Wind resource and site assessment - Wind Power Generation and Wind Turbine Design - page 75

Figure 17: WAsP wind speed prediction error as a function of difference in rug-
gedness indices between the predicted and the predictor site [20].

5 Siting and site suitability

5.1 General

During the process of siting (also referred to as micro-siting), the locations of the
future wind turbines are determined. Apart from the wind resource this process is
driven by a number of other factors such as technical risks, environmental impact,
planning restrictions and infrastructure costs.
Technical risks can very often be mitigated by adjusting the layout to suit the
site-specifi c conditions. High turbulence as a main driver of fatigue loads can be
avoided by maintaining suffi cient distances between the turbines, keeping clear of
forests and other turbulence-inducing terrain features like cliffs. Steep terrain
slopes should be avoided to reduce stresses on the yaw system and blades and
thereby improve the energy output.

5.2 Turbulence

5.2.1 Ambient turbulence
The turbulent variations of the wind speed are typically expressed in terms of the
standard deviation s u of velocity fl uctuations. This is measured over a 10-min period
and normalised by the average wind speed — and is called turbulence intensity I u :

s u

=

I (10)

u

U