Wind turbines have to be built very strong, or they will be blown apart when the wind gets strong.
Anyone can build a “fair-weather turbine”. But they don’t last. The wind is brutal. 
I thought so. If we go back to our last discussions, this would mean that, one way or another, a kite with one or more turbines on board is unlikely to be viable over the long term. If the turbines are strong enough, their weight is restrictive. If no, the turbines would not be robust enough. And for fast crosswind flygen rigid kites, the power would be concentrated on high-rpm small turbines that are likely to have a limited lifespan.
That’s hypothesis, not an analysis. To test your hypothesis you could do an analysis looking at, say, the work Makani and others have done, what generators were used and their characteristics and so on. I’d be interested for example in the possible cooling effect of an added heat sink moving so quickly through the air, or / and active cooling of the generator.
Yes, it is only a hypothesis. But before considering the solutions, if there are any, let’s try to better understand what exists.
We can wonder if the higher rpm of Makani type (after all barely higher than the 1800 rpm of wind turbine generators in general) limits the lifespan, or if the problem could rather come from the apparent strong wind (but which is also a cooling factor) and the concentration of power on small wind turbines. There is a lack of experimental data.
As a start about rpm of wind turbines:
https://www.google.com/search?q=rpm+wind+turbine+generator&oq=rpm+wind+turbine+generator&aqs=chrome..69i57j0i22i30l2.14103j0j7&sourceid=chrome&ie=UTF-8
What is the rpm of a wind turbine generator?
Large-scale turbines typically rotate at 20 rpm, while domestic sized turbines tend to revolve at roughly 400 rpm . In most large-scale turbines, the low speed shaft is connected to a gearbox. The gearbox increases the rotational speed of the shaft, up to 1200-1800 rpm.
Then a discussion about Makani vs Sky Windpower, Reference: Makani vs Sky Windpower :
Oct 3, 2012 #5 bhaazee 80 0 Ofcourse. If accurate answers are required then actual measurements needs to be made. However, based on working principle I think Makani would provide the best power generated to weight ratio. Coz, the propeller rpm in makani compared to sky windpower is far greater (thnks to the circular flight path in makani). Also, in sky windpower since the device has to hover, I think this limits the unit size that can be built. and regarding the tether, assuming the flight can reach heights of 600 m, what if the flight severes itself (bad wind or mechanical wear…) from tether. The flight can be landed safely. But is it possible for the tether to fall without causing any damage? However, since the airborne wind power is something new, a conclusion regarding the effectiveness, problems, etc. can only be reached after 3 to 5 years of usage.
After more than 10 years, we are very far from 3 to 5 years of usage.
Basic FlygenKite:
Option below.
Replace the bar carrying the turbine with a wing profile generating a little lift to partially compensate for the weight and drag of said turbine.
Superimposed wing-turbine modules could make it possible to get closer to the L/D ratio (around 4) of the flexible tethered kite alone, while carrying more turbine surface area.
Below is a sketch representing an option to carry several small turbines without being too penalized by the mass of the bar, which is used only to maintain space between the two lines. The thin (for crosswind version) bar can be a thin foam Sharp rotor or a wing profile in order to generate a little additional lift. The tethers pass through the respective propellers of the turbines, a little like for Kiwee.
For large static versions, inflatable fatter shapewave® Sharp rotors could be used to compensate the turbine weight.
And also this AWES could be used in both static and crosswind versions, by choosing suitable compromises.
A pair of turbines can be enough, the whole remaining more easily handled.
Pierre,
A couple shotgun questions for you:
If I was interested in making a quick Flygen demonstrator, Do you have any recommendations on how to transmit the power to the base station? Would you do AC or DC? Probably AC (the transformer potentially adds a lot of weight)… is there a place to get conductive tethers?
I think a constant loop cycle with the kite engine would be perfect for the flygen, but then some suitable slip ring would be necessary to avoid twisting lines, Also all of the spools with the conductive elements would need slip rings run through the barrel.
If we could come up with a BOM of this list, I would be happy to test it.
It also occured to me you could have a bot that travels up the line as you have them pictured. the bot could have a couple electronics and a power cable going down, this has been done before.
Hi Christian,
The demonstrator had not electric cable.
I experimented something with a heavy electric cable and a multi-meter, but this did not hold with the crosswind movement.
In all cases DC motors are easier for tests, but heavier and less efficient.
For leds (1st video) I used a small DC motor:
https://product.mabuchi-motor.com/detail.html?id=101
Makani rigid wing used a single electric tether with a swivel. For a flexible kite like mine, avoiding the weight of the pod aloft would be a good thing. For my experiments I used figure-eight.
Perhaps a static use would be better and safer if the weight of the turbines is compensated by the lift of the power kite and the Sharp rotors that are used to space the two tethers and increase the space for the turbines, knowing that the gain in crosswind flight is not very high for flexible kites (about 4-6x, and also taking account of irregular power and quick wear). Commercial wind turbines could perhaps do the trick.
Electric cattle fencing is mostly polyethylene with conductive threads wound through it’s length.
It’s made at scale, so it can be cheap to experiment with.
You have at least 2 lines so 1 +ve 1 -ve
Might need to up your voltage for efficiency.
Also in the same shops they have some good winding spools.
Might help
1 Like
Sorry for the deletion above everybody; I wrote a whole response and forgot that I asked about a demonstrator. I am such fanboy of groundgen that I launched into the potential problems. Has anyone put any thought into long conductive tethers aloft? Do wind turbines experience lightning strikes often? Also using a thin tether, is the only option AC? if so does that mean a transformer or some sort of rectification needs to take place aloft?
Even without electric cable, wet tethers can conduct electricity from lightning strikes.
How to reliably protect wind turbines against lightning strikes
To avoid these damages and costs, reliable lightning protection is needed. Lightning protection systems are used to divert incoming electrical current into the ground, protecting rotor blades or nacelles from damage.
The complete lightning protection system of a wind turbine consists of the external lightning protection system and the surge protection system. The external lightning protection is realized by receptors as well as arresters, which conduct the lightning current into the ground via defined paths.
I used DC brushed motors as generators, because the installation was easier, but without being able to test with the kite in crosswind flight, because the wires were torn off too easily. And I measured the power (in W) by using a car, a multi-meter, and the DC motor with the propeller:
However brushless motors and AC are far more efficient (less weight, higher voltage) but required some additional material and are more complex for a first experiment.
Electrical wires can be found at any DIY store.
I did some calculations and I am convinced with todays tech level for conductive wires vs mass, such systems will not scale to many megawatt for one ground station.
Short answer; tether has cubic scaling and at one point the tether shape develops a belly that in turn multiplies the tether drag
1 Like
Makani provided some specifications, including that of the conductive tether. But between what is expected and what is achieved the difference can be significant.
http://www.energykitesystems.net/FAA/FAAfromMakani.pdf
M5 Specs
Rated power: 5 MW
Full rated power wind speed: 9 m/s
Wing mass: 9,900 kg
Generation system: 8 brushless DC motors
Wing
Wing spar: carbon fiber
Wing skin: e-glass
Length: 1,060 m
Tether
Mass: 3,660 kg
Conductor: aluminum
Structure: pultruded carbon fiber
Voltage: 8 kV
Circling radius: 265 m
Operational altitude range: 350m - 650 mM600 Specs
Rated power: 600 kW
Full rated power wind speed: 9m/s
Wing mass: 1,050 kg
Generation system: 8 brushless DC motors
Wing
Wing spar: carbon fiber
Wing skin: e-glass
Length: 440m
Tether
Mass: 250 kg
Conductor: aluminum
Structure: pultruded carbon fiber
Voltage: 8 kV
Circling radius: 135 m
Operational altitude range: 140m - 310m
ChatGPT says: [cut text for briefness, for M600] The weight per meter of the aluminum wire with a cross-sectional area of approximately 33.33 mm² is about 0.09 kg/m (or 90 grams per meter). This calculation assumes the wire is solid aluminum with a density of 2700 kg/m³. In real-world applications, other factors like insulation and manufacturing tolerances may also affect the wire’s weight.
I say: this is the mass of the aluminum core. for the length of the tether, 400 m gives 40 kg aluminum mass. The conpleted tether 250 kg. Sounds right?
Look at the scaling. if we assume M600 at 250 kg tether, the M5 is 2x scale approximately, at 3660 kg. If we would assume cubic scaling we have 250 kg x 2 ^ 3 = 2000 kg. So their actual scaling was even far worse than cubic scaling.
I think though that the power density from m600 to M5 has increased. Quadratic scaling of power should give 600 kW x 2^2 = 2400 kW, and the M5 is rated 5000 kW.
I would say though in hindsight this looks overly optimistic, considering how the flight went.