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I'm trying to design a culture that has access to cooling as a primary source of food preservation.

This culture is set in the desert. They will have access to a magical stone that has the ability to create stable volume "bubbles" off themselves. These bubbles would be robust, but temperature permeable. This could be used to say, hold a bubble full of water, rock, or food without risk of breaking or cracking. (Yeah, I get that I could just make them "cooling" stones, but there is a narrative reason for them working in this way specifically).

The question is, what is a simple low tech way to give this culture easy access to freezing? Do they need access to electricity? I'm also hoping to be able to apply isochoric-freezing into this if possible.

I know temperature differences are important for normal freezing technologies, and pressure plays a role as well. Would it be possible to have a passive system using heat from the sun and temperature difference below the surface? If so, what sort of temperature differences are needed? How much of your system would be needed to bring to freezing temperature a certain volume of space?

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  • $\begingroup$ Isoochoric means "constant volume." I'm thinking that the equipment required to do isochoric freezing would not qualify as "low tech." You need to be able to confine the freezing material so it does not expand. That's because expanding ice crystals are a big part of why freezing damages many materials when frozen. And you need to be able to measure the expansion. That's not "low tech" stuff, especially desert nomadic type stuff. $\endgroup$
    – puppetsock
    Jul 7 at 18:08
  • $\begingroup$ Yeah -- ever seen the demo where a cast iron sphere is filled with debubbled water, plugged, and frozen (with ice and salt, or better dry ice)? Hint: wear safety glasses or better yet stand behind a Mythbusters shield. $\endgroup$
    – Zeiss Ikon
    Jul 7 at 18:13
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    $\begingroup$ Also see en.wikipedia.org/wiki/… . tl;dr it was possible to profitably ship ice in wooden ships over long distances with only low tech insulation. Therefore, depending on how flexible your scenario is, the transport tech available, and how cool the winter months of your desert is, transportation of ice for local storage and use may be possible from a distant location. $\endgroup$ Jul 7 at 19:28
  • $\begingroup$ Thanks for everyone's ideas! Specifically helpful comments from @GrumpyYoungMan I'll likely end up using a combination of Yakhchāl and night ice skimming methods. $\endgroup$ Jul 8 at 13:56
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Desert cultures made and stored ice centuries before the Romans harvested snow from the mountains for their feasts.

The key is that the desert environment loses heat rapidly to the sky after dark; rapidly enough that a skim of ice can form on a shallow puddle by dawn, even when the previous day and the next will approach 50 °C. This ice is gathered before the new day begins to warm and stored (straw for insulation, and an underground chamber with carefully limited airflow, so it collects cold air like an ice cave).

End result is that when weather conditions are right, it's possible for a single family to produce tens of kilograms of ice each night, and accumulate hundreds of kilograms per month -- in a climate where you can literally cook eggs on exposed rocks in the afternoon.

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    $\begingroup$ Also remember that not all deserts are extremely hot all year around. Many (like much of the western US) are located in temperate latitudes and at relatively high elevations, meaning they experience cold winters. Winter ice can be stored in an ice house or similar. $\endgroup$
    – jamesqf
    Jul 8 at 4:42
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    $\begingroup$ There are even deserts that are covered in permafrost and glaciers. One of the best known is called Antarctica. $\endgroup$
    – Aron
    Jul 8 at 10:07
  • $\begingroup$ The specific technique I referenced is one I read about in Scientific American 30-40 years ago, specific to the Arabian Peninsula and Iraq/Iran. $\endgroup$
    – Zeiss Ikon
    Jul 8 at 11:06
  • $\begingroup$ "the desert environment loses heat rapidly to the sky after dark; rapidly enough that a skim of ice can form on a shallow puddle by dawn" Critically because the desert is also extremely dry. The puddle loses most of its heat through evaporation. $\endgroup$
    – J...
    Jul 8 at 14:22
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    $\begingroup$ @ZeissIkon No, agreed, both are equally important. If it's humid it won't work. If it's cloudy it won't work. $\endgroup$
    – J...
    Jul 8 at 14:44
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While I don't know of any in desert regions, in some areas there are sinkholes that have an outflow of cold air that can even cause ice to build up.

Here is one example, https://en.wikipedia.org/wiki/Coudersport_Ice_Mine. This is real natural feature, not a fake tourist trap.

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    $\begingroup$ I've been inside ice caves in Idaho. The ones there seem to depend on density trapping rather than the heat exchange postulated for Coudersport. $\endgroup$
    – Zeiss Ikon
    Jul 7 at 18:01
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The rapid release of pressure

Have you ever wondered how your air conditioner, refrigerator, or vehicle a/c actually work? Ever wondered why that can of compressed air you used to clean your keyboard became uncomfortably cold? Electricity isn't needed. Ice can be manufactured in a completely mechanical way:

Rapid decompression is endothermic

The result is cold, and when you do it with enough pressure or enough times, what you get is freezing temperatures.

So what your people need to do is use a bellows to fill a bladder in a confined space (call it an "ice house"), they pump up the bladder, then twist a wooden valve to release the pressure all at once. Repeat as necessary.

And that's all your (e.g.) car A/C is doing. The motor runs a compressor that forces the refrigerant into a high pressure area, which leads to the cooling vanes where that pressure is decompressed rapidly.

As an aside, electricity-dependent cooling can't be done in many ways

In fact, I can only think of one off the top of my head, the use of a thermoelectric generator.

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  • $\begingroup$ This is equivalent to 19th century refrigeration -- air was compressed, cooled in a water bath heat exchanger, then expanded to chill brine which was then used in another heat exchanger to freeze fresh water. This could make tons of ice in a day, if large enough (which is why modern refrigeration and A/C units are sometimes rated in "tons"). Problem is, it requires metal tubing in quantity and the ability to compress air much more than bellows can do (you don't get enough temperature change for this below about 10 bar). $\endgroup$
    – Zeiss Ikon
    Jul 8 at 11:22
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    $\begingroup$ @ZeissIkon You're making one of the classical blunders! The first is never fight a land war in Asia! The second, and only slightly less famous, is that real life shouldn't be an overriding limitation for any question unless specifically requested by the OP. The goal is suspension of disbelief, not can-be-done-in-real-life (the whole point of the site is to diverge from real life...). $\endgroup$ Jul 8 at 15:44
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    $\begingroup$ Oh, sure, and next you'll be telling me you're Sicilian. $\endgroup$
    – Zeiss Ikon
    Jul 8 at 15:51
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gas-absorption refrigeration technology has been known of for over 150 years. The Crosley IcyBall was marketed 90 years ago for home refrigeration before electricity was commonly available. This would not be passive, a source of heat such as a kerosene burner was needed but in the desert solar heating may be enough.

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  • $\begingroup$ Given a large enough reflector, you can concentrate even weak sunlight sufficiently to vaporize metals, so solar heat would definitely be sufficient. $\endgroup$
    – jwdonahue
    Jul 9 at 0:41
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Heat pipe. Take a 30 foot long chuck of standard 2 7/8" gas pipe. Weld a cap on one end. Other end put a cap with a good valve on it.

Put one end in the ground roughly midway. Fill about half full with propane. Allow propane to boil until about 1/3 of the propane boils off. Close valve.

Put a ring of these around your cold cellar.

In operation in winter when it's cold, propane will boil at the hot (lower) end of the pipe, hit the cold end, condense, and run back down. Net result is that you have a plug of ground that is roughly at the temperatue of the coldest month of the year.

In summer when it's warmer at the upper end, the propane just sits there.

The working fluid can be any stable material that is a gas at storage temperatures, but liquifies under pressure at that temperature. Propane, ether, butane are simple molecules that work.

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Magnetization cooling

Put a suitable material (Gadolinium for example) in between two permanent magnets. The magnetic moments of the Gadolinium will align with the magnetic field of the magnets and slightly heat the Gadolinium. Let it cool down to ambient temperature. Then, move the permanent magnets away from each other. The magnetic field which the Gadolinium experiences decreases. The magnetic moments can reorient themselves, absorbing thermal energy in the process. Now you have Gadolinium which is below ambient temperatures.

Note: Usually you would use electromagnets to create strong magnetic fields. I am not sure how well the permanent magnets would work.

EDIT: There seems to be some concern that this technology is not low-tech enough. From my understanding, this technology can be operated by hand and does not require any sophisticated machinery. You could even hold the magnets with bare hands and move them appropriately, even though I think you would have to repeat the cycle many times to accomplish sufficient cooling.

However, producing the materials in the first place can be quite challenging. Gadolimium has been discovered in the late 19th century, Manganese, Nickel and Copper (suitable replacement for Gadolinium) have been used for thousands of years, albeit without being recognized as such. So if the culture is sufficiently smart, they could identify and use those materials likely without high-tech equipment.

Another option would be to have external trade of some kind where the production can happen elsewhere. We see this quite often in the real world where researchers in developed countries try to invent low tech technology to be used in other regions of the world where people do not have sufficient resources to procure and continually use the established high tech solutions.

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    $\begingroup$ The effect was first discovered in 1881, and a room-temperature proof-of-concept didn't arrive until 1997. Is this really feasible for a low-tech society? $\endgroup$ Jul 8 at 13:05
  • $\begingroup$ Low tech freezing system does not mean the society is not smart. And the 2001 proof of concept was only so late because they used industrial grade materials and permanent magnets. The first working magnetization cooling fridges were reportedly constructed in 1933 (not sure about temperature range and pretty sure electromagnetic). $\endgroup$
    – laolux
    Jul 8 at 13:16
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    $\begingroup$ Absolutely agree that low-tech does not preclude intelligence, but when the example requires an exotic material that wasn't even discovered until the late 19th century using spectroscopy, I'd say we're well past "low-tech". The fact that it took a high-tech society a century between the theory and implementation a room-temperature system suggests that there are significant engineering challenges in making this work. $\endgroup$ Jul 8 at 14:08

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