Environmental Engineering Reference
In-Depth Information
3. Load measurements with extrapolations for extreme conditions. Obviously this
requires field testing, whereas the first two methodologies can be used at the
design stage.
The SLM is unique to the small wind turbine standard IEC 61400-2. It was
developed as a cheap alternative to the complex and expensive aero-elastic
modeling that is routinely done for large turbines. The ''cost'' of the SLM is the
high safety factors.
This chapter describes an assessment of the three-bladed 500 W, 2 m diameter
turbine, shown in Fig.
6.1
and discussed extensively in that chapter and
Chap. 8
.
Although the rotor swept area is larger than 2 m
2
, the tower is not considered in
detail because a special purpose tower was required for roof installation, that would
not be suitable for normal installation. In addition, tower design, which must
satisfy standards additional to IEC 61400-2, is discussed in detail in
Chap. 10
.
Much of the notation used in IEC 61400-2 is also used in this topic, but there
are some major differences which are collected in Table
9.1
. Many of the equa-
tions of this chapter are taken from the Standard. This is indicated by a footnote for
each equation giving its number in IEC 61400-2.
The first step in applying the SLM is to decide on the turbine class as defined in
Table
9.2
. Table
9.3
taken from IEC 61400-2, defines the ten load cases com-
prising the SLM. Note that only Case A is a fatigue load.
Section 9.2
describes the determination of the loads for the 500 W turbine
considered in
Chaps. 6
and
8
. The following sections show how to combine the
loads to give the component stresses and present the partial load factors.
Section 9.6
introduces an excel spreadsheet to simplify the analysis of this and any
other turbine. The extra testing required for IEC certification is described in
Sect. 9.7
. The last section contains the concluding remarks. A number of short-
comings and errors in IEC 61400-2 are identified in this chapter. This is done with
the aim of stimulating further measurements and analysis of wind turbine loads to
Table 9.1 Differences in
notation from IEC 61400-2
Definition
Symbol
used here
Symbol used
in IEC 61400-2
Moment of inertia (kg m
2
)
J
I
Number of blades
N
B
Wind speed (m/s)
U
V
Number of cycles to fatigue
N
cycles
N
Yaw rate
x
x
yaw
Blade angular velocity (rad/s)
X
n (in rpm), x
n
Table 9.2
The IEC turbine
Class
I
II
III
IV
Special (S)
classes
U
ref
50
42.5
37.5
30
Values to be specified
U
ave
10
8.5
7.5
6
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