Wind energy typically conjures up images of towering turbines being revved up by massive, spinning airfoils. At a much smaller scale, however, it's also possible to harvest wind power from the friction of flapping flags.

A study published today in Nature Communications describes an experimental generator, which builds a charge using mechanics that are similar to rubbing a balloon on your nephew's head. 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.

All the flags were small—less than 5 inches in length, and 4 in width—but varied in their dimensions. The researchers built them from a synthetic textile coated with gold, a highly efficient conductor. Each counter plate sandwiched another piece of this gold-coated fabric between a stiff baseboard and a Teflon-like polymer called PTFE that's a triboelectric superstar. 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.

The research team built several of the devices to test different length and width configurations. In a low breeze, longer, thinner flags generated more charge because they contacted the counter plate more often. When the wind picked up, the long flags got too chaotic and their performance fell off. High wind also made the short flags flap chaotically, but they still made regular enough contact with the counter plate to hold a significant charge.

The researchers also took the generators outside for field tests. In one, they attached a flag generator into a weather vane. Flapping in a light breeze (about 8-11 mph) generated a tiny charge: It would take thousands of them to power an energy efficient LED (10 watts). The researchers generated substantially more energy by mounting a flag on top of a car cruising along at about 45 mph, but still not enough to make meaningful fuel offsets.

The flags were pretty durable, though. After over 12 million flutters, a test flag began to tatter, but showed only a tiny decrease in power output.

Because each flag creates only a small amount of electricity, the authors of the paper suggest that building arrays of flag generators might be necessary to generate substantial amounts of power.

The top series shows a shorter flag that contacts the counter plate only once. The bottom series shows a longer flag that contacts the counter plate twice. Jihyun Bae et al./Nature Communications

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