Converting Heat from Mobile Phones Back into Electricity
Silicon nanowires may lead the way to converting waste heat into electricity, according to research reported yesterday in the journal Nature. Two separate teams, one at Caltech and the other at the University of California, Berkeley, reported that they could increase silicon's ability to convert heat into electric current by as much as 100 times.
The difference in temperature between two sides of a chip [red is hot, blue is cold] cause electrons to flow in a roughened silicon nanowire
An array of nanowires [green] convert heat from the temperature difference between two slivers of a microchip. Current in flowing through a heater [red] causes the temperature difference
The application could take surplus heat generated within mobile phones during use, or even from the human body when in standby and convert it to electricity.
Thermoelectric conversion relies on a difference between hot and cold areas in a device. Heat flowing from the hot side to the cold side creates current, which can be captured and used to power a device or stored for subsequent use. Bulk silicon has traditionally been considered a poor material for thermoelectric conversion, because its thermal conductivity is too high; heat travels across it so well that it's difficult to create the necessary temperature differential.
"If you were going to make a high-performance thermoelectric, you would never use silicon, because as a bulk material it's pretty lousy," says James Heath, a chemist who led the research at Caltech. He was surprised by his own results; he expected some increase in efficiency, but not as much as he got.
Thermoelectric conversion efficiency is measured by a number dubbed ZT. Several factors go into that number, and it can be increased both by lowering the thermal conductivity of a material and by increasing its electrical conductivity. Whereas bulk silicon at room temperature has a ZT of 0.01, the Berkeley team increased that to 0.4, and the Caltech team increased it to 0.6. That puts silicon nanowires about on par with bismuth telluride, the compound from which commercial converters are made despite the fact that it is relatively expensive and challenging to work with. Making thermoelectric devices out of silicon, which is abundant, cheap, and easily handled, could help create a new market for the devices.
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The difference in temperature between two sides of a chip [red is hot, blue is cold] cause electrons to flow in a roughened silicon nanowire
An array of nanowires [green] convert heat from the temperature difference between two slivers of a microchip. Current in flowing through a heater [red] causes the temperature difference
The application could take surplus heat generated within mobile phones during use, or even from the human body when in standby and convert it to electricity.
Thermoelectric conversion relies on a difference between hot and cold areas in a device. Heat flowing from the hot side to the cold side creates current, which can be captured and used to power a device or stored for subsequent use. Bulk silicon has traditionally been considered a poor material for thermoelectric conversion, because its thermal conductivity is too high; heat travels across it so well that it's difficult to create the necessary temperature differential.
"If you were going to make a high-performance thermoelectric, you would never use silicon, because as a bulk material it's pretty lousy," says James Heath, a chemist who led the research at Caltech. He was surprised by his own results; he expected some increase in efficiency, but not as much as he got.
Thermoelectric conversion efficiency is measured by a number dubbed ZT. Several factors go into that number, and it can be increased both by lowering the thermal conductivity of a material and by increasing its electrical conductivity. Whereas bulk silicon at room temperature has a ZT of 0.01, the Berkeley team increased that to 0.4, and the Caltech team increased it to 0.6. That puts silicon nanowires about on par with bismuth telluride, the compound from which commercial converters are made despite the fact that it is relatively expensive and challenging to work with. Making thermoelectric devices out of silicon, which is abundant, cheap, and easily handled, could help create a new market for the devices.
source
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