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?