Acoustic Energy Harvesting

At the recent Audio Engineering Society Convention, one of the most interesting talks was in the E-Briefs sessions. These are usually short presentations, dealing with late-breaking research results, work in progress, or engineering reports. The work, by Charalampos Papadokos presented an e-brief titled ‘Power Out of Thin Air: Harvesting of Acoustic Energy’.

Ambient energy sources are those sources all around us, like solar and kinetic energy. Energy harvesting is the capture and storage of ambient energy. It’s not a new concept at all, and dates back to the windmill and the waterwheel. Ambient power has been collected from electromagnetic radiation since the invention of crystal radios by Sir Jagadish Chandra Bose, a true renaissance man who made important contributions to many fields. But nowadays, people are looking for energy harvesting from many more possible sources, often for powering small devices, like wearable electronics and wireless sensor networks. The big advantages, of course, is that energy harvesters do not consume resources like oil or coal, and energy harvesting might enable some devices to operate almost indefinitely.

But two of the main challenges is that many ambient energy sources are very low power, and the harvesting may be difficult.

Typical power densities from energy harvesting can vary over orders of magnitude. Here’s the energy densities for various ambient sources, taken from the Open Access book chapter ‘Electrostatic Conversion for Vibration Energy Harvesting‘ by S. Boisseau, G. Despesse and B. Ahmed Seddik ‘.

EnergyHarvesting

You can see that vibration, which includes acoustic vibrations, has about 1/100th the energy density of solar power, or even less. The numbers are arguable, but at first glance it looks like it will be exceedingly difficult to get any significant energy from acoustic sources unless one can harvest over a very large area.

That’s where this e-brief paper comes in. Papadokos and his co-author, John Mourjopoulos, have a patented approach to harvesting the acoustic energy inside a loudspeaker enclosure. Others had considered harvesting the sound energy from loudspeakers before (see the work of Matsuda, for instance), but mainly just as a way of testing their harvesting approach, and not really exploiting the properties of loudspeakers. Papadokos and Mourjopoulos had the insight to realise that many loudspeakers are enclosed and the enclosure has abundant acoustic energy that might be harvested without interfering with the external design and without interfering with the sound presented to the listener. In earlier work, Papadokos and Mourjopoulos found that sound pressure within the enclosure often exceeds 130 dBs within a loudspeaker enclosure. Here, they simulated the effect of a piezoelectric plate in the enclosure, to convert the acoustic energy to electrical energy. Results showed that it might be possible to generate 2.6 volts under regular operating conditions, thus proving the concept of harvesting acoustic energy from loudspeaker enclosures, at least in simulation.

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