Kites flying in high-altitude winds could provide clean electricity

Jun 24, 2009 by Christine Blackman
A Sky WindPower kite of turbines might capture wind energy with spinning rotors and send electricity to the ground through the wire that tethers it. (Ben Shepard)

(PhysOrg.com) -- At any moment, the winds in high-altitude jet streams hold roughly 100 times more energy than all the electricity being consumed on Earth, according to a study by Stanford environmental and climate scientists Cristina Archer and Ken Caldeira.

To capture that energy, designers are dreaming up models of wind-turbine kites that fly so high, cruising airliners would have to steer around them. The tethered kites would float high enough for powerful to flow through their turbines more than 10 times faster than winds would flow near the ground.

The spinning rotors of the kite turbines would convert the wind’s to electricity and send it back down the wire 30,000 feet to a distribution grid.

Harnessing these high-flying currents could open up an effectively unlimited source of electricity, the researchers say. Kites may have the potential to be cost competitive, Caldeira said, and the study shows that the wind resource is huge and relatively reliable.

The researchers reached this conclusion by analyzing 27 years of data from the National Center for Environmental Prediction and the European Centre for Medium-Range Weather Forecasts. By studying the distribution of in the atmosphere, by location and time, they found that winds at altitudes around 32,000 feet have the highest wind power density. “The wind power density tells you how much wind energy would flow through a wind turbine,” Caldeira said.

The researchers used the data to compile the first global survey of high-altitude wind energy. Archer is an assistant professor at Stanford University and California State University-Chico and Caldeira is an associate professor at Stanford and a researcher at the Carnegie Institution of Washington. Their findings were published in the journal Energies last month.

High-altitude winds hold a huge energy potential waiting to be harnessed. “If you tapped into 1 percent of the power in high-altitude winds, that would be enough to continuously power all civilization,” Caldeira said. In comparison, similar solar cells would cover roughly 100 times more area than a high-altitude wind turbine, he said.

Archer and Caldeira found the highest wind densities over Japan, eastern China, the eastern coast of the United States, southern Australia and northeastern Africa. Included in the analysis were assessments of wind energy above the world’s five largest cities: Tokyo, New York, São Paulo, Seoul and Mexico City. “New York … has the highest average high-altitude wind power density of any U.S. city,” Archer said.

Tokyo and Seoul also have high wind power density because they are both affected by the East Asian jet stream. Mexico City and São Paulo are located at tropical latitudes, so they are rarely affected by the polar and sub-tropical jet streams. As a result they have lower wind power densities than the other three cities.

In order to capture the energy in these jet streams, manufacturers are developing a variety of kite turbines that convert kinetic energy in wind to electricity. Manufacturer Sky WindPower designed a model consisting of a single tethered kite of four connected turbines, each with spinning rotors. The kite transfers the electricity back to a hub on the ground through its tether.

Another model, being developed by Kite Gen, looks like a rotating carousel, based on the ground, with several kites tethered to it. Each kite’s flight pattern is controlled from the ground to capture the most wind, and as the kites circle in the air, they catch the wind and tug on their tethers. The tension triggers a pulley system that converts the energy of motion to electricity.

Though sky-high currents offer huge potential, kite fliers face the challenge of a fluctuating wind. “While the winds at high altitude are much more consistent than the winds at the surface, they’re still not consistent enough,” Caldeira said. For example, if you flew a kite turbine in your backyard to power a house, at some times the wind would blow and at other times, it wouldn’t, he said. As a result, there would be gaps in the flow of electricity.

Archer and Caldeira suggest a large-scale electrical grid to transfer excess to areas that have more demand than production. “Winds are always blowing somewhere, so if you had a large enough electrical transmission grid you could transmit the electricity from where it is blowing to where it isn’t blowing,” Caldeira said.

Making batteries large enough to supplement the wind fluctuations seems improbable, and backup generators would be expensive, Caldeira explained.

Another minor hurdle may be interference from airplanes. Manufacturers such as Sky WindPower and Kite Gen say that air traffic is not a huge problem. Kite users would simply need to obtain flight restrictions above their air space, just as nuclear power plants and refineries do. Sky WindPower also suggests that kites be flown in areas that are rural but not too far from the urban spots that the researchers identified as high-flow.

Wind kites are not yet in use, but manufacturers predict that the cost of high-altitude wind power will range from 2 to 4 cents per kilowatt-hour.

Provided by Stanford University (news : web)

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User comments : 29

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pubwvj
5 / 5 (9) Jun 24, 2009
And when the wicked wind mill drops on your house it goes splat.
NeilFarbstein
2.1 / 5 (12) Jun 24, 2009
Too dangerous! Solar is safer
Ant
4.3 / 5 (6) Jun 24, 2009
The tether would of course require a pair of built in conductors it would not work using a single conductor as all generated energy needs a return path for the electrons.
Ant
3.5 / 5 (4) Jun 24, 2009
The voltage fluctuations caused by differing rotational speeds would be also be a major problem as it is for all wind turbines, static and offshore generators use induction motors as the generator because by feeding back the resulting voltage to the rotor via slip ring conections has the effect of slowing the rotor thereby smoothing out the overall voltage produced.
Ant
3.2 / 5 (6) Jun 24, 2009
pubwvj isnt that what happens with aircraft but no-one worries?
poi
3.2 / 5 (5) Jun 25, 2009
yah.. it can parachute down like a helicopter.
Birger
5 / 5 (1) Jun 25, 2009
As I mentioned in a similar article, for more *in-depth articles* of the concept,
see New Scientist: "Flying windmills could harness the jet stream", 26 July 2007 and "Reach for the sky" 23 September 2000.
New Scientist also presents the idea developed by Kite Gen in "High flyers", 17 May 2008.
Ethelred
4.3 / 5 (3) Jun 25, 2009
Gosh that was a lovely post. Equal parts vitriol and idiocy.

I am curios as to where it came from. You have two other posts and neither show this level of bizarre writing. Was it intended as a joke?

Ethelred

QubitTamer

Quantum Physicist, torturer of AGW religious zealots like Ethelred because i laugh at his hysterics.


QubitTroll will be released from my sig at the end of June.
Ohmaar
5 / 5 (4) Jun 25, 2009
Has anyone stopped to consider how much 30,000 feet of high-tension tether cable would weigh?
Ant
5 / 5 (4) Jun 25, 2009
Ethelred/Ohmaar

I agree, this is the typical university student third year project which goes nowhere simply because they dont think through all the additional problems first, such as the weight of the tether, getting the device up there, and no wind consequencies.
david_42
4.5 / 5 (4) Jun 25, 2009
Repeat from 1987 and 1961.

Up next, repeat of "Clean Power from the Gulf Stream".
PinkElephant
1.4 / 5 (5) Jun 25, 2009
Ohmaar, I'm guessing it would weigh less than the load-carrying capacity of some heavy-lift transport helicopters (such as the Chinook) -- particularly if the tension component is a high-tensile-strength fiber (e.g. mylar) and the conductor is light-weight (e.g. aluminum foil.) Of course but for affordability, carbon nanotube cables would really be a killer app for this... (weren't we promised we'd have those by, like, yesterday?)

pubwvj and others, far as dangers of falling turbines, having more than one rotor per system (e.g. four rotors) makes it a safe bet that it should be able to auto-rotate down to the ground, resulting in a fairly well-controlled landing. Obviously, these things would be best installed over shorelines or deserts, so that if they do have to crash-land, they'd be unlikely to hit anything valuable (or any human being, for that matter.)
Ant
3.7 / 5 (3) Jun 25, 2009
Hi pink elephant
you have obviosly not seen the size of appropriate generators. I dont see any mention of lifting rotors, or did a miss that? I thought these were supposed to be kites as mentioned in the title.
PinkElephant
1 / 5 (3) Jun 26, 2009
Ant, take another look at the picture on top, and the caption at the side of the picture. That particular model has 4 rotors on it. And just like with any wind turbine, those rotors are simply wings moving through the air: an onrushing wind makes them rotate, and so their attached motors act as generators. However, if the flow of electricity was reversed, those generators become motors, and the rotors become propellers. (How else did you think they were planning to achieve 30,000 ft altitude in the first place?)
Koen
5 / 5 (4) Jun 26, 2009
This was an idea of Dutch astronaut and engineer Wubbo Ockels. The energy kite probably does not have any value, because the production/operational costs of such a kite are much higher than the financial value of the energy savings. The fact that something is possible does not mean it is economical.
Dodgy
5 / 5 (4) Jun 26, 2009
This is a very old proposal. I recall it from over 15 years ago, and it has a well-known practical difficulty which has prevented it ever being realised. yet it keeps popping up, like a bad penny, repeated by lazy ignorant journalists who can't think critically...

The first point to make is that you hardly need to put the kites in the jet stream (which is regularly changing its position by hundreds of kilometers anyway). There is quite a lot of energy in normal high-altitude winds. And it's quite easy to design a kite generator to extract it.

The second point is that the kite is the least of your problems. Your problem is designing the cable. The requirements for strength, lightness, flexibility and power transmission put this well beyond the current state of the art. Note that, since it is a catenary, the length will be much greater than 30kft, indeed well over double with any credible tension. Here is a simple calculator. Try the figures... http://www.spacea...cabl.htm

That's why these generators don't exist.
Ant
2 / 5 (2) Jun 26, 2009
Hi PinkElephant

Thans for that I hadnt enlarged the picture. BUT your idea is riddled with problems for starters the control needed in wind turbines to try and ensure a constant voltage will not allow the generator to be used as a motor. In a toy model it might work but not on a practical comercial machine. Others and myself have previously indicated other problems without repeating them again but autorotation of the blades would not produce lift and energy.
jerryd
5 / 5 (3) Jun 27, 2009
Details!! A wind gen rotor can't be a propeller because it's shape is the opposite needed.
Doing variable pitch to auto-rotate adds too much weight, cost, complications.
The cable weight will be way too high.
Aircraft being sliced in half!!
This just is not going to fly.
As for the Gulf steam it is viable as I've already built tidal generators that would do the job if scaled up.
gwrede
1.3 / 5 (3) Jun 28, 2009
For steering it simply has to have variable pitch blades. It also has to have a transformer. It has to have a controlling unit with software. And for getting up, it simply has to use its propellers -- it's unthinkable that they'd be dragged up by helicopter! (Or eve stupider, by plane.)

I agree, the tether is the technologically most challenging part.

For service and bad weather, they'd land. But, like everything else, things do break, which simply means that you can't have them deployed above habited areas. It would be politically impossible to have even one person getting killed by a crashing kite.

Why the transformer? The higher the voltage, the lower the current. And it's the current that dictates the lead thickness, and weight.

PinkElephant
3 / 5 (3) Jun 28, 2009
jerryd & Ant,

I assume variable pitch generally would be a good idea anyway, if one wants to optimize power generation depending on wind conditions. And yes, to enable safe landings through auto-rotation, you would need either variable pitch or the ability to flip the rotor axis by 180 degrees. However, for lift to "orbit" the system of wind and rotor is symmetric. For the rotor to rotate, wind must be moving through it's front "face". If there's no wind and you forcefully spin the rotor in the correct direction (by using its generator as a motor), then the rotor will _induce_ the air to move through its front "face". Hence, the rotor is in fact a potential propeller; it just depends on what its attached electric motor is doing at the moment.

Anyway, I agree with gwrede that the tether is the main obstacle. I still think this would be doable, but only with a carbon nanotube cable as a tether (the same technology that might enable space elevators.) Such a tether would be ideal: very light weight, great flexibility, very high tensile strength, and at the same time very good electrical conductance (so, ideally satisfying both structural and functional requirements in one shot.) Only problem is, we still don't have the technology to make enough high-quality nanotubes cheaply enough, and then weave them into strong cables of any significant length. Maybe in another 15-20 years...

Personally, I think the government ought to dump a few billion dollars into carbon nanotube R&D. This material has so many revolutionary applications in so many areas, that it's puzzling we're not giving it a priority at least equivalent to ethanol from corn...
Damon_Hastings
5 / 5 (2) Jun 28, 2009
Personally, I think the government ought to dump a few billion dollars into carbon nanotube R&D. This material has so many revolutionary applications in so many areas, that it's puzzling we're not giving it a priority at least equivalent to ethanol from corn...

All you have to do is find a way for farmers to grow carbon nanotubes. Then just watch the subsidies roll in!!
Dodgy
4 / 5 (1) Jun 29, 2009
"...I still think this would be doable, but only with a carbon nanotube cable as a tether (the same technology that might enable space elevators.).."

No, it's a harder application than a space elevator. Your wires will have to be insulated. That will make the weight competely impractical, even if we imagine technology which does not exist yet...
DozerIAm
4 / 5 (1) Jul 01, 2009
Interesting comcept, but I have a couple of concerns - As Dodgy pointed out, the jet stream shifts around a bit. Since the area around these "kites" would have to be declared a "no fly" zone for commercial and military aircraft for safety reasons, we would end up with ever shifting "no fly" zones. Sounds like a recipe for an air disaster. My other concern is that the global winds are part of a closed system - if we start putting these things up there is quantity, the energy we take from the wind could be measurable, and could possibly have global implications. The latter concern is WAY past my education and training, so I will concede that I could be way off base on this concern.
KBK
2.3 / 5 (3) Jul 01, 2009
If they can UV proof the 'Spectra' fiber, that will most certainly do the job.

Pound for pound..50,000 times stronger than steel (tensile).

Also known as 'vectran'. Look it up. It makes Kevlar ropes look like a joke.

The concern could also be the disruption of those wind patterns, as previous posters mention. Mankind has a habit of being in the dickweed department, at times, when it comes to calculating consequences that disagree with desire.
gopher65
2 / 5 (1) Jul 01, 2009
KBK, you don't know what you're talking about:

1) Vectran has a maximum tensile strength of 3.2 Gpa. Kevlar is stronger than Vectran, having a maximum tensile strength of 3.6 Gpa.

2) Vectran is *very* heavy for a given volume, coming in at about 1.5 times the density of ice (about 1400 kg/m^3). Kevlar is slightly denser than Vectran (about 1440 kg/m^3).

These figures come from Vectran's manufacturer's website, so they are beyond dispute.

I'll add that while the best polyethylene fibres ("Spectra fibre") has a strength to weight ***ratio*** ~10 times that of steel. It's actual strength per unit of volume is comparable to steel.
gopher65
3 / 5 (1) Jul 01, 2009
I'll also note that polyethylene fibres have about the same density of ice. Too heavy. Waaaaaaaay too heavy.
PinkElephant
not rated yet Jul 01, 2009
@dodgy,

"No, it's a harder application than a space elevator. Your wires will have to be insulated. That will make the weight competely impractical, even if we imagine technology which does not exist yet..."

Why? We don't insulate high-voltage power lines on land, so why would we need to insulate them at altitude? The air is a good-enough insulator regardless of how high off the ground you go.

Though of course, the darn things could act as lightning rods on steroids. But the kites would simply be grounded while storm fronts are passing (otherwise you get all sorts of other issues anyway, from strong wind loading to ice buildup.)
Lord_jag
not rated yet Jul 03, 2009
You don't need clouds to have statically charged air masses. That lightning rod will have constant static discharges, even in good weather. If we could find a way to smooth the puled power out to a nice even flow we would have it made!
eliecerjorfoas
not rated yet Aug 09, 2009
Just a basic question here, if take the energy from the wind, who is gonna move the clouds?