Instructables
So I was browsing the Self Sufficient 'ish' sometime last year and came upon the article titled Keeping your cool, Replacing the Humble Refigerator by a guy named Nev Sweeney. He digs around some alternatives and dredges up the pot-in-pot system developed by one Mr. Mohammad Bah Abba in Nigeria, Africa. The gist is this: take a big, unglazed terra cotta pot and fill it with a little sand then put a smaller pot inside and fill the gap between with more sand. Add water and allow evaporation to provide the cooling. Mr. Sweeney indicates he was impressed with the results.

Well, I decided to go a bit further.

First, the science behind the whole thing is that of the wet-bulb thermometer. You know: the one that measures the dewpoint? The gist is that terrestrial air has some amount of moisture in it and that vaporizing (liquid) water to dissolve it in the (gaseous) air requires energy which comes from the heat that is present in the air. Because heat allows the transfer, a thermometer with a "wet bulb" will read a cooler temperature than a one with a "dry bulb". That is, something that is wet is always a little cooler than something that is dry. All because of evaporation.

Second, how can one convert this to something practical? Like, say, the ever present problem of keeping one's beer ... er ... beverage cool on a hot day.

So I developed the idea of a terra cotta beverage cooler. It works like this: you add water to elicit evaporation which causes cooling which keeps your beverage cool. Even on a hot day. And it's not just passive: it actually reduces the temperature of a warm beverage to something cooler.

The only catch is (as I found out) that it requires a low dewpoint.

Update:

The cooler didn't work all that well out in the desert. The used flower pots I started with had very little porosity, presumably all the pores had filled with minerals and dirt over time. Also, the sand wasn't easy to use, so I'd recommend trying small pebbles instead. You could even fill it with ice just to set the height of the concrete cap.
 
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tomc3uk7 years ago
Could you make some kind of evil super cooler using petrol or ethanol?
jolshefsky (author)  tomc3uk7 years ago
I thought about trying rubbing alcohol ... with a continuous supply and a lot of wind I figure it would get very cold. Maybe do the same thing but title it "Evaporative Terra Cotta Ice Cream Maker". :-)
Yeah. Not so environmentally friendly though (or cheap). Oo maybe with ethanol farmed from corn or whatever they use. And if the thing caught fire.. dear me.
If it caught fire, you would have a bread maker. Seriously though, thought-provoking Instructable. I can see some potential for a larger unit. I would think it would be a good idea, too, to spread a wet cloth over top. Couldn't you also leave the cement off the top and put a layer of ice on top of the sand so that it would melt and absorb into the sand as well as assist cooling?
Uh... if you have ice, who needs an evaporative cooler?. Put the ice in a container (preferably insulated) and put your beverage can in the ice.
true. i am rightly rebuked.
I believe the word you are looking for is "DOH!" <G>
In my experience, placing the ice directly into the beverage would dilute the beverage, at an increasing rate as things warm up.

Theoretically, placing the ice on top, in place of the cement cap should work well. Issue comes in, where does one obtain ice in the middle of the desert, with the closest civilization being 50+ miles away?
Actually based on the physical principles involved you would get more cooling with water. Water has the highest specific heat of any convenient liquid (ammonia being higher, DO NOT USE AMMONIA!). The reason that alcohol feels cooler is because of the high rate of evaporation. Adding more surface area with water as the liquid would result in cooler inner chamber.
leobunyan7 years ago
does it work with a lid??
Yeah, the cooling happens (again) through the terra cotta. A lid would actually be pretty nice to try to keep as much heat from re-entering the beverage as possible.
Mastros3 years ago
One more thing that I forgot.
There wont be a problem with a cover, because that will only isolate the inside container from the hot environment. If its a terracota cover, then all the better, because it can be kept wet and contribute to the whole cooling.
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TheGameBall4 years ago
Could we hear some kind of rule of thumb on the "low dew point" requirement?
It shouldn't be simply the concentration of dew in the air, but also related with how much wind you have. So, best answer should probably come from practice.

Taking the idea from another comment, by Royski, a simple test could be to wrap a wet towel around a container of water, and see how much it gets cooled. If the result is satisfactory, you could go buy a pot and construct the cooler.
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Mastros3 years ago
Very nice improvenment of an ancient practice. I'll use it in my camping campaigns this summer.

Before modern era, our people here in Kypros (Zypern) used to keep their water in pots, with an opening on top from where you poured water out. This mouth was kept covered with a simple cloth, or with the drinking cup itself, to keep dust out. In this way, water was always cool and refreshing. Slices of watermelon were also cooled by being left under the hot sunshine!!

Pots were known since millenia all over the world. So, keeping water in ceramic pots and then noticing that it becomes cool must have been a similarly ancient and universal practice. Asking who invented it is like asking who invented the wheel. And thinking that it was invented by the people of one's village..., hey, we all love our people, right? so, what's the problem if one thinks it was invented by an old man from his village?
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tim_n4 years ago
I've made a slightly larger (and simplier?) version of the zeer on instructables: http://www.instructables.com/id/Garden-Camping-Festival-no-electricity-Fridge/

Didn't see yours (though I searched a few times before I submitted) and found it shortly after submitting - but nice for a single beer - looks more portable than mine which does weigh some!
This principal of cooling is referred to as a "Zeer Pot."

You can view a detailed article here:

www.theruralindependent.com
Funny that the technology is being "rediscovered" by the mainstream... (happens over and over... thats where Archimedes found his water screw system... and the eastern documents he found came from other sources) The scientist noted with this invetion came from a Nigerian potter by the name of Mohammad Bah Abba. He used a glazed pot in the center, and an unglazed one on the outside with sand in the middle. The history if this device dates back to the most ancient times, where these kinds of pots were used as cold storage. There are many references you can find on google, wikipeida, encyclopedia brit. etc. According to an amateur chem book, "The temperature inside can fall as far as 14 deg C (25 deg Fahrenheit) below that of the outside". This may be where the use of an unglazed coilpot, or where allot of the "ribbing" textures on the outsides of ancient clay pots were for. I dont suggest these devices be used as permanent devices as they need cleaning etc... but... interesting if you think of the device also as a filter. salts and minerals build up on the clay pot and in the soil. if you're using dirty water, organic materials are also filtered. The sand or woodshavings of such a type of device would then become excellent materials for composting. in more desert-like conditions this can be invaluable for creating soils that can support life. just a thought.
DeadlyDad7 years ago
I wouldn't use cement to seal the top, because, as is mentioned in the link at the top,

One thing that has been an issue is that with use a crusty skin of salt forms on the inner and outer surface of the cooler, from salts leached out and then left behind by the evaporating water. Whether the salts are from the water, terracotta or the sand I don't know, but it is still coming through and it needs to be periodically scraped off or it interferes with the evaporation process and the set up seems less effective. A bit of water and Scotchbrite (or equivalent) seems to do the job admirably well.

Perhaps a plastic lid with appropriate holes cut out.
jolshefsky (author)  DeadlyDad7 years ago
I hadn't thought of that -- I assumed the salts appeared at the evaporation surface (i.e. the outside of the pot) but I guess they could gum things up inside too. I'll have to see what mine does when I get to test it in the desert in 2 weeks.
I lived in ElPaso, not quite like BurningMan but similar environment. We use evaporative air coolers for houses there, and use 'tap water' (plumbed in of course). it works great for all but about 2 days a year. Then you go to the movies and malls ;) ... Anyway, the scale (salt) does build up on the pads used to drip the water across so the air can evaporate it on the way into the house. Once a year, you tend to throw the pads away, or chemically clean them if you use expensive pads. ... I liked the ones made from wood shavings or cheap un-faced insulation. The water runs through them slowly, and we can just replace the shavings/insulation each year. At the same time, I kept 2 water pumps, and just swapped which one was in the unit each year. So I had an entire year to break the used one down and make sure it is in good shape (again, cleaning off the scale/salt buildup) for next year.
.That depends on the mineral content of the water used, I would guess. I would think anything in the Terra Cotta would build up on the outside only as the water wash flushing minerals out of it. any the point was keep it accessible for maintenance, not they why With my well water it would be a problem. Collecting rain water and using it for the chiller should reduce any problems.
nnygamer6 years ago
An excellent and simple design that I think would work great at Burning Man, very low humidity, no power (well only what you bring with you) just bring extra water to keep it moist.
carpespasm7 years ago
i think i've seen larger ones of these suggested as a good means of making food coolers for people in places like africa. They can be made with local materials, are cheap, and only require water to keep running. good stuff.
Google coolgardie safe for info on the old tech. Again works best when the humidity is low. I have heard of servicemen chilling cans of beer in open pans of gasoline. Then today who could afford that, if the risk is managed well Many places a deep hole can do a fair job, but requires planning ahead.
bdl7 years ago
Can't you simply soak the terracotta pot in water, stand the beverage in it (if the pot is big enough) and allow the natural evaporation to cool the drink?

I have seen wine coolers which work on this principle and seem to do a good job.

Wine Cooler
jolshefsky (author)  bdl7 years ago
The question is, how long will a soaked terra cotta pot stay moist in a dry climate? I envision the wine cooler you linked to being used indoors where there isn't much air circulation; that and I wonder just how well it works. My whole Instructable comes together so the terra cotta retains as much water (because the sand is soaked with water) over as large an area as possible (the sand theoretically wicks the water all over the pot), and the cooling vessel doesn't float (the tin can is stuck down by the cement). I'll get a chance to see just how well it performs in a couple weeks.
Woodenbikes7 years ago
Literally cool instructable. Perhaps it would be more water efficient (BTU's useful cooling / gallon) if the pot and can were reversed. For example if the item to be cooled was put in the terra cotta pot that was itself inside a larger can with sand in between. Then the coolest surface of the moist terra cotta would be facing the item to be cooled.
I don't think this would be as efficient. The terra cotta is porous and it is the water leaching through then evaporating which causes the cooling (plus from the sand's top surface). Butter used to be kept in dampened terra cotta containers. On holiday in Italy I very succesfully cooled my overnight drinkig water by wrapping it in a wet towel.
I'd love to make something like this... but, sadly, I live in georgia, and the humidity stays around 50-70% water just doesn't evaporate well here... however, I know stuff like this works great in the desert! simple things such as mist will keep you cool! (here you just get wet... and barely refreshed)