[Column] Dilemma of Using Waste Heat as Energy

Dec 25, 2008
Tetsuo Nozawa, Nikkei Electronics

Please watch the video of a trick - or rather, a demonstration - first, in which heat is converted into sound. In this experiment, a wire sheet with a mesh gauge of several millimeters was set in a simple plastic tube and heated. With this simple method, they produced the sound.

This is called "thermoacoustic phenomenon," a nonlinear phenomenon in which heated air autonomously transforms into sound when passing through small mesh holes in a wire sheet. The phenomenon itself has been known for long in Japan. For example, at Kibitsu Shrine in Okayama City, Okayama Prefecture, "Narukama Shinji," a ritual to read people's fortunes from this sound, seems to have been practiced from ancient times.

However, the phenomenon has usually been seen as a nuisance that causes unnecessary acoustic oscillation in thermal products and has hardly been utilized.

Only recently, as interest in more efficient and new energies is growing, the thermoacoustic phenomenon began to draw attention. For example, the research group led by professor Yoshiaki Watanabe of Doshisha University, which showed the demonstration above, is currently developing a cooling technology based on thermoacoustic phenomenon. Specifically, the technology converts heat into sound, transmits the sound through a tube and converts it into heat again.

The sound in the tube corresponds to 100W, or 160dB in terms of sound volume, Watanabe said.

"Right under an aircraft engine, noise is about 120dB," he said. "So, 160dB is equivalent to the sound coming from 100 aircraft engines."

Fortunately, the sound is safely sealed in the tube and does not rupture our eardrums.

Still, despite its high cooling capability, this technology has many challenges that need to be overcome in terms of energy efficiency. Especially, when a device becomes smaller, the ratio of the tube's surface area to its volume increases; as a result, the energy efficiency decreases due to the larger loss during sound transmission, Watanabe said.

On the other hand, some researchers in the US came up with an idea of converting the energy of sound into electricity.

They are addressing a research to generate electricity by using a microphone in a tube through which an enormous volume of sound is passing although only little can be heard from outside. And some claim to have achieved an efficiency of more than 40% and generated hundreds of watts.

LED lamp powered by candle flame

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