Wind resource and site assessment - Wind Power Generation and Wind Turbine Design - page 80

Figure 22: An example of fl ow separation over a cliff.

case to reverse fl ow imposing the potential of serious damage to the wind turbine
(Fig. 22). The vertical and horizontal extent of such unsuitable areas can be
estimated using CFD.
The international design standard IEC [25] assumes an infl ow angle of maximum
8° for load calculations.

5.4 Vertical wind speed gradient

The loads on the rotor depend on the wind speed difference between the bottom
and the top of the rotor. The gradient is normally expressed by the wind shear
exponent a from the power law (eqn (6)). Most load cases assume a gradient of 0.2
between the bottom and the top of the rotor [25]. Note that in some cases, loads
will be larger for very small or negative

a .
The gradient causes changing loads of the blades as the angle of attack changes
with each rotation. Thus the gradient adds to the fatigue loads of the blade roots.
Furthermore the rotating parts of the drive train are stressed. The gradient is
affected by four different phenomena:

• Terrain slope . The logarithmic profi le can be heavily distorted by terrain slopes.

While in fl at terrain the wind speed increases with height, steep slopes might
lead to a decrease with height. This is particularly likely for sites where the fl ow
separates and does not follow the shape of the terrain anymore (Fig. 12). As a
consequence the wind speed exponent might exceed the design limit in some
sections of the rotor.

• Roughness/obstacles . If wind turbines are located closely behind obstacles like

for example a forest the vertical wind speed profi le might be again heavily dis-
torted and areas of the rotor are exposed to large gradients. The degree of defor-
mation of the profi le depends not only on the geometry of the obstacle but also
on its porosity.

• Layout . As explained above, the wake of a wind turbine is a conical area behind

the rotor with increased turbulence and reduced wind speed since the rotor
of the wind turbine has extracted kinetic energy from the fl ow. Figure 23 shows
the effect of the wake on the profi le, where the dotted line represents the free
fl ow and the straight line the profi le 5.3 rotor diameter behind the wind turbine.