FlygenKite

FlygenKite is an old project comprising a flexible wing carrying at least one onboard turbine. It was presented in flight at AWEC2013 and in Kite Festival Dieppe 2012 , the generator feeding a lead tape as shown on the video below.

Some tests were also made with larger generators but without pushing them:

In the calculation the induction factor was neglected as for Loyd’s model. The rotor area should be larger to lead to a lesser induction factor.

Although the paper below indicates:

On-Board Generation
On-board generation devices produce electricity by having added drag on the wing via rotor blades that generate continuously while flying in a crosswind motion (see Figure 3). In this case, the lift coefficient is enough to compensate for the device plus the added drag.The high lift-to-drag ratio requires a hard wing that results in increased performance when travelling across the wind. The power produced on-board is then transmitted down a cable connected to the ground.

I think a more careful study of flexible flygen could be useful, even if the observation above is confirmed. As for yoyo mode, a flexible wind should use about 10 times kite area to produce an equivalent power as a rigid kite. As an example the expected 600 kW for Makani M600 using a roughly 40 m² wing with a lift coefficient of 2, would be achieved by using a flexible 400 m² kite with a lift-to-drag ratio of 4. An advantage would be a slower flight, so a lower cut-in wind speed. The relatively slow flight could also lead to a better maximization of the space, above all when more scalability can be reached.

Some improvement of the way to fix the module was and is studied and several options are envisaged.

I think there is an issue with low G_e (lift-to-drag) in flygen. Because the onboard blades are much more efficient if the airflow around them is much higher (i believe though I cant prove it right now). Slower flight equals larger blades, larger generators due to slower rotational speeds etc.

Though I do see that this option may have been underexamined.

Tallak

The point you raise is also mentioned in my post in the quoted publication. That looks to be relevant and I hear the argument of a low glide ratio several times.

However I would want to reexamine that because I saw that the rotor area (33 m²) is close to the wing area (36-40 m²) for Makani M600, as for KiteKRAFT wing. It is a huge value and that leads to relatively large rotors. Loyd’s paper recommended a value of 0.5 for the drag (thrust) of the rotors compared to that of the wing. If the drag coefficient of this wing is even a high value like 0.15, the drag of the (40 m²) wing (without tether drag) would be 6, while the full thrust of the onboard (33 m²) rotors should be about 30: it looks not possible. Indeed with the full rated power wind speed of 9 m/s (data provided by Makani), assuming the speed of flight would be 50 m/s, 33 m² rotors should produce 2 MW. Perhaps (I can be wrong) the thrust coefficient of the rotors should be lower (as well as the power coefficient) as the induction factor is lower when the thrust of the rotors is higher compared to the drag of the wing (see figures 7, 8, 9). See also the relation between axial induction factor and thrust coefficient on


Thank @tallakt for possible explains and corrections about what I just wrote.

A second observation is the limited full rated wind speed of 9 m/s for Makani M 600: this limit can be due to a too high TSR of secondary turbines. As the flight of FlygenKite is slower, it could undergo higher winds, keeping its efficiency, avoiding a too high TSR of secondary turbines.

I think while what I said initially about high G_e is true, it is also true that there are numerous practical issues related to flying very fast like Makani opted for. So I agree that it is maybe worthwhile looking into some less ambitious designs that are more feasible in practical terms.

I dont quite understand your calculations. If the kite has a G_e of 4, you must load it such that the sum G_e with power production «drag» force is G_e’ = \frac{4}{1 + 0.5} \approx 2.7. This allows you to calculate kite speed (around 1.5-2 times wind speed in practice perhaps, no more than 2.7 times windspeed). Next one may calculate how big the drag force of the power producing blades must be in this airspeed. My guess is that for optimum induction factor the blades must be fairly large.

If we eg. consider a AWE like this flying 2x the windspeed, the onboard blades will produce 4x the power compared to a ground based HAWT. This means the onboard blades must be half the diameter of the ground based HAWT.

Such calculations suggest that drag mode AWE works better at higher G_e even than 4.

I do think a glide number for a foil kite could approach 8. With a flying speed of 4x the wind speed, the diameter of the airborne blades would be 1/4 of the ground based windmill. And so on.

To take this to the ultimate conclusion, the blades you propose will be too heavy unless you also use kite blades for the airborne blades. Using something like @Rodread’s Daisy rather than a HAWT like blade should make it possible to increase swept area drastically onboard without a huge weight penalty. In theory of course. Handling should be disastrous

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Yes, this is roughly the speed I expected for a kite of L/D of 4 before being loaded by a turbine adding 0.5 drag in regard to the kite drag, leading to: 4 x 2/3.

Not 8x, the airborne blades being 1/4 of the ground-based windmill?

Great idea: difficult but not impossible, as for some other centrifugally stiffened rotor or/and using bank angle for the blades.

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My reasoning about the diameter if the blades was based on the assumption that they must sweep a certain fraction of the area. So with 2x windspeed, the energy per area is 4x. Then if the diameter of the blades is reduced by \frac{1}{2} the swept area is \frac{1}{4}, and the output energy is comparable.

Do you support your assumption because in Loyd’s formula the L/D ratio is squared (so unlike the real wind speed which is cubed)?

Flying speed (apparent wind) I gather is G_e w (glide number times wind speed). Power is apparent windspeed squared.

The reason I subtract some is due to gravity slowdown, tether strength limit, gusty wind, non optimal flight conditions and control etc. Hard to eliminate all such in real life

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I am not sure to quite understand your calculation. L/D ratio (where does the apparent wind depend) is squared but the power on the onboard turbine is determined by the cubed apparent wind speed if I am right.

Let us take an example (with some approximations) from the onboard turbines: the diameter of a rotor of Makani M600 is 2.3 m. So 8 rotors lead to a rotor area of roughly 33 m². The nominal wind speed for Makani is 9 m/s. If the L/D ratio is 12, then 6 with the onboard turbines and the losses you mention, that lead to a reasonable kite (apparent wind ) speed of 54 m/s. Air density is assumed to be 1.2, power coefficient is assumed to be 0.2 (losses of efficiency due to a desired lower induction factor leading to larger rotor area; other losses because their trajectory is not rectilinear; other losses…) :

1/2 x 33 x 1.2 x 54³ x 0.2 = a little more than 600 kW.

Now with FlygenKite with L/D ratio of 2 with onboard turbines and losses as you suggest, the same “power” (in fact it is thrust, the real power is that on the wing, even if onboard turbines transmit power to their respective own generators) of 600 kW is captured by multiplying 33 by 27 = 891 m². Too much rotor area, unless I am wrong.

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Like how you are laying this out…
When analysing and comparing top level concepts, It’s probably best to keep aparent wind calculation simple.
A fuller analysis of aparent wind contains the true wind vector and is cyclical with the phase of rotation.
I think the Freiburg group proposed a fuller analysis of the changing energy state of pitched rotary wings which could be used later.