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 Homemade Airconditioning

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PostSubject: Homemade Airconditioning   Sat Aug 11, 2007 6:28 am

Evaporation - Pot-in-pot System

This little invention is absolute genius and exceeded my expectations from the very first. It was invented by Mr Mohammad Bah Abba in 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.

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. evaporative cooler.

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. This is also on the list of things I want to do, but it may be a while before I get to it.(I've dug the clay and refined it, but I am a pretty darn
ordinary potter at this point!)

I made a couple of them in one afternoon, but I bought the pots from (you guessed it) the Reject shop, at $6 for the smaller pots and $12 for the larger so my coolers cost $18 a piece to set up, 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, so only put a small amount of water in at a time or put a weight in the bottom of the inner pot.
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. Apart from putting a couple of layers of wet hessian over the top to keep the heat out the job is done. Once one was fully charged with water I stuck a few coke cans in it and the 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 bloody ripper! Being of a scientific bent however I though that I better test it to see if it was cold, so I used the most scientific test I know and applied the "cold" can to the upper part of my wife's arm. The resultant scream and beating confirmed (somewhat painfully) that I was onto a winner.

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 is a bit much for it. 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 scaped 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.

This is a simple and very effective invention, to help people in the lesser developed parts of Africa, but we can use the same technology in the cause of selfsufficientish. 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!

Nevin Sweeney
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