New way to turn waste heat to energy

Industrial processes and electric power plants generate large volumes of waste heat. Researchers around the world have come up with different techniques to harness some of this wasted energy.

Researchers at Stanford University and the Massachusetts Institute of Technology have found a new alternative for low-temperature waste-heat conversion into electricity - that is, in cases where temperature differences are less than 100°C.

They developed a four-stage process that uses waste heat to charge a battery. First, an uncharged battery is heated by waste heat. Then, while the battery is still warm, a voltage is applied. When fully charged, the battery is allowed to cool, which increases the voltage. Once the battery has cooled, it actually delivers more electricity than was used to charge it.

“Virtually all power plants and manufacturing processes, like steel-making and refining, release tremendous amounts of low-grade heat to ambient temperatures,’ said Yi Cui, an associate professor of materials science and engineering. “Our new battery technology is designed to take advantage of this temperature gradient at the industrial scale.” The findings are detailed in a study, published in the journal Nature Communications, by Seok Woo Lee and Yi Cui at Stanford and Yuan Yang and Gang Chen at MIT.

Photo credit: Jose-Luis Olivares/MIT News Office.

The Stanford-MIT system aims at harvesting heat at temperatures below 100°C, which accounts for a major part of potentially harvestable waste heat. “One-third of all energy consumption in the United States ends up as low-grade heat,” said Yang, a post-doc at MIT.

In the experiment, a battery was heated to 60°C, charged and cooled. The process resulted in an electricity-conversion efficiency of 5.7%, almost double the efficiency of conventional thermoelectric devices.

This heating-charging-cooling approach was first proposed in the 1950s at temperatures of 500°C or more, said Yang, noting that most heat recovery systems work best with higher temperature differences.

“A key advance is using material that was not around at that time” for the battery electrodes, as well as advances in engineering the system, said Chen, a professor of mechanical engineering at MIT.

“This technology has the additional advantage of using low-cost, abundant materials and manufacturing processes that are already widely used in the battery industry,” added Lee.

While the new system has a significant advantage in energy-conversion efficiency over conventional thermoelectric devices, it has a much lower power density - that is, the amount of power that can be delivered for a given weight. The new technology also will require further research to assure long-term reliability and improve the speed of battery charging and discharging, Chen added. “It will require a lot of work to take the next step.”

There is currently no good technology that can make effective use of the relatively low-temperature differences this system can harness, Chen said. §This has an efficiency we think is quite attractive. There is so much of this low-temperature waste heat, if a technology can be created and deployed to use it.”

Other authors of the study are Hyun-Wook Lee of Stanford and Hadi Ghasemi and Daniel Kraemer of MIT.

The Stanford work was partially funded by the US Department of Energy (DOE), the SLAC National Accelerator Laboratory and the National Research Foundation of Korea. The MIT work was partially funded by the DOE, in part through the Solid-State Solar-Thermal Energy Conversion Center. 

This article is based on a report from the MIT News Office.