Evaluation of a simple analytical model for offshore wind farm wake recovery by in situ data and Weather Research and Forecasting simulations

Abstract

The recovery of offshore wind farm wakes in the German Bight was analyzed by a unique in situ data set, measured on‐board the research aircraft Dornier Do‐128 during the WIPAFF project in 2016 and 2017. These observations were used to validate a simple analytical wake recovery model in five case studies. The observed recovery rates were compared with the results of the mesoscale Weather Research and Forecasting (WRF) model. The airborne data show that the wake recovery can be described by an exponential function as expected by the analytical model and strengthens the hypothesis that the vertical downward moment flux has an important influence on the wake recovery. However, the predicted wake recovery rates (by the analytical model) do not always fully agree with the observations. Although, as a first‐order approximation, the model seems to perform well, further optimization has to be implemented to account for wind park layout, turbine induced turbulence, and horizontal momentum flux. WRF simulations reveal an exponential recovery, although the mesoscale model does not reproduce the correct atmospheric conditions for most of the cases. Therefore, the wake recovery rates estimated by WRF disagree with the measured data in most of the studied cases.

The Role of Atmospheric Stability and Turbulence in Offshore Wind-Farm Wakes in the German Bight

“…The length at which the wind speed inside the wake has recovered to 95 % of the free stream characterizes the maximum length of the wake. The initial wind-speed deficit D r 1 is found to range up to 43 % compared with the undisturbed flow, which supports the finding of the previous analyses by Platis et al (2018), Cañadillas et al (2019, Platis et al (2020), andPlatis et al (2021). However, in this study all flights of the WIPAFF campaign have been considered now, leading to more representative results.…”

“…In contrast, for the flow from open sea from the northern direction, the conditions are predominantly convective. The presented results fit well in the framework of existing studies of the dynamic stability parameters in the German Bight (Westerhellweg et al 2010;Sathe 2010;Emeis et al 2016;Platis et al 2021). As outlined in Emeis et al (2016), this behaviour is typical over the ocean for the northern hemisphere in the temperate westerly wind belts: warm sector winds most frequently come from the south-west and are thus followed by rather stable conditions, whereas cold sector winds come from the north-west and predominantly bring convective conditions.…”

“…The first distinct relationship between stability and the wake length using synthetic aperture radar data of a wind farm far field was identified by Djath et al (2018), showing that at the wind farm Alpha Ventus in the German Bight, the wake length increases with stronger lapse rate, and could be described by a linear function. Platis et al (2018), Cañadillas et al (2019) and Platis et al (2021) used in situ measurements to show that stable conditions favour longer wakes, though, only case studies are presented.…”

On the effects of inter-farm interactions at the offshore wind farm Alpha Ventus

“…Therefore, understanding these inter-farm interactions has been a topic of interest for research, as well as the industry. Previous studies based on airborne measurements (Cañadillas et al, 2020;Platis et al, 2018) have identified wind speed deficits and increases in turbulence downstream of offshore wind farms and clusters. They showed that these effects are visible up to a level of 50 km downstream, especially in neutral atmospheric conditions.…”

“…Emeis (2009) suggested a simple analytical model to calculate the speed deficit accounting for atmospheric stability, surface roughness, the turbine's thrust coefficient, and the Monin-Obukhov length. Calibration and evaluation of the model can be found in Platis et al (2020). In Nygaard et al (2020) an engineering model accounting for both speed deficits and wake-induced turbulence is suggested.…”