Salient White Elephant

April 1, 2009

Highly Scalable Direct Drive VAWT

There are many variations of the turbine described in this post. I’ll just go through the ones I have time to present one by one.

Two Cables Move With Symmetrical Airfoils

This variation may move along a polygonal path that approximates a circle.

Aerial View Showing Polygonal Path That Is Approximately Circular

Alternatively, the entire path (cables, towers, and all) may be yawed to keep its long dimension at a right angle to the approaching wind:

Aerial View Showing Oblong PathAirfoil Rounding TowerOne Of The Airfoils On Cable Between Towers

Single Cable Moves With Inverting Airfoils on Circular (Polygonal) Path

This variation moves along a polygonal path that is approximately like a circle. The following diagram shows how the airfoil center of mass is near the center of the cable. Because the gravity moment is small, the orientation of the airfoil will be controlled by aerodynamic forces whenever it is not traveling directly into or directly out of the wind. In these cases, the airfoil has only one stable orientation with respect to its aerodynamic forces. That stable orientation is the one in which the blade tips are downwind from the cable. Any other orientation is unstable. Whenever a change in the direction of the airfoil velocity vector causes it to become unstable, it will flip over to the stable orientation. This allows the use of pitched, asymmetrical airfoils. (Pitched, asymmetrical airfoils deliver better aerodynamic performance than the zero-pitch, symmetrical airfoils that are typically used on a Darrieus turbine.) When the airfoil is traveling directly into or directly out of the wind, its orientation will be controlled by the gravity moment. In these cases, the only stable orientation has the blade tips lower than the cable. From an aerodynamic perspective, we don’t care how the airfoil is oriented when traveling into or out of the wind, since in these cases it cannot produce power anyway.

Airfoils Pulling Cable

In the following diagram we can see that a gearbox is not required for this turbine. This is true because the airfoils and the cable are traveling at what we normally think of as “tip speed”. Suppose the wind speed is 20 miles per hour (mph). If the tip speed ratio is 6, then the airfoils travel at 120 mph, or 176 feet/second. If we want the generator to spin at 1,200 rpm, then the wheel that engages the cable must be about 2.8 feet in diameter, which sounds like a pretty reasonable number to me.

Airfoil Rounding Tower

The diagram above has been simplified – it does not show how the airfoil is prevented from hitting the tower or the generator. The following diagram adds a semi-circular guide that rotates the airfoil whenever its orientation is going to cause it to strike the tower or the generator:

Semi-Circular Guide Rotates Airfoil 90 Degrees Clockwise

The wheel that is closest you in the diagram above is the one that is situated at 3 o’clock. (The one with the cable directly behind it.) It is possible that the airfoil will strike this wheel exactly in its center. The design of the closest wheel causes it to move slightly upward and toward the tower whenever this happens, thus diverting the airfoil down the lower part of the guide.

Single Stationary Blade Guide Hangs From Cable, Yawing Oblong Path

Aerial View Showing Oblong PathCross-Section of Blade Guide Showing Integration of Blade Support and Direct Drive Components

Looking Down Cable at Blade Between Towers

Airfoil Rounding Tower

When an airfoil is supported at both ends using two blade guides like the ones pictured above, inverting the airfoil isn’t a big deal. It would seem at first to be impossible to support an inverting airfoil with only a single blade guide, since in this case the structure that connects the airfoil to the blade guide will always hit the blade guide’s supporting structure when going around the tower with one of the two airfoil orientations. But here’s how we get around this problem. In the diagram above, the orientation of the airfoil as it approaches the tower is such that the blade supporting structure (black) will not hit the blade guide support (purple). We wait until the airfoil has rounded the tower and is ready to begin its long linear path to the next tower, then we invert the airfoil. In other words, we wait unti the airfoil has passed the blade guide supports before inverting it. Now when the airfoil is approaching the tower on the other end of the oblong path, we invert the airfoil before it gets to the blade guide supports. For this reason, when the airfoil completes its trip around that tower, it already has the correct orientation for traveling its long linear path back to the other tower.

Another interesting twist to this variation would use a solid weight for toggling the airfoil center of mass instead of a liquid weight. In this case, the motion of the weight during inversion can be damped by adding two small holes that vent each end of the cylindrical cavity to the outside air. With this design, the weight moves slowly from one end of the cavity to the other because it must push air out through one ventilation hole and pull air in through the other. If it isn’t practical to vent the cavity without risking the intrusion of rainwater, then the holes at each end of the cavity may be connected with another very small diameter cylindrical cavity. Neither of the two cavities is vented to the outside air in this design. As the weight travels from one end of the large cylindrical cavity to the other, it must first transfer air from one side of the large cavity to the other by forcing it through the small diameter cavity.

Motion Damping Mechanism for Weight Inside Airfoil

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