A new energy harvesting device converts low-frequency vibrations into electricity. The device, the size of a U.S. quarter, is shown mounted on a stand.
Photo: Arman Hajati
Applied Physics Letters - Ultra-wide bandwidth piezoelectric energy harvesting
Here, we present an ultra wide-bandwidth energy harvester by exploiting the nonlinear stiffness of a doubly clamped microelectromechanical systems (MEMSs) resonator. The stretching strain in a doubly clamped beam shows a nonlinear stiffness, which provides a passive feedback and results in amplitude-stiffened Duffing mode resonance. This design has been fabricated into a compact MEMS device, which is about the size of a US quarter coin. Based on the open circuit voltage measurement, it is expected to have more than one order of magnitude improvement in both bandwidth (more than 20% of the peak frequency) and power density (up to 2 W/cm3) in comparison to the devices previously reported
“In order to deploy millions of sensors, if the energy harvesting device is $10, it may be too costly,” says Kim, who is a member of MIT’s Microsystems Technology Laboratories. “But if it is a single-layer MEMS device, then we can fabricate [the device for] less than $1.”
Bridging the power divide
Kim and Hajati came up with a design that increases the device’s frequency range, or bandwidth, while maximizing the power density, or energy generated per square centimeter of the chip. Instead of taking a cantilever-based approach, the team went a slightly different route, engineering a microchip with a small bridge-like structure that’s anchored to the chip at both ends. The researchers deposited a single layer of PZT to the bridge, placing a small weight in the middle of it.
The team then put the device through a series of vibration tests, and found it was able to respond not just at one specific frequency, but also at a wide range of other low frequencies. The researchers calculated that the device was able to generate 45 microwatts of power with just a single layer of PZT — an improvement of two orders of magnitude compared to current designs.
“If the ambient vibration is always at a single frequency and does not vary, [current designs] work fine,” says Daniel Inman, professor of aerospace engineering at the University of Michigan. “But as soon as the frequency varies or shifts a little, the power decreases drastically. This design allows the bandwidth to be larger, meaning the problem is, in principle, solved.” Inman adds that going forward, the MIT group will have to aim lower in the frequencies they pick up, since few vibrations in nature occur at the relatively high frequency ranges captured by the device.
Hajati says the team plans to do just that, optimizing the design to respond to lower frequencies and generate more power.
“Our target is at least 100 microwatts, and that’s what all the electronics guys are asking us to get to,” says Hajati, now a MEMS development engineer at FujiFilm Dimatix in Santa Clara, Calif. “For monitoring a pipeline, if you generate 100 microwatts, you can power a network of smart sensors that can talk forever with each other, using this system.”
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