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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Wakes: Do we need different models for onshore and offshore wind farms?' taking place on Wednesday, 12 March 2014 at 16:30 -18:00. The meet-the-authors will take place in the poster area.

Sergi Roma Solanellas Alstom Renewable Power, Spain
Co-authors:
Sergi Roma Solanellas (1) F P Christiane Montavon (2) David Madueno (1) Amanda Moragrega (1) Sonia Espana (1)
(1) Alstom Renewable Power, Barcelona, Spain (2) ANSYS, Abingdon, United Kingdom

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Abstract

Sensitivity of wake losses to wind turbine diameter for an offshore wind farm

Introduction

The power production loss due to wakes is one of the main sources of yearly energy losses in large offshore wind farms. The aim of the current study is to quantify the effect of increasing the rotor diameter in terms of wake losses in order to optimize the size of an offshore wind turbine rotor.

Approach

With the purpose of quantifying the variation of the wake losses with increasing rotor size, the array efficiency is simulated with ANSYS’ WindModeller, a CFD tool using actuator disks to model the wake losses which has been validated against some existing offshore wind farms. Three turbines types with identical rated power but different rotor sizes are considered for the computations.
The sensitivity of the wake losses to surface stability conditions is also investigated for some specific cases.


Main body of abstract

When using fast engineering models to estimate wake losses, an array efficiency matrix for a given layout is traditionally derived by steps of 1 degree for the entire wind rose and steps of 1 m/s for a wind speed range of typically 5 to 30 m/s. When assessing wake losses with CFD models, such a large number of simulations becomes prohibitive. Instead, an optimised approach was devised, using a variable directional resolution and limited number of wind speed, chosen so as to provide a good representation of the array efficiency as a function of wind speed curves. The resulting coarse resolution array efficiency matrix is then interpolated to a finer resolution and used in conjunction with the site wind data to calculate an overall array efficiency.
This procedure is used to assess the array efficiency of the wind farm layout for three turbines types with same rated power, but with different thrust coefficient curves and diameters. The simulations are carried out with ANSYS’ WindModeller, assuming a well-mixed (neutral) boundary layer with stable stratification above. Results are compared with those obtained with standard commercial software.
For the most frequent directions, the sensitivity of the array efficiency to stable surface conditions is quantified.


Conclusion

For directions with turbine alignment, the resulting array efficiency strongly depends on the wind speed regime. For wind speeds below rated, arrays with the larger rotors show a reduced efficiency (but still a higher power) compared to the smaller rotors. For wind speeds above rated, the larger rotors outperform the smaller ones for both efficiency and power. This behaviour has been studied when computing the overall array efficiency, which depends also on the wind distribution and the layout orientation.
For the analysed offshore wind farm, the increase in rotor diameter is justified both in terms of efficiency and power production.



Learning objectives
This study will demonstrate the applicability of CFD models for assessing overall large array efficiencies of a specified layout, and quantify the gains, in terms of energy output, that an increase in turbine diameter can deliver, for a given layout and turbine rated power.