sin αst

CONCEPTUALISATION
issues due to turbulence. A yaw mechanism is needed to keep upwind rotors aligned with
the wind direction, while downwind rotors are self-aligning. The majority of HAWTs use
the upwind rotor design [12, p. 3].
Figure 4.6: Upwind and downwind rotors [19, p. 18]
HAWTs are equipped with a power control system that wastes excess energy to avoid
damage of the wind turbine in case of strong winds. Large commercial wind turbines use
either stall or pitch control. The latter involves pitching the blades slightly out of the wind
to reduce the power output. Stall control typically involves limiting the power output by
designing the geometry of the rotor blades so that flow separation is created on the down-
side of the blade when a critical wind speed is exceeded [16, p. 3-4]. Small wind turbines
often use a simple furling mechanism that will both limit the power production and func-
tion as a yaw system.
The power coefficient of horizontal-axis drag-based windmills is 0.3 at the most, while
modern lift-based HAWTs can achieve a Cp value of more than 0.5 [10, p. 78].
HAWTs are widely used for projects in developing countries. In the 1980s the
Nordic Folkecenter for Renewable Energy developed both windmills and wind turbines for
battery charging. The wind turbine used in the hybrid energy project, mentioned in section
1.2, is also of the horizontal-axis type. Pioneers within the field of small wind turbines for
developing countries, such as Hugh Piggott, advocate the use of HAWTs due to their high
power efficiency and technical superiority compared to other wind turbine concepts [20].
20
Upwind Downwind