Abstract
The aim of the present paper is to obtain a better understanding of long distance wakes generated by wind farms as a first step towards a better understanding of farm to farm interaction. The Horns Rev I (HR) wind farm is considered for this purpose, where comparisons are performed between microscale Large Eddy Simulations (LES) using an Actuator Disc model (ACD), mesoscale simulations in the Weather Research and Forecasting Model (WRF) using a wind turbine parameterization, production data as well as wind measurements in the wind farm wake. The LES is manually set up according to the wind conditions obtained from the mesoscale simulation as a first step towards a meso/microscale coupling.The LES using an ACD are performed in the EllipSys3D code. A forced boundary layer (FBL) approach is used to introduce the desired wind shear and the atmospheric turbulence field from the Mann model. The WRF uses a wind turbine parameterization based on momentum sink. To make comparisons with the LESs and the site data possible an idealized setup of WRF is used in this study.The case studied here considers a westerly wind direction sector (at hub height) of 270 ± 2.5 degrees and a wind speed of 8 ± 0.5 m/s. For both the simulations and the site data a neutral atmosphere is considered. The simulation results for the relative production as well as the wind speed 2 km and 6 km downstream from the wind farm are compared to site data. Further comparisons between LES and WRF are also performed regarding the wake recovery and expansion.The results are also compared to an earlier study of HR using LES as well as an earlier comparison of LES and WRF. Overall the results in this study show a better agreement between LES and WRF as well as better agreement between simulations and site data.The procedure of using the profile from WRF as inlet to LES can be seen as a simplified coupling of the models that could be developed further to combine the methods for cases of farm to farm interaction.
Highlights
As the number of wind farms offshore increases there will be more occasions where offshore wind farms will be situated relatively closely to each other due to the limited number of suitable sites and the desire to use the best sites first
A comparison is performed between Weather Research and Forecasting Model (WRF), Large Eddy Simulations (LES) and available site data with an emphasis on increasing understanding of the modelling of flows behind wind farms
To get an increased understanding of the modelling of long distance wakes behind whole wind farms the Horns Rev I (HR) wind farm has been considered for comparisons between microscale Large Eddy Simulations (LES) using an Actuator Disc model (ACD), mesoscale simulations (WRF) using a wind turbine parameterization, site data for production as well as wind measurements in the farm wake
Summary
As the number of wind farms offshore increases there will be more occasions where offshore wind farms will be situated relatively closely to each other due to the limited number of suitable sites and the desire to use the best sites first. It is worth noting that compared to onshore wakes the long distance wakes offshore are more persistent since the lower roughness and turbulence levels mean a slower recovery of the velocity behind the wind farm. An understanding of the wake inside wind farms and the impact of farm to farm interaction due to the long distance wakes behind wind farms is needed to be able to perform accurate estimations of production for offshore wind farms in wind farm clusters
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