The Kite Turbine

Since this is the first post of this free technology project in english, I will summarize its purpose and the progress so far as well as describing the new improved generation software. Here are all posts on this topic, although up until now everything written on the topic is in Swedish.

Kite Turbine Overview:

A ”kite turbine” is a turbine whose blades are composed entirely out of soft or flexible materials. The blades raise themselves in the wind mainly because of their aerodynamic properties as opposed to the inherent stiffness of the materials themselves. The wind power generated is transmitted to the hub via cables.

These are some pictures of the latest kite turbine design, which will be explained in this particular post.

”Strength through softness”

The main purpose of the Kite Turbine is to increase solidity-per-weight ratio. The solidity of a turbine determines how large fraction of the sweep area that the blades cover. This means a larger part of the area can be covered using lighter building materials.

Some scientific reports support the evidence that a higher solidity could be beneficial for wind turbines.

The reson why it is possible to reduce weight, is similar to why cable bridges can be much more light-weight compared to bridges made out of welded metal boxes or other steel structures. It is not that the material of the cable bridge is stronger in itself, as the cables also consist out of steel. It is just that the flexibility of the cables enables the cable bridge uses its strength optimally. The forces of the bridge is applied along flexible cables, as opposed to perpendicular to rigid structures that can easily tear or crack.

To give some additional insight to the technological DNA of the kite turbine, here is a clip a friend of mine sent me that shows a pretty though umbrella. It shows that it is possible for a semi-soft structure to raise itself in the wind due to its aerodynamic properties, similar to how a kite turbine works.

Just to give some idea about the possible benefits of a kite turbine, consider that a traditional wind turbine, just the blades of it, could weight around 100 metric tonnes. A kite turbine could potentially offer an ultra light-weight alternative that can be folded for easy transport.

General Assessment:

For the kite turbine, we can summarize the possible mechanical benefits as follows:

• It will be possible to create wind turbines of higher solidity using less building materials. Some research indicate that a higher turbine solidity would increase efficiency over a range of wind speeds.
• Reduced turbine weight could make it possible to build larger turbines, that reaches higher into the skies.
• The mass reduction could make the turbine easier to start at lower wind speeds.
• Less mass and inertia of the turbine could possibly lessen the structural demands of the wind turbine attachments.

On the other hand, there are some potential drawbacks:

• There is a possibility that a kite turbine could vobble if the blades gets out of synch. Wether this actually is a problem, or if it could be solved with cables between the blades is an open question.
• Generator construction becomes quite different with a slower moving turbine of higher torque.

There are practical aspects of the Kite turbine:

• A possibility to fold or disassemble the blades could make it easier to transport and mount a larger turbine on to a tower.
• It might be possible to create a large sized turbine for sailboats, to be placed near to where humans and animals move, without risk of ”chopping someones head off”. Since the blades could be made out of foam, bent inwards and covering low tension draglines, it would be very safe even to walk directly into the fast spinning turbine.
• A turbine that is foldable could be made portable, taking not much more space than the generator itself. Because of its low blade inertia and softness, it is easier to mount it on a makeshift, low quality pole. This could be used for for hiking, or for disaster area power generation.
• In Sweden, the airforce has been complaining that traditional wind turbines confuse their radar systems which confuse them with helicopters, and they want them placed far from their bases. It is likley that a slower moving construction made out of cloth and draglines will have a more suble radar profile.

This sums up the purley technical and practical potential benefits and drawbacks. But there are aesthetic considerations:

• As a kite turbine is basically shaped as a submarine propeller and rotates slowly because of its high solidity, it is likely that it will be extremely silent. Making it possible to place such a turbine closer to residential areas.
• Since the kite turbine rotates more slowly because of its high solidity, it is probably more pleasant to rest the eyes on. This will make it easier to place the turbine in areas of denser populations without protests.
• The spiraling shape will make the movement of the kite turbine appear softer and more organic. However, will it have a hypnotic effect on whoever beholds it?
• A kite turbine could be made out of transparent materials, making it blend into its surroundings even more.

Previous progress:

As far as I know this is the only Kite Turbine project available. It draws inspiration from other wind power generation concepts such as Italian KiteGen, and USA based Magenn. So far this is a 100% non profit project. I present all insights on my blogg, and share my blade generation source code with anyone who wishes to take part in this narrow technical niche for whatever reason.

Currently I have proved that various prototypes of this turbine design actually do work. This first working version shows a quite vivid turbine, even though it was created with very little sophistication. The shape of the blade is basically created using ”rule of thumb” and ”hands-on-handcraft”.

Later I created a computer program to generate the blade shape in a more orderly fashion, and I experimented with longer blades that would require a smaller hub.

However, the aerodynamic properties of this turbine was not ideal. For example, the blade tip just ends sharply and would probably cause turbulence. The blade width also remain constant along the blade, and even if this makes it similar to an ordinary ”propeller style” turbine, I believe that a kite turbine would preferably have a blade that gets wider towards its tip to fully utilize the best part of its sweeping area.

The current aim is to kind of cross-breed these turbines. Make a computer generated turbine with wider blades towards the turbine edge, and with blade tips of better shape.

In addition to this, a number of other things has also been improved in the turbine generation software. For example spiral generation, adaptation to the hub, templates etc. The new generation software is basically better in most aspects. The first picture of this blog post is an example of what kind of turbine it can generate.

The New Generation Program

This time I present a new generation program that can create a wide range of differnet kite turbines with different number of blades, angle, spiral shape etc. etc. The source code is available for download here.

Instructions: Download the single file and place it in the ”plugins” folder of your installed Google Sketchup. Restart Google Sketchup or type ”load ‘SuperBlade.rb'” in the ruby console to activate the plugin. Use the plugins menu to generate some different kinds of kite turbine.

The spiral

In previous generation I have used archimedes-spiral to generate the curve. While this seemed quite straightforward solution, it wasn’t as good idea as I first thought. One problem is that the tangent of the curve is completely wrong near to the hub, and somehow the shape was not right so I had to mix it with a curve that grows stronger near angle = 0. So in the end it was just conceptually messy and not worth the effort.

What I really wanted was a curve that started as some kind of circular curvature, and then at some point it would continue as a curve that is orthogonal to the tangent of the hub at every step. This would make the top blade tip orthogonal to its drag-lines, which could make some sense.

To define a radial curve that is orthogonal to the hub tangent at every point seemed like a very difficult thing to do mathematically, so I had to find some kind of numerical approximation. What I did was to create a parametric curve that just bends at a constant angle at the start, until the angle of the curve gets close to being orthogonal to the hub tangent. At that point, the angle of every curve segment could be approximated by some interpolation between the angles that would make either side of the curve segment orthogonal to the hub tangent.

The last part of the curve could also end in a perfect orbit around the hub. In the final turbine generation program it is possible to interpolate between either by setting a certain variable.

The idea is that this would in general create a suitable curve.

Lastly, there are mechanical issues involved in the spiral that this model completely ignores. To get an optimal shape, it would probably be necessary to co-optimize the spiral shape together with the blade shape in order to find a mechanical and aerodynamic equilibrium. But this is simply outside the scope of this humble hobby project.

Easier to fit the turbine to a hub

With the new program it is much easier to fit the turbine to any given hub. The reason is that relevant measures are now given directly to the model, for example hub depth etc.  Here is the quite extensive list of parameters taken from the program.

# Attr accessors for fields that can be set as options
attr_accessor :hubRadius, :hubCircumference, :hubDiameter

attr_accessor :prototypeBladeNumber

attr_accessor :spiralStartAngle, :spineLength, :spiralNominalCurvature, :turbineRadius, :turbineDiameter, :turbineCircumference, :turbineCeilingness, :ceilingCurvature

attr_accessor :bladeAttachFrontZ, :bladeAttachBackZ  attr_accessor :frontDraglineAnchorZ, :backDraglineAnchorZ  attr_accessor :bladeDepthAtTop

attr_accessor :wallWidthAtHub, :fractionOfSlotAtHub  attr_accessor :squareTreshold, :wallWidthAtTreshold, :fractionOfSlotAtTreshold

attr_accessor :wallWidthAtTop, :fractionOfSlotAtTop, :bladeDepthAtTop  attr_accessor :bladeIncline
attr_accessor :wallWidthLinearGrowth
attr_accessor :ceilingHeight, :ceilingFractionOfDistanceToHub
attr_accessor :ceilingAlignment, :wallAlignment
attr_accessor :ceilingSweepZ, :wallSweepZ
attr_accessor :tipTopAttackAngle, :tipBottomAttackAngle, :wingProfilePrototype
attr_accessor :numberOfSections, :spiralSegmentCurvature, :profilePerNSection, :draglinesPerNProfile

Some of these parameters are necessary, some are redundant, making it possible to express the same thing in a number of different ways. For example, the angle of the blade against the wind direction can be set in a number of ways. Either give it directly, or for example set the width and depth of the blade at either the tip or at the hub.

(Note: The terminology of the program uses the word ”ceiling” to denote the part of the blade that generates forces away from the hub, and ”wall” for the part of the blade that generates rotational forces. The blade has ”wall” properties near the hub, and gradually assume ”ceiling” properties near its tip. )

The Z-alignment and blade sweeping:

Previously I had the front or the back of the cross sections align themselves Z-wise. Now since the blade can get much thicker at the tip, such simple alignment would not work. What I settled for was simply two linear functions that blends as the blade transcends from mostly generating rotational force, to mostly generating expansion force.

In the first image shown the base of the blade has a slight sweeping, and no sweeping at all for the part that only generates expansion force. However, considering how the Senz umbrella works, it would maybe make sense to have no sweeping at all, making the turbine stand straight out from the hub. This could perhaps reduce Z forces on the turbine as a whole?

Cross section Generation

As I mentioned in the previous post, the general idea is to create each cross section inside an euclidian box that has height =0 just at the hub, and then possibly grows along the length of the blade depending on the parameters. In this euclidian space, the force generated upwards will expand the kite blade, and the force generated rightwards will make the blade turn. The euclidian geometry is then warped around the hub. This is for example why there is a very nice fit around the hub at the very base of the blade.

The reason why a box is used, is because a cross section created along the diagonal can have aerodynmical properties that relate to both the turing and the expanding force of the blade at the same time. This is for example why the very tip of the kite turbine blade can create additional rotational force, rather than just cause turbulence as in my previous design.

In the last post I discussed having a wing profile of some scale that was just beeing rotated along the diagonal axis. However, when I tried to setup some way to configure the wing profile scale, it made more sense to actually relate the wing profile scale and direction directly to the local wind-flow somehow. I did this in a number of steps.

1. 1. Determine the plane normal for the cross section. This is created from two vectors: a) The XY-plane normal of the parametric spiral that is the back bone of the blade. b) The diagonal of the euclidian box.
2. 2. Create a wind vector, given the angular speed of the turbine at that point. This is essentially an XZ-plane projection of the diagonal of the euclidian box.
3. 3. Create the orthogonal projection of the wind vector onto the cross section plane, using results from 1 and 2.

Now we have the plane of the cross section and the wind vector’s projection upon it. This is the basic information we need to generate the cross section. Now we only need to determine how the blade’s upper and under side should be angled in comparison to the wind. This is simply done by setting two angles. So the cross section generation is finished by following these steps for the upper and under side respectively:

1. 4.1. Rotate the wind vector projection from previous step 3 by either A or B.
2. 4.2. Calculate a normal of the blade by creating the difference between the vector from 4.1 and the orthogonal projection of this vector onto the diagonal of the euclidean box.
3. 4.3. Calculate a scale that would fit the first vertex of the wing profile prototype on the rotated vector from 4.1, assuming that the base of the wing profile is stretched along the euclidean box diagonal.
4. 4.4. Create the cross section vertexes by adding the top left tip of the cross section, a fraction of the euclidean box diagonal and the blade unit-normal scaled according to the scale from 4.2.

Now that these vertexes are finished in euclidean space, warp them around the hub and then the cross section is finished!

So, what I have is a pretty neat way to generate the wing profile. I can basically create any shape I want, so now it is just a question of what the optimal wing profile should look like. Maybe I should try to find some expert on airfoil design, and try to gain some insight?

Improved templates

When I built the last turbine I noticed it was cumbersome to get some things right. Therefore, If I will ever build another prototype, I made some improvements on the auto-generated templates. First of all I now have comprehensive ”hub layout” that can be printed out and attached to the hub so it is easy to see where to attach all lines and blades etc.

In addition, I added a print out of the lines themselves:

All sorts of turbines:

When I started this project, I though about putting wind turbines on blimps. However, considering that the earth is potentially running out of helium resources, blimps dependent on helium seems like a bad idea for renewable energy. Essentially what happens to helium that leaks into the atmosphere, is that it drifts out of our atmosphere, never to be seen again. On the other hand, with Japan showing what can happen to Nuclear plants if there is no power, maybe power generating helium blimps could make sense for disaster area usage. If the safety could be maintained somehow, perhaps unmanned hydrogen blimps could also be a future option.  Here is what a kite turbine on a blimp could look like:

Here is what a Kite Turbine could look like if used on a regular tower. However, considering that the kite turbine is potentially lighter, maybe it could influence the design of the tower itself? Would it be possible to use cables in the tower construction somehow when the forces sideways due to wind interaction become more of an issue, as opposed to the sheer weight of the turbine? I noted that new design for a Norwegian floating wind turbine had cables in the tower construction, so maybe it would be something to consider.

The generation program also allowed me to visualize another Idea i have had for a Kite turbine. I like to go camping in the summer, and sometimes I think it would be cool to have your own way of generating green power without renting an electricity pole. Since such a turbine would have to direct itself into the wind, I tried to make a kind of ”parachute” shape that maybe could help in doing that. Note how the sweeping is negative at the tip of the blade for this purpose. At least I think it looks cool :-).

The generation software could generate all kinds of strange turbines. Here are some of the more strange ones:

This last picture reminds me of another cool windpower design I came across that is called Spiral Airfoil. The efficiency estimates for the spiral airfoil also indicates that higher solidity makes more sense for wind turbines.

Raising the turbine in the wind

I would like to make a remark about how to make it easier for a Kite Turbine to start, and prevent entangling of its blades. Even though a wind turbine could raise itself in the wind, it might be a good idea to give it some assistance by creating some flexible structure that is just strong enough to raise the blades themselves in the wind.  Any strong force generated from the wind however, will be taken care of by the cables that transfer that force to the hub, so the flexible structure need not be able to withstand such forces.

Raising the turbine when there is no wind can be  done in a number of ways.

For smaller turbines, the blades could have a core out of spring-steel or carbon/glass-fibre, or the blades could be filled with plastic foam of some sort.

A friend of mine, Tobias came up with the idea of having a thin ring of some flexible material circling around the turbine like a thin faeries wheel with cables that raise the turbine.  This idea could possibly also work for larger turbines.

For very large turbines, for example if mounted on a blimp or a tower, blades pressurized from the inside with helium or air could be considered.  If helium was used, the turbine itself would have neutral buoyancy which could be interesting in itself.

If the purpose is just to prevent completley soft blades from getting entagled, a thin and long glass- or carbon-fibre sprout at the back of the turbine could be sufficient. This way blades from the top of the turbine will not topple over and reach down to the blades at the bottom.  The blades on the top will just rest over the sprout when there is no wind.

The next step

For now I am starting to feel quite satisfied with my achievements in this project. It was something that was fun doing, but I start to feel that I have reached the limit of what I could do.  There are other interesting projects, such as perhaps make another go at building a new programming language.

Perhaps I will use the templates this summer to build a turbine together with my son just for the fun of it, but it is not for sure.

Anyone who reads this on the other hand is free to pick of where I left. Use my generation program to create something fun!