Every year, Lawrence Livermore National Laboratory in California publishes an amazing chart cataloguing all the energy used in the United States. The striking thing about it is the "rejected energy" portion of the chart: in 2013, more than 60 percent of our 97.4 quads (around 293 billion kilowatt-hours) of energy were rejected, or wasted. Most of that dissipates as heat: from smokestacks and manufacturing processes, and from your tailpipe. Capturing some of that heat has long been a dream that would improve our efficiency and reduce energy use and emissions, and new research takes a novel approach to achieve it.
Most waste heat ideas involve the thermoelectric effect, where a voltage is created based on a temperature difference. This approach has been limited by materials and the need for very high temperatures and gradients, but the new idea, using the thermogalvanic effect can operate at much lower temperature differences. The researchers responsible for this idea, from Stanford and MIT, describe thermogalvanics as "the dependence of electrode potential on temperature."
Basically, it works like this: an uncharged battery is heated by waste heat (from, say, some manufacturing process in a factory), and then it is charged. The battery is then allowed to cool, and discharges only at a cooler temperature. The charging voltage is lower at higher temperatures, meaning the charging voltage is lower than the discharging voltage; that means more energy can be extracted via discharge than was inputed via charging. The extra energy comes from the heat differential.
They aim to use this type of system at temperatures below 100°C; in tests, at 60°C they achieved a conversion efficiency of 5.7 percent. The battery used in the tests consisted of a copper hexacyanoferrate cathode and a Cu/Cu2+ anode.
One investigator, Yi Cui of Stanford University, said in a press release that the potential for this idea is enormous. "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."
To be sure, though, they are far from actually sending these batteries to a steel mill to capture a bunch of lost energy. It has a much lower power density than thermoelectric devices, meaning huge versions of this battery might be required to deliver enough power. And charging and discharging probably needs to be sped up substantially before the technology becomes viable as well. But with wasted energy pouring out of every smokestack in the world, this idea isn't likely to dissipate any time soon.