The new method takes advantage of the fact that the voltage of rechargeable batteries depends on their temperature

Using batteries to capture waste heat

Batteries can be used to harness the low-temperature waste heat given off by industrial processes, according to new research.

To date most efforts to harness this plentiful source of energy have focused on thermoelectric devices, solid-state materials that can produce electricity from a temperature gradient, but the efficiency of such devices is limited by the availability of materials.

But by taking advantage of the fact that the voltage of rechargeable batteries depends on their temperature, researchers at MIT and Stanford University have found they can efficiently capture this energy in cases where temperature differences are less than 100°C by combining the charging and discharging cycles with heating and cooling using the ambient heat.

"Virtually all power plants and manufacturing processes, like steelmaking and refining, release tremendous amounts of low-grade heat to ambient temperatures. Our new battery technology is designed to take advantage of this temperature gradient at the industrial scale," said Professor Yi Cui of Stanford who co-authored a paper published in journal Nature Communications today.

The new method involves heating the uncharged battery using the waste heat before charging it while at an elevated temperature.  The battery is then allowed to cool, but 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 was used to charge it.

That extra energy comes from the heat that was added to the system and the method can efficiently harness even relatively small temperature differences. In a demonstration with waste heat of 60°C the new system exhibited an estimated efficiency of 5.7 per cent.

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

“It will require a lot of work to take the next step," cautions co-author Gang Chen, a professor of mechanical engineering at MIT.

The system aims at harvesting heat of less than 100°C, which accounts for a large proportion of potentially harvestable waste heat. According to Chen there is 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 says. "There is so much of this low-temperature waste heat, if a technology can be created and deployed to use it."

Co-author Seok Woo Lee, a post-doc at MIT, added: "This technology has the additional advantage of using low-cost, abundant materials and manufacturing process that are already widely used in the battery industry."

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