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October 11, 2008

New Improved Thermoelectric Materials and ImprovingThermoelectric science

1. Large Enhancements in the Thermoelectric Power Factor of Bulk PbTe at High Temperature by Synergistic Nanostructuring


Power was increased by 71% and the figure of merit increased to 1.5

Northwestern University researchers (along with colleagues from University of Michigan and the Jet Propulsion Lab) discovered that adding two metals, antimony and lead, to the well-known semiconductor lead-telluride, produces a thermoelectric material that is more efficient at high temperatures than existing materials. The results are published online in the journal Angewandte Chemie. [H/T Nanowerk

Thermoelectric materials are only 5 to 6 percent efficient today, but the new generation of materials based on recent discoveries including this one at Northwestern, could produce devices with 11 to 14 percent efficiency, says Kanatzidis. The long-term goal is to reach 20 percent.

"The thermal conductivity was not any lower than our earlier results, but we discovered a net gain in electrical conductivity at high temperatures that we didn't expect," said Kanatzidis. "This means we had a net gain in power [71%] coming out of the material that we didn't have before. This was very surprising."

Interestingly, the researchers also discovered that adding lead or antimony alone to the lead-telluride did not produce an improvement. Lead and antimony both had to be present in the lead-telluride to produce the electrical conductivity gain. The electrons scatter less and move faster with the two inclusions than with just one or none.

"This phenomenon will stimulate new scientific inquiries and generate new ideas on how to design even more efficient thermoelectric materials in the future," said Kanatzidis.







FURTHER READING

2. RTI has been funded for a thermoelectric project with targets in the 20-30% range.

3. The University of Copenhagen is making progress with understanding thermoelectric materials in the group of clathrates, which create crystals full of 'nano-cages'.

Why Thermoelectrics Are Huge for Energy Efficiency
Industrial waste heat is 7 quads in the USA. There is more waste heat from power plants and from cars. Applying thermoelectrics to our current power plants would be like adding 10-30 nuclear power plants and 150-375 coal plants and 500-1500 natural gas plants that would not use any more fuel because it would be from more efficient use of existing power plants.





Heat flows in a car and using thermoelectrics to tap the waste heat. The standard combustion engine is about 30% efficient, but regular diesel engines are about 38% efficient. New diesel engine and free piston engines can reach 50% efficiency or more. The energy for cooling can also be reduced using thermoelectrics.


The 30% energy efficiency is triple the efficiency of todays common thermoelectrics and double most advanced systems and would get to range of using solid state thermoelectrics to replace refrigerators [thermoelectrics can help cool as well as convert heat to electricity] and many small car sized engines. Typical conversion systems become less efficient as they are scaled down to small size. This means there is a crossover point: below some power level thermoelectric technology will tend to be more efficient. Increasing ZT will move the crossover point to higher power levels, increasing the range of applications where thermoelectrics compete. Thus the ZT of 3 to compete with current best car size and refrigerator mechanical systems.

Past coverage of thermoelectrics and refrigerators.



Thermoelectric Figure of Merit
The primary criterion for thermoelectric device viability is the figure of merit given by:



which depends on the Seebeck coefficient, S, thermal conductivity, λ, and electrical conductivity, σ.

High temperature thermoelectrics and ZT, figure of merit

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