First in situ evidence of wakes in the far field behind offshore wind farms

Abstract

More than 12 GW of offshore wind turbines are currently in operation in European waters. To optimise the use of the marine areas, wind farms are typically clustered in units of several hundred turbines. Understanding wakes of wind farms, which is the region of momentum and energy deficit downwind, is important for optimising the wind farm layouts and operation to minimize costs. While in most weather situations (unstable atmospheric stratification), the wakes of wind turbines are only a local effect within the wind farm, satellite imagery reveals wind-farm wakes to be several tens of kilometres in length under certain conditions (stable atmospheric stratification), which is also predicted by numerical models. The first direct in situ measurements of the existence and shape of large wind farm wakes by a specially equipped research aircraft in 2016 and 2017 confirm wake lengths of more than tens of kilometres under stable atmospheric conditions, with maximum wind speed deficits of 40%, and enhanced turbulence. These measurements were the first step in a large research project to describe and understand the physics of large offshore wakes using direct measurements, together with the assessment of satellite imagery and models.

Micrometeorological impacts of offshore wind farms as seen in observations and simulations

“…Christiansen and Hasager (2005) observed offshore wakes via sythetic aperture radar satellite images and showed that these wakes can propagate 20 km downwind. These results were confirmed, recently, by airborne measurements taken downwind of large offshore wind farms at the North Sea (Platis et al 2018). During stable atmospheric conditions, these offshore wakes can be longer than 70 km at offshore sites (Platis et al 2018).…”

U.S. East Coast Lidar Measurements Show Offshore Wind Turbines Will Encounter Very Low Atmospheric Turbulence

“…Offshore wind power plant wakes can extend tens of km downwind (Platis et al, ; Siedersleben, Platis, et al, ; Siedersleben, Lundquist et al, ) in low‐turbulence, stably stratified conditions. These wakes undermine offshore power production (Nygaard, ).…”

“…Offshore ϵ has, on average, smaller values compared to those found in comparable studies onshore, with a weak diurnal cycle. The small average values of ϵ are conducive to strong and long‐lived wakes such as wakes observed in stable stratification in the North Sea (Platis et al, ; Siedersleben, Lundquist et al, ; Siedersleben, Platis, et al, ). Moreover, this extended persistence of wakes will occur throughout the diurnal cycle, given the absence of strong turbulence dissipation even in convective daytime conditions.…”

“…Turbulence also plays a key role in the development and subsequent erosion of wind turbine and wind farm wakes, whose spatial extent can be particularly long offshore (Platis et al, ; Siedersleben, Lundquist et al, ; Siedersleben, Platis, et al, ), with observed wakes extending beyond 45 km in some cases. While the erosion of wakes is related to ambient turbulent kinetic energy, the dissipation of turbulence within the wake, which represents the first step of wake erosion, is defined by the turbulence dissipation rate.…”

Applications of satellite winds for the offshore wind farm site Anholt

“…The SAR wind maps contribute with information about the far-wake field, which is typically not available from other sources. Recently, the first airborne in situ measurements of the far-wake field became available showing that wind farm wakes frequently extend over several tens of kilometres (Platis et al, 2018).…”