An ancient refrigerator in the desert
Long before electric refrigeration, engineers in Persia and Central Asia built massive conical structures called yakhdāns, which translates to "ice pits". These structures were storage units and sophisticated passive cooling machines capable of making and preserving ice through scorching summer months. Records show these structures were in use as far back as 400 BCE.
The visible part of a yakhdān is a large, cone-shaped dome, often reaching heights of over 20 meters. This dome is built over a deep, subterranean storage space which could have a volume of up to 5,000 cubic meters. The walls, sometimes up to two meters thick at the base, were constructed from a special water-resistant and heat-resistant mortar called sarooj. This mortar was a mixture of sand, clay, lime, ash, goat hair, and even egg whites, creating a highly effective insulating barrier. The entire structure's design minimized solar heat gain during the day.
Ice was typically "harvested" in the winter. Water, often supplied by underground aqueducts called qanats, was channeled into shallow pools located on the north side of the yakhdān. These pools were shaded during the day by tall walls. On clear, dry winter nights, the water would lose heat rapidly to the cold sky through a process called radiative cooling, allowing it to freeze even when ambient air temperatures were above 0°C. Workers would then collect the ice before sunrise, break it up, and move it into the deep pit for storage.
The physics of staying frozen
The yakhdān's design brilliantly exploits fundamental principles of thermodynamics to maintain freezing temperatures. The conical shape of the dome is an important element, is a solar chimney. Any warm air that enters the structure naturally rises to the top of the tall dome. Vents at the apex of the cone allow this warm air to escape.
This upward movement of warm air creates a continuous convection current. Cooler, denser air is drawn in through openings at the base of the structure, often channeled from the associated qanat system where it is further cooled by the underground water. This constant, passive airflow carries heat away from the stored ice, while the deep underground pit and thick earthen walls insulate the ice from ground heat. The combination of thick insulation, heat-reflecting shape, and natural convection allowed ice to remain frozen for months, providing a source of refrigeration for food preservation and for making chilled desserts like faloodeh through the summer.