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 often fruitless investment is the area of high-altitude wind generation. Numerous companies have products which are hyped as replacing the iconic white tower with three rotating blades that grace 240,000 individual sites around the world today. At most, some of these products will be useful in high-cost niches where more cost effective alternatives have very specific limitations.
Why is that, and why should investors be wary of claims by inventors promising to revolutionize wind energy?
[3], [6], [7]
The designs range from inflatable, elongated toroid blimps with wind turbine blades in the hole, to flying kite-wings which fly figure eights and have small turbines on them generating electricity to actual fabric kites which are reeled in out-of-the-wind and haul cable out in-the-wind, generating electricity at the ground point. There are other companies with other designs, but these have received the most hype in the past year or so.
There are three fundamental challenges with airborne wind turbines that will relegate them to niche roles:
- Flight hazards and resultant economicsThey create an effectively invisible flight hazard over a remarkably large and changing range. Generally the intent is ‘fly’ them high off the ground where the winds are strongest and most consistent. This requires 2-4 times the length of cable as the height of the kite. As the turbines are intended to fly 500 meters and more off of the ground, the cable will be kilometers long. As the cable must be strong enough to withstand substantial tension, 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.
This in turn generally means that there are no transmission lines of sufficient capacity in the area and that must be factored into the economic costs. In most jurisdictions, this will also require additional insurance which is hard to quantify at present, but not inexpensive. Finally, their ranges will become no-fly zones, which requires regulation changes, which requires legal and potentially lobbying costs.
- Failure safety and economicsIn the rare instances when an industrial turbine fails due to wind load + bad design + component failure, there is a limit to the potential area of damage. Where current set back regulations are due to noise, they vastly exceed likely blade throw distances. 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 240,000 wind turbines working today and thousands more replaced with more modern wind turbines, there has been exactly one home where a window was broken due to blade throw.[4]
- Maintenance and economicsMost airborne turbines require 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.
As pointed out above, these dynamic tensioning winches are going to be located long distances from any centers of population. If a winch fails, the most likely result is kilometers of heavy cable spread across the landscape downwind to the grounded turbine.These factors mean that the requirement for maintenance intensity and regularity is both much higher and more expensive than for standard turbines.
The basic engineering of airborne wind turbines is a mostly solved problem, but with ongoing challenges in low-wind launches and keeping the kites flying using automated controls. The economics and viability of airborne wind turbines and appropriate siting locations and willing insurers are much more problematic.
It’s worth reviewing some of the findings of NASA’s Airborne Harvesting study [1] to see that there are people working on a subset of these problems and that they are not universal.
It’s also worth reviewing the Wikipedia entry on this for historical views [2]. Note that Wikipedia in this instance is a bit of a grab bag of what’s been tried, with relatively little clarity on what actually works. As always, reviewing the bibliographic links will be of high value.
Summary:
High-altitude wind generation is an interesting idea and an engineering challenge that is fun to solve for those inclined, but like vertical-axis wind turbines, it’s at best a niche technology.
References:
[1] http://awtdata.webs.com/annotatedbibliography.htm
[2] http://en.wikipedia.org/wiki/High_altitude_wind_power
[3] http://www.altaerosenergies.com/AltaerosPressRelease032712.pdf
[4] Wind Energy: What is the percentage likelihood of a wind turbine catching fire?
[5] http://www.makanipower.com
[6] http://www.skysails.info/english/power/power-system/skysails-power-system/
