarpment Using Ground-Based and Airborne Measurement Devices

“…Not only wind speed but also wind direction is a relevant parameter of wind power output. Several studies have shown that the occurrence of large directional change results in lower turbine performance [45,46]. The measured and simulated wind direction at the WINSENT test-site are presented in Figure 9 Figure 9b.…”

Baseload power potential from optimally-configured wind, solar and storage power plants across the United States

“…The rotor-equivalent formulation area weights the effect of the variables to account for variations as a function of height within the wind turbine rotor-layer. Therefore, in case of wind speed, the rotor equivalent formulation takes into account effect of wind speed shear and wind direction veer each of which has shown to have, on an average, an effect of around 5% on wind power generation 22,23 . Once the rotor equivalent values are calculated, the wind power can be estimated using:…”