Researchers at MIT and Stanford have demonstrated a new way to transform waste heat into electricity, particularly in situations where the temperature gradient is small.

The new approach is based on a phenomenon called the thermogalvanic effect – the fact that the voltage of rechargeable batteries depends on temperature. The new system combines the charging-discharging cycles of these batteries with heating and cooling. The system can efficiently harness even relatively small temperature differences and aims at harvesting heat of less than 100 Cº, which accounts for a large proportion of waste heat.

To begin, an 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 electric energy comes from the heat that was added to the system.

One of the lead researchers, Professor Gang Chen of MIT, pointed out that there is currently no other good technology that can make effective use of the relatively low-temperature differences that this system can harness. He said that “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.”

However, he pointed out that a lot of work is required to take the technology to the next stage – assuring reliability over a long period of use and improving the speed of battery charging and discharging.