Physicists at the University of Zurich have come up with a surprisingly simple device allowing heat to flow temporarily from a cold object to a hot object without an external power source. Curiously, the process initially seems to contradict the basic laws of physics.
If you put a teapot of boiling water on the kitchen table, it will gradually cool down. However, its temperature should not fall below that of the chart. It is precisely this daily experience that illustrates one of the fundamental laws of physics – the second law of thermodynamics – which states that the entropy of a closed natural system must increase with the time. Or more simply: the heat can circulate itself only from a hot object to a colder object, not the opposite.
Cooling below room temperature
The results of a recent experiment carried out by the research group of Professor Andreas Schilling of the Department of Physics of the University of Zurich (UZH) seem at first glance to question the second principle of thermodynamics. The researchers managed to cool a piece of copper nine grams higher than 100 ° C at a temperature well below room temperature without external power. "Theoretically, this experimental device could turn boiling water into ice, without using energy," says Schilling.
Creation of oscillating thermal currents
To achieve this, researchers used a Peltier element, a component commonly used, for example, to cool minibars in hotel rooms. These elements can transform electrical currents into temperature differences. The researchers had previously used this type of element in previous experiments, in conjunction with an electric inductor, to create an oscillating heat current in which the heat flux between two bodies was constantly changing direction. In this scenario, the heat also circulates temporarily from a cold object to a warmer object, so that the colder object is cooled further. This type of "thermal oscillating circuit" actually contains a "thermal inductor". It works in the same way as an oscillating electrical circuit, in which the voltage oscillates with a constantly changing sign.
The laws of physics remain intact
Until now, the Schilling team had operated these oscillating thermal circuits only with the help of an energy source. Researchers have now shown for the first time that this type of thermal oscillation circuit can also operate "passively", that is to say without external power supply. Thermal oscillations were still occurring and, after a while, the heat passed directly from the colder copper into a warmer heating bath with a temperature of 22 ° C, without being temporarily transformed into another form of heat. ;energy. Despite this, the authors have also been able to show that the process does not contradict any physical law. To prove it, they examined the entropy change of the whole system and showed that it was increasing over time, in accordance with the second principle of thermodynamics.
The potential application is still far
Although the team found a difference of about 2 ° C compared to the ambient temperature during the experiment, this difference was mainly due to the performance limits of the commercial Peltier element used. According to Schilling, it would theoretically be possible to achieve a cooling down to -47 ° C under the same conditions if the "ideal" Peltier element – yet to be invented – could be used: "With this very simple technology, large amounts of hot, liquid or hot gaseous matter could be cooled well below ambient temperature without energy consumption. "
The passive thermal circuit can also be used as often as you want, without it being necessary to connect it to a power source. Schilling admits however that a large-scale application of the technique is still far away. One reason 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.
Contested established perceptions
The UZH physicist believes that these works are more significant than a simple study of "proof of principle": "At first glance, the experiments seem to be a kind of thermodynamic magic, thus calling into question in a to some extent our traditional perceptions of heat flow. "
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