Converting Waste Heat to Electricity Without Thermoelectrics
A great number of systems and processes produce significant amounts of heat that is simply lost, so it makes sense that many, many groups and institutions are working on ways to capture and use that heat. Typically thermoelectrics are looked to as a means to convert the heat directly to useable electricity, but they do not work well in all situations. Researchers at MIT though have refined a method that puts the thermogalvanic effect to work.
The thermogalvanic effect relates to changes to the charging-discharging cycle of batteries and their temperature. When a battery is warm, it stores energy more easily than when it is cool, and if it is discharged at a lower temperature, you will actually get more electricity out than was put in. The extra energy comes from the heat, which means this effect could convert waste heat into useable electricity. Unlike most thermoelectrics though, this method can capture heat less than 100 ºC and operate with temperature differences less than 100 ºC. When tested at just 60 ºC, this technique achieved 5.7% efficiency.
This approach to harvesting waste heat was first proposed in the 1950s, but it was not until modern materials were available for it to be possible. As it is now though, it still lacks in power density compared to thermoelectrics and its long term reliability is unknown. More research is needed to determine its feasibility and to optimize it, but it is still a very promising solution to the problem.