Wind resource and site assessment - Wind Power Generation and Wind Turbine Design - page 78

Table 6: Defi nition of IEC classes [25].
Class

I II III IV S

U ref (m/s) 50 42.5 37.5 30 Site-specifi c

U ave (m/s) 10 8.5 7.5 5

I_w

0.5

I_w_mod

0.4

Andros

Taff Ely

0.3

AlsVik

Vindeby

0.2

0.1

0.0

0 2 4 6 8 10

Separation in Diameters

Figure 20: Wake-induced turbulence intensity I wake [ 24 ] .

5.2.2 Wake-induced turbulence
The wake of a wind turbine is characterised by the velocity defi cit in comparison to
the free fl ow and an increased level of turbulence. While the velocity defi cit leads
to a reduced energy output (Fig. 23) the turbulence in the wake leads to increased
mechanical loads. The wake-induced turbulence I wake is of a different nature than
the ambient turbulence I 0 as the length scales are different. The wind turbine has to
withstand both types of turbulence added up depending on its class (Table 6):

2 2

=+

(12)
Figure 20 shows the empirically measured, maximum wake turbulence as a
function of distance between two wind turbines expressed in rotor diameters. As a
rule of thumb wind turbines should not be closer than the equivalent of 5 rotor
diameters in the main wind direction. Perpendicular to the main wind direction, a
minimum distance of 3 rotor diameters should be maintained.
Turbulence particularly affects the blade root fl ap-wise as well as leading to
varying torsion in the main shaft which is fed through the gearbox into the genera-
tor. Additionally turbulence causes thrust loads in the tower. The international

I I I

wind farm 0 wake