Brief description

The Zeer pot, also known as a pot-in-pot refrigerator, is an evaporative cooling device that does not require electricity. It keeps food fresh for much longer in hot, dry climates.

Use / Function

Its practical purpose is the preservation of perishable foods and cooling of water without electric power.

• Primary use: Extending the shelf life of vegetables, fruits, and liquids.
• Secondary uses: Cooling water for drinking, storing heat-sensitive medicines.
• Scale: Domestic, ideal for rural areas or survival situations.

Operating principle

It relies on evaporative cooling (simple thermodynamics):

1. Water contained in the wet sand between the two pots evaporates through the porous walls of the outer pot.
2. Evaporation is an endothermic process, meaning it absorbs energy (heat) from the surroundings to change from liquid to gas state.
3. As it evaporates, the water draws heat from the inner pot and its contents, reducing the temperature.
4. The temperature can drop significantly below ambient temperature, depending on the relative humidity of the air.

How to create it

Minimum functional version

Two porous clay (terracotta) pots of different sizes are required, such that one fits inside the other leaving a gap of about 2-3 cm (1 inch).

1. Plug any drainage holes in the bottom of the pots with clay, cork, or wax.
2. Place a layer of sand at the bottom of the large pot.
3. Place the small pot inside the large one, centering it on the sand.
4. Fill the gap between the two pots with clean sand, up to the rim.
5. Pour water into the sand until it is fully saturated, but ensure no water enters the inner pot.
6. Place food inside the inner pot.
7. Cover everything with a damp cloth.

Required technical level

Basic. Requires no complex tools or advanced knowledge, just simple materials.

Materials needed

• Essential materials:
  • 2 clay pots (unglazed terracotta) of different sizes.
  • Sand (preferably fine and clean).
  • Water.
  • Cloth or fabric (to cover).
• Tools:
  • None specific (hands, container for pouring water).
• Possible substitutes:
  • The inner pot can be metal (conducts cold better) or plastic (though glazed clay is ideal for the interior to be waterproof while the exterior must be porous). Note: For classic operation, the outer pot MUST be porous.

Variants and improvements

• Early versions: Using amphorae buried in wet sand.
• Modern versions: Using solar fans to increase the evaporation rate.
• Improvements: Elevate the fridge on a permeable base to allow airflow underneath. Place it in a breezy, shaded spot.

Limits and risks

• Climate dependence: Only works efficiently in dry, hot climates. In climates with high relative humidity, evaporation is slow, and cooling is minimal or null.
• Maintenance: Water must be added to the sand regularly (typically twice a day).
• Does not freeze: It does not replace a freezer; it lowers temperature but does not freeze.
• Risks: If the sand dries out, the effect is lost. If hygiene is poor, mold can grow on the damp cloth (wash the cloth regularly).