Using Nanowire Silicon Thermoelectrics to Capture Waste Heat
January 21, 2008 3:17 AM
Professor Peidong Yang led the University of California, Berkley team who discovered silicon nanowire's thermoelectric properties.
(Source: University of California, Berkley)
A scanning electron microscope image of a thin silicon nanowire stretching between two heating pads, one as a heat source, the other as a sensor.
(Source: A. Hochbaum)
Silicon heat capture could allow cheap refrigeration and energy saving
Among the many valuable properties of silicon is its ability to
capture solar energy
to create electricity via its photoelectric character. Now scientists are discovering that silicon, when properly prepared, can form a very good thermoelectric as well. This opens the door to a plethora of uses, including refrigeration, solar heat power generation, and power generation from other heat sources, such as computer waste heat or car heat.
has two basic modes of operation. When a thermoelectric is placed over a heat gradient, it generates an electric current. The other mode is the reverse; when a thermoelectric is exposed to an electric current, it creates a heat difference, cooling one side of it, and warming the other side. Thus thermoelectrics are applicable to power generation, refrigeration and heating.
Traditional thermoelectrics, which have been around since the 1960s, rely on either bismuth telluride or lead telluride. These materials are relatively expensive due to scarcity and lack of a large manufacturing infrastructure. They are also bulky and require more material, which further increaese their cost. While thermoelectric coolers have achieved modest commercial usage in seat coolers and picnic coolers, they have yet to realize their full potential.
may change that. Professor Peidong Yang and his colleagues at the University of California, Berkley published in last week's
journal the results of years of research into using silicon as thermoelectrics. Their results show that silicon can be a viable thermoelectric.
The key is in the preparation. The researchers prepared thin nanowires of silicon. When these wires are exposed to a temperature difference, they generate electricity. Standard silicon is a poor thermoelectric, but according to Dr. Yang, "the performance of the nanowires is already comparable to the best existing thermoelectric material."
A good thermoelectric needs to have two key properties -- good electrical conduction, and poor heat conduction. Silicon typically conducts both very well, but by producing 50 nm nanowires, the heat conduction of silicon is reduced to one hundreth of its normal levels, while electrical conduction remains unchanged. The material is comparable to commercial thermoelectrics.
Two possible uses of the technology are to generate electricity from waste heat of car engines. Current thermoelectrics are too expensive and large to make this a practical possibility. Nanowire silicon layers, though could provide a means to recapture some of the energy lost to heat during the conversion to mechanical energy in a car engine. This extra savings could be stored in batteries, to give next generation electric vehicles, such as
the Chevrolet Volt
, even better efficiency.
It could also find a home in solar power cells. By coupling it with traditional photoelectric cells, much higher efficiencies could possibly be reached. Yet another application is to put the materials in computers to provide energy savings, which would be particularly valuable to mobile computing. Further, it could be used in refrigeration applications, as well.
Much work needs to be done before the process is perfected. The physics behind why nanowires of silicon lose their heat conduction is not understood, which stands in the way of refining the efficiency of this class of devices. Further creating a thermoelectric on the macroscopic scale, by creating a network of nanowires, has yet to be accomplished. Still, the discovery of these properties in silicon promise a way to eventually use replace current less ideal thermoelectrics with an abundant material with a large processing infrastructure.
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