ECN publication
The effect of wind farming on mesoscale flow
Published by: Publication date:
ECN Wind Energy 9-4-2009
ECN report number: Document type:
ECN-M--09-049 Conference Paper
Number of pages: Full text:
9 Download PDF  

Presented at: Global Wind Power 2008, China Wind Power 2008, Beijing, China, 29-31 oktober 2008.

This paper addresses the modification of the wind profile due to a wind farming, and the impact of wind farm design parameters and meteorological parameters on the wind profile. The wind profiles were obtained with the planetary boundary layer method MFwWF. Resolved profiles show how most of the wind speed change occurs in the lower part of the boundary layer whereas most of the wind direction change occurs in the upper part, and that the thinner the boundary layer or the larger the surface roughness, the larger the wind direction change. Near a 5 MW wind turbine with a rotor diameter of 100 m operating at full load the velocity deficit is of the order of 5%, the wind direction change is increased with 1 ... 2 deg, and the velocity recovery distance is 20 rotor diameters. For a wind farm with 22 of these turbines these numbers separated at 10 rotor distances are 15%, 2 ... 3 deg, and at least 2 wind farm length scales. Initial velocity deficits and velocity recovery distances show the impact of nominal power density and geostrophic velocity for a wind farm which consists of 22 wind turbines with a nominal power of 5 MW. The initial velocity deficit relative to the upstream velocity decreases with increasing geostrophic velocity in general, and ranges from 6% (at a turbine separation of 14 rotor diameters) to 32% (at a separation of 5 rotor diameters) if the velocity at hub height is halfway cut-in and nominal. At this hub-height velocity the absolute initial velocity deficit reaches a maximum (of 1.2 m/s in the case of a nominal power density of 5 MW/km2) and the velocity recovery distance relative to the wind farm length scale is of the order of 20. The relative velocity recovery distance for other geostrophic velocities varies between 0 (at low geostrophic velocities) and a limit value of the order of 40 (at high velocities).

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