Pot-in-pot refrigerator
A pot-in-pot refrigerator, clay pot cooler[1] or zeer is an evaporative cooling refrigeration device which does not use electricity. It uses a porous outer clay pot (lined with wet sand) containing 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. The device can cool any substance, and requires only a flow of relatively dry air and a source of water.
History
Many clay pots from around 3000BC were discovered in the Indus Valley civilization and are considered to have been used for cooling as well as storing water.
Since 2000, several international NGOs began to work on the dissemination of this technology in various African countries: Practical Action in Sudan, Humanity First in Gambia and Movement e.V. in Burkina Faso.[2]
Extensive research has also been done in Mali by D-Lab, in partnership with World Vegetable Center.[3]
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.[4]
The device cools as the water evaporates, allowing refrigeration in 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 or cement[1] 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. A strap can be used to tie the inner pot down instead of using sand to prevent it from floating.
Alternatives to the Pot-in-Pot construction include various versions of a simple Pot-in-Dish. For larger storage capacity, evaporative cooling chambers (ECCs) can be constructed from a double walled brick structure with a straw and wood cover. The same basic operating principles apply. Detailed information on construction materials and methods can be found in the D-Lab best practices guide.[5]
Operating conditions
Several key considerations are important for determining if an evaporative cooling device will provide effective cooling and storage. ECCs and clay pot coolers provide the most benefits when they are used in low humidity climates (less than 40% relative humidity), the temperature is relatively high (maximum daily temperature higher than 25 °C), water is available to add to the device between one and three times per day, and the device can be located in a shady and well-ventilated area. If any of these key criteria cannot be met at the time when improved vegetable storage is needed, then ECCs or clay pot coolers may not provide sufficient benefits to justify their use.[5]
Effectiveness
The effectiveness of evaporative cooling varies with the temperature, humidity and airflow. Given a constant flow of cool dry air, evaporative cooling can achieve temperatures as low as the wet-bulb temperature, the 100% humidity condition at the given temperature. Documented tables show the minimum temperature that can be achieved at different starting temperatures and percent humidities.[6]
To determine the effectiveness of evaporative cooling chambers for specific uses it is helpful to consider the following:
- Type of vegetables or other products needing improved storage
ECCs or clay pot coolers provide benefits if post-harvest vegetable spoilage is the result of exposure to high temperatures, low humidity, animals, or insects. Some examples of vegetables that are particularly vulnerable to these conditions include eggplants, tomatoes, leafy greens, peppers, and okra.[5] Non- electric evaporative cooling devices – such as ECCs and clay pot coolers – are not suitable for items that require sustained temperatures below 20 °C (medicine, meat, and dairy products) or foods that require a low humidity environment (onions, coffee, garlic, millet, and other grains).
- Volume of vegetables stored at any one time[7]
If the vegetables can fit into a clay pot with a capacity of 150 liters or less, then a clay pot cooler is suitable. Storing larger amounts of vegetables requires a larger ECC. A brick ECC can be designed to accommodate the storage volumes between roughly 500 and 5,000 liters, see the "Construction of Evaporative Cooling Chambers" section of the Best Practices Guide.[5]
- How often is it needed?
Variations in the need for improved vegetable storage can arise due to seasonal growing and harvest cycles, vegetable production surpluses relative to local demand, and climate variations. It is important to determine if proper operating conditions exist for evaporative cooling to effectively provide benefits during the time when vegetable storage is needed, and if the need for improved vegetable storage is frequent enough that the value an ECC or clay pot cooler can provide is greater than its cost.
In Sudan, Practical Action and the Woman's Association for Earthenware Manufacturing have been experimenting with the storage design of Mohammed Bah Abba. The aim of the experimentation was to discover how effective and economical the Zeer storage is in conserving foods. Zeer is the Arabic name for the large pots used. The results are shown in the following table.
| Produce | Shelf-life of produce without using the Zeer | Shelf-life of produce using the Zeer |
|---|---|---|
| Tomatoes | 2 days | 20 days |
| Guavas | 2 days | 20 days |
| Rocket | 1 day | 5 days |
| Okra | 4 days | 17 days |
| Carrots | 4 days | 20 days |
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.[4]
- 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.
- 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.
- The ability to store vaccines and medicines that would otherwise be unavailable in areas without refrigeration facilities.[8]
Dependencies
- Evaporative cooling
- Refrigeration
- Pot-in-pot refrigerator
See also
References
- ↑ 1.0 1.1 "The clay pot cooler – an appropriate cooling technology" (PDF). Peter Rinker / Movement website. Archived from the original (PDF) on 14 July 2014. Retrieved 17 June 2014.
- ↑ Rinker, Peter (15 April 2014). "The clay pot cooler – an appropriate cooling technology" (PDF). Movement e.V. p. 2. Retrieved 26 December 2016.
- ↑ Eric Verploegen, Ousmane Sanogo, Takemore Chagomoka. "Evaporative Cooling Technologies for Improved Vegetable Storage in Mali" (PDF).
- ↑ 4.0 4.1 "How a zeer pot fridge makes food last longer". Practical Action website. Archived from the original on 9 August 2011. Retrieved 24 December 2010.
- ↑ 5.0 5.1 5.2 5.3 Eric Verploegen, Peter Rinker, Kukom Edoh Ognakossan. "Evaporative Cooling Best Practices, Producing and using evaporative cooling chambers and clay pot coolers" (PDF).
- ↑ "Build an evaporative refrigerator - no moving parts, no electricity | Rebuilding Civilization". Archived from the original on 25 June 2012.
- ↑ prkhitman (27 June 2009). "Cold water in rural India : matka(clay)". fuel efficiency.org. Retrieved 9 February 2012.
- ↑ Abraham, Martin A. A (16 December 2005). "The Twelve Principles of Green Engineering". Sustainability Science and Engineering: Defining Principles (Google ebook ed.). Elsevier. pp. 30, 31. ISBN 9780080481272. Retrieved 4 January 2014.
External links
| This article uses material from the Wikipedia article Pot-in-pot_refrigerator, which is released under the Creative Commons Attribution-ShareAlike 3.0 Unported License (view authors). | 
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Bibliography
- Oluwemimo Oluwasola: Pot-in-pot Enterprise: Fridge for the Poor. United Nations Development Programme, New York 2011. (Online pdf)
- Peter Rinker: Der Tonkrugkühler – eine angepasste Kühlmöglichkeit. Bau- und Nutzungsanleitung. Movement e. V., Teningen 2014. (Online article (pdf files in German/English/French are also linked))