Effect of Upstream Turbine Tip Speed Variations on Downstream Turbine Performance

AbstractA wake study and combined power output analysis of an array of two model wind turbines is presented. In a wind farm arrangement wakes behind the upstream turbines directly affect the performance and structural loads of the downstream turbines. In this analysis the characteristics of the mean and turbulent wake flow behind an upstream model turbine is directly related to the performance characteristics of a downstream rotor located at three different downstream locations. First the influence of the upstream turbine's tip speed ratio variation from design conditions on the wake flow and the downstream turbine performance is analyzed. Thereafter, also the turbulence intensity level at the wind tunnel inlet is varied from low (laboratory conditions, TI=0.23%) to high (atmospheric conditions, TI=10.0%). Finally, the combined power output of the two turbine array is evaluated for a matrix of the different scenarios.A significant influence of the background turbulence level on the wake recovery is observed, especially for the intermediate separation distance of x/D=5. Controlling the upstream turbine's tip speed ratio away from its design point does not result in a significant increase in combined power output. Only for the case of low turbine separation distance (x/D=3) and low background turbulence the added kinetic energy in the wake can be recovered by the downstream turbine. For higher turbine separation distances and higher background turbulence, the added kinetic energy diffuses into the freestream flow and cannot be recovered anymore. In average, the combined efficiency is observed to increase by about 2.5% with every additional rotor diameter of turbine separation distance. Thus, this analysis suggests an accurate management of the upstream turbine tip speed ratio in dependence of background turbulence and turbine separation distance when optimizing the power output of a wind farm

Wind turbine wake characterization in complex terrain via integrated Doppler lidar data from the Perdigão experiment

During the intensive period (May-June 2017) of the Perdigäo experiment, three sets of Doppler lidar were operated to scan the wake of the wind turbine (WT) on the southwest ridge. CU operated a Doppler scanning lidar in the valley bottom approximately 1 km northeast of the WT and conducted multiple arc scans and two RHI scans every 10-minutes centred on the WT. DTU used a dual Doppler lidar system scanning almost horizontally from the northeast ridge. Two of the three DLR lidars were in-plane with the WT for the main wind direction, one in the valley and one on the distant mountain ridge. The third DLR lidar was on the southwest ridge. All three systems (CU, DTU and DLR) were operated such that in data processing vertical and/or horizontal profiles of the wake can be derived at different distances from the WT. The paper describes the strategies used to scan the wake by the three groups and compares wake characteristics derived from the different systems