This post reviews the weird and wonderful world of high altitude wind power. It looks into the reasons for wanting to go high, explains tethered flight and explores the main competing technologies of 1) airborne generation (Google Makani) and 2) ground based generation (KiteGen) and compares their strengths and weaknesses.

Executive Summary

High altitude (500 to 2000 m) wind power machines are still at a relatively early stage of development with the first tests run about 12 years ago.

Over most of the land surface of The Earth, average wind speeds at surface are below 5 m/s, well below the velocity required for conventional ground-based wind turbines to operate. Hence traditional wind turbines can only be deployed in selected areas where surface winds regularly blow strongly and steadily.

250m above the surface winds blow more strongly and steadily providing usable wind speeds over most of the land area of The Earth. As a rule, the higher you go the wind speed increases. The challenge is how to access this wind to generate electricity.

Many companies are engaged in developing prototypes. Most are based on the principle of tethered flight where an aerodynamic kite flies at altitude tethered to the ground by ropes. The wind keeps the kite aloft where the flight speed is related to the aerodynamic efficiency and wind speed. In turn, power is linked to flight speed.

These kites fly cross wind, that is in a direction that is transverse to the wind direction.

There are two main families of tethered kite. One where a simple kite flies a figure of 8 pattern in the sky drawing rope from a ground-based drum that turns a generator. The second is based on kite-like gliders that carry generators on board where the flight movement turns rotors generating electricity that is transmitted to ground via a conducting tether.

There are three obvious risks associated with this technology 1) the risk of crash, 2) the risk to aviation and 3) the risk to birds. Much of the current research and development is going into flight control systems to mitigate the risk of a crash. If the technology can be proven to work then it promises to deliver cheap, subsidy free renewable energy in which case governments may clear selected air space for technology deployment.

These risks are balanced by a number of clear benefits of accessing high altitude wind:

Wind power can be accessed at altitude where wind speed at surface is insufficient to drive a traditional turbine. The wind blows more steadily at altitude reducing intermittency. Power increases by the cube of wind speed. Thus going a little higher may produce a lot more power. High altitude devices are significantly less massive (20 tonnes for 3 MW) than traditional turbines of equivalent rating (417 tonnes of steel and 902 tonnes of concrete). Hence, high altitude devices will be significantly cheaper to make and may produce subsidy free electricity. The low mass translates into less energy used to create high altitude wind capture systems. Back of the envelope style calculations suggest ERoEI >300 for a KiteGen stem device. This is massive compared to other forms of renewable energy apart from hydro and offers scope to mitigate for intermittency by sending some of the power to chemical energy storage. The wind front of a ground-based turbine is defined by the area swept by the turbine blades. A 3 MW turbine may sweep 8000 m^2. This limits the amount of kinetic energy a turbine can extract from the wind defined by Betz’ law to be 59.3% of the available kinetic energy. In comparison, because a kite flies across the sky, a massive wind front of about 1,000,000 m^2 is available (1500 m rope length, 50 degree elevation).

Introduction

Several weeks ago I received an email from Massimo Ippolito, the founder of KiteGen, enquiring about advertising on Energy Matters. KiteGen are a world leader in the development of high altitude wind technology. While I was delighted at the prospect of selling some advertising space, the enquiry came with some strings attached. Massimo wanted me to write an article on high altitude wind power. I explained I knew nothing about this having not followed the technology, believing it to be a bit ‘bonkers’. Massimo confided that everyone thought that to begin with.

And so I did a little research and found some information that caught my attention. For starters, there are many companies active in this arena and one of them, Makani, had recently been bought by Google for $10 million. Secondly KiteGen claimed that high altitude kites had high energy return on energy invested (ERoEI) >300. This was the real hook, because if true, this would make high altitude wind power dirt cheap and this could substantially ease, or remove altogether, issues with intermittency (see Appendix 1 on ERoEI). And finally I came across a very unkind article called Airborne Wind Energy: It’s All Platypuses Instead Of Cheetahs that was published in Clean Technica. It struck me as rather odd that a Green Tech blog should publish an article that focussed only on the potential weaknesses of high altitude wind, totally ignoring the strengths. Could there be any truth in the notion that Green Tech does not want a cheap renewable solution?

This post provides an overview of high altitude wind covering basic principles, the main types of competing technologies, ERoEI and some of the basic physics.

Why Go High?

The primary reason for seeking wind at high altitude is that the wind tends to blow faster and more constantly the higher you go (Figure 1)(see also Figures 6 and 7). Add to that the nominal power increases with the cube of wind speed and you will understand the attraction of reaching for the sky.

Power=0.5*r*A*V^3

r=air density

A = area

V= wind speed

The reason for lower wind speeds at surface is wind shear between the circulating atmosphere and the surface of The Earth. Surface topography and features break up the circulation. It is useful to imagine the flow of a river that will normally be much slower at the edge than in the middle where the main volume of water can flow unimpeded by boulders and branches etc.

Figure 1 Average Dutch wind speed with altitude. At 2000 m the wind blows on average three times as fast as at ground level and given that power increases with the cube of wind speed, 27 times the power is available 2000 m up. High altitude wind devices are all about trying to access that additional high altitude power. While there is often talk about accessing wind in the Jet Stream 10,000 m high, the focus at present is to tap into the layer between 500 and 2000 m. Data for chart provided by Massimo Ippolito.

This is the reason that ground based turbines have grown taller and taller as they reach upwards to access that better wind resource (Figure 2). But this has also been the Achilles heel of wind turbines since as they have grown taller they have grown more massive and ever larger quantities of steel and concrete (embedded energy) are required in their construction. A three MW turbine (Vestas 3112) contains 417 tonnes of metal in the tower and nacelle and 902 tonnes of concrete in the foundations.

This provides the second reason to reach for the sky since the designs of the high altitude devices are much lighter than turbines they promise to deliver much higher energy return (ERoEI) and lower cost electricity. Hence the opportunity is to access greater power using lighter and cheaper devices.

Figure 2 The evolution of wind turbine size. In 1995 the hub height of a 750 kW machine was 50m. In 2015, the hub height of a 6 MW machine had grown to 120 m. Image from Makani.

Figure 3 The evolution of hub height with time.

Figure 4 The evolution of wind front (area swept by rotor) and power rating with time.

Ground based turbines have an operational wind speed range of approximately 3.5 to 25 m/s (these numbers will vary with turbine size and design). 3.5 m/s is the cut-in speed where the rotor is turning with sufficient force to drive the actuator (the generator) and 14 m/s the speed at which the turbine reaches its maximum power rating (Figure 5). Turbines may continue to operate beyond 14 m/s but will not produce any more power. And when it gets too windy at say 25 m/s the turbine needs to be shut down. These features define the all too familiar intermittency issue with wind. No wind – no power, too much wind – no power.

Figure 5 Wind speed versus power for a “typical” turbine.

14 m/s = 50.4 km/h = 31.3 miles / h

Makani has produced a couple of nice wind speed maps of the Earth (Figures 6 and 7). I live in windy Scotland and take wind for granted and was therefore surprised to see that windy places are the exception rather than the rule. Bearing in mind an operational range of 3.5 to 14 m/s for ground based turbines, Figure 6 shows that ground-based turbines are quite simply not viable in most areas. Climb to 250 m however and a useful wind resource can be found over most of the inhabited land mass of the planet.

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.

However, these average wind speed maps do not tell us how steady the wind is blowing. In his PhD thesis, Lorenzo Fagiano [1] provides an estimate for capacity factors at 500 m altitude in Italy and Holland which average to 0.54 (Appendix 1). Altitude therefore reduces but does not solve intermittency alone. To solve intermittency completely depends upon the argument made for high ERoEI (Appendix 1) where a portion of production can go to chemical storage, for example hydrogen or synthetic fuel, that can be used for generation when the high altitude winds drop. The round trip efficiency of storage is of the order 30%, hence a significant portion of production may be lost, but that is the price of dispatch. The anticipated lower cost of high altitude wind makes this acceptable.

The Competing Concepts

Exploiting high altitude wind power is an innovation space still at a very early stage of development. For example major player The Kite Power Research Group of Delft University of Technology initially tested a first experimental prototype of 3 kilowatts (kW) traction power in 2007. While KiteGen tested their first 30 kW prototype a year earlier in 2006. This is only 10 years ago.

Figure 8 Some of the main players and sponsors of high altitude wind power technology. Image credit KiteGen.

These innovators have filed dozens of patents and this has driven innovation in a number of bizarre directions as new entrants seek to secure a toehold in what may develop into a multi-billion dollar industry. This reminds me of the early days of aviation (see video 6 at the end of the post).

All of the main-stream electricity generation concepts involve tethering a high altitude airborne device to the ground. The basic physics principle is to harness the kinetic energy of the wind and convert some of this kinetic energy to electricity. With dozens of players I’m going to summarise the three main concepts:

Lighter than air devices that lift a turbine to altitude using a helium-filled balloon (Air Gen) (The Altaeros Blimp) Kites that fly through the air with rotors and generators attached making more electricity the faster they fly (Air Gen) (The Makani Kite) Kites that fly through the air with ropes that uncoil from a drum on the ground that rotates and drives an actuator (Ground Gen) (The KiteGen Stem)

The blimps are not serious contenders for grid-scale generation but are more intended for niche applications. The kites (2 and 3) are both serious contenders with different strengths and weaknesses.

The Altaeros Blimp

I will kick off with one of the most unlikely entrants which comes from US company Altaeros. Their concept is to simply lift a rotor-based turbine into the sky using a helium filled balloon. The balloon is tethered to a ground station by two tethers one of which must obviously be conducting to transfer power.

Figure 9 The Altareos lighter than air generator.

Figure 10 Altareos reaching for the sky.

Video 1 2 mins 19 secs

I have not found any detailed presentations from Altaeros nor have I found the power rating of their turbine, but it doesn’t look very large. Somewhat confusingly they call their balloon a BAT which I have to guess means Balloon Airborne Turbine?

This concept does not really capture the power of the wind in the way that a kite does (see the Makani and KiteGen videos below) and one can envisage in a strong wind that the device would get blown towards the ground losing any advantage that altitude might offer.

Reading the Altaeros web site I get the feeling that this is not a serious entrant for full scale power generation but it may instead have niche applications:

Power to remote communities An elevated platform for telecommunications systems An elevated platform for defence surveillance systems

The Altaeros BAT has the advantage of being simple, and mobile and it may turn out to be the first commercial generator of high altitude wind but I don’t see it replacing nuclear power baseload any time soon.

The Makani “Quadracopter” Kite

Makani is Hawaiian for wind and was founded in 2006 by Saul Griffith, Don Montague, and Corwin Hardham. Makani is interesting, partly because of their technology concept but also because long-term supporter, Google, acquired the company into Google-x in 2013 for an undisclosed sum rumoured to be $10 million. If you are a tech company committed to reducing CO2 emissions through renewable energy innovation, then high altitude wind power is at the top of most investor’s shopping list.

Makani won the competition for the most avantgarde device that is a hybrid kite / glider (Figure 11). Makani have a simple but informative web site and several informative videos two of which are linked below. Make no mistake, this is a serious high tech venture aiming to capture industrial scale energy from high altitude winds. Whether or not it can be made to work is of course another question. You just have to watch the Makani kite whizzing round in giant circles to get a feel for what might go wrong (Video 3).

Figure 11 The Makani kite. For take off and landing, the generators turn into propellors enabling the kite to hover like a helicopter.

Video 2: 5 mins 23 secs

Video 3: 6 mins 17 secs

The Makani concept is a glider tethered to the ground and so when the wind blows across the aerofoil this produces lift that may keep the glider aloft indefinitely for so long as the wind blows strongly enough. At this point I need to introduce the concept of flying cross wind. To understand this you really need to watch the video linked above where you will see the Makani flying at high speed in giant circles in the sky transverse to the wind direction. Electricity is generated by the rotors linked to actuators and is transmitted to the ground via a conducting tether. It is conceptually easy to understand how flying faster will generate more power. And at this point it is worthwhile introducing a second equation that links flight speed to wind speed and aerodynamics:

flight speed = wind speed * aerodynamic efficiency

The aerodynamic efficiency is the ratio of lift to drag of the aerofoil + tether rope. The drag of the Makani kite is also increased by rotors that will slow the flight speed and reduce power but it is the braking of the flight that at the same time produces power.

Figure 12 The Makani concept emulates that of a wind turbine but can reach much higher altitude and generate more electricity using a lighter device.

The Makani system is comparable to the motion described by the tips of a turbine blade but has the advantage of being much lighter. And not being constrained by the height of the tower it can fly higher to access faster winds.

Some vital stats gleaned from the videos includes a 1 MW Makani will weigh about 1/10th of an equivalent turbine (10 tonnes versus 100 tonnes) and will cost about 1/2 as much. The flight speed is 100 mph (67 m/s).

Saul Griffith, one of the founders, provides an audacious vision in Video 2.

Figure 13 A 230Kw Makani would be the size of a Cessna.

Figure 14 A 1.3 MW Makani would be the size of a Gulfstream.

Figure 15 A 6MW Makani would be the side of a Jumbo Jet.

Figure 16 Plans for a 600 kW Makani armed with 8 turbines.

Conceptually I have problems imagining a kite the size of a jumbo jet whizzing around the sky but can easily imagine the mess and loss of capital that would follow should it crash. But one of the big plusses for Makani is that the rotors can quickly be turned into propellers providing the option of powered conventional flight and the ability to hover giving control over take off and landing.

The KiteGen Stem

KiteGen was founded in 2007 by Massimo Ippolito. The venture began as a hobby flying kites to see if electricity could be generated. In 2006 the first prototype was tested. This was a truck mounted with 30 kW nominal output using a fabric sports kite at low altitude. The first patent application was filed in 2003 jointly with others including the University of Delft and the first patent granted in 2009.

The essence of the patent that underpins KiteGen technology today was a kite or aerofoil attached by two ropes to a control system on the ground where varying the forces on the two ropes both controlled the kite and drove an electrical generator located on the ground.

Figure 17 One of several patents linked to high altitude wind power owned by KiteGen.

Figure 18 provides a simplified view of the basic technology comprising a kite, two ropes that wrap around two drums located on the ground. As the kite is pulled up and away by the wind, the drums rotate individually generating electricity. Figure 18 shows that the kite is fitted with motion sensors that communicate with the kite steering unit (KSU) on the ground.

Figure 18 Simplified view of the KiteGen showing a kite attached by two ropes that are individually wrapped around drums anchored to the ground. When the kite flies away from the ground station the ropes spool out turning the drums that turn a generator. Graphic from ref 1 figure 2.3.

Alert readers will grasp this simple concept but will wonder how the kite is recovered when it reaches the end of its ropes. The answer to this is given in Figure 19.

Figure 19 The KiteGen flies across the sky and away from the ground station tracing figures of 8 patterns. The power produced is a function of the force on the ropes and the speed at which the ropes unwind the drums. The KiteGen is designed to maintain a uniform force on the ropes and so power varies with wind speed and reel out speed (Appendix 3). Graphic from ref 1 figure 4.12.

Figure 20 Schematic layout of the KiteGen stem showing the main components.

Figure 21 Turin, 26th April 2016. Rope drums and actuator on a real KiteGen. The actuator (generator) is the black drum to the right of the yellow label. This prototype is being used to perfect the layout design and components testing.

In keeping with most groups testing kite power systems, the KiteGen flies in a cross wind pattern figure of 8. The aerofoil shape of the kite generates “lift” as it flies with tremendous forces transmitted to the two ropes (up to 20 tonnes per rope). The kite flies away from the ground station tracing out figures of 8 in what is called the power or traction phase (Figure 19). At the end of its ropes, the kite is manoeuvred into an aerodynamic neutral position where it loses all of its lift and the ground generators turn into electric motors to recover the kite at tremendous speed to its starting position in what is called the recovery phase. Recovery consumes only a small fraction of the energy used in the power phase (Figure 22, sideslip recovery).

Figure 22 Left electrical output (kW), right rope length (m) from a 42 minute test flight. The first half of the test shows the kite gaining altitude, but also generating electricity as the ropes unwind. The kite then reaches its target altitude of 1400 m for a rope length of 2000 m (45˚ elevation) and switches to the yo-yo operational mode, flying out to 2,500 m before being pulled back in. In pink is power consumption used to pull the kite back and shows the difference between doing so in full power wing mode and using the side slip manoeuvre. Clearly pulling the kite back without losing lift will consume as much energy as is produced. Click chart for a large version.

The KiteGen is furled into a wind neutral position using a side-slip manoeuvre that is also patented by KiteGen. This is best understood watching Video 4 below.



Video 4 4 mins 47 secs. Note this video switches between animation and real flight footage. The sideslip demonstration comes in at about 2 mins 20 secs.



Video 5 6 mins 28 secs. Flying figures of 8 and generating electricity.

It is only the patented two rope system that can perform the patented side-slip. Competing systems that use one rope have developed their own manoeuvres that are aerodynamically less efficient. In other words more energy is required for the recovery phase.

This operational mode is called the yoyo configuration and produces electrical output as shown in Figure 20. With several KiteGens operating together the yoyo cycles can be adjusted to eliminate the intermittency.

One aspect of this operational mode that it is important to grasp is that most of the power is generated by the aerodynamic lift of the kite. The faster the kite flies in its figure of 8, the more lift is generated transmitting more power via the ropes to pulleys, drums and generators on the ground (Figure 19). Equally the aerodynamic efficiency of the kite determines how much force is transmitted through the tethers to the ground.

Figure 23 The operational mode of the KiteGen aims to maintain a constant force on the ropes (the nominal force) (another KiteGen patent). As shown here 250 kN total for a 50 m^2 kite. For the 120 m^2 power wing (see below) the nominal force is 300 kN (150 kN per rope). For the 120 m^2 power wing to reach the nominal force a wind speed of 5 m/s is required. When that wind speed is exceeded the ropes begin to reel out and electricity is generated. The reel out rate continues to increase with wind speed until a reel out rate of 10 m/s is reached (wind speed = 15 m/s) at which point the KiteGen has reached its nominal power rating of 3 MW. The system aims to maintain the nominal force and power rating by adjusting the position of the kite in the sky.

Aerodynamic efficiency is defined as the ratio of lift / drag, some examples are given below.

Sport kite ~ 6

Jumbo jet ~ 12

KiteGen power wing ~ 28

Glider ~ 70

All of the tests conducted to date by KiteGen, and other groups pursuing similar concepts, have been made using small fabric kites. But fabric wind surfers kites are not suited to commercial power generation. KiteGen are now at the stage where they have designed and built a 120 m^2 semi-rigid composite material power wing that is enormously more powerful than the sports kites used before. The prototype is nearing completion awaiting first test flights in 2017 (Figure 22).

Figure 24 The 120 m^2 power wing prototype manufactured on-site by KiteGen. It is a semi-rigid composite construction designed to be light and strong. The core is honeycomb paper covered in fibreglass. The leading edge is milled by a robot from a large block of polystyrene and then coated in fibreglass. The sections are connected using high performance zippers. Production models will use Kevlar instead of fibreglass (lighter and stronger) and the leading edge of the aerofoil will be made of carbon fibre instead of polystyrene. The kite is studded with motion sensors that provide on-board communication describing the shape of the kite and communication between the kite and the ground. Massimo Ippolito (right), Eugenio Saraceno (left) leaving me in the centre.

One final aspect of the operational aerodynamics is the impact that the drag of the ropes has on the wing. The aerodynamic efficiency is the ratio of lift / drag. The ropes provide only drag that increases with altitude as more rope is deployed, compensated by faster winds the higher you go. Calculations show that the aerodynamic efficiency of the power wing falls to roughly 18 when the ropes are included.

The ropes used by KiteGen are made of Dyneema, an extremely light and strong polyethylene fibre. 9 mm ropes are able to withstand 10 tonnes of force.

Risks: The Makani and KiteGen Compared

The Makani quadracopter can quickly change from a kite into a powered glider that offers clear advantages over the KiteGen in take off, landing and flight control. But on the opposite side of that same coin with the Makani quadracopter most of your money is in the sky and one must expect to lose devices. And the Makani studded with 8+ rotors flying at 67 m/s does not appear to be avian friendly, although I’m unsure what bird life is found at the planned operational altitude.

The KiteGen kite is simple, lightweight and cheap and may almost be viewed as a consumable item. Thus in the event of a crash significant financial loss is not incurred. But technical challenges remain in achieving automated take off and landing of a large kite. The KiteGen power wing is huge and should be clearly visible to birds. But seen from a distance it will appear as the size of a bird soaring in the sky. Near ground level, the ropes do not move very fast. Eugenio tells me that at operational altitude it would mainly be large flocks of migrating birds that may be encountered. These can be detected by radar and avoided.

To become serious contenders in the commercial power generation market, both technologies need to prove reliability through extended field trials. These alas are not cheap to conduct.

The Other Players

I don’t have time and there is little need to review all of the players on the high altitude wind field. But three other companies / concepts merit a brief mention.

KitePower headed by Dr. Roland Schmehl at the University of Delft (The Netherlands) is one of the early entrants to this field a little over 10 years ago. Their kite flies from a single rope and the kite therefore needs a control box that flies below the kite. The box creates drag compensated by the single rope creating less drag. But the kite control unit needs the muscle to fight large forces via the bridle system. And this also requires a conducting tether to transmit electricity from ground to the kite.

Figure 25 The components of the KitePower set up has a lot in common with the KiteGen stem. Though note the single rope and kite control unit that flies below the kite.

Kite Power Solutions are actually based in Scotland. Their concept, as far as I can tell, is very similar to KitePower with a single rope and a kite control unit flying beneath the kite. Figure 26 gives a feel for the bridal control system.

Figure 26 Detail of the bridal ropes of the Kite Power solutions kite.

Ampyx Power is a hybrid concept between the KiteGen and the Makani using a glider to fly figures of 8 with a single tether that unspools cable from a ground based drum / generator.

Figure 27 The Ampyx “glider” flies figures of 8 similar to the conventional kites.

Concluding Thoughts

Is trying to access high altitude wind energy using tethered kites that fly cross wind driving generators a crazy idea? Maybe!

But one just needs to read the 6 benefits I list in the Executive Summary to understand that this is an energy resource that Mankind will pursue and that a range of technology concepts must be given the opportunity of commercial scale field trials. And trials must be afforded the opportunity to fail in pursuit of eventual success.

Conceptually, I find it impossible to conceive that this technology could be deployed in windy, blustery Scotland. But this technology is not intended for windy areas but for those vast areas of the globe where wind speed at ground level is low but a few hundred meters up there is this steady but invisible flow of 10 m/s wind (Figure 7).

Ten years ago I did not believe that the commercial deployment of high altitude kite power would have been feasible. The enabling technologies today are the huge advances made in solid state motion sensors combined with GPS and computing power that in theory enables automated control of kite geometry and flight.

When I visited KiteGen at the end of April, I was impressed by their engineering and physics skills that spans aeronautics, mechanical engineering, electrical engineering, automation and simulation. When I asked Massimo what drove him, his reply was simply this:

To solve the global energy problem.

I think it is fitting to conclude with this video of the Wright brothers first flight at Kitty Hawk in 1903. It took a long time for Man to reach this point. 102 years later the Airbus A380 took to the sky.



Video 6 2 mins 46 secs. Classic footage of the Wright Brothers first flight at Kitty Hawk, 1903. Its well worth watching 😉

Appendices

The 4 appendices go into greater detail about the physics and engineering of the KiteGen system that may only be of interest to those with physics and engineering backgrounds. I personally have struggled to grasp and understand all the concepts. Parts are copied from KiteGen documents.

1) An Estimate of ERoEI for a KiteGen Stem

KiteGen have presented a back of the envelope style ERoEI calculation for their 3 MW stem indicating a value of 562 which is incredibly high. I have done my own calculation using a variant of their methodology and my own input variables. The idea is to try and estimate the energy intensity of a wind turbine structure and to interpolate that into a KiteGen stem. This involves making many weak assumptions but should be good for arriving at a ball park number.

I will begin with estimating the energy intensity of a wind turbine based on the mass of the superstructure. According to Vestas, their V112 3 MW turbine contains 372 tonnes of metal in the tower and nacelle (I will ignore the 947 tonnes of steel and concrete in the foundations for the time being).

I am going to make the following assumptions:

ERoEI = 18 [1]

Capacity factor = 0.3

Lifespan = 20 years

Power = 3 MW

Mass of superstructure = 372 tonnes

Energy produced during life time = 3*24*365.25*20*0.3 = 157,788 MWh

Energy required to create and maintain machine = 157,788 / 18 = 8,766 MWh for an ERoEI of 18.

Energy intensity = 8,766 MWh / 372 tonnes = 23.6 MWh / tonne

In his PhD thesis, Lorenzo Fagiano provides the following table for the theoretical capacity factors for a KiteGen [2]:

The average is 0.54, which is used in the calculation below. The capacity factor is higher than a turbine because high altitude winds (500 to 2000 m) blow more steadily than at the surface. Assumptions for a 3 MW KiteGen stem:

Capacity factor = 0.54

Lifespan = 20 years

Power = 3 MW

Mass of superstructure = 20 tonnes

Energy produced during lifetime = 3*24*365.25*20*0.54 = 284,018 MWh

Energy required to create and to maintain machine = 20 tonnes * 23.6 MWh / tonne = 472 MWh

ERoEI = 284,018 MWh / 472 MWh = 602

This is an astonishingly high and difficult to believe number but it is borne out of the much lighter weight and higher capacity factor for the KiteGen. In his calculation, Massimo Ippolito got a number of 562 using an energy intensity of 40 MWh / tonne. Using that figure, my ERoEI estimate falls to 355 which is perhaps more realistic.

It is this aspect of the KiteGen and high altitude wind that really caught my attention. Many years ago when I first began looking into global energy issues I believed the problem may be easily solved by a combination of wind power and partial conversion of surplus power to an energy store such as hydrogen. Unfortunately there are many who still believe this is a solution. The problem with this approach and conventional turbines is the low ERoEI of wind turbine electricity, that makes it expensive combined with round trip energy losses in going to storage such as hydrogen, that are typically of the order 70%. With 70% losses, the ERoEI of a wind turbine – hydrogen system falls to 5.4 (ERoEI of 18 * 0.3) and we drop off the net energy cliff (Figure 28). In other words, with a wind turbine – hydrogen system you take expensive electricity and waste 70% of it to mitigate for intermittency. Consumers and economies don’t like this!

Figure 28 The estimated ERoEI for a 3 MW KiteGen plotted on the Net Energy Cliff. The electrical output from a KiteGen is not smooth. This is partly mitigated by on-board super-capacitors that can store and discharge power to smooth out the supply. To convert the output to dispatchable power, a very conservative approach would be to convert all the electricity to an energy carrier like hydrogen and then combust the hydrogen in a gas turbine to generate electricity. This will consume about 70% of the available energy, but even doing this leaves an ERoEI > 100. In reality some of the output power can be sent direct to the grid while some can be stored to mitigate for intermittency. For explanation of the net energy cliff see ERoEI for Beginners.

The KiteGen stem is a complex machine, but it is lightweight and cheap to build and to install. IF it works according to expectations then it may produce large quantities of cheap, unsubsidised electricity. A KiteGen – hydrogen generator would still have ERoEI of 355*0.3 = 107 which is still huge compared to most other forms of electricity generation today. A KiteGen may also be used to make synthetic fuels. An important point is that a KiteGen may be able to make the liquid fuel to mine materials and make the electricity to manufacture more KiteGens with ample energy left over for the rest of society to use. But all this depends on the assumptions made above holding true and the machines actually working to specification.

[1] Charles A.S. Hall n, Jessica G. Lambert, Stephen B. Balogh: EROI of different fuels and the implications for society: Energy Policy 64 (2014) 141–152

[2] LORENZO FAGIANO PhD thesis 2009. Control of Tethered Airfoils for High–Altitude Wind Energy Generation

2) The Physics of Kite Flight and Power Generation

The various aspects of the Kite power system can be described by 5 basic equations. Wind power systems operate on a basis of extracting kinetic energy from moving air and the equations therefore are rooted in the equation for kinetic energy.

[1] KE = 1/2 mv^2

m = mass of air

v = velocity of air, in this case wind speed.

From this one can see that the area of the wind front will determine the mass of moving air. And since the KiteGen is operating at different altitudes the density of the air is an additional variable that will control the mass of air. These variables combine to give the equation for power.

[2] Power = 1/2*r*A*V1^3

r = density

A=area of the kite

V1 = wind speed

We see here a crucial factor in that power increases by the cube of wind speed.

[3] flight speed = wind speed * aerodynamic efficiency

As mentioned previously, much of the force on the ropes is generated by the efficiency of the aerofoil. Flight speed increases with wind speed and the aerodynamic efficiency.

We end with two equations that describe the force on the ropes. Equation 4 uses the lift coefficient of the wing. Equation 5 uses the aerodynamic efficiency.

[4] force on the ropes = 1/2 * flight_speed ^2 * lift coefficient * area of the wing * air_density (lift coeff. CL = 1.2 -:- 1)

[5] force on the ropes= 1/2 * wind speed^2 * aerodynamic efficiency^2 * area of the wing * air_density (aerodynamic efficiency E=8-:-60 KiteGen wing =28)

3) The Different Modes of the Operational Cycle.

KiteGen have some excellent internal documentation that helps one further understand the physics of kite based power generation.

“In a groundgen concept such as that of KiteGen Stem, the reel-out velocity and the cables tension are fundamental factors for determining the power output. It is possible to show that when the reel-out speed is zero the tension is maximum and when the reel-out speed is maximum, i.e. equal to the absolute wind speed, the tension is zero. Since a first assessment of the power output is given by the product of the cables tension by the reel out velocity, zero reel-out speed means zero power output and maximum reel-out speed means again zero power output. Therefore, the KiteGen Stem must operate with a non-zero reel-out speed and a non-zero tension. It is possible to find an optimal reel-out speed that maximizes the power output. In his article, Miles L. Loyd demonstrated the optimal power output is achieved at 1/3 of the wind speed. In this scenario, having a greater absolute wind speed means achieving greater power by increasing both the reel-out speed and the cables tension.

Despite the physical optimality of the Loyd’s model [2], KiteGen chose to follow a different practical approach in order to both manage practical design and operations and decrease the cost of energy. What happens when the absolute wind speed is increased is shown in Figs. 1 to 4 (summarised in Figure 29) which represent the Kitegen Stem power curve in four different hypothesis. The power curves look similar to those of conventional windmills, however, whereas windmills can only regulate their power, the KiteGen Stem regulates both force and power. Note that this curve is valid during the reel-out phase only and therefore does not take account for the duty cycle of the machine.”

Figure 29 The four phases of KiteGen operation. Upper panel is power, middle panel is force and the lower panel reel out velocity. The key feature is “stage B” where reel out and power production are zero as the system builds to the nominal force, in this example 200,000 N in each rope (middle panel) achieved at wind velocity of 5.42 m/s. Upon reaching nominal force, the ropes begin to unwind and power is produced (stage C) and power continues to rise with wind speed (compare upper and lower panels) while maintaining uniform force on the ropes. At 16.97 m/s the nominal power of the machine is reached, in this case 4 MW. Note that the reel out velocity is wind speed minus 5.42 m/s.

The KiteGen Stem power curve is divided into four zones depending on the absolute wind speed.

– Zone A: The machine is switched off because the absolute wind velocity is below the cut-in velocity.

Zone A and Zone B are separated by the cut-in velocity

– Zone B: The machine does not reel-out the cables, the force in the cable is below the nominal force.

Zone B and Zone C are separated by the full-force velocity

– Zone C: Full Force Zone: the absolute wind speed is above the full-force-wind-velocity. The machine operates with nominal tension in each cable. Power generated varies and is below the maximum power. The machine is regulated in order to maintain a constant nominal force in the cables.

Zone C and Zone D are separated by the full-power velocity

– Zone D: Full Power Zone: the absolute wind speed is above the full-power-wind-velocity. The force in the cables is maintained equal to the nominal force and the generated power is equal to the maximum power. The machine is regulated by positioning the kite outside the power zone.

In this approach the cables operate at their nominal tension and the generators operate at the nominal torque and/or power.

4) Betz’ Law

Wind turbines function by extracting some of the kinetic energy of the wind moving through the turbine blades. We can imagine a cylinder or pipe of air flowing through the blades that extract some of the kinetic energy. Let us imagine we could extract all the kinetic energy, then the exit velocity of the wind would be zero and no air would pass across the blades, hence energy production would be zero. This tells us that we cannot extract all the energy. Similarly, at the other end of the scale, if the wind blows straight through we extract no energy. It is clear that somewhere between these end members there may be an optimum amount of energy that a turbine can extract from air flowing through the blades.

Betz law from 1919 defines this optimum amount of energy a wind turbine can extract from the wind. It turns out that 59.3% of the kinetic energy of the wind can be extracted when the outlet velocity of the air flow is equal to 1/3 of the inlet velocity.

M. Ippolito has claimed that Betz’ law does not apply to the KiteGen and I believe he may be correct. Betz’ calculation is applied to the flow of wind across a turbine blade. The removal of kinetic energy applies only to the cylinder or pipe of air flowing through the turbine defined by the circular area of the rotating blades more commonly known as the wind front.

For a Kite flying circular or figure of 8 paths through the sky there are two ways to define the wind front. The first is the area of the kite and the second is the area of the sky that is swept by the flight pattern. Because the kite is moving continuously it is always sampling fresh wind that has maximum kinetic energy. The area swept is vast compared with the area of the kite that will never notice the slowing of the wind caused by its flight path. With a rope length of 1500 m and 50 degree elevation the swept area is approximately 1,000,000 m^2. That is 125 times larger than the area swept by an equivalent turbine rotor. I believe it may be the case that a kite should never manage to extract more than 59.3% of the energy in the swept flight path area. But in practice, a kite will only ever manage or need to extract a fraction of 1% of the kinetic energy of the wind that is available to it.

[1] LORENZO FAGIANO PhD thesis 2009. Control of Tethered Airfoils for High–Altitude Wind Energy Generation

[2] Miles L. Loyd (1980) J. Energy: Crosswind KitePower

[Note added 18th August 2016: After 45 days the comments are closed on all Energy Matters posts. Hence the comments closed here today just as interesting commentary was emerging. I am proposing to open a new commentary thread on High Altitude Wind Power (HAWP) in a week or so.]