Salient White Elephant

April 23, 2009

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.

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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 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 9, 2009

Helium Magnus Effect Turbine

Helium Magnus Effect Turbine

The balloon has an electric motor and flywheel inside of it. When the motor spins the flywheel, Newton’s Third Law (for every action there is an equal and opposite reaction) causes the balloon to spin in the opposite direction. Alternatively, a motor on the ground could spin the cable first in one direction, and then in the other. Interestingly, only the generator and reciprocating arm on the ground need be yawed. The balloon could float at high altitude to take advantage of wind shear.

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