Starting and Low Wind Speed Performance - Small Wind Turbines: Analysis, Design, and Application - page 101

Chapter 6
Starting and Low Wind Speed
Performance
6.1 Introduction
It will be shown in this and the next chapter that designing blades only for efficient
power extraction using Eqs. 5.12a , b and 5.13a , b for the optimum chord and twist
respectively, can result in a high cut-in wind speed. The turbine will then be of
little practical use except in rare situations where the wind blows regularly and
strongly. Even a turbine like the 1.94 m diameter one shown in Fig. 6.1 , whose
power curve, Fig. 6.2 , demonstrates a good cut-in wind speed of 3.5 m/s, may not
extract maximum power in low winds.
There are at least three reasons to be particularly interested in performance at
low wind speed. First, many small turbines are located close to the load they
supply and this may not be a good wind site. Secondly, very few small turbines
have pitch adjustment, so that a blade designed for optimal power extraction at a
high tip speed ratio, will present high angles of attack when it is stationary. As
shown in Fig. 4.9 these high angles occur at low Reynolds numbers which makes
it even more difficult to generate sufficient lift to turn the blades. Thirdly, turbines
start only when the aerodynamic torque generated on the stationary blades exceeds
the resistive torque in the generator and drive train. Recall that Fig. 1.12 shows
typical resistive torques of PMGs used in small turbines as a fraction of rated
torque. For comparison, Sect. 6.2 describes an estimation of the ratio of aerody-
namic starting to rated torque and shows that it can be smaller than the resistive
torque ratio. In the absence of significant Reynolds number effects, the aerody-
namic torque scales as R 3 —as shown by Eq. 2.9 Figure 1.12 suggests that the
resistive torques decrease less rapidly than R 3 as R decreases; in fact they increase
as a proportion of rated torque. Thus starting is a particularly important issue for
micro-turbines. This is most likely the reason why micro-turbines for power on
yachts, for example the Rutland 913 in Fig. 1.3 , often have five or more blades.
The turbine in Fig. 6.1 had a permanent magnet generator (PMG) with a maxi-
mum static resistive torque of 0.36 Nm. It was mounted for many years on the roof of
the Engineering Building at Newcastle University and has been investigated in