Electronic Design

Parallels And Divergences In Energy Harvesting

Earlier this month, I attended the first nanoPower Forum on energy harvesting, hosted by market research firm the Darnell Group. I later received a press release from IMEC, a Dutch research center in nanoelectronics and nanotechnology, about its latest energy-harvesting product.

The IMEC energy harvester (I'm going to call products like this "micro generators" to suggest they're in the same class as wind turbines and solar panels, but at a smaller scale) is an R&D project that produces electricity from mechanical vibrations to power wireless sensors. IMEC says its piezoelectric generator put out 40 µW for an input vibration with a resonance frequency of 1.8 kHz and an amplitude of 180 nm.

For more information about the micro generator, I turned to the paper that IMEC presented at the nanoPower Forum. In it, IMEC described its wide-ranging energy-harvesting research: "The design of piezoelectric devices is similar to the classical design of accelerometers: a bending structure is connected to a vibrating frame," it said. "The beam supports a piezoelectric capacitor and a mass. The vibration of the frame induces a vibration of the mass and a bending of the cantilever. The strained piezoelectric layer generates charges that flow in the external circuit."

Going back to IMEC's press announcement provides more information: "The device consists of a piezoelectric capacitor formed by a platinum electrode, a PZT \[lead zirconate titanate\] layer and a top aluminum electrode. This capacitor is fabricated on a cantilever that supports a mass on its tip. As the harvester is subjected to oscillations, the mass causes the piezoelectric layer to be stretched. By doing so, it induces an electrical power when an electrical load is connected to the device." Several things struck me when I read the press release.

For one, the pre-stressed cantilever design is similar to Face International's "Lightning" piezo switch (see "Zombies And Energy Harvesting," ED Online 15788). Face was demonstrating switches at the forum, but the company's reps also talked to me about cantilevered piezo microgenerators. It occurred to me that Face licenses its piezo Lightning technology from NASA, leading me to wonder whether IMEC will have any patent problems. Alternatively, how much of this technology is covered by prior art?

The next question had to do with the need for bandwidth in vibrational microgenerators. Whether piezo or magnet-in-a-coil, they need to be mechanically tuned to the resonant frequency of the vibration. But if the thing that's vibrating doesn't cooperate and vibrate at that exact frequency, output falls off. In practical terms, even the best existing microgenerators have a very narrow resonance band.

Another factor is the vibrational frequency itself. Piezo microgenerators like Face International's and IMEC's respond to higher frequencies than electromechanical microgenerators such as those made by Perpetuum in the U.K., which resonate around 100 Hz. (There's more on Perpetuum in "Zombies and Energy Harvesting.")

In terms of bandwidth, Perpetuum is ahead of the electromechanical pack with electromagnetic microgenerators that respond across a band of 2 whole Hz—from 118 to 120 Hz. That top end of the bandwidth is selected for ac induction motors that run on North American 60-Hz power lines, and Perpetuum has a different model for Europe with an effective range from 98 to 100 Hz. The resonance bandwidth is needed because induction motors "slip" under load, so they're not exactly synchronous with the power-line frequency, except at no-load.

Increasing the Q of the mechanically resonant part of the microgenerator, as IMEC discusses in its release, would seem to further narrow the frequency range across which the microgenerator would respond—a classic tradeoff issue. IMEC doesn't mention bandwidth in its paper or in the press release.

For increased bandwidth, a Boeing paper at the conference presented one way of making a microgenerator that responds to a fairly wide range of vibration frequencies. (The 787 may incorporate a range of vibrational and thermocouple energy harvesting schemes.) Presented in a sketch in a foil that wasn't included in final conference proceedings, Boeing's idea would employ a circular array of cantilevered masses of varying length (hence, resonant frequency), all connected to a common piezo generator to broaden frequency response.

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