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Inventor of Low Tech Fridge Wins Award
juju2112 writes "Mohammed Bah Abba of Nigeria won a Rolex award for his pot-in-pot invention. Here's how it works. You take a smaller pot and put it inside a larger pot. Fill the space in between them with wet sand, and cover the top with a wet cloth. When the water evaporates, it pulls the heat out with it, making the inside cold. It's a natural, cheap, easy-to-make refrigerator."
Time Magazine invention of the year for 2001
But, cut the guy a break. The cool thing here is that he's done it with readily available local materials which is pretty much one of the key features for a real engineer. To paraphrase the old saw:
Anyone can make you an evaporative cooler for $100; this guy's done it for $1.
It is not a Time Award but a Rolex Award and indeed very old news (2000!).
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The article didn't mention the effectiveness of the device. Say, on a hot summer day, RH of 80%, if we keep the pot under the shade, could we achieve 15 degree C. A temperature ideal for beer.
The cooling effect has been scientifically studied. Here is this article describing it (Google-translated from Spanish).
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.from a Mexican. (This serves as an object lesson to me. Even the experts might well overlook simple and obvious tricks that "every child" knows. Even if that expert is me). The water evaporates from the Tshirt drawing heat out of your body.
.for the enviroment. Much of the mythology surrounding the "magical" abilities of the Australian aborigine come from the same source, their technology being too advanced for a European to understand. It was lost technology to them.
No, not particularly. It's a very old trick to make cold water by putting it in an unglazed clay pot, which is porous, and allowing evaporation of the seepage to draw the heat out. I learned it from Mexican Indians 35 years ago and it was effective enough to make water cold enough to make your teeth hurt even in the tropical rainforest. It works even better in the desert where evaporation happens quicker due to the low humidity.
European bicycle racers have been wrapping their water bottles with a damp cloth covering to keep the water chilled for decades as well.
Until a couple of weeks ago I thought everybody knew you could keep cool by wearing a dampened T-shirt, and then I learned that the Pardy's, those paragons of sea lore and self-sufficiency without electrical power, had only just learned this trick. .
Wrap something damp around a pot, as is done with the water bottle, and the air inside the pot chills, as does anything inside the pot. Wrap a porous outer layer around the damp cloth, such as another pot, and you moderate the evaporation rate.
This "invention" seems to miss a few of the finer points of the device, thus requiring the damp cloth over the two pots. You need to use an unglazed pot for the outer one. Then you can even put a real cover on the thing and it still works. Better. Longer. Some sort of batting works better as a wick than sand, although sand will do and is certainly freely available.
I don't mean to denigrate this man's intellectual accomplishment. If he thought it up on his own from basic principles the intellectual feat is equal to the first man that did it.
But it really does amount to the reinvention of folklore that exists in one place in some other place.
And the people from Rolex think of it as a new invention because they are modern, mechanistic folk who don't know how to go about living without modern power and machines or what people who do not have such devices already know about doing so.
The Zapotec Indians I lived among for some months knew lots of tricks that had been handed down over thousands of years for surviving with nothing but what you could make with your own two hands. I've got a poncho just about eight feet from me right now that was woven by them on a backstrap loom they made themselves, with wool from sheep they had grown themselves, sheared themselves, carded themselves, spun themselves, using weaving techniques their ancestors had invented themselves (even though many people throughout the world had invented the same thing). Living with them for a few months taught me more about how to think about living than any number of survival books and hiking expeditions had ever done.
Many of the things they did appeared as magic to me, because I was just an ignorant Americano and their technology was sufficiently advanced. .
I was in Mexico in the late 60s (that's where I first heard Abbey Road). The Zapotecs are starting to lose it too now as they begin to sell their weaving to touristas so that they may buy Tshirts and blue jeans. Most of them buy neon colored acrylic yarn from the store now instead of using their own lovely wool, because the Americanos really like the bright "native" colors instead of the natural tones of wool.
Well, their lot will certainly improve with more money at their disposal, and I certainly won't begrudge them that. Doctors cost serious money no matter how "self-sufficient" they are, and they coul
I'm from South Africa and I remember a visit to a friends farm about 20 years ago, where he showed me this big black metal box (about 6 feet, 180cm high) he had in his back yard which he used for storing spiced and salted dried meats (locally called Biltong, a bit like beef jerky I think). It worked on the same principle in that it was double walled with the space inbetween the wall filled with sand and a large grating on top which needed to be replenished with water every now and again. It was amazingly cool in the African summer heat.
He had replaced the box after the one from his grandfather finally rusted to pieces after just over 75 years of continual use.
Truckers in South Africa also used to also carry a water bag in a wet sand filled canvas bag outside their trucks to provide a constant source of cool water.
I think the principle is probably much older than this, probably going back to the first person realising that the wind chilled him more after taking a dip in a lake that when he was dry.
Basically, the outer clay pot is porous. The water evaporates and escapes through the pores in the clay. This all happens very quickly because the air is so dry. So assuming that 1 kg of water evaporates each hour, this means about 2kJ of energy, and thus heat, is sucked from the pot. So for you non-metric heads, this means that every gallon of water equals 8,000 BTU. For reference, a typical family refigerator might use 7,700,000 BTU/yr, or 900BTU/hr.
You'd be surprised at the massive amount of energy that a liquid-to-vapor phase change can carry away. In fact, six times more energy is needed to turn one molecule of 100C liquid water to one molecule of 100C vapor water than is needed to heat liquid water from 0 to 100C!
Boiling, which is a similar phenomenon, is the most efficient way to transfer heat known to science.
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Evaporative coolers such as units from bonair are excellent in dry, hot climates. They constantly draw in dry hot air from outside, drop it by about 10 degrees C and duct it through your house to escape through open doors and windows.
Where I live at present (Mount Isa, Queensland), just about every house and business has at least a 6000cfm evaporative air conditioner. Humidity can often get below 30%, meaning that they work particularly well. In fact, they can theoretically cool to the dew point, which if you take note of the last 72 hr readings from Mount Isa can pull down to 10 degrees or so when it's dry.
They are of course completely fucking useless for about 3 weeks of the year when it's hot and humid and you get storms in the afternoon at 35 degrees and 90% humidity. You just sweat like a pig then, or retreat to the refrigerative airconditioner you normally keep in reserve in your bedroom.
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There is a lot of hype here.
This isn't actually very different to the way an electric fridge or air conditioner works. The main difference is that in a fridge, the refrigerant is contained within a closed cycle; in this simple evaporative scheme it is lost to the surrounding air. Since it's only water, few people are likely to be bothered about that. That's why, if you have a CFC fridge and it's still working, there's no point getting rid of it ..... the CFCs are sealed up nice and tight inside it, till you scrap it {there's not much you can actually do to get rid of unwanted CFCs, except leak them into the atmosphere when nobody's looking; which is almost certainly what will happen to the CFCs in your fridge, even if you don't put a chisel through the evaporator in a defrosting accident} and making a new one uses up more energy and resources than keeping an existing one going.
The idea that an evaporating liquid draws heat from its surroundings is nothing new.
Basically, the difference between a liquid and a gas is how much the molecules are vibrating: if the vibration is weak, the molecules' affinity for each other bonds them loosely together so they follow one another around, assuming the shape of a container but occupying a definite volume. If the vibration is stronger than that attractive force, then they just fly apart, occupying the whole of the container and exerting a pressure on it. Heating, of course, makes the molecules vibrate more strongly, which is why liquids turn into gases when heated.
If you try to force more molecules into a space, eventually they will be forced into colliding with one another often enough to form a liquid. This is what goes on in a cigarette lighter: there are just too many molecules to behave as a perfect gas, so some of them are forced together and behave as a liquid.
Pressure, volume and {absolute -- i.e. in Kelvins, 0C = 273.15K} temperature are related by the equation: P * V = n * R * T, where n = number of moles of gas and R is the Ideal Gas Constant. No gas is truly ideal, because the assumption is that the individual molecules have neither mass nor volume; however, the relationship holds reasonably well in real life, only deviating sharply around the point where liquefaction actually occurs.
A fridge or air conditioner has three main parts: the compressor, the condenser and the evaporator. The refrigerant gas is first compressed. Pressure goes up and volume goes down, so temperature also goes up. It is then pumped around some pipes at the back of the fridge {or in the outdoor part of the air conditioner; portable units don't have an outdoor section, so the condenser is cooled by blowing air over it and out of a window through a length of flexi-flue -- uncouple this and you've got yourself a de-humidifier} to allow it to cool down. Once the refrigerant has cooled to ambient temperature and become a liquid again, it is forced out by its own pressure through a tiny hole into a larger space {the evaporator - usually the outer jacket of the icemaking compartment of a fridge, or the coil of pipe in the indoor part of an air conditioner that gets covered with ice crystals}. Now the pressure is not sufficient to keep the refrigerant molecules together, so it becomes a gas again. Pressure goes down, volume goes up, so to satisfy the laws of physics, temperature must go down.
The compressor's intake draws the low-pressure refrigerant out of the evaporator and the whole thing starts again. {In an air con., the whole process has to be stopped every so often to allow the accumulated ice to melt off the surface of the evaporator. Plumbed-in units have a permanent drain, portable ones have a tank which needs emptying periodically. The meltwater is pure enough to be used anywhere demineralised water is required.}
You can also get a terracotta butter cooler which works on this principle: the inside of the tray and dome are salt-glazed, the outsides are unglazed. You soak the whole thing in water, which then evaporates slowly from the outer surface, keeping the butter usefully cold {not rock solid, but not runny either}.
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Evaporative cooling has been use in kitchens for millenia, although it is usually used to keep water cool (unglazed pots). For storage of more than a few hours, a cellar, solid stone building, or cave is less hassle. You easily get guaranteed 70F or below long-term storage in most regions of the world, and if you are architecturally clever, you can actually get lower-than average-long-term temperatures without any maintenance or needing to re-fill water into little jugs.
We homebrewers have used this trick for a long time too.
... well, lagers.
If you're making a lager, you are supposed to keep it at a relatively cool temperature for an extended period of time while it
If you're not fortunate enough to have an extra fridge (with appropriate temperature regulator), or be living in a cold climate with a cool garage/basement, you can use this technique to keep it fairly cool.
Just put your carboy (or other fermenter) in a tub with a couple inches of water in it and wrap the vessel in a towel (my favorite was a thick Bugs Bunny terrycloth) with the bottom edge of the towel in the water. Just water your beer every couple of days and you're good to go.
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Here is a link to research being done using a similar approach, but more efficient evaproration (not water), and a vacuum, so it can actually produce 2kg of ice a day. (Not in production yet, due to deterioration of the system after a couple of years, but doesn't sound too far off.)
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The point is, and apparently it was novel enough an application to merit $75,000, is that this makes a dry cool space, a refrigerator, that can keep food cold, and has an enormous economic impact -- "Eggplants stay fresh for 27 days, instead of the usual three. Tomatoes and peppers last for up to three weeks" -- not just a pleasantly cool cup of water.
NB, this was reported in Time magazine in 2001.Slashdot is keeping its fine tradition of reporting "news" years late. Expect the dupe tomrrow.
I live in New Delhi, India-where summer temperatures of 45C are not uncommon. We have what we call 'desert coolers', which are much better than ACs for cooling. Imagine a large metal box with a big fan on one side and straw mats on the other three-which are wetted by water drawn up from the tank below by a pump. The air sucked in by the fan evaps the water, losing heat in the process, and becoming quite cool. I have a large one at home-and I've observed the room temp drop to 22-23C when its above 40 outside. This stuff consumes about 10-20% of the power consumed by an AC-so it's quite good. (power consumption depends on the wattage of the fan, u can put as powerful a fan as you like). They are also quite cheap to make, and it's almost like a cottage industry here-every summer, local shops stock these coolers in various sizes-huge 8' high ones with industrial grade exhaust fans, to cool large areas, to dinky little 'personal coolers'.
However, during the monsoons, or rainy weather-the humidity renders them useless, as evaporation on the straw mats reduces.
Oh, and clay pots have been used in India too, for generations, for keeping water cool-though not in the way mentioned.
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don't look at the heat on the roof as an enemy, it's a free energy source. If you can collect it, you can use it with an ammonia evaporative refrigeration unit. You could also use it (possibly) to generate some useful amounts of electricity.
Another way to get free cooling in the summer is to have a lot of plastic pipe buried down in the yard below the surface effect heating. That's a variable that you'll have to determine, the depth, but should be easy to find out. In northern climes, it's roughly equivalent to the mean average frost depth. The pipes (long enough, some hundreds of feet are needed to cool say around a 1500 - 2000 sq ft structure) have a single entrance to them coming out of the ground at the farthest away, lowest, shadiest/coolest spot you have in the yard. They come into the building and have a vent at the lowest most central point, then are open to the room. Depending on how many stories your building is, you have floor vents that may be opened and closed, all the way to the roof, where another vent is located. Heat rises, you are creating a thermo-siphon effect. Air enters at the outside pipe, travels underground through the pipes and gets cooled. The roof vent, being the highest and hottest point, acts as the draw, the pump if you will, drawing the cooler air upwards and out, cooling as it travels. That's why you need a lot of buried pipe, but once constructed, it's relatively maintenance free, just needs take care on adequate screening at both ends to prevent insects and dirt entering, etc, and to keep rainwater out, relatively easy with normal conical vent caps. It's a chimney effect, low tech, no moving parts, but you can get some decent cooling from it. I don't have a link real handy, but I imagine that googling will find you some drawings and real-world examples of this technique in action.
The water based evaporative coolers are in large scale use around the world. Local to me is a rather large commercial poultry operation, all the buildings there have massive evaporative coolers installed, they work fairly well, and save many thousands in electric costs, in fact, I doubt they could operate the farms at a profit without them. Basically they are just huge screens that have water dripping down them, and the exhaust fans in the building draw the air through them.
Large commercial sized greenhouses mostly all have them as well.
Your insulation efforts are bang on. Nothing beats massive insulation as a heat/cold moderator. It's the most productive and efficient way to spend the energy dollar once any sort of artificial heating/cooling is required. In some places, the technique is called "superinsulation", with a usual targeted goal of R-55 to 60 range, as opposed to (in the US anyway) the normal R-18 or so. I've worked on two of those projects, they work pretty well for dropping costs (increasing effieicney really) for both cooling and heating.
What is new about this is his effort. He maximized his design for over two years to get the maximum affect (prolonging the life of produce) for the least cost. Then he built two factories to produce them and distributed them to rural villagers for free (using his own money). Imagine the changes it made on a culture where food grown would only last 1-2 days once picked if it could now last a month or more!
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The real "invention" here is his efforts toward making a positive change in the villager's lifestyle. Obviously if someone is awarding $100,000 dollars there is more to it. You folks should do some more research before you nock it!! He plans to use the $100,000 to distibute the pots more widely and to increase his education efforts!
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