"Dynamic soaring in wind turbine wakes"

Engineering Flight Control
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I have just seen the first elements of this recent study (June 2023), which appears to be a seminal publication, leading to a possible new main path of AWE. Indeed it could lead to a significant energy generation (by wake revitalization) without the requirement of tethers, generators, motors, additional land use…So a specific topic is needed.

Further studies could be envisaged, covering numerous fields, starting with the safety of unmanned gliders in dynamic soaring, in regard to the proximity to the blades of the wind turbines.

I mentioned this study on (Dynamic) Soaring - #4 by PierreB and AWEC 2024 CfA Ideas Discussion - #6 by PierreB.

The full PDF is available on
https://www.researchgate.net/publication/371620671_Dynamic_Soaring_in_Wind_Turbine_Wakes/comments

Congratulations to the authors:

Also on https://www.sciencedirect.com/science/article/abs/pii/S0947358023000717. Some excerpts of the description are below:

Abstract

Dynamic soaring for UAVs is a flight technique that enables continuous, powerless periodic flight patterns in the presence of a wind gradient. However, sufficiently large wind gradients are uncommon over land, while at offshore locations the largest wind gradients are located close to the ocean surface, thereby limiting the scope of practical application. An intrinsic feature of wind turbines is that they inherently produce very sharp wind gradients in the near wake. Therefore, in this paper, we propose and investigate periodic stationary dynamic soaring trajectories in the near wake of wind turbines. We additionally consider the potential of dynamic soaring for revitalizing the wind turbine wake. To this end, we apply periodic optimal control based on a simplified model for the glider dynamics and the wind profile in the wake. The cost function maximizes the revitalization of the wake. We compute optimal orbits for a range of different wing spans and different mass-scaling assumptions. The largest glider configuration, with a wingspan of 10 m and a mass of 222.6 kg, achieves a wake revitalization of about 0.94% of the total turbine thrust.

Introduction

Modern wind turbines are often clustered together in large farms in order to reduce balance-of-system costs and optimize the use of land or sea concession areas. However, wind turbines generate wakes that impair the efficiency of downstream turbines when spaced too closely together. In modern wind farms, with turbine spacings of 5 to 7 turbine diameters, overall efficiency losses due to wakes can be up to 40% and more, depending on wind direction and atmospheric conditions [2]. In the current work, we propose and investigate the potential of using glider planes to speed up wake recovery, by flying dynamic soaring loops between a wind turbine near wake and the surrounding free stream. […]

Thus, e.g., at the Betz–Joukowsky limit (a =1/3), wake dissipation amounts to approximately 50% of the turbine power production. For a 10 MW turbine, this amounts to 5 MW.

In the near wake, the described wake dissipation power is still available in the velocity difference between wake and free stream, and can thus be harvested by UAVs using dynamic soaring maneuvres [13]. Dynamic soaring is a flight technique that enables continuous, powerless periodic ‘loiter‘ flight patterns in the presence of a wind gradient. These patterns have been computed, analyzed, and experimentally validated in Bencatel et al. [3], Zhao [12], Zhu et al. [13] for typical wind scenarios.

In this work, we propose and investigate periodic stationary dynamic soaring trajectories of a glider in the near wake of wind turbines. This could be applied to achieve indefinite flight times so as to generate a quasi-continuous stream of inspection data or environmental measurements. However, our aim is to investigate how dynamic soaring can additionally be used to transfer momentum from the free stream to the wake, thus enhancing wake recovery. To this end, we apply optimal control based on a simplified glider model to compute periodic flight orbits that maximize wake revitalization and investigate how this objective depends on the glider wing span and mass.

The remainder of this paper is organized as follows. Section 2 introduces a model for the wind profile and the glider plane, while Section 3 states the optimal control problem formulation and solution strategy. Section 4 describes the obtained numerical solutions and discusses the results of a parameter sweep with respect to the glider plane size. Section 5 gives a conclusion. […]

Conclusion

In this paper, we proposed and investigate a novel way to fly a glider plane in dynamic soaring trajectories in the wake behind a wind turbine. We formulated and solved an optimal control problem to maximize the revitalization of the wake. We showed that stationary trajectories exist for an Ampyx AP2 glider, and, after performing a grid search on other possible parameters, we found that with a larger plane, we can revitalize the wake by approximately 0.94% of the turbine thrust. Future work…[…]

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Unless that is a typo, that doesn’t seem that promising.

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I asked the authors:

From the abstract:

“The largest glider configuration, with a wingspan of 10 m and a mass of 222.6 kg, achieves a wake revitalization of about 0.94% of the total turbine thrust.”

And from the introduction:

“Thus, e.g., at the Betz–Joukowsky limit (a =1/3), wake dissipation amounts to approximately 50% of the turbine power production. For a 10 MW turbine, this amounts to 5 MW.”

Would this mean that it would take around a good fifty of these 10 m wingspan gliders to revitalize the “wake dissipation amounts to approximately 50%”?

We would perhaps understand better with the complete PDF, but this value of 0.94%, although disappointing at first glance, would seem to indicate that each of the 50 gliders would prevent to dissipate a part of wake for approximately 100 kW, which is not so bad, and which would correspond to an optimized tethered AWES version for the mentioned 10 m wingspan glider at 10 m/s wind speed. But I could be wrong. There remain safety problems aggravated by the quantity of gliders near the blades.

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I have a doubt about the relation between “a wake revitalization of about 0.94% of the total turbine thrust” and “wake dissipation amounts to approximately 50% of the turbine power production”, because thrust and power are not the same. So I am waiting for their reply, and the PDF.

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I have to say, this idea sounds very farfetched to me. Kind of reminds me of “DaBiri” with his idea for counter-rotating small vertical-axis wind turbines placed below regular windfarms - grasping at straws.

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Hi Doug,

This and Dabiri’s ideas sound great. The difference may lie in economics. The discussed study aims to use regular (not vertical-axis) wind farms and achieve wake revitalization using unmanned gliders in dynamic flight.

If your objections are scientific, technical or economic, please argue your objections.

If your objections are of a different nature, then I think we can do without it. The main reason is that any of these supposed reasons could apply to any AWES we have been discussing for years.

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Hello Pierre:
You are often a great source of information and insights. But this is not one of those times.
I’ve already spent plenty of time debunking the Dabiri idea of placing small, counter-rotating, vertical-axis turbines below windfarm turbines. Here’s one that was down the street from my place, damaged by a strong wind, being dismantled after less than a year of operation.

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Vertical-axis turbines use way more material to deliver less power, and always break down. Try to find a happy owner of a vertical-axis turbine anywhere in the world. Find a vertical-axis turbine that is still running, anywhere in the world.
Vertical-axis advocates are well-known in wind energy to have a mental departure from reality. Not that any wind energy inventor, including me, doesn’t have a few pet vertical-axis ideas, but at the same time, you need to know the landscape of wind energy technology to understand why you don’t see vertical-axis windfarm turbines. one big clue is why Dabiri retreated to small vertical-axis turbine below an existing windfarm rather than promoting a windfarm using full-size vertical-axis turbines. Anyone who understands wind energy realizes the towers place the turbines up into smoother, faster air. The Dabiri idea would be equivalent to raising the ground level, slowing the wind going through the regular turbines above. All in all, (sorry to say) it’s a dumb idea in more than one way - a compound-dumb-idea, just someone without real ideas, nonetheless trying to somehow “fit in” or “make a difference”.
It;s easy to see how someone who doesn’t really understand wind energy could come up with such an idea, but for people who know better, it is easy to see the holes in his theory. Even the company that made the Windspire turbines Dabiri’s effort used, is out of business.

When a windfarm is upgraded, it is called “repowering”. They remove the old, shorter turbines, and install larger turbines. Do you think none of these companies ever thought of just leaving the smaller turbines in place? Of course they would have considered it, then moved on. I doubt if any would have proposed replacing the smaller turbines with vertical-axis turbines. Why? They simply know better. They pay attention.

Just as I was able to cross a 4-lane highway at 5 years old without doing mathemat9ical calculations of the speeds and directions of the cars, but just by common sense, I can say the idea of drones or gliders or whatever flying around a windfarm trying to redirect the wakes of turbines is a ridiculous idea. But for someone who can’t see that, they are also likely to say Dabiri has a great idea.

I think the difference between my expressing a valid opinion and you saying your opinion is that my opinion is wrong, and yours outweighs it, is reflected in the fact that I have been involved in wind energy for my whole life, that I’ve designed and manufactured wind turbines that people have been happy with, that the building I am in is powered by a wind turbine that I refurbished, on a tower right outside, and so this message is wind-powered. I’m sure your message is powered by a utility, like most people.

At some point, just as a musician gets so familiar with their instrument that they can just play what they hear in their mind, without thinking about it very much, I’ve been in wind energy long enough to know good ideas from bad at a glance.

There is only so much energy in the stream of wind going through a windfarm. Gliders soaring between the turbines would use (remove) some of that energy. There are other ways to redirect the wakes of turbines such as slight adjustments of aim.

Just as I said you can scold me about my skepticism over the giant solar-hydrogen-powered airship, once you can show it to me, go ahead and tell me how wrong I was once we have airplanes flying through our windfarms to redirect the wakes of the turbines. Some ideas are so ridiculous they don’t require more than the common sense of a five-year-old to see through. :slight_smile:

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I will try to summarize your reasoning.

  1. J. Dabiri: using vertical axis wind turbines is bad. So the work to optimize wind flow in a vertical axis wind farm is also bad.

  2. As a result, the work of optimizing the wind flow within a wind farm (although not vertical axis wind turbines) according to the present study is also poor.

I think we call this kind of reasoning a sophism.

In this case, you should make incremental improvements to three-blade wind turbines, instead of discussing the questionable concepts of airborne wind turbines for years, only to constantly lament the predictable lack of results.

My point of view is that we must examine the innovations (at least theoretical) as carefully as possible, or else convert to the gradual improvement of wind energy as it exists.

So I am still waiting for your comments on this project, and not on vertical axis wind turbines.

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Pierre, you were the one who said you thought the Dabiri idea of virtually lowering the height of existing wind turbines by placing vertical-axis turbines underneath, effectively raising the ground level to reduce windflow, and flying drone gliders between turbines on existing windfarms to somehow magically eliminate the fact that the first turbines in a series naturally get more energy, were both great ideas. You were the one who essentially equated both dumb ideas by saying they were both great ideas in the same sentence. And they are both dumb ideas for similar reasons. It’s people desperate to find some way to make a difference in wind energy, without actual good ideas, but bad ideas instead, which is not only common, but par for the course in wind energy.

Just as it is easy to debunk vertical-axis turbines on the facts, it’s easy to debunk this other dumb idea.

  1. flying gliders between wind turbines would USE energy to power the gliders. That is less energy left for the turbines.
  2. The gliders would use the exact same materials that could be used to build more wind turbines, which would actually produce more energy.
  3. It would be inevitable that at some point, a glider would contact a wind turbine blade, resulting in a catastrophic failure.
  4. This idea would make a dreaded chain-reaction windfarm failure more possible. Rather than one turbine exploding and the flying pieces hitting other turbines, causing a cascade of exploding turbines, now you would add crashing airplanes into the equation, possibly destroying an entire windfarm due to a single crash.
  5. As usual, idiots sitting behind a desk propose all sorts of ridiculous ideas in wind energy, never knowing how brutal a windfarm-class wind resource actually is. A windfarm is not a good place for aircraft to be flying around. Experienced wind energy people, and pilots for that matter, know this without having to think too hard about it.
  6. The gliders would use valuable resources, cost valuable capital, and require additional windfarm personnel to babysit. The reduction of humans operating windfarms is a major factor in keeping wind energy affordable.
  7. As with wind turbine blades, such aircraft would require periodic replacement, becoming toxic waste after they are worn out.
  8. And, like I said, anyone who has actually been involved with real, working wind energy for many years could tell you this is a loser of an idea at first glance, just as I was able to safely cross a 4-lane highway at age 5 without needing to do engineering calculations.
  9. I have given my opinion. You might have noticed how many times my opinion has turned out to be correct, even when it goes against everything you read in the “press-release-breakthroughs” so prevalent on the internet these days. Someone has to stand up for reality. I have a right to an opinion, and I have a right to express that opinion. In fact, I feel I have an obligation to flag proposed bad ideas and indeed utter stupidity when I see it, in order to hopefully steer people away from wasting more of their time, energy, and capital on ineffective or even destructive ideas. Witho9ut any dissenting voices, you are left in an echo-chamber of nonsense. is that where you want to be, in an echo-chamber of nonsense where nobody has any ability to discern good ideas from bad?
  10. rather than castigating me for expressing my opinion, why don’t you try to prove in some way what a great idea the gliders flying between wind turbines and eventually hitting them and destroying the whole windfarm in a cascade of exploding turbines really is! :slight_smile:
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Also seems quite heavy?

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Isnt it more like VAWT have been proven to be inferior so many times, yet still peple build them like lemmings running off a cliff. Why waste time with the lemmings?

I think if the starting point is poor enough it is fair to dismiss the idea right away. Because there are just to many lemmings.

I am talking about the YAVAWT syndrome here (yet another VAWT), not the paper mentioned initially.

Also mention my personal knowledge of VAWT is pretty small, as is my curiosity on the subject. Regarding the paper, it is not public and I cant be bothered to ask for it. So maybe if it shows up on Scihub later I may look and comment

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Ok, but these arguments are very fragile, because we dont know how kites/gliders affect the overall cost picture

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Your presentation is tendentious. My opinion about Dabiri’s concept was expressed:

It is about wake effect and optimization of a whole wind-farm, not about VAWT as such. Before, perhaps on the old forum, I calculated that these numerous small VAWT were uneconomic, but it is not the question.

It is funny that you praised John Dabiri’s work on:

The publication mentionning John Dabiri, and also your comment, are on your linked publication. I put again this link, and also two quotes (about John Dabiri’ statement and your comment):
https://nawindpower.com/stanford-researchers-boost-wind-productivity-with-wake-steering?utm_medium=email&utm_source=LNH+07-03-2019&utm_campaign=NAW+Latest+News+Headlines

“To meet global targets for renewable energy generation, we need to find ways to generate a lot more energy from existing wind farms,” says John Dabiri

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Selsam

Selsam

4 years ago

This is what I’ve been explaining with regard to the line of rotors in a Selsam SuperTurbine™, offset from the wind direction to bring fresh wind to each rotor. The rotor skew directs the rotor wakes away from downwind rotors.

The wake effect and the optimization of a wind-farm has be discussed on this forum, for example on:

“…this can drop to a little as 1 W/m² in very large installations” could justify the purpose of the currently discussed study.

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The paper (at least the conclusion), refers to Ampyx AP2 glider. The table 1 page 9 indicates some specifications: mass of 36.8 kg, wing span of 5.5 m. By applying the cube law of mass scaling, we achieve a value very close to the given value of 222.6 kg with a wingspan of 10 m. So the estimation looks correct.

The comment could be relevant when my sentence is taken alone. But my complete statement is:

So there are two propositions (1 and 2) which constitute the whole of the reasoning which I attribute to @dougselsam. IMHO 2 cannot result from 1.

Apart from all this, I would like to mention a few points.

An interesting statement from the Introduction seems indicate that both “glider wing span and mass” should be taken into account:

To this end, we apply optimal control based on a simplified glider model to compute periodic flight orbits that maximize wake revitalization and investigate how this objective depends on the glider wing span and mass.

Basically, the only reservation that I would have (while waiting for the PDF and more precise information) and which I have expressed, is the safety problem posed by many gliders flying relatively close to the blades. If this problem proves too important to allow industrial application, the fact remains that the idea, scientifically speaking, is interesting. Other safer solutions could yet be studied, such like a few taller wind turbines, or rather taller wind rotors (without generators) among the wind farm (?).

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Hello Pierre: I stand by my assertion that the Dabiri idea of placing shorter, vertical-axis turbines below windfarms is a bad idea. Thank you for reminding me that he was also involved with the skewed rotor wake-steering announcement. I either forgot he was involved in promoting the wake-steering idea, or never noticed, but it looks like it might actually be an idea from one of his grad students. Nonetheless, my mention of the wake-steering idea was just to point out that it was what I had been saying for well over a decade with regard to SuperTurbine rotors’ ability to get fresh wind even when placed fairly close together, due to all of them being skewed to the wind direction together.

I was not so much deriding Dabiri per se about the vertical-axis turbines, just using his name to identify the concept of placing vertical-axis turbines below an actual windfarm, which is a combination of bad ideas, all rolled into one. As I stated, it is easy to see how an outsider to wind energy could assume the vertical-axis turbines were viable, and not realize that placing turbines below regular turbines would tend to slow the wind going through the windfarm as a whole by raising the apparent ground level.

You know what? I don’t really have time to sit here literally wasting the whole day away responding to every little word you want to try and misinterpret. I know a bad idea when I see one. I know a project that will never happen when I see it. Out of all the goofy ideas and actual projects I’ve debunked, I don’t think you’ll find too many times I haven’t been correct in the fullness of time.

Who immediately declared Magenn a ridiculous idea the moment it came out, as major magazines and even NASA were using the Magenn images to promote the idea of AWE? Who was the ONE PERSON who immediately said it was a dumb idea, taking the least-effective wind turbine category and making it many times as expensive? Me, that’s who. Either nobody else had the analytical skills and wind energy experience to immediately see how bad of an idea it was, or nobody had the guts to say so.

And who had the guts to “tell it like it is” regarding the equally-highly-publicized “Altaeros” project, declaring publicly that it was NOT EVEN RUNNING as all the online and in-print publications were worshipping the concept and saying it was powering X homes in Alaska?. I explained the envelope was obviously too frail to survive a strong wind, and finally even called the local Alaskan newspaper that had first published a story that it would be powering X homes in their remote town, only to find nobody at the newspaper even knew what I was talking about and that there was indeed no such AWE system operating in their little town.

Later when they announced some project with Oman, which I had also been offered, I stated that it also would never happen. (They sent a couple guys over here to let me know who to contact to tell them how many wives I would want when I moved to Oman! All their literature was printed in gold leaf. Pretty impressive. I was flattered, but not ready to pick up and move all the way over there, thinking how many people in Oman would love to live in Southern California)

Altaeros’ concept of the tubular blimp wasted a lot of what could have been enclosed space for the helium, and used more material to make a weaker, heavier envelope. Few others even bothered to properly analyze it. No other “journalist” bothered even to make a simple phone call to Alaska to see if such a project was even actually operating there. Nope, they just repeated yesterday’s false stories as though they had any idea what the heck they were talking about.

Of course people have short memories - don’t we all. I will say, I’m probably a better source than most to give the actual facts on whacky wind energy ideas, because I have been involved with not only novel configurations, but also making real wind turbines survive in a windfarm wind resource for many years.

As I’ve been saying for years, which Roddy repeated in his most recent interview, “the wind is brutal”, and most people have no idea of what it takes for a system to survive. A major factor is keeping things as simple as possible, and also making your system able to dump excess power when the wind gets too strong. Your average armchair inventor has no idea of any of this, being content to build light cardboard and plastic models powered by household fans as their “research”. It’s a good start, but gets nowhere near what it takes to have an economically sensible system that is still even there, let alone still running, after a storm, or even able to survive just good productive winds!

I will place a link to one of my videos from a few years ago where you can see wind turbines surviving a strong wind. Without overspeed protection, you will miss out on your most productive days in fear that your shit will be blown away. :slight_smile:
(2) A Windy Day in Oak Hills, California - YouTube

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You can continue with all the AWE ideas that are talked about or not. I say them all, without exception, and you will be right. If you want, give me any idea about AWE or even AWES prototype, I’ll tell you in a few words why it doesn’t work and probably never will.

You therefore only have two possibilities: launch into three-bladed wind turbines (the only wind turbines that work or will work in the identifiable future), or continue to predict the failure of projects that wind energy professionals ignore.

I encourage you to continue on this second path: you will always be right.

Now, if we come back to our topic (finally!), we see that it mainly includes three-blade wind turbines, that is to say something that has proven itself. The possible breakdown would therefore only concern the dynamic soaring part, in other words the AWE part. This is therefore progress compared to exclusively AWE projects.

Another possibility is trying to discover possibilities, knowing that failure is the expected reward. I think this publication presents something new to consider.

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Hi Pierre: Thanks for an engaging discussion, but I do not feel it is a good use of my time to endlessly discuss at length what I feel are simply bad ideas. :slight_smile:

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I just received the PDF. From what I understand, the calculations did not take a 10 MW wind turbine as a basis, but one with approximately 3 times less power. So a 10 m span glider would prevent to dissipate a part of wake for approximately 30 kW?

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Wind energy for non-wind energy people. Land of the lost.

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Uninteresting comment.