Mini-Windmills Generate Power in the Dark

Miniature windmills could help power devices where sunlight is not available, such as in tunnels or the shadows of mountains, valleys or forests, researchers now reveal.

Such windmills could power devices as humdrum as home alarms and lamps as well as sensors for monitoring border security, climate changes or forest fires, the inventors said.

After solar power, wind power is the most effective source of mechanical energy in nature, explained materials engineer Shashank Priya at the University of Texas at Arlington.

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"Solar energy is very cheap now, just $25 for a one-watt solar panel. They're a very effective source of power when you have sunlight," Priya said. "But if there's less light, their efficiency at generating power can go from about 10 percent down to 3 percent. If windmills can generate that amount of power or more, than they can become a helpful technology."

For instance, tunnels are obviously too dark for solar panels. "You can have cars passing at high speed through them, generating big gusts of wind. So you can imagine windmills inside to power the lights, replacing electrical wires that, if they caught on fire, would be hard to extinguish," Priya said.

The windmills could also be helpful in shadowy environments. "You can imagine networks of sensors at national borders near forests that can transmit information wirelessly to the Internet," Priya said. "Or you can imagine them helping monitor forests to capture information on fires as early as possible.

"You can think of them monitoring traffic in the mountains, powering red and green lights where sunlight doesn't reach the roads to let people know whether there's opposing traffic coming on the curving roads or not," he added. "You can also imagine them monitoring climate as weather stations."

Priya also said the windmills could find more innocuous uses at the home and supplement solar panels "to help harvest power day and night. You can imagine the little windmills looking rather decorative at rooftops, lawns or backyards."

The researchers had to start their windmill design from scratch, as the large windmills used to generate power in wind farms do not work well when scaled down in size.

Piezoelectric ceramics are at the core of the windmills the researchers have made so far. These materials convert mechanical stress into electricity. Priya initially worked on piezeoelectrics that help power corrosion, fatigue and heat sensors in aircraft by converting vibrations felt throughout planes into electricity.

The best windmill design the researchers have so far possesses three aluminum alloy fan blades capable of capturing wind from all directions, working with winds ranging from three to 15 miles per hour. The rest of the windmill is made cheaply from plastic. In total, the windmill is 3.5 inches by 4 inches by 5.5 inches in size, a little larger than a can of soda, and roughly 1.3 lbs. in weight.

The windmill converts wind energy into mechanical energy, which piezoelectric components then convert to electricity for storage in batteries or supercapacitors. It can generate 5 milliwatts of continuous power at average wind speed of 10 miles per hour.

The researchers are now adding magnetic elements into future windmills that will, along with the piezoelectric components, help transform mechanical energy into electricity. These ought to boost windmill power roughly tenfold, Priya estimated.

"We're getting very close to a cost-effective generator to make a commercializable system," Priya said. He and his colleagues reported their findings in the latest issue of the journal Applied Physics Letters.

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