In order to study the effect of vertical staggering in large wind farms, large eddy simulations (LES) of large wind farms with a regular turbine layout aligned with the given wind direction were conducted. In the simulations, we varied the hub heights of consecutive downstream rows to create vertically staggered wind farms. We analysed the effect of streamwise and spanwise turbine spacing, the wind farm layout, the turbine rotor diameter, and hub height difference between consecutive downstream

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... turbine rows on the average power output. We find that vertical staggering significantly increases the power production in the entrance region of large wind farms and is more effective when the streamwise turbine spacing and turbine diameter are smaller. Surprisingly, vertical staggering does not significantly improve the power production in the fully developed regime of the wind farm. The reason is that the downward vertical kinetic energy flux, which brings high velocity fluid from above the wind farm towards the hub height plane, does not increase due to vertical staggering. Thus, the shorter wind turbines are effectively sheltered from the atmospheric flow above the wind farm that supplies the energy, which limits the benefit of vertical staggering. In some cases, a vertically staggered wind farm even produced less power than the corresponding non vertically staggered reference wind farm. In such cases, the production of shorter turbines is significantly negatively impacted while the production of the taller turbine is only increased marginally. KEYWORDS atmospheric boundary layer, large eddy simulation, power production, turbulence, vertically staggered wind farm, wake model INTRODUCTION Wind power is a very promising clean and sustainable energy form. The recent decades have shown a rapid development of the wind industry with an ever increasing contribution to the total energy production worldwide. 1 A recent trend is the construction of very large wind farms. The flow dynamics in these wind farms is complex due to the interaction between the different wind turbine wakes and the atmospheric flow above the wind farm. Unfortunately, the operation of wind turbines is negatively impacted by wakes created by upstream turbines. For reviews on wind farm modeling, we refer to other studies. 2-4 An overview of research challenges for the wind energy community is discussed by van Kuik et al. 1