BioEd Online

Tapping the power of the wind doesn't have to mean peppering the skyline with vast turbines. The inventor of a pocket-sized windmill says his tiny device could power wireless networks of sensors in remote locations.

Wireless networks have freed us from miles of cumbersome wiring needed to carry information, but the electronic 'nodes' of such networks still need power. If geologists want to place hundreds of sensors on a mountain to monitor seismic activity, for example, they either have to supply electricity using cables or hike out to each sensor every six months or so to replace batteries.

"The problem is keeping the nodes powered all the time," says Shashank Priya, an electrical engineer from the University of Texas, Arlington, adding that wind power could be the answer.

His windmill is about 10 centimetres across, and is attached to a rotating cam that flexes a series of piezoelectric crystals as it rotates. Piezoelectric materials generate a current when they are squeezed or stretched, and are commonly used to make a spark in gas lighters.

Priya has found that a gentle breeze of 16 kilometres per hour can generate a constant power of 7.5 milliwatts, which is more than enough to keep an electronic sensor running. He unveiled his windmill earlier this year¹, and has now followed up with precise details of the device's abilities, presented in Applied Physics Letters².

Sunshine savers

The first trials of completely wireless sensor networks used photovoltaic cells for power. But these did not always work, since a spate of cloudy days could cause a node to shut down. "The idea failed because light isn't available everywhere and all the time," says Priya.

So researchers have turned to piezoelectric materials that can harvest 'ambient power' from the vibrations around them. Some scientists are trying to use the vibrations of passing cars to power sensors that monitor the structural health of bridges, or to tap the shaking of an aircraft to run sensor networks without needing to pack more wiring into an already crowded hull.

"We're all trying to scavenge that wasted vibrational energy," says Priya. His clever contribution to the field is to create vibrations where they do not already exist, from the power of a breeze.

Small but efficient

The piezoelectric generator is much more efficient way of converting wind energy on a small scale than the conventional generators that create energy for the national power grid from wind turbines.

A conventional generator that used a 10-centimetre turbine would convert only 1% of the available wind energy directly into electricity. A piezoelectric generator ups that to 18%, which is comparable to the average efficiency of the best large-scale windmills, says Priya.

"It's an interesting idea," says Henry Sodano, a mechanical engineer at Michigan Technological University, Houghton. Piezoelectric harvesting is a rapidly developing field, he adds, which will probably become much more common as electronic circuits get smaller and require less power.

Priya has also patented a much smaller device, measuring just 0.5 centimetres a side. This is driven by a smaller turbine fitted with tiny wind-catching cups, just like the devices used by meteorologists to measure wind speed. He is developing this system to capture energy from even lighter winds.