Salient White Elephant

April 23, 2009

Extremely Tall Direct Drive Wind Turbine

No Wind, Extremely Tall Direct Drive Wind Turbine

Aerial View, Extremely Tall Direct Drive Wind Turbine

Wind Blowing, Turbine Producing Power, Extremely Tall Direct Drive Wind Turbine

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Wind Farm Hydro Drive

Filed under: Wind Turbine Drive — Tags: , — Salient White Elephant @ 10:47 pm

The gearbox and generator are eliminated with this hydro drive. Instead of electric cables carrying power, water pipes carry power from each turbine to a central collection point. The water pressure at the central collection point is converted to electricity with a water turbine. An air tank buffers pressure changes.

Wind Farm Hydro Drive

Wind Farm Hydro Drive Driving Generator

I guess this idea needs a means of regulating pressure on a per turbine basis. That way if the wind speed is lower for one turbine than for all the others, the low wind speed turbine will still be able to actuate the hydro-drive. On the other hand, I guess this could be solved by providing each turbine with a variable speed transmission. Actually, this would be pretty easy to do if a crankshaft is used to drive the water pump piston. All you need to do is vary the radius of the off-axis part of the crankshaft.

Reciprocating Electric Generator Variation

This variation eliminates the water turbine at the central station where water pressure is converted to electricity. The water turbine is replaced with a reciprocating piston exactly like the kind that the turbines use to raise water pressure in the first place. So you just reverse the process, and the reciprocating piston drives the electric generator.

Airborne Wind Dam

A blimp suspends a giant flow accelerator with a small high-power turbine in the middle of it. In other words, a blimp suspends a turbine that looks kind of like the one described in this earlier post:

Wind Turbine with Flow Accelerating Shroud

This rotor will spin at high rpm, so it is easy to make it a direct drive machine. As described in prior posts, the bottom of the flow accelerator can attach to the top of a supporting tube in order that the blimp doesn’t have to carry the entire gravity load of the shroud and turbine. The tube is on wheels and a control system causes it to follow the blimp around with changing wind speed and direction.

Here’s another variation:

Airborne Wind Dam

Airborne Wind Dam with Lattice Support

Blimp With a Hole Variation

In this variation, the blimp has a cylindrical hole right through the middle of it (running down its longitudinal dimension). The small high-speed direct drive turbine rotor is inside this cylinder. A large shroud like the ones depicted above encircles the blimp in order to further accelerate flow through the cylinder and direct drive turbine rotor.

Airborne Wind Dam, Cylindrical Hole in Blimp

Skyscraper Variation

In this variation, the blimp is moored to the top of a skyscraper. When the windspeed gets too high the blimp simply detaches from the building and flies to a nearby airport where it lands until the storm passes. I hate to make the next suggestion, because somebody might actually do it. The blimp could be moored to the top of a mountain. This would produce a lot of visual pollution, so I don’t think it’s a very good idea.

Lightweight Electrical Components Variation

The blimp is big and round. It has plenty of room for a ring generator. Of course, you don’t need a ring generator to make this machine direct drive, because the accelerated flow through the cylinder is already sufficient for making the turbine rotor spin at high rpm. But if the turbine drives a very large diameter ring generator, then electricity can be generated with high voltage. If voltage is high, then current is low, and the weight of electrical components such as the electrical cables is minimized. (I’m assuming insulation weighs less than copper.)

Structural Electrical Cable Variation

This option develops electrical cable technology that is suitable both for conducting electrical power and for carrying a tensile mechanical load. This minimizes the weight of the mooring cable, since the electrical cable simultaneously provides both mechanical and electrical functions.

Aerodynamic Transmission Variation

The diagram below is a little ridiculous, but I’m a terrible artist, and I’m using 2D software to create these diagrams, so for this diagram I decided to come up with something that just shows the general idea. And the general idea is to reduce the weight of airborne components by using a light-weight hollow tube to moor the blimps. The hollow tube transmits the high air pressure that accumulates at the center of the dam (shroud) to the ground. A small high-speed turbine rotor drives a generator at the ground level end of the tube. The turbine rotor is high speed, and so it doesn’t need a gearbox. The airborne system carries no electrical or mechanical devices, and so it is light in weight.

Airborne Wind Dam With Aerodynamic Transmission

This idea suggests an interesting question – what happens to the Betz Limit when de-energized air doesn’t flow away from the turbine on the downwind side of the turbine “rotor”?

Of course, the aerodynamic transmission may also be applied to more convention turbine designs. Perhaps a shroud is positioned at the top of a conventional wind turbine tower, and the tower itself is used to route the high pressure air to a turbine rotor and generator on the ground.

Pressure Differential Aerodynamic Transmission Variation

A wall is added to the inside of the aerodynamic transmission tube. This separates the tube into two halves, just as though there were two tubes instead of one. One half of the tube opens on the high pressure side of the shroud, and the other half opens on the low pressure side. This pressure differential is carried to the ground where one side of a high-speed turbine rotor encounters the high pressure, and the other side of the rotor encounters the low pressure. Air thus flows through the rotor and turns an electric generator.

Cross-Section of Aerodynamic Transmission Tube Showing Transmission of Pressure Differential

Rotating Drive Shaft Variation

Hollow, light-weight, rotating tubes are connected end-to-end through universal joints. The tubes are attached to the mooring cable and so are suspended beneath the mooring cable. A high-speed rotor at the top transmits power to the ground through the rotating drive shaft tubes. The spins at high rpm so drive shaft torque is low.

Reciprocating Blimps

Two blimps are moored to each end of a horizontal tube or lattice structure that is near the ground. The tube rotates about a vertical axis that goes through the center of its longitudinal dimension. Each blimp is equipped with a giant light-weight parachute that looks like an old World War II parachute. These parachutes can be opened up to create a high drag profile, or closed to create an extremely low drag profile. When one blimp opens up its parachute, the other closes its parachute. The blimps take turns opening their parachutes in order for the wind to drag alternately drag each in the downwind direction. The reciprocating action of the horizontal mooring tube drives a generator.

Alternatively, perhaps a large “venetian blind” structure hangs from each blimp. The slats in these blinds can be opened up to create near zero drag profile, or closed to create a high drag profile. This variation is modeled after the turbine depicted below:

Here’s an embodiment that uses venetian blind like slats:

One of a Pair of Reciprocating Blimps

Rotary Variation

This is the same idea, except that the horizontal tube rotates instead of reciprocating. As the end of the tube begins its 180 degree “downwind sweep”, the blimp that is moored to it closes its slats to create maximum drag, and the other blimp opens its slats to minimize drag.

Adding HAWT Rotors

Instead of using drag devices, a giant HAWT rotor can be attached to the tail of the blimp. The rotor has very long, very light weight blades that rely on centrifugal stiffening to maintain their extended shape. These minimize drag by feathering their blades, and maximize drag by pitching their blades to a position similar to a HAWT turbine that is producing power. The drag action is similar to the lifting action of an autogyro. In other words, the HAWT rotors are not present to generate power directly. They are only present to control the drag profile of the blimp. On the other hand, perhaps a means can be devised for allowing them to augment the power produced by the system. In order to keep the weight of airborne components to a minimum, they do not produce very much electricity, but they do add a little bit to the electricity that is produced on the ground.

Blimp VAWT

Since no people will ride in the blimp, I guess it could be filled with hydrogen.

Blimp VAWT

I’m always drawing 2 bladed VAWTs when I really mean 3. A three bladed VAWT produces relatively constant power, and more importantly in this case, thrust. Since wind speed will normally increase with altitude, blade radius increases with altitude so as to maintain a more constant tip speed ratio. For simplicity, I drew only 2 guy wires. In reality, at least three would be required.

The turbine’s rotational axis can tilt, and the gearbox and generator tilt along with the rotor axis. This allows the wind turbine to seek its optimum orientation given wind speed and other parameters. Alternatively, perhaps the lower end of the wind turbine (the generator) is on wheels and is able to move if the turbine wants to move. Since the structure will be very heavy, the control system would likely control the movement of the generator this way and that along the ground as wind conditions change. But if we let the lower end of the turbine move, then we may as well save some weight by using only one guy wire or lattice to moor the blimp. In this case, the turbine will “yaw” by roughly tracing out a circle about the guy wire anchor point as the wind direction sweeps through a 360 degree arc.

The teardrop streamlined shape of the blimp is critical. If any old shape is used for the floatation device, then the wind will tend to carry it downstream. I don’t know what the optimum aerodynamic profile would be, but you would want to minimize drag. For this reason, perhaps a very elongated shape (kind of like a cigar) might be used.

Kites and Airfoils

Additional lift may be generated if a kite is positioned on top of the blimp. Imagine, for example, those modern parachutes that look like a big airfoil made out of something like nylon. If one of these is mounted on a vertical pole that extends upward from the blimp, then it will “take flight” when the wind speed reaches a sufficient value. In this case it will be available to add further support for the turbine whenever the turbine is running and producing power. Some kind of inflatable airfoil might do the same trick. If a kite or modern parachute is used, it will be suspended by its supporting structure in a shape and orientation that “prepares it” for taking flight when the wind picks up.

Other Variations

I think there must be many ways to develop better configurations if more than one blimp is employed. For example, two blimps could be moored to a giant circular railroad track. The anchor points are “yawed” so that a line connecting them forms a right angle with wind direction. The blimps are connected by a cable, and multiple VAWT turbines are suspended from the cable that connects the two blimps.

Weight is obviously one of the primary issues with airborne turbines. Perhaps some sort of inflatable airfoils can be used. The airfoils may even themselves be filled with hydrogen. Or a design similar to those light-weight modern parachutes might be used for the blades.

April 21, 2009

VAWT Forest

Many tall slender VAWT rotors are optimally dispersed and mechanically linked to drive a single generator.

Savonius Forest

Walmart Rooftop VAWT Forest

A weather protection shroud (not shown above) runs along the same path as the chain and protects components from rain and other weather. It may also be desirable to explore the option of using a rotating drive shaft to transmit power from turbine to turbine, and finally to the generator. I won’t draw this option, because I’ve already drawn it for one of the variations below.

In the following embodiment, rotors form a wall along the four sides of the roof. A tarp that is supported by a lightweight metal frame runs along each of the four sides of the roof and beneath the rotors. The tarp minimizes turbulence.

Walmart Rooftop VAWT Wall

Variable Scale Deployment – Rotors Suspended From Tubes

This embodiment is suitable both for utility scale deployment, such as a large VAWT Forest suspended over a cornfield, as well as for small scale deployment, such as a rooftop turbine for a residence.

One Row of VAWT Forest Suspended From Tubes

In this variation, rotors are suspended from a bunch of tube segments. The tubes are connected by flexible joints that allow them to approximate the troposkein shape of a cable. Suspended VAWT rotors drive a chain that runs inside the tubes along their longitudinal dimension. A universal joint connects the turbine rotor shaft to a second shaft above the turbine rotor. The weight of the turbine rotor keeps its shaft approximately vertical. However, in high winds the lower end of the rotor shaft can swing in the downwind direction, thus regulating power output and thrust load on the supporting tubes. The yielding rotor shaft also provides a low drag profile storm wind shut down mechanism. The drive chain bears only a relatively low torque load, since tubes carry the gravity load, and since the diameter of the turbine rotor is relatively small (so that the rotor turns at relatively high rpm). Whatever the chain load, it acts to lift the row of turbines up against the force of gravity, and it also cancels some of the gravity load supported by the tubes. Only a single chain runs down one row of turbines because the return path for the chain is through the next higher level of turbines:

Path of Drive Chain, VAWT Rotors Suspended From Tubes

Rotors Suspended From Cables, Counter-Rotating Torque Tubes

VAWT Forest With Counter-Rotating Drive

VAWT Forest With Counter-Rotating Drive (Zoomed In View)

The universal joints not only allow for transferring mechanical power to a slightly different axis, they also permit the rotor to yield in response to wind gusts, and to assume a nearly horizontal orientation when the machine is shut down for storm winds. Note that if the wind direction is at a right angle to the cable, then the cable will twist in response to rotor thrust. This does not present a problem because the orientation of the torque tubes is fixed with respect to the cable, and so the torque tubes move along with the cable when it twists. The orientation of the top rotor shaft is also fixed with respect to the cable, so it rotates along with the torque tubes when the cable twists.

Reciprocating Drive

It is also possible to use a reciprocating drive for all of these machines. In this case, a tube is pushed and pulled by a crankshaft. Perhaps the simplest (and least expensive) option would have the crankshaft driving a cable. I will add some drawings of these options if I have more time later.

For more related discussion, see VAWT Wall.

HAWT Wall

Filed under: Horizontal Axis Wind Turbine (HAWT) — Tags: , — Salient White Elephant @ 2:50 pm

HAWT rotors are configured in various ways within a plane, and are mechanically linked to drive the same generator.

HAWT Wall

See VAWT Wall.

VAWT Wall

A great number of very tall and very slender VAWT rotors are arranged into a circular wall. All rotors are mechanically linked to a common drive mechanism (chain and sprocket?) so that all drive a single generator:

Savonius Wall

Alternatively, the wall is separated into (say) 30 degree arc segments. All of the rotors that are contained in a given 30 degree arc are mechanically linked to drive a single generator, but each arc drives a separate generator. This way the arcs that are largely parallel to wind velocity will not drag down the arcs that are largely orthogonal to wind velocity. (And those arcs that are largely parallel to wind velocity may still produce a small amount of power.)

Of course, the same idea may be applied to any type of tall slender vertical axis wind turbine.

Suppose 2 bladed H rotors or 2 bladed Darrieus rotors are used. In this case the rotational angle of each rotor may be phase shifted somewhat with respect to its neighbors, and the power output of the machine will be very smooth.

Of course, it isn’t necessary for the wall to be circular in shape. I’m guessing that a circular wall would produce the most energy and the highest capacity factor at sites that have completely unpredictable wind directions. But if a site has a somewhat predictable wind direction, then the wall can definately be shaped to take advantage of the prevailing wind direction. Also, the wall may assume a non-circular shape in order to make its way around some obstruction like a barn or something like that.

Note also that the smaller the rotor diameter, the higher the rotor rpm! This characteristic might allow for eliminating the gearbox (or at least for reducing the speed change required, and thereby increasing the efficiency of mechanical power transmission.)

Seems like this machine may turn out to be suitable for almost any application you can think of – from small to gigantic utility scale wind turbines! Imagine a wall of these rotors on top of a Walmart. The rotors could be small diameter, fairly tall, and made of plastic. Even if a rotor flies apart it shouldn’t create a hazard since its only light weight plastic. And it certainly seems like this idea would be just absolutely perfect for low cost, clean energy in the third world! Imagine a machine like this driving the low-tech water transport pump I posted earlier:

Dirt Cheap Ultra-Simple Efficient Third World Water Transport Pump

Muti-Speed Transmission

Yawing VAWT Wall

This machine doesn’t need a yaw drive, as its yaw angle is self regulating:

Downwind Yawing Savonius Wall

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.

Torque-Speed Decoupling HAWT

I don’t have near enough time to do justice to this idea. I’ll just post it in very abbreviated form and leave the rest to your imagination.

Torque-Speed Decoupling HAWT, Downwind View

Torque-Speed Decoupling HAWT, Side View

Torque-Speed Decoupling Mechanism

The chain-like structure that protrudes from the inner perimeter of the rim of the spoked wheel is essentially just a bunch of rollers whose axes are parallel to the rotor axis of rotation. The fact that these engaging mechanisms are rollers (rather than just fixed pegs) permits very efficient transfer of mechanical power from the rotor blade to the chain drive.

Variations

The high speed generator shaft can be attached to a second smaller (aerodynamic) rotor. This permits more efficient conversion of energy flowing through the axial region of the rotor disk, and eliminates the big fat twisted small-radius section of the larger rotor blade.

The chain drive may be replaced with a high-speed rotating shaft whose axis of rotation is coaxial with the longitudinal dimension of the blade (coaxial with the blade spar). The rotating shaft goes down the center of the blade just like the chain drive. A tire at the blade tip engages a ring that looks like a giant washer and that protrudes from the inside of the rim of the spoked wheel. The tire drives the high-speed shaft that goes through the center of the blade. Assuming a 2 bladed turbine, the two shafts will be counterrotating. One of these shafts drives the generator rotor and the other drives the “stator” in the opposite direction. In this design, the generator rotates with the blades and slip rings will be required to transmitt power from the rotating generator.

Yet another variation has the chain drive or high-speed rotating shaft running longitudinally inside the spar of an H-Rotor Darrieus. In this embodiment, either the generator can rotate with the blades and transmitt its power out through slip rings, or else another sprocket near the tower protrudes from the spar and drives a rotating ring which in turn drives a generator that is fixed with respect to the turbine foundation.

April 18, 2009

Pseudo-Lifting Fractal Turbine

When I first started learning about wind turbines, I wondered why a wind turbine with 2 or 3 blades was described as “aerodynamic”, or “highly efficient”.

Sleek, Highly Efficient, Modern HAWT
So much of the swept area seems unaffected by the blades. Surely adding more blades would allow for harvesting the energy in the wind that seems to be “escaping” through the large regions that are between the blades in the diagram above. This issue is discussed with the aid of the word “solidity”, which refers to the ratio of the area occupied by the blades divided by the total swept area. Surely the higher the solidity the better… no?

High solidity is a desirable feature with turbines that do not employ lifting surfaces. But a turbine that uses airfoils, like the one in the diagram above, is perfectly able to influence air “from a distance”. Imagine a long straight line of people dancing “the train”. Each has her hands on the shoulders of the person immediately before her. You’re standing there beside the line, and just as the last person goes by, you jump in line behind her. Then you put your hands on her shoulders and start dancing with the rest of the people in the train, but you kind of “drag your feet”. That is, you move more slowly than the rest of the train. Because the train is outrunning you, your arms will have to stretch out forward. And since you’re pulling back on the shoulders of the girl in front of you, she will have to slow down too. Her arms will have to stretch out forward, and so she slows down the guy in front of her, and so on. In this way, you cause the whole train to slow down, though you’ve only touched the girl immediately in front of you.

I think this is exactly how a lifting wind turbine manages to draw kinetic energy from air that has already “escaped” through the empty spaces between the blades:

Airfoil Exerting Influence From A DistanceYou may have heard that wind turbines with lifting devices are far more efficient than those that rely on drag forces alone. I can think of two ways of looking at this. First, the lifting force created by suction on the low pressure side of the airfoil is much larger than the lifting force created by ram air pressure on the other side. I read once that the suction force is 5 times greater than the ram air force. Second, most drag devices create turbulence. Why is this bad? Imagine the long line of air molecules “dancing the train” past the high-speed airfoil rotor. Their momentum is in the downstream direction, and their motion is very orderly. It’s hard for a molecule to make its way back toward the rotor because everybody is moving downstream. It’s kind of like trying to drive slow on a Los Angeles freeway when everybody else is barreling by at 90 mph. But turbulence is disordered motion. Imagine a crowd of people in a riot. Everybody’s running this way and that, banging into each other and so on. Now imagine the air molecules passing a turbine rotor in this chaotic fashion. Not only is it easier to make your way back to the rotor, but in fact somebody may bang into you and push you back toward the rotor. For this reason, the rotor cannot maintain low pressure on its downwind side. This eliminates the suction force that the rotor might otherwise generate, and therefore there is far less force to torque the rotor against the generator and generate electricity. Of course, we didn’t need to go through all this analysis to realize that the drag rotor turns less of the wind’s kinetic energy into electricity. All we had to do is consider all the chaotic energy in the “wind riot” that is left in the wake of the rotor. All of this chaotic energy is energy that was not converted into electricity, and it therefore represents an inefficiency in the system. We’d like to design a rotor that slows all of the air gently and evenly, leaving the least amount of disturbance in the wake.

Now let’s talk about thunder – what is it? When lightning pierces a region of air, it heats the air in the region tremendously. But lightning is only present for a very short time, and then it disappears. The air that was in the path of the lightning bolt is very hot, meaning it has expanded. But immediately after the lightning bolt is gone, this heat is dissipated into the surrounding air. Now the pressure of the air that was left in the wake of the lightning is very low because it is still expanded. So the surrounding air collapses into this evacuated space with a tremendous slap and a deafening roar. How does this relate to the issue of solidity and turbine rotors exerting influence from a distance? The fewer blades a turbine has, the lower the solidity. In order for a 2 bladed turbine to exert as much aerodynamic influence as a 3 bladed turbine, it must spin faster. But if it can exert influence from a distance, why does it need to spin faster? Because it can only exert influence from a limited distance. Unlike the human train, each molecule in the wind train exerts just a little less backward force on the molecule that is next in line downstream, until eventually molecules exert no backward force at all on their downstream neighbors. This makes sense. If turbines could draw kinetic energy from wind that is 100 miles downstream, we’d be powering entire nations with just a handful of wind turbines. A 2 bladed turbine spins faster than a 3 bladed machine so that the wind that passes through the “empty regions” between the rotor blades doesn’t have time to escape the region of aerodynamic influence before the rotor has extracted its energy. (Although not relevant to the Pseudo-Lifting Fractal Turbine, I may as well go ahead and explain why a turbine cannot extract all of the kinetic energy from the wind that passes through its rotor. If a rotor took all of the energy from the wind, then the wind would not be moving at all, and we’d be left with giant piles of air molecules lying around the turbine! The good news is that this renewable energy waste is not radioactive; the bad news is that we’d have to hire a bunch of truck drivers to haul it off to the official government renewable energy waste dump. If you want to learn more about this phenomenon, look up Betz’ law in Wikipedia.)

You would think the rotor characteristics just discussed would lead us to select a 2 bladed rotor design in order to cut the cost of one blade from the cost of the turbine. Unfortunately, things are not that simple. A 2 bladed turbine makes more noise than a 3 bladed turbine. This is because a high speed 2 bladed rotor blade pierces the wind like a lightning bolt. Yes, the evacuated region on the downwind side of the airfoil is able to draw kinetic energy from wind that is already considerably downstream of the rotor, but the evacuated region that realizes this action subsequently collapses just like the collapsing air in the wake of a lightning bolt. The first large wind turbine I ever worked on was the 2 bladed Carter 300 kW machine. One fascinating aspect of this machine was that it was able to extract energy from wind traveling as fast as 40 miles per hour! Standing beneath this turbine in a 40 mph wind was like standing beneath a heavy military helicopter – VOP!!!… VOP!!!… VOP!!!… VOP!!!… VOP!!!… Although it belonged to the “featherweight class” of turbines, it produced the electricity of an expensive heavyweight champion. But a machine like this pays the penalty of noise. Such a turbine can’t be sited near places that have people – residential areas and so forth. And its high-speed airfoils are difficult for birds to see, so this further limits siting options.

Now that modern wind turbines have reached a scale of several megawatts, you can imagine the difficulty of designing a robust blade that’s 40 yards long. A wind gust hits a region of one blade at a point (say) 75% down its longitudinal dimension (near the blade tip), and not only must the 40 yard long slender blade dissipate the huge lifting force generated by this gust, but in fact that mechanical shock ripples throughout the entire slender turbine structure. It’s a little like trying to design a giant “stick man” using nothing but drinking straws, and requiring that he cannot be knocked down by even 100 mile per hour storm winds.

The Pseudo-Lifting Fractal Turbine

The Pseudo-Lifting Fractal Turbine proposes to solve many of these issues. It moves very slowly, and I think it should be extremely quiet. It won’t kill birds. It poses little danger in the case of an accident. (I can’t think of a way for this turbine to “run away” and fly apart like an unloaded traditional rotor.) It can’t impress an admiring green public with lightning flash signs of power, but it will just absolutely dazzle mechanical engineers with the simplicity of its scalable mechanical design characteristics. I am hypothesizing that this turbine delivers the efficient aerodynamic performance of a high-speed modern rotor, but with the low velocity of a drag rotor. I’ve prefaced its description with a long theoretical discussion because I developed this machine from that theory.

The turbine design begins by finding the “perfect” drag shape:

The Perfect Drag Shape

Now we fractalize this shape one iteration:

Fractalized Drag Shape
Now we approximate the small arcs with clamshell airfoils that can open up into the fractal shape, and that can close up into a shape that has a low drag profile:

Clammified Airfoils Approximate Ideal Drag Profile
(I’ve drawn the clamshells packed together like sardines. Not sure why I did this. If the rotor truly generates aerodynamic suction, then the long arguments presented earlier would seem to indicate that it isn’t desirable to have 100% solidity. But while thinking about this topic, I want to point out something interesting about the open clamshells above. Doesn’t it look like the ram air pressure would be higher than it would be were it only hitting a single angled airfoil!? And I wonder if the low pressure might also be augmented somewhat, since low pressure air can’t reach the high pressure side of the airfoil by zipping around its trailing edge.)

Now we use two clamshell airfoil fractals to make a reciprocating rotor. (The following diagram is zoomed out to the extent that you can’t see the smaller arcs… only the large ones.)

Pseudo-Lifting Fractal Turbine
When the clamshells on one side close, the clamshells on the other side open, and vice versa. I am hypothesizing that if this rotor is carefully designed to avoid the generation of turbulence, and if the rate at which the clamshell airfoils open and close is just right, then this turbine should use aerodynamic suction to siphon kinetic energy from the wind in a manner that is very similar to the action of a high-speed modern turbine rotor.

How is the machine yawed? Both the tower and the tube at the top of the tower reciprocate as the machine produces power. (The ring and the generator are at ground level.) But the tower-top tube is also able to rotate with respect to the tower. Its motion is fixed to the motion of the tower whenever a brake that connects the two is activated. When the controller detects a change in wind direction, it simply releases this brake at the appropriate time and for the appropriate duration to cause the tower-top tube to rotate with respect to the tower to the correct yaw angle.

The turbine is shut down during high winds by closing all clamshells and releasing the brake that controls the yawing action.

Variations

I guess there are a lot of variations on the ideas presented here. Here’s an example:

Reciprocating Savonius-Like Turbine

Earlier I asked whether the high pressure and low pressure might be augmented by the open clamshell airfoil shape. Here’s a diagram that examines this issue in different way:

Clamshell Airfoil

And if this works, then why not something along the lines of the following?

Staggered Clamshell Airfoil

Maybe if the staggered clamshell airfoil produces lift, then maybe it would additionally provide the advantage of mixing air of different energy levels. If so, it might eliminate the “dead air” that is present with some airfoil designs. Another interesting question is what kind of vortex activity might be generated by this device? On the one hand, I can see how it might make zillions of vortices, but on the other hand I wonder if they would mostly cancel each other?

I wonder if anybody has ever tried to think of ways to use fractal shapes to make a wind turbine. Seems like there might be some amazing new possibilities there doesn’t it?

April 17, 2009

Lifting Savonius

Lifting Savonius

April 15, 2009

20 Megawatt VAWT

This is a 3 or 6 bladed machine, but I’ve drawn only 2 blades (and 2 towers) in the diagrams to make them easier to understand.

20 Megawatt VAWT

Aerial View Showing How Circular Shape of Stationary Ring is Maintained With Cables Configured Like Spokes

20 Megawatt VAWT Drive System Detail

Aerial View of One Possible Drive System

The 20 Megawatt VAWT Exceeds the Betz Limit

The rule of thumb for separating horizontal axis turbines is that two turbines should not be closer than two or three rotor diameters. Apparently, after the streamtube has traveled two or three rotor diameters beyond the turbine, it will have re-absorbed kinetic energy from the surrounding wind to the extent that its energy density will have roughly returned to its original value. It’s easy to see how this will apply to a very large diameter VAWT. Simply imagine a ridiculously large turbine. Say for example the diameter of the VAWT described here is equal to the diameter of New York City. In this case it is obvious that the wind that passes through the downstream 180 degree arc will have just as much energy as the wind that passes through the upstream 180 degree arc. From this example we can see that the maximum fraction of energy that a very large diameter VAWT may extract is 2 x 60% = 120%. Darrieus rotors have a reputation for delivering lower efficiency than their horizontal axis cousins. However, given the doubling of the ceiling on energy extraction, it is reasonable to believe that a very large diameter VAWT may prove to be quite efficient. This is wonderful news, since one frequently cited reason for the gradual disappearance of the Darrieus from the windfarm landscape is that it has the larger weight per power ratio. The extraordinary scalability of the VAWT described here together with its ability to exceed the Betz limit may very well reverse this state of affairs.

Though I believe that a very large machine is possible with the configuration presented here, the design may also be useful for smaller machines. In this case, it seems like the outside towers might be eliminated:

Less Than 20 Megawatt VAWT

Another Variation

20 Megawatt VAWT

Aerial View Showing How Circular Shape of Stationary Ring is Maintained With Cables Configured Like Spokes

20 Megawatt VAWT Supporting Structure Detail

20 Megawatt VAWT Drive System Detail

Torque Tube Engaging Tires That Drive Generators

Another Variation

20 Megawatt VAWT (Zoomed Out) (Legend)

20 Megawatt VAWT (Zoomed Out)

Close Up Cross-Section of Airfoil Engaging Stationary Ring

Aerial View Showing How Circular Shape of Stationary Ring is Maintained With Cables Configured Like Spokes

In the interest of clarity, the diagram has a number of simplifications. For example, at least three towers are required, but more than three towers may be used if needed. Why would you need more than three towers? Because the diameter of this machine can be made very very large.

Aerial View of One Possible Drive System

One design alternative would have the generators on the ground. In this case, the central tower is replaced by the torque tube, and the torque tube extends to the ground where it engages the tire that drives the generator. Another alternative would replace the cables in the drive system with symmetrical airfoils. Why use airfoils instead of tubes? To minimize drag. Why airfoils instead of cables? Look again at the diagram that shows the whole machine:

20 Megawatt VAWT (Zoomed Out)

If the (vertical) airfoils are tall, the cable might make them bow too much or might put too much vertical load on the rings. Using a streamlined airfoil to transfer torque to the torque tube would eliminate these problems.

Single Airfoil Support Variation

20 Megawatt VAWT (Blade Supported in Middle, Zoomed Out)

20 Megawatt VAWT (Blade Supported in Middle, Drive Mechanism)

20 Megawatt VAWT (Blade Supported in Middle, Drive Mechanism with Symmetrical Airfoil Strut)

This post shows improvements on ideas from earlier posts. I won’t duplicate earlier discussions here, but I’ve pasted links below in case you want to read more. One very important point that is explained in earlier posts is that the wind machine proposed here can exceed the Betz limit. In fact, this machine can extract 120% of the kinetic energy flowing through the swept area instead of just 60%.

April 13, 2009

Wind Turbine Stethoscope

Small inexpensive microphones are embedded into the wind turbine tower, blade spar, gearbox, generator, and various other places. Engineers and technicians can download short audio samples (30 seconds?) via the SCADA system in order to listen for abnormalities. The turbine controller occasionally takes samples of the various audio signals, and performs a spectral analysis on the data. The resulting spectra may be compared against several baseline spectra. Possibilities for baseline spectra include:

  • An average of many spectra taken from healthy turbines,
  • An average of many spectra taken from turbines having approximately the same total lifetime number of operating hours as this turbine,
  • the spectra recorded and stored during this turbine’s first day of operation,
  • the spectra recorded and stored after this turbine has been “broken in” (after 30 days of operation?),
  • the spectra of this turbine that was recorded and stored yesterday.

If the controller determines that the current spectrum is significantly different from the baseline spectra, it sends an email to the service department warning of a possible problem.

Microphones may also record aerodynamic noise of the blades. Small, inexpensive cameras (like “web cams”) provide visual feedback on the condition of the surface of turbine blades. These cameras are mounted at the blade root, and look longitudinally down the blade. In addition to damage, accumulation of smashed bugs and other foreign matter may be observed.

I got these ideas from troubleshooting turbines in the field. It always amazed me at how much you could learn about a wind turbine simply by listening to it. Usually I’d just walk around the wind park listening, but sometimes I’d put my head against the tower in order to hear the highly amplified intimate details of the turbine’s inner life. I don’t know why I was surprised by how revealing this audio information was… after all, doctor’s diagnose many illnesses with a stethoscope, so why not turbines? As a matter of fact, this idea might be applied to a wide variety of machinery, from bulldozers to airplanes.

For some strange reason, engineering has long suffered under a trend of making everything “idiot-proof”. You don’t troubleshoot an electronic control system anymore, you read an error code off of a display that tells you which circuit board isn’t working right. Then you replace the bad board with a good one, and send the bad one back to headquarters. This is simply a waste of resources. Sure, if you can speed up troubleshooting in the field, that’s a good thing. But most technicians who do this kind of work have skills that are not being leveraged (and certainly not being developed) by the “idiot-proof” ideology. Some of the technicians I’ve worked with in the wind industry were quite talented. Why waste a resource like that? A better approach is to find the right balance, where on the one hand you wouldn’t be soldering transistors onto a circuit board at the wind park, but you also wouldn’t pay good money for an over-engineered solution when field technicians are perfectly capable of performing a certain amount of on-the-spot troubleshooting. The irony of the idiot-proof ideology is that people who have all of the challenge siphoned out of their jobs eventually lose even the skills they started with.

The problem with idiot-proof technology is that it creates idiots.

A person can acquire an amazing amount of knowledge through troubleshooting, and thus becomes more valuable to the company every day simply by showing up for work. And the feedback and input of an experienced technician like this can greatly improve the design of the next generation wind machine (assuming it is in some way derived from the current generation machine).

Think for a moment of how much you know about your own body on the basis of sound and feeling. For example, when I go to the gym and get on the stairmaster, I know that my right knee will always “tick” when it passes through a certain angle. It always does this, and it’s always at the same angle. Is this normal? No way! It certainly indicates some kind of a problem, even if it’s only an insignificant one. But this example illustrates the need to record audio that is outside of the audibal spectrum, especially the sub-audio. Sub-audio is important because it includes what you might call “vibration”. You can’t hear vibration, but it is obviously very important. I would classify the tick in my right knee as vibration – it is something I feel, not something I can hear.

High Altitude Guyless Darrieus

High Altitude Guyless Darrieus
The key to this idea is that the torque tube encircles the top of a tubular tower (like you’d expect to find on a modern multi-megawatt horizontal axis wind turbine). Aerodynamic forces are approximately balanced with respect to the middle of the torque tube (that is, halfway down its longitudinal dimension). And the middle of the torque tube is the part that engages the bearing at the top of the tower. So aerodynamic forces inflict only a small overturning moment on the torque tube, and this overturning moment can easily be carried by some kind of wheel or bearing at the bottom of the torque tube that also engages the outside of the tower. The gearbox and generator are situated at the top of the tower just like they would be in a horizontal axis machine. Alternatively, the center part of the torque tube (barely visible in the above diagram) can extend all the way down to ground level where it connects to the gearbox and generator, all of which live in the comfortable surroundings of the inside the bottom of the tower.

I favor three bladed VAWTs, but I usually just draw two blades because it makes the diagram easier to understand.

The rule of thumb for siting horizontal axis wind turbines is that turbines should not be closer than two or three rotor diameters. I assume this means that by the time the streamtube has traveled two or three rotor diameters past the rotor disk, it has been re-energized by the surrounding wind. For this reason, I keep wondering what would happen if the diameter of a Darrieus rotor were three times larger than its height. Wouldn’t this mean that the maximum amount of energy that the machine could extract from the wind would be not 60%, but 120%? (60% for the upwind arc plus 60% for the downwind arc.) If so, wouldn’t this mean that the efficiency of a large diameter Darrieus might rival or even surpass the efficiency of a horizontal axis machine? In this case, we can build very robust arms that extend from the torque tube, two arms for each blade, and make the diameter very large. The parts of the arms near the axis of rotation can be large, very stiff, and very robust because the velocity is low near the hub, so we don’t need to worry about the arms generating too much drag.

April 11, 2009

Reciprocating System for Transferring Wind Turbine Power Down the Tower

Reciprocating System for Transferring Wind Turbine Power Down The Tower

April 9, 2009

Magnus Effect VAWT

Magnus Effect Turbine With Yawing Oblong Track (Aerial View)

Magnus Effect Turbine Spinning Cylinder in Blade GuideBoth the lifting force generated by the spinning cylinder and the drag force on the stationary cylinder push the wheel against the downwind side of the blade guide. For this reason, the cylinder will always spin with the correct polarity for propelling the cylinder along the blade guide, regardless of whether it traverses the track in the clockwise or counterclockwise direction. It would seem like this kind of turbine might be pretty good at starting itself. But how is it prevented from starting up in the wrong direction? Maybe the wheels could be ratcheted so that they can turn in one direction without the cylinder spinning. In this case, it would seem that the blade would eventually experience a perturbation that would tend to start it spinning with the power-producing polarity, and if so then this machine is self starting. Rotating tires may be used for reversing the spin polarity as the cylinder rounds one end of the track:

Spinning Cylinder Passes Between Rotating Tires To Reverse Spin PolarityWriting this post has clarified one aspect of the Magnus Effect Rotor – if the rotor diameter is large then the time it takes to reverse the spin polarity of the cylinders may perhaps be insignificant.

Although the aerial view above shows an oblong blade guide (oblong track), a single blade guide could be used as well. A single track is possible if the turbine has only one blade (one spinning cylinder) and in this case the cylinder would stop when it gets to one end of the guide, and then travel in the opposite direction back to the other end of the blade guide. If it is not desirable to yaw the blade guide, then a giant circular guide may be used instead. Note that regardless of which blade guide configuration is selected, the cylinder will always spin with the correct polarity, regardless of whether it travels around the track in the clockwise or counterclockwise direction.

Many posts on the Salient White Elephant (such as this one or this one) describe ideas for generating electricity with turbine configurations that are similar to the Magnus Effect VAWT, so I won’t duplicate the descriptions of those ideas here.

A Magnus Effect Rotor that is similar to an H-Rotor may also be designed using the ideas presented here. In the case of an H-Rotor, the spin polarity for the spinning cylinders can be reversed using a mechanism similar to that described above. That is, the cylinders always push downwind, and this causes them to engage one spinning mechanism during the upwind arc of the rotor, and the other spinning mechanism during the downwind arc of the rotor. However, in the case of the H-Rotor, it isn’t necessary to synchronize or gear the cylinders to the rotation of the rotor itself, as their aerodynamic forces are sufficient for generating the rotor moment. In this case the spinning cylinders can be controlled by an electric motor. This makes it easy to reverse the spin polarity slowly and at the proper rotational angle of the rotor. In this application, the main lesson we learn from the Magnus VAWT described here is that the rotor diameter should be very large so that the time taken to reverse the spin polarity is insignificant compared to the time it takes the rotor to complete one revolution.

Since no utility scale wind turbines that use the Magnus Effect have been built (not to my knowledge anyway), I have to assume there’s some big disadvantage to this approach. But before closing, let me point out one big advantage of this type of rotor – cylinders are a lot stiffer than blades, especially given weight. From this perspective, I wonder why a Magnus Effect rotor similar to the turbine below wouldn’t be effective?

Airfoils for Very Large Darrieus Wind TurbineVery Large Darrieus Wind Turbine

April 3, 2009

Closed Loop Direct Drive for Highly Scalable Horizontal Axis Wind Turbine

Closed Loop Direct Drive for Highly Scalable Horizontal Axis Wind Turbine

This drive system has sensors in the turbine blade that measure how much each blade is bent by lift and other forces. The shrouds that house direct drive generator windings are moved in the upwind or downwind direction in order to properly position them for the approaching turbine blade. What kinds of sensors might provide the proper information to the closed loop control system that adjusts the position of the generator windings? I don’t know. Strain gauges embedded into the blade spars that communicate with the controller via a radio link? A cylindrical cavity inside the blades that runs in the longitudinal direction and that has a laser at one end and a mirror at the other?

Another idea would give the turbine blade a permanent magnet that can protrude (longitudinally) from the blade tip. With this design, the shroud that houses the generator windings move to a location that is completely outside of the rotor disk (i.e. the swept area). As a blade approaches the windings, its magnet is pushed out of the blade tip. Once it passes the generator windings, the magnet is retracted so that it doesn’t create drag during the rest of the blade’s trip around the rotor disk.

Well it’s a couple of days after I wrote this, and re-reading it, it seems like a pretty stupid idea. I guess I just keep thinking about how once the turbines starting getting really big (1 megawatt or so), then the really sexy power electronics came onto the scene. Making this blog has proven to me that wind turbine innovation is likely to come from integrating other already well-known technologies, like power electronics. They didn’t invent power electronics for wind turbines, but once the machines got big enough to justify the cost of integrating these well understood devices and circuits, the addition of them provided much improved performance, and probably cut the cost of energy as well. So I’m just thinking that closed loop control technology might be one of the next things. Like for example, suppose it’s difficult to manufacture a giant ring generator and still maintain the tight tolerances required for good performance (given temperature changes, etc). Then maybe strain gauges of other sensors could be added, and a closed loop control system could adjust the radius of the rotor or stator at various angular positions… kind of like the way you turn the little nut-like things on a bicycle wheel to make it perfectly round and so that it won’t wobble side to side. So instead of trying to make the perfect ring generator, you just let the control system continually adjust the various tolerances based on sensor readings taken every minute or so. One thing’s for sure… pretty much every wind turbine configuration that can be thought of has already been thought of over all these centuries. So I wish I was in a position to look at how other technologies might be integrated in order to lower the cost of wind electricity… but I’m just not in a position like that. (Another thought that comes to mind is self-health checks… like they have in avionics systems… they have raised the detection of equipment failure or underperformance or abnormal performance to a high art form.)

Magnets In Rotating Shroud

Magnets In Rotating Shroud
This design is applied to the Highly Scalable Horizontal Axis Wind Turbine. I would put the rotating shroud on the other side of the nacelle, away from the blades, but that would require the mechanical power to go through the shaft, and I was trying to avoid that. I wonder if you could just have a rotating shroud with no magnets in it, and wedge its trailing edge between two tires and directly drive an off the shelf generator?

Rotating Shroud Drives Tires

April 2, 2009

Transmitting Mechanical Power Down the Tower

Filed under: Horizontal Axis Wind Turbine (HAWT) — Tags: , , , , — Salient White Elephant @ 4:05 pm

I always wondered why nobody ever tried to put a HAWT gearbox and generator on the ground rather than at the top of the tower. I always just assumed that if I were a mechanical engineer, then I’d know that the numbers just don’t work. But then it occurred to me that this is exactly how the Darrieus rotor’s power gets to its gearbox and generator. Of course, only half of the torque comes from the top of the tower, but still… it got me to wondering if it might somehow be feasible to put the HAWT gearbox and generator on the ground. And my elementary school teachers always assured me that there’s no such thing as a stupid question, so…

(bear in mind that these same teachers would always ask me if I wouldn’t like to spend a month of my summer vacation coming in for English grammar tutoring so I could get a hundred points of extra-credit…)

Transmitting Mechanical Power Down the 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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