A research team at the University of Houston’s physics department and the Texas Center for Superconductivity (TcSUHh) are working on an innovation that they say could boost motor vehicle mileage by 5 percent and power plant and industrial processing performance as much as 10 percent.
TcSUH is a large multidisciplinary university-based superconductivity and advanced materials research center, with over 200 faculty, postdoctoral fellows, graduate and undergraduate students, housed in the Houston Science Center and several other buildings on the University of Houston campus.
“Telluride has been studied for years,” says Dr. Zhifeng Ren, M.D. Anderson Chair professor of physics at UH and lead author of a paper describing the work, published in The Proceedings of the National Academy of Sciences.
Qian Zhang, a research associate in Ren’s group who designed the experiment, says she ultimately decided to add another element, known as a dopant, to alter the electrical properties of the tin telluride. In this case, she added indium to boost its conducting properties.
The PNAS paper, entitled “High thermoelectric performance by resonant dopant indium in nanostructured SnTe,” (Proceedings of the National Academy of Sciences, 2013; 110 (33): 13261 DOI: 10.1073/pnas.1305735110), is co-authored by Dr. Ren with Qian Zhang, Bolin Liao, Yucheng Lan, Kevin Lukas, Weishu Liu, Keivan Esfarjani, Cyril Opeil, David Broido, and Gang Chen, variously of the IH Department of Physics and Texas Center for Superconductivity, the Department of Mechanical Engineering at the Massachusetts Institute of Technology at Cambridge, Massachusetts and the Department of Physics, Boston College, Chestnut Hill, Massachusetts.
In the paper’s abstract, the researchers note that from an environmental perspective, lead-free SnTe would be preferable for solid-state waste heat recovery if its thermoelectric figure-of-merit could be brought close to that of the lead-containing chalcogenides. Consequently, in this work, they studied the thermoelectric properties of nanostructured SnTe with different dopants, and found indium-doped SnTe showed extraordinarily large Seebeck coefficients (a measure of the magnitude of an induced thermoelectric voltage in response to a temperature difference across that material) that can not be explained properly by the conventional two-valence band model. The co-authors attribute this enhancement of Seebeck coefficients to resonant levels created by the indium impurities inside the valence band, supported by the first-principles simulations. This, together with the lower thermal conductivity resulting from the decreased grain size by ball milling and hot pressing, improved both the peak and average nondimensional figure-of-merit (ZT) significantly.
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In a UH release, Dr. Ren explains that earlier work had faltered because lead-containing telluride, despite its strong thermoelectric properties, can’t be used commercially because of the health risks associated with lead, which has sparked the rush for a similar, but safer compound. Dr. Ren observes that without lead, there is a much better chance for the process to be commercialized.
The UH research shows the potential for building a device that can capture waste heat – from vehicle tailpipes, industrial smokestacks, power plants and other sources – and convert it to electricity to boost productivity.
Dr. Ren and his research team arrived at UH in January from Boston College, and their work continues his long-standing research into nanostructured thermoelectrics and thermoelectric energy conversion. The research was conducted with colleagues from the Massachusetts Institute of Technology and Boston College.
In one example of a potential practical application of the research findings, Dr. Ren notes that a device could be developed to capture heat from a car’s tailpipe and convert it to power the car’s electronics, improving the car’s mileage by about 5 percent, Ren says, observing that “Even 1 percent, every day, would be huge,” considering how much crude oil is consumed worldwide.
The United States and China, the world’s most energy-intensive nations, consumed 18.6 million barrels and 10.3 million barrels of crude oil daily respectively in 2012, according to the U.S. Energy Information Administration, and energy consumption in other countries is growing. ExxonMobil, in its annual energy forecast for the next 30 years, predicts that despite measures being implemented to slow or more optimistically — reverse — the process of global warming, global energy demand will increase 35 percent by 2040, a projected increase in demand that makes even a small gain in efficiency valuable, Dr. Ren says, predicting that the waste heat capture process he and his team have developed could be made more efficient in the future.
Moreover, capturing car exhaust and converting it to electricity is only one example of how the process can be used. Dr. Ren suggests that the technique could also be used in power plants, potentially boosting the conversion rate of coal-fired power plants from 40 percent to as much as 48 percent– with similar gains possible in other industrial plants that use large amounts of energy, in some instances yielding efficiency gains as great as 10 percent.