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4.2.1 Horizontal-axis wind turbines
CONCEPTUALISATION
Horizontal-axis wind machines have been known since the 10th century. Some of the earli-
est types were windmills, fixed permanently to face costal winds and used to grind cereals.
Later followed more versatile mills that functioned as sawmills, threshing mills and as
wind pumps, used for land drainage and for water supply. Several historical horizontal-
axis wind mills are shown on figure 4.4 [10, p. 1-13].
(a) (b) (c)
Figure 4.4: (a) Dutch windmill, (b) American wind pump and (c) Thai wind pump [15]
Today horizontal-axis wind turbines (HAWT), which produce electricity, are the most
commonly available wind machines. In fact all presently grid connected commercial wind
turbines are of this type [16, p. 2]. Figure 4.5 shows a modern commercial offshore wind
turbine and a small commercial household wind turbine.
Figure 4.5: On the left a 3 MW Vestas offshore wind turbine at Kentish Flats Offshore Wind
Farm [17]. On the right a 160 W battery charging small wind turbine [18]
The common denominator for modern HAWTs is that the rotor, shaft and generator are
mounted on the top of a vertical tower. Distinction is made between downwind and up-
wind rotors, see figure 4.6. Upwind rotors face the direction of wind and thereby avoid the
shade effect of the tower that exists on downwind rotors, which is known to create fatigue
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