Thermoelectric devices, which convert heat into electricity, can potentially recapture useful energy from heat sources such as the hot exhaust gases of vehicles. But a theoretical study in Physical Review Letters shows that, if such devices are to remain relatively efficient while producing substantial power, they can’t be any smaller than a minimum of perhaps a few millimeters across. This size limit is determined by the laws of quantum physics that govern the behavior of electrons, even though such quantum effects are usually associated with much smaller size scales.

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