Effect of low-level jet height on wind farm performance

TL;DR

This study uses large-eddy simulations to analyze how the position of low-level jets relative to wind turbines affects wake recovery and energy extraction, revealing complex interactions that influence wind farm efficiency.

Contribution

It provides new insights into the impact of LLJ height on wind farm performance, highlighting the role of turbulence and shear in wake recovery and energy entrainment.

Findings

Wake recovery is faster when LLJ is above turbines due to turbulence.

Low turbulence above LLJ causes slow wake recovery when LLJ is below turbines.

Negative shear can enhance energy extraction from the jet when LLJ is below turbines.

Abstract

Low-level jets (LLJs) are the wind maxima in the lowest 50 to 1000 m of atmospheric boundary layers. Due to their significant influence on the power production of wind farms it is crucial to understand the interaction between LLJs and wind farms. In the presence of an LLJ, there are positive and negative shear regions in the velocity profile. The positive shear regions of LLJs are continuously turbulent, while the negative shear regions have limited turbulence. We present large-eddy simulations of wind farms in which the LLJ is above, below, or in the middle of the turbine rotor swept area. We find that the wakes recover relatively fast when the LLJ is above the turbines. This is due to the high turbulence below the LLJ and the downward vertical entrainment created by the momentum deficit due to the wind farm power production. This harvests the jet's energy and aids wake recovery. However, when the LLJ is below the turbine rotor swept area, the wake recovery is very slow due to the low atmospheric turbulence above the LLJ. The energy budget analysis reveals that the entrainment fluxes are maximum and minimum when the LLJ is above and in the middle of the turbine rotor swept area, respectively. Surprisingly, we find that the negative shear creates a significant entrainment flux upward when the LLJ is below the turbine rotor swept area. This facilitates energy extraction from the jet, which is beneficial for the performance of downwind turbines.