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The pot-in-pot refrigerator or zeer (الزير) is a refrigeration device which keeps food cool without electricity by using evaporative cooling. A porous outer earthenware pot, lined with wet sand, contains an inner pot (which can be glazed to prevent penetration by the liquid) within which the food is placed. The evaporation of the outer liquid draws heat from the inner pot.

Although the principle behind the device has been known for centuries, one modern device was developed by lecturer Mohammed Bah Abba for use in modern Nigeria and distributed by his company, Mobah Rural Horizons. . Mohammed Bah Abba received a Rolex 2000 Laureate, Applied Technology http://www.rolexawards.com/profiles/laureates/mohammed_bah_abba

The devices can be constructed with simple components and are beneficial for areas of the developing world where electricity is unreliable or non-existent.

History
There is some evidence that evaporative cooling was used as early as the Old Kingdom of Egypt, around 2500 B.C. Frescos show slaves fanning water jars, which would increase air flow around the porous jars and aid evaporation, cooling the contents. Many earthenware pots were discovered in Indus Valley Civilization around 3000 BC which were probably used for storing as well as cooling water similar to present days ghara or matki used in India and Pakistan.

Despite being developed in Northern Africa, the technology appears to have been forgotten with the advent of modern electrical refrigerators. However in the Indian Subcontinent, ghara, matka and surahi, which are different types of earthenware water pots are used to cool the water. In, Spain the popular botijos, porous clay containers to keep and to cool water, have been in use for centuries (and are still relatively widespread), working in the same way as the pot-in-pot refrigerator, favored by the low Mediterranean climate humidity; locally the cooling effect is known as "botijo effect".

The technique of using evaporative cooling with clay pots for food preservation was re-discovered by Mohammed Bah Abba, a Nigerian teacher from a family of pot-makers, who patented a pot-in-pot refrigerator in 1995 to help Sudanese families preserve food. Bah Abba was awarded the $75,000 Rolex Award for Enterprise in 2000 and the World Shell Award for Sustainable Development in 2001 for its development.

Development and distribution
Mohammed Bah Abba developed his zeer based on his grandmother's craft of traditional pottery. Wanting to use traditional methods to help the rural poor of northern Nigeria, he hit upon the idea of refrigerating food by placing it in a pot, then insulating the pot with some material contained in a larger pot. It took him about two years to develop and test the idea, and during this time he experimented with many different types of insulating material, including foam from an old mattress. In the end, he found that wet river sand was the best material for his cooler. He passed his ideas onto the Intermediate Technology Development Group, who in turn worked with the University of Al Fashir to run experiments on the zeer to measure its ability to maintain food's nutrient content and extend its shelf life. After tests were concluded, Bah Abba began to distribute zeers. He gave away the first 5,000 pots for free, taking the cost from his lecturer's salary. He also tried several methods to publicize the pots for largely illiterate villages, and eventually found that it was most effective to record a play in which the zeer featured, at which point a publicity team took the video around the villages and projected it onto the walls of houses in the evening when workers were coming home from the fields. In this way large numbers of people were exposed to the zeer when they turned up for the free entertainment.

Construction
A zeer is constructed by placing a clay pot within a larger clay pot with wet sand in between the pots and a wet cloth on top.

The device cools as the water evaporates, allowing food stored in the inner pot to be kept fresh for much longer in a hot, dry climate. It must be placed in a dry, ventilated space for the water to evaporate effectively towards the outside. Evaporative coolers tend to perform poorly or not at all in climates with high ambient humidity, since the water is not able to evaporate well under these conditions.

If there is an impermeable separation layer between the food and the porous pots, undrinkable water such as seawater can be used to drive the cooling process, without contaminating the food. This is useful in arid locations near the ocean where drinkable water is a limited commodity, and can be accomplished by using a pot that has waterproof glaze applied to the inner wall where the food is stored.

Extended operation is possible if the pots are able to draw water from a storage container, such as an inverted airtight jar, or if the pots are placed in a shallow pool of water.

Impact
Pot-in-pot refrigeration has had multiple positive impacts on the population that uses them beyond the simple ability to keep food fresh for longer periods of time and decreasing instances of food-related disease.


 * Increased profits from food sales: As there is no rush to sell food to avoid spoilage, farmers are able to sell their produce on demand and can command higher prices.
 * Increased opportunities for women: Women can sell food directly from their homes, decreasing their dependence on their husbands as sole providers. Also, because girls traditionally take food to market to sell, and because food in the zeer stays fresh long enough that they can go to market once a week rather than once a day, there is more time for them to attend school.
 * Rural employment opportunities: Farmers are able to support themselves with their increased profits at market, slowing the move into cities. Also, the creation of the pots themselves generates job opportunities.
 * Increased diet variety because food is available for longer into the year.

A zeer costs about 150 naira (approx. US$1.00 in 2011) to make, and they sell for 180-200 naira (US$1.20 to US$1.30 in 2011). Bah Abba sells about 30,000 per year.

Reinvention
In 2006 Emily Cummins designed a similar device.

The device can be made for commonly available material such as cardboard, sand and recycled metal. The device is composed of two cylinders. The inner metal cylinder is fitted inside the outer cylinder which can be made from whatever materials the manufacturer has access to, including wood or plastic. Space is left between the inner and outer chamber to be filled with organic material which can include sand, wool or soil that is then saturated with water. As heat from the sun evaporates the water, the inner chamber cools reducing and maintaining the temperature at 43 °F.

Cummins's device is used in areas of Africa such as Zambia, Namibia, and South Africa in areas where electricity is often not readily accessible to help preserve perishable foods such as meat and dairy products. It is not yet being used for vaccines.