In the arid deserts of ancient Persia (modern-day Iran), a remarkable feat of engineering allowed people to beat the scorching heat and enjoy cool refreshments, even during the hottest summer months. The ingenious solution was the “Yakhchal“, a type of evaporative cooler and ice house that harnessed the principles of radiative cooling to create and store ice.
Yakhchāls are ancient Persian ice houses that were used to store ice and sometimes food. They have a domed shape above ground, a subterranean storage space, shade walls, and ice pools. The structures take advantage of the physics of evaporative cooling and radiative cooling in the arid desert climate. Yakhchāls have been built since at least 400 BCE and many still remain standing today. They are constructed using a unique water-resistant mortar called sarooj, which acts as effective insulation. The tall, conical shape of the building optimizes the solar chimney effect to keep the interior cool. Shade walls, often as high as 10-15 meters, are built near the yakhchāl to minimize convection losses and provide shade. Water from a qanat is channeled along the north side of the wall to pre-chill it before entering the yakhchāl. Many yakhchāls also contained ice pools, which were used to provision the structure with water for evaporative cooling, to prep or transport ice to storage, or to produce ice. The pools could freeze water overnight due to their negative energy budget resulting from the desert’s cold nights and radiative cooling.
Architectural Design and Working Principle
A typical Yakhchal consisted of a domed structure above ground, with a deep, subterranean storage chamber below. The thick walls, often over 2 meters thick at the base, were constructed using a unique water-resistant mortar called “sarooj,” composed of sand, clay, egg whites, lime, goat hair, and ash. This special mixture provided excellent insulation, preventing heat transfer from the outside.
The key to the Yakhchal’s cooling ability lay in its clever design, which took advantage of the principles of evaporative cooling and radiative cooling, as well as the arid desert climate’s low humidity levels. During the winter months, water from nearby qanats (underground aqueducts) was channeled into shallow pools or trenches adjacent to the Yakhchal. At night, when temperatures dropped below freezing, the water in these pools would freeze due to radiative cooling, where heat is lost through infrared radiation to the cold night sky. The process was aided by the low humidity in the desert air, which facilitated more efficient radiative cooling.
Over several nights, layers of ice would accumulate in the pools, eventually reaching a thickness of up to 50 cm. The ice was then cut into blocks and transferred to the underground storage chamber of the Yakhchal, where the thick insulating walls and the cooling effect of the qanat water kept the ice frozen throughout the scorching summer months.
Enhancing Cooling with Windcatchers and Shading
Many Yakhchals incorporated additional features to enhance their cooling capabilities. Some had badgirs (windcatchers) or wind towers that funneled cool air from the qanat into the storage chamber, further lowering the temperature through evaporative cooling. Strategically placed shade walls were also common, shielding the ice pools and the Yakhchal itself from direct sunlight during the day. This prevented the absorption of solar heat, which would have hindered the freezing process and melted the stored ice more quickly.
Applications and Legacy
The ice produced and stored in Yakhchals served various purposes, from preserving perishable foods to making refreshing treats like faloodeh (a traditional Persian frozen dessert) and chilled sherbets for royalty during the sweltering summer months.
Faloodeh – a traditional Iranian cold dessert
While modern refrigeration technology has largely replaced the need for Yakhchals, these ancient structures remain a testament to the ingenuity and resourcefulness of Persian engineers. Their principles of passive cooling through radiative and evaporative processes have inspired modern architects and engineers in the pursuit of sustainable, energy-efficient building designs. The Yakhchal’s ability to create and maintain cool environments in the harshest of desert climates, without relying on external energy sources, aligns remarkably well with the goals of the MIRACLE project.
By developing photonic metaconcrete with radiative cooling capabilities, MIRACLE aims to revolutionize the construction of nearly zero-energy buildings, much like how the Yakhchals provided passive cooling solutions centuries ago.
As we strive for more sustainable and energy-efficient technologies, the ancient wisdom embodied in the Yakhchals serves as a reminder that nature itself can offer ingenious solutions, if we are willing to observe, understand, and harness its principles.