Thermodynamic magic allows cooling without energy consumption – ScienceDaily



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Physicists at the University of Zurich have developed an incredibly simple device that allows heat to flow temporarily from a cold to a hot object without an external power source. Curiously, the process initially seems to contradict the fundamental laws of physics.

If you put a pot of boiling water on the kitchen table, it will cool gradually. However, the temperature should not fall below the table. It is precisely this daily experience that illustrates one of the fundamental laws of physics – the second law of thermodynamics – that states that the entropy of a closed natural system must increase with time. Or, more simply, heat can flow itself from a hotter object to a colder one, not the other way around.

Cooling below room temperature

The results of a recent experiment conducted by Prof. Andreas Schilling at the Department of Physics at the University of Zurich (UZH) seems at first glance to challenge the second law of thermodynamics. The researchers were able to cool a 9-gram piece of copper from over 100 ° C to a level significantly below room temperature without an external power supply. "Theoretically, this experimental device could turn boiling water into ice without using any energy," says Schilling.

Creating Oscillating Heat Currents

To achieve this, the researchers used a Peltier element, a commonly used component, for example, to cool minibars in hotel rooms. These elements can transform electrical currents into temperature differences. Researchers had already used this kind of element in previous experiments, in connection with an electric inductor, to create an oscillating heat stream in which the heat flow between two bodies perpetually changed direction. In this scenario, the heat also temporarily flows from a cooler object to a warmer one, so that the cooler object is cooled further. This type of "thermal oscillating circuit" actually contains a "thermal inductor". It functions in the same way as an electric oscillating circuit, in which the voltage oscillates with a constantly changing signal.

Laws of physics remain intact

So far, Schilling's team has operated only these thermal oscillating circuits using a power source. The researchers have now shown, for the first time, that this type of thermal oscillating circuit can also be operated "passively," that is, without external power supply. Thermal oscillations still occurred and after some time the 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. Despite this, the authors have also been able to show that the process does not contradict the laws of physics. To prove this, they considered the change in the entropy of the whole system and showed that it increased with time – fully according to the second law of thermodynamics.

Potential application still very far

Although the team recorded a difference of only about 2 ° C compared to the ambient temperature in the experiment, this was mainly due to the performance limitations of the Peltier commercial element used. According to Schilling, it would be possible in theory to cool to -47 ° C under the same conditions if the "ideal" Peltier element – still to be invented – could be used: "With this very simple technology large quantities of solid materials, liquid or gaseous could be cooled to well below room temperature without any energy consumption. "

The passive thermal circuit can also be used as many times as desired, without the need to connect it to a power source. However, Schilling admits that a large-scale application of the technique is still a long way off. One reason for this is that the Peltier elements currently available are not efficient enough. In addition, the current configuration requires the use of superconducting inductors to minimize electrical losses.

Established perceptions challenged

The UZH physicist considers the work more significant than a mere "proof of principle" study: "At first glance, experiments appear to be a kind of thermodynamic magic, challenging to some extent our traditional perceptions of heat flow."

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