September 20, 2012 | 5 Comments
Northwestern University scientists collaborating with scientists and mechanical engineers at Northwestern and Michigan State University have developed a thermoelectric material that is the best in the world at converting waste heat to electricity resulting in the world record ZT of 2.2.
Using a very environmentally stable material, the common semiconductor called lead telluride; the material is expected to convert 15 to 20 percent of waste heat to useful electricity. These numbers suggest thermoelectrics could see commercial applications installed by industry.
Currently huge amounts of heat are just lost to the atmosphere and out to space. Thermal energy rises from vehicle exhaust pipes, furnaces, water heaters, power plant smokestacks, industrial facilities, and nearly all manner of energy uses. The best waste heat temperatures range from 400 to 600 degrees Celsius (750 to 1,100 degrees Fahrenheit), the sweet spot for thermoelectrics use.
Mercouri G. Kanatzidis research team leader and a senior author of the paper said, “Our system is the top-performing thermoelectric system at any temperature. The material can convert heat to electricity at the highest possible efficiency. At this level, there are realistic prospects for recovering high-temperature waste heat and turning it into useful energy.”
Vinayak P. Dravid, one of Kanatzidis’ close collaborators is the Abraham Harris Professor of Materials Science and Engineering at the McCormick School of Engineering and Applied Science and also a senior author of the paper adds, “People often ask, what is the energy solution. But there is no unique solution – it’s going to be a distributed solution. Thermoelectrics is not the answer to all our energy problems, but it is an important part of the equation.”
The efficiency of waste heat conversion in thermoelectrics is governed by its figure of merit, or ZT. This number represents a ratio of electrical conductivity and thermoelectric power in the numerator (which need to be high) and thermal conductivity in the denominator (which needs to be low).
Dravid explains, “Now, having a material with a ZT greater than two, we are allowed to really think big, to think outside the box. This is an intellectual breakthrough.”
Kanatzidis fills the comment out and looks ahead with; “Improving the ZT never stops – the higher the ZT, the better. We would like to design even better materials and reach 2.5 or 3. We continue to have new ideas and are working to better understand the material we have.”
There is a catch, as Dravid explains, “It is hard to increase one without compromising the other. These contradictory requirements stalled the progress towards a higher ZT for many years, where it was stagnant at a nominal value of 1.”
The team has been at this a while. In early 2011 they published about a thermoelectric material with a ZT of 1.7 at 800 degrees Kelvin. This was the first example of using nanostructures (nanocrystals of rock-salt structured strontium telluride) in lead telluride to reduce electron scattering and increase the energy conversion efficiency of the material.
That’s very old news now – the performance of the new material reported in Nature is nearly 30% more efficient than its predecessor.
The team achieved this by scattering a wider spectrum of phonons, across all wavelengths, which is important in reducing thermal conductivity. A phonon is a quantum of vibrational energy, and each has a different wavelength. When heat flows through a material, a spectrum of phonons needs to be scattered at different wavelengths (short, intermediate and long).
The breakthrough lies in showing that all length scales can be optimized for maximum phonon scattering with minor change in electrical conductivity.
Kanatzidis explains, “Every time a phonon is scattered the thermal conductivity gets lower, which is what we want for increased efficiency. We combined three techniques to scatter short, medium and long wavelengths all together in one material, and they all work simultaneously. We are the first to scatter all three at once and at the widest spectrum known. We call this a panoscopic approach that goes beyond nanostructuring.”
“It’s a very elegant design,” Dravid said.
The team brought the improved long-wavelength scattering of phonons by controlling and tailoring the mesoscale architecture of the nanostructured thermoelectric materials to a new level.
But this story isn’t anywhere near over yet. The team says the successful approach of integrated all-length-scale scattering of phonons is applicable to all bulk thermoelectric materials.