11 January 2008

Harvesting Waste Heat: Nanowire Thermoelectrics

Silicon nanowires offer us yet another way to turn waste heat from heat engines into electric power. Silicon is less expensive than previously used bismuth and tellurium.
The thermoelectric effect occurs when one end of a material, such as a wire, is heated. Electrons travel to the colder end, producing an electrical current. However, to harness this energy effectively it is critical that the temperature difference is maintained. This requires a material with an unusual combination of properties - excellent electrical conductivity but poor heat conductivity. As silicon conducts heat well, no-one was expecting it to be a good candidate.

...James Heath of the California Institute of Technology led one of the teams. 'It was a surprise,' he told Chemistry World. 'We found that when you make extremely small silicon nanowires there are some fundamental differences in how these materials behave and transfer heat.'...Heath believes that with further research the efficiency of these materials could be boosted even further. The technology could help to cool and power microchips in the future, he added.

...Francis DiSalvo has worked extensively on thermoelectric materials at the Cornell Fuel Cell Institute, Cornell University, New York. 'This research is exciting but there is still some way to go,' he told Chemistry World...One important role for thermoelectric materials would be capturing heat that is lost during energy production by power stations or car engines, DiSalvo said. 'If these materials can be made cheaply enough, capturing even a fraction of this wasted heat would make them worthwhile,' he added.

One reason nanowires are able to perform better than bulk materials is that thermoelectric efficiency is dependent upon "ZT", the thermoelectric figure of merit. A ZT of 1 or greater provides a higher thermoelectric conversion efficiency. T is temperature, but Z = S**2σ/κ, where S is the Seebeck coefficient, sigma is the electrical conductivity, and kappa is the thermal conductivity. It was recently discovered that in nanowires, the ratio of electrical and thermal conductivities can be better controlled in the higher, more favourable direction.
“Bulk silicon is a poor thermoelectric material at room temperature, but by substantially reducing the thermal conductivity of our silicon nanowires without significantly reducing electrical conductivity, we have obtained ZT values of 0.60 at room temperatures in wires that were approximately 50 nanometers in diameter,” said Yang. “By reducing the diameter of the wires in combination with optimized doping and roughness control, we should be able to obtain ZT values of 1.0 or higher at room temperature.”

...When scaled up, thermoelectric modules could eventually be used in co-generating power with gas or steam turbines.

By capturing an ever increasing percentage of waste heat, society is able to "turn back entropy" somewhat.

Now we simply need to develop ways of "turning back the entropy" that has taken hold in educational systems, political systems, and nihilistic religions and philosophies.

H/T Brian Wang

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