General Aerodynamic Considerations
The governing equation for power extraction is given below:
-
(1)
- where: P is the power, F is the force vector, and U is the speed of the moving wind turbine part.
The force F is generated by the wind interacting with the blade. The primary focus of wind turbine aerodynamics is the magnitude and distribution of this force. The most familiar type of aerodynamic force is Drag, this is the same force that is felt pushing against you on a windy day. Another type of force is lift, this is the same force that allow most aircraft to fly. The direction of the drag force is parallel to the relative wind, while the lift force is perpendicular. Typically, the wind turbine parts are moving so this alters the flow around the part. An example of relative wind is the wind one would feel cycling on a calm day.
To extract power, the turbine part must move in the direction of the force. In the drag force case, the relative wind speed decreases subsequently so does the drag force. The relative wind aspect dramatically limits the maximum power that can be extracted by a drag based wind turbine. Lift based wind turbine typically have lifting surfaces moving perpendicular to the flow. Here, the relative wind will not decrease in fact it increases with rotor speed. Thus the maximum power limits of these machines is much higher than drag based machines.
Read more about this topic: Wind Turbine Aerodynamics
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