Abstract
Abstract. Wind farm sites in complex terrain are subject to local wind phenomena, which have a relevant impact on a wind turbine's annual energy production. To reduce investment risk, an extensive site evaluation is therefore mandatory. Stationary long-term measurements are supplemented by computational fluid dynamics (CFD) simulations, which are a commonly used tool to analyse and understand the three-dimensional wind flow above complex terrain. Though under intensive research, such simulations still show a high sensitivity to various input parameters like terrain, atmosphere and numerical setup. In this paper, a different approach aims to measure instead of simulate wind speed deviations above complex terrain by using a flexible, airborne measurement system. An unmanned aerial vehicle is equipped with a standard ultrasonic anemometer. The uncertainty in the system is evaluated against stationary anemometer data at different heights and shows very good agreement, especially in mean wind speed (< 0.12 m s−1) and mean direction (< 2.4∘) estimation. A test measurement was conducted above a forested and hilly site to analyse the spatial and temporal variability in the wind situation. A position-dependent difference in wind speed increase of up to 30 % compared to a stationary anemometer is detected.
Highlights
Complex and mountainous terrain gains importance for wind farm development due to land use conflicts and high wind potential caused by speed-up effects at escarpments and steep ridges
Within a wind farm in complex terrain, which was analysed by Ayala et al (2017), the annual energy production (AEP) of single wind turbines varied by up to 25 %, wake effects seem neglectable when taking into account the park layout and prevailing wind directions
After the data acquisition was started, the unmanned aerial vehicles (UAVs) heads to the first measurement point, where it is holding position for 5 min 100 m above the start level, before moving on to the waypoint at the same height
Summary
Complex and mountainous terrain gains importance for wind farm development due to land use conflicts and high wind potential caused by speed-up effects at escarpments and steep ridges. Such orographic features as well as obstacles, roughness differences and jet and tunnel effects result in a complex wind field. On these sites, the risk of annual energy production (AEP) overestimation is increased (Lange et al, 2017). Remaining uncertainties and long computation times make extensive measurements for sites in complex terrain mandatory for a bankable site assessment (International Electrotechnical Commission, 2009; Measnet, 2016; Fördergesellschaft Windenergie und andere Dezentrale Energien, 2017)
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