It’s Not Rocket Science #3: Yakhchal

By 400BC, Persians had developed a system for making ice in winter and storing it throughout the summer and in a hot desert climate, in buildings they called yakhchal.

Most of what you’ll find written about yakhchal on the internet seems to come from wikipedia. Most diagrams come from one academic paper but images are from several private sources. Yakhchal and the idea of ice cream before refrigerators appeals to many people – some for its sheer ingenuity and inventiveness, some for reasons of cultural grandstanding, some for the genius of vernacular problem solving, and some for the low-energy aspects of energy storage that may have application today. Not least of all,

thermal energy storage using ice is practical because of the large heat of fusion of water. One metric ton of water, one cubic metre, can store 334 million joules (MJ) or 317,000 BTUs (93kWh or 26.4 ton-hours). In fact, ice was originally transported from mountains to cities for use as a coolant, and the original definition of a “ton” of cooling capacity (heat flow) was the heat to melt one ton of ice every 24 hours. [W]

The following is a summary of what’s involved.

1. Even cities in Persian deserts were serviced by a system of underground canals called qanat. Here’s how they work.
2. In winter, water from these qanat was led into channels and allowed to freeze overnight. High walls shaded these channels from the sun from the south and often from the east and west as well. The walls also protected the channels from the wind to facilitate freezing. Ice was made in layers over several evenings, and when it was about 50cm thick, was cut into blocks and stored in the domed yakhchal building. The door was sealed at a special ceremony and opened in summer at another.
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3. This all sounds very simple. However,if we have a piece of ice in shallow ponds with a determined height, and some water with a diameter equal to “s”, the following relationships can be expressed to determine the size of the ice made, where Eq 1 describes the cooling of water up to freezing point and Eq 2 describes the freezing of water in a layer of water at zero degrees.
   Eq 1: (Qr1+Qe1+Qc1-Qs1)∆t1= ρw*As*Cw*twi
   Eq 2: (Qr2+Qe2+Qc2-Qs2)∆t2= ρw*As*hif
   where:
   Qr1: transitive heat of radiation
   Qe1: transitive heat of evaporation
   Qc1: transitive heat of convection
   Qs1: transitive heat of water above ice surface
   ∆t1: Time to cool the water to zero degrees C
   ∆t2: Time to freeze the water
   ρw: Density of water
   As: Area of ice surface in cavity
   hif: Freezing enthalpy
   twi:  Primal temperature of the water
   Cw: Specific Heat of waterTherefore, the size of the ice the can be made can be calculated from Eq 3.
   S´=ρw * s/ρi* ∆t
   where ∆t=∆t1+∆t2 [thanks Mahdavniejad and Javanrudi] I only mention this to show that even just making the ice involves many factors that need to be understood empirically, if not theoretically. Just filling a hole with water on a winter’s night will not guarantee timely, sufficient and good quality ice.
4. The yakhchal storehouse itself was designed to keep the ice frozen for as long as possible and it did this by several means.
   1. Thermal mass: The section shows two parts – the dome and the pit beneath it. Warmer air rose and was vented from the top of the dome, whilst cool air remained in the underground portion that was already insulated by the ground. The pit could be as large as 5,000m³ and the dome span as much as 11m.
      
   2. Insulation: The walls of the dome were at least two metres thick at the base, and made of mud brick coated with a special waterproofing render composed of sand, clay, egg whites, lime, goat hair, and ash. This mortar had excellent insulating properties. I can’t find any information for how the optimum ingredients or mix for the mortar were discovered. I can imagine the goat hair may have functioned like the glass fibres do in fibreglass, but what properties do the egg whites add to the render? And how did anyone know they had those properties? There must be some lost science here because the idea of adding egg whites and seeing what happens is not random. Somebody thought about it and said “I think adding some egg whites might do the trick!”
   3. Evaporative cooling (?): Here, accounts differ. One of the most reblogged sentences about yakhchal states that the “continuous cooling waters that spiral down its side keep the ice frozen throughout the summer” but I could find no reference to these domes being shaped to allow water to spiral down the exterior, evaporating, and thus evaporatively cooling it as it “spirals down”. Presumably, somebody would have to climb onto the building and pour water over it. However, Hosseini and Namazian write that
      

      One of the advantages of these vaults was that they could be built step-like. They used stairs to help workers to cover the external crust of the vault with thatch to protect it from rain, snow, sun and atmospheric variations. They built smaller stairs between these stairs so workers could maintain or repair them easily.

      This seems more likely, especially since the domes don’t look especially spiral. (See also archnet.) However, it could just be that the stairs were used to cover the vault with straw during the day to prevent heat buildup, and the straw removed at night to facilitate night sky radiant cooling of the dome. We already know that the ancient (and not so ancient) Persians knew about night sky radiant cooling to make ice, so why not use it to cool the dome as well? Hosseini and Namazian say the straw is to “protect the dome from atmospheric variations” – and so it would during the day but, removing the straw at night would cause the dome to act in synergy with atmospheric variations.

Applications: The ancient Persians had already worked out how to passively cool  buildings. Their knowledge of orientation, sun angles and their invention of different types of wind towers for different uses is well documented. The most common type of wind tower draws air out of the building so it can be replaced by air cooled by those same underground qanat.  Such wind towers (called bagdir = Persian: بادگیر‎ bâdgir: bâd “wind” + gir “catcher”) were often positioned in fours around water cisterns. In the sense that yakhchal use the off-peak power to store energy in the form of ice, they are the forerunners of modern-day thermal energy storage systems. Since A/C was sorted, the manufactured ice was used in summer to cool drinks and make

Faloodeh, faludeh or fālūde (Persian: فالوده‎), which is a Persian cold dessert consisting of thin vermicelli noodles made from corn starch mixed in a semi-frozen syrup consisting of sugar and rose water and is often served with lime juice and sometimes ground pistachios. [W]

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The recipe for this has not changed since 400BC. At that time, faloodeh, ice cream and other chilled desserts and drinks would have been only for the wealthy only but, by 1870

there were plenty of yakhchals in Isfahan; some of them were for private use. Nevertheless, the poor could also use the yakhchal to cool water. Sherbets and fruit were preserved with ice in all shops. Huge chunks of ice were carried by donkeys and sold all over the province. In Isfahan, people could buy ice either in the bazaar or straight from the yakhchal building. (Ernest Holster*)

Further reading:

(1) “An Overview of Iranian Ice Repositories, An Example of Traditional Indigenous Architecture”, Bahareh HOSSEINI, Ali NAMAZIAN 

(2) “Historical Ice Houses: Remarkable Example of Iranian Cultural Heritage”, Amirkhani ARYAN, Okhovat HANIE, Pourjafar Mohammad REZA, Zamani EHSAN

(3) “An Overview of Some Vernacular Techniques in Iranian Sustainable Architecture in Reference to Cisterns and Ice Houses”, Amir Ghayour KAZEMI & Amir Hossein SHIRVANI 

(4) “Assessment of Ancient Fridges: A Sustainable Method to Storage Ice in Hot-Arid Climates”,  M. MAHDAVINEJAD, Kavan JAVANRUDI

*Ernest Holster was a German photographer kicking around Persia at the time.