Scanning electron microscope images of the novel thermoelectric material hot-pressed at a) 1123K, b)1173K, c) 1273K and d) 1373K. Images: University of Houston.
Scanning electron microscope images of the novel thermoelectric material hot-pressed at a) 1123K, b)1173K, c) 1273K and d) 1373K. Images: University of Houston.

With energy conservation expected to play a growing role in managing global demand, materials and methods that make better use of existing sources of energy are becoming increasingly important. Now, in a paper in the Proceedings of the National Academy of Sciences, researchers report a step forward in converting waste heat – from industrial smokestacks, power generating plants or even automobile tailpipes – into electricity.

Taking a thermoelectric material composed of niobium, titanium, iron and antimony, the researchers succeeded in raising its power output density dramatically by using a very hot pressing temperature – up to 1373K, or about 2000°F – to create the material.

"The majority of industrial energy input is lost as waste heat," the researchers wrote in the paper. "Converting some of the waste heat into useful electrical power will lead to the reduction of fossil fuel consumption and CO2 emission."

Thermoelectric materials produce electricity by exploiting the flow of heat from warmer areas to cooler areas; their efficiency is calculated as the measure of how well the material converts heat into power. For example, a material that takes in 100 watts of heat and produces 10 watts of electricity has an efficiency rate of 10%.

That's the traditional way of considering thermoelectric materials, said Zhifeng Ren, professor of physics at the University of Houston (UH) and lead author of the paper. But having a relatively high conversion efficiency doesn't guarantee a high power output, which measures the amount of power produced by the material rather than the rate of the conversion.

Because waste heat is an abundant – and free – source of fuel, the conversion rate is less important than the total amount of power that can be produced, said Ren, who is also a principal investigator at the Texas Center for Superconductivity at UH. "In the past, that has not been emphasized."

The researchers, who in addition to UH came from the Massachusetts Institute of Technology, Morgan State University and Boston College, tweaked a compound made up of niobium, iron and antimony, replacing 4–5% of the niobium with titanium. Processing this new compound at a variety of high temperatures suggested that a very high temperature – 1373K – resulted in a material with an unusually high power factor.

"For most thermoelectric materials, a power factor of 40 is good," Ren said. "Many have a power factor of 20 or 30." The new material has a power factor of 106 at room temperature, and the researchers were able to demonstrate an output power density of 22 watts per cm2, far higher than the 5–6 watts typically produced.

"This aspect of thermoelectrics needs to be emphasized," he said. "You can't just look at the efficiency. You have to look also at the power factor and power output."

This story is adapted from material from the University of Houston, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.