Cool objects in the room

Physical magic enables cooling without the use of energy

Physicists at the University of Zurich have developed an astonishingly simple arrangement in which heat temporarily flows from a colder to a warmer object without any external energy supply. However, this process only appears to contradict the laws of physics.

If you put a jug of boiling water on the kitchen table, it will cool down over time. But its temperature will never drop below that of the table. It is precisely this everyday experience that describes one of the fundamental theorems of physics: the second law of thermodynamics. It says that the entropy of a closed system must increase over time. In simple terms, this means that heat can only flow from a warmer to a colder object and not vice versa.

Cooling below room temperature

The research group of Prof. Andreas Schilling at the Physics Institute of the University of Zurich (UZH) has now carried out an experiment, the result of which appears to violate the Second Law of Thermodynamics at first glance. The researchers have succeeded in cooling a nine-gram piece of copper from over 100 ° C to well below room temperature without any external energy being supplied. “Theoretically, you could use this experimental set-up to solidify boiling water into ice without using any energy,” says Schilling.

Generate oscillating heat flows

The researchers used a so-called Peltier element for this - a component that is used, for example, to cool the minibar in hotel rooms. It has the ability to convert electrical currents into a temperature difference. With the help of such an element, in connection with an electrical inductance, the scientists generated an oscillating heat flow in previous experiments, in which the heat flow between two bodies constantly changes direction. At times, heat is also transferred from a colder object to a warmer object, so that the colder object cools down further. Such a "thermal oscillating circuit" actually contains a "thermal inductance". It works in the same way as an electrical oscillating circuit in which the electrical voltage oscillates with a constantly changing sign.

Physical laws remain intact

Until now, Schilling's team had only operated such thermal oscillating circuits with the addition of energy. The researchers have now been able to show for the first time that such a thermal oscillating circuit can also be operated passively - i.e. without any external energy supply. Thermal oscillations also occurred and over a period of time heat flowed directly from the colder copper to a warmer heat bath of 22 ° C without being converted into another form of energy in the meantime. The authors were also able to prove that no laws of physics were violated during this process. To do this, they calculated the change in the entropy of the entire system and showed that this increases over time - just as the Second Law of Thermodynamics requires.

Potential application still a long way off

The difference to room temperature achieved in the experiment was only just under 2 ° C, but this is mainly due to the properties of the commercial Peltier element used. According to Schilling, a (not yet existing) “ideal” Peltier element would be able to cool down to -47 ° C under the same conditions: “With this very simple technique, theoretically large amounts of hot material, regardless of whether liquid or gaseous, cool below ambient temperature without any expenditure of energy. » The passive thermal circuit could be used as often as desired without energy having to be supplied.

Schilling admits, however, that a large-scale application is not yet foreseeable. On the one hand, the currently available Peltier elements are not efficient enough. On the other hand, the current experimental setup requires superconducting inductances in order to keep the electrical losses as small as possible.

Old ideas turned upside down

For the UZH physicist, it is not just the first-time proof of principle that is important: "At first, the experiments seem like thermodynamic magic and thus to a certain extent shake our common ideas about heat flows."