A consequence of the second law of thermodynamics is that heat spontaneously flows from hot to cold, not the other way around. But now researchers in Switzerland have now shown that, if two reservoirs at different temperatures are connected using a passive thermoelectric element, “thermal inertia” can allow the hot reservoir to cool down to below the temperature of the cold reservoir. While this does not violate the second law, thermoelectic materials available today are not good enough for the effect to be exploited in practical devices. However, the researchers believe it could one day be used in refrigeration.
The thermoelectric effect is a well-known phenomenon whereby some materials convert a temperature difference into a potential difference. Heating one end of a metal, for example, causes the excited electrons at the hot end to diffuse towards the cold end. The effect also works in reverse: applying an electric current to a thermoelectric material leads creates a temperature gradient. This is the basis of thermoelectric coolers, which are widely used in computers, hotel minibars and other situations where the compressor required by a standard refrigerator would be unfeasible.
In thermoelectric coolers, heat flow from cold to hot is driven by external energy input. In this latest research, Andreas Schilling and colleagues at the University of Zurich dispensed with an external power supply. They started from two reservoirs at different temperatures. If these were placed in thermal contact, heat would simply flow from the hot reservoir to the cold until the two reached thermal equilibrium. Instead, however, they connected them electrically through a thermoelectric element, allowing the heat flow to drive an electric current. Crucially, they also added an electrical inductor.
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