How Wind Speed Shear and Directional Veer Affect the Power Production of a Megawatt-Scale Operational Wind Turbine

Patrick Murphy, Julie Lundquist, Paul Fleming

Research output: Contribution to journalArticle

22 Scopus Citations

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 a (the log-law wind shear exponent), ..beta..bulk (a measure of bulk rotor-disk-layer veer), ..beta..total (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, REWS..theta.., although statistically significant differences in power production do not occur between REWS and REWS..theta.. 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.
Original languageAmerican English
Number of pages46
JournalWind Energy Science Discussions
DOIs
StatePublished - 2019

Bibliographical note

See NREL/JA-5000-78107 for final paper as published in Wind Energy Science

NREL Publication Number

  • NREL/JA-5000-74314

Keywords

  • directional veer
  • turbine
  • wind energy
  • wind speed shear

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