Design and development of small wind turbines - Wind Power Generation and Wind Turbine Design - page 262

about 8 10 m rotor diameter. Downwind passive yaw machines can be found up

to about 15 m rotor diameter. Large wind turbines virtually are all of the active
yaw, upwind design.
The upwind design appears more popular. The Jacobs design has been around
for many decades (Fig. 8), and Bergey produce a well-known upwind turbine
( Fig. 9 ).
Upwind turbines require a tail vane to orient the machine into the wind, whereas
downwind turbines naturally track the wind without the need for a tail vane. The
rotors on downwind machines are subject to “tower shadow” each time a blade
passes behind the tower. The blade briefl y sees reduced and more turbulent winds
behind the tower, resulting in cyclical moments on the low-speed shaft and turbine
mainframe which do not exist on an upwind machine. This increases fatigue cycles
on the turbine. This must be traded off against the simplicity of the downwind
design. In large wind turbines driven yaw is needed, and there is no reason not to
have the rotor upwind of the tower.

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1.2 Small wind turbine rotor design

In general the same issues in blade design exist for small turbines as for large wind
turbines. These are discussed elsewhere in this topic, and so only the unique aspects
of small wind turbine blade design will be discussed in this section. These issues

Figure 8: Jacobs 10kW (credit: AWEA, Ed Kennel).