History
This little invention is absolute genius and exceeded my expectations from the very first. It was invented by Mr Mohammad Bah Abba (see pic below) a teacher from Northern Nigeria, Africa, he took a strong local tradition of pottery and found another use for it. I caught the end of a documentary about the idea, but wanted to know more, so I plugged “pot in pot” into the net and got a bit more information. He invented it back in the 1990s and in 2001 got the Rolex Award for Enterprise and used the money he recieved to make his invention available throughout Nigeria. Unfortunately, he passed away in 2010 at the age of 46. This article commemorates a great man!
(pic stolen from the 'net)
The idea is fiendishly simple, which usually means it takes a special kind of intellect to think of it. Take one large, unglazed terracotta pot and a smaller unglazed terracotta pot put the smaller pot inside the larger one and fill the space between them with coarse sand and then saturate the coarse sand with water. The water moves by capillary action into both unglazed pots and evaporates from the inside and outside clay surface. Bingo….one evaporative cooler.
The pot colour gets darker as the water comes to the surface
The idea is that the locals make the pots specially and use all local materials, everybody wins except the multinationals, life’s hard ain’t it? Theoretically you could win the clay (dig it from the ground) refine it, make the pots dry them and then fire them, all using low tech, local materials and processes.
Construction
I made a couple of them in one afternoon, but I bought the pots from the Reject shop (they no longer appear to sell terracotta pots in 2023), at $6 for the smaller pot (27cm diameter) and $12 for the larger pot (35cm diameter). So my coolers cost $18 a piece to set up (2005 costing), plus a bit for the sand (locally in Africa they go for 40c a set). I suppose you could use the “flower pot” style pot, but the more rounded ones can store more food for the same size pot. So the ones I bought are much more spherical than the traditional pot. The pots made for this purpose also contain no drain hole so the first job was to put some putty in the drain holes of both pots and then cover the putty with a square of plastic sheeting to stop any leakage. I then placed a layer of about 25mm of sand in the bottom of the larger pot and sat the smaller pot in the larger one, it is then a simple matter to pour more coarse sand into the gap between the pots.
The pots are now ready to be charged with water, and there are a couple of points to note –
- If you leave the sand down about 12 mm instead of filling the space entirely it makes putting in the water much easier, and
- If you put too much water in initially the smaller one will tend to float out of the larger one.
Filling the sand with water takes some time because it has to percolated down through the sand, and if you do a bit and then come back to it, it gives the water time to soak into the unglazed terracotta, important for keeping the inside pot heavy.
The next thing is to arrange a lid or cover of some description, my original go-to was a couple of layers of wet hessian bags over the top, like potato sacks or the bags coffee beans are imported in. This worked fairly well but the inner pot still had a tendency to float. I got an appropriately sized terracotta pot saucer (mine was 32cm) to fit on top as a lid when upturned. The pot saucer was much more robust and rot resistant than the hessian bag, and also heavier. If that did not stop the inner pot from floating, a brick strategically placed on top of the pot saucer certainly did.
Testing
At the time I put them together I carried out a qualitative temperature test: once it was fully charged with water I stuck a few coke cans in it and then walked away for a while. When I came back a few hours later and reached in for the can it felt COLD! The thing worked like a ripper! Being of a scientific bent however I thought that I better test it to see if it was cold, so I used the most scientific test I had at the time and applied the “cold” can to the upper part of my wife’s arm. The resultant scream and beating which I received confirmed (somewhat painfully) that I was on to a winner.
Having said that, I do like quantitative tests so I set up the cooler again and left it to cool. To check temperatures I used our Infrared thermometer. To get something close to the setup, but not affected by the evaporative cooling I took the temperature of the brick sitting on top pot saucer acting as the lid of the cooler and it showed a temperature of 28.0⁰C, I then removed the lid (complete with brick!) and checked the temperature of the inner wall of the cooler and it showed 24⁰C. This was not as much of a difference as I was expecting, but the humidity is currently at 82% which is pretty high for where we are. This does point out one of the disadvantages of the evaporative cooler (or anything that requires evaporating water to work), that it will be much more efficient and effective in a lower humidity environment.
Other stuff
A full charge of water lasts several days, depending on ambient conditions and it seems to work inside or outside, so long as it is in a shady spot, full sun easily overpowers the evaporative cooling. For best operation, site the cooler in a place that is out of the sun but exposed to the wind
One thing that I have found to be an issue with this particular set up originally 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 itself (tank water so unlikely), the terracotta pots or the sand I don’t know. It came through initially and needed to be periodically scraped off or it would interfere with the evaporation process and make the set up less effective. A bit of water and scotchbrite (or equivalent) seems to do the job admirably well. It has been some considerable number of years since this was a problem.
Another thing perhaps worth noting: If you store your coolers when not in use somewhere out of the way, ours is under the back deck, be aware that the space between the inner and outer pot is ideal as a home for tiny critters, especially spiders. The moral of the story is: look before you grab!
Final words
This is a simple and very effective invention, to help people in the lesser developed parts of Africa, but anybody can use the same technology in the cause of sustainability. While researching this invention on the net I came across a company who were going to make and sell an “improved” model with a solar powered fan, I can’t help getting the feeling that they have missed the point though!
