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Intuitively, it seems like heat engines and refrigerators require a good bit of technology to make a working substance pass through different thermodynamic states and then finally return to the initial state. Usually, at the very least, some tubing and pumps are involved. Thus such devices appear to be pretty special occurrences in nature, requiring a human to make them.

On the other hand, there are all sorts of material cycles in nature, like the water cycle and the carbon cycle. But I'm not sure if those constitute thermodynamic cycles.

So the question is, are there any genuine thermodynamic cycles that are not man-made? Of course we will count approximate cycles, because even man-made cycles are only approximate. Refrigerators are constantly leaking a few molecules of refrigerant, for example. And the initial state may never be precisely reached again, although it gets close enough to be functional.

EDIT: What makes me hesitate to immediately consider the material cycles to be examples of thermodynamic cycles is the difficulty in identifying a thermodynamic system that stays together through the whole cycle. The system has to be small enough that it takes definite values of thermodynamic quantities such as temperature and pressure at a given time. But over time matter gets thoroughly mixed together, making it hard for the initial system to stay together and remain plot-able on a thermodynamic diagram. Perhaps there is a conceptual way around that issue that I haven't thought of yet.

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    $\begingroup$ would you regard the heat engine of a tropical storm acting in a 'cycle'? $\endgroup$
    – Neil_UK
    Commented Jul 3 at 13:04
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    $\begingroup$ From what I understand, what you expect is for the physical body to be in equilibrium every so often. This is not achievable in real world, complex problems, except when you confine your system to be small enough (one molecule, maybe?). In general, you have nonuniform (even chaotic) events that persists throughout the thermodynamic cycle that it is practically impossible for individual components of the cycle to "go through it together". Indeed, this may be one good reason why you don't usually see thermodynamic plots for these phenomena. $\endgroup$
    – hendlim
    Commented Jul 3 at 23:43
  • $\begingroup$ @hendlim I am coming around to the view that even man-made cyclic processes exhibit the same mixing effect that I brought up regarding the water cycle. $\endgroup$
    – ether
    Commented Jul 4 at 2:56
  • $\begingroup$ @Neil_UK Perhaps I need to expand my idea of what constitutes a thermodynamic cycle. I had been thinking of the working substance as a closed system, but maybe it is better to think of open systems. As I understand it, the storm is constantly sucking up new moist warm air and then spitting out colder dry air. The same air is not reused. Similarly with the water: the ocean acts as a reservoir of water, with only a small amount of it passing through the storm. $\endgroup$
    – ether
    Commented Jul 4 at 3:14
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    $\begingroup$ "Life" as the hardest cyclers: A New Physics Theory of Life: An MIT physicist has proposed the provocative idea that life exists because the law of increasing entropy drives matter to acquire lifelike physical properties. (Paper, Talks) $\endgroup$
    – David Tonhofer
    Commented Jul 4 at 7:22

2 Answers 2

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As its name suggests, a thermodynamic cycle is a sequence of thermodynamic processes (e.g. heating, cooling, expansion, contraction, etc.) that happens, well, in a cycle. A man-made machine is one example, but thermodynamic cycles are everywhere in nature. You should be able to spot a thermodynamic cycle by looking at whether a thermodynamic quantity is involved and whether it changes in a cycle.

My favorite example is probably ocean circulation, which is partly driven by the ocean temperature (a thermodynamic quantity) and salinity that affect the water density (another thermodynamic quantity). Here is one great video explaining ocean circulation qualitatively.

Another example I can think of is the water cycle. Water on bodies of water heats up due to the sun, evaporates, condensates into clouds when it is cold enough up there, hangs out in the sky, and then comes back down as rain when it gets dense enough, completing the cycle. In this cycle, the water undergoes a thermodynamic cycle in which it undergoes phase transitions from liquid, to gas, back to liquid. You should admit that it is cooler when it is cloudy, which makes the water cycle one, big air cooler.

Why do textbooks provide examples using man-made machines? I am not a book writer, myself, so I would not know. However, I can say that man-made machines are simpler than whatever goes on in nature, which may sound more abstract when we try to spot the thermodynamic part of the process. The mathematical description is also simpler since we can describe man-made machines pretty well using simple thermodynamic cycles such as the Carnot cycle, Otto cycle, etc.

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    $\begingroup$ In a water cycle, the water itself undergoes the thermodynamic cycle: it evaporates into water vapor and condenses back into liquid water, which makes a cycle. In ocean circulation, salt should be considered a part of the system since it plays a part in changing the density of the water. You may also consider the salt in a water cycle: evaporation is due to heat of the sun and salt is responsible for changing the evaporation rate. However, I would say the effect is so minimal that you may as well ignore it. $\endgroup$
    – hendlim
    Commented Jul 3 at 4:51
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    $\begingroup$ If you want to model something, it is inevitable for you to decide on what you would consider as the system. You may model it however you see fit. For example "one kilogram of water" is a legitimate physical model. You then have one kilogram of water plus the sun as a basic system and from there you may add everything else (wind, salt, etc.) to make your model more complicated, but more realistic. As I stated in my answer, I think your usual textbook does not use the phenomena I mentioned because they are more complicated than man-made machines, hence more effort to model correctly. $\endgroup$
    – hendlim
    Commented Jul 3 at 5:29
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    $\begingroup$ Then you consider either one drop of water or all the water that is spread out all over the world. The choice is yours. Being spread out means that another interaction is at play, e.g. wind. You can technically observe a simple water cycle by containing some amount of water in an isolated container where the bottom part is heated and the upper part is cooled. The water will evaporate and the vapor will condense near the top, drop back down via the wall of the container, and repeat the cycle. You may not observe any cloud, though. $\endgroup$
    – hendlim
    Commented Jul 3 at 5:42
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    $\begingroup$ I think Cepheid variable stars constitute thermodynamic cycles. They have a well defined photosphere radius (and thus volume) and surface temperature that very cyclically. $\endgroup$
    – shockburner
    Commented Jul 3 at 15:31
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    $\begingroup$ @ether For a standard refrigerator the refrigerant doesn't stay in a single coherent thermodynamic state through the cycle. Instead, its a continual process and different bits of refrigerant take on different thermodynamic states at different points in time. In advanced systems, you have multiple cooling loops and split distribution and multiple condensers, such that any particular drop of refrigerant doesn't even have a consistent path through the system. -- Similar things apply for the water cycle, just with additional layers of complexity, as befits a world-spanning process. $\endgroup$
    – R.M.
    Commented Jul 3 at 17:10
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Growing up near Yellowstone, my mind goes to geysers. Simplified:

  1. Water fills up underground chamber
  2. Heat converts water to steam
  3. Steam pressure ejects water
  4. Repeat

As a biologist, I also think of plant photosynthesis during the day/respiration at night, plankton vertical migrations in response to sunlight, etc, but I'm not sure these would count from a strict physics perspective.

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