The unsteady wind profile in the atmospheric boundary layer upstream of a modern wind turbine is measured. The measurements are accomplished using a novel measurement approach that is developed and demonstrated for wind energy applications. The measurements of the unsteady 3D velocity field have to be resolved in a low dynamic head environment and over large flow angles around a modern wind turbine (rotor diameter 80–120m and tower height 60–100m). The novel measurement approach is comprised of an autonomous Uninhabited Aerial Vehicle (UAV) that is equipped with a seven-sensor fast-response aerodynamic probe (F7S-UAV). The autonomous UAV enables high spatial resolution (∼9% of rotor diameter) measurements, which hitherto have not been accomplished around full-scale wind turbines. The 7-sensor fast-response aerodynamic probe developed at ETH Zurich is the key-enabling technology for the measurements. This measurement system is realized as a light, compact measurement chain that conforms to the limited payload area and weight restrictions of the UAV. The time-averaged wind profile from the F7S-UAV probe is found to be in very good agreement to an independently measured profile using the UAV. This time-averaged profile, which is measured at a wind turbine that is located in moderately complex terrain, differs by as much as 30% from the wind profile that is extrapolated from a logarithmic height formula; therefore the limited utility of extrapolated profiles, which are commonly used in site assessments, is made evident. The time-varying wind profiles show that, at a given height, the velocity fluctuations can be as much as 44% of the time-averaged velocity, therefore indicating that the wind turbine and its components, notably the gearbox, will experience substantial loads that may impact the fatigue life of the components. Furthermore, the shear in the velocity profile also subjects the fixed pitch blade to varying incidences and loading. Analysis of the associated velocity triangles indicates that the sectional lift coefficient at mid-span of this modern turbine would vary by 12% in the measured time-averaged wind profile. These variations must be accounted in the structural design of the blades. Thus the measurements of the unsteady wind profile accomplished with this novel measurement system, demonstrate that it is a cost effective complement to the suite of available site assessment measurement tools.

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