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Re: [AWES] Re: Betz for Kites: Power Generation Using Tethered Wings
 Hi Sean,Your draft paper is very promising. There is not much to improve on, but a few small points.A seeming gap in the conceptspace you present is that opensource AWE has lately found strategies to improve AWES "Betz" streamtube efficiency over the classic Loyd singlekite paradigm. This work is well documented by Joe Faust and on the AWES Forum, in many posts. Rod Read has many variants modeled in 3D. Even given modest kite L/D ratios, the Betz efficiency of these new airborne architectures is roughly predictable to approach 20%, far more than the sparse models.We identify the relevant streamtube as the rectangle defined by regulatory ceilings (like the provisional FAA 2000" ceiling) and the kitecell landfootprint diameter. The windgradient with altitude is an important added dimension. The resulting rectangle is topologically equivalent to the Betz discassumption.We developed highdensity kite array concepts and methods that maximally fill this frontal airspace. A kite arch that spams the entire space and supports dense WECS arrays is typical of proposed solutions. Houska's multikite summedpower theorem points toward this, but Rod and others want more emphasis of this solution path.Your identification PowertoMass as a a critical metric is quite correct. A note to older readers that this corresponds to legacy "powertoweight" aeronautical usage and associated heuristics.A burning open question in relating powertomass to the extreme cases of singleskin softwings like the NASA Power Wing v. a rigid composite like Ampyx. Softwings may sweep slowly, but are so large, by equivalent mass, in our most probable windvelocity. Here is where a clear conclusion statement based on your data is hoped for.Thank You for a very fine effort in understanding kite energy.dave santosPS Please disregard the comment by a willful nonreader who missed that you correctly find that ordinary kite models have a Betz efficiency so low its almost meaningless :)
 That's a good analogy allright. Actually the point I'm making in the paper is that Betz efficiency is rather meaningless for many of the kitepower systems currently being investigated. Kites can extract wind from such a large area, that they will likely never approach Betz's 59%.
Sean
 In AirborneWindEnergy@yahoogroups.com, "Doug" <dougselsam@...> wrote:
>
> Hi Sean:
> Good to always remember Betz when discussing wind energy system design. Without reading your paper specifically, I would point out that Betz always seems to apply in any wind energy situation. Saying one can overcome Betz, beat Betz, etc. has always turned out to be not true. The air molecules must be left with enough kinetic energy to leave the area, so you can process more molecules, to extract more energy.
>
> In the case of airborne wind energy, nothing changes. Betz is still officiating, still in charge.
>
> I liken a turbine to a movie theater turnstyle: You can only charge a certain portion of a person's income or they will be broke and unable to afford the ticket to push thru the turnstyle. If you try to extract ALL of a person's income for a movie, you will have an audience of zero. There's a balance that the movie theaters have found: they can extract 59% of a person's income to have them sit in a dark room, doing nothing, looking at an empty mirage that is nothingness, for 2 hours. "If you give us 59% of your income we will give you absolutely nothing but a few strategicallyplaced photons in return!  sound good?" (for most people that sounds like a good deal  another topic) Then, like the air molecules wonder where all their kinetic energy went, the movie patron wonders what has gone wrong with their life. Where did all their money go? Why are they treading water? Why does their actual life have no drama so they have to pay for artificial drama? Because they were at the movies!
> To explain Betz you have to put it in language people can understand.
> :O..............
>
>  In AirborneWindEnergy@yahoogroups.com, "seanccostello" <seanccostello@> wrote:
> >
> > Hi,
> > I'm the first author of that paper. Any feedback/discussion is very welcome. Could you be more specific?
> > Sean
> > sean.costello@
> >
> >
> >  In AirborneWindEnergy@yahoogroups.com, "roderickjosephread" <rod.read@> wrote:
> > >
> > > No it certainly does not.
> > > And it raises many more questions than it attempted to answer.
> > >
> > >  In AirborneWindEnergy@yahoogroups.com, "Joe Faust" <joefaust333@> wrote:
> > > >
> > > > Betz for Kites: Power Generation Using Tethered Wings
> > > > <http://infoscience.epfl.ch/record/181236/files/Report0.pdf>
> > > > "This paper describes how to evaluate the maximal power generating
> > > > potential of any kitepower system"
> > > >
> > >
> >
>  Hi Rod,
Thanks for such detailed feedback! This kind of feedback is really very precious.
So I'll try to answer those points onebyone:
QUESTION 1:> in the paper itself , it is stated that...
ANSWER 1:
> The generic highaltitude system we will consider....
> So the kite has to be a singular unit, & do all the lifting and work all
> by itself ? why?
True, my claim is that this paper "describes how to evaluate the maximal power of any kitepower system" is perhaps too strong. The claim I would stand behind is that the theoretical part of the paper (section 4) gives one the mathematical TOOLS to analyse the maximal power of any kitepower system.
Section 5 applies this result to particular cases to get more tangible numbers. These cases are not all encompassing as there are so many diverse ideas out there. I defined the 'generic' kite system such that "the `kite system', may be any combination of wings, attachments, weights or balloons". So this part of the analysis does not hold for wings with turbines on them for example. However, it is easy to apply the theoretical result from section 4 to wings with turbines on them. I've done it myself, I just didn't include it in the paper.
COMMENT 2:> I'm miffed that you didn't show how your method demonstrates power
ANSWER 2:
> available in a transverse driven endless band
> or from a ganged curtain of tacking jib like sails
I will certainly try to, if you give me a precise definition of your system. Do you mean like a Ladermill?
COMMENT 3:> at heights of 10 km it is a significant burden which the wings must
ANSWER 3:
> sustain; this means a largeportion of the systemâs power
> producing potential is used to negate gravity, rather than to generate
> power.That's not dissimilar to ground based turbine mass distribution
> ... but they're still worthwhile products.
I disagree, the wind generally does not exert any upwards force on a wind turbine. A kite must always derive an upwards force from the wind simply to maintain its altitude = deflect wind downwards = put energy into accelerating the air downwards = loss of overall power producing capability.
QUESTION 4:> What if the kites are designed to rotate? Surely we can consider that.
ANWER 4:
> Do they absolutely have to always work in their optimal condition only?
No, they don't, but you should use the kite's maximum L/D in this analysis as it only gives you an UPPER bound. If they spend time operating at lower L/Ds you will generate less power than the upper bound. I know, an upper bound is relatively meaningless if you're nowhere near it. It doesn't tell you what you WILL produce, but at least you know where the ceiling is in terms of power generation..
QUESTION 5:> In one line dragging groundgen configuration. Why not design the force
ANSWER 5:
> takeoff to come from the direction the kite is going instead?
sorry, don't follow you there.
COMMENT 6 + ANSWER 6:
Certainly, the restraining force on the kite need not always be from tethers. Again, I only assume this in the 'application' section 5.
Like I said, if you have a particular architecture, give me a detailed spec of it and I will try to apply my theory to it and give you a maximum power generating capacity.
Sean
 In AirborneWindEnergy@yahoogroups.com, "roderickjosephread" <rod.read@...> wrote:
>
> Hi Sean,
> Thanks for being welcoming of critique.
> I hope I'm competent enough to reply.
> And please accept my sincerest thanks for producing what is an excellent
> work.
>
> My specific issue was the statement
> "This paper describes how to evaluate the maximal power generating
> potential of any kitepower system"Whereas in the paper itself , it is
> stated that...
>
>
> The generic highaltitude system we will consider....
>
>
> So the kite has to be a singular unit, & do all the lifting and work all
> by itself ? why?
>
> I'm miffed that you didn't show how your method demonstrates power
> available in a transverse driven endless band
> or from a ganged curtain of tacking jib like sails (I had a wayfarer
> dingy planning just on a jib yesterday, massively windy day out)
>
> at heights of 10 km it is a significant burden which the wings must
> sustain; this means a largeportion of the systemâs power
> producing potential is used to negate gravity, rather than to generate
> power.That's not dissimilar to ground based turbine mass distribution
> ... but they're still worthwhile products.
>
>
>
> Kites are wings (just like the blades of a turbine, the wings of an
>
> airplane or the sails of a yacht) which are connected to the ground
> using a system of cables.
>
>
> really different to most turbine blade profiles, but ok
>
>
>
>
> Wings are generally designed to operate most efficiently (achieve the
>
> greatest lift to drag ratio) for zero sideslip, i.e.Î²= 0.
>
> What if the kites are designed to rotate? Surely we can consider that.
>
> Do they absolutely have to always work in their optimal condition only?
>
>
> Iâd be surprised if 1 ever has except ~nearly a standard turbine.
>
>
>
> If we could design a configuration such that this force is aligned with
> the wind
>
>
> it would be easy to operate the wing optimally at all times. The only
> way to
>
> exert a force in line with the wind would be if the wing was flying just
>
>
> above the ground.
>
>
> In one line dragging groundgen configuration. Why not design the force
> takeoff to come from the direction the kite is going instead?
>
>
>
>
>
> But the whole point of kite power is to harness winds at
>
>
> altitudes of at least several hundred meters.
>
>
> Not when theyâre made for generation itâs not. The whole
> point is cheaper generation. Yes efficiency drives everything but ROI
> advantage is ROI advantage.
>
>
>
>
> Is that max power zone diagram starting to resemble a spinning cone?
> Yes.
>
>
>
> This is because in general the direction of the restraining forces
> (transmitted using tethers)
>
> Does it always have to be...? NO
>
>
>
>
> The results also highlight the importance of operating the wings and
> tether at full load.
>
> Loads of well filled sails, networked to be controllable will do me just
> fine thanks.
>
>
> Being such an open source fan boy, my other issue was that;
>
> A confidential document put itâs assumptions at the end.
>
> Letâs be up front with our weaknesses instead and build on the
> meagre scrapings of emerging potential instead.
>
>
> I'm maybe not smart enough to get your approach yet ...
> But it just seems to be missing a dimension
>  Hi Dave,
Thanks for taking the time to go through the draft paper. As I said to Rod, critical feedback from people who read the paper is very much appreciated.
It's true that the conceptspace will need an update for the final version of the paper (the current version is a draft, written one year ago).
I looked for Joe Faust's posts on Betz efficiencies for, I searched the for the keyword Betz on the forum, but only found one topic. Perhaps you could point me towards some relevant keywords.
I am certainly not suggesting in this paper that kite's have low Betz efficiencies, and are therefore 'worse' than wind turbines. Rather, for the solutions I currently see under development by startups, Betz efficiencies are pretty meaningless. That is why we tried to develop an alternative efficiency metric in this paper.
You would like a clearcut conclusion as to whether rigid wings (heavy, high L/D) are better than soft wings (light, low L/D). I can only answer part of that question, because the answer also depends on economic factors I know little about.
In method we outline for analyzing a (certain class of) kite system's maximum power producing factor is the following:
1. Determine the kite's P_max (this is proportional to the kite's area, its max L/D squared and the windspeed cubed)
2. Determine 3 reduction factors:
a) Due to the kite's strength/weight ratio (i.e. max aerodynamic force the kite can
withstand/weight)
b) Due to the tether sag (depends on the kite's operating height and the tether material
properties)
c) Due to the ground tether angle
Now let's assume that for the same cost you can build a very big flexible kite, and a small rigid kite, such that they both have the same P_max. You are flying both wings using tethers made with the same material, at the same height. Both kites are trimmed to operate at their maximum L/D, and the wind is such that the maximum wing loading is achieved.
Now let's look at the reduction factors for each wing:
b) and c) will be the same for both wings.
In general a) will be worse for the rigid wing, because as L/D increases, maximum wing loading/wing weight tends to decrease. (about 50 for a surf kite, 16 for the Ampyx rigid wing).
So all other factors being equal (launching difficulties, cost of manufacture, etc..), the wing's strength/weight ratio is key. And if you have to keep your kite in the air in suboptimal winds (i.e. where the wing is not operating at full loading), then the reduction factor a) is likely to completely kill your power generating capacity if you are using a rigid wing, whereas the lighter flexible wing will be much less affected.
I hope that answers your question. Bear in mind that this paper is only part (but an important part I think) of a jigsaw puzzle that an entrepreneur would need to assemble if he really wanted to compare different kite power concepts from an economic point of view.
Best Regards,
Sean Costello
sean.costello@...
 In AirborneWindEnergy@yahoogroups.com, dave santos <santos137@...> wrote:
>
>
>
> Hi Sean,
>
> Your draft paper is very promising. There is not much to improve on, but a few small points.
>
> A seeming gap in the conceptspace you present is that opensource AWE has lately found strategies to improve AWES "Betz" streamtube efficiency over the classic Loyd singlekite paradigm. This work is well documented by Joe Faust and on the AWES Forum, in many posts. Rod Read has many variants modeled in 3D. Even given modest kite L/D ratios, the Betz efficiency of these new airborne architectures is roughly predictable to approach 20%, far more than the sparse models.
>
> We identify the relevant streamtube as the rectangle defined by regulatory ceilings (like the provisional FAA 2000" ceiling) and the kitecell landfootprint diameter. The windgradient with altitude is an important added dimension. The resulting rectangle is topologically equivalent to the Betz discassumption.
>
> We developed highdensity kite array concepts and methods that maximally fill this frontal airspace. A kite arch that spams the entire space and supports dense WECS arrays is typical of proposed solutions. Houska's multikite summedpower theorem points toward this, but Rod and others want more emphasis of this solution path.
>
> Your identification PowertoMass as a a critical metric is quite correct. A note to older readers that this corresponds to legacy "powertoweight" aeronautical usage and associated heuristics.
>
> A burning open question in relating powertomass to the extreme cases of singleskin softwings like the NASA Power Wing v. a rigid composite like Ampyx. Softwings may sweep slowly, but are so large, by equivalent mass, in our most probable windvelocity. Here is where a clear conclusion statement based on your data is hoped for.
>
> Thank You for a very fine effort in understanding kite energy.
>
> dave santos
>
> PS Please disregard the comment by a willful nonreader who missed that you correctly find that ordinary kite models have a Betz efficiency so low its almost meaningless :)
>  "So all other factors being equal (launching difficulties, cost of manufacture, etc..), the wing's strength/weight ratio is key. And if you have to keep your kite in the air in suboptimal winds (i.e. where the wing is not operating at full loading), then the reduction factor a) is likely to completely kill your power generating capacity if you are using a rigid wing, whereas the lighter flexible wing will be much less affected....
I hope that answers your question. Bear in mind that this paper is only part (but an important part I think) of a jigsaw puzzle that an entrepreneur would need to assemble if he really wanted to compare different kite power concepts from an economic point of view "
 Sean Costello
Thanks, Sean.
John Oyebanji
Hardensoft International Ltd
NigeriaSent from my BlackBerry wireless device from MTNFrom: "seanccostello" <seanccostello@...>Sender: AirborneWindEnergy@yahoogroups.comDate: Tue, 23 Jul 2013 10:49:17 0000To: <AirborneWindEnergy@yahoogroups.com>ReplyTo: AirborneWindEnergy@yahoogroups.comSubject: [AWES] Re: Betz for Kites: Power Generation Using Tethered WingsHi Dave,
Thanks for taking the time to go through the draft paper. As I said to Rod, critical feedback from people who read the paper is very much appreciated.
It's true that the conceptspace will need an update for the final version of the paper (the current version is a draft, written one year ago).
I looked for Joe Faust's posts on Betz efficiencies for, I searched the for the keyword Betz on the forum, but only found one topic. Perhaps you could point me towards some relevant keywords.
I am certainly not suggesting in this paper that kite's have low Betz efficiencies, and are therefore 'worse' than wind turbines. Rather, for the solutions I currently see under development by startups, Betz efficiencies are pretty meaningless. That is why we tried to develop an alternative efficiency metric in this paper.
You would like a clearcut conclusion as to whether rigid wings (heavy, high L/D) are better than soft wings (light, low L/D). I can only answer part of that question, because the answer also depends on economic factors I know little about.
In method we outline for analyzing a (certain class of) kite system's maximum power producing factor is the following:
1. Determine the kite's P_max (this is proportional to the kite's area, its max L/D squared and the windspeed cubed)
2. Determine 3 reduction factors:
a) Due to the kite's strength/weight ratio (i.e. max aerodynamic force the kite can
withstand/weight)
b) Due to the tether sag (depends on the kite's operating height and the tether material
properties)
c) Due to the ground tether angle
Now let's assume that for the same cost you can build a very big flexible kite, and a small rigid kite, such that they both have the same P_max. You are flying both wings using tethers made with the same material, at the same height. Both kites are trimmed to operate at their maximum L/D, and the wind is such that the maximum wing loading is achieved.
Now let's look at the reduction factors for each wing:
b) and c) will be the same for both wings.
In general a) will be worse for the rigid wing, because as L/D increases, maximum wing loading/wing weight tends to decrease. (about 50 for a surf kite, 16 for the Ampyx rigid wing).
So all other factors being equal (launching difficulties, cost of manufacture, etc..), the wing's strength/weight ratio is key. And if you have to keep your kite in the air in suboptimal winds (i.e. where the wing is not operating at full loading), then the reduction factor a) is likely to completely kill your power generating capacity if you are using a rigid wing, whereas the lighter flexible wing will be much less affected.
I hope that answers your question. Bear in mind that this paper is only part (but an important part I think) of a jigsaw puzzle that an entrepreneur would need to assemble if he really wanted to compare different kite power concepts from an economic point of view.
Best Regards,
Sean Costello
sean.costello@...
 In AirborneWindEnergy@yahoogroups.com, dave santos <santos137@...> wrote:
>
>
>
> Hi Sean,
>
> Your draft paper is very promising. There is not much to improve on, but a few small points.
>
> A seeming gap in the conceptspace you present is that opensource AWE has lately found strategies to improve AWES "Betz" streamtube efficiency over the classic Loyd singlekite paradigm. This work is well documented by Joe Faust and on the AWES Forum, in many posts. Rod Read has many variants modeled in 3D. Even given modest kite L/D ratios, the Betz efficiency of these new airborne architectures is roughly predictable to approach 20%, far more than the sparse models.
>
> We identify the relevant streamtube as the rectangle defined by regulatory ceilings (like the provisional FAA 2000" ceiling) and the kitecell landfootprint diameter. The windgradient with altitude is an important added dimension. The resulting rectangle is topologically equivalent to the Betz discassumption.
>
> We developed highdensity kite array concepts and methods that maximally fill this frontal airspace. A kite arch that spams the entire space and supports dense WECS arrays is typical of proposed solutions. Houska's multikite summedpower theorem points toward this, but Rod and others want more emphasis of this solution path.
>
> Your identification PowertoMass as a a critical metric is quite correct. A note to older readers that this corresponds to legacy "powertoweight" aeronautical usage and associated heuristics.
>
> A burning open question in relating powertomass to the extreme cases of singleskin softwings like the NASA Power Wing v. a rigid composite like Ampyx. Softwings may sweep slowly, but are so large, by equivalent mass, in our most probable windvelocity. Here is where a clear conclusion statement based on your data is hoped for.
>
> Thank You for a very fine effort in understanding kite energy.
>
> dave santos
>
> PS Please disregard the comment by a willful nonreader who missed that you correctly find that ordinary kite models have a Betz efficiency so low its almost meaningless :)
>
 On 20130723 12:49, seanccostello wrote:
> I searched the for the keyword Betz on the forum, but only found one
Hi Sean,
> topic. Perhaps you could point me towards some relevant keywords.
please, type in your browser "Betz limit"
You will receive about 20 000 hits
The following link contains a very nice fleshsimulation, a lot of links
and a clear explanation.
http://www.flapturbine.com/betz_limit.html
By the way, it schould be called as "LanchesterZhukowskyBetz limit",
since these three researcher discovered this law, independently.
See: http://en.wikipedia.org/wiki/Betz_limit
Regards,
Gabor  Thanks again SeanAssuming I get enough childcare organised I can post a new design concept on Mothra arch kite configuration by tomorrow.Check out some of my youtube stuff for a glance of where I'm trying to go with this...My point on q3 was the large part of the mass of a standard turbine used to "lift" the turbine is still essential to their useful operation. We can consider many kiteforms other than just arches and webs capable of the same lift only function.Large arch kites and webs of kites can be set to collectively give an overall drive vector.Asymmetric kites can be conjoined to induce group spinning around a distance from an axis...If I may continue / elaborate...Sean,What can I say but, thank you so much for your well considered answers.
This kind of discussion is vital for overall progress.
My Q1, Comment 3, Q4 and Q5 are all trying to get at the same things.ANSWER 3:QUESTION 1:ANSWER 1:> So the kite has to be a singular unit, & do all the lifting and work all
> by itself ? why?
True, my claim is that this paper "describes how to evaluate the maximal power of any kitepower system" is perhaps too strong. The claim I would stand behind is that the theoretical part of the paper (section 4) gives one the mathematical TOOLS to analyse the maximal power of any kitepower system.
Section 5 applies this result to particular cases to get more tangible numbers. These cases are not all encompassing as there are so many diverse ideas out there. I defined the 'generic' kite system such that "the `kite system', may be any combination of wings, attachments, weights or balloons". So this part of the analysis does not hold for wings with turbines on them for example. However, it is easy to apply the theoretical result from section 4 to wings with turbines on them. I've done it myself, I just didn't include it in the paper.
COMMENT 2:ANSWER 2:> I'm miffed that you didn't show how your method demonstrates power
> available in a transverse driven endless band
> or from a ganged curtain of tacking jib like sails
I will certainly try to, if you give me a precise definition of your system. Do you mean like a Ladermill?
COMMENT 3:> at heights of 10 km it is a significant burden which the wings must
> sustain; this means a largeportion of the systemâ€™s power
> producing potential is used to negate gravity, rather than to generate> power.That's not dissimilar to ground based turbine mass distribution
> ... but they're still worthwhile products.
I disagree, the wind generally does not exert any upwards force on a wind turbine. A kite must always derive an upwards force from the wind simply to maintain its altitude = deflect wind downwards = put energy into accelerating the air downwards = loss of overall power producing capability.
QUESTION 4:ANWER 4:> What if the kites are designed to rotate? Surely we can consider that.
> Do they absolutely have to always work in their optimal condition only?
No, they don't, but you should use the kite's maximum L/D in this analysis as it only gives you an UPPER bound. If they spend time operating at lower L/Ds you will generate less power than the upper bound. I know, an upper bound is relatively meaningless if you're nowhere near it. It doesn't tell you what you WILL produce, but at least you know where the ceiling is in terms of power generation..
QUESTION 5:ANSWER 5:> In one line dragging groundgen configuration. Why not design the force
> takeoff to come from the direction the kite is going instead?
sorry, don't follow you there.
COMMENT 6 + ANSWER 6:
Certainly, the restraining force on the kite need not always be from tethers. Again, I only assume this in the 'application' section 5.
Like I said, if you have a particular architecture, give me a detailed spec of it and I will try to apply my theory to it and give you a maximum power generating capacity.
SeanRod Read
Windswept and Interesting Limited
15a Aiginis
Isle of Lewis
HS2 0PB
07899057227
01851 870878  I don't see what is new here.
The notion that most kite systems or AWE systems, as currently envisioned, would be unlikely to maximize the Betz power available in a given area has been discussed years ago on this group. Once the basic beginner concept of the Betz coefficient was introduced to this group, the notion of currentlyenvisioned AWE systems not reaching Betz was one of the first observations/predictions FROM this group.
I think even Dave S. was able to grasp that and was onboard with the discussion. We already know you can't "beat" Betz. Is the idea here to point out once again that currentlyenvisioned AWE systems will be unlikely to match the nearlyperfect area sweep by a fixedposition rotor? Sorry but I'm trying to understand what new information is offered.
:)
 In AirborneWindEnergy@yahoogroups.com, Rod Read <rod.read@...> wrote:
>
> Sean,
> What can I say but, thank you so much for your well considered answers.
> This kind of discussion is vital for overall progress.
>
> If I may continue / elaborate...
> My Q1, Comment 3, Q4 and Q5 are all trying to get at the same things.
>
> Asymmetric kites can be conjoined to induce group spinning around a
> distance from an axis...
>
> Large arch kites and webs of kites can be set to collectively give an
> overall drive vector.
>
> My point on q3 was the large part of the mass of a standard turbine used to
> "lift" the turbine is still essential to their useful operation. We can
> consider many kiteforms other than just arches and webs capable of the same
> lift only function.
>
> Check out some of my youtube stuff for a glance of where I'm trying to go
> with this...
> Assuming I get enough childcare organised I can post a new design concept
> on Mothra arch kite configuration by tomorrow.
>
> Thanks again Sean
>
>
>
> QUESTION 1:
> > So the kite has to be a singular unit, & do all the lifting and work all
> > by itself ? why?
>
> ANSWER 1:
> True, my claim is that this paper "describes how to evaluate the maximal
> power of any kitepower system" is perhaps too strong. The claim I would
> stand behind is that the theoretical part of the paper (section 4) gives
> one the mathematical TOOLS to analyse the maximal power of any kitepower
> system.
> Section 5 applies this result to particular cases to get more tangible
> numbers. These cases are not all encompassing as there are so many diverse
> ideas out there. I defined the 'generic' kite system such that "the `kite
> system', may be any combination of wings, attachments, weights or
> balloons". So this part of the analysis does not hold for wings with
> turbines on them for example. However, it is easy to apply the theoretical
> result from section 4 to wings with turbines on them. I've done it myself,
> I just didn't include it in the paper.
>
> COMMENT 2:
>
> > I'm miffed that you didn't show how your method demonstrates power
> > available in a transverse driven endless band
> > or from a ganged curtain of tacking jib like sails
>
> ANSWER 2:
> I will certainly try to, if you give me a precise definition of your
> system. Do you mean like a Ladermill?
>
> COMMENT 3:
>
> > at heights of 10 km it is a significant burden which the wings must
> > sustain; this means a largeportion of the systemâs power
>
> > producing potential is used to negate gravity, rather than to generate
> > power.That's not dissimilar to ground based turbine mass distribution
>
> > ... but they're still worthwhile products.
>
> ANSWER 3:
> I disagree, the wind generally does not exert any upwards force on a wind
> turbine. A kite must always derive an upwards force from the wind simply to
> maintain its altitude = deflect wind downwards = put energy into
> accelerating the air downwards = loss of overall power producing capability.
>
> QUESTION 4:
>
> > What if the kites are designed to rotate? Surely we can consider that.
> > Do they absolutely have to always work in their optimal condition only?
>
> ANWER 4:
> No, they don't, but you should use the kite's maximum L/D in this analysis
> as it only gives you an UPPER bound. If they spend time operating at lower
> L/Ds you will generate less power than the upper bound. I know, an upper
> bound is relatively meaningless if you're nowhere near it. It doesn't tell
> you what you WILL produce, but at least you know where the ceiling is in
> terms of power generation..
>
> QUESTION 5:
>
> > In one line dragging groundgen configuration. Why not design the force
> > takeoff to come from the direction the kite is going instead?
>
> ANSWER 5:
> sorry, don't follow you there.
>
> COMMENT 6 + ANSWER 6:
> Certainly, the restraining force on the kite need not always be from
> tethers. Again, I only assume this in the 'application' section 5.
>
> Like I said, if you have a particular architecture, give me a detailed spec
> of it and I will try to apply my theory to it and give you a maximum power
> generating capacity.
> Sean
>
>
> Rod Read
>
> Windswept and Interesting Limited
> 15a Aiginis
> Isle of Lewis
> HS2 0PB
>
> 07899057227
> 01851 870878
>
> http://kitepowercoop.org
>  Doug wrote "I don't see what is new here."What would be new to you is to finally see AWE as serious aviation, not just an adjunct to conventional windpower. Advanced AWES Forum aviation thinking recasts the old "Betz discassumption" to the rectangular frontal airspace streamtube being newlydefined by earlyphase FAA restrictions, as our formally available resource.As usual, the expertlevel Forum is first to explore such new conceptual angles (albeit mixed with tedious "beginner" wind lessons). These new ideas seem too often to occur in blindspots of your understanding. If you review all related Forum discussion, it does duly contain the "Betz Limit" truism, but mostly as your irrelevant idee fixe. Reread carefully the broader discussion that consistently corrects such simplistic discourse (even rejecting the "Betz" label for a physical limit discovered by several scientists, as Gabor rightly notes in reminder).The true AWES Forum understanding is to refer to "Streamtube Efficiency" as the basic measure of total kite thermodynamic efficiency. This is why Sean's search on "Betz" in the forum did not naturally lead to our best thinking about this topic. He could perhaps substitute Streamtube usage for Betz in the title and general discussion of his paper.
 Hi Gabor,
I am of course aware of the Betz limit, I was referring to postings regarding kites and the Betz limit on the AWES forum that Dave Santos pointed me to.
Sean
 In AirborneWindEnergy@yahoogroups.com, Gabor Dobos <dobosg001@...> wrote:
>
> On 20130723 12:49, seanccostello wrote:
> > I searched the for the keyword Betz on the forum, but only found one
> > topic. Perhaps you could point me towards some relevant keywords.
> Hi Sean,
> please, type in your browser "Betz limit"
> You will receive about 20 000 hits
>
> The following link contains a very nice fleshsimulation, a lot of links
> and a clear explanation.
>
> http://www.flapturbine.com/betz_limit.html
>
> By the way, it schould be called as "LanchesterZhukowskyBetz limit",
> since these three researcher discovered this law, independently.
>
> See: http://en.wikipedia.org/wiki/Betz_limit
>
> Regards,
>
> Gabor
>  Hi Rod,
I took a look at some videos of your 'Mothra' arches, very interesting indeed.
If I understand correctly your system distinguishes between aerodynamic structures used to provide a constant, nearvertical force, and wings that fly crosswind.
Our analysis can be applied quite easily in this case (if you want to locate the relevant part of the paper it's at the end of Section 4). Let's say you have two wings (or any structure that generates lift and drag): Wing 1 for vertical lifting and Wing 2 for crosswind. They have area and glide ratio A1, GR1 and A2, GR2 respectively. The upper bound for the power that can be generated by the two wings is (note, I've very slightly simplified the expressions here):
P < (0.5*rho*wind_speed^3)*(4/27)*(A1*GR1^2 + A2*GR2^2)
The key here is that each wing contributes an amount proportional to A*GR^2 to the overall maximum power generating capacity. The bottom line is that if wing 1 has a low GR (= L/D) relative to wing 2, it is not going to help you generate much more power than if you just had wing 2 flying alone. Let's take an example where you have a lifting wing that does not perform well aerodynamically (I imagine that the lifting wing must have a reasonably low L/D to remain stable) compared to the crosswind wing:
Wing 1 (vertical lifting wing): GR1 = 3, A1 = 10m^2 > A1*GR1^2 = 90m^2
Wing 2 (crosswind wing): GR2 = 7, A2 = 10m^2 >A2*GR2^2 = 490m^2
So in this case you might be better forgetting about Wing 1 and just fly Wing 2 alone. However, if you can produce a very large area of Wing 1 extremely cheaply, then the scheme starts to make sense.
Can you estimate L/D ratios and areas for the different components in your system?
Best Regards,
Sean
 In AirborneWindEnergy@yahoogroups.com, Rod Read <rod.read@...> wrote:
>
> Sean,
> What can I say but, thank you so much for your well considered answers.
> This kind of discussion is vital for overall progress.
>
> If I may continue / elaborate...
> My Q1, Comment 3, Q4 and Q5 are all trying to get at the same things.
>
> Asymmetric kites can be conjoined to induce group spinning around a
> distance from an axis...
>
> Large arch kites and webs of kites can be set to collectively give an
> overall drive vector.
>
> My point on q3 was the large part of the mass of a standard turbine used to
> "lift" the turbine is still essential to their useful operation. We can
> consider many kiteforms other than just arches and webs capable of the same
> lift only function.
>
> Check out some of my youtube stuff for a glance of where I'm trying to go
> with this...
> Assuming I get enough childcare organised I can post a new design concept
> on Mothra arch kite configuration by tomorrow.
>
> Thanks again Sean
>
>
>
> QUESTION 1:
> > So the kite has to be a singular unit, & do all the lifting and work all
> > by itself ? why?
>
> ANSWER 1:
> True, my claim is that this paper "describes how to evaluate the maximal
> power of any kitepower system" is perhaps too strong. The claim I would
> stand behind is that the theoretical part of the paper (section 4) gives
> one the mathematical TOOLS to analyse the maximal power of any kitepower
> system.
> Section 5 applies this result to particular cases to get more tangible
> numbers. These cases are not all encompassing as there are so many diverse
> ideas out there. I defined the 'generic' kite system such that "the `kite
> system', may be any combination of wings, attachments, weights or
> balloons". So this part of the analysis does not hold for wings with
> turbines on them for example. However, it is easy to apply the theoretical
> result from section 4 to wings with turbines on them. I've done it myself,
> I just didn't include it in the paper.
>
> COMMENT 2:
>
> > I'm miffed that you didn't show how your method demonstrates power
> > available in a transverse driven endless band
> > or from a ganged curtain of tacking jib like sails
>
> ANSWER 2:
> I will certainly try to, if you give me a precise definition of your
> system. Do you mean like a Ladermill?
>
> COMMENT 3:
>
> > at heights of 10 km it is a significant burden which the wings must
> > sustain; this means a largeportion of the systemâs power
>
> > producing potential is used to negate gravity, rather than to generate
> > power.That's not dissimilar to ground based turbine mass distribution
>
> > ... but they're still worthwhile products.
>
> ANSWER 3:
> I disagree, the wind generally does not exert any upwards force on a wind
> turbine. A kite must always derive an upwards force from the wind simply to
> maintain its altitude = deflect wind downwards = put energy into
> accelerating the air downwards = loss of overall power producing capability.
>
> QUESTION 4:
>
> > What if the kites are designed to rotate? Surely we can consider that.
> > Do they absolutely have to always work in their optimal condition only?
>
> ANWER 4:
> No, they don't, but you should use the kite's maximum L/D in this analysis
> as it only gives you an UPPER bound. If they spend time operating at lower
> L/Ds you will generate less power than the upper bound. I know, an upper
> bound is relatively meaningless if you're nowhere near it. It doesn't tell
> you what you WILL produce, but at least you know where the ceiling is in
> terms of power generation..
>
> QUESTION 5:
>
> > In one line dragging groundgen configuration. Why not design the force
> > takeoff to come from the direction the kite is going instead?
>
> ANSWER 5:
> sorry, don't follow you there.
>
> COMMENT 6 + ANSWER 6:
> Certainly, the restraining force on the kite need not always be from
> tethers. Again, I only assume this in the 'application' section 5.
>
> Like I said, if you have a particular architecture, give me a detailed spec
> of it and I will try to apply my theory to it and give you a maximum power
> generating capacity.
> Sean
>
>
> Rod Read
>
> Windswept and Interesting Limited
> 15a Aiginis
> Isle of Lewis
> HS2 0PB
>
> 07899057227
> 01851 870878
>
> http://kitepowercoop.org
>  Sean,You should find that opensource AWES densearray methods do far exceed the common singlekite paradigm in potential streamtube efficiency, and that there are many other LCOE advantages as well. As a simple membrane with rope loadpaths, Mothra Arch wing area costs less than 1% of the cost of highperformance composite wing area. This sort of nonsweeping wing used for lift and control enjoys enhanced L/D by operating at lower wingloading, at a lower flow velocity. The arch also enjoys a wingingroundeffect boost. Of course, a fullyloaded hot wing has a lowered L/D; unloaded L/D is hardly a correct "working" value.Mothraarches support high L/D WECS in dense arrays by physically constraining them like a team of draft animals. The arch itself is constrained from yaw/roll instability by its double anchorpoint (structural stability). There is always an energetic cost to control (starting with Information Theory), and passive control like a Mothra offers is equatable to the active control demands of a single wing sweeping patterns.Perhaps I missed your notice of the parasitism of the control power demand (sensing, computing, actuation), but your power calculations must not omit this energetic cost to control, which can be considerable in the case of dynamically unstable kiteplanes forced to fly an aerobatic pattern. Passive methods have been found for dynamic stability, and the Low Complexity design approach may not just be energetically superior, but far simpler and cheaper.We keep in mind that LCOE is the ultimate predictor of AWES success, and that a focus on raw kite power cannot necessarily predict the LCOE winner. Thanks for accounting for these key ideas in your worthy studies,daveS
 Dave S.:
How can you take such a simple concept and try to make it so complicated. The Betz concept is a simple one and as real wind energy discussions have always agreed, the type of wind turbine or shape of the blade path is irrelevant. Interesting that you cannot control your urge to argue with me, but you have nothing to say. Why use so many words to say nothing? Why not just sit back and relax and stop arguing with every fact I bring to the table? Didn't anyone ever tell you, you can't argue with proven facts?
:)
Doug S.
 In AirborneWindEnergy@yahoogroups.com, dave santos <santos137@...> wrote:
>
> Doug wrote "I don't see what is new here."
>
>
> What would be new to you is to finally see AWE as serious aviation, not just an adjunct to conventional windpower. Advanced AWES Forum aviation thinking recasts the old "Betz discassumption" to the rectangular frontal airspace streamtube being newlydefined by earlyphase FAA restrictions, as our formally available resource.
>
> As usual, the expertlevel Forum is first to explore such new conceptual angles (albeit mixed with tedious "beginner" wind lessons). These new ideas seem too often to occur in blindspots of your understanding. If you review all related Forum discussion, it does duly contain the "Betz Limit" truism, but mostly as your irrelevant idee fixe. Reread carefully the broader discussion that consistently corrects such simplistic discourse (even rejecting the "Betz" label for a physical limit discovered by several scientists, as Gabor rightly notes in reminder).
>
> The true AWES Forum understanding is to refer to "Streamtube Efficiency" as the basic measure of total kite thermodynamic efficiency. This is why Sean's search on "Betz" in the forum did not naturally lead to our best thinking about this topic. He could perhaps substitute Streamtube usage for Betz in the title and general discussion of his paper.
>
>
>
>
>
>
>
> >
>  That's why I'm into Open Source Hardware kites development.
A motivated contributor, provided a better answer than I could before I'd had a chance to get to my laptop.The answer and these forum discussions serve as a change log.Essential to successful OS project development.If you consider recent criticism of opensourceecology.orgWhere most discussion was controlled, edited and filtered by 1 overbearing founder...VSThe success of the Bio Brick Foundation biobricks.org DNA database.They obliterate IP roadblocks & promote democratisation of synthetic biology. Openness seems critical for life.At their latest synthetic biology conference all of the work presented was application driven and industry ready. Bio Brick Foundation has moved far beyond it's initial hype phase of development.
Interestingly, 1 of the presentations was on Bacterial cellulose materials, complex seamless forms were presented as biocoture. "fashion"If you need a replacement for opensourceecology.orgit already exists. Initial contributors are regrouping their materials.Another, essential to OS, strength we have on this forum is Attribution.
Joe Faust take a bow, you make it possible.Rod Read
Windswept and Interesting Limited
15a Aiginis
Isle of Lewis
HS2 0PB
07899057227
01851 870878  Don't know if you missed this:Page 6 and appendix DRegardsAndrea P
 Dear Andrea,
Houska and Diehl's Betz summary is a good picture of 2007 thinking. They took as the chosen streamtube only the small circle swept by a single kite; not the whole reachable airspace of the "kite window".The biggest development since then is that the US FAA* has defined a provisional frontal airspace streamtube for early AWES a 2000ft altitude limit. KiteLab's kitefarm model presumes a 2000 x 4000 airspace. This new view determines a very specific maximal streamtube for us."Backofthenapkin" calculations have been presented onForum that compare singlekite, megaarch, and densearray configurations. Costello, et al, also see that almost all the potential wind energy bypasses a single kite, and can use the new FAAderived airspace to calculate how much of the regulatorallydefined AWE resource is lost under the singlekite paradigm, compared to megaarch and densearray concepts,Ciao,daveS* In AWE, ICAO will likely harmonize with the FAA, over time.  So the key to AWE will be to figure out the best placement of kites in a kite window, right? OK I think I'm catching on. It's all about kite placement. Place your kites right and you are making lots of electricity.
Every time I think I know where my kite window is, the wind shafts direction. Yesterday's kite window is today's safe place to stand. Every time I try to fly a kite it quickly gets too windy and I am lucky to pull it down before it rips apart. By the way I think I may have a workable idea for how a Mothra could make significant amounts of power.
 In AirborneWindEnergy@yahoogroups.com, dave santos <santos137@...> wrote:
>
>
>
> Dear Andrea,
>
> Houska and Diehl's Betz summary is a good picture of 2007 thinking. They took as the chosen streamtube only the small circle swept by a single kite; not the whole reachable airspace of the "kite window".
>
> The biggest development since then is that the US FAA* has defined a provisional frontal airspace streamtube for early AWES a 2000ft altitude limit. KiteLab's kitefarm model presumes a 2000 x 4000 airspace. This new view determines a very specific maximal streamtube for us.
>
> "Backofthenapkin" calculations have been presented onForum that compare singlekite, megaarch, and densearray configurations. Costello, et al, also see that almost all the potential wind energy bypasses a single kite, and can use the new FAAderived airspace to calculate how much of the regulatorallydefined AWE resource is lost under the singlekite paradigm, compared to megaarch and densearray concepts,
>
> Ciao,
>
> daveS
>
>
>
> * In AWE, ICAO will likely harmonize with the FAA, over time.
>  Thanks Andrea,
Yes, I've gone through all of Diehl and Houska's work. Some of the first researchers to develop dynamic kite models I believe.
They apply Betz's result in their paper, I am actually doing the opposite: I claim that Betz efficiency is meaningless for many kite systems and I derive a different efficiency measure.
Best Regards,
Sean
 In AirborneWindEnergy@yahoogroups.com, Andrea Papini <andrea.papotto@...> wrote:
>
> Don't know if you missed this:
> http://www.kuleuven.be/optec/files/Houska2007.pdf
> Page 6 and appendix D
> Regards
> Andrea P
>  If you take the total swept area of your kite's path, the upperend of the available total power will be bounded by Betz, as all windenergy systems are. Meanwhile, my guess as to what you're trying to say is that you cannot envision a movingkitebased wind energy system that could achieve the Betz coefficient as applied to its entire swept area? Or are you saying you can beat Betz (famous last words), or what? What's your "efficiency measure"?
 In AirborneWindEnergy@yahoogroups.com, "seanccostello" <seanccostello@...> wrote:
>
> Thanks Andrea,
> Yes, I've gone through all of Diehl and Houska's work. Some of the first researchers to develop dynamic kite models I believe.
> They apply Betz's result in their paper, I am actually doing the opposite: I claim that Betz efficiency is meaningless for many kite systems and I derive a different efficiency measure.
> Best Regards,
> Sean
>
>  In AirborneWindEnergy@yahoogroups.com, Andrea Papini <andrea.papotto@> wrote:
> >
> > Don't know if you missed this:
> > http://www.kuleuven.be/optec/files/Houska2007.pdf
> > Page 6 and appendix D
> > Regards
> > Andrea P
> >
>  Sean wrote " I claim that Betz efficiency is meaningless for many kite systems and I derive a different efficiency measure."Dear Sean,Why spark idle controversy by featuring Betz at all? Its been years since we put the Betz Limit to rest on this Forum. If your thesis abstractly disregards AWES streamtube efficiency, in seeking to formalize powertomass efficiency, this focus could be helpfully reflected in the final choice of the paper's title.Houska and Diehl's early noting of streamtube efficiency as an AWES critical parameter applies in common cases of restricted land and airspace.* "Powertoweight" is also an essential performance dimension (welldiscussed on the Forum), so your work is of great interest and very appreciated,Best wishes,daveS* Many odd factors may critically drive AWE LCOE viability, like energymarket conditions and aviation liability insurance rates.
 Hi Doug,
Our efficiency measure works like this this:
You tell me:
a) the Area and L/D of all the wings in your system,
b) the windspeed,
c) THE ANGLE OF THE OVERALL RESTRAINING FORCE (i.e. force not due to the wind) acting on the system.
I give you a maximum power you cannot exceed.
Betz works like this:
You tell Betz
a) the swept area of your system
b) the windspeed
Betz gives you a maximum power you cannot exceed.
Many of the kite power systems being developed could only achieve a tiny fraction of Betz's
limit, so the Betz limit tells them little. For those systems our limit gives a TIGHTER upper bound = more useful.
Here's a small example:a single 200m^2 kite, on a 1km tether, flying at around 500m altitude ( tether force = restraining force, has an angle of about 30 degrees to the ground), let's say the kite has an L/D of 6 and the wind is 10m/s.
Betz's maximum power = 470 MW (swept area = 1.6 million sqm.)
My maximum power = 350 kW
As you can see, my bound gives a much more sobering figure. If you want to impress investors perhaps you'll do your sums with the Betz limit, but if you want a more realistic cap on the power your kite can generate, my bound is more useful in this case.
Best Regards,
Sean
ps. I've simplified things a bit here. A lot of details are given in the paper to take into account more system parameters, such as system weight, tether weight, lift coefficient..
 In AirborneWindEnergy@yahoogroups.com, "Doug" <dougselsam@...> wrote:
>
> If you take the total swept area of your kite's path, the upperend of the available total power will be bounded by Betz, as all windenergy systems are. Meanwhile, my guess as to what you're trying to say is that you cannot envision a movingkitebased wind energy system that could achieve the Betz coefficient as applied to its entire swept area? Or are you saying you can beat Betz (famous last words), or what? What's your "efficiency measure"?
>
>  In AirborneWindEnergy@yahoogroups.com, "seanccostello" <seanccostello@> wrote:
> >
> > Thanks Andrea,
> > Yes, I've gone through all of Diehl and Houska's work. Some of the first researchers to develop dynamic kite models I believe.
> > They apply Betz's result in their paper, I am actually doing the opposite: I claim that Betz efficiency is meaningless for many kite systems and I derive a different efficiency measure.
> > Best Regards,
> > Sean
> >
> >  In AirborneWindEnergy@yahoogroups.com, Andrea Papini <andrea.papotto@> wrote:
> > >
> > > Don't know if you missed this:
> > > http://www.kuleuven.be/optec/files/Houska2007.pdf
> > > Page 6 and appendix D
> > > Regards
> > > Andrea P
> > >
> >
>  The Betz efficiency is truly meaningless to most wind energy inventors since, as you point out, they are unlikely to even approach, let alone challenge, the Betz coefficient as applied to the entire swept area.
On the other hand, a flying turbine that travels across the wind can overcome what Betz would allow for the same turbine, hardmounted.
:)
Doug S.
 In AirborneWindEnergy@yahoogroups.com, dave santos <santos137@...> wrote:
>
>
>
> Sean wrote " I claim that Betz efficiency is meaningless for many kite systems and I derive a different efficiency measure."
>
>
> Dear Sean,
>
> Why spark idle controversy by featuring Betz at all? Its been years since we put the Betz Limit to rest on this Forum. If your thesis abstractly disregards AWES streamtube efficiency, in seeking to formalize powertomass efficiency, this focus could be helpfully reflected in the final choice of the paper's title.
>
> Houska and Diehl's early noting of streamtube efficiency as an AWES critical parameter applies in common cases of restricted land and airspace.* "Powertoweight" is also an essential performance dimension (welldiscussed on the Forum), so your work is of great interest and very appreciated,
>
> Best wishes,
>
> daveS
>
>
> * Many odd factors may critically drive AWE LCOE viability, like energymarket conditions and aviation liability insurance rates.
>
>
> >
>