sin αst

CONCEPTUALISATION
Figure 4.2: The forces and the moment acting on an airfoil subjected to a 2D air flow [12, p.
103]
The flow velocity on the convex upper surface of the airfoil is increased, creating a low
pressure zone above the airfoil in accordance with Bernoulli’s principle. This results in the
lift force that is perpendicular to the direction of the oncoming airflow. Both the lift force
and the drag force are considered acting on the chord line, ¼ of the chord length from the
leading edge. To describe the forces completely it is additionally necessary to identify the
pitching moment, which is generally placed at the same reference point as the force com-
ponents [11, p. 6].
Tip-speed ratio
The term tip-speed ratio (TSR) is defined as the ratio: � = Ut/V, where Ut is the tangential
velocity of the rotor blade tip and V the velocity of the wind. The tip-speed ratio is related
to the power coefficient that has an optimum at a specific TSR. For a drag-based device the
TSR will never exceed a value of 1, but it is higher for a lift-based device. [10, p. 69]
4.2 Survey of wind turbine concepts
This section contains a survey of numerous different wind turbines and their key proper-
ties. The survey focuses on the most prevailing types of wind turbines that have the poten-
tial of being used in rural areas. This means that so called fantasy turbines or
unconventional turbines that have not been effectively proven in practice are left out of the
survey. Examples include diffuser-augmented wind turbines (DAWT) and airborne wind
turbines, both shown on figure 4.3.
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