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(a)
6.0
Onshore
5.0
5 RD
Offshore
4.0
3.0
2.0
7 RD
1.0
10 RD
0.0
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
Turbine thrust coefficient
(b)
1.0
0.9
0.8
10 RD
0.7
0.6
7 RD
Onshore
0.5
5 RD
Offshore
0.4
0.3
0.00
0.20
0.40 0.60 0.80
Turbine thrust coefficient
1.00
1.20
1.40
Figure 16-8.
(a) Wind farm equivalent roughness
z
00
for an infinite turbine array as a function
of thrust coefficient and array spacing in rotor diameters (RDs), for both offshore (ambient
z
0
=
0
.
001 m) and onshore (ambient
z
0
=
0
.
03 m) projects. (b) Asymptotic hub-height wind
speed as a fraction of the free-stream speed for the same cases.
Source:
AWS Truepower, based
on equations in Frandsen (2007).
where
C
t
is the turbine thrust coefficient and
s
d
and
s
c
are the mean downwind and
crosswind spacings, respectively, in rotor diameters. Figure 16-8a shows
z
00
for a
range of
C
t
and mean array spacings
5
. The roughness is strongly dependent
on the spacing, much less so on the background or ambient roughness (here assumed
to be 0.001 m for offshore arrays and 0.03 m for onshore arrays).
0
.
(
s
d
s
c
)
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