My characters live on a tropical island on another planet. Although civilized, by their standards, they have not been through an iron age or bronze age since the metals oxidize when exposed to their air. They do not have electricity.
How can such people make ice? I have heard of yakchals in the desert being used to make ice. However, I believe this would only work in a hot desert environment. Is that true? Is there any other way of making ice in this world?
If they have the means to compress air, they can make both hot and cold.
Compressing a gas, plain old air would do, heats it up. This heat can then be allowed to exit into the environment. When the compressed air is released, it cools down. Enough of this cooling, will form water Ice.
It is easier to do if you have access to metals, and mechanization, and electricity.
But none of these is necessary, merely convenient.
A large clay or wooden or glass cylinder, with a tightfitting seal (think oversize bicycle pump) will do well enough.
Compress your air at one point, get rid of that heat, (running water comes to mind), then pipe the compressed air to another location, where it is released while in contact with the water you want to freeze.
You don't need extraordinary pressures for this to work. For example, nice sturdy glassware piping will do fine. Maybe even for the primary compression cylinder.
According to these handy online calculators:
Energy from compression change in Air
and
Latent heat of Fusion for water
If you compress 1.2m3 of air at 30C down to 1m3, let it cool, then release
you cool the air exiting to -3.1C
And absorb enough heat to freeze 125g of cold water to Ice, or cool 330g of water down from 30C to 0C (but still liquid)
Even a wooden cylinder can take 0.2 bar of pressure without strain.
The Hawaiian islands are probably the stereotypical tropical islands, yet they have plenty of ice on the top of their highest peaks, like Mauna Kea.
If they have a similar situation, they can simply harvest and transport ice from the peak, covered in straw to insulate it.
Producing ice by night cooling only works when the air is very dry and the sky clear, while usually tropical islands have high humidity.
If instead of ice you want to get cooling, they can use something akin to the pot-in-pot refrigerator which however do not work amazingly well in humid conditions
The effectiveness of evaporative cooling varies with the temperature, humidity and airflow. Given a constant flow of cool dry air, evaporative cooling can achieve temperatures as low as the wet-bulb temperature, the 100% humidity condition at the given temperature.
saltpetre
It can be found in caves -- the Romans found it there -- or dung heaps. It can be mixed with water to cool it, even to the point of freezing. It has to be pure water, though; you can't freeze ice cream with it, for instance.
