Published on March 31st, 2014 | by Mike Barnard44
Sky Windpower’s High-Altitude Generation Aspirations Aren’t Realistic
March 31st, 2014 by Mike Barnard
Sky Windpower is an airborne wind generation system that is often given glowing reviews in the inadequately critical popular and technical press. In 2008, they were listed in TIME Magazine’s Best Inventions of the Year. In 2011, they were featured on the front cover of Popular Mechanics.
They continue to be active in the space, and are Gold Members of the Airborne Wind Energy Consortium.
Sky Windpower’s proposed high-altitude wind generation solution raises major concerns:
- For serious generation it would require building the world’s largest, most powerful, fully autonomous quadcopter, and many of them.
- It would need to get the price per unit for these devices down to a fraction of the price of the world’s largest helicopters today.
- It would need to consume more land without secondary uses than comparable conventional wind generation.
- It would require that significant additional airspace be declared to be fully restricted up to and including passenger jet altitudes.
- It is likely limited to non-winter operation.
- Tether weight alone will be greater than the largest load lifting helicopter capacity today.
- It would require turning maintenance to flight ratios for rotorcraft on their heads.
- The organization is very optimistic regarding capacity factors due to weather- and maintenance-induced landing requirements, as well as energy costs to fly the devices to their operating altitude and back.
- It is unlikely that Sky Windpower have fully analyzed and accounted for failure conditions in their projections; testing required to satisfy aviation authorities and insurers would likely take more than a decade by itself.
What is Sky Windpower’s solution?
At heart, it’s simple. Fly a quadcopter up to 4,500 to 9,000 metres (15,000 to 30,000 feet) where the wind is consistently stronger and faster, have the blades spin in the wind like a kite, generate electricity from reversing the electric motors similar to regenerative braking on an electric car and transmit the electricity to the ground down a conductive tether. They have a small (1:13 or 1:17 scale) prototype that they have tested as of December 2011 flying with additional safety tethers through a limited range of the required manoeuvres. More testing may have been done and not publicized on their website.
Sky Windpower is foregoing most of the cubing of wind velocity Makani takes advantage of through cross wind flight while still challenged by a large loss of swept area. To get significant generation with this relatively static quadcopter approach, they intend to fly it in the 15,000-30,000 foot (4500-9000m) range. To be conservative this implies a minimum 9 kilometer electrified tether which is at any given time somewhere in a half sphere with a 18 kilometre diameter with a volume of about 1500 cubic kilometres. More realistically, it would require an 18 kilometre electrified tether for a half sphere of 36 kilometres with a volume of around 12,000 cubic kilometres.
One person who is not with Sky Windpower has said that Sky Windpower has abandoned the high-altitude goal and is targeting a 2000′ foot ceiling, but this is not reflected in their publicly available material as of March 2014; it is worth assessing the high-altitude implications regardless.
What are the implications for scaling?
Sky Windpower projects one quarter the swept area over four rotor sets for equivalent generation faceplate capacity to a conventional wind turbine. Let’s consider a five MW conventional wind turbine as a comparison. It has about a 128 m rotor diameter and a swept area of just under 13,000 square meters.
A small amount of math using Sky Windpower’s factor tells us that the device would require four 32 m (112 ft) diameter rotors with a total craft size of about 70 meters (245 ft) per side including blades. For context, the MI-26 heavy lift helicopter has a rotor diameter of 32 m with its eight blades; check the rotor droop and imagine four MI 26′s bolted together with less fuselage. Considering a two MW conventional wind turbine, the Sky Windpower device would have 23 m rotor (80.5 ft) diameter and the device would be roughly 51 m (179 feet on each side including blades.
- MI 26 heavy lift helicopter
This tells us that the Sky Windpower generation devices would need to be much bigger than the biggest rotorcraft today to be at a scale where useful generation might occur. This is a red flag in and of itself, but not necessarily insurmountable given advances in material science and use of electrical versus gas turbine motors. It does represent significant engineering challenges which are not addressed or called out in their public documentation.
What about the tether weight and length?
Tether weight is glossed over on the Sky Windpower site. Makani’s one km, carbon fibre and aluminum tether is projected to weigh 3,660 kg; let’s assume that is accurate and representative. The minimum length 9 km conductive tether would weigh about 33,000 kg assuming adequate strength. A 9 km ceiling would require a roughly 18 km tether weighing 66,000 kg. The MI 26, one of heaviest lifting helicopters in the world, has a maximum lift of about 20,000 kg above its own 36,000 kg weight and a hover ceiling of 1700 m.
It would be unsurprising if a 9 km carbon fibre and aluminum conductive tether cost more than the mast of an equivalently generating conventional wind turbine, and for tether plus device to cost significantly more than a conventional wind turbine of equivalent capacity. For comparison, an MI 26 by itself costs $15-18 million, and a 5 MW Sky Windpower device would be much bigger and fully autonomous. Also for comparison, the MI 26 weighs about 36,000 kg including fuel; take off most of the fuselage, replace the turbines with electric motors, remove the fuel tanks and then multiply by four. It’s likely that a 5 MW equivalent Sky Windpower device would weigh 20,000-30,000 kg by itself. The device would have to lift 53,000 – 63,000 kg to 4.5 km to get into its operational zone. No weight or dimension calculations for the Sky Windpower device are included in their public documents, so these are necessarily estimates.
Sky Windpower asserts that helicopter limitations don’t matter due to using rotor kiting; they don’t show their math so its difficult to determine which limitations they are referring to, and whether they are correct or not.
What about weather conditions for safe flying?
Sky Windpower’s current plan per public material is to land the devices when lightning is forecast. They make no mention of how long it will take their device to ascend to its 4.5 km operating minimum or descend from that altitude safely, but they project future developments which will make this necessary less often. They are silent on other weather conditions which would require grounding of the device; there are easily half a dozen which would require this. They are also silent on energy required to lift the potentially 20,000 kg device and 33,000 kg of cable to operating height and what the net energy output impact will be. For context, one conventional wind farm’s draw from the grid for blade start and other peripheral items was 1:320th of the annual generated electricity.
Like Makani, Sky Windpower does not address temperature variation as a concern in their public material. However, while Makani can safely operate in tropical and subtropical climates, or in summers in temperate climates, the Sky Windpower device is projected to fly at altitudes where the temperatures are often below zero regardless of surface temperatures. The rule of thumb is 3.5 degrees F (2 degrees C with rounding) per 1000′ of altitude gain. At minimum suggested operational altitude of 15,000′, the Sky Windpower would be operating in temperatures colder than surface temperatures by about 53 degrees F or 30 degrees C. At 30,000′, operating temperatures will be 105 degrees F (60 degrees C with rounding) lower on average.
In their publicly available material, there is no mention of de-icing technology requirements on the ground, or operational factors which will prevent ice build up aloft on the device or along the tether. It is possible that when operating, airframe vibrations will prevent ice build up, as will tether oscillations, but these assumptions are unstated. Regardless, de-icing on the ground is a significant unstated operational requirement for any climate which experiences winter conditions. To be clear, there is apparently nothing in the literature from any airborne wind generation organization which addresses this.
What about maintenance?
Like Makani, maintenance requirements will be much higher for this device than for a conventional wind turbine. The ratio of maintenance to flight hours for helicopters is often in the 3.5-4.5 to 1 range, meaning that a helicopter flies one hour for every 3.5 – 4.5 hours of maintenance. Sky Windpower is silent on why the quadcopter technology is expected to achieve the inverse of that ratio at a minimum, and why this is going to be better than the guaranteed 95% availability modern conventional wind turbines have while under warranty. With icing and weather related downtime as well as significant energy loss to get their device to altitude it’s unclear that they will achieve the significantly greater capacity factors to overcome other challenges. Remember that new conventional wind turbines in the US operate at 35% capacity factor in lower wind category zones, and up to 47% capacity factor in the best wind category zones per LBNL. The best modern wind farms in the US and Brazil are seeing 50% capacity factors regularly. Achieving significant improvements on those numbers is non-trivial.
What about spacing and safety?
Spacing requirements will be large. Ground equipment and tethers are not worked out in any detail on their public site. Given multiple devices at multiple altitudes with multiple tether angles ascending and descending, turbulence at different altitudes, tether slack potential and tether oscillation, a single 70 m (245 ft) per side device will need a large amount of operating leeway on all sides. If a device manages to get the cable of a nearby device caught in its blades, both are likely to be in serious trouble and this would very likely cause a domino effect in other devices which would have to be taken account of in spacing.
This randomly chosen winds aloft MAPS sounding from a paragliding site is instructive. At around 5000 ft, the wind direction is about 180 degrees from the wind direction at 15000 ft and very strong. Tethered devices ascending and descending through this will have very significant tether angle and direction differences. The highly idealized pictures of farms of airborne devices have them all pointing the same way, at the same altitude with tethers that are all parallel; this is highly unrealistic. This is a very significant safety factor that those proposing these devices downplay or or ignore entirely.
For an idea of the best possible scenario of a high-strength cable caught in large rotor blades, watch this video of a helicopter catching a cable.
A worst case scenario is a higher-altitude Sky Windpower device having a tether broken a kilometre or two below the device due to an airplane strike or a malfunction on another Sky Windpower device. The Sky Windpower device would be blown downwind rapidly, with the 3700-7400 kg of severed tether penduluming beneath it. Once it stabilizes, it will not have power so it probably won’t be able to make headway and will likely still be going downwind if more slowly, and with limited onboard control ability due to lack of significant power. All devices downwind will have the tether dragged through their operating airspace and potentially fouling their rotors.
These factors imply at minimum very large gaps between devices as well as a very large amount of synchronous flying automation given the oscillating tether. While quadcopter automated synchronous flying is advancing, at present it is only done with tiny quadcopters in safely enclosed indoor volumes. Sky Windpower asserts that they could bring a device down safely in the event of a severed tether, but it’s unclear that they have thought through the scenarios. In any event, the implication for spacing is that safety requirements for the foreseeable future will required significant spacing, likely more than conventional wind turbines just as with Makani. Given the chaotic nature of atmospheric conditions, solving this would require significant instrumentation to determine and communicate precise tether locations and automation software that can plot safe courses through a sea of moving cables.
What about land usage?
The area under a Sky Windpower farm will be the equivalent of an airport. Due to very large devices landing and taking off, it is likely that no other economic uses would be allowable due to workers safety and insurance reasons. Unlike Makani, maintenance would likely not require nearby generating devices to be shutdown however. This once again is a comparative disadvantage to conventional wind farms which are mostly interspersed with agricultural and other land uses.
Sky Windpower claims 1:400th of US airspace would be required to provide all of the USA’s electrical demand, so they’ve done some density math, but unfortunately don’t show it. Generous assumptions would be:
- five MW devices,
- 60% capacity factor and
- 1000 m spacing between devices.
At roughly 3500 TWh per year annual consumption, roughly 133,000 of these devices covering about 2% of the total land mass of the contiguous United States with technology that makes any other uses unsafe would be required. It’s unclear what Sky Windpower’s calculation of 1:400th — an order of magnitude less than this calculation — is based upon, but it is highly suspect as a power density calculation without engineering or safety considerations.
What about flight permissions?
Sky Windpower indicates that they have had initial discussions with the US Federal Aviation Authority on operating conditions and requirements, and have some ideas on how to achieve acceptance of their device. Their comparison to blimps is disingenuous just as Makani’s comparison to antenna tower guy wires is disingenuous; blimps have passive lift and are much safer than an auto rotating quadcopter with four rapidly rotating blades of the scale of the Sky Windpower projected device. Imagine four MI 26 heavy lift helicopters bolted together auto rotating down in a housing complex in a high wind. Imagine a blimp coming down in the same place. The FAA and insurers will imagine this.
Like Makani, Sky Windpower is more optimistic about the FAA accepting their arguments than I am, and if this moves forward it will likely after be a decade or more of dedicated and restricted testing. One comparison to blimps may be apt, however; currently blimps carrying radar equipment at 15,000 feet do not require tether markings in restricted airspace according to a point made on the airborne wind energy systems forum. It’s unclear if aviation authorities will be convinced that the more passively safe blimps are equivalent to massive quadcopters however.
What does this all add up to?
Sky Windpower is projecting building the biggest multi-rotor helicopter ever, the largest autonomous helicopter ever, the heaviest lifting helicopter ever, the highest-flying autonomous helicopter ever and for a required price point much lower than a single MI 26. The company does not have appeared to have worked out even a small fraction of the engineering challenges. It’s fairly clear that it’s not possible given air density with increasing altitude to lift a 33,000 kg tether in addition to the airframe weight, making their high-altitude solution non-viable based on that factor alone. Unlike conventional wind turbines which co-exist well with farming and secondary uses, no secondary uses would be reasonable within a farm of these devices. There is not even the most basic of reasons why a federal aviation authority would consider setting aside large swaths of restricted airspace for these devices.
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