Thermodynamic magic promises refrigerators that do not expend energy

Chilling out: in principle, a thermal inductor could help convert boiling water to ice without using any external energy. (Courtesy: A Schilling and AC Mangham)

Not for the Law of Thermodynamics

Physicists at the University of Zurich in Switzerland have developed an incredibly simple device that allows heat to temporarily flow from a cold object to another hot object without requiring an external power source, as it does in traditional refrigerators.

Curiously, the process seems at first glance to contradict the fundamental laws of physics, more specifically the Second Law of Thermodynamics , which states that the entropy of a closed natural system must increase with time. Or, more simply, the heat will flow itself from a warmer object to a colder one, not the other way around.

Andreas Schilling and his colleagues were able to cool a nine-gram piece of copper from over 100 ° C to a level significantly below room temperature without any external power supply, which seems at first glance to challenge this Second Law of Thermodynamics.

"Theoretically, this experimental device can turn boiling water into ice without using any energy," says Schilling.

The arrows represent the direction of the heat flux from light / yellow or dark / purple to the respective hottest object. [Image: A. Schilling et al. - 10.1126 / sciadv.aat9953]

Cooling without energy consumption

To do this, the team used a Peltier element , a commonly used component, for example, to cool minibars in hotel rooms or in solid-state (non-gaseous) refrigerators used in automobiles. These thermoelectric elements can transform electrical currents into temperature differences.

The team had already used a Peltier plate, along with a coil, to create an oscillating heat chain in which the heat flow between two bodies perpetually changes direction. In that experiment, the heat also temporarily flows from an object colder to a warmer one, so that the colder object is cooled further. This type of "thermal oscillating circuit" contains a "thermal inductor", functioning in the same way as an electric oscillating circuit, in which the voltage oscillates with a constantly changing signal.

However, until then, these thermal oscillating circuits - essentially thermal diodes - only operated using an external source of energy.

Now, for the first time, Schilling has shown that this type of thermal oscillating circuit can also operate "passively", ie without external power supply. Thermal oscillations still occur and after some time the heat flows directly from the coldest copper to a warmer solution with a temperature of 22 ° C, without the heat being transformed into another form of energy in the way.

Despite this, the team made the calculations to show that the apparatus does not contradict the laws of physics. To prove this, they considered the change in entropy of the whole system and showed that it increases over time - fully according to the Second Law of Thermodynamics.

Another team has already found that the Second Law of Thermodynamics fails on an atomic scale . In fact, there seem to be several Second Laws of Nanoscale Thermodynamics . [Image: Iñaki Gonzalez / Jan Gieseler]

Applications depend on additional developments

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 used Peltier element, which was purchased commercially. According to Schilling, it would be possible in theory to achieve a cooling down to -47 ° C, under the same conditions, if Peltier's "ideal" element - still to be invented - could be used.

Another factor that leaves large-scale applications of this technique still distant in the future is that the current configuration requires the use of superconducting inductors to minimize electrical losses.

But this does not seem to make the team discourage or even dislike their experiment.

"With this very simple technology, large amounts of solid, liquid or gaseous materials can be cooled to a temperature well below room temperature without any energy consumption. At first glance, the experiments seem to be a kind of thermodynamic magic, challenging our perceptions heat flow, "said Schilling.


Heat flow from hot to cold without external intervention by using a thermal inductor
A. Schilling, X. Zhang, O. Bossen 
 Science Advances 
 Vol .: 5, no. 4, eaat9953 
 DOI: 10.1126 / sciadv.aat9953 

 LC-circuit calorimetry 
O. Bossen, A. Schilling 
 Review of Scientific Instruments 
 Vol .: 82, 094901 
 DOI: 10.1063 / 1.3632116

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