Body Heat Could Charge Your Cellphone

Berkeley (CA) - Remarkable research findings on energy conversion have been published in the most recent edition of Nature: Scientists believe to have discovered a much more efficient way to use silicon to convert heat into electricity - for use in a variety of products ranging from cars to portable electronics.

Imagine a time in which you would not have to plug in your cellphone or iPod over night to recharge the battery. Instead you would power and charge a device simply by wearing it close to your body. The concept of converting waste heat into electricity isn't exactly new, but it never really materialized due to efficiency hurdles. Now, scientists from the U.S. Department of Energy's (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley think they may have found a key increase the conversion efficiency by a factor of 100.

In contrast to previous silicon nanowire-based converters, researchers have used what they describe as "rough" silicon nanowires. The material is created in a process of "electroless etching" in which arrays of silicon nanowires are synthesized in an aqueous solution on the surfaces of wafers. According to the paper published, the "technique involves the galvanic displacement of silicon through the reduction of silver ions on a wafer's surface". In contrast to common silicon process methods, this technique results in vertically aligned silicon nanowires that apparently feature exceptionally rough surfaces. And it is that roughness that is believed to be critical to the surprisingly high thermoelectric efficiency of the silicon nanowires.

"The rough surfaces are definitely playing a role in reducing the thermal conductivity of the silicon nanowires by a hundredfold, but at this time we don't fully understand the physics," said Arun Majumdar, a mechanical engineer and materials scientist with joint appointments at Berkeley Lab and UC Berkeley, who was one of the principal investigators behind this research. "While we cannot say exactly why it works, we can say that the technique does work," he said.

While the research is far from being completed, the scientists already believe that the results could allow us to use energy much more efficiently. Potential applications of this technology include DOE's hydrogen fuel cell-powered "Freedom CAR," and personal power-jackets that could use heat from the human body to recharge cell-phones and other electronic devices.

"You can siphon electrical power from just about any situation in which heat is being given off, heat that is currently being wasted," said Majumdar. "For example, if it is cold outside and you are wearing a jacket made of material embedded with thermoelectric modules, you could recharge mobile electronic devices off the heat of your body. In fact, thermoelectric generators have already been used to convert body heat to power wrist watches."

On a slightly larger scale, conversion modules could also be used to convert the heat from automotive exhaust into supplemental power for a Freedom CAR-type vehicle, or provide the electricity a conventional vehicle needs to run its radio, air conditioner and power windows. Above that, thermoelectric modules could eventually be used in co-generating power with gas or steam turbines, the researchers believe.

The meaning of the discovery will depend on whether these rough nanowires will be efficient enough to make commercial sense. However, if we consider that nearly all of the world's electrical power, approximately 10 trillion Watts, is generated by heat engines, giant gas or steam-powered turbines that convert heat to mechanical energy, we know that much of this heat worth another 15 trillion Watts of electricity is not converted but released into the environment. It doesn't take much to see that there is an enormous potential of increased efficiency in this discovery.

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