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.
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.
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.
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.