Researchers Achieve New Breakthrough in Converting Waste Heat into Electricity

As energy conservation is expected to play an increasing role in managing global demand, both materials and methods that use existing sources of energy have become more and more important.

Now, scientists have revealed a breakthrough in converting waste heat, produced from power generating plants, industrial smokestacks or automobile tailpipes, into electricity. They reported their findings this week in the Proceedings of the National Academy of Sciences.

The scientists involved in this study successfully used a thermoelectric compound consisting of titanium, iron, antimony and niobium to dramatically increase the material’s power output density at a very hot pressing temperature up to 1373 K or about 2,000°F to create the material.

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

Thermoelectric materials generate electricity using the heat-current flow from a warmer area to a cooler area. The efficiency of these materials is measured based on how well the material changes waste heat, which is often produced by power plants or other industrial processes, into power. For instance, a material that takes in 100 W of heat and generates 10 W of electricity has an efficiency of 10%.

This is the conventional method of considering thermoelectric materials, said Zhifeng Ren, MD Anderson Professor of Physics at the University of Houston and lead author of the paper. However, obtaining a relatively high conversion efficiency may not guarantee a high power output that calculates the amount of power generated by the material rather than the conversion rate.

Ren, who is also a principal investigator at the Texas Center for Superconductivity at UH, said that waste heat is an abundantly and freely available source of fuel and so, the conversion rate is less significant than the total amount of power that can be generated. “In the past, that has not been emphasized.”

In addition to Ren, other investigators involved in the research include Ran He, Jun Mao, Qing Jie, Jing Shuai, Hee Seok Kim, Yuan Liu and Paul C.W. Chu, all of UH; Daniel Kraemer, Lingping Zeng and Gang Chen of the Massachusetts Institute of Technology; Yucheng Lan of Morgan State University, and Chunhua Li and David Broido of Boston College.

The investigators tweaked a compound consisting of iron, antimony and niobium, replacing 4 to 5% of the niobium with titanium. Processing the compound at various high temperatures suggested that an extreme high temperature, 1373 K, produce a material with an extraordinary high power factor.

For most thermoelectric materials, a power factor of 40 is good. Many have a power factor of 20 or 30.

Zhifeng Ren, Professor of Physics, University of Houston

The newly created material has a power factor of 106 at room temperature, and investigators were able to reveal an output power density of 22 W/cm², which is higher than the 5 to 6 W typically produced, Ren said.

“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.”