DURHAM, N.C. -- By taking advantage of the vagaries of the natural world,Duke University engineers have developed a novel approach that they believecan more efficiently harvest electricity from the motions of everyday life.

Energy harvesting is the process of converting one form of energy, such asmotion, into another form of energy, in this case electricity. Strategiesrange from the development of massive wind farms to produce large amountsof electricity to using the vibrations of walking to power small electronicdevices.

Although motion is an abundant source of energy, only limited success hasbeen achieved because the devices used only perform well over a narrow bandof frequencies. These so-called "linear" devices can work well, forexample, if the character of the motion is fairly constant, such as thecadence of a person walking. However, as researchers point out, the paceof someone walking, as with all environmental sources, changes over timeand can vary widely.

"The ideal device would be one that could convert a range of vibrationsinstead of just a narrow band," said Samuel Stanton, graduate student inDuke's Pratt School of Engineering, working in the laboratory of BrianMann, assistant professor of mechanical engineering and materials sciences.The team, which included undergraduate Clark McGehee, published the resultsof their latest experiments early online in Applied Physics Letters.

"Nature doesn't work in a single frequency, so we wanted to come up with adevice that would work over a broad range of frequencies," Stanton said."By using magnets to 'tune' the bandwidth of the experimental device, wewere able verify in the lab that this new non-linear approach canoutperform conventional linear devices."

Although the device they constructed looks deceptively simple, it was ableto prove the team's theories on a small scale. It is basically a smallcantilever, several inches long and a quarter inch wide, with an end magnetthat interacts with nearby magnets. The cantilever base itself is made of apiezoelectric material, which has the unique property of releasingelectrical voltage when it is strained.

The key to the new approach involved placing moveable magnets of opposingpoles on either side of the magnet at the end of the cantilever arm. Bychanging the distance of the moveable magnets, the researchers were able to"tune" the interactions of the system with its environment, and thusproduce electricity over a broader spectrum of frequencies.

"These results suggest to us that this non-linear approach could harvestmore of the frequencies from the same ambient vibrations," Mann said. "Moreimportantly, being able to capture more of the bandwidth makes it morelikely that these types of devices could someday rival batteries as aportable power source."

The range of applications for non-linear energy harvesters varies widely.For example, Mann is working on a project that would use the motion ofocean waves to power an array of sensors that would be carried inside oceanbuoys.

"These non-linear systems are self-sustaining, so they are ideal for anyelectrical device that needs batteries and is in a location that isdifficult to access," Mann said.

For example, the motion of walking could provide enough electricity topower an implanted device, such as a pacemaker or cardiac defibrillator. Ona larger scale, sensors in the environment or spacecraft could be poweredby the everyday natural vibrations around them, Mann said.

Source: Duke University