Rotor Design: Part 3

Whereas the FX 63-137 has a large Cl/Cd over a relatively wide range of alpha, the Cl/Cd vs. Alpha curve of the NREL S822 airfoil exhibits a much sharper peak and a restrained max Cl/Cd.

FX and S822 Polars

The S822 airfoil can be used effectively in the primary and the tip regions of the blade. The rotor should be operating at or near the design TSR during normal operation, at which alpha will stay constant up to the maximum rpm allowed by the permanent magnet generator (PMG). This quality allows the engineer (me) to set the twist of each section in such a way that the entire length of the blade is capitalizing on the peak Cl/Cd during normal operation. In fact, there are many tools available that allow engineers to easily optimize the twist of a blade for AEP using blade element momentum theory.

S822 at Small Alpha

The resistance of the generator can be increased at very high wind speeds, holding the rotational speed of the rotor constant and causing the TSR of the wind turbine to decrease; this increases alpha, forcing the blade to operate outside of the peak Cl/Cd and causing stall along the primary and tip regions of the blade. As a result, the turbine can continue to function in high wind speeds, albeit at a much lower efficiency.

S822 at Large Alpha

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