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How Cold Does a Pot-in-Pot (Zeer) Fridge Get?

Tim Lash, June 24, 2011

Introduction

The pot-in-pot fridge is exactly what the name describes. It has been used to prolong food storage in poverty-stricken places. There are many websites that describe the pot-in-pot fridge, which is also known as a zeer. However, those places do not provide much info on how cold the zeer gets. An experiment was performed to find an answer this question.


Materials/Parts

The following parts were used:

  • 5 gallon plastic bucket

  • small ceramic bowl

  • Aluminum soda-can (a 8.3 FL OZ. Red-Bull can to be exact with top cut off)

  • water (enough to moisten sand)

  • play sand (about 25 pounds)


Test Equipment

The following pieces of test equipment were used:


Background

Having an electric refrigerator to keep food cold is something that is taken for granted in western civilization. In places of severe poverty, such as Sudan, refrigeration is a luxury. It is possible to make a pot-in-pot fridge, also known as a zeer, to prolong food storage.

The zeer is basically two pots. One pot is smaller than the other. Put the smaller pot inside of the bigger pot. Fill the space in between pots with sand and water. Cover the inner pot with a lid of some sort. The fridge works by evaporation cooling. In climates with low humidity, the water will evaporate easily. The water carries away heat as it evaporates, thus cooling the sand and inner pot. On average, water must be added to the sand twice a day to keep it wet.

Three great places online to learn more about the zeer are:

The pot-in-pot fridge is capable of keeping the food at a temperature lower than the ambient air temperature. How much lower though? How many degrees Centigrade or Fahrenheit lower and under what ambient conditions? Practical Action states, “This evaporation brings about a drop in temperature of several degrees, cooling the inner pot and extending the shelf life of the perishable food inside.” The Mail Online article about Emily Cummins' invention states, “Without using any power, temperatures stay at around 6c.” 6 Centigrade is 43 degrees Fahrenheit. 6 Centigrade but under what ambient conditions?

Not satisfied with these generalized performance statements, this engineer set out to measure the performance. It's also cool (no pun intended) to see and feel things work right before your eyes.

Note that the zeer operates on the same evaporation cooling principle that a swamp cooler works. A swamp cooler only works well in climates where there is low humidity. A USA Today article, “When it's too humid for swamp coolers,” details how much a swamp cooler can drop the temperature inside a house.


Procedure

The figure below is a horrible looking drawing of what was built:

pot in pot fridge construction diagram
A photo of the experiment are shown below:

pot in pot fridge experiment setup

The climate where this experiment was done is rather humid. Since the pot-in-pot fridge needs low humidity to work, two things were done to help reduce humidity. One, the zeer was kept indoors with air conditioning that maintained temperatures between 75'F (24'C) and 80'F (27'C). Two, a fan was used to blow air across the top of the fridge to maximize evaporation.

The USB-502 sensor was placed near the fridge to measure ambient air temperature and humidity. The USB-501 sensor was used to data log temperatures inside the soda can. The soda can was considered the inside of the fridge. Data logging was done for about 20 hours.


Results

Results are available in the room_and_can_2011-06-18.csv file.

The outside air temperature and pot-in-pot temperature are graphed below:

graph of ambient air temperature and pot in pot fridge temperature versus time

The temperature difference and ambient air relative humidity are graphed below:

graph of pot-in-pot fridge temperature difference and humidity versus time


Discussion

The fridge does work after an initial cool down period. This is indicated from the data gathered. The temperature inside the can is never above 70'F (21'C). This is lower than the lowest room temperature of 75'F (24'C). The temperature differential is probably around 8'F between ambient air and inside the fridge.

The air relative humidity probably averages about 45%. The USA Today article mentioned in an earlier section predicts about 10'F (6'C) temperature drop at 75'F (24'C) ambient air temperature and 45% relative humidity. Thus, the fridge temperature drop is within 20% of an expected value.

So the fridge is 8'F (4'C) colder than the surrounding air at 45% relative humidity. The temperature drop is more for lower humidity. It would be interesting to see how the pot-in-pot fridge performs under lower humidity. If I have time in the future and the weather cooperates, I will do the experiment.






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