New preprint on large-eddy simulation of airborne wind energy farms

New preprint on Large-eddy simulation of airborne wind energy farms available in open access in Wind Energy Science (WES), the house journal of the European Academy of Wind Energy (EAWE). The interactive peer review is open and we invite interested readers from the scientific community to provide comments. Got to “Discussion” and “Post a Comment” to help the authors improving the quality of the manuscript.

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This article, like the previous one I posted, contains a good number of theoretical predictions supported by a significant number of equations, and also some interesting perspectives, all of this being of an impressive scientific level. And yet, no (informal) comments on the forum. So let’s try.

Abstract. The future utility-scale deployment of airborne wind energy technologies requires the development of large-scale multi-megawatt systems.

It is therefore assumed that a solution is proposed. So let’s take a look.

Abstract. In this study, we consider ground-based power generation pumping-mode AWE systems (lift-mode AWES) and on-board power generation AWE systems (drag-mode AWES).

As usual.

Abstract. For the lift-mode AWES, we additionally investigate different reel-out strategies to reduce the interaction between the tethered wing and its own wake. […] Wake-induced performance losses increase gradually through the downstream rows of systems and reach in the last row of the parks up to 17 % for the lift-mode AWE park and up to 25 % and 45 % for the moderate and dense drag-mode AWE parks, respectively. For an operation period of 60 minutes at a below-rated reference wind speed of 10 m s−1, the lift-mode AWE park generates about 84.4 MW of power, corresponding to 82.5 % of the power yield expected when AWE systems operate ideally and interaction with the ABL is negligible. For the drag-mode AWE parks, the moderate and dense layouts generate about 86.0 MW and 72.9 MW of power, respectively, corresponding to 89.2 % and 75.6 % of the ideal power yield.

These very interesting perspectives are well explained within the paper, above all page 17 with figure 3, page 19 mentioning “the targeted wingspan of 60m” , page 20 mentioning “[…] that for lift-mode AWE systems, the wing mass is reduced by 25% in order to account for the absence of on-board turbines”, and page 23 with figure 7 and mentioning that the “drag-mode AWE system operates at a constant tether length l ≈ 650 m and follows an near-circular flight path of diameter D ≈ 200 m at a mean elevation of about 17 degrees”, all that in order to allow understanding the basis; then until the end to understand the different effects according to the two AWES types (lift and drag) and their respective farms.

My feeling is that, a time more, the efficiency by wing area, the wing being taken alone, then reconsidered within a farm, is maximized, but without consider the price to pay: a 6-7 ton wing flying at 60 m/s with a mean elevation of 17 degrees leads to a low altitude of the tether-wing set preventing any secondary use, preventing ships from passing under, so leading to a very low power-to-space-use ratio, increasing the risk of crashes.

Perhaps it would be desirable to have other premises than the usual AWE drag and lift systems, even if one of them might work. Unless of course to consider the AWE research as a field which must remain purely theoretical.


The data are: “the targeted wingspan of 60m […] tether length l ≈ 650 m […] flight path of diameter D ≈ 200 m […] mean elevation of about 17 degrees”.

So, by my approximations with possible slight errors, the mean height of the top of the tether is about 182 m. The height of the 200 diameter path extends over about 191 m for the tether, + 57 m (wingspan of 60 m, assuming the tether clip is central), so about 248 m. During the path the top of the tether goes down to 86.5 m height, and only 58 m with the (half) wing; then goes up to 286.5 m, 315 m with the wing.

58 m high at the bottom of the blade, this is what we already have with current wind turbines, and without the multitude of risks.

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The analysis shows encouraging progress.

I’m looking forward to when a similar study identifies the potential benefits of stacking.
Like having multiple KiteKRAFT drag mode AEWS on 1 line or multiple Kiteswarm & Ampyx mixed on 1 line. Or maybe since they are real, a Kite Turbine too.

Im worried radius-to-wingspan ratio of 1.67… is that slightly realistic?

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From the abstract:

The aircraft have wingspans of approximately 60 m and fly large loops of approximately 200 m diameter centred at 200 m altitude.

A simple calculator taking account of velocity and bank angle:

At first glance this seems to fit. But if we consider the wing span (60 m), the inner tip would make a loop of only 80 m diameter. Perhaps also take account of the required minimal speed of a heavy rigid 60 m span wing, knowing that the inner tip will fly slower. So it seems to be a very (too?) tight loop.

Makani indicates a roughly doubled diameter for a similar wing : : the circling radius is 265 m, the 5 MW wing span is 65 m, its mass is 9,900 kg.

Apart from that this topic is related to New preprint on large-eddy simulation of airborne wind energy farms.

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“How many angels can dance on the head of a pin?”

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The lift at one tip will be 54% of the lift at the other tip. How to balance that and still retain flightworthiness in other phases.

It could be genious or poor stupid. I dont know. But clearly I am vary here

I think your question is out of the scope of this research. It looks like the authors just copied the specs from other publications, so the question is probably more relevant to those.

I also don’t think it is so interesting which specific architecture they simulated as we can’t look into the future to see which specific architecture is going to fly. I would have chosen something big anyway to see an effect at all and to be able to see the effects of manipulating variables, and to have my research actually be relevant to large scale systems.

It would be a rather important input if you are simulating a farm how much space each unit needs to function…

Since you are discussing this new publication in the journal Wind Energy Science, in which I am the responsible associate editor for AWE-related submissions, a few remarks and suggestions:

The journal uses an open interactive peer-review process. Once the authors submit a first draft of their manuscript this is published immediately as a “discussion paper”. A minimum of 2 referees from the scientific community are then assigned the task to provide a review within a few weeks’ time. In this period of time, also the public can post comments. Once such a comment is posted and is considered relevant by the editor, it will be published immediately and has to be taken into account by the authors when compiling a revision. This is a great way to engage with the authors early on and help them improve the quality of their work. In the end, all comments and replies to comments will be published to provide a completely transparent peer-review process. You can already look into this for the article under discussion (this review has indeed only the two assigned referees).

Here you see a discussion paper in which an additional comment (CC1) was posted next to the assigned reviewers (RC1 and RC2).

Considering the above, I encourage everyone here to check out new AWE-related submissions to Wind Energy Science and actively engage during the peer review process. This interactive process should eventually lead to a faster buildup of knowledge compared to the traditional peer-review process. Also, this will allow feedback from practitioners with much experience but who often do not engage in scientific debates.


Author’s response:

Author’s tracked changes:

Reading these two documents, the very sharp remarks of referees 1 and 2, the answers as well as the chiseled corrections, the very high scientific quality of the publication, its scope, one could think that AWE field is at an advanced stage of development.

Indeed, it cannot be ruled out that, as with the conquest of space, most of the theoretical work precedes most of the achievements. Or…

I quickly scanned the document looking for other ways to reduce eddy effects. I might have missed some:

Can’t we stagger the rows so that kites are not directly behind the the upstream row? This will naturally happen if there are changes in wind direction.

Can’t we vary the operating height of kites in downstream rows or coordinate the rotation of downstream kites so that the position of the kite in the upstream row is opposite the position of the kite in the downstream row?

In the case of ocean wind farms can’t we relocate the anchor points of the individual units so that the pattern of the wind farm can be modified slightly?

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I think it is too soon to do this analysis. Why would you make an AWE farm before you know the parameters and behavior of a single unit? Do the single units even have an option of moving their path due to other kite’s wake? And is the radius-to-wingspan of 0.6 even feasible (for me that would require some explanation of how that could be done aerodynamically, because its extremely tight).

All in all I think the efforts would be better spend on preparing the first single unit, eg investigating the impact on radius-to-wingspan on the flight maneuverability and the effect on aerodynamic efficiency.

Though if the authors are not really trying to build wind power, it seems any topic is as good as another, as long as the method is of academic interest

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I wondered if the diameter has been confused with the radius. By using a radius of 200 m and a diameter of 400 m, we would have a radius-to-wingspan of 0.3, which seems more plausible and more in line with the Makani reference (the circling radius is 265 m, the 5 MW wing span is 65 m), on which the authors rely.

But on reflection I am certainly wrong and 200 m diameter is the right value for the author.

Indeed page 1107:

The drag-mode AWE system operates at a constant tether length l ≈ 650 m and follows a near-circular flight path of diameter D ≈ 200 m at a mean elevation of about 17◦.

17° is a very low value and involves in a very high bank angle (73°?), leading to a potentially tighter loop. That said I wonder how the nominal power of 5 MW (12 m/s wind speed) is achievable in these conditions, if such a small loop is really feasible.

I think it is not too soon to do this analysis because we need to also show the potential of the technology on large scale, to the best of our knowledge today. I write “potential” because it still requires that we solve the challenges that we are facing with smaller units. Without showing this potential we will not be able to get the necessary funding.

Hi @rschmehl ,

Countless scientific publications have already mentioned the almost unlimited potential of airborne wind energy systems for more than 10 years, both in relation to the resource (high altitude wind energy) and in relation to existing technologies (HAWT) in terms of power to mass ratio.

What investors want now are prototypes, even if they are limited to units of about 20 kW, tests, curves, measurements, flight times in operation…

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From what I know, investors want both. Because the “countless publications” were all based on countless assumptions that we more and more have to question. Only with validated wind resource data and validated energy harvesting models, that can also be used to simulate scaled-up AWE systems, we can make meaningful predictions. I must say that I have not seen this yet in literature. Individually, yes, but not in combination. There is still a lot of research to be done.

And that does not mean that we do not need prototypes that reliably work for extended times. We also need those, yet, we must rely here more on the companies because academic research groups can certainly not develop such systems.

The questions here are perfect for the upcoming AWEC in Milan. Please join and ask the experts in the panel discussions. Download the final program (27-05-2021) from here.


I may come across often times as a hardass. In reality I hate to ask the hard questions. But in the end, I have been on a mission for a while now, and that mission is to see if I can make some part in some progress on advancing wind energy. And when a publication like this starts with the parameters 60 m wingspan and 200 m looping diameter, we are either dealing with some secret tech or otherwise some loss of contact with the realities involved. I hate talk of «teorists» vs «practical folks», but this is the kind of stuff that could fuel that fire, because it should be so apparent to even an outsider to AWE to see that this may be a problem.

I am afraid with that starting point, for me its «garbage in, garbage out». I also see some other things that doesn’t make fully sense to me. If this is the case then it does not matter if investors or anyone else wants this. It will not help anyone.

Its also a bit sad that the paper otherwise seems sound (I admit being unable to read all of it after encountering 60/200). Maybe by working on the input parameters, the paper could easily be genuinely useful. But some damage is already done, how could we trust that the authors did not make some other odd assumptions.

Also making things worse is that the paper presents some power density numbers for a windfarm, and obviously those number would not look as good if you selected a wingspan og 60 m and a looping diameter of, say, 400 m.

Still with the caveat that the 60/200 numbers may actually be well thought out.