One disadvantage of the traditional Darrieus Turbine is that it requires symmetrical airfoils of zero pitch. This is unfortunate, since a pitched asymmetric airfoil has much better aerodynamic characteristics. Because a given side of the Darrieus airfoil must serve as the high pressure side for half a rotor turn, and then as the low pressure side for the other half turn, Darrieus machines have not been able to take advantage of the superior performance of the pitched asymmetric airfoil.
This post describes a technique for inverting the airfoils twice per rev. This permits the Darrieus to employ pitched asymmetric airfoils:
I believe in three bladed Darrieus machines, but it’s usually easier to use two bladed machines in diagrams and explanations, and that is what I have done here. As the two-bladed Darrieus approaches the rotational angle at which it produces no power, the airfoils may rotate freely. This doesn’t matter because they aren’t torquing the rotor anyway. As the rotor enters the other half of its power producing arc, the net wind velocity vector shifts so that it is no longer parallel to the airfoil’s velocity vector. Now the airfoil is mechanically stable, but unless its high pressure side is upwind and its low pressure side is downwind, it is aerodynamically unstable. If unstable, it will flip over so that the polarity of the airfoil is appropriate for the given power producing arc.
The embodiment depicted in the above diagrams is only one of many variations. Basically, it boils down to this:
- Each airfoil is able to rotate about about a horizontal axis tangent to its circular path of motion. (If the airfoil is pitched, then its chord will not be parallel to its axis of rotation. In this case, the chord will sweep out the surface of a cone if the airfoil is rotated 360 degrees. However, the axis of rotation is still tangent to its circular path of motion.)
- The mass of each airfoil is balanced with respect to its axis of rotation. (“Axis of rotation” here means the airfoil axis – not the rotor axis.) Because the airfoil is balanced, centrifugal force will not cause it to rotate.
- The airfoil is aerodynamically stable when the high pressure side of the airfoil is upwind and the low pressure side is downwind. The airfoil is unstable when oriented with the opposite polarity. In this case, aerodynamic forces will rotate the airfoil, causing it to flip to the other side.
A similar approach would have the controller actuate the airfoils. In this case, the controller has a wind vane just like a horizontal axis machine, and uses this information to determine when to flip the airfoils.
Note that if the circular path of the VAWT blade has a large diameter, centrifugal forces are greatly diminished. In this case it may not be necessary to counterbalance the inverting blade.