The diagram indicates the effect of different surface roughnesses on the wind speed at varying heights. The wind speeds over each terrain are indicated by the distance between the vertical and curved lines.
The mean wind speed increases with height above ground level. In addition the flow tends to become less turbulent with increasing height. Both these factors mean that it is sensible to mount a turbine as high as is possible. the way in which the wind speed profile changes with height depends upon the 'roughness' of the terrain.
The diagram indicates the effect of different surface roughnesses on the wind speed at varying heights. The wind speeds over each terrain are indicated by the distance between the vertical and curved lines.
The gradient wind is the wind at an altitude where it is unaffected by the terrain below. Rough terrain reduces the wind speed at higher altitudes than smooth.
The tool models the effect of hub height on turbine output using a log law:
Energy output as a proportion of energy output with a 50m hub height = ( log(H/R) / log(50/R) )^3
where log is the natural logarithm (to the base e), H is the hub height in metres and R is the roughness length of the terrain. The values used for the roughness lengths are:
| Terrain description | Sheltered | Open plain | Coastal | Hilltop |
|---|---|---|---|---|
| Roughness length (m) | 0.4 | 0.05 | 0.03 | 0.03 |