A Yakhchal was an ancient type of ice house that functioned using evaporative cooling. Above ground, the structure had a domed shape, but had an underground. This was often used to store ice, but sometimes was used to store food as well. The underground space, combined with the thick, heat-resistant material, insulated the storage space year round. The structures were mainly built and used in ancient Persia, and many that were built hundreds of years ago are still standing.

How they worked

A yakhchal takes advantage of the low humidity in desert climates which promotes the evaporation of water (making evaporative cooling more effective) and promotes rapid cooling once the sun sets (water vapor inhibits radiative cooling in less arid climates). In some deserts, temperatures drop below freezing at night. Water was often channeled from a qanat (Iranian water delivery system) (possibly a topic for another megathread) to the yakhchal, where it freezes when the temperature is low enough. A wall is usually built in an east–west direction near the yakhchāl. Incoming water is channeled along the north side of the wall so that radiative cooling in the shadow of the wall cools the water before it enters the yakhchāl. Ice was also sometimes brought in from nearby mountains and stored in the yakhchāl.

The building allows cold air to pour in from entries at the base of the structure and descend to the lowest part of the yakhchāl, large underground spaces up to 5,000 m3 (180,000 cu ft) in volume. At the same time, the tall conical shape of the building guides any remaining heat upward and outside through openings at the very top of the building, and through this active process the air inside the yakhchāl remains cooler than the outside.

The heat-resistant material that allowed the yakhchal to function was known as sarooj. This was composed of sand, clay, egg whites, lime, goat hair, and ash in specific proportions. Sadly, this means that sarooj was not :im-vegan:, but I’m sure you can find substitutes for the egg whites. It was, however, very resistant to heat transfer, and it was thought to be impossible for water to travel through it. Sarooj was effective year-round, and could be as much as two meters thick at the base.

The ice stored within was used year-round, especially during hot summer days. Some uses would be preservation of food, to chill treats, or to make faloodeh, a traditional Persian frozen dessert.

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