Because the charging voltage is lower at high temperatures than at low temperatures, once the battery has cooled it delivers more electricity than what was used to charge it—i.e., converting heat to electricity.

Schematic voltage plot of the thermally regenerative electrochemical cycle (TREC) for thermal energy harvesting. Typical charging (dashed blue) and discharging (solid blue) voltage curves at the same temperature forms closed loop, the area of which means energy loss during a cycle. Negative temperature coefficient of the cell shifts down the charging curve at high temperature (solid red) below the voltage curve of discharging at low temperature (solid blue). The area of the closed loop between charging at the high temperature and discharging at the low temperature is the energy obtained during the cycle. Lee et al., SI. Click to enlarge.

The electrode materials have low polarization, high charge capacity, moderate temperature coefficients and low specific heat. These features lead to a high heat-to-electricity energy conversion efficiency of 5.7% when cycled between 10 and 60 °C, opening a promising way to utilize low-grade heat, the researchers said.

The system aims at harvesting heat of less than 100 ˚C, which accounts for a large proportion of potentially harvestable waste heat.

The basic concept for this approach was initially proposed in the 1950s, Chen says, but “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.

The earlier work was based on temperatures of 500 ˚C or more, Yang adds; most current heat-recovery systems work best with higher temperature differences.

A design for heat recuperation in TREC with heat exchangers (HXs). This design shows four heat recuperation steps, but it can be extended to any number of recuperation steps by adding more counter-flow heat HXs and cells. Lee et al. Click to enlarge.

While the new system has a significant advantage in energy-conversion efficiency, for now it has a much lower power density than thermoelectrics. It also will require further research to assure reliability over a long period of use, and to improve the speed of battery charging and discharging, Chen says.

Chen, the Carl Richard Soderberg Professor of Power Engineering and head of MIT’s Department of Mechanical Engineering, says there’s currently no good technology that can make effective use of the relatively low-temperature differences this system can harness.

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.

—Gang Chen

Stanford’s Profesor Cui noted that virtually all power plants and manufacturing processes, such as steelmaking and refining, release tremendous amounts of low-grade heat to ambient temperatures. The new battery technology would take advantage of that temperature gradient at the industrial scale.

By exploring the thermogalvanic effect, [the MIT and Stanford researchers] were able to convert low-grade heat to electricity with decent efficiency. It is a very promising technology. … This is a clever idea, and low-grade waste heat is everywhere.

—Peidong Yang, a professor of chemistry at the University of California at Berkeley, who was not involved in the work

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

Resources

• Seok Woo Lee, Yuan Yang, Hyun-Wook Lee, Hadi Ghasemi, Daniel Kraemer, Gang Chen & Yi Cui (2014) “An electrochemical system for efficiently harvesting low-grade heat energy” Nature Communications 5, Article number: 3942 doi: 10.1038/ncomms4942