Simulation of the effect of atmospheric stratification on the power production of a wind farm

Abstract

The wind energy industry is growing more than ever before and wind energy as a renewable energy source has shown quite a potential over the years. Unfortunately, the power yield of a wind farm can fluctuate largely over time, which originates from fluctuating wind speed magnitudes. Convection of air, turbulence, humidity and radiation of heat are processes in the atmospheric boundary layer, the lowest region of the troposphere above Earth’s surface, that are responsible for these fluctuations. This region is characterized by a diurnal cycle in which two examinable cases can be found, namely the convective boundary layer (CBL) case during the daytime period and the stable boundary layer (SBL) case during the nighttime period of the diurnal cycle. In the SBL case, the earlier mentioned physical processes result in a cool surface layer and relatively little turbulence is present, whereas in the CBL case a relatively warm surface layer and high turbulence level are present. The Dutch Atmospheric Large Eddy Simulation (DALES) model is used in which wind turbines are implemented with the help of former TU Delft master student P.A. van Dorp. With DALES, a simulation is performed for each case and the output data of the two simulations are analysed and compared to find the optimal case in terms of total power yield of a configuration of two wind turbines with the second fully in the wake produced by the one in front. An arbitrary distance of 600m between the wind turbines is chosen, which corresponds to 7.5 times the diameter length of the wind turbines, and a hub height of 80m is chosen. First, the simulations are validated by examining the turbulent kinetic energy profiles. Then, the wind speed profiles over the domain are analysed, corresponding to the turbulent kinetic energy profiles and showing that the average wind speed at hub height for the SBL case is larger than for the CBL case. Furthermore, the wake profiles behind the turbines are displayed and compared for the two cases, showing little difference. Finally, the total power yield of this specific wind turbine configuration is calculated. With an average power yield of 1.71MW for the SBL case compared to 0.90MW for the CBL case, the SBL case is shown to be the optimal atmospheric boundary layer case in terms of power production for this specific wind turbine configuration.