Thermodynamic “magic” could cool with no energy consumption

SWITZERLAND: Physicists at the University of Zurich have developed a device which could, in theory, turn boiling water to ice, without using any energy.

It allows heat to flow temporarily from a cold to a warm object without an external power supply. Intriguingly, the process initially appears to contradict the fundamental laws of physics.

The researchers managed to cool a 9g piece of copper from over 100°C to significantly below room temperature without an external power supply. 

“At first sight, the experiments appear to be a kind of thermodynamic magic, thereby challenging to some extent our traditional perceptions of the flow of heat,” explained Professor Andreas Schilling of the department of physics at the University of Zurich.

“Theoretically, this experimental device could turn boiling water to ice, without using any energy.”

To achieve this, the researchers used a Peltier thermoelectric cooler, a component commonly used to cool minibars in hotel rooms. 

The researchers had already used this type of element in previous experiments, in connection with an electric inductor, to create an oscillating heat current in which the flow of heat between two bodies perpetually changed direction. In this scenario, heat temporarily flows from a colder to a warmer object so that the colder object is cooled down further. This kind of “thermal oscillating circuit” in effect contains a “thermal inductor”. It functions in the same way as an electrical oscillating circuit, in which the voltage oscillates with a constantly changing sign.

Until now, Schilling’s team had only operated these thermal oscillating circuits using an energy source. The researchers have now shown for the first time that this kind of thermal oscillating circuit can also be operated “passively”, ie with no external power supply. Thermal oscillations still occurred and, after a while, 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 were also able to show that the process does not actually contradict any laws of physics. To prove it, they considered the change in entropy of the whole system and showed that it increased with time – fully in accordance with the second law of thermodynamics.

While a difference of only about 2°C compared to the ambient temperatures achieved, this was mainly due to the performance limitations of the commercial Peltier element used. According to Schilling, it would be possible in theory to achieve cooling of up 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 solid, liquid or gaseous materials could be cooled to well below room temperature without any energy consumption,” he said.

The passive thermal circuit could also be used as often as desired, without the need to connect it to a power supply. 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. Furthermore, the current set-up requires the use of superconducting inductors to minimise electric losses.