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Thermoelectric materials convert temperature gradients into voltages and vice versa. If one end of such a material is hot and the other is cold, a voltage is generated, which can then be used to create electrical power.
For a material to have good thermoelectric properties, however, it must be a good electrical conductor and a poor thermal conductor. Because bulk silicon is good at conducting both electricity and heat, scientists had ruled it out as a possible thermoelectric material. Two teams have now independently discovered that by nanostructuring silicon, they can reduce its thermal conductivity, making the material promising for thermoelectric applications (Nature 2008, 451, 163 and 168).
"It confirms a growing sense in the science community that proper nanostructuring of materials will yield very significant enhancements in thermoelectric performance," he says.
Next Step: Although the cells' electron transport was better, their overall light conversion efficiency was low compared to that of some nanoparticle-based solar cells (which have achieved efficiencies of up to 10 percent). Zinc oxide harvests electrons from the dye less efficiently than does titanium dioxide -- a material more commonly used in nano solar cells. The researchers are now making their nanowires out of titanium dioxide, a more challenging manufacturing process. The nanowires also have a smaller surface area than a network of nanoparticles, so they carry less light-absorbing dye. The researchers are consequently shrinking their nanowires to 10 nanometers in diameter so that they can fit more nanowires onto their arrays and increase the total surface area. Yang predicts that with thinner and more numerous titanium wires, his team will be able to achieve a conversion efficiency of 10 percent or more, which could make these nano solar cells a viable source of energy.
Dr. M. M. Zhang
Materials Research Department, Toyota Technical Center, Toyota Motor Engineering & Manufacturing North America (TEMA) Inc.
2350 Green Road, Ann Arbor, MI 48105 (USA)
Researchers at McMaster University, in Ontario, say that they have grown light-absorbing nanowires made of high-performance photovoltaic materials on thin but highly durable carbon-nanotube fabric. They've also harvested similar nanowires from reusable substrates and embedded the tiny particles in flexible polyester film. Both approaches, they argue, could lead to solar cells that are both flexible and cheaper than today's photovoltaics.
Solar proponents love to boast that just a few hundred square kilometers' worth of photovoltaic solar panels installed in Southwestern deserts could power the United States. Their schemes come with a caveat, of course: without backup power plants or expensive investments in giant batteries, flywheels, or other energy-storage systems, this solar-power supply would fluctuate wildly with each passing cloud (not to mention with the sun's daily rise and fall and seasonal ebbs and flows). Solar-power startup Ausra, based in Palo Alto, thinks it has the solution: solar-thermal-power plants that turn sunlight into steam and efficiently store heat for cloudy days.
"Fossil-fuel proponents often say that solar can't do the job, that solar can't run at night, solar can't run the economy," says David Mills, Ausra's founder and chairman. "That's true if you don't have storage." He says that solar-thermal plants are the solution because storing heat is much easier than storing electricity. Mills estimates that, thanks to that advantage, solar-thermal plants capable of storing 16 hours' worth of heat could provide more than 90 percent of current U.S. power demand at prices competitive with coal and natural gas. "There's almost no limit to how much you can put into the grid," he says.
Silicon, in the form of photovoltaic cells, is good at generating electricity from sunlight. New research shows that it could also make a good thermoelectric: a material that converts heat into electricity and vice versa. Since silicon is more abundant than the leading thermoelectric materials and has a vast manufacturing infrastructure behind it, it could eventually yield cheap devices for generating power from engines' waste heat or from solar heat.
In this week's Nature, University of California, Berkeley, chemistry professor Peidong Yang and his colleagues report having fabricated silicon nanowires that generate electricity when a temperature differential is applied across them. Until now, silicon has been considered a bad thermoelectric material. But according to Yang, "the performance of the nanowires is already comparable to the best existing thermoelectric material."
Cool customer: This image, produced by a scanning electron microscope, shows a rough silicon nanowire bridging two heating pads--one serving as a heat source and the other as a sensor. Researchers have found that 50-nanometer-wide silicon nanowires have drastically lower heat conductivity than bulk silicon but retain their electrical conductivity. Thus the nanowires show potential as thermoelectric materials--ones that convert heat into electricity and vice versa.