Salient White Elephant

April 10, 2009

Variable Pitch Savonius Turbine

Variable Pitch Savonius with Aerodynamically Adjusting Vanes

This post describes a modified Savonius rotor that might be useful for low power applications in undeveloped areas with no power grid. We’ve all seen homemade Savonius rotors built by sawing an empty barrel in half. Let’s begin by imagining just such a barrel, but instead of sawing it in half, we cut a large slot into one of its sides:

Slot Cut In Side of Barrel

Now we cut an identical slot into the opposite side of the barrel. Next, the slots are adorned with two nylon vanes. These vanes can bow like the sail of a sailboat. They can bow in towards the axis of rotation (so that they are actually inside the barrel), or they can bow out away from the axis of rotation:

Aerial View, Variable Pitch Savonius

Downwind View, Variable Pitch Savonius

Instead of nylon vanes, the metal that was removed to make the slots in the side of the barrel can be made into little swinging doors on hinges.

Comparing the Variable Pitch Savonius with the Standard Savonius, notice that the vanes of the Variable Pitch Savonius cannot impede the motion of the rotor. The same cannot be said of the Standard Savonius. For this reason, the Variable Pitch Savonius might turn out to be more efficient than the Standard Savonius. Note that if the Variable Pitch Savonius is attached to a Darrieus rotor in order to make the Darrieus self-starting, then once started, the Variable Pitch Savonius rotor will not load down the Darrieus rotor. (This is so because the vanes of the Variable Pitch Savonius cannot impede the motion of the rotor.)

Standard Savonius and Variable Pitch Savonius Compared

An Alternative Approach

Variable Pitch Savonius, An Alternative Design

Each of the arc shaped vanes in the diagram above are rotated as far as they are able to go in normal wind conditions before hitting stops that limit their further rotation. Two of the vanes are rotated in toward the axis of rotation (red “X”), and two are rotated away from the axis of rotation. In high winds, the vanes are able to rotate beyond the angle permitted by the stops. This happens as the restraining forces of springs (not shown above) are overpowered. In this case, the rotor assumes a low drag configuration approximately as depicted in the following diagram:

Variable Pitch Savonius, An Alternative Design, High-Wind Low-Drag-Profile Mode

Alternatively, maybe springs could draw the vanes toward their nominal position (which is where every point on the vane is equidistant from the rotor axis of rotation). In this case, as the wind blows harder and harder, the vanes sweep a larger and larger angle. Finally a wind speed is reached where the rotor shuts down as depicted above.

The arc shaped vanes might be made of plastic, kind of like a whiffle-ball bat. In this case, they might be so light in weight that it wouldn’t be necessary to counterbalance them to prevent centrifugal force from causing them to always be swung out away from the axis of rotation. Also, the arc shaped vanes should have rounded ends like the leading edge of an airfoil. Hopefully, this would reduce the amount of turbulence generated by the interaction of the arcs and the wind.

Vane of Variable Pitch Savonius Has Rounded Ends (Aerial View of Vane)

Variable Pitch Savonius, Alternative Design, On Guyed Tower

I haven’t had nearly enough time to think about the Variable Pitch Savonius, especially how it might be configured on a guyed tower, and how the various levels of rotors would interface with the electric generator. Hopefully I’ll have time later to come back and add more to this post.

When stacking multiple levels of VAWT rotors, it might be good to keep in mind the issues discussed in the Helically Stacked Darrieus or Savonius Rotor.

Another alternative would have a rotor with many short pivoting arcs like the two depicted above. Also, airfoils could replace the arcs:

Self-Starting Variable Pitch Darrieus

It seems intuitive that a drag type rotor needs more solidity, because it is less able to act on air from a distance. But an inexpensive turbine designed for rural or third world application needs to be reliably self-starting. The last diagram above shows a rotor that uses airfoils. Because this rotor uses lift, it needs less solidity, and some of the airfoils could be eliminated from this design. Unfortunately, eliminating some of the airfoils would seem to reduce startup torque. But here’s an idea to get around this problem. Here we allow the airfoils to rotate through a very large angle when the rotor isn’t turning. This produces a lot of startup torque. As the rotor spins faster and faster, the airfoils are more and more biased toward their nominal (zero pitch) orientation. This is realized using a flyweight. Because this idea is very three dimensional, and I’m a terrible artist, I couldn’t do it justice in a diagram. But I’ll go ahead and post what I’ve got, and try to fill in the rest with a written explanation:

Self-Starting Centrifugally Regulated Variable Pitch DarrieusThe radial arm that supports the airfoil should be on top of and on bottom of the airfoil in the diagram above. But this would block your view of the centrifugal pitch regulating mechanism, so I tried to draw a see-through ghost arm instead. A cable attaches the trailing edge of the airfoil to a centrifugal flyweight. When the rotor is stationary, the wind can easily push the airfoil away from its nominal zero pitch orientation because it has only to lift the flyweight (which will be hanging down toward the ground when the rotor is stationary). As the rotor spins faster and faster, the trailing edge of the airfoil is pulled towards its nominal zero-pitch orientation with greater and greater force. (The part of the arm that supports and routes the cable is ridiculous in the diagram above. In an actual design, it would probably extend from the axis of rotation to the trailing edge of the airfoil along the chord line. But I had to draw it this way in order for all the parts to be visible.)

Check This Out!

Just found a similar rotor on the web that’s pretty cool:

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