By looking at examples found in nature, engineers at Duke University, Durham, N.C., have developed an approach that they believe can more efficiently harvest electricity from the motions of everyday life. The Office of Naval Research is supporting the experiments. “Energy harvesting” is the process of converting one form of energy, such as motion, into another form of energy, such as electricity. Although motion is an abundant source of energy, only limited success has been achieved because today's devices only perform well over a narrow band of frequencies. These “linear” devices can work well, for example, if the motion is fairly constant, such as the cadence of a person walking. However, as the researchers point out, the pace of someone walking changes over time and can vary widely.
“The ideal device is one that could convert a range of vibrations instead of just a narrow band,” explains Samuel Stanton, graduate student in Duke's Pratt School of Engineering. “Nature doesn't work in a single frequency, so we wanted to create a device that would work over a broad range of frequencies. By using magnets to ‘tune’ the bandwidth of the experimental device, we were able to verify in the lab that this new nonlinear approach can outperform conventional linear devices.”
Although the device they constructed looks simple, it was able to prove the team's theories on a small scale. It is basically a small cantilever, several inches long and a quarter inch wide, with an end magnet that interacts with nearby magnets. The base is made of piezoelectric material, which has the unique property of releasing electrical voltage when strained. Key to the new approach is placing moveable magnets of opposing poles on either side of the cantilever end magnet. By changing the distance of the moveable magnets, the researchers “tune” the interactions of the system with its environment, and thus produce electricity over a broader spectrum of frequencies.
“These results suggest that this nonlinear approach could harvest more of the frequencies from the same ambient vibrations,” says Mann. “Being able to capture more of the bandwidth makes it more likely that these types of devices could someday rival batteries as a portable power source.”
The range of applications for nonlinear energy harvesters varies widely. For example, Mann is working on a project that would use the motion of ocean waves to power an array of sensors that would be carried inside ocean buoys. Another use is that the motion of walking could provide enough electricity to power an implanted device, such as a pacemaker or defibrillator. On a larger scale, sensors in the environment or spacecraft could be powered by everyday natural vibrations around them, according to Mann. For more information, visit www.duke.edu.