# Cheap Solar Fridge

## Step 8: How it works

The action that allows it to stay cool is the evaporation of the water surrounding the smaller pot.  As the water evaporates, heat draws out from the smaller pot, keeping the contents inside much cooler the the surrounding environment.

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gsteele says: Oct 2, 2012. 8:12 AM
Very "cool" project. I am wondering if it might be easier to use a circular egg crate foam top. It would hold more water and be less messy than a wet towel. I am imagining an eye bolt through the center of the foam, with a large plastic washer and nut on the bottom side (to keep from tearing the foam and make it easy to pull off the lid)
ibrent says: Jul 19, 2012. 6:45 AM
Beware: this only works well in dry climates. The coolest that it will get is determined by the 'dew point' which is how cool the atmosphere has to be before water vapor condenses out. If the humidity is 100%, then the dew point is equal to the current ambient temperature and this device won't cool at all. Where I live, today's forecast high temperature is 81 degrees, and the high dew point is forecast to be 70 and the low dew point is 66. This means that in the peak of today's heat, this device will at most cool down to 70, which may not be sufficient to prevent food spoilage, or even feel terribly cool.
fzbw9br in reply to ibrentJul 19, 2012. 8:37 AM
the relationship of DP to actual temp is not linear. I'll have to check at work as we use electronics now to calculate the DP, but from memory, I think the DP is actually lower than the WetBulb...

I'll check it out, and try to reply here...
ibrent in reply to fzbw9brJul 19, 2012. 10:31 AM
You're exactly right, the dew point is always lower than the wet bulb, see the various psychrometric charts running around. There's all kinds of factors like barometric pressure, etc. In my example, with a dew point of 70, and dry bulb of 81, the wet bulb reading would be 73. So 73 would be the maximum cooling one could expect from an evaporative refrigerator such as this. So is it worth the fuss for an 8 degree cooling, which is what I'd get where I live? Probably not. In dry climates it's very effective. Of course, in dry climates getting the water for this may be the real trick.
Tumunga in reply to ibrentJul 19, 2012. 12:30 PM
I betcha Bear Grylls, if in a desert, would be able to get some "water" for it...if you know what I mean.
nekko fox in reply to TumungaJul 19, 2012. 2:06 PM
Gotta say it, he's a terrible survivalist if he thinks drinking his own urine is a means of retaining water in dire situations...

Still, that joke will forever follow him to his grave.
Ole bally in reply to nekko foxJul 20, 2012. 3:35 AM
heheheh! Lots of little blunders in his programs! Swimming at night in the Zambezi when crocs are sleeping?!!!
I kind of like the other guys approach better where he chats with the locals to see how they live on the land before making his program...Bear would benefit a lot by asking around too! I think he eats some of that stuff just for the 'gross' factor! :)
espdp2 in reply to Ole ballyJul 20, 2012. 5:21 PM
He's made a good living by grossing people out...
Ilan Voyager in reply to ibrentJul 19, 2012. 7:57 AM
I do agree with you and your explanation is very informative. More the air is dry better it works. The traditional water cooler uses a a porous ceramic (botijo in Spanish) suspended in a shadowed place with lots of wind. There are several techniques for cooling for example fruits.
The same principle was used in Iran for cooling the houses with a wind tower.
peabody1929 says: Jul 19, 2012. 10:22 AM
Suggested improvement to make it really solar powered. Get a small fan that is driven by a small solar array. Place the fan to blow air at the wet surface. It will increase the evaporation rate, cooling things faster.
ideamaster in reply to peabody1929Jul 19, 2012. 9:52 PM
SunDanzer is a brand of DC powered refrigerators and freezers. They work pretty well and you can reduce your power requirements for a solar installation because you don't have large ac loads when the motor of a regular appliance turns on.
digitalia says: Jul 19, 2012. 7:20 AM
This would be great to take to the lake. I'd get it there dry and sandless, and assemble it on the beach, using an old beach towel. Great idea!
scantelcorps in reply to digitaliaJul 19, 2012. 4:20 PM
Yep, and use your bubble gum to seal the pot holes. LOL.

Actually, the air at the beach might be a bit too humid to allow this method to work well.

Now, if you were to bury a barrel deep in the sand with inlet and outlet ports, and connect that to a solar chinney above ground, the sun-heated hot air above would rise... thus drawing cool air from the barrel to the surface. You could keep your food as cool as 56 degrees F... much cooler! Still not the 40 degrees you want to avoid spoilage though. :(
justrelax says: Jul 16, 2012. 2:29 PM
isnt it heavy to carry ? :)
macrumpton in reply to justrelaxJul 16, 2012. 5:26 PM