Fluttering flags to harvest power looks promising to researchers

Fluttering flags to harvest power looks promising to researchers
Credit: Nature Communications 5, Article number: 4929 doi:10.1038/ncomms5929
"Flutter-driven triboelectrification for harvesting wind energy," published this week in Nature Communications, is a study of note for those interested in what researchers are exploring as sources of clean and sustainable energy. The authors have various affiliations with Samsung Institute of Technology (SAIT), Seoul National University, Korea Advanced Institute of Science and Technology (KAIST), Samsung Electronics, Chonnam National University and Georgia Institute of Technology. Their generator uses contact electrification caused by the self-sustained oscillation of flags. The authors said that "flutter-driven triboelectric generation is a promising technology to drive electric devices in the outdoor environments in a sustainable manner." Translation: Power is obtained from the fluttering motion of a flag-like structure. Business Insider Australia's Chris Pash said, "The flutter-driven triboelectric generator is based on the principle of charge transfer when two materials are rubbed together, similar to when a balloon is rubbed against clothing and then sticks to a wall."

Nick Stockton in Wired described the generator as building a charge "using mechanics that are similar to rubbing a balloon on your nephew's head." He explained the process: "when a breeze hits the small contraption, the electrode-coated flag stirs into motion, brushing against a conducting counter plate. This rubbing action builds a static charge on the counter plate's polymer surface, in what's called the triboelectric effect. A small capacitor gathers the charge."

The material used was a synthetic textile coated with gold, a highly efficient conductor, added Stockton. Each counter plate, he said, sandwiched another piece of the gold-coated fabric between a baseboard and polymer called PTFE. "When the gold flag flaps against it, it builds up a nice static charge, which the gold in the baseboard then conducts into the capacitor." Over 12 million flutters later, a test flag began to tatter but showed only a tiny decrease in power output, said Stockton.

What about capturing otherwise wasted from such sources as walking, wind blowing, vibration, or ocean waves? Zhong Lin Wang of Georgia Tech in December last year gave an interesting presentation in a video on the energy around us. Georgia Tech researchers are developing a family of power generators that take advantage of the triboelectric effect, to produce small amounts of electricity for portable devices and sensors.

Professor Wang in the video said, "We look for new energy but energy is around us, everywhere, all the time." Referring to an illustration of a top and bottom of a shoe-box like structure, he said, "What you see here is A materials and B materials." When they become physically in contact, he said, there is a charge transfer. If they are separated by a gap, there is a voltage generated .In a second mode, if one part is sliding against the other, there is also a charge. One mode is physical contact and the other one, sliding. He said, "This is the two-phase mode we use." The triboelectric effect, is used to create "surprising amounts of electric power by rubbing or touching two different materials together," said a Georgia Tech News Center report. "The fact that an electric charge can be produced through triboelectrification is well known," Wang explained. "What we have introduced is a gap separation technique that produces a voltage drop, which leads to a current flow in the external load, allowing the charge to be used. This generator can convert random mechanical energy from our environment into electric energy."

Harvesting energy from a rotatable flutter-driven triboelectric generator in outdoor. Credit: Nature Communications 5, Article number: 4929 doi:10.1038/ncomms5929

Each flag creates a small amount of electricity; the authors suggest building arrays of flag generators might be required to generate substantial amounts of power, Stockton said.

Harvesting energy from a stacked flutter-driven triboelectric generator packaging module on the highway. Credit: Nature Communications 5, Article number: 4929 doi:10.1038/ncomms5929

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Capturing wasted electricity with triboelectric generators

More information: Flutter-driven triboelectrification for harvesting wind energy, Nature Communications 5, Article number: 4929 DOI: 10.1038/ncomms5929

Abstract
Technologies to harvest electrical energy from wind have vast potentials because wind is one of the cleanest and most sustainable energy sources that nature provides. Here we propose a flutter-driven triboelectric generator that uses contact electrification caused by the self-sustained oscillation of flags. We study the coupled interaction between a fluttering flexible flag and a rigid plate. In doing so, we find three distinct contact modes: single, double and chaotic. The flutter-driven triboelectric generator having small dimensions of 7.5 × 5 cm at wind speed of 15 ms−1 exhibits high-electrical performances: an instantaneous output voltage of 200 V and a current of 60 μA with a high frequency of 158 Hz, giving an average power density of approximately 0.86 mW. The flutter-driven triboelectric generation is a promising technology to drive electric devices in the outdoor environments in a sustainable manner.

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Sep 25, 2014
If they make this work efficiently in air than it could only get better utilizing the design for water.

Sep 25, 2014
If they make this work efficiently in air than it could only get better utilizing the design for water.

No, it wouldn't work at all in water because you need a natural source of flowing water. However, all of the natural sources of flowing water are full of dissolved ions of various minerals and salts that make the water very conductive whereas the air is a pretty good electrical insulator. That conductive water would dissipate any charge differentials as soon as they were generated, rendering the device inoperative.

Sep 25, 2014
At an astounding 7% efficiciency. Meanwhile real wind turbines are 20 to 30% efficient, and don't involve banging pieces of gold together thousands and thousands of times.

Sep 25, 2014

We tested the corollary of this, the fan of flapping sheets of piezoelectric materials. They were covered in some kind of flexible plastic and would not short out in water. But they flapped (buzzed), continuously for years. It was back in the mid 1980's, and I was with PG&E.

Scorn at your own risk of embarrassment.

Sep 26, 2014
We tested the corollary of this, the fan of flapping sheets of piezoelectric materials. They were covered in some kind of flexible plastic and would not short out in water.
Piezoelectric crystals generate electric fields within the crystal in response to mechanical stress applied to the crystal. The system described in the article is based on transfer of electrons from the surface of one material to the surface of another. The system you describe is only an analogue in the sense that something is flapping and can be made to operate underwater. A system such as the one in the article cannot operate in naturally occurring water because the dissolved ions in natural water increase it's conductivity to the point that it would inhibit the electron transfer between the materials and would also quickly dissipate any electrons that managed to transfer from one material to the other.

Sep 26, 2014
At an astounding 7% efficiciency. Meanwhile real wind turbines are 20 to 30% efficient, and don't involve banging pieces of gold together thousands and thousands of times.

Is your point that unless initial experiments in a new technology immediately show it to be superior, no further development should be done? With that view, we wouldn't ever have developed electricity, so a moot point.

Sep 26, 2014
Efficiency figures by themselves are meaningless. What is important is the combination of cost and efficiency. If you can build e.g. a solar cell that is only 7% efficient but only costs $1 per square metre on a roll, and is tough enough to last 20 years stuck to a wall, roof, path, pavement or road then that is a game-changer.

Sep 26, 2014
If they make this work efficiently in air than it could only get better utilizing the design for water.

Not really, since the effect is better the faster stuff flutters and the larger the range of motion. You can get a membrane to flutter a lot faster in air than in water.

(If we think large scale then that could be an answer to reducing bird fatalities from wind energy generation)

(Unrelated note: In the US they'd probably put their national colors on the flags. They can't seem to get enough of thse fluttering around. Looking through history books I can only find one similar example of flag-fixation..and that isn't too...erm...flattering)

However, all of the natural sources of flowing water are full of dissolved ions of various minerals and salts that make the water very conductive

In water I'd sheath the entire thing in a polymer and put it where there are turbulences.
Wind turbines like laminar flow. These flags like turbulences. They could complement each other

Sep 26, 2014
Code Warrior does not know what we tested, and he is incorrect.

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