Abstract
Low-level jets (LLJs) are the wind maxima in the lower regions of the atmosphere with a high wind energy potential. Here we use large-eddy simulations to study the effect of LLJ height on the flow dynamics in a wind farm with $10\times4$ turbines. We change the LLJ height and atmospheric thermal stratification by varying the surface cooling rate. We find that the first row power production is higher in the presence of a LLJ compared to a neutral reference case without LLJ. Besides, we show that the first row power production increases with decreasing LLJ height. Due to the higher turbulence intensity, the wind turbine wakes recover faster in a neutral boundary layer than in a stably stratified one. However, for strong thermal stratification with a low-height LLJ, the wake recovery can be faster than for the neutral reference case as energy can be entrained from the LLJ. Flow visualizations reveal that under stable stratification the growth of wind farm's internal boundary layer is restricted and the wind flows around the wind farm. Wind farms extract energy from LLJs through wake meandering and turbulent entrainment depending on the LLJ height. Both effects are advantageous for wake recovery, which is beneficial for the performance of downwind turbines. This finding is confirmed by an energy budget analysis, which reveals a significant increase in the kinetic energy flux in the presence of a LLJ. The jet strength reduces as it passes through consecutive turbine rows. For strong stratification, the combined effect of buoyancy destruction and turbulence dissipation is larger than the turbulent entrainment. Therefore, the power production of turbines in the back of the wind farm is relatively low for strong atmospheric stratifications.
Highlights
The atmospheric boundary layer is dynamic and undergoes continuous transitions during the day due to changes in, for example, the surface heat flux and the geostrophic wind
The presence of a wind farm is felt by the upwind flow as a roughness change, and due to the continuity constraint, the flow accelerates over the wind farm
The study was carried out by systematically increasing the cooling rate at the surface, which results in lower level jets (LLJs) height and a reduction of the atmospheric turbulence
Summary
The atmospheric boundary layer is dynamic and undergoes continuous transitions during the day due to changes in, for example, the surface heat flux and the geostrophic wind. The vertical wind veer in the Ekman spiral causes a skewed spatial structure of the turbine wake, which enhances the shear production of turbulent kinetic energy leading to larger flow entrainment and faster wake recovery [41] It is a common practice in the wind energy community to use periodic boundary conditions in the spanwise direction, which results in “infinitely” wide wind farms [20,26,42,43].
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