Researchers at the University of California, Santa Cruz, are leading a collaborative effort to develop new technology that could increase the efficiency of internal combustion engines by converting waste heat into electric current. The project will use nanoscale engineering of materials to develop efficient technology for direct conversion of thermal energy to electric energy. It is funded by a five-year grant from the Office of Naval Research that provides up to $1.2 million per year spread among seven institutions.
"The Navy is interested in electric ships, but this technology will also be useful for electric cars and any other vehicle that needs electricity," said Ali Shakouri, an associate professor of electrical engineering at UCSC and principal investigator on the grant project.
Car engines are notoriously inefficient: Only about one-third of the energy they generate goes to propelling the car. The rest dissipates as heat, and keeping the engine cool presents an engineering problem of its own. Devising a way to turn some of that heat into useful energy would enable a car to travel further on less fuel.
"The current systems for converting heat to electricity are turbine-based, heavy, noisy, and not very efficient," Shakouri said. "What we are working on has no moving parts."
Shakouri heads the multi-institutional team of researchers working on the project. The researchers will explore the capacity of thinly layered materials to channel the random jostling of heat energy into the orderly flow of electricity. They hope to reach 20 percent efficiency for this conversion.
Scientists have understood the principles behind the conversion, known as the Seebeck effect and thermionic emission, for some time. When one side of a material is hotter than the other, electrons on the hot side move faster. In the process, some will find their way to the cold side, and this flow creates electricity. But previous attempts to harness this principle to generate power have required huge temperature differences and attained only about 6 percent efficiency.
Shakouri's team hopes to change this using the revolutionary new techniques of nanotechnology, which enables scientists to manipulate the properties of a material or device by precisely positioning its atoms.
"The ability to do nanoscale engineering of the materials has been a major advance," Shakouri said.
Three key properties will enable a material to efficiently convert heat energy to electricity, Shakouri said. First, it must allow "hot" electrons to move easily into the cold area, a characteristic known as electrical conductivity. Since electrons will flow only as long as the material maintains a temperature difference, the material must also be a thermal insulator. And finally, "hot" electrons must flow more easily than "cold" ones. Putting tiny vacuum gaps between the hot and cold sides of the device are one way this might be achieved, since hot electrons, but not heat itself, could jump across the gap.
Shakouri and some other members of the team have worked on a similar problem before, but with a different goal: refrigeration. They are currently improving a nanoscale cooling device that can be applied as a thin layer on a computer chip. The electric current flowing through the chip creates a temperature difference between the layers of the device, and suctions heat away from the chip.
The car engine device would essentially reverse this process, surrounding the engine and using the temperature difference between the engine and its surroundings to generate electric current.
"In this project, both solid-state materials and small vacuum gaps will be investigated to improve the preferential flow of hot electrons," Shakouri said.
The research team is divided into five smaller groups, each assigned a particular task, such as theoretical modeling, making or testing materials, or demonstrating the overall system. Though work on the project will go on at institutions from Santa Barbara to Cambridge, Massachusetts, its center will be Shakouri's lab, which will perform device design and system analysis and will determine which paths the research should take.
The institutions involved are UC Santa Cruz, Harvard University, Massachusetts Institute of Technology, North Carolina State University, Purdue University, UC Berkeley, and UC Santa Barbara.
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Note to reporters: You may contact Shakouri at (831) 459-3821 or ali@soe.ucsc.edu.
