Clean Power

Published on March 31st, 2014 | by Michael Barnard


Sky Windpower’s High-Altitude Generation Aspirations Aren’t Realistic

March 31st, 2014 by  

magazinecoverSky Windpower is an airborne wind generation system that is often given glowing reviews in the inadequately critical popular and technical press. In 2008, they were listed in TIME Magazine’s Best Inventions of the Year. In 2011, they were featured on the front cover of Popular Mechanics.

They continue to be active in the space, and are Gold Members of the Airborne Wind Energy Consortium.

So, how do they fare compared to Makani and the engineering compromises of other airborne wind generation devices?

Sky Windpower’s proposed high-altitude wind generation solution raises major concerns:

  • For serious generation it would require building the world’s largest, most powerful, fully autonomous quadcopter, and many of them.
  • It would need to get the price per unit for these devices down to a fraction of the price of the world’s largest helicopters today.
  • It would need to consume more land without secondary uses than comparable conventional wind generation.
  • It would require that significant additional airspace be declared to be fully restricted up to and including passenger jet altitudes.
  • It is likely limited to non-winter operation.
  • Tether weight alone will be greater than the largest load lifting helicopter capacity today.
  • It would require turning maintenance to flight ratios for rotorcraft on their heads.
  • The organization is very optimistic regarding capacity factors due to weather- and maintenance-induced landing requirements, as well as energy costs to fly the devices to their operating altitude and back.
  • It is unlikely that Sky Windpower have fully analyzed and accounted for failure conditions in their projections; testing required to satisfy aviation authorities and insurers would likely take more than a decade by itself.

What is Sky Windpower’s solution?

At heart, it’s simple. Fly a quadcopter up to 4,500 to 9,000 metres (15,000 to 30,000 feet) where the wind is consistently stronger and faster, have the blades spin in the wind like a kite, generate electricity from reversing the electric motors similar to regenerative braking on an electric car and transmit the electricity to the ground down a conductive tether. They have a small (1:13 or 1:17 scale) prototype that they have tested as of December 2011 flying with additional safety tethers through a limited range of the required manoeuvres. More testing may have been done and not publicized on their website.

Artist's rendition of Sky Windpower device
Artist’s rendition of Sky Windpower device

Sky Windpower is foregoing most of the cubing of wind velocity Makani takes advantage of through cross wind flight while still challenged by a large loss of swept area. To get significant generation with this relatively static quadcopter approach, they intend to fly it in the 15,000-30,000 foot (4500-9000m) range. To be conservative this implies a minimum 9 kilometer electrified tether which is at any given time somewhere in a half sphere with a 18 kilometre diameter with a volume of about 1500 cubic kilometres. More realistically, it would require an 18 kilometre electrified tether for a half sphere of 36 kilometres with a volume of around 12,000 cubic kilometres.

One person who is not with Sky Windpower has said that Sky Windpower has abandoned the high-altitude goal and is targeting a 2000′ foot ceiling, but this is not reflected in their publicly available material as of March 2014; it is worth assessing the high-altitude implications regardless.

What are the implications for scaling?

Sky Windpower projects one quarter the swept area over four rotor sets for equivalent generation faceplate capacity to a conventional wind turbine. Let’s consider a five MW conventional wind turbine as a comparison. It has about a 128 m rotor diameter and a swept area of just under 13,000 square meters.

A small amount of math using Sky Windpower’s factor tells us that the device would require four 32 m (112 ft) diameter rotors with a total craft size of about 70 meters (245 ft) per side including blades. For context, the MI-26 heavy lift helicopter has a rotor diameter of 32 m with its eight blades; check the rotor droop and imagine four MI 26’s bolted together with less fuselage. Considering a two MW conventional wind turbine, the Sky Windpower device would have 23 m rotor (80.5 ft) diameter and the device would be roughly 51 m (179 feet on each side including blades.

MI 26 heavy lift helicopter

This tells us that the Sky Windpower generation devices would need to be much bigger than the biggest rotorcraft today to be at a scale where useful generation might occur. This is a red flag in and of itself, but not necessarily insurmountable given advances in material science and use of electrical versus gas turbine motors. It does represent significant engineering challenges which are not addressed or called out in their public documentation.

What about the tether weight and length?

Tether weight is glossed over on the Sky Windpower site.  Makani’s one km, carbon fibre and aluminum tether is projected to weigh 3,660 kg; let’s assume that is accurate and representative. The minimum length 9 km conductive tether would weigh about 33,000 kg assuming adequate strength. A 9 km ceiling would require a roughly 18 km tether weighing 66,000 kg. The MI 26, one of heaviest lifting helicopters in the world, has a maximum lift of about 20,000 kg above its own 36,000 kg weight and a hover ceiling of 1700 m.

It would be unsurprising if a 9 km carbon fibre and aluminum conductive tether cost more than the mast of an equivalently generating conventional wind turbine, and for tether plus device to cost significantly more than a conventional wind turbine of equivalent capacity. For comparison, an MI 26 by itself costs $15-18 million, and a 5 MW Sky Windpower device would be much bigger and fully autonomous.  Also for comparison, the MI 26 weighs about 36,000 kg including fuel; take off most of the fuselage, replace the turbines with electric motors, remove the fuel tanks and then multiply by four. It’s likely that a 5 MW equivalent Sky Windpower device would weigh 20,000-30,000 kg by itself. The device would have to lift 53,000 – 63,000 kg to 4.5 km to get into its operational zone. No weight or dimension calculations for the Sky Windpower device are included in their public documents, so these are necessarily estimates.

Sky Windpower asserts that helicopter limitations don’t matter due to using rotor kiting; they don’t show their math so its difficult to determine which limitations they are referring to, and whether they are correct or not.

What about weather conditions for safe flying?

Sky Windpower’s current plan per public material is to land the devices when lightning is forecast. They make no mention of how long it will take their device to ascend to its 4.5 km operating minimum or descend from that altitude safely, but they project future developments which will make this necessary less often. They are silent on other weather conditions which would require grounding of the device; there are easily half a dozen which would require this. They are also silent on energy required to lift the potentially 20,000 kg device and 33,000 kg of cable to operating height and what the net energy output impact will be. For context, one conventional wind farm’s draw from the grid for blade start and other peripheral items was 1:320th of the annual generated electricity.

Like Makani, Sky Windpower does not address temperature variation as a concern in their public material. However, while Makani can safely operate in tropical and subtropical climates, or in summers in temperate climates, the Sky Windpower device is projected to fly at altitudes where the temperatures are often below zero regardless of surface temperatures. The rule of thumb is 3.5 degrees F (2 degrees C with rounding) per 1000′ of altitude gain. At minimum suggested operational altitude of 15,000′, the Sky Windpower would be operating in temperatures colder than surface temperatures by about 53 degrees F or 30 degrees C. At 30,000′, operating temperatures will be 105 degrees F (60 degrees C with rounding) lower on average.


In their publicly available material, there is no mention of de-icing technology requirements on the ground, or operational factors which will prevent ice build up aloft on the device or along the tether. It is possible that when operating, airframe vibrations will prevent ice build up, as will tether oscillations, but these assumptions are unstated. Regardless, de-icing on the ground is a significant unstated operational requirement for any climate which experiences winter conditions. To be clear, there is apparently nothing in the literature from any airborne wind generation organization which addresses this.

What about maintenance?

Like Makani, maintenance requirements will be much higher for this device than for a conventional wind turbine. The ratio of maintenance to flight hours for helicopters is often in the 3.5-4.5 to 1 range, meaning that a helicopter flies one hour for every 3.5 – 4.5 hours of maintenance. Sky Windpower is silent on why the quadcopter technology is expected to achieve the inverse of that ratio at a minimum, and why this is going to be better than the guaranteed 95% availability modern conventional wind turbines have while under warranty.  With icing and weather related downtime as well as significant energy loss to get their device to altitude it’s unclear that they will achieve the significantly greater capacity factors to overcome other challenges. Remember that new conventional wind turbines in the US operate at 35% capacity factor in lower wind category zones, and up to 47% capacity factor in the best wind category zones per LBNL. The best modern wind farms in the US and Brazil are seeing 50% capacity factors regularly. Achieving significant improvements on those numbers is non-trivial.

What about spacing and safety?

Spacing requirements will be large.  Ground equipment and tethers are not worked out in any detail on their public site.  Given multiple devices at multiple altitudes with multiple tether angles ascending and descending, turbulence at different altitudes, tether slack potential and tether oscillation, a single 70 m (245 ft) per side device will need a large amount of operating leeway on all sides. If a device manages to get the cable of a nearby device caught in its blades, both are likely to be in serious trouble and this would very likely cause a domino effect in other devices which would have to be taken account of in spacing.

Screen Shot 2014-03-31 at 9.37.20 AM

This randomly chosen winds aloft MAPS sounding from a paragliding site is instructive. At around 5000 ft, the wind direction is about 180 degrees from the wind direction at 15000 ft and very strong. Tethered devices ascending and descending through this will have very significant tether angle and direction differences. The highly idealized pictures of farms of airborne devices have them all pointing the same way, at the same altitude with tethers that are all parallel; this is highly unrealistic. This is a very significant safety factor that those proposing these devices downplay or or ignore entirely.

For an idea of the best possible scenario of a high-strength cable caught in large rotor blades, watch this video of a helicopter catching a cable.


A worst case scenario is a higher-altitude Sky Windpower device having a tether broken a kilometre or two below the device due to an airplane strike or a malfunction on another Sky Windpower device. The Sky Windpower device would be blown downwind rapidly, with the 3700-7400 kg of severed tether penduluming beneath it. Once it stabilizes, it will not have power so it probably won’t be able to make headway and will likely still be going downwind if more slowly, and with limited onboard control ability due to lack of significant power.  All devices downwind will have the tether dragged through their operating airspace and potentially fouling their rotors.

These factors imply at minimum very large gaps between devices as well as a very large amount of synchronous flying automation given the oscillating tether. While quadcopter automated synchronous flying is advancing, at present it is only done with tiny quadcopters in safely enclosed indoor volumes. Sky Windpower asserts that they could bring a device down safely in the event of a severed tether, but it’s unclear that they have thought through the scenarios. In any event, the implication for spacing is that safety requirements for the foreseeable future will required significant spacing, likely more than conventional wind turbines just as with Makani. Given the chaotic nature of atmospheric conditions, solving this would require significant instrumentation to determine and communicate precise tether locations and automation software that can plot safe courses through a sea of moving cables.

What about land usage?

The area under a Sky Windpower farm will be the equivalent of an airport. Due to very large devices landing and taking off, it is likely that no other economic uses would be allowable due to workers safety and insurance reasons. Unlike Makani, maintenance would likely not require nearby generating devices to be shutdown however. This once again is a comparative disadvantage to conventional wind farms which are mostly interspersed with agricultural and other land uses.

Sky Windpower claims 1:400th of US airspace would be required to provide all of the USA’s electrical demand, so they’ve done some density math, but unfortunately don’t show it. Generous assumptions would be:

  1. five MW devices,
  2. 60% capacity factor and
  3. 1000 m spacing between devices.

At roughly 3500 TWh per year annual consumption, roughly 133,000 of these devices covering about 2% of the total land mass of the contiguous United States with technology that makes any other uses unsafe would be required. It’s unclear what Sky Windpower’s calculation of 1:400th — an order of magnitude less than this calculation — is based upon, but it is highly suspect as a power density calculation without engineering or safety considerations.

What about flight permissions?

Sky Windpower indicates that they have had initial discussions with the US Federal Aviation Authority on operating conditions and requirements, and have some ideas on how to achieve acceptance of their device. Their comparison to blimps is disingenuous just as Makani’s comparison to antenna tower guy wires is disingenuous; blimps have passive lift and are much safer than an auto rotating quadcopter with four rapidly rotating blades of the scale of the Sky Windpower projected device. Imagine four MI 26 heavy lift helicopters bolted together auto rotating down in a housing complex in a high wind. Imagine a blimp coming down in the same place. The FAA and insurers will imagine this.


Like Makani, Sky Windpower is more optimistic about the FAA accepting their arguments than I am, and if this moves forward it will likely after be a decade or more of dedicated and restricted testing. One comparison to blimps may be apt, however; currently blimps carrying radar equipment at 15,000 feet do not require tether markings in restricted airspace according to a point made on the airborne wind energy systems forum. It’s unclear if aviation authorities will be convinced that the more passively safe blimps are equivalent to massive quadcopters however.

What does this all add up to?

Sky Windpower is projecting building the biggest multi-rotor helicopter ever, the largest autonomous helicopter ever, the heaviest lifting helicopter ever, the highest-flying autonomous helicopter ever and for a required price point much lower than a single MI 26. The company does not have appeared to have worked out even a small fraction of the engineering challenges. It’s fairly clear that it’s not possible given air density with increasing altitude to lift a 33,000 kg tether in addition to the airframe weight, making their high-altitude solution non-viable based on that factor alone. Unlike conventional wind turbines which co-exist well with farming and secondary uses, no secondary uses would be reasonable within a farm of these devices. There is not even the most basic of reasons why a federal aviation authority would consider setting aside large swaths of restricted airspace for these devices.

Sky Windpower just isn’t viable at high altitudes. And if it’s targeting lower altitudes under 2000′, it still has enormous challenges and is giving up significant wind resources. This just isn’t a viable alternative for any significant generation.

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About the Author

For the past several years Michael has been analyzing and publishing reports and articles on decarbonization technologies, business models and policies. His pieces on electrical generation transformation and electrification of transportation have been published in CleanTechnica, Newsweek, Slate, Forbes, Huffington Post, Quartz, RenewEconomy, RenewablesInternational and Gizmag, as well as included in textbooks. Third-party articles on his analyses and interviews with Mike have been published in dozens of news sites globally and have reached #1 on Reddit Science. Much of his work originates on, where Mike has been a Top Writer annually since 2012. He also has published a climate-fiction novel, Guangzhou Future Tense.

  • Peter Gray

    The ultimate kiss of death: appearing on the cover of Popular Mechanics. Has _anything_ displayed there _ever_ gone on to practical reality? Maybe occasionally (it’s a rhetorical question), but I see this as an anti-endorsement.

    Seriously, it looks like this thing entails a record-breaking number of independent fatal flaws. A few more questions and engineering challenges to consider…
    1) Aside from whether carbon fibre is the right material (it’s conductive and reactive with aluminum, so kevlar or spectra is probably a better bet), a 9- or 18-km long line will need to run at fairly high voltage. What about the weight, bulk, and drag of the necessary insulation? Any estimates of total tether diameter?
    2) As the machine goes up or is brought down, the tether will need to be stored on reels, with full power running through it. An additional insulation challenge, as well as extremely strong magnetic fields. We’ve seen how anti-wind kooks react to relatively quiet turbines and supposed health effects of infrasound – wait till they get hold of this!
    3) A 10+ km tether would need to have very high tensile strength to support its own weight, with a strong pyramid effect with altitude. At best, the low end of the tether would tend to drape low over the ground, further complicating the spacing and interference issues.

    This has all the signature features of an investor-fleecing machine and nothing more.

    • Good points regarding glide ratio and potential generation Peter. I’ll take that into account in future. However, I think flying the device so that the tether doesn’t tangle with other devices blades or tethers is an extremely difficult feat, so my concerns are merely reduced, not eliminated.

      • Peter Gray

        I’m with you on that, Mike. It sounds like an awful mess if even one or two little things go wrong. Imagine a serious wind farm with hundreds of units. I agree on your main points. Whether the device can glide upwind would be a distant secondary consideration. But there’s no need to weaken a good critique with assertions that are easy to refute or legitimately doubt.

        • Absolutely. I always appreciate fact-based suggestions for improvement.

          • Peter Gray

            That attitude shows in the quality of your work, and it’s one reason you’re among the top writers on this site. I didn’t pore over every sentence, and I’m not thoroughly informed on the details, but it wasn’t easy to find even a minor flaw in your long and technical, but quite readable article.

  • Mike, look at this video: Then you’ll immediately understand why your consistent comparisons with helicopters is invalid. This machine lifts with a power of a mere 0.5 kW!

    Helicopters are completely different things, propelled by infinitely complex internal combustion engines and designed to safely carry people at reasonable speed. The reason why Sky Windpower can claim such low maintenance is the fact that it is propelled by electric motors. Because the construction does not transport humans, that also means less safety regulations. It doesn’t need to move distances at speed, etc etc.

    However I do agree with your general assessment that success seems highly unlikely.

    Tether weight is a factor, but maybe a leap in CNT cabling technology will save the day. (see for instance:

    I believe the ‘low tech’ kite approach is much more feasible. Kites are very cheap to build, don’t pose a threat when they break loose and land in a random location, the (heavy) generating equipment stays on the ground, the tether does not need to conduct energy and can thus be made from strong and ultralight materials like kevlar. See:

    • Peter Gray

      Wow, what a stretch! That’s a nice video and a fun project, but it mainly serves to demonstrate, in case we need reminding yet again, that human-powered flight will _never_ be practical. Yes, I said never. Sure, the machine “lifts with only 0.5 kw,” but it’s lifting a gross weight of < 100 kg, in ground effect, in a windless hangar. What in the world does that have to do with a tethered generator that must lift 30 to 70 tonnes of cable alone?

      You have a point about electric motors needing less maintenance than a turbine engine, but another explanation for the low advertised requirements could be that the SW guys have never built a real device at anywhere near full scale, and they're just making up numbers, as they do everywhere else.

      Whenever someone points to future technology saving the day, I figure (s)he's displaying the last card in his hand. New supermaterials might make this closer to possible, but won't they also make terrestrial wind turbines cheaper and more efficient at the same time?

      I think light, flexible kites might be closer to practical, esp. for pulling ships. Some promising ones were demonstrated a few years ago, but then nothing seems to have come of them.

      • Hi Peter, see my answer above to Mike for my reasons for coming up with that video. It’s not because I don’t like his conclusion. I fully agree with him, but was not convinced by his comparisons with helicopters, machines that serve a totally different purpose.

        And yes I am fully aware of the limitations of that demonstration (minor nitpick: don’t forget to include the mass of the pilot, so it is > 100 kg).

        But the Sky Windpower device will have orders of magnitude more power available, so a sturdier, heavier machine will be able to lift itself on reasonable power requirements. But that is not where the challenges are, it’s mostly the tether. As stated here, the weight, aerodynamic loading and safety (what happens when the tether breaks?) are all potential dealbreakers.

        You should look at human powered aeroplanes instead of helicopters. Human powered aeroplanes are a lot less of a challenge, even though they too will never be practical. The human motor is simply too weak. The athlete in my video can do > 700W for 1 minute. An average human can deliver 100-150 W continuous power.

        • Peter Gray

          Thanks for a more polite and friendly reply than I deserved, Arne.

          You’re right that helicopters have a different purpose, but I’d lean toward Mike’s citing them as a pretty good analogue for the points he made about cable collisions and such. In terms of mass and energy, existing helicopters seem closer to the SW notion than any of the model or human-powered quadcopters.

          I did mean to include the pilot in the total mass, since I’d expect a rather light pilot, and a very light airframe. The gross might be > 100kg, but I’d bet not by much.

          It’s a matter of opinion, but I’d call those safety, cost, and load issues nearly certain fatal flaws rather than potential dealbreakers. Remember that the device must compete with ground-based turbines, which have driven kWh costs rather low.

          As it happens, I worked for a couple years on the Raven human-powered airplane project about 20 years ago, as the head of the propeller development/production team. So I’m familiar with the power output stuff, and I got a firsthand education in why these airplanes will never be practical. The Raven goal was 100 miles (over water in Puget Sound), but the beautifully conceived craft never really got off the ground, as much because of poor people management as anything else. Also surprisingly poor understanding of the scientific method and unbiased testing.

          I’ve also flown hang gliders, including as a test pilot and tandem instructor, for more than 35 years, and paragliders for 18 or so years. I’m naturally intrigued by the idea of making power with kites, but Mike has convincingly shown that it’s not likely to pan out.

          • “I worked for a couple years on the Raven human-powered airplane project”

            That has been a project that I followed with great interest at the time, when I was heavily involved in the HPV scene in the Netherlands.

            Cool that I now ‘know’ someone who actually worked on that project. 🙂 Pity it never panned out.

            “Mike has convincingly shown that it’s not likely to pan out.”

            O sure, no disagreement there. I only think that helicopters are not good benchmarks to compare this stuff with, just as a 747 is not a good reference if you want to build a hang glider.

            fatal flaws rather than potential dealbreakers

            That is perhaps a better way to put it. Although by nature I am very careful with such definitive statements. How many people have said that the weight and cost of batteries were the fatal flaws of ev’s, when in retrospect they only turned out to be technological hurdles?

          • Peter Gray

            So we don’t stray too far off topic, clog the interwebs, and bore everyone else, I’d be happy to take this offline. I’m at peter underscore gray at wsu dot edu.

    • Yes, I’m aware that a human being powered a lift-effect helicopter for a flight of about a minute in a hangar. I’m afraid that’s immaterial.

      Regarding your assertion that helicopters are completely different things, I think you’ll find that scaling up toys and tiny prototypes to the size required for any serious generation makes quadcopters extremely complex devices as well. And that quadcopters share the attribute of “Beating the air into submission” as a famous quote goes with helicopters. Assertions that these devices will vastly exceed helicopter maintenance cycles and approach wind turbine maintenance cycles (2% downtime annually due to maintenance) are rank speculation and not based on anything but optimism that I’ve been able to discover.

      If you have references for your belief that electric quadcopter-based approaches would even approach 75% availability, please provide them.

      Regarding fabric kite based generation, please see my analysis of the design choices of the airborne wind generation space here for my thoughts:

      • Since the machine that Sky Windpower intends to build has never been built before we both are scrambling to find benchmarks. You chose the helicopter, I went out to look for something else at the other end of the spectrum (my large, light, fragile, simple and slow vs. your compact, heavy, sturdy, fast). The Sky Windpower machine is somewhere in the middle.

        I agree with most of your assessment and conclusion, but your persistent comparison with helicopters is invalid and undermines your arguments.

        If you have references for your belief that internal combustion engined helicopters, designed for carrying people and cargo are a suitable benchmark for evaluating the Sky Windpower device, please provide them.

        • My apologies but it’s a very large powered rotorcraft and helicopters are large powered rotorcraft. This strikes me as a remarkably easy comparison.

          If you would prefer we could talk about airplanes which also have a much lower maintenance availability than wind turbines in any class you want to look at.

          Flying objects require more maintenance than objects that stay on the ground due to some very basic tradeoffs. As part of my analysis of AWE maintenance I looked up stats on aircraft of all sorts. It’s not encouraging for AWES. This isn’t a controversial statement.

          I’m afraid the onus is on you to provide support for your claim to the contrary. Extraordinary claims etc.

          I’m glad we agree on most of the points and I’m quite willing to agree to disagree on this point in he absence of data to the contrary.

          • Mike, I please spell it out for me, what did I exactly claim that needs support?

          • Perhaps we are merely agreeing violently. Let me restate: Sky Windpower will not achieve 98% availability, and is extremely unlikely to achieve 75% as that would exceed every heavier-than-air craft ever, whether manned or unmanned, fixed wing or rotorcraft.

            I don’t think you disagree with any of those statements, you just think that the helicopter comparison is invalid.

            At present helicopters are lucky to achieve 25% availability on a 24/7/365 basis. Some categories of maintenance such as fuelling obviously go away, and to your point they will be unmanned devices so aircrew safety systems will be gone as well. And the engine will be simpler. But these changes will not make them as robust and simple as fixed-wing craft as they will still be beating the air into submission. The comparison is a useful one because it points out how far down the availability scale rotorcraft start and how far any rotorcraft solution would have to go to achieve three-quarters of the annual availability of ground-based wind turbines.

            I make the same comparison in my analysis of Makani to fixed wing maintenance availability make clear that it’s extremely unlikely to achieve the same availability as a useful and instructive comparison. I do the same for soft-wing kites to known metrics.

            This calibrates the discussion and allows useful statements. Your assertion that it is not apt or useful isn’t well supported or articulated.

            Perhaps if you were to provide an alternative mechanism for calibrating likely maintenance downtime…

          • “and is extremely unlikely to achieve 75% as that would exceed every heavier-than-air craft ever, whether manned or unmanned, fixed wing or rotorcraft.”

            See: which claims an operational availability of >80%.

            For years I have fought against EV naysayers that were adamant EV’s would never make it due to fundamental technological and economical limitations of batteries. Yet practical and affordable EV’s are now on the road.

            For years I have fought as a hydrogen vehicle naysayer that was adamant that the hydrogen car would never make it due to fundamental technical and economical limitations in fuel cells and hydrogen supply. Yet the introduction of hydrogen cars by Hyundai and Toyota is now very close.

            Technical limitations have a nasty habit of being temporary. So I have learned to be cautious and never say never (unless there is a fundamental law of physics in question).

            And by definition, all analogies have limitations. Imo the helicopter analogy has too much limitations to be of much use, and I think we can limit our disagreement on that, as the other points you make are good ones.

          • You mistake the aircraft industry metric for the generation metric. The aircraft metric is available flying days per year and excludes maintenance downtime daily. Generation metric is 24/7/365 as stated in an earlier comment.

            Once again, no heavier-than-air craft has come close to 75% by generation standards which are the applicable comparisons. The 4-5 hours maintenance to flight hours of helicopters is the comparable statistic and useful starting point for comparison.

            We will have to agree to disagree on this point I suspect.

          • The main disagreement was on the helicopter-as-reference-point thing I believe, but never mind.

            I for one am not ready to make a guess due to the lack of suitable comparison material. Normal airline operations include all kinds of pesky details that this machine doesn’t need to worry about: flight schedules, cleaning toilets, having to land regularly so people can get on and off, turbine engines, etc. You’d have to analyse the data and compensate for these differences. I guess the only way to prove their number is for Sky Windpower to build a prototype.

          • Bob_Wallace

            The Zephyr 7 flew for 336 hours and 22 minutes. Fourteen days and change.

          • Yup. That’s not an annualized maintenance cycle but a fixed wing aircraft designed to stay aloft without tether stress etc that has not gone beyond prototypes achieving test demonstrations. Once again, not a good starting point for comparisons of this type.

            There are lots of cool demonstrations out there, but the production helicopter comparison is for maintenance to flight ratios and remains the strongest starting point for that discussion.

            I don’t think this is actually controversial, I just think we’ve ended up discussing it in the absence of substantive points related to the Sky Windpower analysis. Peter Gray’s point about gliding rotorcraft while still generating power is a much more useful correction or improvement.

            My apologies to both Bob and Arne, but I’m not going to belabour the point any further.

          • Bob_Wallace

            “At present helicopters are lucky to achieve 25% availability on a 24/7/365 basis.”

            With helicopters you’re using a fuel engine. With these devices an electric motor. There’s more similarity to a wind turbine than a helicopter. Take away fueling, crew issues, and the higher servicing needs of an ICE.

            (Making no inferences about the feasibility of these puppies.)

          • The point that I’m failing spectacularly to make is helicopters need much more maintenance than fixed wing aircraft so all else being equal, rotorcraft based generation will also require more maintence.

            Yes, your points are valid and already agreed upon, but achieving sufficient availability to not have significant impacts on capacity factors will be difficult and / or very costly.

          • Bob_Wallace

            I think you’re failing because you’re starting with a piloted, ICE vehicle and not looking at it as a room fan turned on its back.

            A properly sized electric motor can run for years without service.

            An electric helicopter wouldn’t have the advantage of a lifting wing, it would have to work harder than an electric airplane motor. But electric motors can do immense amounts of work for long periods. It’s just a matter of supplying them with ample power and making sure the bearings are good. —

            Lifting/supporting the tether, producing competitively priced electricity, finding airspace where they would be permitted to operate – all different issues.

  • Peter Gray

    Excellent report, Mike. I’d love to see more pieces on CT with this level of quality, research, and yes, skepticism.

    You’ve demonstrated how much writing it takes to cover even a partial list of this idea’s fatal flaws. Here’s one more that comes to mind: rotor tip speed. What would that look like at 30k feet, and what kind of design and power compromises would be needed to keep the blades subsonic?

    It would be nice to see a rough graph of potential power output vs. altitude, taking tether weight, wind speed, and air density into account. Seat of the pants, it feels to me like the thing would hit steeply declining returns well below 30k.

    The highest point a rotorcraft has ever achieved was just under 41k feet, 12.4k meters. But that was a special machine stripped down to the bare minimum, not carrying a huge load that gets heavier the higher it goes – more than linearly, due to tether sag and tension limits. For practical operations, helicopters top out at about 20k.

    • Yes, I left the physics of rotorcraft lift vehicles and performance ceilings alone. I haven’t felt like finding out what the coffin corner for helicopters is. There are definitely additional challenges beyond the ones I identified, some solvable, some not so much.

      • Peter Gray

        Same here; it’s not really necessary or relevant. I did look up the helicopter altitude record, which is surprisingly high at 41k feet. But really, how many fatal flaws does one concept need before we dismiss it?

  • JamesWimberley

    Perhaps these toys will come in handy when humans start exploring the atmospheres of gas giants.

    • Jouni Valkonen

      Not, the atmosphere of gas giants, but the atmosphere of Venus. We should have a floating Venus atmospheric probe within the next 10 years.

      If we ever needed to evacuate humanity from Earth and settle on other planet, Venus would be the choice, because at 50 km altitude Venus has the most Earth like environment. Temperature and pressure are close to ideal.

      Also floating in the Venus atmosphere is essentially as easy as manufacturing a habitat that is a cross between Hindenburg and Titanic. Carbondioxide is heavier than air, so breathable air acts as strong lifting gas in Venus.

      At 50 km altitude there are strong jet streams and wind velocity difference between atmospheric layers can be utilized. Also temperature gradient can be utilized. But naturally solar power production is just overwhelming in Venus.

      • Peter Gray

        That easy, huh? It is appropriate that you chose those two vehicles as prototypes.

        Kidding aside, I hope you’re not serious about evacuating humanity to another planet. It’s a dangerous notion when it encourages people to think we can trash this planet, because, hey, we’ll just move to another one!

        At the very best, Venus would be 1,000 times less hospitable and a million times more expensive to get to than the South Pole, the middle of the Sahara, or the bottom of the ocean. Look how successful we’ve been at establishing sustainable human life in any of those places.

        • Jouni Valkonen

          We would certainly evacuate the population of Earth onto Venus, if we detected 100 km asteroid that is on the collision course. It would be too large to deflect and it would vaporize all oceans and sterilize Earth. My guess is that if all Earth resources would be used for evacuation, we could evacuate Earth in 20 or 30 years.

          It is true that if Earth is polluted, it will be always better place than Venus. By the way, pollution is not the major issue on Earth, but the agriculture is the major environmental Evil. Therefore we need vertical greenhouses ASAP!

          I recommend for you to read Geffrey Landis paper on prospects of the feasibility of colonizing Venus. It is very stimulating to read. Venus is by far more hospitable place for permanent space colony than e.g. Mars. The major drawback is the deep gravity well that it is almost impossible to get away from Venus. Floating rocket launchers are probably beyond current rocket technology.

          Colonization of Venus

          • Bob_Wallace

            Why don’t you look up the energy it would take to move 7 billion people to another planet?

            Perhaps what you mean is not “evacuate” as in getting everyone into lifeboats and off to safety, but colonizing another planet with a small breeding stock. That might be doable, but we could much easier move a significant portion of the population underneath the Earth’s surface during the hottest weather, venturing out to grow crops in the coolest.

            But why spend time playing with these ideas? We have the opportunity to prevent extreme climate change. Let’s do that.

          • Jouni Valkonen

            The point was to prepare on asteroid impact that melts 10 km of Earth crust.

            The worst case scenario for climate change is that Oceans will boil and Earth becomes another Venus, so it really does not help to go into Venus that has already seen a runaway greenhouse effect and its possible early oceans have been boiled away.

            It is good to ponder what will happen in extreme disaster that will end the life on Earth.

            And if we have small colony on Venus, this would help a lot actual evacuation process if that is needed.

            This is the entire philosophy of Elon Musk that Life on Earth must become multiplanetary in order to prepare extreme disaster that sterilizes Earth.

          • Peter Gray

            So we can’t gather the collective will to spend a few percent of GDP on known methods for climate change mitigation/avoidance, but we could get everyone on earth to contribute everything they have, to an effort to ferry everyone to Venus? (and live in abject poverty for 20 or 30 or however many years) The paper is interesting and novel, but it’s still a fantasy, and wishful thinking doesn’t translate to reality.

            With that kind of lead time, why not send rockets to the asteroid and nudge it off course? At such a great distance, it wouldn’t take much energy to make it miss Earth – surely far less than hauling 7B people and all their luggage and inflatable cities to Venus. Then hoping the whole elaborate, untested project will work out on the first attempt. Don’t just read that one paper; look at the actual history of space exploration.

            I’m with Bob on this. If you enjoy such fantasies, there must be other places to discuss them. It’s a waste of time and space on a clean energy site.

          • Jouni Valkonen

            This is perhaps the reason why Elon Musk is as far removed from you that can be possible. Elon can think outside the box and have wider perspective on things.

          • Bob_Wallace

            Elon works on practical solutions.

            Peter is making statements of fact.

            Moving 7 billion people to another planet is an extreme and unrealistic fantasy.

          • Jouni Valkonen

            It is not fantasy if we use 99 % of World’s GDP on evacuating the planet, it can be done. At least several hundreds of millions can be transferred out from this planet. I would assume that quite considerable portion of people will leave voluntarily behind and will e.g. ensure that their children will get evacuated.

            Elon Musk and SpaceX is preparing to transport 80 000 people annually on Mars at a cost less than $500 000 per seat.

            World economics are rather different if the whole world is working towards common goal.

            Actually it is quite refreshing to think what humanity could achieve, if the whole population would work towards different kinds of megaprojects. It is of course sad that there are folks like Peter, who openly admits that he lacks the vision for better world.

          • Bob_Wallace

            Here, I found you a site where you can enjoy discussing a mass exodus from the planet with others.


            This is not an appropriate topic for CT.

          • Peter Gray

            Any interest in responding to the substance of what I wrote, or are you fine leaving it as a personal attack?
            Musk has done a great thing with Tesla, but it’s a common mistake among highly successful, even brilliant people – and their idolators – to think they can take on any and every challenge, even those far outside their expertise. Several emminent scientists come to mind: Linus Pauling, Fred Hoyle… I suspect that Musk’s California vacuum train idea will fall victim to that kind of overextension, but we’ll see.

          • Off topic I know, but Musk himself said he was open sourcing his Hyperloop idea because he couldn’t execute on it in addition to Space X, SolarCity and Tesla. He took on Tesla fairly reluctantly as well. He knows he’s overextended. But of course he’s overextended at levels well above what most of us can imagine.

          • Bob_Wallace

            He has turned the Hyperloop idea over to others. Work is underway to build a working scale model in order to prove the concept.

            I suspect Elon’s greatest skill is surrounding himself with very competent people. He doesn’t have the background to design and manufacture cars and spaceships. But he seems to know how to assemble teams that know how.

  • patb2009

    Mike for high altitude tether vehicles it’s important to look at tether drag forces. the drag forces on tether at any reasonable altitude start dominating, a strong wind will start heavily loading the cable and creating tremendous force. this makes the tether be larger and heavier which starts to increase down force on the kite. Take a look at high altittude balloons, the tethers get big.

    the tether and mooring station here are not small.

    • Valid point. I considered tether drag more relevant for cross-wind kite generation than the sheer weight of the conductive tether for high-altitude generation, but I’d be interested in any references you have to total tether drag for high altitude solutions.

    • Peter Gray

      Yes, and those don’t need two conductors and a lot of insulation.

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