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I've spent some time in South Sudan lately and it is hot, with daytime temperatures of 40 -50 C pretty well all the time. Given the predictable high temperatures, I would not bother with evaporation methods especially when sunlight is plentiful.

My advice would be to go for a 12 volt fridge (as used in caravans and boats) a solar panel and a battery for night time. The battery should be large enough to provide power for all the hours the sun does not shine, while the solar panels should have an output sufficient to both run the fridge and charge the batteries.

To gain some margin it would be useful to over specify the panels and battery. Besides keeping your salad fresh a fridge would also any keep medicines and drinks cool.

A solar power system would also power radios, lights (LEDs) etc. and recharge laptops, mobile phones and sat equipment. Solar systems are pretty common in parts of Africa, (though nothing much is common in more remote parts of South Sudan).

VSO might want to think about providing a standard pack of solar charged battery systems to serve the above needs. Most of it is off the shelf kit these days.

Paul