Yakhchāl

A yakhchāl ( "ice pit"; yakh meaning "ice" and chāl meaning "pit") is an ancient type of ice house, which also made ice. They are primarily found in the Dasht-e Lut and Dasht-e-Kavir deserts, whose climates range from cold (BWk) to hot (BWh) desert regions.

In present-day Iran, Afghanistan, and Tajikistan, the term yakhchāl is also used to refer to modern refrigerators.

The structure typically had a domed shape above ground, a subterranean storage space, shade walls, and ice pools. It was often used to store ice, but sometimes was used to store food as well as produce ice. The subterranean space and thick heat-resistant construction material insulated the storage space year-round. These structures were mainly built and used since ancient times in Persia.

History
Records indicate that these structures were built as far back as 400 BCE, and many that were built hundreds of years ago remain standing, where Persian engineers built yakhchāls in the desert to store ice, usually made nearby. The ice created nearby and stored in yakhchāls is used throughout the year especially during hot summer days, for various purposes, including preservation of food, to chill treats, or making traditional Persian desserts like faloodeh and sorbets.

Although many have deteriorated over the years due to widespread commercial refrigeration technology, some interest in them has been revived as a source of inspiration in low-energy housing design and sustainable architecture. And some, like a yakhchāl in Kerman (over a mile above sea level), have been well-preserved. These still have their cone-shaped, eighteen meter high building, massive insulation, and continuous cooling waters that spiral down its side and keep the ice frozen throughout the summer.

Design
A yakhchāl's engineering is optimized to take advantage of the physics of evaporative cooling and radiative cooling, and the fact that the arid, desert climate is low in relative and absolute humidity. The low relative humidity increases the efficiency of evaporative cooling due to the vapor pressure differential, and the low absolute humidity increases the efficiency of radiative cooling because the water vapor in the air otherwise inhibits it. In addition, in some desert climates, like those at high altitudes, temperatures drop below freezing at night. Their design is generally split into three areas:


 * The ice house or reservoir
 * The shade walls
 * The ice pits or pools

However, they varied greatly, as some used all three components, whereas others were simply a large shade wall over a thin pool.

Ice house
Most yakhchāls operate like a traditional ice house. The tall, conical shape of the building is to optimize the solar chimney effect, creating a convection current to guide any remaining heat upward and outside through openings at the very top of the building. Through this passive process, the air inside the yakhchāl remains cooler than the outside. At the same time, the building allows cold air to pour in from entries at the structure's base and descend to the lowest part of the yakhchāl: large underground spaces up to 5,000 m3 in volume.

The yakhchāl is built of a unique water-resistant mortar called sarooj. This mortar is composed of sand, clay, egg whites, lime, goat hair, and ash in specific proportions, is resistant to heat transfer and is thought to be completely water-impenetrable. This material acts as effective insulation all year round. The sarooj walls are at least two meters thick at the base.

They also often have access to a qanat (Iranian aqueduct), and are sometimes equipped with bâdgirs (windcatchers or wind towers) built of mud or mud brick in square or round shapes with vents at the top which funnel cool air down through internal, vertically placed wooden slats to the water or structure below. A bâdgir can also function as a chimney, releasing warm air out the top and pulling cool air in from a base opening or a connected qanat (air in a qanat is cooled by the underground stream). It is this construction that allows the ice house of a yakhchāl to take advantage of evaporative cooling, keeping the structure cool to well below ambient temperatures.

The ice inside the structure was often separated using wood and straw to separate the layers of ice and keep them from sticking to each other. Furthermore, most designs incorporated a hole at the bottom that would connect back to the qanat, or simply act as a well for drainage.

Shade walls
The temperature differences between shaded and non-shaded area in most areas where the yakhchāls were constructed often have temperature differences of nearly 15 C-change to 20 C-change cooler, making shade walls necessary for production and storage, as well as giving workers extra time to harvest ice. A wall is usually built in an east–west direction near the yakhchāl, often as high as 10 m and sometimes as high as 15 m to minimize convection losses as well as to provide shade. Due to their height, the base of the walls were often significantly thicker, and in some design the walls were arched and/or buttressed in order to support the load (as pictured at the yakhchāl at Sirjan).

Water is often channeled from a qanat to a yakhchāl, which is used to fill the provisioning pools or used to power the evaporative cooling throughout the ice house. Incoming water is channeled along the north side of the wall so that radiative cooling in the wall's shadow pre chills the water before it enters the yakhchāl (as pictured at the yakhchāl at Kowsar). Ice is then brought from either the ice pools covered by the walls, or from nearby mountains to be stored in the reservoir.

Ice pools
Many yakhchāls contained ice pools. These pools were constructed to either provision the yakhchāl with water needed for it evaporative cooling to function, so ice could easily be prepped or transported to storage units within, or for the production of ice. Sometimes these pools were channels that were square in shape of dimensions roughly 100 m x 10 m with a depth of 40 cm to 50 cm, akin to a reflecting pool. Often, no special material was used to finish the channel surface.

By night time, the ice pools would often have a negative energy budget:
 * Heat conduction into the pool would be minimal due to the construction of the shade walls throughout the day and/or straw covering over the pool bed during the day.
 * Hot air convection towards the pools would be minimal, either by location or again due to the height and position of the walls.
 * Evaporation would take heat away from the pools, as with the rest of the yakhchāl.
 * Due to the low moisture content in the air, reflected radiated heat from the pool back onto the pool would be minimal, meaning that the pool's heat could be properly emitted.

This meant that ice pools could use the cold of the desert nights and/or radiative cooling to freeze water which would later be transported to storage as ice.