Physicists at the University of Zurich have developed an amazingly simple device that allows heat to flow temporarily from a cold to a warm object without an external power supply. Interestingly, the process initially seems to contradict the basic laws of physics.
If you place a teapot of boiling water on the kitchen counter, it will gradually cool. However, it is not expected that its temperature will fall below that of the table. It is precisely this everyday 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 over time. Or, more simply, heat can flow from a warmer to a colder object, not vice versa.
Cooling below room temperature
The results of a recent experiment by the research group Prof. Andreas Schilling of the Department of Physics at the University of Zurich (UZH) seem at first glance to question the second law of thermodynamics. The researchers succeeded in cooling a 9-gram piece of copper to well below room temperature without an external power supply of more than 1
Generation of oscillating heat fluxes
For example, the researchers used a Peltier element, a commonly used component to cool minibars in hotel rooms. These elements can convert electrical currents into temperature differences. The researchers had already used this element in previous experiments in conjunction with an electrical inductance to produce an oscillating heat flow in which the heat flow between two bodies constantly changes direction. In this scenario, heat also temporarily flows from a colder to a warmer object, causing the colder object to continue to cool. This type of "thermal resonant circuit" actually contains a "thermal inductor". It works like an electrical resonant circuit where the voltage oscillates with a constantly changing sign.
Physical laws are retained
So far, Schilling's team had operated these thermal circuits with only one energy source. The researchers have now shown for the first time that this type of thermal resonant circuit can also be operated "passively", ie. H. Without external power supply. Thermal vibrations still occurred and after some time the heat flowed directly from the colder copper to a warmer heat bath at a temperature of 22 ° C, without being temporarily transformed into another form of energy. Nevertheless, the authors were also able to show that the process actually does not contradict any physical laws. To prove this they considered the entropy change of the entire system and showed that it increased with time in accordance with the second law of thermodynamics.
Possible application still in the far distance
Although the team found a difference of only about 2 ° C compared to the ambient temperature in the experiment was mainly due to the performance limitations of the commercial Peltier element. According to Schilling, theoretically cooling down to -47 ° C under the same conditions would be possible if the "ideal" Peltier element could still be invented: "With this very simple technology, large quantities of hot solid, liquid or gaseous Materials could be cooled significantly below room temperature without energy consumption. "
The passive heat cycle could also be used as often as needed without having to supply a power supply. Schilling acknowledges, however, that a large-scale application of the technique is still far away. One reason for this is that the currently available Peltier elements are not efficient enough. In addition, the current device requires the use of superconducting inductors to minimize electrical losses.
Questioning established perceptions
The UZH physicist considers the work more important than a mere proof-of-principle study: "First Apparently the experiments seem to be a kind of thermodynamic magic, which is our traditional perceptions of the heat flow to some extent. "
Materials from University of Zurich . Note: Content can be edited by style and length.