Are airborne wind turbines a plausible source of cheap clean energy?

One of the interesting things about wind energy is that while the market has spoken loudly and clearly about what makes economic sense for wind generation, every week there’s another news story on wind generation innovations such as relatively ineffective vertical-axis wind turbines that are going to replace horizontal-axis, three-blade wind generators.

An ongoing area of enthusiasm and to date fruitless investment is the area of airborne wind generation. Numerous companies have concepts or designs which are hyped as replacing the roughly 250,000 iconic white, tri-blade towers around the world today. However, there are several fundamental challenges with airborne wind turbines related to flight hazards, safety, technical viability, economic viability, maintenance challenges and winter weather operation that will prevent them from filling any but minor niche roles for the foreseeable future.

Airborne wind energy has been in research and development for 70 years without generating any useful amounts of electricity in a production capacity anywhere. Investors as well as journalists and bloggers covering energy should be very dubious of claims.

What is airborne wind energy?

Airborne wind energy is based on the long-known reality that winds get stronger and more reliable the higher off the ground you go. This is why conventional wind turbines have gotten much taller over the past forty years.

In general, proposed airborne designs which are claiming large improvements over conventional wind turbines for utility scale generation fall into two approach categories. The first is lower altitude but still above the range of wind turbines, up to 650 meters or so, and uses the speed of the kite flying in figure eights or circles to maximize aerodynamic lift. This either maximizes speed past small turbines mounted on the wing, or increases tension against a regenerative winch on the ground.

The second is very high-altitude devices in the 4.5 kilometre to 9 kilometre range. These are intended to use the very strong winds at those altitudes directly. Both are conceptually sound approaches that have significant real world challenges. Other reasonably worked out approaches are static in lower winds closer to the ground, so the physics don’t work for them generating large amounts of electricity; there may be good small wind generation options in that space especially for remote or emergency situations, but not utility scale generation. There are some purely blue-sky conceptual approaches that are so poorly worked out as well as unlikely as to be easily ignored at this time.

Designs range from Altaeros‘ inflatable, elongated toroidal blimps with wind turbine blades in the hole, to flying kite-wings such as Makani’s which fly figure eights or circles and have small turbines on the front of the wing, to rotorcraft adaptations such as Sky Windpower‘s which use quadcopters as kites / turbine, to actual fabric kites such as Skysails which use tethers stripping out of regenerative winches to generate electricity. There are other companies with other designs, but these are the dominant models.

1. Flight hazards

Airborne wind generators would create an effectively invisible flight hazard over a remarkably large and changing range if not lit and marked both on the tether and device. Tether lengths will typically be two to four times the target altitude.  As utility scale devices in this class are intended to fly at altitudes of 400 meters to 9000 meters, the tethers will be one to 18 kilometres long.  The tether must be strong enough to withstand substantial tension, so it’s also strong enough to seriously damage aircraft.  Generally this will require that the only appropriate areas for this technology are those with no near-earth flying, which means very sparsely populated areas, and likely offshore. This in turn generally means that there are no transmission lines of sufficient capacity in the area and this must be factored into the economic costs.

Aviation authorities will require lighting and marking of at least the devices themselves and likely the tethers, and these requirements have often been ignored during design and engineering of proposed approaches with the hope that they will be waived. While some discussions with the US Federal Aviation Authority (FAA) have discussed the possibility of flying tethers without marking or lights, the FAA has not agreed to this, and likely would not permit unmarked tethers in many categories of these devices (more on this later).

In most jurisdictions, this will also require additional insurance which is hard to quantify at present, but will likely be much more expensive than for current wind generation approaches.  Their ranges will become no-fly zones potentially up to 9 kilometres or air passenger altitudes, which likely requires regulation changes which in turn requires legal costs and probably lobbying costs. Radar blimps have restriction zones that were approved as a matter of national security; it’s difficult to assert that the same political pressure would be brought to bear to support airborne wind generation.

2. Failure safety

In the rare instances when a utility-scale wind turbine fails due to wind load plus bad design plus component failure, there is a limit to the potential area of damage. Current set back regulations for noise annoyance mitigation of 400 meters or more greatly exceed probable blade throw distances which rarely exceed 200 meters.  With airborne turbines however, the turbine could be over a very large range of downwind real estate in the event of a failure, and high enough up that throw distance of failed components is much longer. This requires additional engineering to reduce failures, a very sparsely populated or unpopulated downwind range of kilometers and additional insurance again. Note that while there are roughly 250,000 utility scale wind turbines working today building from tens of thousands in the 1970s, there has been exactly one home where a window was broken due to blade throw and no one injured in any way.

A large group of proposed airborne devices have hard airframes and propellors or rotors that act as wind turbines. Makani’s proposed 600 KW onshore generator is a 1050 kg flying wing with eight rapidly spinning two-meter diameter propellors.  A device of this mass and characteristics hitting homes, schools or shopping plazas potentially miles downwind would have great impacts. Sky Windpower’s conceptual approach if scaled up to 5 MW capacity would potentially weigh in the 20,000 – 30,000 kg range, have four 32 meter diameter rotors and be 70 meters on a side. The rapidly spinning and heavy blades would present a very large safety risk if it was forced down in any inhabited area.

Tethers present another class of problems. Their length, strength and in some designs electrification make them an extraordinary danger if they were to be draped over roads, buildings or power lines, or if a person were to be hit by a fast moving tether.  A kite with a dangling snapped tether can fly downwind for miles.

The people working in airborne wind energy systems and the aviation authorities are fully aware of these safety concerns and are attempting to engineer safety features and put in places setbacks to accommodate for them, but actual solutions are at best in early testing phases and in many cases purely conceptual.

3. Maintenance

The flying devices are going to be more complex with more moving parts than conventional wind turbines and the launch and landing cradles are going to be complex as well. Increased complexity leads to increased maintenance, all else being equal. With large, heavy objects banging into one another in potentially high winds, failure rates will be higher. The conceptual rotorcraft devices especially are going to be higher maintenance; helicopters typically fly one hour for every 3.5 to 4.5 hours of maintenance and there is little reason to believe that kiting rotorcraft will reverse that ratio as they scale.

Most airborne turbines require at least dynamic tensioning from the anchor point on the ground.  This enables them to both fly in the most effective range of wind and height, and be returned to the ground in the event of low-wind conditions or maintenance requirements. The winches that are doing the dynamic tensioning require motors, require components that step down generated power to run the motors or connections to the grid, require lubrication and must deal with heavy cables and cable loads.

Winter conditions have so far been ignored by public documentation of the majority of reasonably worked out generation schemes. Icing of flying devices is a serious maintenance and safety concern, and approaches to solving icing for airplanes are significant maintenance and operations expenses by themselves. Further, high altitude devices will often or even usually be flying in below zero temperatures so icing and frost build up will need to be addressed.

As pointed out above, these devices are likely to be located long distances from any centers of population.  These factors mean that the requirement for maintenance intensity and regularity is both much higher and more expensive than for standard turbines.

4. They might not work at all

Tethers for crosswind and high-altitude designs are highly problematic in two different ways that challenge these technologies working at all.

While Makani has not included flight speed information in their publicly available documentation, calculations indicate that the devices would have to fly in the 130-140 KPH range to achieve their projected power outputs. Under these conditions, tether drag becomes a critical factor. Their submission to the FAA requests that their 440-1060 meter tethers not be required to be lit or marked, as the additional drag entailed would eliminate effective use. If aviation authorities require marking or lighting of tethers, Makani’s solution will simply not work; this is true of many if not all cross-wind approaches.

High-altitude solutions have a different problem: tether weight. Makani has actually provided weights for very light, strong, conductive tethers made of carbon fibre and aluminum. Scaling up to  tether lengths required for 4500-9000 meter altitudes suggests that the tethers alone would weigh 33,000 – 66,000 kg. One of the heaviest lifting helicopters in the world has a maximum lift capacity of 20,000 kg. Anchor points for 33,000 kg alone is a non-trivial engineering exercise. There is little evidence that any of the proposed high-altitude solutions will actually be able to even lift the tethers, regardless of working otherwise. And if they can’t carry tethers to the required heights, they just don’t work. While blimp-based approaches have the potential to lift these weights, they have other constraints.

Further, the rotorcraft approach if scaled up to a useful 5 MW range would require the biggest rotorcraft ever built, for example Sky Windpower‘s quadcopter would have 32 meter diameter blades, be 70 meters on a side and weigh 20,000-30,000 kg. And they’d have to be the largest autonomous aircraft ever built. To be competitive, these enormous, autonomous devices would have to be a fraction of the cost of current large helicopters. The engineering challenges are very high, and the economic factor makes them extremely unlikely to be viable.

5. Actual economic benefits are not obvious compared to conventional wind energy

Assessments of two of the better worked out concepts in crosswind and high-altitude wind energy, Makani and Sky Windpower don’t make it obvious that their devices will be cheaper than conventional wind turbines with similar capacities. They require complex, carbon-fibre, autonomous flying aircraft of great size and long — often extremely long — carbon fibre and aluminum conductive tethers. The combination will not be cheap even if manufactured in bulk. It’s not apparent that their capacity factors will be better due to maintenance requirements and weather-related groundings. It does make it clear that farms of airborne wind energy devices will need to be more widely spaced than conventional wind turbines, and that the land will typically be unusable for any other purpose due to safety concerns (e.g. high-speed tether movement, high-speed flying devices and 70 meter by 70 meter quadcopters landing and taking off), making real estate costs a very significant factor by comparison.

Meanwhile, conventional wind generation prices have been dropping for decades and are now cheaper than any new form of generation except unconventionally extracted gas. It’s difficult to consider significant investment in alternative means of generating electricity from the wind when the current approach works so well.

6. There is zero history of production after decades of attempts

Kites are a well-known technology and have been for thousands of years. The earliest record of kites being considered for electrical generation appears to be from 1943. The seminal whitepaper on the subject was published in 1980. The first successful demonstration of electrical generation was performed in 1986.

Depending on where you choose to consider the history of airborne wind generation starting, it’s 27 to 70 years old. And there isn’t a kite borne wind generation system in production anywhere in the world today even for remote site generation of small amounts of electricity. And there doesn’t appear to be a prototype generator that has flown for more than a few hours at a time. There doesn’t appear to be a single recorded capacity factor from a working device in this category.

For comparison, this is the first electricity generating wind turbine in the world. It was built in 1891 by James Blyth to power his cottage. While a low-efficiency Savonius design, it just sat there and worked, likely achieving 10-15% capacity factors.

James Blyth's 1891 electricity generating wind turbine

As far as can be determined, this was the first attempt to use a wind turbine to generate electricity. It undoubtedly took tinkering and a great deal of maintenance by modern standards, but it worked pretty much the first time and more importantly, produced useful amounts of electricity where it was needed.

This is better than the airborne wind energy has been able to do in its 70 year history of trying. And to be clear, the people attempting to make airborne wind energy work are often extremely smart, very well qualified and very creative. There are university research programs in airborne wind energy, an emergent industry consortium, a Google-owned startup and several aerospace-experienced engineers at least tinkering in the field. That so much brainpower has been unable to get to end of job on any approach to generating electricity from airborne systems is a strong indicator that it might never actually work usefully.

Summary:

Airborne wind generation is an interesting idea and an engineering challenge that is fun to play with for those inclined, but it’s at best a niche technology. After decades of research and attempts, there are still enormous unresolved engineering challenges and significant outstanding safety and regulatory issues. Investors expecting any near term profits should stay clear. Technology and environment journalists and bloggers should be very skeptical.

14 comments

  1. AeroFox · · Reply

    Altaeros could be plausible , even mainstream given the rate of melting polar ice in the Northern Hemisphere (Artic), eroding North Atlantic and Mid-Atlantic coastal states and communities. A rising ocean will reclaim the Eastern Seaboard of the US, just like the pope is Catholic and water is wet.

    It makes sense for flood plains as emergency power management, interim power management , remote facility load management and island nations that are under threat of submerging. Will these fly in urban centers, probably not. Could it make sense as a peaker power plant in centralized wind farms-on-shore-it absolutely could. Capacity factors in centralized wind generators average anywhere from 15% (older farms) to 35% (newer machines in optimized arrays-Class VI). Depending on age of farm, tower altitude, it could breathe life into old wind farms as a high altitude repowering model or optimize newer wind farms with load balancing at higher altitudes. As a stand-alone generator or array, it will be challenged. But as an optimizer or peaker plant to wind farms, it has value.

    It’s wrong to assume that wind turbines (3 Blade HAWT) will always have the best sites favorable to transmission. Those sites are running out. In its simplest form, this design solves a real estate problem. If the wind farm can’t widen horizontally on land, for reasons of poor wind resources, environmental restrictions, and prohibitive transmission costs, then build up..way up, like at 600-1000ft. but as part of the centralized wind farm.

    1. Fair enough. However, I’ve been reading deeply on airborne wind energy systems recently and the challenges are much more significant than I had realized. I’ll be updating this material with my findings some day soon. Net-net: FAA regulations prohibit most of this and it will take ten-fifteen years for that to change if it does, physics and air regs limits these devices to under 2000′ and most to under 1000′ so they are competing with modern HAWTS, reliability of the devices is extraordinarily low, they have to be landed for lightning, high winds and low winds, air regs require a pilot and a spotter be in attendance at all times, generation aloft is seriously limited by conductive tether limitations in both capacity and length, ground generation goes from max to negative in three minute cycles so expensive storage must be added for any significant generation and finally nobody in the tiny community of people working on this agrees on anything except that it’s the future.

      1. Aerofox · ·

        Regarding FAA,
        1. Centralized wind farms (3 blade HAWT) are already designated and nationally registered with the FAA. Their locations are known and pilots, usually general aviation aircraft, are the only type aircraft that would be affected. FAA issues bulletins to these pilots on VFR/IFR rules of flight when a pilot files his or her flight plan-advisories are issued notifying the pilot of VFR/IFR altitude limitations if the flight path enters the space of the aerostat. This is a common procedure now for non power producing equipment that might be loitering at AGL 2000′
        A no fly zone is not needed.

        2. This design flies under 2000′ feet anyway or I would assume that it does, but altitude is not topography specific but land level or sea level based. Hint-put it on a ridge or foothill next to the wind farm to save on cable length.

        3. This is not competing against a 3 Blade Hawt, but enhancing the farm output with peaking power, load balancing, power optimization. Key here is selling the peaking power which can and does fetch above market rates depending on the ISO needs (summer/winter). Land based turbines in that farm aren’t natural peakers due to wind intermittency. With good forecasting the premium from delivering peaking power from the wind farm that the aerostat supplements could cover the deployment, recovery labor cost and still leave a profit to the wind farm operator.

        4. This helps buffer or offset the farm from profit loss from curtailment periods.

        No storage needed just flow more renewable electrons onto the grid and boost the farm revenue. I don’t know any wind farm owner/operator that is going to say no to increasing peaking power availability.

        It has value as a total systems approach.

        Do post FAA regs as you find them.

      2. I’ll email you the couple of useful documents I’ve received. The draft FAA regs are very useful. The fundamental thing is that the devices are aircraft and as such have an additional set of regs that ground wind generation do not have. Registering a wind farm and not putting it at the end of runways is much different than adhering to FAA flight regs.

  2. rabi · · Reply

    Hi Mike. Nice informative article. Do you think Makani design would be better by having generator at ground station instead of aloft? It should make both kite and tether lighter.

    1. Ground based generation has it’s own challenges, but a soft wing and ground generator has the lowest potential liabilities due to slower speeds and mostly fabric aloft. There are three or four companies trying to make a go of that model including SkySails, Windlift and a University of Delft team. Soft wings with ground generators have challenges with lower generation resulting in non-utility scale levels of electricity and difficulties with requirements for electricity at the wing for lights and bridle-located winches. This requires micro generation aloft — added complexity and drag, hence reduced power generation, or batteries hence more maintenance.

      Soft wing companies also seem to be opting for inflatable spar kites like kitesurfing kites over para foils like paragliders. This increases ability to deal with rain and offshore solutions, but will require having pressure sensors and pumps aloft in any production solution as well, once again adding complexity.

      Ground generators and soft wings appear more technically simple and elegant initially but run into problems as new requirements are introduced. I’ve been led to believe Makani’s initial model was in line with this but they moved to hard wings and in air generation due to a more complete requirement set.

      There are teams trying out inflatable structure wings that look like wings instead of kites with ailerons etc. They appear to achieving greater speeds hence power potential, but are a long way from proving that their set of compromises is any more viable.

      1. rabi · ·

        Interesting trade-offs. Power for kite itself could be achieved by using a small turbine/generator or solar cell and small battery, although that would increase some weight and drag. They could be useful for fast but temporary installation scenarios, like for areas with humanitarian crisis or to support remote temporary mining, scientific operations. High seas tugging ships should be more efficient as kinetic energy is directly transferred without electrical conversions. Looks like besides basic engineering, to make it compatible with aviation industry and address all safety concerns as you have explained, would require a significant R&D.

      2. Regarding the remote niche, that’s where realistic firms like Altaeros, Magenn (now defunct) and Wind Lift are aiming. But they face competition from much simpler mast-based small wind turbines that pack down smaller and auto furl such as Uprise Energy’s STAR turbine.

        Regarding towing ships, that’s where SkySails started although there system isn’t on that many ships today; it’s still very much in early adopter space.

  3. As utility-scale airborne wind energy systems (AWES) are difficult to install onshore.Perhaps the only possibility could be farshore,after a first row of conventional turbines offshore, then after a second row of floating turbines,where deep sea is a big problem for floating turbines but not for airborne systems.If some elements have a better technology (among issues time-life of kites regarding UV) the lightness of AWES can be a serious advantage,in spite of reliability and magagement issues.Grid connections should be possible with the last row of conventional turbines…A good test to know the interest of AWES:do the manufacturers of conventional wind turbines want develop AWES?Now the answer is no,but in case of some precisions about goals for AWES and substantial technical improvements allowing both an economical interest and a common management…

    1. I’m not as sure that airborne wind is much easier to float than standard turbines actually. Wave clearance is a problem. A non fixed winch adds another oscillation dynamic. Launching and landing are much more complicated when the base is moving with the waves and will be complicated by higher winds when conditions require mass landings. Rogue 100′ waves like the one that a guy just surfed in Portugal are more of a problem for landed airborne wind devices than for conventional masts. Salt spray rime buildup will likely be more of a challenge regardless leading to more maintenance.

      I’m not sure about offshore at all right now. Big unresolved engineering problems that are not-trivial.

      1. On http://www.youtube.com/watch?v=5wyfQ83mE_s there is,in French language,experimentation of floating turbine,towards a better knowledge of the performance in electrical production when the turbine moves with both water and wind.
        The question could be partially the same for airborne system,differences being the important variations of trajectories following the floating station and,of course launching and recovering.
        It is interesting to note the logic of the possible implemented technologies according to the deep of water:progression from fixed turbines,then floating turbines,then perhaps tilted floating turbines where the rotor is partially airborned or pushed downwind and where the tilted mast works (for some configurations) a little like an always tilted tether;towards (perhaps) airborne systems.Indeed more is the deep of water,less are fixed elements.In the end if airborne systems have some advantages (lightness,light structure allowing implementation in deep sea)above disadvantages (difficult management of launching and recovering,other disadvantages like important space used being lesser due to the important distance of the coast), their implementation could be made by manufacturers (with some partnership) of turbines offshore,in the generation following floating turbines.The link between these different turbines is the grid undersea.The complexity and the important cost of electrical cables is enough high to require (at least partially) said manufacturers for all systems offshore.

      2. Thanks for the link. My French is much poorer than your English but it’s clear that they are darling with a fixed wind device not an airborne device.

        I’m less concerned with production than with trying to safely and reliably land an airborne wind energy device when the floating mast is oscillating due to waves, the device is oscillating due to wind and the tether is oscillating due to both. It strikes me as a complex problem space that is difficult to solve for and is currently poorly accounted for offshore airborne wind systems. SkySails is closest due to oceanic experience and soft wings but I don’t see evidence that they have solved for automated landing on bobbing small masts.

      3. Yes,the link shows a floating turbine.In a fixed turbine there is no movement (in fact bending of blades).In a floating turbine there is a single movement of the whole machine according to the effects of wind and waves,while for an airborne system there are three movements for the floating station-mast (due to waves),for the tether (due to both waves and wind as you write) _ being able to be divided according to the place within said tether_,and for the wing (due to wind,and inderectly to waves).In spite of such complexity,winding allows the wing to touch the mast in all situations.SkySails seems to control automated landing and recovering on a big ship.In what the use of a small mast would it aggravate the problem? Maybe because movements are more numerous and hasten for a small floating mast than for a big ship? For a soft wing precision is maybe not required at a same level than for a rigid wing like Makani’s device needing an own launching as helicopter-mode.Another problem is the control of cycles by using the method of reel-in/reel-out,and by taking account of said three combined movements.

  4. […] a few things in general, but a few specific things for Makani’s model as documented in their Tethered Aircraft […]

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