[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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LED lamp powered by candle flame

Next, please look at the photo on the right. This is a demonstration in which the heat of a candle flame was converted into electric power to turn on an LED. The photo was shot when I interviewed Nextreme Thermal Solutions Inc of the US.

"Most energy of a candle flame turns into heat," the company said. "So, an LED lamp powered by the heat glows brighter than the candle flame itself."

This technology is based on a well-known principle called "thermoelectric conversion." Many manufactures are paying close attention to it because it requires no microphone speaker or any other moving parts, making it easy to enhance durability and reduce size unlike the aforementioned thermoacoustic technology.

The technology had once been developed in Japan's national project. And it seems that it will be commercialized in several fields in the near future.

These technologies are all an effort to efficiently convert waste heat, which has been a nuisance thus far, into electricity. Potential heat sources include boilers, hot water pipes, automotive emissions and engines, ICs, microprocessors, transmitters in high-speed telecommunication devices, the sun and human bodies (body warmth).

Merely technologies for old cars?

While each of those technologies are very interesting and exciting, there is one small concern: They are useful only when applied to devices with a low energy efficiency. For example, if automotive engines become more efficient and generate less waste heat, the technologies might become unnecessary.

In fact, I heard in one of my interviews that, "Even with gasoline cars, the temperature of waste heat is decreasing as engines have been downsized and become more efficient."

When researching for the story on the waste heat utilization technologies, I found that only the US and European automakers such as General Motors Corp (GM) and BMW AG are enthusiastic about those technologies.

Should GM develop a practicable thermoelectric conversion technology or an electric vehicle? And should we reuse the heat of incandescent bulbs using a waste heat utilization technology or replace them all with fluorescent or LED lamps? The waste heat utilization technologies are not very competitive among the technologies for enhancing energy efficiency.

Nevertheless, such technologies will not disappear, either, as long as body temperature or solar heat exists. I hope these researchers take a long-term perspective on those technologies.