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Should do an effi calc. They have all 4 walls wet and porous. Side 1 is inlet, side 4 is outlet (don’t want wet), and sides 2 and 3 have flow parallel to surface (cooling outside air). I think it would be best to use a few watt fan and insulate not just the top and bottom, but also sides 2 and 3.
I think the temp of the air would approach wet bulb exponentially, i.e. behave like C = 1. This means that with NTU = 3, effe = 95% (1- e^-3), so I would assume higher effe (of course this is an optimization with the cost of charcoal).
If you could compare to an electric system, the walls might be R-6/in foam that are 2 inches thick, with 1ftx1ftx1ft, this requires ~0.26 W/K thermal. So if we are keeping it 15 K cooler than surroundings, this is 4 W thermal, and with COP = 2, this is 2 W elec.
If you require 40 L/day, this is 1000 W thermal of evaporation, which is 2.5 orders more! So basically your flow rate is so high that your internal temp is nearly equal the temp of the air after going through the mat. I would say this has a thermal effi of 4 W/ 1000 W = 0.4%. What you should do is, say you can get to 20 C air, then you want the internal to go from 20 C at the inlet to 25 C at the outlet, or 22.5 C on avg. Then if you insulate well, since evap went from 35 C to 20 C, and the air absorbed heat from the enclosure from 20 C to 25 C, you should be about 33% thermal efficiency, so 100x less water use! This also means the air velocity should be ~100x less. The tricky part is the radiation loss from the front of the wet pad. I think this would be minimal as long as there was not much water flow on the outside of the pad.
This only works in dry areas where you typically don’t have a lot of water, so water use in very important.
It looks like you have put a lot of work into this, but I would add just a very rough economics section. For instance, if water has to be carried 100 m, and a person can carry 20 kg, this might take 6 min round trip. At 25 cents/hr, this is 2.5 cents, or 2 cents/gal, and this doesn’t count the cost of the container or the well. And if surface water, the distance is likely to be much farther. C.f. US cost of 0.3 cents/gal.
My rule of thumb is that something lasts 10x as long in a fridge as room T. I’ve also heard that biological activity roughly doubles every 20 C, so I guess going from 3 C to 23 C would only be a quadrupling, so these aren’t consistent. Your number of 20 days for 17 C vs 2 days for what – 37 C? Of course it could be this hot during the day, but not at night. It could be that even going to 30 C could get you to 4 days, which would be a significant adv. This could be done with very little water (near 100% thermal effi b/c the air would warm up nearly to ambient before leaving). Also, it means that it would work in humid climates, too!
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Great work!, --Lonny 13:50, 13 June 2011 (PDT)
Hello, thanks for an innovative idea....i must say it is a fabulous task. I am from mid India where hot & dry climate prevails. So Charcoal cooler would be a great option for the people who live in villages where access to electricity is a major problem. Here i want to ask about the type of charcoal which should be used. Whether it should be activated charcoal which can be given any shape & density or it should be charcoal came out of timber having appx density 208kg/m3,
please help me on this matter,