Hub height optimization to increase energy production of wind farms

The objective of this study is to assess the effect of hub height optimization on the annual energy production (AEP) of a wind farm. The only optimization variable was the hub height of each wind turbine and all other characteristics of the wind farm, including the base location of the wind turbines and the rotor diameter, remained unchanged. A greedy search algorithm was coupled with geometry-based models to perform the hub height optimization at a test wind farm (Lillgrund in Sweden). The optimization was carried out with 360 wind directions and wind speeds ranging from the cut-in to the cut-out wind speeds under neutral atmospheric conditions. Large eddy simulations were also conducted to provide further details on the flow characteristics and to validate the performance of the optimal layout. Although the optimization algorithm was designed so that the hub height of each wind turbine could have any magnitude between the minimum Hmin and the maximum Hmax recommended by the manufacturer, the optimization process ended up with a layout that included only two different hub heights, either "short" turbines with Hmin or "tall" turbines with Hmax, and no hub height in between was assigned to any of the wind turbines. The location of the short and tall turbines within the wind farm depends on the relative frequency of wind directions at the site; an irregular distribution was achieved for the Lillgrund wind farm studied here, with half of the turbines short and half tall. The AEP of the optimized wind farm with multiple hub heights was approximately 2% ( 5 GWh) higher than that of the wind farm with a single hub height Hmax. The hub height optimization not only improves the AEP of a wind farm, it also decreases the capital costs, as half of the wind turbines are cheaper to build because their towers are 20% shorter.