Page 418 - Wind Energy Handbook

392 COMPONENT DESIGN

Bond and Ansell (1998). Note the relatively low stress ranges at R ¼ 0:84 (¼ UCS/
UTS), which may be due to simultaneous occurrence of compressive and tensile
damage. Despite this, the simplification of the constant life diagram to a series of
straight lines between the R ¼ 1 stress range for each fatigue life and either the
UTS or UCS is reasonably accurate.

Material safety factors

The material safety factor applied when timber is used in building construction is
normally high, e.g., about 3–4. However, there are a number of reasons for
adopting a much lower value in blade design:

(1) laminated construction is used, so any defects are very localized;

(2) the moisture content is carefully controlled during manufacture, and the blade
skin is then very effectively sealed against further moisture ingress;
(3) creep effects are negligible as the gravity loads change direction because of
blade rotation and the wind loads are temporary in nature.

Accordingly a partial safety factor of only about 1.5 is normal for design against
extreme loads.

7.1.8 Governing load cases

Extreme loading during operation: stall-regulated machines

As described in Chapter 5, wind turbine design codes specify a number of load
cases consisting of various combinations of defined wind speed and direction
changes – some of them involving external or machine faults – which are an
attempt to define an envelope of the worst loadings to be expected in practice. It is
instructive to take one such code, IEC 61400-1, and compare the blade loadings
arising from the different load cases for a particular design. The WTG chosen is a
40 m diameter, 500 kW stall-regulated Class II machine fitted with TR blades (see
Figure 5.2(a)) and operating at a single rotational speed of 30 rpm. The rated wind
speed, U r , and cut out speed, U o , are 16 m/s and 25 m/s respectively. The shaft tilt
with respect to the horizontal is taken as 58 so, allowing for a 88 inclination of the
flow to the horizontal as specified in the code, the maximum shaft tilt with respect
to the flow is 138. Category A turbulence characteristics are assumed.
The table below summarizes the conditions applicable to the power production
load cases (apart from those involving a machine fault) and compares the resulting
peak out-of-plane blade bending moment at 60 percent radius, ignoring dynamic
effects. In the cases involving a change of wind direction, the maximum yaw error
is simply taken as equal to the maximum direction change i.e., ignoring any action
taken by the control system to correct it.