The Second Law of Thermodynamics is bent but not broken.
The second law of thermodynamics is safe. A new device — which could theoretically turn boiling water to ice, without using any energy — only ‘appears’ to break this fundamental rule of physics
Physicists at the University of Zurich have developed a deceptively simple device that allows heat to flow temporarily from a cold object to a warm object without an external power supply. The process initially appears to contradict the fundamental laws of physics — primarily, the second law of thermodynamics.
If you put a cup of hot coffee in a room of average temperature, the drink will gradually cool down — eventually reaching the same temperature, there or thereabouts as the air is the room. We describe this as it reaching thermal equilibrium with the room.
What he shouldn’t expect to see is heat continue to flow from the cup after equilibrium is reached. The temperature of the coffee is not expected to fall below that of the room.
This everyday experience demonstrates 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 put: Heat can flow by itself only from a warmer to a colder object — not the other way round.
Cooling below room temperature
The results of a recent experiment carried out by the research group of Prof. Andreas Schilling in the Department of Physics at the University of Zurich (UZH) appear at first sight to challenge the second law of thermodynamics.
The researchers managed to cool a nine-gram piece of copper from over 100°C to significantly below room temperature without an external power supply.
Schilling says: “Theoretically, this experimental device could turn boiling water to ice, without using any energy.”
To achieve this quite remarkable result the team used a Peltier element — a heat pump which transfers heat from one side of a device to another. Peltier elements are commonly used, for example, to cool minibars in hotel rooms.
These elements can transform electric currents into temperature differences. The researchers have 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 such a scenario, heat also 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.
Fear not: Laws of physics remain intact
Schilling’s team — who had had only operated these thermal oscillating circuits using an energy source — have now shown that this kind of thermal oscillating circuit can also be operated “passively” 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.
The authors were also able to show that the process does not actually contradict any laws of physics. To prove that the second law of thermodynamics remains intact — 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.
The potential application still a long way off
Although the team recorded a difference of only about 2°C compared to the ambient temperature in the experiment — mainly due to the performance limitations of the commercial Peltier element used.
Schilling believes that it would be possible in theory to achieve cooling of up to -47°C under the same conditions were the “ideal” Peltier element to be used. Unfortunately, such an element has yet to be created.
Schilling continues: “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.”
Although the passive thermal circuit could also be used as often as desired, without the need to connect it to a power supply, Schilling admits that a large-scale application of the technique is still a long way off.
One reason for this is that the current generation of Peltier elements simply aren’t efficient enough. In addition to this, the current set-up requires the use of superconducting inductors to minimize electric losses.
Despite these limitations, Schilling considers the work more significant than a mere “proof-of-principle” study.
He concludes: “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.”
Original research: http://dx.doi.org/10.1126/sciadv.aat9953