Hi Pierre: If a generator in wind energy is making a large amount of continuous power, overheating of the generator can become a major issue.

Hi Doug, can the fly-gen generators benefit from cooling thanks to the strong apparent wind during crosswind figures?

Hello Pierre:
Yes of course - wind is the main thing providing cooling. I’m sure faster wind flow would help. However it has its limitations. In my experience, overheating will begin at a downwind location of the generator that is relatively shielded from the direct wind. After the generator burns out, turning into a very smelly blackened mess, one will be able to examine the generator, even taking it apart if you can stand the smell, and see where the overheating started. Once overheating starts, it spreads, due to the increased electrical resistance of the wire at higher temperatures. Overheating is a runaway situation, like trying to stop a truck going fast downhill. Like a runaway freight train. There are tables that show the reduced life of generator windings’ insulation at various high temperatures. Any increased heat shortens the life of the wire insulation. This is why most generators are so big and heavy. It is easy to get huge amounts of power from a small generator for short bursts, but for sustained high output, you need a big, beefy generator.


And the propeller diverts the apparent wind out of the generator. Many elements plead in favor of a generator on the ground, at least concerning the current technical art.

Hi Doug, please can you tell why?

It seems to me that (as a possible reason not excluding other causes), as we have already mentioned in the forum, the wing of Makani was much heavier than expected. As a result, the wing struggled to recover after gaining momentum downward with each loop.

Perhaps a way to get a lighter but still performing package would be to combine Wind Catcher downwind of a semi-rigid wing.

Pierre what blows my mind is the number of supposed “engineers” and other supposed super-high-end “talent” hired, and the stories told. That many “engineers” should have been able to predict the way the craft would operate. Why did the “story” go on for so long? I’ve seen and in many cases debunked so many wind energy schemes ahead of the fact, often promoted by “the smartest people in the world”. Few to none are still even under consideration, let alone operation. Why can I analyze the facts in 1 minute using no computer and with no engineering staff, while these “teams” with all that “expertise” and millions of dollars go on for years with what turn out to have been silly expectations? There is an aspect of giddy insanity in the field, and I think it also often turns into greedy deception where these companies may realize they are wasting everyone’s time and money but hope to exit at a profit before the bad news becomes too obvious to ignore. The giddy deliriousness results from only hearing enthusiastic promotional side of the story, never considering or even understanding all that can go wrong. I think the backdrop of “saving the planet” adds an almost religious fervor to it all, in many cases. Under that dynamic, the “press-release” aspect, and the articles and coverage that ensue, encourage ignoring reality as long as the “story” can be held together. When I saw the videos of the flights, I just thought “really? This is what all that money and talent came up with, and they are promoting it?”. At some point I just don’t know what to say. Now you’re talking about an early prototype with a limited stated use for camping. There are aspects I like about it, but also, I think it might find a problem with high winds and longevity, like almost every other wind energy device. If anyone pays attention to the realities of small wind turbines, with most every manufacturer having gone bankrupt, the reason is they can’t withstand what Mother Nature dishes out, over and over again. As I said over a decade ago, the wind is invisible so people can imagine it doing whatever they like, but it does what it does, and that often means destroying small turbines.


And what do you think Makani could have done better, other than staying on a smaller scale?

Pierre: Thanks, I appreciate the interest in my opinion. At some point I would just say that anyone is free to contact Selsam Innovations / U.S. Windlabs, and if they have that many millions of dollars to waste, why not spend it on someone who has a clue already, like me and my team of experienced people who already understand wind energy? Some aspects are so obvious to me that it pains me to see others not recognizing them - but hey, that’s just my take. If I spill out every thought on these forums for free, what do I have left? Then again, why trust me? Maybe, like all those “top-notch engineers”, I don’t know what I’m talking about either, right? Hmmmm, well people can judge for themselves at some point.
One thing I find interesting is if you go to the big-name organizations, for example, who there, even the highest on the food-chain, has ever actually designed and/or built a well-running, long-lasting wind energy system?

Maybe with a few exceptions most only made incremental changes to a small part of the whole thing. Thats the inevitable truth of companies growing bigger than a handfull of people.

Fo AWE, fewer people need to be able to do so. Because the teams are smaller, and the tech dev steps are huge. And these few people need to have that good understanding in order to have a fighting chance.

That is the logical side. But in practice the hierarchy in a company is mostly not decided by choosing the optimum knowledge from the entire team. For a company like Makani, I would expect one or a few people made most of the important decisions and then said engineers implemented these ideas one bit each.

Now if this were the case, Makani would be very dependent on the foresight of these key people.

I think they maybe did some good stuff, but I cant quite see how they could miss roll control in «real» winds while hovering. There was no inherent stability, nor active control. So I think we could chalk this down to not forseeing this problem or not taking it seriously (prioritization). Both as deadly sins IMHO.

Roll control is one example, there are surely more examples in the Makani history. But I think at least this one thing lead to the last offshore crash and turned to be the last nail in the coffin of Makani.

Following this reasoning other key people may have succeeded in the Makani bubble. But it would surely look somewhat different.

During this current AWE hype-cycle, I’ve compared many AWE efforts as “showing up at a Formula-one car race with a wheelbarrow, wondering why your machine can’t even qualify for the race, let alone win.”
With as many tools that engineers have today, it perplexes me that so much could be spent for such lackluster results.
Seems like with that many people on a team, someone should know what they are doing. That is a lot of hype, a lot of expense, a lot of getting people excited, and a lot of years of talent, to come up with a wheelbarrow.

If I remember … might be wrong … Makani used differential gain of contra-rotary thrust for roll control via motor torque. e.g. a CW and CCW rotor on each side… but it wasn’t enough roll control authority against the tether with spare power to also combat yaw instability in hover or something like that am I right?

Could be. I expect such a control scheme would not be able to generate large amounts of torque, as props in hover are undoubtly running at «full steam» and this strategy involves slowing down a subset of props and speeding up others. Also momentum equilibrium must be maintained at the same time. I havent given it any thought, I dont see it as probably viable.

I would be more into things like thrusters or drag members on the wing tips or moving the bridle attatchment point, or maybe a support bridle just for these operations.

Or just not hovering like that…

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Flygen AWES require light generators aloft that rotate at fairly high rpm. However, as an example (see page 44/51), life expectancy of the EMRAX motor is the same as life expectancy of the bearings that are mounted in the motor. And for longevity, low speed (1800 RPM) is superior to high speed (3600 RPM).

Magnetic bearings have a higher lifetime and are used for gas microturbine generator type which rotates at very high rpm while its lifetime is high, being about 50 000 - 80 000 hrs, the higher value shown on this table.

See also (page 1) High-speed synchronous electrical machines with permanent magnets (HSEMPMs) operating at a rotational speed of 30,000-60,000 rpm and a power of 50-300 kW are of high interest for the aerospace industry.

And to see more: micro generator for gas turbine , High Speed Generator for Gas Microturbine Installations , Development of a high-speed turbo-generator supported by active magnetic bearings, describe generators with rpm in the tens of thousands.

So there are some possibilities for both lightweight (about 5 kW/kg) and durable generators.

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Hi Pierre: I remember the “comedy” section at Windpower trade shows, where you’d typically see “innovative” drag-based vertical-axis demo mini wind turbines powered by an off-the-shelf household fan from Walmart. Of course the promoters had to mention “magnetic” or “maglev” bearings, as though overcoming the tiny amount of resistance in ball bearings was a valid “rescue”, or even relevant, for their naive repetition of the least-efficient type of turbine commonly pursued by beginners too lazy to check into what had already been tried and found lacking. Not to say magnetic bearings will not find eventually a place in wind energy - (who knows?), but as of today it is a typical “buzzword”-based blunder, like “3-D printing” or partially-realistic 3-D computer models that make unworkable configurations seem promising. There is often a big difference between economic energy solutions and stuff that just sounds “cool” or “futuristic” based on neophyte overenthusiasm due to global warming derangement syndrome (GWDS).

Hi Doug, I don’t think that magnetic bearings are massively used in current wind energy, because their rpm is too low, and MW wind turbines are too heavy.

Things can be different for flygen AWES whose generators have to be both light and durable, in spite of the fairly high rpm of generators aloft.

Gas micro turbine generator type has both high lifetime and rpm thanks to magnetic or air bearings which remove frictions. So their technology can be useful for some flygen schemes.

What I was not clear about is whether any gas micro turbine uses magnetic or air bearings today, or is it just on a list of future possibilities?
And would any wind energy system, airborne or otherwise, spin fast enough, and/or last long enough, to merit such exotic, complicated, and heavy bearings?
As usual, I’m looking to see whether any AWE system is realistically viable as a basic source of energy, before such exotic fine-tuning steps are considered, same as the vertical-axis drag-based turbines mentioned. 3-D printing anyone?

Indeed as specified on High Speed Induction Generator for Applications in Aircraft Power Systems on JSTOR

Electric generators have higher power density when the operating speed is increased. High speed electric generators direct coupled to gas turbines provide an ideal source of electric power for airborne applications because of reliable operation and high power density. To function reliably in the speed range of 60000 RPM to 120000 RPM, the rotor of the electric generator must be robust. Examples of the robust rotor technologies for the generator include: permanent magnet (PM), induction and switched reluctance. The objective of the present paper is to describe the current activities in the field of high speed induction generators and associated controllers. A range of induction generators and controllers rated from 5 kW to 200 kW operating at speeds to 62000 RPM is currently under development. The rotor is designed using high strength magnetic materials for the magnetic paths, and high strength alloys for the conductors that form the squirrel cage. Tests have been successfully conducted to demonstrate the high yield strength of the rotor construction materials. High power density of the induction generator designs is demonstrated by the electromagnetic weights of the induction generators: 5 kW weighs 1.7 Lbs.,-30 kW weighs 5 Lbs., and 200 kW weighs 37 Lbs. in electromagnetics.

For what I see on several documents (see numerous links above) the density increases when rpm increases, but up to a certain value because of the heavy bearing.

20 000 rpm or more are useless for flygen AWES like Makani M600 whose rpm is far lower (perhaps not much higher than for current wind turbines); but the more propellers are small the more rpm is high…

And also I think about some other schemes. And high-speed (20 000 rpm and more) generators for aircraft or for gas micro turbines already exist.
Some extracts:

High-speed Motor Operation

Electric machines can be much smaller and have better efficiency when they can operate at a higher speed than the 3,600 rpm limit imposed by 60 Hz power systems.

High Power Density Motor Performance

For applications where light weight is of utmost importance, such as in aerospace and motorsports, Calnetix’s high-speed motors and generators are designed to provide maximum power density while maintaining high operating efficiency and good thermal performance consistent with design requirements. An example of a power dense machine that we have designed and manufactured is rated at 100 kW and weighs 7.7 kg in a 2.3-liter volume, which is equivalent to a machine power density of 13 kW/kg and 43 kW/liter. Calnetix is able to achieve such high-power density due to the following:

  • Optimization of electromagnetic design, including magnetization topologies
  • Optimization of the machine’s L/D (length over diameter) aspect ratio
  • Optimization of the magnetic circuit
  • Selection of materials, including the grade of magnets, the stator laminations and their thickness, the magnet retention sleeve and other materials

Calnetix High Power Density

Optimized Performance by Application

Calnetix’s high-performance motors and generators can be designed along with magnetic bearings and high efficiency variable speed drives (VSD) completely in-house. This enables us to achieve the most compact, optimized solution for each application without compromising any inherit benefits, such as high-speed, high-efficiency operation and zero maintenance requirements. The special construction features and fabrication of our high-performance stators and rotors provide some unique benefits, including:

  • Optimized stator configuration due to utilizing in-house tools, such as fill factor, end-turn bundle and neutral termination techniques
  • High back EMF per magnet volume with high magnetizing efficiency due to the rotor’s ability to magnetically couple to the stator with minimal flux fringing in the axial and transverse directions
  • Minimal rotor harmonic currents due to surface mounting techniques that keep circulating electrical paths isolated without loss of electromagnetic or structural integrity
  • Minimal rotor air gap and air gap cooling requirements due to high strength composite sleeve that has near-zero losses from harmonic eddy currents
  • Advanced electromagnetic efficiency, rotor tip-speeds and industry-leading power densities due to the proprietary technologies in Calnetix’s PM rotors


Calnetix Rotor

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Are you saying that the flygen mode would be more suitable?

Thats the last 1%. Though I am honestly not sure that tether transfer of energy is the best way either, seeing new concepts aiming for 50 MW in a single windmill…