Everyone can appreciate the cooling effect of a cold, iced drink on a hot summer day. How did people in hot climates even survive before there was electricity and freezers? Surely it would have been impossible to have ice at least? Well, as a matter of fact it wasn’t, and people in the deserts of Iran were enjoying frozen drinks well before electricity, thanks to the Persian ice house. Although ice houses that store natural ice and snow harvested during the winter have existed in many countries, the Persian ice house is unique because of the way the ice was made in the desert at temperatures above freezing.
The process of ice making
Even though winter nights can be very cold in the deserts of Iran, temperatures rarely drop below freezing. But ice could be made even at temperatures just above freezing, thanks to a phenomenon known as night sky radiation or radiative cooling.
Outside the ice house there were shallow pools or channels, and these were shaded with walls during the day so that they remained as cool as possible. These pools were then filled with water on clear winter nights, and how much water was poured in the pool varied depending on how cold it was. The water came from the qanat system of underground aqueducts that brought water into the desert cities.
In the desert the sky is usually clear, and while this means that it gets very hot during the day, at night the clear sky can be used as a heat sink. A clear night sky has a very low temperature, and on clear nights surfaces facing the sky radiate heat into the sky. In this way the temperature of the water in the pool could drop below the temperature of the air, and on cold nights it would drop below 0°C, thus creating ice. The walls that provided shading during the day would also protect the water and ice from warm winds. The ice was then harvested before sunrise, and the process was repeated each night until the ice house was full.
Ice storage in the ice house
Once the ice was harvested, it was stored in the ice house, known in Persian as yakhchal. The ice was stored in a deep pit in alternating layers of ice and insulating straw. The structure of the ice house is very thick which helps keep it cool in the hot climate, thanks to the large thermal mass that regulates heat. The ice house has a domed roof, so that the hottest air would rise to the top, while the coolest air remained at the bottom where the ice was. Once the ice house had been filled, the building was sealed and only opened in the summer when the ice was used to make iced drinks.
One of the finest details is the hole at the top of the dome. The purpose of the hole is to let in daylight, because this eliminates the need for people to carry candles or other light sources that would heat up the space and melt the ice. But an uncovered hole would also let the cool air inside the ice house escape. The solution was to place a piece of marble on top of the hole, because the marble would block the air but let enough light pass through.
In this way every feature of the ice house ensured that the ice kept from melting as long as possible, and the ice could last until the end of summer. The Persian ice house and technique of ice making really show the potential that passive cooling without electricity can have when the local climate and principles of physics are understood.