Sustainable Energy

Good Vibrations

Tiny devices that harness the energy from ambient vibrations could one day be used to power a variety of wireless sensors.

A miniature device that harnesses ambient vibrations and converts the energy into electricity has been developed by engineers in the United Kingdom. They claim that the energy-scavenging machine is considerably more efficient than similar devices and capable of generating 10 times more power.

Shake to wake: The prototype accelerometer (above) is powered by a microgenerator that harvests energy from ambient vibrations.

“The motivation is to power wireless devices,” says Steve Beeby, an engineer at the University of Southampton, in the United Kingdom. “It’s a parasitic energy source.” The device, which generates power using the natural vibrations going on around it, could be embedded in sensors in bridges or the airframes of planes. It would be particularly useful in situations in which it would be difficult to access power wires or replace batteries.

The device, which measures less than a cubic centimeter, has shown that it can generate 46 microwatts when vibrated at just 52 hertz. This is typical of the kinds of vibrations found at an industrial plant, and it would be enough power to run a device like a temperature or pressure sensor.

It’s not the first time such devices have been made. Larger cup-size commercial generators that monitor equipment in industrial plants or in oil refineries are being produced by some companies, including Perpetuum, a spinout firm from the Southampton lab. Similarly, some wristwatches are now powered by the movement of the wearer’s hand or by heat from his or her skin.

But making small generators capable of creating significant amounts of power has been problematic, partly because electromagnetic devices don’t scale well, says Beeby. “The smaller you go, the less power you get.”

“The biggest challenge is that the power levels are very small,” says Eric Yeatman, an engineer at Imperial College London, in the United Kingdom, who works on similar devices. One reason is that the amount of motion being harvested and the available frequencies of the vibrations tend to be low, which means the available energy is pretty limited to start with.

Beeby’s device works by having four small, high-performance magnets made out of neodymium iron boron attached to the end of a springy cantilever. The magnets are arranged around a fixed disc-like ring made of coiled copper wire. When the device is shaken, the cantilever oscillates, causing the magnets to move backward and forward across the coil. Their fluctuating magnetic fields induce an electrical current in the coils in much the same way that regular electrical generators work.

Other researchers have tried similar approaches in the past but struggled to generate decent amounts of power, says Yeatman. “Previously, they have tended to have quite low output voltages,” he says.

Beeby says he and his team solved this problem by making their coil out of extremely narrow copper wire measuring just 12 thousandths of a millimeter in diameter. They are able to wind it very tightly, squeezing 2,300 turns onto a coil just 2.5 millimeters in diameter. The voltage output of generators is very much dependent on the number of turns in a coil, says Beeby.

To test the device, he rigged it up to a simple accelerometer circuit and found that it was able to convert 30 percent of the available kinetic energy into electricity. Although it’s difficult to make direct comparisons with other devices because of the differences in design, energy source, and size, Beeby’s group nevertheless carried out a comparison that tried to take these factors into account. According to these calculations, the group’s device performed very well and was the most efficient yet.

The work has been published in the Journal of Micromechanics and Engineering and was carried out as part of a wider European project called Vibration Energy Scavenging, or VIBES. The device is designed to mop up energy from vibrations of particular frequencies. (The prototype, for example, was designed to work with vibrations typical of manufacturing equipment.) But by varying the parameters, the same design could be used to work with other frequencies for other applications, says Beeby.

According to Yeatman, however, the most likely commercial applications will be in situations in which low-cost devices can’t be easily reached or accessed, such as in wireless sensor networks for bridges and other large structures.

Another example of this is the use of microgenerators to power medical implants, says Beeby. One suggestion is to employ them to power devices like pacemakers, possibly even using the motion of the heart as the energy source. With these kinds of devices, one of the big risks is the need for a battery change, says Andrew Grace, a cardiologist at Cambridge University, in the United Kingdom, and a consultant at Papworth Hospital, also in Cambridge. “The idea that the heart could provide the energy to recharge the pacemaker, almost like a wristwatch, sounds very attractive,” he says.

Beeby suspects that there are better parts of the body from which to scavenge energy, such as the limbs. And he feels that there are other types of implants that would use less energy than a pacemaker, such as biomedical sensors or drug-delivery systems.

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