Neat idea, and a physics nitpick

Enviropundit links to an article about the Zeer Pot-in-pot cooler. What grabbed my attention at once was the following:

The principles of the technology are very simple. When heated by the sun, water contained in wet sand between two jars evaporates, cooling the contents of the innter pot.

Um...

That's taken directly from the abstract of the paper, but unless I missed something major in my physics classes, heat flow doesn't work that way. The lowest temperature you should be able to achieve with an evaporative cooling system is the dew point. Placing the system in direct sunlight shouldn't do anything more than throw more heat into the system, causing the water to evaporate faster.

On reading further, I see:

The innovative cooling system that Abba developed in 1995 consists of two earthenware pots of different diameters, one placed inside the other. The space between the pots is filled with wet sand that is kept constantly moist, thereby keeping both pots damp. Fruit, vegetables and other items such as soft drinks are put in the smaller inner pot, which is covered with a damp cloth and left in a very dry, ventilated place. The water contained in the sand between the two pots evaporates towards the outer surface of the larger pot where the drier outside air is circulating. The evaporation process causes a drop in temperature of several degrees, cooling the inner container and extending the shelf-life of the perishable food inside.

Oh. That's what I thought.

It works because of the low humidity and the ventilation; the sand is a water reservoir, additional wicking material, and presumably additional insulation.

I guess whoever wrote the abstract didn't read the article very carefully.

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This is more than just scoring points at Enviropundit's expense. It's easy for people to get details wrong. If they're never fixed, you may wind up thinking a bit of technology can do more than it really can. Or, you may use the technology in a way decreases its efficacy.

My hunch, backed up by at my (admittedly two-decades-old) knowledge of thermo-goddamics, is that putting one of these coolers in direct sunlight will, contrary to what is implied in the abstract, shorten the life of whatever is stored within it. Now, it may be the sand and ceramic insulates the contents well enough that the difference is negligable. But on the off chance the difference is significant, the end user should be aware of what direction it runs.