Design and development of small wind turbines - Wind Power Generation and Wind Turbine Design - page 266

control methods eliminate the need for stalling airfoils when used in conjunction
with a grid-tie inverter as discussed in the electrical system section below.
Another approach, not commonly used, is the so-called “tip brake” (see Figs 5
and 6). Centrifugally deployed fl aps are mounted at the end of each blade, and an
overspeed condition causes them to deploy and face the wind, resulting in signifi -
cant drag at the blade tip, thus limiting rotational speed. Tip brakes typically do not
automatically reset, as they should only deploy when other problems exist (brake
failure, generator failure, etc.) which would require the attention of a service tech-
nician. It is interesting to note that tip brakes do not signifi cantly degrade rotor
effi ciency during normal operation since, although there is increased drag at the
blade tip, they tend to prevent blade tip losses.
Other braking systems whose main function is not rotor overspeed control will
be discussed in Section 1.3.
1.2.3 Rotor manufacturing considerations
It is generally easier to build small wind turbine rotors than those for large wind
turbines. The rotor weight plays a less role in the design, and there is more focus
on using minimum cost manufacturing techniques (such as injection moulding
for the smallest machines). While glass reinforced plastic is the most common
material (as in large machines), it is also easily possible to use wood or recyclable
thermoplastics.
Aerodynamic effi ciency is sometimes sacrifi ced in favour of ease of manufac-
turing. For example, it is possible to extrude plastic blades, such that there will be
twist but no taper. The effect on effi ciency is illustrated on Fig. 12. The lower
Figure 12: Effect of adding taper to a blade with twist only.