Back to the programme printer.gif Print




Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Real world power curves: A new era for wind resource assessments?' taking place on Tuesday, 11 March 2014 at 14:15-15:45. The meet-the-authors will take place in the poster area.

Volker Barth DEWI GmbH, Germany
Co-authors:
Alexander Tsegai Wassie (2) F P Volker Barth (1) Kai Mönnich (1)
(1) DEWI GmbH, Oldenburg, Germany (2) Carl von Ossietzky Universität, Oldenburg, Germany

Printer friendly version: printer.gif Print

Abstract

Micrositing using wind shear corrected power curves - one step forward or aside?

Introduction

Wind shear variation has been recognised as one of the main drivers for power curve uncertainties. The equivalent wind speed concept has been developed to address this issue and will be included in the upcoming release of IEC 61400-12-1. 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.

Approach

As a first step, we developed an appropriate way of introducing wind shear via the equivalent wind speed concept in the standard micrositing workflow, which has so far been tailored to hub height wind speed only. In a second step, we explored the differences of the classical method (power curves and wind speed measurements at hub height only) with the new workflow for various site conditions and wind shear regimes. Additionally, uncertainties related to wind shear measurements are estimated and compared to reduced power curve uncertainties when using the equivalent wind speed.

Main body of abstract

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. The magnitude of these differences reinforces the demand for supplying the wind shear for which a power curve has been determined.
On the other hand, this also raises the necessity to accurately determine the wind shear at the site. Otherwise, the equivalent wind speed concept is of no much help. This requires at least two additional high-quality measurement heights, preferably close to hub height and with reasonable distance to each other. Unfortunately, experience shows that this is often not easily achieved: met masts often do not even reach hub height, and frequently only one useful time series is available. In these cases the wind profile often can only be estimated by models with considerable model errors. In any case, estimating the wind shear from either additional measurement heights or from models enhances the uncertainty of the wind measurement, so that the advantage of having a less uncertain power curve is at least partly offset in the overall energy yield result.

Conclusion

While the equivalent wind speed concept is considered to be suitable to eliminate wind shear effects from the power curve measurement, power generation still depends on the wind conditions experienced by the individual turbine. This imposes the need to properly assess the wind shear at the site during micrositing studies, which implies new challenges given the existing equipment and/or the complexity of the site. Energy yield uncertainties will therefore not drop as much as suggested by the reduced power curve uncertainties.


Learning objectives
(1) Be aware that it is the combination of uncertainties that counts in the end.
(2) Keep wind shear issues in mind when setting up a wind speed measurement at a wind farm site.