A Practical Zeer Pot (non-electrical refrigerator)

 by Berkana

Picture of A Practical Zeer Pot (evaporative cooler / non-electrical refrigerator)
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A zeer pot is an evaporative cooler used in rural Africa and parts of the Middle East to keep vegetables fresh. They consist of two terra cotta pots, one nested inside the other, with the gap between them filled with wet sand. (The term “zeer” apparently means “basement” in Persian.) The sand serves as a thermal mass that keeps the pot chilly once it cools down, and acts as a wick to spread the moisture up the walls of the pot. When placed in a shady, breezy location, the evaporation of water off their outer surface chills the pot. If you have a good breeze all the time, or perhaps a small fan powered by a solar panel, the pot can get quite cold. Imagine that chill you get when you step out of a pool when the wind is blowing. Now imagine that wet wind chill going on all day. That’s what the pot feels with a constant breeze. Unless the air is very dry and the pot is exposed to a constant breeze, they generally do not chill down as cold as a refrigerator, but they will usually keep vegetables fresh for most of a week compared to leaving them out. If you do have cool dry air and a constant stiff breeze, the interior of a zeer pot can chill down to around 40˚F.

Think of it as an open-cycle refrigerator. Conventional refrigerators evaporate a refrigerant in a closed circuit to absorb heat from their interiors, then compress the refrigerant vapor in the coils in back to condense it and to expel heat. The zeer pot simply uses water as its refrigerant, and leaves the condensation to nature.

In the under-developed parts of Africa and the Middle East, zeer pots use custom made pots prepared by local potters. Here in the developed world, we need to settle for pre-made pots from the hardware store. There are some drawbacks, but also some advantages afforded by these limitations, as you will see.

Note: won’t work as well in high humidity

If you live in a hot and humid area, the zeer pot probably won’t work well; high humidity results in much less evaporative cooling. (However, a friend of mine who used a zeer pot to cool water in a humid part of Africa tells me that even with the humidity, it worked surprisingly well, so this is not definitive.)

What makes this zeer pot practical? 

There are other zeer pot instructables out there, but I designed this one to optimize for practicality. If the capacity is too small, the cooling capacity too low, or if it an eyesore or is annoying to use nobody would want to use it. This zeer pot uses a large glass pot lid, has an interior basket divider, and sits on a rolling cart. It even has a layer of decorative pebbles over the sand to make it look pretty. The terra cotta pot legs hold the pot off the rolling cart with enough clearance to let the bottom surface contribute to evaporation. The inner pot is bolted down so that it doesn’t float up when you charge the pot with water. Nearly everything I used in this project was purchased at a hardware store, and it can be made in a few hours, plus a day to let the sealants cure.

I will be building an zeer pot array for A Place for Sustainable Living in Oakland (California) based on this design. The goal is to use an array of zeer pots to displace the use of at least one of their refrigerators. The design shown here is the outcome of my experimentation with making zeer pots for them.

(Be sure to read all the notes to all of the photos. Many important details are listed there.)

Parts list with prices 

Prices are rounded to the nearest dollar

Items are mostly from Orchard Supply Hardware (Berkeley, CA). The pot lid was from Kukje market (Daily City, CA), and the sandwich basket was from Web Restaurant Store.

  • 18″ terra cotta pot— $30 @ Orchard Supply Hardware 
  • 14″ terra cotta pot—$15 @ Orchard Supply Hardware 
  • terra cotta pot feet, quantity: 7— $1.79 each, so about $13 @ Orchard Supply Hardware 
  • heavy duty planter caddy with five casters—$30 @ Orchard Supply Hardware 
  • 50 lbs of sand; the finer the better, pre-washed — $6 @ Orchard Supply Hardware 
  • 4″ long 1/2″ bolt 
  • 2″ washers for a 1/2″ bolt, quantity: 5 
  • jamb nuts for 1/2″ bolt, quantity: 4 
  • Refrigerator thermometer: $8 @ Orchard Supply Hardware 
  • Silicone Sealant—$5 @ Orchard Supply Hardware 
  • 13.5″ glass pot lid — $8 @ Kukje Market (Korean markets usually have these; wherever you are, if there’s a korean market or housewares shop, you’re in luck.) 
  • sandpaper 
  • 12″ diameter sandwich basket— $7 at Web Restaurant Store
  • 8″ eyelet or hook bolt and a pair of nuts and washers— about $6 @ Orchard Supply Hardware
 
 
 

Step 1: Prepare the central bolt, bolt shut the hole in outer pot

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(Be sure to view all of the photos above; much of the explanation is contained in the photo notes.)

In this practical zeer pot design, we have a limitation that I turned into an opportunity. The terra cotta pots that we have access to have holes in the bottom for drainage; these need to be plugged so sand and water don’t drain out the outer pot, nor into the inner pot. I have found that it is insufficient to merely plug the holes; zeer pots also have another annoying problem where the inner pot will try to float up as you wet the sand in the space between the pots. In order to solve both of these problems, I use a 4″ long 1/2 diameter bolt, and a bunch of nuts and broad washers and a bit of silicone sealant to seal the holes in both pots. The secondary benefit of this is that the inner pot can’t float up because it is bolted to the same bolt that seals the outer pot. This way, you can be generous with charging the sand with water without worrying about the inner pot floating up.

The first thing you need to do is to put a pair of broad washers on your bolt, put some silicone sealant on the threads, and tighten them down with a nut. Then, seal around the nut with more sealant. 

While the sealant is curing, use your sanding block to remove the clay burr around the hole of both pots. Be sure to sand off the burr both on the inside and the outside of the hole until the washer can lay flat against the pot. If you do not remove the burr, water will leak past the hole.

Now, it is time to seal the hole in the outer pot. This works best with an assistant helping you. Lay the pot on its side, and put some silicone sealant on the clay around the hole both the inside and outside the pot; insert the shank of the bolt through the hole, and have your assistant thread on a washer and bolt it down from the other side. Wipe up any sealant that squeezes out around the washer. Then smear sealant around the nut and bolt to prevent water from leaking out around the threads. 

 
 

Step 2: Prepare the washer on the bolt to seal the inner pot

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Turn the outer pot upright, and rest it on pot feet on your rolling pot cart. Put three pot feet into the pot near the bot, and thread another washer onto the bolt such that when you put your washer down on the nut, the top level of the washer is just a tiny bit higher than the top of the pot feet. The washer must not have its upper surface lower than the upper surface of the pot feet; if it does, the inner pot will rest on the pot feet, and won’t have its hole pinched tightly by washers from above and below.

Add a little bit of sand to the outer pot; you want just enough to fill the places that will be hard to get to once the inner pot is in place. Be sure there isn’t sand on the washer or pot feet; the sand may prevent the inner pot from sitting flat on the washer, and will prevent a good seal. If you decide to dampen the sand to make it more easily shapable, do so BEFORE you put it into the pot, and add water using a spray bottle only until the sand has the consistency of brown sugar. Any more, and the sand sticks to everything.

Once the washer is in place, seal the threads with silicone, and thread on the nut that holds the washer in place. Put some silicone on the washer so that it will seal against the bottom of the inner pot, and rest the inner pot on that washer. Then put some sealant around the hole on the inside, add another washer to the bolt, and pinch it down with a nut. Seal the threads and the gap around the nut with silicone, and wipe up any silicone that squishes out around the washer.

(Unfortunately, I don’t have a picture of the last step where we tightened down another washer to pinch down the inner pot.)

 
 

Step 3: Sand down the inner pot if pot lid won’t fit

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When you buy your inner pot, you should bring your pot lid to find one that fits well. However, please be aware that terra cotta is an imperfect material, and the pots will not likely be perfectly round. Because of this, you should use some sand paper to sand the inner pot’s upper lip to make the lid fit.

Try to get the lid to fit as well as you can, and note where it contacts the terra cotta. Mark those areas either with pencil or perhaps with chalk. Use the sand paper to sand away the contacting areas, and keep doing this until the lid fits right. In our case, it took several hours of very careful checking and sanding to get the two zeer pots to fit their lids well.

Wipe up or vacuum up all the terra cotta dust before continuing.

 
 

Step 4: Optional, for the sheet metal blackbelts: add a thermometer.

Picture of Optional, for the sheet metal blackbelts: add a thermometer.
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If you really want your zeer pot to have that extra nice touch, get a piece of sheet metal, and cut out a bracket to mount your refrigerator thermometer. You’ll need a bot and nut that fits the vent hole of your lid to lock down one end of the bracket; the other end will thread through the same screw that holds the handle onto the pot lid.

With the thermometer mounted on the lid, you can tell the temperature at a glance. Since the warmest air in the zeer pot rises to the top, you’ll know that the temperature inside will be cooler than what the thermometer indicates.

 
 

Step 5: Fill the gap with sand, add water, and top with decorative rocks

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Firstly, adjust the four pot feet on your rolling platform at this time so that they are evenly distributed. Once the sand is in and the pot is wet, the zeer pot will be very heavy.

Use a funnel to add sand. You will likely end up using all 50 pounds of sand. Add more if needed; we ended up adding a couple of quarts of extra sand you want about 1/2 inch of a gutter going all the way around, which you will fill with decorative rocks after you wet it all down.

Add water one quart at a time; the sand will settle, and you will probably need to add a bit more sand when the pot is saturated. When the sand is saturated, you can add the decorative rocks, and perhaps a bit more water. The rocks are not just for looks; when you re-wet your pot, the rocks dissipate the water that you’re pouring into the gutter so that the stream of water doesn’t cut a pit into the sand. 

Once the pots have their sand saturated with water, the pot will darken from water wicking through the terra cotta. Put the pot in a breezy area for the initial cool down. A breeze is absolutely necessary; it simply will not cool down enough without it.

A decorative trim is one of those things that sets this zeer pot apart; people are more inclined to use beautiful things. A zeer pot that looks really nice is not going to be something you’re embarrassed to use.

 
 

Step 6: Making the inner basket

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Get your eyelet bolt, and a pair of nuts and washers, and bolt it onto the sandwich basket as close to the middle as you can. The basket should be pinched tight between two washers. This inner basket gives you an extra platform to put stuff on, and is one of those features that makes this zeer pot practical compared to some of the other “first world” hardware store zeer pots you may have seen.

Insert the basket into the zeer pot, and put the lid on. Adjust the eyelet bolt such that it is as high as possible without touching the glass lid. You want the eyelet bolt to be useful as a handle even if the basket is filled with fruit or other items.

 
 

Step 7: Root cellar variant, and important notes about usage and limitations of the zeer pot

You should moisten the zeer pot with a quart of water three times a day. If you store your water in the zeer pot, or have a dedicated water chilling pot, you will have the best results, since the water you add will be cold already, and won’t increase the temperature of the pot.

Root cellar variant

One neat variant for keeping root vegetables and scallions fresh is to build your zeer pot, and to fill the inner pot half way up with damp sand. Then, burry your carrots and beets in the sand to store them. Damp sand will keep your root vegetables as fresh as possible by keeping them alive. You will find that they remain crisp for longer this way. Also, if you have scallions, burry the root parts in damp sand to keep them fresh. This works even better than simply keeping them cold.

Please note:

  • The zeer pot will eventually accumulate mineral build-up. Use hot water and a sponge, or perhaps a bit of lemon juice to dissolve away the minerals that crust up on the outside of the zeer pot. 
  • The zeer pot will not be as cold as a refrigerator; it will be cool, and it will keep your food cool, but it will not chill a hot container of food down to safe temperatures. 
  • The zeer pot needs a breeze to cool. If you have a good breeze all the time, or perhaps a small fan powered by a solar panel, the pot can get quite cold. Imagine that chill you get when you step out of a pool when the wind is blowing. Now imagine that going on all day. That’s what the pot feels with a constant breeze. 
  • High humidity will result in reduced performance. However, a friend of mine who used a zeer pot in a hot humid part of Africa told me that it still worked “shockingly well”, so it might just work. But keep it in the shade, with a breeze.
  • Zeer pots actually perform better than refrigerators for many vegetables; vegetables wilt in the refrigerator because the condensation on the cooling tubes dries out the air. Refrigerators blow a lot of air over a little chilling surface that is really cold, causing the air to dry out. In contrast, the air in the zeer pot is chilled over a much larger surface that is only a little bit colder. This minimizes condensation; also, since the surface inside the pot will be moist terra cotta, the air inside will have as much moisture as possible, which keeps vegetables crisp in spite of not being as cold as a refrigerator.

The zeer pot is a greener option only if you use it according to the following rule: 

  • These things can evaporate a couple gallons of water a day if you have a good breeze, especially if the weather is dry and warm. If you multiply this water consumption by several zeer pots, this can be a considerable water consuming appliance. I don’t intend to unleash upon the world a device that wastes water in lieu of using electricity, especially in California, where we are experiencing a drought; I expect that everyone who uses these zeer pots to use a bucket to catch the gallons of water that you would normally waste while waiting for the shower to warm up, and to recover this water for the zeer pot. That way, you’re saving electricity without using any additional water. Or, go ultra-sustainable and use captured rain water.
 
 

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