Aerodynamic Lift – something for nothing?

Readers may have noticed that I have been largely absent from these pages for a few weeks. That is because I’ve been doing a consulting job for KiteGen assisting with a presentation to be made to the CleanTech investor summit in Rotterdam in November (see disclaimer at end). In the course of doing this work certain things came to light that explains the power generated by kites, flight reliability and the move away from fabric to composite materials. I kick off with an amazing movie of a kite powered trimaran / hydrofoil (below the fold).

The video shows the kite flying cross wind. The kite is not a parachute simply dragging the boat down wind, but it is a wing where the aerofoil shape and the speed provides the power. The aerodynamic lift is creating large forces pulling on the boat. But since the boat has a keel the only direction it can move is forwards. This is exactly the same principle as sailing. The main difference is the kite catches a lot more wind at altitude and the hydrofoil reduces the drag of water on the keel. If you look at the picture up top you will see that the kite is flying in a direction roughly perpendicular to the wind as shown by the waves. The kite is flying cross wind in the power spot.

The sail boat videos are a good way to visualise the power created by aerodynamic lift (there are more videos at the end of this post). In sailing, the kinetic energy of the wind is converted to the kinetic energy of the boat. In kite power generation, the aerodynamic lift pulls rope off a drum connected to a generator making electricity.

Aerofoils are shaped so that the curvature of one surface gives it a larger area than the opposing surface. The pressure of the air flowing over the curved surface is reduced creating a net upwards force that is called lift. The question is, does this give us something for nothing?

“C” shaped sports kites and the KiteGen PowerWing have this aerofoil shape. The faster they fly, the more lift they generate.





And so to re-visit one of the main equations from my earlier post High Altitude Wind Power Reviewed:

force on the ropes = 1/2 * wind speed^2 * aerodynamic efficiency^2 * area of the wing * air_density

From which it should be clear that wind speed, aerodynamic efficiency and wing area are all key variables.

In the last post I did on high altitude wind, there was a lot of discussion in the comments about flight security and test flight data. Let’s say this was a matter of some irritation for Massimo Ippolito, the main innovator at KiteGen. In the course of many discussions with Massimo it has become clear that the fabric sports kites are mere toys compared with the composite PowerWing that has been designed and built for the industrial scale 3 MW machine. One variable that was missed in my earlier article was the tensile strength of the kites. Fabric sports kites are typically 10 kN while the composite PowerWing is 100 kN. The lower tensile strength of the fabric wing sports kites probably sets an upper limit of around 100 kW for power generation that is too low for a commercial generator. Bespoke fabric kites made from heavier duty material that is braced by bridles may exceed 100 kW, but the bridle system increases drag.

A relaxed Massimo Ippolito, Turin, September 2016.

“During tests using fabric sports kites we burst hundreds of kites that were simply not designed for the purpose of power generation.”


The Tables below compare the key variables for fabric and composite wings. The examples are worked with a wind speed of 15 m/s, which is not really that high, and shows that the PowerWing produces about 100 times the force of a sports kite. It is the higher aerodynamic efficiency of the PowerWing that gives it this enormous advantage. The 100kN tensile strength of the PowerWing provides a safety factor of 2.4 over the maximum forces expected.

The 400 kN total nominal force is reached at a wind speed of about 3.5 m/s. Once that force is exceeded the ropes must unspool on the drums converting the excess force to electricity. (Note this uses only the aerodynamic parameters of the wing / kite. The ropes add both mass and drag and this reduces the efficiency so that the nominal force is not reached until 5.4 m/s.)

The PowerWing reaches maximum 3 MW output at wind speed of 17 m/s. At higher wind speed it must then fly out of the power spot to contain the total force within machine design parameters.

The PowerWing is designed in this way so that it can produce maximum power output under lower wind conditions. Under low wind conditions, the kite will be flown towards the power spot where it generates maximum lift. In higher wind conditions it is flown away from the power spot, the power spot being defined as the direction of maximum lift when the wing is flying perpendicular to the wind direction.

I think it is worthwhile showing the following figure again. The upper panel (red) shows electric power production and the lower panel (blue) rope reel out velocity. The middle panel (green) shows the force curve. The cut-in wind speed is 2 m/s. That is the speed required for the kite to fly. Between two and 5.42 m/s the force on the ropes builds to the nominal force of 200kN on each rope. No power is produced at this time. >5.42 m/s  the ropes begin to unwind (lower panel) and electricity is produced reaching maximum power output at 17 m/s. Beyond 17 m/s the kite must be manoeuvred to a position where it sees 17 m/s wind speed either by going lower or by flying out of the power spot.

I want to finish with three more movies that demonstrate the awesome power of aerodynamic lift using both kites and semi-rigid sails.

This amazing video shows a kite foiler playing in the surf 2 mins 39 secs.

This video 9 min 25 secs shows Vestas Sailrocket team going after the world speed sailing record which they break at 65 knots. The fun starts at about 5:30. This is achieved using a rigid sail on an out rigger. Language advisory!

This video 4 min 21 secs shows a land based version of the Sailrocket called Greenbird that goes at 126 mph!


will be strictly moderated. Comments should be strictly limited to the technical concept of aerodynamic lift and how this can be converted to useful work that may be used by Mankind. Does it provide something for nothing?


In the following months I will be working for KiteGen as an unsalaried consultant, assisting in a third round equity raising exercise. Remuneration will be in the form of commission upon a successful conclusion. I have worked for over ten years as an unsalaried blogger, hoping that a white knight may ride across the horizon one day to restore financial security. And so I am very grateful for this opportunity. If you wish to protest about this then please do so via email and not in the comments.

I hope to find time to write at least one post per week and to maintain the objectivity of the blog. There may be a couple more but not too many more articles on kites.

KiteGen are at an interesting and vital stage of their development. They have secured a new test site in The Alps with consent to fly up to 5000 m and space for 9 machines. The current plan is to deploy the first fully functional 3 MW stem next spring with field trials of the PowerWing over the summer months. They are also moving to new premises adjacent to open ground that may permit limited testing of PowerWing launch and retrieval procedures.

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101 Responses to Aerodynamic Lift – something for nothing?

  1. 1saveenergy says:

    126 mph in a wind powered land yacht using fluid dynamics:
    – Coincidently;
    also using fluid dynamics but in a totally different way –
    In 1906 a Stanly steam car achieved 127 mph at Ormond Beach, Florida.

    LNER Steam Locomotive ‘Mallard’ got the world speed record for steam locomotives (125.88 mph ) at Stoke Bank in 1938.

    & just using explosive grunt …
    The first person to claim the land speed record on British soil was Victor Hémery, at 126mph in a 21.5-litre 4cyl engine Benz at Brooklands race track in 1909.

  2. Adam Gallon says:

    “I want to finish with three more movies that demonstrate the awesome power of aerodynamic lift using both kites and semi-rigid sales.”

  3. ralfellis says:

    I still do not see how you turn rope tension into useable energy. I heard you pull one kite against another, on the same spool. But how much tension can you release on the one being pulled in? And if that is not much, you end up with the released kite negating the forces of the tentioned kite. And if you release too much tension, the kite falls to the floor.

    Having watched para-surfers, I see too many problems here with keeping the kites airborne. In the hands of a novice, they always crash to the ground. So is this a system that can be automated? And if not, then who is going to sit there 24/7, playing with kites? And what will the spacing between kite sites be, to stop entanglement? While I am not a fan of windelecs (turbines), at least they run fairly autonomously.


    • Euan Mearns says:

      This is more complicated to explain than I’d like. A good starting point is to imagine a salmon that has been hooked on a rod and line. It may swim vigorously away from the angler unspooling line very fast on the reel. The salmon then tires and swims back towards the angler. KiteGen have their patented sideslip where the kite is moved to a position where it loses all lift (the tired salmon) and can be reeled back for about 1/10th the energy (or maybe less) used on the power part of the pumping cycle.

      A key component of the KiteGen technology is to maintain the same force on the ropes all the time. Force builds between 2 and 5 m/s (see central green chart). Thereafter, the force is kept constant, actually at 150 kN on each rope for 3 MW, by either allowing the ropes to unwind on the drum or, if there is not enough wind, by recalling the kite. The latter is important since it ensures the kite always has lift. Its very unusual at altitude to have wind speeds less than 2 m/s which is required to keep the kite aloft. The actuators work as either electric motors or generators.

      Hopefully someone from KiteGen will call by to provide the physical explanation with equations.

      Automation is another key element. One of Massimo’s earlier inventions was a triaxial acceleration computer – I think maybe the first solid state motion sensor. It was this and kite surfing that gave him the idea for KiteGen. The kite carries motion sensors, a GPS, a radio, a micro-turbine and battery to power the onboard electronics. The ground station always knows where the kite is and what it is doing. First sign of any trouble then side slip and recall.

      The stem that follows the kite also has motion sensors that tells the control system what the kite is doing.

      In the KiteGen business plan, low machine availability / reliability is assumed in the early years. But it is also assumed that there will be a steep learning curve so that reliability builds with time. Initial commercial deployment is envisaged in uninhabited wilderness. Only when months / years of reliability is proven will this move into more inhabited areas.

      • A C Osborn says:

        Euan, do you know if they have considered adding the new light weight “fabric” type Solar cells to the top of the Kite for a bit of extra power?
        Or does the extra weight negate any gains?

      • oldfossil says:

        Purely out of curiosity I’d like to know if the triaxial acceleration computer is the same as that used in avionics such as the AIR Avionics “Butterfly” variometer.

        • yes it is a precursor of a variometer, and is a 20 year old product of our company now is updated and we adopted the common name that is IMU, an inertial platform that include accelerometers, gyro, gps, altimeter and compass.

    • @Ralph
      You are fully right, the two kites on the same spool, is a wrong idea that somebody else is proposing, I imagine for patent novelty and some freedom to operate.
      No needs at all to have the complication to synchronise the two reciprocating behaviors and flight paths. The timing of the kite recalling maneuver is much shorter, therefore is energetically harmful to wait for the active kite completing the power harnessing cycle. It is far better to control each kite independently optimising the full strokes. Some small energy accumulation and the alternator, that seamlessly could behave as motor, could provide the function of an electrical axis apparently recalling back the wing when the others are lifting.

  4. 1saveenergy says:

    The first part of my post (don’t know where it went ??)

    “Does it provide something for nothing?”
    Only in the way a heat pump works.

    I understand power is generated on reel-out but …
    1; how long does that last (don’t say as long as a piece of string).
    2; what % of production is used to reel back.
    3; what happens in gusty / no wind conditions.

    • Euan Mearns says:


      I think the analogy with a heat pump is good. A heat pump is a device designed to extract heat from air. An aerofoil is designed to extract kinetic energy from moving air.

      But there is one major difference. For a heat pump, for 1 unit of energy spent you get 3 back (ignoring the embedded energy). For a KiteGen operating at close to optimum conditions, for 1 unit of energy spent (on the machine) you get >300 back. The low energy return of a heat pump makes them expensive – that’s why most of us aren’t using them. The projected high energy return for KitGen promises cheap electricity. This is central to Massimo’s philosophy.

    • Euan Mearns says:

      The x-axis scale is in seconds. Test flight data for 40 minutes. Compare the production phase in green with recall (sideslips) in pink. Massimo will need to give the exact figure, but recall uses <10% of the energy produced. This is vital to the efficient operation.

      • 1saveenergy says:

        On that graph the pink area looks a lot more than 10% but the final ~ 11mins look promising.

        • @1saveenergy
          consider that the work done with the composite wing is aimed to multiply by almost 100 the active phase, while the sideslip has no reasons to worsen.

        • Euan Mearns says:

          1save – if you look at the pink energy expenditure, from left to right you see 3 small negative spikes that represent kite management. Short recalls to boost lift until steady high altitude wind is reached. Then there are two large negative spikes that represent kite recalls at full power, i.e without side slip. Then the last two negative “spikes” where the kite is recalled using side slip. You need to look at my earlier post to learn about the side slip that is vital to economical functioning of the PowerWing. Basically the kite is manoeuvred to a position where it loses all lift.

          • Willem Post says:


            I think the kite scheme would be useful for smaller cargo ships traveling long-haul, steady routes, i.e., the Canaries to the Caribbean.

            Steady winds, a capable crew managing the sail.

            Multiple sails may not be feasible, due to entanglements.

            The ship’s engines would assist as needed

            Regarding electricity production, entanglements and the sheer number of units to serve, say a city, would be enormous, i.e., not practical.

            Factory-built, low-cost, MODULAR nuclear, 100 to 250 MW would be the way to go.

          • gweberbv says:


            just google “SkySails”. Unfortunately, they went recently bankrupt.

        • heavyweather says:

          The two large pink spikes are full recalls. Sideslip would be normal operation.

      • Roberto says:

        If I look at the graph I see large fluctuations, on time scales similar to those of ‘conventional’ wind power turbines.
        What ‘spinning reserves’ to keep frequency and other important network parameters in the right range are foreseen for kitegens?…
        I hope it is not the same as wind power now, where regulation and balancing is left to OTHER forms of generation…

        • It is not left to “others”. There is a pool of generation to draw from for frequency regulators.
          Batteries are going to take that business. Younicos comes to mind.

          • Roberto says:

            Ok… Who PAYS for it, then?
            Is it going too be like now, when neither PV nor wind have any responsibility for regulation and balancing, or is there any thought on the part of the proponents of kitegen as to how to solve the problem?
            That is the vexata quaestio.

          • Willem Post says:


            Wind and solar cannot start a grid, are asynchronous sources tending to destabilize the grid. Both are weather-dependent, supplementary sources. Wind takes reactive power from the grid.

            Synchronous sources, coal, oil, gas, nuclear, biomass, etc., can start a grid, and provide stability. They provide reactive power to the grid, are not weaker-dependent.

            Germany had an installed capacity of 39,612 MW at the end of 2014. During 2014, the output varied from 29,687 MW (74.9% of capacity briefly during December) to 24 MW (0.06%). The average output was 5,868 MW (14.8% = the capacity factor). The production was 51,405.8 GWh.

            Output was between 0 to 10% of capacity for 45.5% of the time (3986.75 h).
            Output was in excess of 50% of capacity for 5.2% of the time (460.75 h).

            A graph of wind output during 2014, does not reveal any available base load, i.e., a reliable minimum output to rely on.

            During winter, solar is near zero, if panels are covered with snow and ice.
            Solar is near zero, or zero, about 75% of the hours of the year.
            Wind is near zero, or zero, about 40% of the hours of the year.
            Solar + wind is near zero during many hours of the year
            Solar and wind are variable energy, requiring 24/7/365 baby-sitting by the traditional (fossil, hydro, nuclear, bio, etc.) plants.
            Any missing energy, to satisfy demand at any time, must be provided by almost ALL traditional generators at least some of the time.
            All must be kept in good working order, staffed, fueled, ready to go, as needed.
            They do not need wind and solar to function, but wind and solar definitely cannot function without these generators, i.e., wind and solar are supplementary, are grid-disturbing cripples, 24/7/365, i.e., as unsteady drunks disturbing church service.
            Without output of these generators, the economy could not function.

            During high wind periods, Germany has excess energy production, which it exports to the grids of nearby countries to make its wind energy work, but that likely will become less of an option in the future. See URLs.


          • I was talking about batteries. They are blackstart capable on their own. Thermal plants do have to be kept blackstart capable and they get paid for doing so.
            Supplementary or not, wind and solar safe fuel that would have to be imported otherwise. They lower the cost of electricity and keep the German Industry competitive.
            PV isn’t getting more expensive either.
            And then there is KiteGen.

            Sure the grid doesn’t look like 50 years ago. If there was a superior solution there would be someone adopting it and drawing advantage.

          • Willem Post says:


            Batteries cannot start anything on their own except a car.

            In UPS systems, the batteries serve to keep critical systems going in case of a power failure.

            They provide enough power until emergency diesel generators are up and running.

            Once the DGs are running in synchronous mode, about 1 – 3 minutes, they provide power to critical systems, and maybe also to a local power plant, etc., if that power plant does not have its own DG sets.

            Wind and solar would be totally useless for that purpose.

            I used to design such systems for hospitals, trading floors, etc.

          • WEMAG and Younicos don’t share your opinion. Things change. It just needs new people sometimes to do envision and do things. There will be more turbines and more PV and more gridscale batteries.
            Kites batteries… isn’t that beautiful.

          • @Willem post
            your point about the grid bootstrap reveal a quite sophisticated knowledge of how the grid work.
            I agree that start an island grid with current static converters is tricky, however to recover from a blackout the EHV backbone need to be up and powered with enough reserve to progressively reconnect electrical districts. The KiteGen carousel concept could store enough kinetic energy in the rotor to deal with heavy district re-connections absorbing huge drawn pulses. Then the wings can modulate the power provided to the rotor in order to strictly follow the balance. The Stem, the single kite version generator is provided of super-caps for internal functions that could occasionally provide a quota of the bootstrap power.

            I’m often stressing that in energy cannot exist opinions, energy is so math oriented and thermodynamic dependent that is always matter of good computing practices. To boot a grid with little and sparse generators the re-connection strategy needs smaller blocs to be added in progression.
            Perhaps this is the real meaning of the mysterious “smart grid” ->how to effectively recover from blackout without synchronous generators.

          • Euan Mearns says:


            On this note I’d point out that I’m giving a talk to The Scottish Oil Club in Edinburgh on 3rd November with the simple title “Blackout”. With closure of Cockenzie caol (1.2 GW) and Longannet coal (2.4 GW) Scotland is now based on a system of 2.2 GW nuclear and about 7 GW of wind dangling on the end of 3.5 GW interconnection with England.

            I cannot be in Turin and Edinburgh at the same time 😉

        • Greg Kaan says:

          One advantage that the kites have over wind turbines is that the generation and retrieval periods are controllable (assuming sufficient wind for generation) so multiple kites in a farm can be set with time offsets, flattening the output. To what degree this can be achieved is to be seen but we KNOW that wind turbines are a dead end.

          The final farm output may still need some battery storage to act as an output filter but that’s what a trial is needed for. I’d much rather our governments fund a decent feasibility trial for the KiteGen concept than continue to throw money towards deployment of the mature, intermittent technologies that are guaranteed to fail.

        • Euan Mearns says:

          Roberto, intermittency is at two scales. The first scale is fluctuations in the output from a single stem. This is easily managed through coordinating the production cycles of a number of machines. The second scale is daily and weekly variations in wind speed. Maximum power is reached at 17 m/s. Ability to access that 24/7 will be site specific. But these machines are really light weight and cheap to build with high ERoEI. And so the theory is that it will be possible and economically viable to convert some power to a chemical store in order to provide dispatch. This is the same theory as for wind turbines where the theory does not work because we are dealing with expensive subsidised electricity where it is out of the question wasting two thirds to provide dispatch.

        • Eugenio Saraceno says:

          Just to recall the past discussions the large pink fluctuations are full power recalls made during a test to be compared with sideslip recalls (the little pink fluctuations)
          The fluctuations to deal with are of course the small pink ones as no one wants to recall the wing with full power as it is possible to do it by sideslip.
          For a single kite installation the sideslip energy consumption is covered by a supercap rack that also smooths the production peaks. The graph of the black box would look like a trapezoidal waves with short zero power intervals.
          An array of kites working with different phases would smooth these intervals and look like a constant output with small fluctuations that is far better for the grid than a wind farm.
          The overall power output of the farm may vary as the wind can change to weaker or stronger but the kites could adaptively change the operation altitude to try to keep the power constant or to vary within a timeframe compatible to the grid specifications.

  5. Greg Kaan says:

    Euan, you can add the incredible speeds achieved in the last America’s Cup by the AC72 yachts as a demonstration of the power that can be produced by lift from rigid sails.

    Unfortunately, the staggering cost of these yachts and the immense amount of training needed to sail them means we may never see them in action again.

    All the best for Massimo and his Kitegen team and to you in this venture – it is by far the most exciting development I have seen for wind powered electricity generation

    • Greg Kaan, thank you very much for the appreciation.
      After looking at your links my thought is that cheap energy availability is precursor of an easier and more amusing life for everybody, despite that, is much easier to involve professionals and spend a lot of money for such spectacular ventures.
      i.e. Don Montague, that is the protagonist of the exciting Kiteboat development, has gifted us of several sport kites, of his production, for testing with KiteGen and only in change of the picture when finally blasted.
      He preferred by far the adrenaline, spending his life flying at full speed over the sea instead to help with the development of the special big wing for energy production :-). Hi Don! ;-).

  6. Hi Euan and Massimo,
    I’m a big fan (forgive the term) of all things Kite energy production. Wish you all the best. I’m also involved in a kite energy scheme, although mine is very low tech compared to the kitegen mega machines.

    • Euan Mearns says:

      Rod, thanks for your supporting words. I had a quick look at your site and surmise that your designs are based mainly on drag (parachutes) and not on lift (aerofoils). There is no doubt in my mind (and that is a tiny geologist’s mind) that the future of high altitude wind lies in aerodynamic lift and not drag. Lift provides the power multipliers.

      • Thanks for taking a look Euan. My rotary kite stacks are continually going cross wind exactly like the sailing configurations you show. Double check with the likes of Aerotrope who helped Vestas on sailrocket. I’m transmitting torsion a lot like a small version of Massimo’s carousel design. It’s easy to stack, pack and track a small ring set like this.

  7. Olav says:

    Kite looks good to use in low wind envirioment like Poland interior,Hungary and similar places. Traditionally wind turbines are to low to work well there. Maybe the wind there is more steady to.

    • Euan Mearns says:

      Olav, you hit one nail on the head here. Over most of the land surface of Earth, the wind speed is too low for ground-based turbines. But at 1500 m there may be enough wing to produce 3 MW continuously from a kite.

      In such areas the atmosphere dynamics are much more stable and favourable for “long term” continuous production from the kite pumping cycle.

      • Olav says:

        Thank for that Euan. This was my thinking during last post you made about this. But I was travelling to much then. I think East European countries struggles with EU obligations. Only solar will work but not at night and winter output is poor.. Wind is more there all the time and especially at high altitude, Maybe Massimo should look in that direction as competition from traditionally wind turbines for founding is less there.
        If a sucessful demo is done there with base load possibility then it is interesting.

  8. pyrrhus says:

    Congratulations! Sounds like a much lower capital cost way of tapping solar energy (prevailing winds) that would be highly scalable. I look forward to more reports!

  9. Rob says:

    Wish you every success it’s tough to land new contracts in the current climate and nice to hear about new British engineering design.

    Good luck


  10. Peter Lang says:

    Euan and/or Massimo Ippolito

    I’ve often asked, but received no satisfactory answer so far, how the risk to aircraft will be mitigated?

    I’ll pose the question a different way this time. Let’s consider a scenario or replacing France’s nuclear power plants with wind kite electricity generators.

    How many cables would be required, reaching what altitude, and where would they be located to feed the existing grid? What argument would you propose to convince the aviation industry they pose negligible risk to aviation?

    France generated 77% of its electricity from nuclear power in 2014; 63.2 GW supplied 416 GWh (net) (i.e. 75% capacity factor).

    Let’s assume France’s fleet of wind kites can achieve half the capacity factor of nuclear with equivalent availability (an optimistic assumption?). Therefore, 2 x 63.2 GW = 126.4 GW (net) of wind kite generation capacity would be needed to supply the same energy as the nuclear plants. At 3 MW (net) per wind kite, 42,000 wind kites, with cables to 5000? m altitude, would be required. These would be distributed around France (like the nuclear power plants).

    Can the aviation industry be convinced to accept the risk to aviation?

    • Aren’t existing nuclear site already flight restricted zones? Why not replace each with a 3-5GW plant and fly between 1000-1500m?
      I guess you could just avoid planes if one really would happen to cross the flight path. Radar stations are also part of the design I guess.

      • Roberto says:

        Each of the 19 nuclear power stations in France occupy a surface on the ground of ~2 km2… On that surface you can squeeze in at most a few 10s MW, certainly not GW…

      • Eugenio Saraceno says:

        Jenny is right (as usual 🙂
        many P-Zones or D-Zones or R-Zones are on the aviation map for many reasons:
        Nuclear plants, Military bases, dangerous chemicals and so on.
        P-zones stays for prohibited and D-zone stays for dangerous, R is restricted.
        It is important to recall that the forbidden zones are cylinders with a maximum altitude so that aircrafts can fly over the zone if it passes over the cylinder.
        Civil airliners usually (except near airports) fly over 6000 m. AWEC systems usually work well under these altitudes. There is no risk for them if kite systems are operated far from airports.
        The risk is for small planes and helicopters flying at low altitudes. If they do not listen to NOTAMs or do not update the maps they can enter in a dangerous zone.
        There is also a problem with helicopter rescue that needs to access also to no fly zones if there is an emergency.
        All these cases can be resolved by different technology layers that can be added to improve the safety of the kite operations.
        As I said the simplest solution is a restriction of flight on the KiteFarm area. (P-Zone or R-Zone or D-Zone) This is very common for nuclear or petrochemical plant. A typical Restricted zone is 5000 ft high from ground level and has a radius of one Nm. This 16,4 km³ volume could contain a medium size farm (up to 150 MW) with virtually no collision risk.
        An active safety strategy would be using radar targetable wings (they must have embedded metal plates) so that a radar station could notify kite-farm control room when a plane is entering the farm zone.Farm. The control room can drive the wings to form a safety corridor in a minute.
        Wings, just as every aircraft does, could be equipped with flarms that communicate position and speed to other aircrafts. When an aircraft has to cross the farm area, the farm control room detects an ongoing collision and drive the wings to move towards areas enough far from the aircraft route. This protocol can be enforced also by a radar centre communication .The Kite farm control room can have a radar equipment to detect ongoing collision and drive wings to safe areas. Radar surveillance could be useful also to manage safe corridors for bird migrations

        • Willem Post says:


          You have a lively imagination,

          With a busy sky and weather events, i.e., the real world, the hundreds of such sails to serve a larger city with an airport would have to be placed about 10 miles from that airport.

          And each unit would have to be placed about a mile from the next unit.

          In summer there are many days with no wind, even at high altitude, so a complete complement of traditional power plants would be required.

          Wind and solar could supplement energy AT TIMES.

          • Eugenio Saraceno says:

            @Willelm Post
            ten miles from the airport? Right, also 20 if you want, what is the problem of staying away from airports?
            one mile from unit to unit? We claim shorter distances but if it was wrong 1 mile is also good if one just figures out how much unused space there is in many countries.
            How many days with no wind in altitude? Do you have a statistics?
            On the availability of wind in altitude please read carefully this, especially chapter 3.
            There could be temporary local wind calms in altitude but this just mean that the constant wind stream has been deviated and it blows strong at some hundred km away. A mesh of farms geographically deployed, having no grid constraint, has always (95% of the time) enough wind to produce at nominal power)

      • Peter Lang says:

        You guys are guessing. Ask the aviation regulator.

        Planes don’t always go where they are supposed to. Read up on air crash investigations. When a plane is having problems and the crew is intent on trying to work out how to address it and land, they have more important things to worry about than restricted areas. The priorities for the crew are: aviate, navigate, communicate.

        Small aircraft also have to be kept safe

        Stop guessing, and find out whether aviation regulators would allow it.

        • gweberbv says:


          in former times ships were allowed to freely navigate through North Sea and Baltic Sea. In a few years from now, significant areas will be crowded by offshore wind turbines with just a few corridors left for seafaring.
          The same might happen for the lower zones of the sky.

          IF power generation by kites is technically feasable and economic attractive THEN regulations will be adapted to the new reality.

        • Eugenio Saraceno says:

          No guess at all,
          Already asked the permit from the aviation regulator and the outcome has been a new a D zone on the map
          (D318) with coordinates
          45°01’41”N 007°15’35”E.
          of radius 2 NM and max altitude 11600 feet AMSL.
          Specifically created to operate a kite system

          From the perspective of the aviation authority there are a plenty of areas of no interest for civil aviation. In this case it is a mountain area where an aircraft would never fly. Even in an emergency situation it would be impossible to avoid a crash when flying so low in the middle of a mountain range, with or without kites.
          Small planes simply must avoid that zone as they must avoid all forbidden zones, kite or not kite. Try to fly over the White House with a cessna.
          We are trying to solve an energy problem that is more important than allowing cessnas fly wherever they want.
          Would you renounce to build highways just because they are dangerous for bikers? Highways are necessary for the modern economy and bikes could have their lanes nearby
          As I stressed in the previous message the only threat is for emergency and rescue flights. We are ready to deal with all the stakeholders to find the specification of an auxiliary radar/flarm/transponder based safety system with protocols to allow safe passing of aircrafts inside the kite farms.

        • Peter Lang says:

          Eugenio Saraceno,

          Your answer dodges the question. The question is about 42,000 cables to 5,000 m elevation distributed around France, – and ultimately cables to 5000 m in all countries if we want to go renewables (an impossible dream) instead of nuclear to power the world in future.

          I notice that Euan and Massimo Ippolito also did not answer this question, which leads me to believe it is a show stopper!

          • my answer is just few step below.

            This very big program to substitute the nuclear fleet in France, if really needed, ….

          • Peter Lang says:

            I saw your comment. It does not address the issue. And your stated capacity factor for your device at a fleet capacity needed to supply 77% of France’s electricity, with the same availability as nuclear, is not credible. You are comparing best case for your device verses long term average at a high proportion of total grid electricity generation with load following to meet demand. I suspect your estimate is overstated by a factor of 2 to 5 or perhaps more.

          • Why not credible?

            you have a preconception about wind turbines that barely touch the huge resource, our effort here was to explain that there is a disruptive novelty in the energy scenario and everybody have to reset their belief.

            This is Spain, not France, but the method is well exposed and the behavior of high wind exploitation is fully baselod more than 99% @ 1500m:

            so 70%, as nuclear, could be considered a worst case, or a guarantied availability, In any case this is a high altitude wind feature, KiteGen has no big merit here.

            What issue isn’t addressed yet?, perhaps I’ve miss the thread, in any case for repetitive base questions we have a FAQ:

          • Peter Lang says:

            Massimo Ippolito,

            In any case this is a high altitude wind feature ….

            What issue isn’t addressed yet?

            The issue of aviation safety with cables dangling from high altitude hasn’t been addressed. It’s a show stopper. It means your device is unlikely to be acceptable. It cannot provide a significant proportion of global electricity, let alone global energy. Conversely, nuclear can. Nuclear can provide a large component of global electricity effectively indefinitely. So why go past the obvious?

          • Euan Mearns says:

            Peter, the first 200 commercial machines will be deployed in Saudi Arabia, over desert where a comprehensive wind study has already been conducted. Do you not think this technology should be afforded the opportunity to prove itself first before you simply declare it to be a show stopper. People can then look at the LCOE and risks and then evaluate if it is worth deploying in their particular country. Massimo may disagree, but I think the KiteGen is designed to be deployed in the white areas of the map where surface winds don’t blow hard enough for ground based turbines.

            Figure 6 Areas of Earth where a sufficient wind resource to drive wind turbines are found at surface (hub height 100 m). Map from Makani Power.

            Else where on the thread someone suggested that Poland could be a candidate country. If it is proven that KiteGen can produce cheap, reliable dispatchable power, do you not think that the Polish government might not set aside a 100 sq kms of forest somewhere and declare it a no-fly zone?

            You’ve made your point.

          • Eugenio Saraceno says:

            @Peter Lang
            Also powering all the world with nuclear for an unlimited amount of time is just a dream, far more difficult than having km of tethers and no fly zones around the skies

          • Peter Lang says:


            In your response to my comment you asked me several questions then ended with “You’ve made your point”, which I interpreted to mean: “no more on this issue, please”.

            Then Eugenio Saraceno @ October 12, 2016 at 5:13 pm, chimes in with his last comment which no one has refuted yet. Am I to interpret that you are going to allow/encourage such comments on Energy Matters to to stand without being refuted? That is the impression I get from your comment and intro – i.e. “not too much challenge to this wind idea, please.”

  11. John ONeill says:

    I don’t think there’s a general restriction over nuclear plants – the US instituted one after 9/11, but I think it was temporary. Finland has a 4 km radius to 6,500 feet over its two plants, as does Hungary. Checking NOTAMs ( Notices to Airmen ) for the UK didn’t seem to show anything.
    The worry isn’t the kites, which are visible, but the cables, which are not. Power lines across valleys have killed a lot of people in New Zealand, even though they’re all mapped and often have big round markers on them. I’ve come a bit close myself once or twice ( on a hang glider you’ve got more time to look for things, but no power to climb over them or, sometimes, penetrate upwind. )

  12. steve says:

    How do the kites and ropes avoid damage by lightning strikes? Planes on airport approaches are often at similar altitudes for 20 miles or more.

    • Eugenio Saraceno says:

      wings and ropes are made from composites and polymeric materials, no conductive paths to the ground. There are no reasons to fear of a special attraction of the lightning nor to exclude that occasionally the lightning could strike a wing.
      The ground machinery in any case is electrically designed to manage the lightning strikes.

      • “wings and ropes are made from composites and polymeric materials, no conductive paths to the ground”

        The fact that they are up there means that there is a conductive path to ground albeit not a very good one surely?

        Still even so, it does not discount the lightening striking the system and the discharge damaging the wing.

        • Eugenio Saraceno says:

          lightening strikes are not to be excluded at all as I said.
          But plastic materials are also unable to resist even at very low temperatures if compared to how a lightening stroke could do. It would be a fuse unable to propagate the discharge.
          It is to be calculated the very low probability of such events and see how much can cost an insurance to refund damages, tacking in account that a operating power wing could earn some 150$/hour and can afford paying its life insurance.

  13. This very big program to substitute the nuclear fleet in France, if really needed, could better achieved with the KiteGen Carousel configuration that improve the power converted by each kite almost four times, thanks the higher speed of the base sliding frame (20m/s vs 5m/s of reel-out of the Stem) maintaining a similar force on ropes and angles of application. The overall capacity factor of Nuclear Plants in France is 70% very similar to HAWP, so the right figure is close to 5000 wings not 42000.
    All 5000 wings weight just 1000 tons and cost approximately 300M€/year as consumable. This for a production of 400TWh/year also said €10billion/year of wholesale electrical energy. The current uranium bars replenishment for the 57 reactors cost around €4billions/year. Those are evidently ballpark figures, however any promise of HAWP if possibly inaccurate could easily maintained with slight adjustments to the operative parameters, tanks the absence of constrains to scalability: improving the wing aerodynamic efficiency or strength, widening the wing surface, elevating the working altitude.

    • John ONeill says:

      The capacity factor for nuclear in France is about 70 %, because they load follow, but the ‘Unplanned Capability Loss Factor ‘ over three years varied between 4.4% and 8.9%. That’s a bit below the trend setters ( Korea, 0.6 to 1.2 % ), but still a lot easier to plan around than 30% weather outage.

    • Peter Lang says:

      By the way, the capacity factor of France nuclear was 75%* in France in 2014. It’s been over 90% in the USA for a decade or more.

      As I stated in my first comment: “France generated 77% of its electricity from nuclear power in 2014; 63.2 GW supplied 416 GWh (net) (i.e. 75% capacity factor).”

  14. Eugenio Saraceno says:

    Some technical notes on the force and power management by the KiteGen team

    Once you have your kite maximum allowed force Fmax (with some safety factor) and the kite efficiency E (by spec of your system) the cut-in wind speed Vcut can be calculated

    let’s recall the cross wind force equation where w is the wind speed




    Over Vcut if you do not allow your kite to unspool the force would overcome the nominal force but you can ask your alternator PID control (proportional integral derivative) to maintain constant the force by allowing the rope to unwind.

    Then each additional m/s of wind speed over your cut-in is useful to reel out and produce power. If you let your rope reel out at w-Vcut THE FORCE REMAINS UNCHANGED Fmax but the kite will do a work Fmax*l where l is the length of the unwound segment of rope. To do this work in a certain time frame it expresses a power P=Fmax*(w-Vcut).

    Then you have also a spec for your maximum reel out speed (say 150 rpm)
    so you may calculate at what wind speed you reach your nominal power Vmax
    If you are not able to limit your max speed you will overload your power electronics and risk overheating the alternators and the servo converters.

    It is needed to fly with a non optimal angle to derate the power, this is like reducing the wind speed. In fact when you fly crosswind (i.e. the wind direction perpendicular to the wing direction) you are in the power spot (i.e. the maximum lift force exploitable). It is enough to change this configuration to reduce the lift force.

    Alternators can express a torque (tangential force F times the radius r of the pulley) and a speed. The power adsorbed by the alternator to make the pulley rotate at a certain speed v with a certain torque is F*v.

    On the other hand the alternator produces a power P=F*v when the rope on the pulley is pulled with a force F making the rope unwind at speed v.
    Limiting the force to Fmax means just commanding to an alternator to exert a torque Fmax*r without moving.

    The alternator will not produce work until some counter force exceeding Fmax is exerted on the rope as exceeding Fmax means make the pulley move at a certain speed.

  15. Leo Smith says:

    Energy from wind depends on exchanging the kinetic energy of the wind relative to the generator(ground) for energy in the generator.

    The swept area of a traditional turbine is the area over which the energy is collected. The wind is slowed over that area.

    I utterly fail to see how a kite on a string actually slows the air over a larger area, or more than, a traditional windmill..


    • Euan Mearns says:

      Leo, have you watched the vids? They become progressively more awesome. The boats convert the KE of the wind into the KE of the boat without visibly slowing the wind at all, because the boats are moving and always sampling fresh wind. Watch the vids and feel the power.

      • Envisioning the energy transfer is a real issue (air being transparent and all that.) A good playful approach helps see what happens with wings. Slide a knife through soft butter, a spatula through a sink of water, a paddle past your stand up paddle-board.
        Forces between the viscous media and foil always balance. The air mass is decelerated (deflected sideways) the boat is accelerated (squirted to the other side (bit like squeezing a lemon pip))
        The deceleration of the air is temporary, it soon gets dragged back up to flow speed by it’s air molecule buddies.

        • 1saveenergy says:

          “The deceleration of the air is temporary, it soon gets dragged back up to flow speed by it’s air molecule buddies.”

          BUT, the whole column of air has been slowed by the amount of energy removed (however diminutive it maybe).

          To be more pedantic; the friction of air molecules ‘dragging’ their buddies back up to (the new) speed, raises the temperature (by a minuscule amount).

          Therefore, it could be said that extracting power from wind contributes to global warming……isn’t physics fun !!!

          • Oh 1saveenergy that’d be quite funny. It’s not true though. And these energies certainly aren’t diminutive.
            Physics is enjoyable & enlightening when scales are applied correctly. It’s good that you can see the large volumes of air being affected.
            Consider wind comes from a heat imbalance in air which causes flow over massive distances. Think of wind KE as heat flux if you like. We can’t create nor destroy energy, only convert it with varying degrees of efficiency. With kites, we remove energy from the wind by affecting the gas flow in a small volume of the larger field. We convert some wind KE into useful forms, we lose some to friction etc. Wind KE is continually being lost to friction through viscous drag on the earths surface. Atmospheric pressure (Caused by density and depth of atmosphere..) is why it’s warm enough on earth…
            I’m not the best to describe the intricacies… try
            An Introduction to Meteorology for Airborne Wind Energy Cristina L. Archer
            or check out some of the other works here
            This one was kinda relevant to what you’re saying

        • Euan Mearns says:

          I may already have said this on this thread? I think a hooked salmon provides a good analogy. When a salmon is hooked by an angler it “runs” i.e. swims away from the angler vigorously unwinding line on the reel. In this analogy the reel is the generator, the rod is the KG Stem, the line is the rope and the salmon is the kite. The angler can alter the tension on the reel making it harder for the salmon to run and it must therefore expend more energy. When the salmon tires, it swims back to the angler – the side slip – before running again – the pumping cycle.

          The alternators are set to 150 kN each. Once that force is exceeded the ropes unwind on the drum generating electricity. Most of the force is derived from the lift of the kite (PowerWing). There are dozens of groups developing prototypes based on similar concepts. It clearly works. More wind = more lift^2

          From Eugenio:


          Over Vcut if you do not allow your kite to unspool the force would overcome the nominal force but you can ask your alternator PID control (proportional integral derivative) to maintain constant the force by allowing the rope to unwind.

          • I love the salmon analogy Euan. I used to spend my summers as a ghillie on Grimersta fishing estate. I had to row about a million oar strokes per summer while the guests fished. A really good place to consider flow.
            A salmon in a river can fight harder than a salmon in a loch… it uses the flow to its advantage.
            There are plenty of small kite designs which can take high wing loading in high wind.
            This one suits this discussion…
            So having seen that… What do you reckon to my idea of using tilted rotary kite networks to transmit torsion?

    • Leo, I really like the way you phrased the first two paragraphs.
      One advantage of kites is that the whole kite surface area is moving at it’s top speed. A common analogy is that a kite acts like the tip of a HAWT blade. The tip sweeps a much larger area than the root. The HAWT blade root is really only there to transfer torsion.

  16. jacobress says:

    What ground area do you need for one 3 MW unit? Or: how tight or close can many units be packed? Or: what ground area do you need to produce 1 GW (at optimal or average wind speed) ?

    • @jacobress
      The wing is an full featured aircraft, with control surfaces and continuous path control, a mean position error of 10 m around the control set-point, appear achievable with a feedback loop of 16ms. The safety factor of 10 adopted in the machine specs suggest a distance of 100m between machines and wings. So the packed array could be in perspective 100 machines km2, so 3 km2 to produce 1 GW, the wings will fly at slight different angles/altitude in order to avoid reciprocal wake effects. Those figures are the result of study and simulations, in an early phase we would prefer a more sparse matrix that could be completed when the MTBF will be better asserted.

  17. Grant says:

    I’m not intending to compare or promote an alternative here but as this news popped up a few days ago on the BBC (complete with the apparent approval of a WWF spokesperson no less) I felt it might be pertinent to see a reference here.

    If nothing else it suggests that some doors seem to be open with occupants prepared to listen.

    Secondly I can’t see a problem of conflict of interest from flying.

    If the Paris agreement does indeed result in countries around the world rushing to achieve a zero “carbon” economy mass air travel and private travel of any kind would almost certainly have do be discontinued so leaving no conflict at all.

    At least that would force the COP meetings and similar to to adopt communication policies that did not conflict with their messages – something they must find unbearably painful from a moral perspective.

  18. Grant says:

    AH! Apologies!

    I see the West Freugh announcement is included towards the end of the last Blowout Week post.

    I had only glanced through it (having been distracted by the Kites and Boats) but not far enough it seems.


  19. David Whannel says:

    Overall can KiteGen + current backup or future backup predictions compare with upcoming MSR prices (Moltex offering £30.35/MW, Transatomic 520MW plant just $1.7bn) and can it be affordably installed offshore over the horizon preferably? Considerations of future nuclear seawater extraction as a renewable becoming more affordable as that technology advances along with all possibility of both being replaced in future decades by fusion or advanced breakthroughs in chemistry such as hydrogen production would also help if any guesses can be added?

  20. I am not sure that the conversion of kitegen versus nuclear or even the comparison of the two is worth while. Clearly this is a prototype or early stage demonstration project and this will have no impact on immediate funding for other energy generators.

    Like Peter Lang, i do have some issues about control and aeroplane issues. However I am not sure that I have a major issue with where to put KitGen. We rarely have flights over our industrial complex (usually military) which if we take near shore, would offer a strip of 5 km, probably reduced to 3 km to avoid shipping. There are a number of former sites around the UK like this which I bet would have limited to no flights overhead (e.g. Redcar). That is before we go to sea or to deserts etc.

  21. Euan Mearns says:

    This one’s for Roderick showing Massimo’s prototype zero from a little more than 10 years ago.

  22. Hi Euan! It’s Islin. I was wondering if there was any work done on the optimal latitude (e.g. Alaska) hat this technology could work at. I’m assuming that with your wind speed simulation, you would have a pretty good idea of optimal latitudes.

  23. Euan Mearns says:

    @ creatingpotential, Hi Islin,

    Figure 6 Areas of Earth where a sufficient wind resource to drive wind turbines are found at surface (hub height 100 m). Map from Makani Power.

    Figure 7 Areas of earth where a useful wind resource is found at an altitude of 250 m. Map from Makani Power.

    The two maps from Makani, show average surface wind speeds at 100 m and at 250 m. You’ll see that S Alaska is one of these windy places where I imagine (and I could be wrong) that its very difficult to handle the big kite on the ground. And I’m unsure about performance in the Arctic where it will clearly be very cold a lot of the time.

  24. Eugenio Saraceno says:

    The most comprehensive work about altitude wind is Archer & Caldeira’s Atlas of the high winds

    One of the project main objectives will be working at nominal power with winds no more than 10-12 m/s so all latitudes are optimal; the tropical latitudes are less good.
    Polar is perhaps much more than the need.
    By the way it can work also with stronger winds as it is not required to work in the power spot (i.e. perfectly crosswind)
    The maximum exploitable wind is 70 m/s as the wings fly at 80 m/s

  25. oldfossil says:

    Have you thought about underwater kites to exploit ocean currents? Less variable, high energy density, apart from a whole lot of fish there’s not much to run into down there.

    • Dead wrong concept IMO. Ocean is a delicate ecosystem that exchange no more than 60TW of kinetic energy, mainly driven by the wind itself, with a balance of 3600TW and 100PWh accumulated in the geostrophic stationary regime. The ocean breathe with currents preserving sea life, braking currents likely isn’t a good idea.

      it is true that water is 800 times denser than air, but is also generally characterized by fluidic speeds well below one meter per second, this it is true in the rivers, in the sea currents, the effect of the tides and waves.

      The energy potential of the fluid depends linearly on the density, but the cube of the speed, it is sufficient that the air to move at least 9.4 times faster than water, the subject of energy utilization, that the gap due to the higher density vanishes, the ‘air that we can exploit is typically 50 times faster than water.

      But the great advantage of faster winds vs the currents, is the minimum amount of material and the weight of energy harvesting devices, in the case of water it is necessary to withstand forces 50 times more intense for the same production performance this implies much stronger and heavier structures that very negatively affect both Capex and life cycle analysis of the generator, this without adding the considerable increase in the cost of maintenance of machinery in water, especially with salty environments.

      • 1saveenergy says:

        it’s only waves that are powered by wind, waves are erratic & unpredictable.

        Tidal streams are powered by gravity (moon & sun) & are totally predictable

        You say
        “The ocean breathe with currents preserving sea life, braking currents likely isn’t a good idea.”

        How much energy do you think we can remove to be able slow the tidal streams ??

        IF extracting energy by slowing a natural process is a bad idea, then we also shouldn’t slow the wind by extracting energy from it.

      • oldfossil says:

        Thanks, I forgot that it’s the cube of the speed.

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