An entirely new approach to harvesting waste heat — one based on a phenomenon known as the thermogalvanic effect — has been developed by researchers at MIT and Stanford University.
The new technique is entirely divorced from the (usually used) thermoelectric effect, which relies on (generally expensive) solid-state materials — instead taking advantage of the fact that the voltage of rechargeable batteries varies depending on the temperature.
The new approach utilizes this reality — combining the charge-discharge cycles of these batteries with heating and cooling, in a way that the discharge voltage is higher than the charge voltage. Via this approach even relatively small temperature differences (ex: a 50 degrees Celsius difference) can be effectively harnessed.
The press release from MIT provides more:
To begin, the uncharged battery is heated by the waste heat. Then, while at the higher temperature, the battery is charged; once fully charged, it is allowed to cool. Because the charging voltage is lower at high temperatures than at low temperatures, once it has cooled the battery can actually deliver more electricity than what was used to charge it. That extra energy, of course, doesn’t just appear from nowhere: It comes from the heat that was added to the system.
The system aims at harvesting heat of less than 100 degrees Celsius, which accounts for a large proportion of potentially harvestable waste heat. In a demonstration with waste heat of 60 degrees Celsius the new system has an estimated efficiency of 5.7%.
This general concept is by no means truly “new” — having been first proposed back in the 50s — but this is, more or less, the first work to make the approach seem truly viable. The key to this is the “utilization of a material that was not around at that time” for the battery electrodes, as well as advances in engineering the system, according to MIT professor Gang Chen.
While the new approach allows for a significantly higher energy-conversion efficiency than thermoelectrics, for the time being it possesses the weakness of a much lower power density. The lifespan of technologies using the approach hasn’t been explored much yet, either — so it’s reliability hasn’t been completely tested yet, the way that is has in many thermoelectric technologies.
“It will require a lot of work to take the next step,” Chen notes.
But there is certainly a niche there for the approach — as 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,” he continues. “There is so much of this low-temperature waste heat, if a technology can be created and deployed to use it.”
Peidong Yang, a professor of chemistry at the University of California at Berkeley who was not involved in this work, states: “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.”
“One-third of all energy consumption in the United States ends up as low-grade heat.”
That last bit really drives the point home. Of course, with technologies like this, no matter how cool it sounds, until the economics have been worked out it’s really hard to say if they have any future at all.
The new work was partially funded by the US DOE and the US Air Force. The new research is detailed in a paper published in the journal Nature Communications.
On the subject of thermoelectrics — interesting new findings concerning the potential use of a metal known as lithium purple-bronze (LiPB) were recently made by researchers at the University of Miami. The findings suggest that the metal (or related ones) could be used to great effect in thermoelectric devices owing to its “surprising” and “interesting” properties.
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