Waste heat could be new source of clean energy

About two-thirds of all the energy we use—in computers, in cars, in making products such as steel—is lost as heat. That energy does not do anything useful, and yet we are paying for it all the same. Assistant Professor Sanjiv Sinha, with the help of collaborators across campus, is making breakthroughs in finding efficient and cost-effective ways to reclaim some of that lost energy.

For Sinha and his group, the answer lies in thermoelectric materials: those in which a temperature difference creates an electric potential (or vice versa). Thermoelectric energy conversion has been studied since the early 1800s, but it was only effective at very high temperatures and the materials were too expensive. If it could be done with a cheap, abundant material that could be altered to be more effective at thermoelectric energy conversion, the implications for energy production would be powerful.

Sinha specializes in the study of electro-thermal transport: how electrical current and heat are carried through a material. These characteristics are very important in a thermoelectric material, especially when the goal is to make the electric conductivity high and the thermal conductivity low, but there are no cheap and abundant materials with these characteristics. They decided to find a material that they could change the characteristics of in order for it to meet the necessary standards.

"Just before our work began," Sinha said, "a discovery was made that if silicon was structured as a nanowire, the thermoelectric energy conversion efficiency gets a significant boost. So our project here involved using silicon nanowires in a scalable fashion, trying to engineer it to be able to boost energy conversion efficiency, and to understand the physics of transport inside this structure."

This effort took interdisciplinary cooperation between faculty members and departmental affiliates from three College of Engineering departments: Professors John Rogers and David Cahill, from Materials Science and Engineering; Associate Professor Xiuling Li, from Electrical Engineering; and Sinha and Department Head Placid Ferreira from MechSE. Nicholas Fang, formerly an assistant professor in MechSE and now an associate professor at Massachusetts Institute of Technology, also collaborated on the project, in addition to approximately 20 students across the three departments.

Waste heat doesn’t just come from polluting sources of energy—thermoelectric energy conversion can be used with all types of energy production, including alternative sources such as solar power and biofuels.

"It’s going to be there for the vast majority of energy solutions that are being talked about," Sinha said. "It’s nature’s garbage. No matter what energy portfolio you create, there will be significant advantage to be able to get waste heat and convert it at low cost."