Offshore wind farm wake recovery: Airborne measurements and its representation in engineering models

We present an analysis of wind measurements from a series of airborne campaigns conducted
to sample the wakes from two North Sea wind farm clusters, with the aim of determining
the dependence of the downstream wind speed recovery on the atmospheric stability. The
consequences of the stability dependence of wake length on the expected annual energy yield of
wind farms in the North Sea are assessed by an engineering model. Wakes are found to extend
for significantly longer downstream distances (>50 km) in stable conditions than in neutral and
unstable conditions (<15 km). The parameters of one common engineering model are modified
to reproduce the observed wake decay at downstream distances >30 km. More significant
effects on the energy yield are expected for wind farms separated by distances <30 km, which
is generally the case in the North Sea, but additional data would be required to validate the
suggested parameter modifications within the engineering model. A case study is accordingly
performed to show reductions in the farm efficiency downstream of a wind farm. These results
emphasize not only the importance of understanding the impact of atmospheric stability on
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offshore wind farms but also the need to update the representation of wakes in current industry
models to properly include wake-induced energy losses, especially in large offshore clusters.