356
10 Wind Turbines
solidity of a horizontal-axis wind turbine for power generation is in the 0.06–0.08
order. The solidity of the machine represented in Fig. 10.5 is about 0.5. With a
greater solidity, the maximum power extraction from the wind is reached at a lower
blade speed and the power coefficient is lower (see Sect. 10.3: performance). A high
torque is obtained, as required for pump driving. Turbines of this type are applied
on locations where electric power supply is difficult, as on meadows and fields.
Dimensions are limited. The rotor is kept in the wind by a vane. It is turned partially
out of the wind at high wind speed. This may be realised by mounting the rotor onto
a frame with a hinge and a spring. The rotor is turned either upwards or sideways
by the axial force on the rotor or by the force on a plate perpendicular to the wind
(systems are not represented on Fig. 10.5).
10.2.5 Vertical-Axis Wind Turbines
Figure 10.6 represents a type with straight prismatic blades and a vertical shaft. A
horizontal section is shown, with one blade in four positions. In the upwind and
Fig. 10.5 Low-speed
horizontal-axis wind turbine
for pump drive
Fig. 10.6 Vertical-axis wind turbine with straight blades (VAWT)