Wind Turbine Airfoils and Rotor Wakes - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 395

Wind Turbine Wake Effects
Modern wind power stations consist of numbers of individual turbines arranged on a
given site so as to best utilize the local wind energy. This appears to call for the placement
of most of the units at the locations of strongest flow. However, such concentrations of
turbines will cause shielding of neighboring units, so that downwind turbines are exposed
to a lower wind speed. Clearly, the station designer is seeking an optimal situation, in
which the most energetic regions of the site are exploited without crowding these desirable
areas with so many units that array interference (or wake interference ) prevents them from
achieving the best energy capture. It has also been suggested that, in complex terrain,
turbines on upwind slopes or near crests of hills and ridges may actually precipitate separa-
tion in the lee flow, significantly lowering the surface wind energy on the downwind slope
and causing even larger wake deficits.
The extent of the loss due to array interference can be a significant factor in the
economics of a wind power station. A representative picture of the loss situation for a 6x6
array of wind turbines is shown in Figure 6-15 [Lissaman and Zalay 1982]. In this
hypothetical example, 36 turbines are arranged in a rectangular grid pattern, spaced 10 dia-
meters apart in the direction of the prevailing wind and at various crosswind distances. An
important parameter in determining the severity of wake effects on downwind machines is
the ambient turbulence intensity of the wind, s 0 /U , which will be discussed later. Figure
6-15 gives the total array energy loss as a function of turbulence and crosswind spacing,
Figure 6-15. General effect of spacing and ambient turbulence intensity on the wake
energy losses in an array of wind turbines on flat ground. [Lissaman and Zalay 1982]