Monday, September 24, 2012

A tip from Rover!

The latest Rover on Mars may have some good news for energy hunters back home!
The Rover depends on a sandwich of semiconducting material that can turn heat into electricity. In this present case, the steady radioactive decay of plutonium 238 warms such thermoelectric material and turns roughly 4 percent of that heat into a steady flow of electrons. A similar radioisotope thermoelectric generator (RTG) on the moon's Sea of Tranquility is still working after decades, as are the RTGs in the two Voyager spacecraft launched 35 years ago.
While 4 percent of heat may seem too small for earthly needs, researchers have discovered a way to at least double the efficiency of such power generators. The most common core of new and old thermoelectric is a compound called lead telluride. When exposed to heat on only one side it induces an electric current as long as the temperature differential is maintained. The challenge has been to keep heat from transferring across the material without also interfering with its ability to conduct electricity.
By engineering the material from the atomic to the individual grain scale, the thermal conductivity of lead telluride can be impeded without affecting its electrical conductivity. The result is a material that can convert at least 8 percent of the heat into electricity—and could theoretically convert as much as 20 percent.
The researchers first melted the lead telluride and then froze it, creating nanoscale crystalline structures out of the atoms. These precisely oriented nanostructures scatter the medium wavelength vibrations, or phonons, that carry heat while allowing electrons to pass unobstructed. But longer wavelength phonons continue to pass through as well, because their wavelengths are longer than the size of the nanostructures. It was this hurdle that the engineering overcame.
Such thermoelectric devices might become practical in harvesting some of the exhaust heat from vehicles—such as marine tankers or trucks—and turning it into electricity. BMW and Ford are already testing similar thermoelectric material in cars. Or the devices could be used in high-heat metallurgical or glassmaking industries.  Scientists at the Massachusetts Institute of Technology have even used such thermoelectric materials to build a device to turn the sun's heat more directly into electricity, rather than employing the vast arrays of mirrors of a conventional solar-thermal power plant.
Now the problem is that lead and tellurium are toxic, but surely material engineering will find new nontoxic alternatives?

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