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John O | April 2019

Device appears to break rules, passively allows heat to flow from cold to warm object

By Josh Perry, Editor


Physicists from the University of Zurich (Switzerland) demonstrated a simple device that appears to break the second rule of thermodynamics, which states the entropy of a closed, natural system must increase over time, by allowing heat to flow from a cold to a warm object without an external power supply.


Theoretically, this experimental device could turn boiling water to ice, without using any energy. (A. Schilling, A.C. Mangham/UZH)


According to a report from the university, researchers cooled a nine-gram piece of copper from 100°C to below room temperature using a Peltier element. Previously, the scientists had used thermoelectric elements to create an oscillating heat current, which pushed heat from the cold to the warm object, but only with an outside energy source.


“The researchers have now shown for the first time that this kind of thermal oscillating circuit can also be operated ‘passively’, i.e. with no external power supply,” the report explained. “Thermal oscillations still occurred, and, after a while, heat flowed directly from the colder copper to a warmer heat bath with a temperature of 22°C, without being temporarily transformed into another form of energy.”


While this appeared to be in contradiction to the rules of thermodynamics, the researchers considered entropy at the system-level and saw that it increased over time, which was in accordance with the second law.


Limited by the Peltier element used in the experiments, researchers were only able to demonstrate a 2°C difference from the ambient but they believe that it could be possible to achieve cooling of up to -47°C. The ideal Peltier element for that level of cooling has yet to be invented, according to the report, and this type of passive device is not close to being practical on a large-scale.


The research was recently published in Science Advances. The abstract read:


“The cooling of boiling water all the way down to freezing, by thermally connecting it to a thermal bath held at ambient temperature without external intervention, would be quite unexpected.


“We describe the equivalent of a ‘thermal inductor,’ composed of a Peltier element and an electric inductance, which can drive the temperature difference between two bodies to change sign by imposing inertia on the heat flowing between them, and enable continuing heat transfer from the chilling body to its warmer counterpart without the need of an external driving force.


“We demonstrate its operation in an experiment and show that the process can pass through a series of quasi-equilibrium states while fully complying with the second law of thermodynamics.


“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.”

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