Abstract
In this study we consider large wind farms in a conventionally neutral atmospheric boundary layer. In large wind farms the energy extracted by the turbines is dominated by downward vertical turbulent transport of kinetic energy from the airflow above the farm. However, atmospheric boundary layers are almost always capped by an inversion layer which slows down the entrainment rate and counteracts boundary layer growth. In a suite of large eddy simulations the effect of the strength of the capping inversion on the boundary layer and on the performance of a large wind farm is investigated. For simulations with and without wind turbines the results indicate that the boundary layer growth is effectively limited by the capping inversion and that the entrainment rate depends strongly on the inversion strength. The power output of wind farms is shown to decrease for increasing inversions.
Highlights
Atmospheric boundary layers (ABLs) are categorized as neutral, stable or unstable based on the heat flux at the earth’s surface
It can be observed that turbulence manages to penetrate into the lower part of the inversion layer and that the estimate of the height of the boundary layer based on the turbulent stresses lies above the inversion base
The purpose of the current study was to determine the effect of the capping inversion on the conventionally neutral atmospheric boundary layer and on the performance of a wind farm
Summary
Atmospheric boundary layers (ABLs) are categorized as neutral, stable or unstable based on the heat flux at the earth’s surface. A very stable, thin layer can often be found between the neutral boundary layer and the stable free atmosphere aloft. This so-called capping inversion is characterized by its thickness, the temperature difference across the layer (the inversion strength), and the height of the inversion base. The turbulent entrainment process at the top of the boundary layer is slowed down and further deepening of the boundary layer is prevented.
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