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MIT researchers simplify energy harvesting device

9/19/2011

MIT researchers have designed a MEMS energy-harvesting device for wireless sensors that can generate two orders of magnitude more power compared to current designs.

Instead of taking a cantilever-based approach, they engineered a microchip with a small bridge-like structure that's anchored to the chip at both ends. The Massachusetts Institute of Technology researchers deposited a single layer of piezoelectric material (PZT) to the bridge, placing a small weight in the middle of it.

When undergoing a series of vibration tests the device was able to respond not just at one specific frequency, but also at a wide range of other low frequencies.

"There are wireless sensors widely available, but there is no supportive power package," says Sang-Gook Kim, a professor of mechanical engineering at MIT and co-author of the paper in a statement. "I think our vibrational-energy harvesters are a solution for that."

A common energy-harvesting design consists of a small microchip with layers of PZT glued to the top of a tiny cantilever beam. As the chip is exposed to vibrations, the beam moves up and down like a wobbly diving board, bending and stressing the PZT layers. The stressed material builds up an electric charge, which can be picked up by arrays of tiny electrodes.

The cantilever-based approach has limitations and simply increasing the number of cantilever beams and PZT layers occupying a chip is wasteful, and expensive, say the researchers.

"In order to deploy millions of sensors, if the energy harvesting device is $10, it may be too costly," says Kim. A single-layer MEMS device can be fabricated for less than $1, according to Kim.

The researchers came up with a design that increases the device's frequency range while maximizing the power density, or energy generated per square centimeter of the chip.

When undergoing a series of vibration tests the device was able to respond not just at one specific frequency, but across a wide range of other low frequencies.

The researchers calculated that the device was able to generate 45 microW of power with just a single layer of PZT - an improvement of two orders of magnitude compared to current designs.

The team published its results in the Aug. 23 online edition of Applied Physics Letters.

Nicolas Mokhoff

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