FlygenKite

Maybe calculate the drag of that tether assuming zero kite drag. Quite possibly TSR of 5-6 is not possible with such massive bridles just due to tether drag

Thanks for your contribution @tallakt . You well know tether drag issues. On my sketch the proportions are not applied: above all the main tether (6) should be far longer, the whole being a sort of Y configuration like MAWES or dancing kites but as a full rotor. So the main tether being motionless (as for all Y configurations) the tether drag is mitigated.

That said it is true that the geometry of thin blades made of tied wings imposes an additional tether length. Indeed in “usual” Y configuration the tether between the wing and the main tether attachment point is straight, while in the tied wing rotor said tether makes an angle whose vertex is the control pod (5). I don’t think that the additional length would be very important: it can be calculated according to the span of the blade in relation to the distance from the attachment point to the main tether via control pod (5).

I think this additional tether length could be avoided or mitigated with a more suitable design, like that on the sketch below, where the control pod (5) is directly on the main tether (6).

What I am concerned with is the many tethers close to the kite plane. These will be moving the same speed as the kites. Also as these are at a large angle to the shaft, they must be a bit thicker to accomodate higher tension.

A Y-split is beneficial for Yoyo (eg Kiteswarms), but I feel in this configuration the benefit is offset by the amount of thinner bridle lines.

I am all for bridle lines, but they must be minimal and necessary.

The TSR will be higher than the average glide number (L/D) as your blades extend very far towards the shaft center, like a windmill. Maybe the inner blades may be slightly in front of the outer blades to help the overall TSR.

I think a good place to start calculations is to split your bridle into segments, then multiply their l \cdot d \cdot v_t [length, diameter, tether travel speed, relative to tip speed]. Then compare that to the drag of the kite themselves. If tether drag is a lot larger than the kite drag, expect a large reduction in TSR. And vice versa. To be more specific

\sum \frac{1}{2} \rho C_{D,t} l d v_t^2

For all tethers

\sum \frac{1}{2} \rho C_{D,k} S v_k^2

For all kites

To make calculations concrete assume eg tip speed 100 m/s. Then adjust that speed depending on the placement/radius in the kite looping plane. Use the speed at the center of the tether.

C_{D,t} is probably close to one. For \rho it is customary to use 1.225. The rest are given by your drawing/design/wing choice. I would expect C_{D,k} to be eg 0.1 for a rigid kite, though this is very implementation specific

Anyway, this will give you a ballpark indication whether tether drag or kite drag is dominating.

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Thanks for the indications.

The tip wings (carrying the secondary turbines, or in yo-yo configuration although, unlike Y Kiteswarms like, depower during reel-in phase would be too difficult due to the number of tied wings) can be likened to Y dancing kites (like Kiteswarms and some others) by removing internal tied wings.

I think continuous tied wings bring a structural cohesion that Y configuration has not.

Intuitively I think (being right or wrong) that tether drag of tip wings in tied wing configuration would be similar to that of the wings in Y (Kitewarms) dancing kite configuration.

Internal tied wings being slower as they get closer to the center, tether drag decreases also.

Also I have come to believe that TRPT is a strategy superior to flygen for drag mode AWE. Maybe you might look at a soft/supported soft shaft for this construction. If indeed TSR > 6 is in the design goals, this would seem feasible

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I sketched a TRPT Daisy-like with tied wings on Medium scales for torque transfer systems onshore and offshore - #107 by PierreB.

The problem I see concerns the size of the required rigid tilted ground ring for the generator transmission when the rotor becomes huge.

And also the achievable altitude is dependent to the diameter of the rotor due to the torque transfer requirement.

For a 1 km diameter rotor (it is quite huge but AWES will be competitive when high dimensions (= more or less tether length) will be reachable) such a rigid ground ring would be 0.2 or 0.3 km diameter and 0.1-0.2 km height. It was the reason why I conceived Rotating Reel System.

The limit of a fly-gen flexible wing is its low lift to drag ratio (perhaps 3 at the best with the turbines, and 5 for the kite alone), leading to a low rpm of the generators aloft, something like rpm 1000 for a 2 m diameter turbine (to be compared to the diameter of 2.3 m of the secondary turbines of Makani M600) of TSR about 4 and for a kite flying at 30 m/s with 10 m/s wind speed. The generators would be too heavy when the system scales.

Perhaps one way to drastically lighten the secondary turbines is to install them at the tip of the blades of at least one secondary rotor (TSR about 3 x TSR about 3 of the secondary mounted rotor, so about 9, but not more, rather less). This would give a much higher speed, something like rpm 10.000 for turbines of say 0.5 m diameter.

A use under a static kite is also a (better?) possibility.

A sketch below:

Hello Pierre:
I’m trying to figure out what the lift-to-drag ratio of a supporting kite has to do with the RPM of a turbine?
Also, the idea of enhancing the wind speed through a rotor by mounting it on the tip of a turbine blade “sounds like” a good idea, but the problem wight become keeping that generator cool. There are two ways high winds destroy turbines:

  1. high RPM and gusts rip the turbine apart;
  2. Sustained high output leads to generator heating until the windings burn up the insulation and/or sometimes melt.
    All these “armchair engineer” ideas sound great as long as everything remains “on-paper”.
    I’ve mentioned in the past, having a visitor - a girl with zero wind or engineering experience, who on the plane ride thought up the idea of putting small turbines on the tips of larger rotor blades. I had to tell her that it is an idea that gets mentioned every so often, and may work OK, but as far as I knew, nobody had ever tried it, and that was maybe 14 years ago?

One big question is: “Why hasn’t anyone tried turbine-reeling on the ground, or blade-tip turbines?” Should they? Are we missing out on the next “big breakthrough in wind energy”? Or is there a lesson to be learned here? If these ideas are absurd on a tower, is it possible they are absurd in the air too?
Anyway, just as it would be possible to do kite-reeling on the ground using an unloaded turbine on a tower on rails, pulling a cable as it rolls downwind, then being reeled back upwind, and seeing how much intermittent energy you could extract from the reel, minus the energy used to reel the turbine back in, GE or anyone else could build a tower-mounted turbine with turbines and generators on the blades.
So why hasn’t anyone done it?
Well, to start, you want the blade tips to be light weight. Adding turbines and generators at the tips would make that impossible.
I think examining whether any concept would seem “viable vs absurd” when applied to tower-mounted turbines can be instructive for AWE.

The higher the L/D ratio, the faster the wing and the higher the rpm.

There are high density generators that work at high rpm, example on

Here the blade tips would be robust.

Peter Jamieson clearly explains the secondary rotor concept in his book page 128.

Doug, I bet that none of the concepts discussed in AWE will have a serious future, be it yo-yo systems, Laddermill, fly-gens, Serpentine ™, and so on. However, there is nothing to stop us from having fun with these concepts, leaving the serious work where it is, in the studies and incremental improvements of regular 3-blade wind turbines.

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If I had just a bit more spare time I would have done it! :smiley: Another big drawback is the added tower clearance requirements. I would imagine this to be less of a problem on a AWE turbine as the inflow velocity is more predictable/constant.

Anyway: Researchers at DTU were looking for commercial partners to develop this idea further some months ago - so it might happen :slight_smile:

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About secondary rotors (see also Figure 2, page 810):

I just proposed to use it for a flexible kite flying crosswind at low speed, in order to obtain a high rpm of the tip turbines, so more lightness.

Hey Pierre: Some of the motors and generators that say they can magically produce high output without overheating have not been exposed to the relentless torture of steady-state high winds. At some point, all that heat has to go somewhere.

I checked out the Linked-in link and even commented - flagged it as one more “Professor Crackpot” idea in wind energy (which 999 out of 1000 are). Here is my comment:

This is an idea commonly mentioned by outsiders to wind energy. Like all concepts for increasing the air velocity through a rotor, this one has dealbreaker issues. At a 6:1 Tip Speed Ratio (TSR), these secondary blades would be spinning at supersonic speeds, therefore Helge says to reduce3 the TSR to 2. That means increasing rotor solidity (more blades). But low TSR, high-soldity rotors cannot even approach the Betz coefficient, because if the blades are not at a high TSR, the kinetic energy goes into wake swirl rather than spinning a generator. This is one more “Professor Crackpot” notion that will probably never be built after further scrutiny, and if built, will go down in the history of failed wind turbine “improvements”. Also, adding a lot of mass to the primary blade tips will require greatly-enhanced primary blade strength = greatly increased rotor weight. This is just one of those ideas that “sounds great” to newbies and people who haven’t fully thought it through. Nice try, but this aint gonna fly! :slight_smile:

Doug Selsam

They are not trying to achieve high Power coefficient for the secondary rotors. What you are trying to optimise is Cp/Ct => leading to a much lower than usual power coefficient.

You should think of them much more like a propeller than a wind turbine (at least in terms of solidity and TSR). Normal propellers can also reach 90% efficiency if I recall correctly.

My answer:

The tip blades [props] need to have a large pitch otherwise they will be producing lots of drag. In that case they will not provide the aero gearing to make this worthwhile. I will provide the insight to see this: for a fixed windmill, downwind force is essentially free (just make the structure strong enough). For a moving windmill though, drag matters a lot. And the faster the blades spin, the more downwind drag. There, I saved someone millions…

Its an interesting thought though: are supersonic windmill blades possible?

Apparently not Makani’s millions…Assuming that secondary rotor drag was a real problem, which I doubt…

There is only one Betz limit, which is that of the whole wind turbine.

Well Pierre, I’ll admit, with two layers of blades, it DOES become a bit of a brain-teaser, and I think these smaller, faster-traveling rotors would still be subject to a Betz coefficient of their own, at 90 m/s, and a TSR of 2, or whatever, but whether we want to invoke Betz or not, rotors need to produce power efficiently, and a low TSR rotor cannot produce power efficiently. It is a matter of momentum exchange versus kinetic energy exchange. MV^2 versus just MV. This is one of many facts about wind energy that I have never seen or heard discussed in AWE circles, yet it is very basic. So now you know. Congratulations. :slight_smile:
Also, crossing over to what Tallak is also talking about - secondary rotor drag: Low TSR secondary rotors will generate less power because the lower-speed blades will just force the wind into a swirling wake, as much as spinning the secondary rotor. That is a waste of power you will never get back. The multi-blade slow-moving rotor acts more like stationary vanes redirecting the flow. You want the rotor to get all the energy, not the wake. A slow rotor leaves energy in the swirling wake. I’m guessing that may translate to more drag, but that doesn’t sound quite right for a rotor with a steeper pitch to have more reverse thrust (then again it has more blades = higher solidity) , but I will stick with the fact that a low TSR, high-solidity rotor, such as found on farm windmills, for example, cannot harness as much energy from the same wind as a high TSR rotor. A high-solidity, low TSR rotor is less efficient and cannot approach the Betz coefficient. This is known. And because it is extracting power FROM the air it is not the same as a multi-blade propeller, or even co-axial, counter-rotating, dual-rotor propeller on, say, a giant Russian cargo plane. This fact of Low TSR, High-Solidity rotors being less efficient is one more reason to beware of the claims made by promoters of ducted turbines. Ducted turbines and Makani-style kite-plane propellers end up adding rotor solidity and reducing TSR, which makes them less efficient at extracting energy from the wind flow.

From the PDF above (Top-level rotor optimisations based on actuator disc theory, author Peter Jamieson):
Table 3 page 813:
Design tip speed, (primary) Vt 40, (secondary) vt 160, (unit) m s−1

Page 814:

Assuming a rated wind speed of Ur = 11 m s−1, and a relative wind speed for the secondary rotors of 160 m s−1 […]

A short video of a secondary rotor carrying secondary turbines, all being old material, then a photo:

As expected the turbines add a lot of drag. It is understandable why early scientists classified fly-gen (Makani) as drag devices.

The full rotor with its tip turbines is destined for a crosswind flexible kite (FlygenKite). One could perhaps say that it is a double drag (drag²) device.