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Every year 10 gigawatts of potential power are squandered as waste heat from industrial processes—enough to light 10 million homes. The thermoelectric effect, in which charges are created by temperature differences, provides a way of transferring this heat into electricity—but only some of it. For decades the temperature differential had to be 500 degrees C or greater to capture any useful amount of energy, explains Yuan Yang, a postdoctoral scholar at M.I.T. That is unfortunate, because the Environmental Protection Agency estimates that a third of all wasted energy in the U.S. every year is lost at temperatures below about 100 degrees C.
Yang, his professor, Gang Chen, postdoc Seok Woo Lee and Yi Cui of Stanford have developed a way to harvest heat at temperatures 10 times lower—as little as 50 degrees C. The trick was to exploit the thermogalvanic effect, a cousin of the thermoelectric effect, in which the entire material's temperature changes, along with the voltage, rather than a gradient within the battery cell. The group took uncharged battery cells with copper-based electrodes, charged the cells while they were hot and then cooled them down. Presto: the batteries delivered a higher voltage than was used to charge them. In other words, the energy used to heat the battery was captured in the form of electricity.
Only in the past two years or so have battery electrodes become efficient enough to convert such low-temperature differentials into electricity, Yang says, and plenty of development remains before the process can be commercialized. But in time, banks of batteries could line the walls of factory smokestacks or power plants, converting low-grade excess heat into electricity. “This is something attractive,” Yang says, “because low-grade heat is everywhere.”