Heat flowing from a colder to a warmer body without external intervention

According to the rules of classical thermodynamics, the flow of heat between two thermally connected objects of different temperatures is determined by Fourier’s law of heat conduction. This law states that the rate of heat flow between these objects increases with growing temperature differences. This is also more generally supported by the second law of thermodynamics, which requires that heat can flow by itself only from a warmer to a colder body.

Researchers from the University of Zürich, Switzerland, carried out an experiment which, at first sight, seemed to challenge the second law of thermodynamics. They managed to cool a nine-gram piece of copper from over 100°C to significantly below room temperature without an external power supply.

To achieve this, the researchers used the equivalent of a “thermal inductor,” composed of a Peltier element and an electric inductance which can cause the temperature difference between two bodies to change sign. It does this by imposing inertia on the heat flowing between them and enabling the heat transfer from the chilling body to its warmer counterpart without the need of an external driving force. They demonstrated its operation in an experiment and showed that the process can pass through a series of quasi-equilibrium states while fully complying with the second law of thermodynamics. The researchers suggest that this thermal inductor extends the analogy between electrical and thermal circuits and could serve, with further progress in thermoelectric materials, to cool hot materials well below ambient temperature without external energy supplies or moving parts.

Original article in Science Advances: https://advances.sciencemag.org/content/5/4/eaat9953

Summary on the University of Zurich website: https://www.media.uzh.ch/en/Press-Releases/2019/Thermodynamic-Magic.html