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Busting myths about the Zeer pot

Zeer pot example - Image credit: Adam from ModernBushman.com
Zeer Pot

If you don't already know, a Zeer pot is a type of evaporative refrigerator.  It is a device that keeps being rediscovered even though various versions of the device have been used for thousands of years. I am going to call them evaporative refrigerators since they operate by evaporation and are used like a refrigerator.  Every new blog entry or YouTube video about them seems to spread misinformation and sometimes outrageous claims.  Let’s clear some things up.

  1.  It can keep a temperature of 6°C  (43°F) from this article about how a British student re-invented the device

    Technically this claim isn’t false but it is misleading.  Cooling depends upon outside temperature and relative humidity.  The table below lists the various ways the device could get to 6°C (43°F). The bottom line is that it requires desert like conditions of 10-30% and it can’t be warmer than 18.3°C (65°F) outside to get the device to cool to that temperature.   This fact isn’t mentioned.  There are probably a thousand videos on YouTube with various claims on performance, but they all have to obey the laws of physics. (More on performance limitations in this article).

    Conditions under which an evaporative refrigerator could reach 6°C (43°F)
    Outside Temperature Relative Humidity
    20.5°C (69°F) 0%
    18.3°C (65°F ) 10%
    15.3°C (59°F ) 20%
    13.3°C (56°F ) 30%
    12.2°C (54°F ) 40%
    10.5°C (51°F ) 50%
  2.  Using acetone or alcohol or another liquid will yield better results.

    It might be fun to do as a party trick but for daily use it would be more expensive than using electricity to run a modern refrigerator.   The rate of evaporation is the key to cooling performance in these devices, so faster evaporating liquids would help to improve performance, but it also matters how much energy they remove as they evaporate.  Acetone removes 29% less energy than the same amount of water.  It would probably make up for that with its faster evaporation rate. The bottom line is that it is going to be very expensive to use acetone for any long term cooling.  

  3. Instead of using clay pots, try metal pots or cloth bags or whatever.

    Because there are so many versions of this device, there are lots of ways to get to the same results.   The most important thing to remember is that the wet bulb temperature is a minimum temperature to which they can cool no matter how they are constructed.   It doesn’t matter if you use charcoal or sponges instead of sand or if you use a bag instead of an outer pot.  Anything that increases evaporation is good.  More detail explained in this article.

  4. Putting the Zeer pot in the sun will aid in evaporation and make it colder

    That will work as long as the additional cooling from evaporation is greater than the temperature gain of the device sitting in the sun. Probably more often than not, placing the device in the shade (preferably in a nice breeze) will be the best way to improve performance. Since you cool yourself the way a Zeer pot does, by evaporation, ask yourself if you feel cooler sitting in the sun or in the shade.

  5. Bury the Zeer pot, put it in your root cellar or inside another Zeer pot to increase cooling

    Those are sure ways to slow down or stop evaporation. If you can elevate the device to get evaporation on all sides, that would be  better. Unless your root cellar is also a wind tunnel?

Bottom line:

 

Evaporative coolers are a neat idea and when used in the correct environments they can provide much needed cooling for very little cost.  Just make sure your expectations are reasonable.  They will very rarely manage to keep temperatures as cool as our modern refrigerators.  Warmer temperatures and less storage space are part of the trade-off for their low cost and simplicity. 

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