Environmental Engineering Reference
In-Depth Information
significant and may easily make the difference between an economic and uneco-
nomic project.
Large wind turbines are often sited in ''complex terrain'' due to hills and valleys
and the like. This complicates the resource assessment, partly because the simple
equations for the vertical variation of the mean wind speed, Eqs.
1.14
and
1.15
are strictly only valid for fully developed flow over flat terrain. Furthermore, the
vertical variation in wind speed usually depends on wind direction, e.g. Ziter and
Lubitz [
4
], and time of day, e.g. Istchenko and Turner [
5
], because of changes to
atmospheric stability. The lower tower heights of small turbines can magnify the
effects of terrain especially in the presence of buildings and trees of similar height.
Table
12.1
demonstrates some of these features of the wind resource. It lists the
daily output from two identical Skystream 2.4 kW wind turbines on 11 m towers
over a 16 day period in 2010. The turbines are on the fringe of the city of Calgary
(Canada) about 8 km apart. Turbine ''A'' is on flat ground about 80 m from a large
two-storey house. Turbine ''B'' is about the same distance from a similarly sized
house but is on a ridge. Despite these site differences, the average outputs
are similar but there are huge variations in daily production. Some of these
correlate with wind direction, as output is seriously reduced if the wind flows over
the house before reaching the turbine or if the wind flows over the house after
passing through the turbine.
It is clear that the wind resource is difficult, and often expensive, to assess, and
some compromises are inevitable. There are a number of possible approaches to
resource assessment:
Table 12.1 Daily output
from identical Skystream
2.4 kW turbines
approximately 8 km apart
Date
Daily output from
turbine A (kWh)
Daily output from
turbine B (kWh)
30 April
0.75
11.15
1 May
1.42
8.48
2 May
8.08
5.16
3 May
17.93
10.93
4 May
0.6
24.56
5 May
6.96
3.39
6 May
4.66
5.65
7 May
11.49
7.32
8 May
38.16
20.8
9 May
21.75
27.79
10 May
3.66
25.28
11 May
22.4
7.13
12 May
7.97
9.95
13 May
13.25
0.98
14 May
28.93
9.29
15 May
13.99
28.65
Average
12.62
12.91
Data from Colin Dumais
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