Wake effect
The reduced wind speed and increased turbulence downstream of a wind turbine, which lowers the output and raises the fatigue loads of turbines located behind it.
A turbine that extracts energy leaves behind a wake of slower, more turbulent air. Close behind the rotor the velocity deficit can be 30–50%; the wake recovers as surrounding air mixes in, but recovery takes many rotor diameters (D). Because power scales with the cube of wind speed, a 10% deficit cuts a downstream turbine's output by roughly 27%.
Layout rules of thumb are 5–9 D spacing in the prevailing wind direction and 3–5 D crosswind. Typical array (wake) losses on annual energy:
- Onshore wind farms: 5–12%
- Large offshore arrays: 10–20%; Horns Rev and Lillgrund measurements showed individual rows losing 30–40% in aligned wind directions
Offshore wakes persist longer because low surface roughness and stable atmospheric conditions reduce mixing. Satellite SAR images and studies of the German Bight show cluster wakes extending 30–60 km, so one wind farm can reduce yields at a neighbouring one, now an issue in North Sea lease planning.
Engineering models include Jensen/Park (top-hat deficit), Bastankhah-Porté-Agel Gaussian models and the FLORIS framework (NREL); large-eddy simulation is used for complex cases. Active wake steering, yawing upstream turbines a few degrees off the wind with the yaw system, has been shown in field trials to raise farm output by 1–3%.
Related terms
All terms →- Offshore wind
Wind turbines installed in seas or large lakes, usually on fixed foundations in water up to about 60 m deep, exploiting stronger and steadier winds than onshore sites.
- Power curve
The relationship between hub-height wind speed and a turbine's electrical output, used with a wind speed distribution to estimate annual energy production.
- Yaw system
The motors, gears, bearing and brakes that rotate a wind turbine nacelle about the tower axis to keep the rotor facing the wind.
- Repowering
Replacing ageing generating equipment at an existing site with newer, usually larger and more efficient technology, most commonly applied to wind farms after 15 to 25 years.
- Capacity factor
The ratio of the energy a power plant actually produces over a period to the energy it would produce running at full nameplate capacity for the whole period.