Micrositing using Wind Shear Corrected Power Curves - One Step Forward or Aside?

摘要

Wind shear variation has been recognised as one of the main drivers for power curve uncertainties. The rotor equivalent wind speed (REWS) concept has been developed to address this issue and will be included in the upcoming release of the IEC 61400-12-1 standard. While this concept can be expected to reduce uncertainties related to power curve measurements, the consequences for the uncertainty of subsequent energy yield assessments and micrositing studies are less clear. Our analysis shows that the differences between energy yields obtained using the classical approach (hub height wind speed and hub height power curve) and energy yields obtained using the equivalent wind speed show a clear dependency on wind shear, and even more important, on the wind shear frequency distribution. The magnitude of these differences reinforces the demand for considering wind shear for energy yield assessments. On the other hand, this also raises the necessity to accurately determine the wind shear at the site. This requires at least two high-quality measurement heights, preferably close to hub height and with reasonable distance to each other. Experience shows that this is often not easily achieved. In such cases the wind profile often can only be estimated by models with considerable model errors and without reasonable information about the frequency distribution. While the additional uncertainty from using REWS instead of hub height wind speed is small, the additional measurement uncertainty cannot be neglected. Therefore, the advantage of having a less uncertain power curve is at least partly offset in the overall energy yield result.