Environmental Engineering Reference
In-Depth Information
Table 9.4 Subscripts and
co-ordinates
Subscript
Meaning
ave
Average
B
Blade
cog
Centre of gravity (mass)
design
Input parameter for the simplified design equations
e50
Once per 50 year extreme (averaged over 3 s)
f
Loads (applied only to safety factors)
hub
Hub
m
Material (applied only to safety factors)
max
Maximum
r
Rotor
s
Shaft
x
Blade: horizontal direction giving a positive
moment in direction of rotation
x
Shaft: horizontal direction such that a positive
moment acts in direction of rotation
y
Blade: horizontal direction such that a positive
moment is in direction of rotation
z
Blade: towards blade tip
for P
design
in Watts. The efficiency, g, is to be taken as
4
0
:
6
þ
0
:
005P
design
for P
design
\20 kW
g
¼
ð
9
:
2
Þ
0
:
7
for P
design
[ 20 kW
The maximum yaw rate, x
yaw,max
, is determined (in rad/s) according to
5
for A
proj
[ 2m
2
3
0
:
01 A
proj
2
x
yaw
;
max
¼
ð
9
:
3
Þ
for A
proj
2m
2
3
where the ''projected'' rotor area, A
proj
,isinm
2
. According to Eq.
9.3
, x
yaw,max
reduces from 3 rad/s for micro-turbines to 1 rad/s for the largest possible small
turbine. A major limitation of the correlation that is apparent from
Chap. 8
is that it
is the same for furling and non-furling turbines. Another shortcoming is that (
9.3
)
gives a yaw rate well in excess of the maximum rate of change of wind direction in
the model of the ''extreme direction change'' (EDC) of the IEC Standards for the
aero-elastic analysis. Since
Chap. 8
demonstrated that wind turbines in yaw
approximate a second order linear system when they are not furling, it is unlikely
that the maximum yaw rate can exceed the maximum rate of change in wind
direction. Thus one way for the designer to ''reduce'' the gyroscopic loading on the
4
(IEC 50).
5
(IEC 27).
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