Kites flying in high-altitude winds could provide clean electricity
June 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 jet streams 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 kinetic energy 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 wind power 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 energy 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)
Jun 15, 2009 |
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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.
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
QubitTroll will be released from my sig at the end of June.
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.
Up next, repeat of "Clean Power from the Gulf Stream".
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.)
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.
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.
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.
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.
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.
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...
All you have to do is find a way for farmers to grow carbon nanotubes. Then just watch the subsidies roll in!!
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...
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.
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.
"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.)