Less Heat, More Energy?
A novel device converts waste heat into electricity with exceptional efficiency.
Written by Agam Shah
ALTHOUGH LONGJI CUI RESIDES IN CHILLY BOULDER, COLO., he’s obsessed with heat. And he can’t stop thinking about better ways to exploit heat for power.
“Heat is a wide-open renewable resource—66 percent of the total energy consumption in the world is not used very well. If people can use heat, that will be a huge thing,” said Cui, an assistant professor in the Department of Mechanical Engineering, Materials Science, and Engineering at the University of Colorado, Boulder.
Solar energy is a popular form of heat-related energy generation, but Cui’s lab focuses on extracting electricity from artificial heat generated by industrial sources—most of which currently goes to waste by dissipating into the environment. “It’s a wide-open space. There are many things that can be done,” he said.
To that end, Cui’s research group began exploring the possibility of generating more energy from heat emitted by nuclear power plants, manufacturing facilities, or even combustion chambers in engines. The result is a compact thermophotovoltaic (TPV) device that has twice the yielded power density of conventional TPV designs.
“What we are showing is that you can improve power density without sacrificing efficiency.”
—Longji Cui, assistant professor at the University of Colorado, Boulder
Their findings were recently published in Energy & Environmental Science: “Enhanced power density in zero-vacuum-gap thermophotovoltaic devices.”
Developing the new device required rethinking how TPV devices—which harvest energy from artificial heat sources—are constructed. Most devices used in factories today are based on 50-year-old designs from the Massachusetts Institute of Technology’s Lincoln Laboratory.
TPV devices include a radiation emitter that, when heated up, will glow and emit infrared light. A TPV cell receives the light through a vacuum and converts the light to electricity.
“What we are showing is that you can improve power density without sacrificing efficiency,” Cui said. “The thermal source is hot, not as hot as the sun, but it also generates a spectrum of radiation, and we harvest a part of that radiation.”
Cui’s team redesigned the architecture by putting a transparent glass material between the emitter and the TPV cell, which doubled the power density. Current designs use only vacuum or air when transferring radiation.
This “zero-vacuum” gap replaces the vacuum with solid glass so more energy can be extracted from the same heat source. The glass allows thermal heat waves to travel through the device without losing strength.
“That effectively increases power density without sacrificing efficiency,” Cui added.
The idea came to Cui while he was experimenting with Planck’s Law—a fundamental law relating temperature and the emissions from objects. One of its often-overlooked aspects is that surrounding an emitter with a transparent material allows more energy to be extracted from a heat source.
The zero-vacuum gap TPV device, designed by the Cui Research Group.
Credit: University of Colorado, Boulder
Doctoral student Mohammad Habibi showcasing one of the group’s TPV cells used for power generation. Habibi was the leader of both the theory and experimentation of this research.
Credit: University of Colorado, Boulder
As a result, when “we put glass, almost like a funnel, you can extract double the energy. That increases power density without increasing temperature,” he said, while emphasizing that he was not redefining Planck’s Law but instead reworking device architecture. Adding glass was a simple trick, but the output was magical.
“TPV’s efficiency is much higher than thermoelectric and even comparable to turbines,” said Cui, adding that it can also generate energy at lower temperatures. “If we double the power density, that means we use half the area that people are already using, which means half the cost.”
While glass has proven the device’s efficacy, other materials may unlock even more power potential. Cui’s team is investigating amorphous silicon next, as it could result in a nearly 20-fold increase in power density.
The TPV device could also pave the way for smaller devices that store energy for the grid. A patent is already pending on the technology, which is based on already available commercial tech. Scaling up this solution may offer heavy industries a new way to harness wasted heat in a renewable way.
Agam Shah is a business and technology writer in Phoenix and an adjunct faculty member at the Walter Cronkite School of Journalism at Arizona State University.
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