Starting and Low Wind Speed Performance - Small Wind Turbines: Analysis, Design, and Application - page 104

Fig. 6.5 Low wind speed
performance of 500 W wind
turbine from Wright [ 1 ]. The
symbols define whether the
rotor is steady, accelerating,
or decelerating. The solid
lines show the boundaries of
the regions as determined by
blade element analysis

600

steady
accelerating
decelerating

500

400

300

ii)

200

100

iii)

i)

0

1 2 3 4 5 6 7 8 9

wind speed (m/s)

• A gust is often required to turn the blades, in this case from about 2 m/s to
nearly 6 m/s around time, t = 3s.
• Starting is dominated by a long period of slow acceleration—called the ''idling
period''—about 85 s in this case. It will be assumed that idling is sufficiently
long to justify a quasi-steady analysis of starting. The analysis to be developed
in this chapter is a modification of the standard, power-producing, blade element
theory of Chap. 5 . Note from Fig. 1.12 that this turbine has a high ratio of
resistive to rated torque which accentuates, but is not uniquely responsible for,
the long idling period.
• Using the high-a formulations of aerofoil lift and drag considered in Chap. 4 ,it
is possible to accurately simulate starting as shown by curve (b) in the figure.
This curve, along with the less accurate predictions labelled (a) and (c) will be
explained in Sect. 6.4 .

Wright [ 1 ] measured about 200 h of low wind speed performance of this tur-
bine. Using the method described by Wright and Wood [ 2 ] he determined whether
the rotor was steady, accelerating or decelerating for a large number of 15 s
sequences taken from the data. Figure 6.5 shows the results in terms of rotor speed
versus wind speed with the rotor state denoted by symbol. The delineation between
the three regions—(1) decelerating, (2) steady, and (3) accelerating as determined
by blade element theory is shown by the solid lines. It is clear that the rotor
accelerated from rest only for wind speeds of approximately 4.8 m/s, the
''starting'' wind speed from Fig. 6.3 . However, the decelerating rotor did not stop
until about 2.5 m/s, which can be called the ''stopping'' wind speed. It is clear that
the cut-in wind speed from Fig. 6.2 is an average of the starting and stopping
speeds and that the key to improving low wind speed performance is to understand
starting performance. The former task begins the next section which discusses the
aerodynamic torque on a stationary blade for comparison with the resistive torque
data from Chap. 1 . Improving starting performance is part of the multi-dimen-
sional optimisation described in Chap. 7 .