Too Darn Hot: Thermoelectric Power Generation

by Gordy Slack


Two attacks in the technological battle against the energy crisis are the search for new and renewable energy sources, and the battle against energy waste. New materials under development by Ali Shakouri, Associate Professor of Electrical Engineering at the University of California, Santa Cruz, stand to address both issues at once by converting energy now wasted as heat into a reusable new source of electricity.


About 65 percent of the energy generated by the cylinders of a car engine leaves the car as wasted heat. If a significant portion of that wasted energy could instead be recovered and put to use, says Shakouri, it could substantially improve automobile efficiency.


Research at the Thermionic Energy Conversion Center focuses on metal semiconductor nanocomposites that selectively scatter cold, low-energy electrons, while allowing hot, high-energy ones to flow easily in one direction.

Shakouri and his colleagues in the UC Santa Cruz-based Thermionic Energy Conversion Center are working to rescue some of that heat and convert it directly into electricity. They have recently received a four-year, 6.8-million-dollar grant from the Defense Advanced Research Projects Agency (DARPA) to further their work and develop thermoelectric materials that will allow them do just that. This research started under a prior, Office of Naval Research (ONR) multi-university research initiative in 2003.


The new project involves scientists from six universities (UC Berkeley, UC Santa Barbara, Delaware, Harvard, MIT, Purdue, and North Carolina State) and an independent company (BSST Inc.), who work together to create new materials that could, according to Shakouri, boost the efficiency of an engine by as much as 20 percent.


For over a century, physicists have known that if one side of a material is much hotter than the other, then free charges, such as electrons, on the hot side move faster, and some of those fast-moving electrons will find their way to the cold side, creating a flow of electricity. This conversion of heat into electricity has never been efficient enough to commercialize profitably, except in very specific applications where efficiency is not vital. But by exploiting new materials that can coax and guide electrons effectively from the hot side of the material into a condensed stream on the cold side, thermoelectric energy generation, as the process is called, may soon become a potent resource, says Shakouri.


To be efficient, the materials have to have high electrical conductivity, low thermal conductivity, and a high “Seebeck coefficient,” that is, the hot electrons must flow much more easily than the cold ones flow.

Shakouri and his colleagues are focusing on solid-state thermionics, developing metal semiconductor nanocomposites that selectively scatter cold, low-energy electrons, while allowing hot, high-energy ones to flow easily in one direction. “The materials are like filters for some electrons,” Shakouri says.


The researchers are initially focusing on recovering heat energy from the catalytic converter and the exhaust rather than the engine because they are simpler targets.
Once the heat is converted into electricity, it could be directed to an electric motor, in the case of a hybrid car, or to operating the car’s air-conditioner, headlights, and other electrical functions.


“The field is so multi-disciplinary,” says Shakouri, “that to assemble a team of the leading experts in the nation for each of these areas, we had to open the project to seven institutional partners.” Collaborating scientists come from such fields as thermal transport, electrical transport, physics, electrical engineering, and material science.


In addition to its six participating universities, the project has a corporate partner, BSST, the research arm of Amerigon. BSST is helping Shakouri’s team develop scalable manufacturing approaches for their thermionic materials. The production methods employed up to now, while demonstrating proof of concept, would scale-up sufficiently for commercial manufacturing, notes Shakouri, who predicts that within a decade his materials, or materials like them, will be used to increase the fuel efficiency of vehicles by 10 to 20 percent.