Within the framework of the extensive measurement and evaluation programme NSW-MEP the effect of wind turbine wakes on energy output has been evaluated in the OWEZ wind farm, based on the SCADA measured 10-minute electric power and availability of the individual machines. The evaluation resulted in a global overall wind farm efficiency.
The wind farm performance has been determined using sector-wise power curves of the whole wind farm. The wind turbine nacelle anemometer has been used to establish the upstream wind speed in the applied wind direction sectors. The nacelle wind speed measurement has been corrected to the undisturbed wind speed via the NTF (Nacelle Transfer Function) determined with turbine #8 and the nearby meteo mast. The annual energy production AEP has been calculated using a representative offshore wind regime per applied sector.
The wind farm efficiency is the ratio of the annual energy production based on the sector-wise power curves and the AEP of the turbines applied at the same location but not affected by surrounding machines. As a reference the power curve of turbine #8, measured with the meteo mast wind speed, has been used. The analyses resulted in a wind farm efficiency of 93.4%. This result is based on a period of 15 months of SCADA data. Only data has been used when wind turbine were 100% available.
In case the average energy production is based on the average power of the 12 turbines in row one, determined with undisturbed wind coming from south-west, the derived efficiency is 91.5%.
For verification purposes the wind farm efficiency has been calculated with the ECN-code FLUXFARM using the same configuration and wind regime. The calculated efficiency is 91.7%; giving confidence to the derived analyses results.
Furthermore the wake effects between wind turbines in a row and between rows has been visualised. It is shown that wind turbines in a row (mutual distance is 7.2 rotor diameters) have a maximum drop in output power of almost 40%. Wakes of wind turbines between rows (11.1*D) have a maximum drop in power of about 27%. The power decrease appeared to be not
depending on the number of upstream turbines.