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It's a Bird, It's a Plane, It's Airborne Wind Power
Jeremy Faludi, 30 Jan 06

airborne_windmills.jpg

Wind turbines are constantly getting taller because everyone knows the higher you get off the ground, the better the wind speeds. But building big towers is expensive, especially if you want one 15,000 feet tall. So why not ditch the tower and make the windmill fly?

Several people are trying to do that. They've been written about before by others, but we've yet to tackle them, so here's a little round-up of the three most notable projects: Sky Windpower, Laddermill, and Magenn. Each is a bit sketchy, but deserves to be given a shot. It seems obvious that once someone creates a workable system, it will become a huge winner, because of the sheer amount of power available up high: 1% of the jetstream's wind power could supply all US electrical demand. Also, one of the main complaints about wind power is its intermittency--the wind doesn't blow all the time, and so (according to Sky Windpower), most wind farms are only operating at their peak capacity 19-35% of the time. The wind is much steadier at altitude, so you get even more advantage over ground-based wind power. A final advantage is ad-hoc generation: devices with a reasonably simple tether-system do not have to be permanently installed in one place, they could be trucked out to any location that needed them.

Sky Windpower

Sky Windpower is the furthest along, with functional prototypes tested in the field. (We should have covered it last year when everyone else did, but here it is now.) Bryan Roberts, an Australian mechanical engineering professor, teamed up with some Americans to commercialize his "Flying Electric Generator" idea--a windmill tethered to the ground but flying like a whirligig in the jetstream, at 15,000 to 35,000 feet altitude.

According to their figures, one flying windmill rated at 240kW with rotor diameters of 35 feet could generate power for less than two cents per kilowatt hour--that would make them the cheapest power source in the world. For greater power needs, several units would be operated in the same location--Sky Windpower says that an installation "rated at 2.81 megawatts flying at a typical U.S. site with an eighty percent capacity factor projects a life cycle cost per kilowatt hour at 1.4 cents." And they would have far better uptime than most windmills--since the jetstream never quits, they should operate at peak capacity 70-90% of the time. Output would also be less dependent on location than it is on the ground, simply because terrain doesn't matter much when you're at 35,000ft; however, since the jetstream and other "geostrophic" winds don't blow much at latitudes near the equator, it would be useful primarily for middle- and higher-latitudes.

They can't promise uninterrupted power all the time, however. In an electrical storm, the power-carrying tether becomes the biggest lightning rod you've ever seen. (Move over, Ben Franklin!) Their website says this problem is "frequently brought to our attention, and must be addressed." Their plan is to take the flyers down to land before a storm gets bad, and wait for it to end.

The flying windmills would initially get in position under their own power, using their motors to drive the propeller blades and helicopter upwards until they reached altitude. Then the motors would turn off and become generators as wind pushes the propeller blades, and the whirligig would float instead of fall because when tethered, the lift generated by the wind would overcome the craft's weight as it also generates power.

The obvious question is safety. What happens if one of these things falls out of the sky? The proposed design has quadruple-redundancy in the propellers used to hold it up and generate power, and the units could be located away from population centers, so that seems reasonable. What about planes running into them, or more likely, their tethers? They would fly in restricted airspace. Sky Windpower points out that there are already many high-altitude tethered balloons in the US that have not had problems, and that enough installations to generate 100% of the US's power needs could fit in 1/400th of the nation's airspace. What about birds getting killed? They say that the flying windmills could make noise that would keep birds away; this would be prohibitive for ground-based turbines, but at high altitude no people will be around to be annoyed by it.

The prototypes Roberts has been making for the last 25 years have gone from wind tunnel to field trial, and they apparently work, with no new technologies required. However, they have been stalled seeking funding for the last three years. The text on their website shows a woeful lack of marketing savvy, leaning towards crackpottyness, so they may not be getting funding anytime soon. But hopefully they will find someone willing to give them a shot, since they seem the most promising of the three.


Magenn

Magenn is a more modest design, which makes it more feasible. The inventor is Fred Ferguson, a Canadian engineer specializing in airships. He envisions a range of devices from the household scale (possibly even luggable by RV's or backpackers) to the megawatt power-plant scale.

Magenn's design is radically different from other windmills on the market--it would not use propeller blades. Instead, it would be a helium blimp, with Savonius-style scoops causing it to rotate around motors at the attachment-points to its tether. The household device would be the size of a mobile-home floating at 150-400ft. altitude, generating up to 4kW of power. Megawatt-size versions of the device would be the size of a normal blimp and would be tethered at 400-900ft. altitude.

The blimp-like design has several advantages: its Savonius scoop design lets it operate in winds as low as 2 mph; it is safer in a crash, because it would fall slowly and be mostly made of flexible material; it is safer for airplanes, because it sits below legally usable airspace; it is safer for birds, because the moving parts are visible and travel with the wind, not perpendicular to it; and it is a less risky investment, because it is smaller, cheaper, and easier to build than one of Sky Windpower's devices. Their estimated price for a household system is $10,000. (It should even be cheaper than existing tower-mounted turbines, watt-for-watt.) There are tradeoffs, though--Sky Windpower's flying windmills would be in higher winds and more constant winds, which would be better-suited to large applications; Magenn says their devices are likely to operate at capacity 40-50% of the time. There may be other bugs to work out, too; Magenn's site is too slick to admit shortcomings of their plan like Sky Windpower's does (e.g. what to do about thunderstorms). The Toronto Star wrote about Magenn and its skeptics in December, but its main critic apparently doesn't know that the limits of legal airspace around cities would prevent planes from hitting the power-blimps.

The critic's valid point is that the devices are currently vaporware, with not even a working prototype built yet. Despite this, Magenn has a distribution partner lined up, once they do get into production. It looks like the power-blimps would be mechanically simple and easy to build, so I would give them good odds of pulling it off by the end of the year, as promised. But it will no doubt take a few years before their invention is optimized and debugged.


Laddermill

Laddermill was a research project at TU Delft in the Netherlands. They imagined a series of kites strung together by cables into a loop hundreds of meters--possibly even a few kilometers--high. The kites would be computer-controlled to change their attitude and generate more lift from the wind on one side of the loop than the other. This would cause the entire loop to rotate, and its rotation would push a generator down on the ground to create electricity. It sounds ludicrous, but the folks at TU Delft are smart, and we need to seriously investigate more out-of-the-box approaches. In some ways, this design is simpler than Magenn or Sky Windpower because it does not have to have the power-generation equipment aloft, and does not require a power-carrying tether, just a mechanical one. The elimination of power-generation hardware from the flying parts of the device also means that it should be safer, because the bits that can crash are small and light.

It appears nothing much has been done on the project since 2003, so it may be dead.

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Comments

What about Kitegen?

http://www.kitewindgenerator.com/


Posted by: Some Guy on 29 Jan 06

There are other shortcomings to this model other than just merely keeping these things aloft. Just as in land-based transmission lines, much energy is lost traversing the conductor. If Sky Windpower's device is located 35,000 miles up, and then over unpopulated areas, the juice would need to travel over many, many miles to get to where it is needed. How is the conductivity of the tether? Does the cost per kwh take this into account? What about the energy required to lift and keep aloft?

Nice Article...Keep 'em coming. You should take a look into Compressed Air Energy Storage (CAES).

Mark Brandon
Sustainable Log - News and Views for Socially Responsible Investors
http://sustainablelog.blogspot.com
http://www.firstsustainable.com
Subscribe to Sustaianble Log, we donate $1 to a cause of your choice through Alternative Gifts International


Posted by: Mark Brandon on 29 Jan 06

This might help address some of the objections to windpower: That turbines are loud, ugly, and that they kill too many birds. I'd think a few tethers hooked to a yoke of kites would leave less of a mark on the countryside than a big turbine farm.


Posted by: EAS on 29 Jan 06

Mark, I think you meant 35,000 feet, not miles. That's less than 7 miles, which seems pretty insignificant compared to how far power is often transferred


Posted by: EAS on 29 Jan 06

If it's doing what aviators call "autorotating", that is to say its blades are spinning in the relative wind, it won't "plummet out of the sky" except in the case of a catastrophic structural failure (all engineering statements should contain that phrase). It would autorotate down gently, possibly steerably, like a helicopter force-landing.


Posted by: Alex on 30 Jan 06

Great article Jeremy.

I knew about Magenn Power from the PowerGen show in Las Vegas in December; but I did not know about Sky WindPower, Laddermill and Kitewindgenerator.

It's amazing that Sky WindPower got their technology to work, great job Bryan Roberts!

I have to agree with Jeremy though, after checking out all the proposed technologies of "Floating Wind Generators", www.magenn.com "power-blimps" seems to be the most viable. I really like the simplicity of the Magenn Air Rotor System, it just makes sense to me.

At any rate, good write-up, and best of luck to all the companies mentioned above. The world needs all the new technology it can get.


Posted by: John Ponting on 30 Jan 06

Actually, Eric, I understood the distinction of feet vs. miles. Thirty five thousand feet is equivalent to about 6 1/2 miles. If the base station is in an unpopulated area, then presumably, the juice would need to flow several more miles to get to a population center, degrading every foot that it travels. Even with our current models of centralized power plants, it is estimated that 1/3 of the juice generated at the plant is wasted by the time it gets to the end user. I am just curious as to whether this factor is taken into account in the cost/kwh calculations, because if the generator needs to put out 2kwh for every 1 that is used, then we are back where we started -- with a premium price for wind power.

Mark Brandon
Sustainable Log - News and Views for Socially Responsible Investors
http://sustainablelog.blogspot.com
http://www.firstsustainable.com
Subscribe to Sustaianble Log, we donate $1 to a cause of your choice through Alternative Gifts International


Posted by: Mark Brandon on 30 Jan 06

Hi Mark, Transmission losses in the US were estimated at 7.5% in 1995 according to The Electric Power Engineering Handbook by Grigsby, so while this would certainly be a factor to consider in the cost calculation, I don't think it would be of major significance. After all, coal, gas, nuclear and hydro plants all tend to be located away from urban centres due to pollution and geographic concerns.


Posted by: Will Sacks on 30 Jan 06

Maybe the cables to the aerial generators could be superconducting and mostly lossless?


Posted by: Pace Arko on 30 Jan 06

I was just reading about Space Data's proposal to fly cell phone repeaters over North Dakota. (http://www.spacedata.net/press122005.htm) I wonder, could you combine power production and low cost wireless communication in one compact unit!


Posted by: Thom Worlledge on 30 Jan 06

What about a design that allows power to flow both ways? That way, you could power the rotor to initially launch it, even if there's no wind at the surface. As the rotor flies higher, eventually wind would provide lift and power, and the power flow would reverse. You could have a microprocessor on board to control the attitude of the rotor so as to maximize power generation and lift, and if the wind dropped too much, you could have it reverse the power flow to keep it aloft until the wind picked back up. This system could also control when and where the rotor was brought down.


Posted by: Paul in Florida on 30 Jan 06

Seven miles of copper wire is HEAVY. Here is a chart of Resistance losses and weight for various AWG wire.

http://www.zelax.ru/english/sprav/awg_multi.html

# 10 wire, 35000 feet would weight over 1000 pounds. I think line losses would be significant as would stress factors - having wire waving around in the air for months and years would certainly take it's toll. Plus where is all this new copper going to come from ? According to my basic electricity learning, you'd also have to run at tremendous DC voltages to over come the internal resistance in the wire - say 500 volts DC +

K


Posted by: kochevnik on 30 Jan 06

So why not carbon nanotube wires - these can be conductive and very light(see space elevator ideas)? I think if people would just invest in the idea this method of power generation could be extremely promising. Unfortunately getting people to make the attitude shift is the hardest thing about all of these technologies. Oh, if only I had a large fortune to give away!


Posted by: Daniel Johnston on 30 Jan 06

I find all of these high altitude options intriguing, and wish any and all of them great success. Regarding line losses: "superconducting and mostly lossless" is a far-fetched hope IMHO.

What about a tether that physically transferred the rotational energy down to the ground? I imagine a hollow plastic tube with a strong wire or lattice of woven wires (or lighter carbon fibers?) rotating in the middle--like a long, flexible axle. I have no idea what kinds of torque problems would occur, or whether the line losses to good old friction would negate the benefit of relocating all the heavy electrical generator equipment to the ground leaving only the blimp and tether up in the sky. I have replaced one far-fetched hope with my own far-fetched alternative!

Nat


Posted by: Nat Olsson on 30 Jan 06

"Regarding line losses: "superconducting and mostly lossless" is a far-fetched hope IMHO."

Never say never. Weirder things have happened in materials science.


Posted by: Pace Arko on 30 Jan 06

I have always wondered if the wind generated by cars and especially trucks as they pass on the highway could ever be harnessed with a series of small windmills attached to guardrails or medians. Unlike natural winds, automobile winds are fairly constant (though they diminish at night). I wonder if anyone has looked into this source of energy.


Posted by: Fiona on 30 Jan 06

You could use the car windpower to run the signs that say "Please drive carefully" ...


Posted by: Adam Burke on 30 Jan 06

Better yet, use car windpower to power road-rule enforcement cameras, so the very act of speeding powers the camera.


Posted by: Young Freud on 31 Jan 06

So who are the REAL heros? The ones that are trying to get us off of foreign oil and the destruction of our environment or the ones out there blowing up brown people for Bush and the oil companies? Hmmm....


Posted by: Cowicide on 31 Jan 06

As for "Never say never" to superconductors, room temperature superconductors would mean a great change in *all* power generation and transmission (and probably storage).

Whether the dust would settle for or against this particular option in that hypothetical case is anybody's guess.


Posted by: Jiri Baum on 31 Jan 06

I like the lighter than air solution and have played with similar circulation lifting systems. By using helium for lift you get past the problem of needing to balance the lifting force with the power generating force.

There are several challenges with the jet stream helictopter idea, including, getting the thing to altitude. Right now, helicopters find it hard to get that high. It takes too much power to develop the lift, so it would be difficult to design a ship that could get to 25 or 30K feet and still have enough reserve lift capability to generate electricity. Remember, the generators take energy from the rotors, slowing them down in effect, so you have to be able to maintain altitude with the helicopter running at less than full power, and in a hover to boot. Not a simple challenge, that. Then of course, the jet stream changes course quite often, so I doubt a fixed land base (one honking big pillar to hold the whole thing in place) could operate at capacity as much as they predict.

I think the flaw with the tethered kite idea is simply, how do you keep the whole mess from flying downwind? Eventually your loop would be horizontal rather than vertical as shown in the picture. (You can't fly a kite directly overhead for very long.)

Re: car wind power. I designed a system in college that used vertical axis circulation turbines (similar to the blimp) as both power generators and guard rails. The biggest problem is that a car hitting the guard rail would cost the state more than it cost the car owner.

Great kudos for these guys coming up with the ideas, we need more like them.


Posted by: Jimm on 31 Jan 06

Hi Jer!

I found this article beacuse some collegues of mine have been researching wind power for demonstrations at the science museum I work at. Thanks for keeping us up to date on the bleeding edge...


Posted by: Kathy Reeves on 31 Jan 06

Why fly the thing up there under power at all? It's essentially an autorotating chopper. To get it up to altitude I can see two better options - one, use a balloon, two, tow it up with a fixed-wing aircraft.


Posted by: Alex on 31 Jan 06

Regarding superconductors:

I used to work (as a lab grunt, while getting my BS in Physics) at a superconductivity lab at the University of Arkansas.

For superconductors to be used in the kind of situation you're proposing, Pace, we would have to solve all of the following problems:

* either provide an excellent thermal insulating sheath or above room temperature superconductors
* find a superconductor with a massively higher current density than currently available
* find a superconductor that could take the physical abuse of being whipped around in the wind, subjected to constant tension, and generally abused

All of the high temperature superconductors (where high temperature means warmer than liquid nitrogen) that we have found so far are ceramics. We can't even build commercially viable "wires" that don't need to flex, much less something that could take constant bending without fracturing and failing.

That is not to say that we won't some day have the superconductor you're looking for - but it is a holy grail on the order of efficient fusion power, or even worse. Such superconductors may simply not exist.


Posted by: Bobby on 31 Jan 06

What about taking an idea from the Lunar Solar Power book? They advocate putting solar panels on the moon to constantly generate electricity. What might be useful to this discussion, however, is how they plan to transmit it to earth. They suggest freeing up portions of the microwave spectrum and using them exclusively for sending down energy. Such an idea might be viable on a SkyWindpower station like that above.


Posted by: Daniel Johnston on 31 Jan 06

As a response to Jiri Baum (4 comments ago):
Helium is expensive- use hydrogen, better lift, isnt that dangerous -no ignision source, usually away from people that could be hurt by the fire. (if it burns anyway)

Also, think you havent read the website of skywindpower about about the generators with the rotors well. You the website mentions the problem with helicopters and those altitudes: (quote)
"We also get asked if FEGs are not subject to the same problems helicopters are when they attempt to fly at high altitudes. The answer is no. Helicopters designed to hover with loads at low altitude have increasing trouble as air density decreases and motion of the air mass has no benefit. However, the faster moving airmass at high altitude far more than compensates for the lower air density in the case of the tethered FEGs."
If you still have objections and checked the site, you could ask the people that made the website yourself. (maybe you contribute)

The ladder idea used a simulation to try to show that stability-wise it would work.
(but i have my doubts about it too)


Posted by: Jasper on 31 Jan 06

sorry made a mistake last post was a response to Jimm, not Jiri Baum (used to having the name on top)


Posted by: Jasper on 31 Jan 06

The biggest problem with the balloon solution is that it requires an extremely low housing density. In a typical suburban area, what percentage of the houses could take advantage of it?

If the cable is 400 feet long, then you probably don't want any house within 400 feet also flying one. A typical suburban lot is, what, 100 feet on a side? So that leaves you with 6% of the houses powered by wind... hmmm...

The megawatt station would be more practical, you could have one flying over the neighborhood green-belt. Sell advertising on it...


Posted by: Peter da Silva on 31 Jan 06

The Sky Windpower concept faces three challenges, which are mitigated by design or application of existing technology:

1) Deployment and retrieval. Their design is helicopter, so they can fly it within a volume limited by the tether. Unlike a helicopter, it does not carry fuel, a crew or any crew support equipment such as air conditioning, oxygen, all of that cabin structure, seats, windows, etc. That means the vehicle can be light weight; motor/generators, a simple airframe, some avionics and the tether. Therefore 35,000 ft may be achievable.

2) Maximizing efficiency. They have to find and maintain that sweet spot in an ever changing jetstream. This can be done with ground based Doppler radar monitoring in combination with standard airborne weather radar equipment found on commercial airliners. This equipment also allows the vehicle to detect and avoid thunderstorms for safety

3) Safety. You can't have the vehicle crash and leave the tether laying over miles of terrain, even if unpopulated. Autorotation is possible for a controlled decent under failure conditions, but they must be able to pilot the vehicle towards the base station as they take up cable slack. It would be best to design for emergency powered flight with loss of one rotor. Not sure if stability with three rotors is possible with their current design.

I don't think the tether and power transmission are challenging or require special materials. If you're worried about weight or power loss just crank up the voltage.

Superconducting transmission cables are being demonstrated now
http://www.amsuper.com/products/htsWire/HTSCables.cfm

Potentially a superconducting tether would be lighter because it could carry one or two orders of magnitude higher power for a given size. However, the liquid nitrogen loop seems impractical.


Posted by: David M on 31 Jan 06

Jasper , if they use hydrogen , what would happen to the balloon on new years eve ?

"Oooo .. Fireworks" :)


Posted by: Christian on 31 Jan 06

Jasper -

Wouldn't it be a problem floating a big balloon of hydrogen on top of "the biggest lightning rod you've ever seen"?


Posted by: Adam Burke on 31 Jan 06

They must have to use Helium. But helium prices are at record levels - I wonder if this has been factored into the cost per kWh ? How often would you have to top this thing up ? Anyway, like the general idea ...
W


Posted by: will on 1 Feb 06

GREAT article, very interesting. I just wonder though when they say 1/400th of the airspace, it doesnt sound like much. I'll have to do a google search to see if "airspace" means by area or by volume. Just calculating roughly - one of the proposed 35 foot Sky Windpowers would produce 250kW. A larger size gas turbine can produce in the area of 250MW, so it would take 1000 of the windmills to produce one turbine worth. I think the general estimate is that 1MW can support about 1000 households.


Posted by: Anant on 1 Feb 06

Moving rotational energy down to the ground! Genious idea. Don't bother with a flexible shaft but use a simple pully system.

On the ground: Generator placed on rotating table. A pully on the shaft. A cable loop runs up to another pully on the windmill shaft rotating at altitude. Especially the Magenn mill would be suitable for this approach since it doesn't need to be powered to reach altitude.

Actually "Sky Wind Power" should not need to be powered to reach altitude. The Auto Gyro effects in a reasonable wind should make the whole thing go up like a kite. This would work, especially when using the pully power transmission method I described above. The weight reduction of the wind mill and no copper cables would make autogyro a viable option.

Also the pully system would reduce the risk for lightning strike and the effects of a crash would be reduced due to a lesser device weight.


Posted by: Paul van Dinther on 1 Feb 06

This whole idea doesn't sound very feasible to me.


Posted by: scalpmed on 1 Feb 06

As far as I know the laddermill idea isn't dead. Current research is concentrating on controllable kites. In fact the TU Delft has recently (December 2005) installed a kite testing system on tog of one of the engineering tower blocks.

See:
http://www.lr.tudelft.nl/asset/webpage/en/news.php


Posted by: Baldur van Lew on 2 Feb 06

Check out this wind power generation concept from kite master Peter Lynn:

http://groups.google.com/group/sci.energy/browse_thread/thread/7e4fdb0421d2afb0/882c3b4181aa1b4d
http://www.inet.net.nz/~cbrent/pete/


Posted by: Oren T on 2 Feb 06

To transfer the amounts of energy that are going to be generated by one of these devices, you aren't going to be able to use heavy-duty household wiring. You will need to use something like 1-inch diameter cable, and you will need 4 strands of it. This works out to approximately 2 pounds per foot of cable, which is about 35 tons of cable. That's a pretty hefty kite string!

You then need to consider the ability of your wind device to hold up this weight. You must also have an anchor point that is capable of withstanding the pull of that 35 tons (plus the weight of the wind machine) when the wind blows. The wind machine can only do work when there is an equal force being exerted at ground level to prevent the wind machine (and it's cable) from moving.

If the wind machine ever does pull up the anchor, you'd better hope you are not upwind from it at the time.


Posted by: Jan on 9 Feb 06



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