Introduction to Wind Turbine Technology - Small Wind Turbines: Analysis, Design, and Application - page 4

output power given by the Betz-Joukowsky limit, derived in Sect. 2.5 , is inde-
pendent of size. On the other hand, there are operational issues that do depend on
size; for example, starting performance and cut-in speed—the lowest wind speed
at which power is extracted. Both of these are more important for small machines
because:
• Small wind turbines are often located where the power is required or adjacent to
the owner's home which may not be the windiest location, whereas wind farms
containing large turbines are deliberately sited in windy areas.
• The generators of small turbines often have a significant resistive torque that
must be overcome aerodynamically before the blades will start turning. Fur-
thermore, pitch control is rarely used on small wind turbines because of cost.
(The precise definition of blade pitch will be given in Chap. 3 .) Thus it is not
possible to adjust the blade's angle of attack to the prevailing wind conditions.
This problem is particularly acute during starting. Starting and low wind speed
performance are discussed in Chap. 6 .
• Small wind turbine aerodynamics is influenced strongly by low values of the
Reynolds number, Re. This hugely important parameter is introduced in the next
section and its influence on blade aerodynamics is a major topic of Chaps. 4 and 5 .
Low values of Re mean, in practice, that small wind turbines bear greater
similarity to model, rather than full-sized, aircraft, and hummingbirds rather
than eagles. The later discussion of airfoil lift and drag and blade performance
calculations identifies many features particular to small turbines.
• Large wind turbines have complex yaw drive mechanisms to align the rotor to
the wind. These are usually deemed too expensive for small turbines, so some
form of free yaw is used. The most popular options are to have a tail fin, like
three of the five turbines shown in Fig. 1.2 , or to have downwind blades, as do
the remaining two. Neither choice is optimal for reasons that will be explained
in Chap. 8 on tail fin dynamics and yaw behaviour.
• Many small turbines rely on furling for overspeed protection—see Chap. 8 —
whereas large turbines usually have a brake on the high speed shaft (after the
gearbox and before the generator). Aerodynamic overspeed protection is dis-
cussed in Chap. 8 .
Virtually all large turbines, such as those seen in Fig. 1.3 , are upwind
machines—the blades are in front of the tower when viewed from the wind direc-
tion—and have three blades. The main differences occur in the drivetrain and
generator. The most common generator types are doubly fed induction generators
(DFIGs) and permanent magnet generators (PMGs), e.g. Burton et al. [ 2 ] and
Bianchi et al. [ 3 ]. DFIGs require a gearbox and are rarely used on small turbines,
but PMGs do not. They and the less-used induction generators (IGs) are described
in Sect. 1.8 and Chap. 11 . There is a much greater diversity of small turbine types as
seen in Fig. 1.2 with the number of blades varying from two to seven, and the most
popular turbines, the Proven and the Skystream being downwind machines.
Most upwind small turbines have a tail fin which keeps the blades pointing into
the wind. The tail fin is designed to minimize h, the yaw angle of the turbine