New review article on AWE published

Topic: New review article on AWE published

New 28 page review article on AWE published: “Electricity in the air: Insights from two decades of advanced control research and experimental flight testing of airborne wind energy systems”. Annual Reviews in Control. Access the article for free until 12 June 2021 via this link.


What an incredible article. A positive and insightful tour de force of AWES academic research.
I find it an astonishingly honest account of AWES history from the academic viewpoint.

I’m going to give a big response to

Electricity in the air: Insights from two decades of advanced control research
and experimental flight testing of airborne wind energy systems.
The title tells you what to expect.

I’m going to be quite blunt

The clarity with which the authors present the breadth of work is brilliant.
This is how AWES was laid out in the books. This is a great guide to consult if you need to know the development of most any particular aspect of AWES research.

The progress, breadth and quality of research work into control systems over those two decades has been incredible. It’s a huge and valuable lot of work. Ground-breaking in many cases. With uses applicable in other industries too.

It’s a remarkable account of a huge academic journey. But it leaves me as ever questioning the logic of the overall approach in academic work, how deep these insights were and how broadly the Kite and AWES research communities outside of academia were consulted or observed.

The abstract is inspiring but foretells the conclusion
we start with…

> Because the flight operation of tethered devices can be adjusted to a varying wind resource, the energy availability increases in comparison to conventional wind turbines. Ultimately, this represents a rich topic for the study of real-time optimal control strategies that must function robustly in a spatiotemporally varying environment. With all of the opportunities that airborne wind energy systems bring, however, there are also a host of challenges, particularly those relating to robustness in extreme operating conditions and launching/landing the system (especially in the absence of wind). Thus, airborne wind energy systems can be viewed as a control system designer’s paradise or nightmare, depending on one’s perspective.

And in the Conclusion

Airborne wind energy (AWE) represents a promising technology that has grown over the past decade from a tight cluster of organizations pursuing initial concept designs to a thriving research and development field consisting of over 60 institutions worldwide. Just as the size of the AWE community has grown over the past decade, so has the maturity of both the control architectures used to harvest the wind energy and the prototypes –

We are stuck in the single line single kite control loop.

You can see why AWES research would start from a controls perspective. Control is a very appealing hammer to wield on this problem. We can control planes and kites, we can control wind turbines - Lets control a flying wind turbine.

There is recognition of early alternative concepts especially laddermill and kitegen carousel - even saying

It is worth noting, however, that several multi-kite setups are still under consideration in recent literature

but then with regard to multi-kites

An examination of the aforementioned concepts, along with the timeline along which the concepts were introduced and (where relevant) experimentally prototyped reveals that last decade has seen a gradual transition towards the utilization of crosswind flight, both with airborne and ground-based generation. Due to this trend and the relative dominance of crosswind systems within the AWE control literature, these types of systems will serve as the focus of the technical content in this review article.

We are stuck in a single kite control loop again
with this thing called crosswind flight which none of these single line kites seem capable of

Do we need controls?
Well you would think so when the standard control model of kite dynamics is called
The Unicycle model yet it ignores side slip
For me this is personally very frustrating :rofl: I genuinely fell of my downhill unicycle about 10 times yesterday. Don’t worry it was mostly over heather. I also have a simple electric unicycle. Amazing machine, just needs an IMU. Thing about unicycling is holding a rail, or holding another unicyclists hand makes unicycle easier… There’s a clue there.
Still on the question do we need controls?

It is also shown in van der Vlugt et al. (2019) that for the ideal case of a massless wing that is not actuated, the angle of attack stays constant on any 3D flight trajectory.

Not really if you just fix a broad enough bridle set. Kites can plough on through an extraordinary wind range … just match the tether angle to keep it in an efficient operational mode.

Here’s more evidence toward AWES research laser focus on power.
Not looking at AWES in the whole.

However, in the scientific literature, the contributions related to launch and landing of AWE systems are by far less numerous than those pertaining to crosswind flight control and power generation, both for ground-gen and fly-gen systems. One reason for this gap is that launch and landing phases can be initially carried out by a human pilot, where the system is subsequently switched to autonomous operation. Indeed, launch and landing phases are rather short and carried out at relatively low speeds, so that a pilot can execute them effectively, in contrast with the power generation phase, which requires a continuous, high level of attention to obtain good orbit repeatability and stabilize the flight pattern. For small-scale systems employing a soft kite, one business model even assumes that re-positioning of the kite after landing is eventually carried out by a crew, thus having non-fully-autonomous operation (see Fig. 26).

As a consequence of the aforementioned factors, the study of the automation problem of launch and landing started several years after that of crosswind flight. For the same reason, the design of systems suitable for fully autonomous, repeatable launch and landing has started in relatively recent times, first for fly-gen systems around 2010, and then for ground-gen systems from around 2014 onward.

OK I’ve been very harsh on AWES academia.
I may have upset some.
That this paper is being shared openly is a great sign of the enthusiasm of AWES academia to progress and develop these incredible technologies cooperatively.
We can only welcome that in these urgent times.
I really appreciate the value of this paper as a guide to the copious valuable works it references so clearly.
I’ll give the last word of my review of this paper back to the paper itself.

Given this record of progress, as documented in this survey paper, a bright future exists for the field.

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Rotary AWES do not require a sophisticated control system compared to crosswind AWES and therefore are of little interest as such for scientific work. What are the academic studies for @Kitewinder Kiwee? It just works. That said crosswind AWES concentrate academic interest because they maximize the power / wing area ratio. Other ratios are discussed on this forum, such as power / weight or power / space use. If the latter is taken more into account, rotary AWES (by construction or by…control) will be able to emerge more.

maximize the power / wing area
Hold my beer

Research focused on the power / wing area ratio, rightly or wrongly, rather wrongly in my opinion, since I mention other ratios such as power / space use which would favor the rotary AWES. Crosswind kites have a better power / wing area ratio, but not power / space use ratio, excepted by using low radius loop paths.

I would make an analogy although it is not quite exact: crosswind and rotary kites could correspond respectively to airplanes and helicopters. Airplanes have a better power / wing area ratio, but require a lot more land use for takeoff and landing operations.

In reality, rotary kites, by their constrained flight, maximize the swept area (in addition to facilitate take-off and landing), compensating the relative (and sometimes negligible when the blades are far apart as for Daisy before scaling up) lack of power / wing area ratio.

However the power / weight aloft ratio favors also crosswind kites compared to rotary kites…

Otherwise, this document seems to show a transition towards a theoretical scientific focus by the importance given to work on control (key word for this topic), which could possibly benefit other industrial fields.

To show the progress of AWES in 20 years, it would take at least a chronology of tests showing duration, and power curves.

In Dec2018 the best Daisy Power/Blade area test data was measured slightly over 2kW/m2 from a single rotor test (total blade area 0.6m2 power peak 1.4kW)
The best minute average was about 1.5kW/m2
Airborne mass 2kg for rotor set, transmission & lifter.

Are traction/retraction systems competitive with that kW/m2 or kW/kg either on peak or cycle efficiency?

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I believe Kitemill in its next release will aim for something many multiples of the highest Daisy minute average power (kW).

I am not sure if kW per square meter wing area is such a descriptive number across designs… I think actually any such number is bound to be deceptive as kite rig power output is highly nonlinear and subject to many saturation values (eg. force, power, limitations on reeling speed and accelleration).

In particular, such a metric punishes slow moving larger wings relative to rigid wings. But the scaling and other aspects of these designs are vastly different.

A better basis for comparison would just state:

  • The average power output vs wind (power curve)
  • The cost of build, installation, maintenance
  • Average “hub” height

This is vague enough to cover many different designs and traditional windmills in one and the same system.

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Well said @tallakt
It’s a multifactorial game of top trumps for sure.
Really Looking forward to seeing the new Kitemill rigs !!
Also in big news… W&I have secured some funding for a phase 1 scoping for an automated 10kW system, with plans developing for phase 2 make, test, … Pretty sure this is coincidental timing but our patent application also got published within 24 hours of that news.

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Thanks for factual data.

This is a direct response to the power / wing area ratio in force in all studies which are based on Loyd’s model. One of the explanations for such efficiency is the spacing of the blades combined with a stationary rotary system.

And if we consider the power / land use ratio, this advantage can only increase.

Now there are questions about the potential for scaling, which, combined with the ease of management, could favor soft wings such as @Kitepower or SkySails kites.

There are still questions about the range (height of the system) of torque transfer and its management (take-off, landing), but also some questions for other systems…

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(here returns mr tough crowd)

The first few pages are really nice. The unicycle model has proven useful to me (though is mass and gravity in there? It remains one of the more interesting things to observe in such a model).

When the control section starts I have the feeling that the authors (perhaps the authors of the referenced papers) are somewhat missing the boat. What use are these optimizations when the real world is so nonlinear. For rigid kite AWE at larger scale the issue seems to be just staying below max tether force and power, and at the same time staying afloat during the upstroke. Hopefully limiting the max force and power may be done using plain boring feedback control using a tether force sensor (or estimate) and the elevator. And why would you even not have maximum power available otherwise?

Optimizing the path? that assumes you could actually make use of many paths. If the tether is at its minimum length thats probably optimal, which means you dont get to choose either path or altitude.

And no mention of optimizing tether diameter, length, aspect ratio, mass, lift coefficuent etc. These thibng seem to have a much larger impact to me.

And why no mention of all the interesting stuff that happens when the kite is not flying along a naive path? And how to estimate the wind where you are flying without this being easy to measure?

Seems to me a lot of these control papers is a bit like jazz - fun for the band more than the audience.

Will try to rectify this at the AWEC conference next time :slight_smile: Oh yes, and I am a controls engineer by education

Final point: some of these lists seem like exhaustive lists. Eg still no picture of rotary rigs. Also in the launch/land section some methods are listed, but surely that list does not cover all the best options?



Of course, anyone without institutional access to the article can contact me for a personal copy of the article, which I will gladly share. Expect that my co-authors would do the same.