(a) Power generation by hand shaking; (b) design of BPEH.
(a) Power generation by hand shaking; (b) design of BPEH.

Energy is a valuable commodity and devices that can capture lost or wasted energy are becoming highly desirable. Piezoelectric materials generate an electrical charge when they are deformed, so make ideal energy harvesters from motion or vibration. Researchers from Harbin Institute of Technology in China have designed a novel, more efficient energy harvester based on the piezoelectric lead zirconate titanate (PZT) [Pan et al., Composites Science &Technology 119 (2015) 34].

Piezoelectric-based energy harvesting devices typically take the form of cantilevers, which deform in response to vibrations in large structures, machinery, or even the human body to generate power. However, such devices are not so efficient in real environments where vibrations span a wide frequency range and are more often then not at the lower end of the spectrum. It would be particularly useful to be able to capture these low frequency vibrations, such as those arising from human movement and water waves.

Fuhong Dai and colleagues believe that their design of piezoelectric energy harvester can do just this. Instead of a cantilever, the new device takes the form of a stack of aluminum and carbon fiber layers, which produces large deformations in response to relatively small vibrations. Because of the thermal expansion mismatch between the aluminum and carbon fiber layers, the bi-stable hybrid symmetric laminate (BHSL) ‘snaps’ back and forth between two stable curved configurations. PZT bi-stable piezoelectric energy harvesters (BPEHs) mounted on each side of the structure experience large deformations as the BHSL snaps between its two stable configurations, generating a large power output when exposed to external low-frequency vibrations.

“The BPEHs have better power performance compared with cantilever-type piezoelectric harvesters with same geometric parameters,” says Dai.

In fact, the new device can generate six times more power than a conventional cantilever-type energy harvester when shaken by hand at a frequency of 5 Hz. The power output of the BPEH reaches 37 mW at 5 Hz and could achieve 0.1 W at 10 Hz, believes Dai. Arrays of devices could even generate up to 1 W of power, the researchers calculate. The bi-stable design also means that the device can work in non-resonant modes, widening the frequency range over which it can operate.

“There are a wide variety of sources of low-frequency mechanical energy, including fluid flow such as wind and water movements or direct human action like walking, which are difficult to harvest by traditional cantilever-type energy harvesters,” says Dai.

The new approach could potentially get around these limitations, creating portable, small-sized energy harvesters for renewable resources, such as wind and hydroenergy, and human activity.