Turning heat into energy
Energy cannot be created or destroyed; however, it can be wasted. The Second Law of Thermodynamics states that heat cannot be converted entirely to mechanical energy. As more and more energy is transformed, more and more of it gets wasted. Scientists like Dr. Mark Lee, professor and head of the Department of Physics at the University of Texas at Dallas, and the team at Texas Instruments Inc. are looking for ways to harvest that energy efficiently and effectively.
“In a general sense, waste heat is everywhere: the heat your car engine generates, for example,” Lee said. “That heat normally dissipates. If you have a steady temperature difference — even a small one — then you can harvest some heat into electricity to run your electronics.”
Thermoelectric generation is a potent green energy source, but its development has faced a number of obstacles, primarily the efficiency and cost to do it. “Thermoelectric generation has been expensive, both in terms of cost per device and cost per watt of energy generated,” Lee said. “The best materials are fairly exotic — they’re either rare or toxic — and they aren’t easily made compatible with basic semiconductor technology.”
So, Lee teamed up with Texas Instruments Inc. to design a better way for electronics to convert waste heat into reusable energy.
Silicon’s Energy-Harvesting Power
Microelectronic thermoelectric generators (TEGs) recycle waste heat into electrical power. However, TEGs and Silicon integrated circuit technology lack compatibility preventing broad adoption in microelectronics. Silicon is the second-most abundant element in the Earth's crust but is is known to be a poor thermoelectric material in it’s bulk, crystalline form. But, new research in 2008 showed that silicon performed much better as a nanowire.
One barrier is that the nanowire is too small to be compatible with chip-manufacturing processes. To overcome this, Lee and his team relied on “nanoblades” — only 80 nanometers thick but more than eight times that in width.
Study co-author Hal Edwards, a TI Fellow at Texas Instruments, designed and supervised the fabrication of the prototype devices. He turned to Lee and UT Dallas to further study what the devices could do.
“A deep dive for these novel measurements, detailed analysis and literature comparisons requires a university group,” Edwards said. “Professor Lee’s analysis identified key metrics in which our low-cost silicon technology competes favorably with more exotic compound semiconductors.”
The team’s circuit-design solution combined an understanding of nanoscale physics with engineering principles. One key realization was that some previous attempts failed because too much material was used.
“When you use too much silicon, the temperature differential that feeds the generation drops,” Lee said. “Too much waste heat is used, and, as that hot-to-cold margin drops, you can’t generate as much thermoelectric power.
“There is a sweet spot that, with our nanoblades, we’re much closer to finding than anyone else. The change in the form of silicon studied changed the game,” he added.
Lee said that the advanced silicon-processing technology at Texas Instruments allows for efficient, inexpensive manufacturing of a huge number of the devices.
“You can live with a 40% reduction in thermoelectric ability relative to exotic materials because your cost per watt generated plummets,” he said. “The marginal cost is a factor of 100 lower.”
Gangyi Hu PhD’19, who finished his doctorate in physics at UT Dallas in May, is the study’s lead author. “We optimized the configuration of our devices to place them among the most efficient thermoelectric generators in the world,” Hu said. “Because it’s silicon, it remains a low-cost, easy to install, maintenance-free, long-lasting and potentially biodegradable.”
These findings could have a profound impact on how circuits are cooled in electronics and provide a method of powering the sensors used in the “Internet of Things (IoT).”
“Sensors go everywhere now. They can’t be constantly plugged in, so they must consume very little power,” Lee said. “Without a reliable light source for photovoltaic energy, you’re left needing some kind of battery — one that shouldn’t have to be replaced.”
“We want to integrate this technology with a microprocessor, with a sensor on the same chip, with an amplifier or radio, and so on. Our work was done in the context of that full set of rules that govern everything that goes into mass-producing chips,” Lee said. “Over at Texas Instruments, that’s the difference between a technology they can use and one they can’t.”
Lee’s research is supported by the National Science Foundation through the Grant Opportunities for Academic Liaison with Industry (GOALI) program, and the work at UT Dallas was performed in the Texas Analog Center of Excellence (TxACE) laboratory.
Gangyi Hu, Hal Edwards & Mark Lee (2019) “Silicon integrated circuit thermoelectric generators with a high specific power generation capacity” Nature Electronics volume 2, pages 300–306 doi: 10.1038/s41928-019-0271-