Wind Energy

61
On the Influence of Low-Level Jets on Energy
Production and Loading of Wind Turbines
N. Cosack, S. Emeis and M. Kühn
61.1 Introduction
The variation of wind speed with height above ground is an important design
parameter in wind energy engineering, as it influences power production and
loading of the wind turbine. In a convective boundary layer we usually expect
small vertical gradients of atmospheric variables like momentum or temperature.
These unstable conditions are the normal case during daytime. At night,
when the thermal production of turbulence vanishes and the mechanical production
of turbulence is small (low to moderate wind speeds), the surface layer
can decouple from the rest of the boundary layer. Such stable conditions lead
to strong vertical gradients of atmospheric variables at night. Especially we
observe very low wind speeds near the ground and higher wind speeds above
the surface layer. This high wind speed band above the surface layer is called
Low-Level-Jet (LLJ). In addition to the change in wind speed, the wind direction
in the various layers can also differ significantly. In the following sections
we evaluate the influence of LLJs that have been measured in the northern
part of Germany on wind turbines by means of computer simulation.
61.2 Data and Methods
Since 1996, the Institute of Meteorology and Climatic Research of the
Forschungszentrum Karlsruhe performed measurements of vertical wind profiles
at various sites in Germany using Sodar. Details on the measurement campaigns
and their results have been presented in [1, 2]. From the vast amount
of data, two typical profiles of change in wind speed with height have been
chosen for the described analysis. Although the change in wind direction with
height has a significant impact on loading and power, only results from profiles
with constant wind direction will be shown here.
The aero-elastic simulation tool Flex5 [3] has been used to estimate the
power production and the loading for two turbine types: a 1.5 MW machine