The Pot-in-pot Cooler, based on by ancient techniques, was designed by Mohammed Bah Abba who won the Rolex Prize and who has been promoting this process thoughout Nigeria. The concept takes advantage of evaporative cooling. Two terra-cotta pots are used, one inside the other, allowing for approximately a 3-4cm (1.5-2in.) gap between the smaller and the larger pot. The gap is filled with sand and then saturated with water. Some of the water will seep into the clay pots as well.
Fruits and vegetables are placed inside the small pot, and the cooler is covered with a wet towel. As the water slowly evaporates from the cooler, heat is taken from clay pots, keeping the food a cool temperature inside. Water will need to be added to the sand depending upon the air temperature outside. Left in the shade, the Pot in Pot Cooler will keep foods fresh for some time.
I decided to make a “pot-in-pot” cooler and here are photos of it. The basic unit has two large unglazed pottery pots which I commissioned to be made without bottom holes by Panama Pottery in Sacramento. One pot fits inside the other with a layer of sand in between the pots. Alternately, I could have used normal pots and sealed their holes with wax or waterproof tape.
I am using my watering pot here to wet the sand layer, which I do every morning. Inside the inner pot, there is a watermelon, a cabbage, a bag of carrots and two apples.
I have a folded towel on top of the pots and produce and I am wetting the towel to provide additional evaporation through the cloth top. I moisten the cloth twice a day.
Folsom, California, is typically hot in the summer, but this is an exceptionally hot day at 109°F in the shade. It is an excellent opportunity to test this cooler.
Here the temperature inside the pot is 74°F degrees. It is 35°F degrees cooler inside the pot than outside. I have now had these fruits and vegetables in the device for a week and they are perfectly fresh. The daytime outside temperature has varied this week from the high of 109°F degrees to 90°F degrees and the temperature inside the pot has varied from 75°F degrees to 62°F degrees.
If you would like to learn more about this device, go to http://rolexawards.com/en/the-laureates/mohammedbahabba-the-project.jsp
Sharon Cousins discovered that it is important to weight the inner pot when initially moistening the sand or it will try to float and the sand will slip down and pile up underneath. This is a very effective way of keeping produce fresh, in many cases keeping vegetable fresh for longer than in the refrigerator. Sharon bought large plant pots on sale and used pieces of plastic jug glued on with Shoe Goo to seal the drain holes. She later discovered, much to her sorrow, that a Northern winter can destroy a pot-in-pot cooler unless it is stored somewhere where it can dry out for the winter (water in the somewhat porous unglazed clay freezes and can crack the pots), and they can be enormously heavy and hard to move. If you live where it freezes or will need to move your pot-in-pot for other reasons, get or make a sturdy base with rolling caster wheels (often sold for heavy plant pots) to help when it is time to move it, or put it together on a sturdy wagon you won't need for anything else (especially if you have one a child outgrew). But make sure it is a strong rolling base, because the weight of the clay and sand and water and vegetables really add up.