Environmental Engineering Reference
In-Depth Information
Obstacles can cause a displacement of the boundary layer from the ground.
This displacement can be considered by the parameter
d
. For widely scattered
obstacles, parameter
d
is zero. In other cases
d
can be estimated as 70 per cent
of the obstacle height.
The
roughness length z
0
describes the height at which the wind is slowed
to zero. In other words, surfaces with a large roughness length have a large
effect on the wind. Table 5.3 shows the classification of various ground classes
depending on the roughness length.
The following example shows the influence of the ground cover. The same
wind speed
v
(
h
1
) = 10 m/s at a height
h
1
= 50 m above different ground
classes is assumed. Equation (5.6) is applied to calculate the wind speed
v
(
h
2
)
at a height of
h
2
= 10 m. The displacement
d
for the boundary layer from the
ground must be considered for higher obstacles in ground classes 6 to 8. Table
5.4 shows the calculated results. The wind speed decreases significantly with
rising roughness lengths
z
0
; thus, it does not make any sense to install wind
power plants in built-up areas or large forests. The wind speed also increases
significantly with height. For instance, the wind speed at a height of 50 m is
30 per cent higher than at 10 m for ground class 4. This must be considered
for the installation of large wind turbines. The usable wind speed at the top of
large wind towers is much higher than at the common measurement height of
10 m. Wind turbines of the megawatt class come with hub heights of between
50 and 70 m for coastal areas (ground class 1 to 3) and even higher for inland
areas with higher roughness lengths.
This example should not give the impression that the wind speed is already
independent of the ground at a height of 50 m. The wind speed usually
becomes independent of the height, where the wind becomes known as
geostrophic wind
, at heights significantly exceeding 100 m from the ground.
Finally, the power law of Hellmann is another relation for the vertical
distribution of wind speeds.
Table 5.3
Roughness Lengths
z
0
for Different Ground Classes
Ground class
Roughness
Description
length
z
0
in m
1 - Sea
0.0002
Open sea
2 - Smooth
0.005
Mud flats
3 - Open
0.03
Open flat terrain, pasture
4 - Open to rough
0.1
Agricultural land with a low population
5 - Rough
0.25
Agricultural land with a high population
6 - Very rough
0.5
Park landscape with bushes and trees
7 - Closed
1
Regular obstacles (woods, village, suburb)
8 - Inner city
2
Centres of big cities with low and high buildings
Source:
Christoffer and Ulbricht-Eissing, 1989
