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.
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.
Step 1: What Will You Need?
- a tin can big enough to fit your beverage container
- an unglazed terra cotta flower pot big enough to fit the tin can and some sand
- some fine-grained sand
- if your pot has a hole, some sealant and a piece of plastic to cover it
And if you want it to be as portable and spill-resistant as mine:
- a 2"-3" piece of flexible hose
- more sealant
Step 2: Seal the Hole in the Bottom of the Pot.
If your pot has a hole in the bottom for drainage, this will need to be sealed. Otherwise, sand and water will leak right out. To do this, cut a small piece of plastic and glue it over the hole.
Step 3: Add the Sand and the Can.
Next, add a little sand to the bottom of the pot. The idea is to provide enough so the tin can will rest on it, but not so much that the can sticks out of the top too far.
Add a little at a time and press the can into the pot. As you get close, the can will start to mash down the sand. Keep adding until the can is resting on sand and there's a small gap between the can bottom and the edge of the pot.
Then it's as easy as setting in the can and filling the edges with sand. Leave a gap -- even if you're not going to use cement to seal it, if you fill it to the lip of the pot then there will be no place to add water and let it sink in.
Step 4: Add the Fill Tube and Cement.
Use a piece of rubber tubing to provide a fill tube. The idea is to make it big enough that you can actually add water (and to provide a reservoir as the water slowly wicks into the sand).
Mix up a tiny batch of cement -- about 1 cup. I'm no expert at mixing it but I know it shouldn't be like wet sand and it shouldn't be like soup. If it's too dry then it won't seal and if it's too wet then it will crumble once it's set up. If you're not familiar, try a batch and see how it works.
Anyway, insert the fill tube along the edge of the sand. Stick it into the sand just a small amount (maybe 1/10" or so) -- just enough that the cement won't flow around and seal it.
Add the cement around the edge and try to seal around the fill tube as best you can. If you want to make it look less industrial, you can add decorations into the cement -- once it cures you can't.
Let the cement cure overnight.
Step 5: Add Sealant Around Fill Tube.
I found that the fill tube didn't seal against the cement so when I added water, it just leaked out around the edges of the tube and barely wicked into the sand. I had to add sealant to make it work. Not much more to say ... just make sure you get into the gaps around the fill tube and go all the way around.
Step 6: Try It Out!
Once everything is good and dry, you're all set to go. I use a funnel to get enough water into the fill tube to let it wick into the sand. It takes a few minutes and the pot gets wet around the edges pretty quick.
Unfortunately, the last few weeks around here have had miserably high dewpoints. I did a test indoors where the air temperature was 77 degrees Fahrenheit but the dewpoint was over 60 degrees, so the inside of the can only got down to 75 degrees. I'll need to test it further when it's drier.
Fortunately, the whole point of the exercise was to make coolers for beverages while I'm in the August desert of Nevada for the Burning Man festival. The air temperatures can get over 120 degrees Fahrenheit but the dewpoints stay very low -- around 35 degrees or so. This is the ideal environment for a terra cotta cooler so I'm looking forward to how well it works.
Step 7: Download As PDF
In case you wanted to take advantage of the Non-Commercial License of this Instructable, feel free to download it as a PDF here.