Pretty much anything we do that involves energy, from generating electricity to using it in a laptop, produces energy in the form of heat that ultimately goes to waste. The problem is that all of the means we have of converting heat into useful energy require large temperature differences. The waste heat, by contrast, is a low-grade energy source, usually involving temperature differences of less than 100 degrees Celsius.

So far, attempts to find ways of producing useful energy from waste heat have largely focused on thermoelectric devices, which directly convert temperature gradients to electricity. But these devices generally cost a lot to produce, so the economics of using them to capture waste heat are pretty questionable. But now researchers have produced a demonstration of a device that acts more like a battery that can be charged or discharged based on temperature differences. Although it’s not especially efficient, the ability to store charges may add significantly to its utility.

The research team involved in the work (a collaboration between people at MIT and Stanford) had previously demonstrated an actual battery that took advantage of temperature differences. In that example, which relied on a copper-based chemical reaction, charging and discharging took place at two different temperatures. At an elevated temperature (perhaps one provided by waste heat), the voltage difference needed for charging was lower than it would have been otherwise. By dropping the battery to room temperature during discharge, their battery produced a higher voltage. In essence, the temperature difference was used to get more useful current out of the battery.