Deserts, by and large, are hot places. By definition they are arid zones, but such places are typically arid because of their high ambient temperatures and water-wicking winds. And as any child will tell you, the great danger for those in the desert is dehydration, and dying of thirst.
Even in the modern day these vast tracts of inhospitable land pose extreme challenges to those who would try to cross them, or even more implausibly survive in them. And yet, people do so. And not just in the present: they have been doing so for thousands of years.
Just how do you survive in the desert? Well, if you lived in Persia around 2,000 years ago, the answer was a yakhchal. Somehow the ancient Persians were able to make ice in the great deserts of their empire.
Sophisticated Temperature Control
A yakhchal is an ice house, a carefully designed and built storage chamber designed to defend the contents against the heat of the day. Remains of yakhchals have been found which date as far back as 400 BC: these structures were built to last, and many appear to have been in constant use for centuries.
The basic design is one of a dome with an open hole at its peak and a sunken interior. A combination of the central dome structure and a system of surrounding walls providing shade meant that water held in pools within could be cooled to the point where it formed ice.
This seems impossible, but a combination of the extreme insulation against the desert heat provided by the thick walls and the water itself inside meant that the yakhchal could reach such low temperatures. But this by itself was not enough: a yakhchal also used the desert’s very nature against it.
It is the arid conditions of a desert which provided the yakhchal with its secret ingredient. The extremely low humidity of the surrounding air promoted radiative cooling from the dome structure, colling the interior in a way which would not be possible in a damper environment.
The ready evaporation of some of the water from the interior, through the hole at the top of the dome, cooled the rest. This, combined with typically high winds which would remove the evaporated water efficiently from the surrounding area, meant that the temperature inside could drop below freezing.
Many structures also took advantage of the extreme cold which deserts typically experience at night. The radiative cooling would continue after dark (the outside air was still very dry) but the mean temperature would drop, and this boost to refrigeration would allow the ice to persist through the day.
The ice pools themselves also form part of the cooling mechanism, providing the water for the evaporative cooling and acting similarly to a radiator, only in reverse. This meant that, alongside the ice held in the interior of the dome, other goods could also be stored. The Persians built their empire on frozen food, and a particular favorite was sorbet.
In point of fact, yakhchal do not need this level of complexity to be effective. A simple shade over sufficient water will produce a cooling effect as the water evaporates into the dry surrounding air. But the yakhchal of the ancient Persians reached an astonishing degree of sophistication, a significant driver in their expansion and success.
Yakhchal survive to this day and are still effective means of ice generation in the desert. Modern energy efficient housing has also tapped the technology of ancient Persia: sometimes modern problems require ancient solutions.
Top Image: A traditional yakhchal in Iran. Source: Shirin57 / CC BY-SA 4.0.
By Joseph Green
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