Salient White Elephant

April 27, 2009

Blimp Supported Linear Turbine

Filed under: Airborne Wind Turbine, Linear Wind Turbine — Tags: , — Salient White Elephant @ 8:40 pm

Blimp Supported Linear Turbine

There’s not much detail in the diagram, and much has been omitted. But I’ve drawn all the mechanisms involved so many times on this blog that I don’t think I’ll draw them again. But let me explain how it works. Heavy cables connect the blimps to the ground and carry the large loads. The symmetrical airfoils travel from the ground to the blimp and then back to the ground again. These airfoils are supported at either ends by moving cables (not shown) that turn a bunch of pulley wheels. The pulley wheels are suspsended from the heavy cables. Power is transmitted from the airfoils through the pulley system to a couple of generators at ground level. Each rail car has one of these generators on top of it.

Alternatively, replace the airfoils in the diagram with relatively small diameter Darrieus or H rotors. Power is still transmitted to the ground mechanically with a pulley system.


April 21, 2009

Semi-Direct Drive Linear Turbine With Yawing Oblong Track

Ideal Path for Wind Turbine Blade

Doesn’t it seem like the ideal path for a wind turbine blade would have the entire blade moving at “tip speed”, spending the majority of its time traveling in a direction that is orthogonal to the direction of the wind?

Description of the Machine

Aerial View Semi-Direct Linear Turbine With Yawing Oblong Track Built Into Cornfield

(Note that the word “power cable” in the diagram below does not refer to an electrical cable, but rather to a cable that carries the mechanical power to the pulley wheels at the generators.)

Side View of Semi-Direct Drive Linear Turbine With Yawing Oblong Track

Looking at Leading Edge of Airfoil Semi-Direct Drive Linear Turbine With Yawing Oblong Track

Cross-Wind View of Tower Semi-Direct Drive Linear Turbine With Yawing Oblong Track

Bowed Airfoil Rides Padded Rims of Pulley Wheels Semi-Direct Drive Linear Turbine With Yawing Oblong Track

Aerial View of Apparatus That Routes Airfoil and Power Cable Round Tower Semi-Direct Drive Linear Turbine With Yawing Oblong Track

Drag on Moving Cable

How much aerodynamic drag does a moving cable add to the system compared to the drag of a strut or the low speed part of a troposkein Darrieus blade? Consider an H Rotor supported by a strut. The instantaneous velocity of a given radial location on the strut is at right angles to its longitudinal dimension. For this reason, it is able to leave a semi-evacuated space behind it as it moves. This space is filled with turbulent air. But the velocity of a given cylindrical cross-section of a cable is along its longitudinal dimension. Because there is always more cable following any cylindrical cross section, it is impossible for the cable to leave a semi-evacuated space in its wake. The only semi-evacuated, turbulated space it can leave is on its downwind side, and the force thus produced cannot drag down the system because it is orthogonal to the velocity of the cable. The only aerodynamic drag that is possible is that drag that is created by the cable’s ability to “stick to the air around it”. I’m not an aerodynamicist, but I’m guessing this might turn out to be pretty small. Also, the moving cable in the machine presented here has small diameter because it moves at “tip speed”, and because it is only required to carry the torque at tip speed. Because tip speed is high, torque will be low, and the cable will have small diameter.

Circular Variation

One obvious disadvantage of the oblong path is the need to yaw the track. We can eliminate this problem by providing the turbine with a polygonal path that approximates a circle:

Aerial View Showing Polygonal Path That Is Approximately Circular

Airfoils Could be Slowed to Go Around Tower

To avoid inflicting excessive centrifugal force on the blades as they go around the tower, maybe the control system could sense the point at which blades are nearing towers, and slow them down somewhat as they make their way around the tower. Alternatively, mechanical support could be provided on the opposite side of the airfoil to prevent excessive bowing due to centrifugal force:

Wheels Support Airfoil as it Rounds Tower

Low Drag Airfoil Variation

The airfoils need to have gaps near the regions where the load carrying cables penetrate them. This is necessary to accomodate the mechanical supports for the apparatus that steers the airfoils around the tower. But if the airfoils carry a small generator, like the generator that rubs against a bicycle tire in order to power a light for the bicycle, then these gaps could be filled with streamlined shapes during the long path between the two towers. The streamlined shapes would be pulled out of the way as the airfoil makes its way around a tower. The actuating mechanism is powered by electricity that is generated from the small generator just described that is located inside the blades.

Alternatively, maybe the streamlining shapes could be pushed out of the way by some mechanical means as the airfoil goes around a tower.

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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March 27, 2009

Direct Drive Linear Turbine With Yawing Oblong Track

Direct Drive Linear Turbine With Yawing Oblong Track

Direct Drive Linear Turbine With Yawing Oblong Track, Close Up View of Blade to Blade Guide Interface

Although the direct drive apparatus isn’t shown in the diagrams, a written explanation should suffice. Permanent magnets are attached to the blade supporting structure that is at each end of the airfoils. The airfoils drive these magnets at a speed that is analogous to the “tip speed” of a more traditional wind turbine. Generator windings are built into the blade guides (aqua colored components in diagram above). Instead of embedding windings into the entire length of the blade guides, windings are separated somewhat. Blades speed up slightly while traversing the distance between windings.

Direct Drive Linear Turbine With Yawing Oblong Track, Aerial View

The blade inverting sections of the track permit the use of asymmetrical, pitched airfoils. (Asymmetrical, pitched airfoils deliver better aerodynamic performance than zero-pitch, symmetrical airfoils. For an explanation of the “helical airfoil inverting section” of the track, see my earlier post: 20 Megawatt Direct Drive Darrieus.)

I wonder if the blade guides can be flexible? If so, the flexibility may provide a number of benefits. For one thing a flexible guide might be cheaper, lighter in weight, and easier to build. For another, it may help to absorb abruptly changing loads due to (for example) wind gusts. It might be interesting to explore the possibility of a blade guide which is flexible enough to resemble, to a degree, a hanging cable, and yet which is still rigid enough to accurately maintain the tight mechanical tolerances that would be required for the direct drive generator components contained within it:

Can Blade Guides Be Flexible?

Two Hanging Cables Variation

Airfoil Suspended Between Two Cables (Drive System Omitted)

(If you haven’t yet read about why you might want to invert the blade, click here.)

Airfoil Suspended Between Two Cables

Close Up of Drive System for Airfoil Suspended Between Two Cables

It might be interesting to explore how the Eye of the Cat Rotor Blades would perform when hanging from two cables. Although there’s no centrifugal forces to balance, remember that the Cat’s Eye Rotor also balances aerodynamic forces. In this case, think about what happens if the blades are longer than the separation distance of the cables (including slack… so that they tend to keep the cables pushed apart).

Single Hanging Cable Variation

In this variation, sensors detect when two airfoils get too close to each other. In this case, the direct drive generator coils for the leading airfoil are switched off until it reaches an acceptable distance from the airfoil immediately behind it.

View Looking Down Structural Cable Cable Hanging Between Towers

View Looking Down Structural Cable, Moving and Stationary Parts, Cable Hanging Between Towers

View Looking Down Structural Cable, Big Picture, Cable Hanging Between Towers

Downwind View, Airfoil Entering Helical Blade Inverter

Aerial View, Airfoil Entering Helical Blade Inverter

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