Help me understand how much heat sink possible with frozen H2O

[maximiliano]

So many questions on this topic...but I guess the starting point would be to explain the reason for the question. That way, maybe someone can help me work backwards to answer the question...and thus I can apply that to other situations.

I have a friend who thinks he can make a personal cooling system, using frozen water (0c) as the coolant. I say that the amount of heat sink capacity of say 128oz of frozen water, is probably way too small to matter, in terms of cooling a person off while climbing a mountain, in 90 F degree heat, for 3 hours. Add to that the weight of the water itself is 8 pounds. It seems to me the ice would be melted and assume the ambient temp so quickly, that little would have happened in terms of cooling the person off...over a 3 hour period of exertion.

So, I'm trying to understand (maybe there's a solid formula for this) how much heat sink is possible, using ice as the sink, and an ambient temp. or 90F...or 99F I guess, since the purpose is to sink energy from the person, which is surely 99F.

I have another question, which is related...but I'll ask it separately.

Thanks in advance as always!

 

[haruspex]

8 lb (3.6kg) ice takes 3.6*334 = 1200 kJ to melt it.
Walking slowly generates 150W. For mountain walking I see a figure of 300W, but I'm not sure whether that is heat generated or power consumed; the difference is that a lot of the energy goes into raising you up the mountain, so does not end up as heat. Let's take 250W.
In practice, some ice will melt as a result of heat from the environment, but suppose we could prevent that; or, at least, suppose that it roughly balances the heat lost by the walker to the environment. 1200kJ/250W = 4800 seconds = 1 hour 20 minutes.
The melted ice would still be cold, so you could get another 20 minutes from that. Better still, use it to wet a bandana, so now you get evaporative cooling too, and the weight reduces.

 

[mrspeedybob]

And the climber has something to drink.

Why start at 0c? Why not start at -40c or -50c? I realize most of the heat is absorbed during melting but starting colder certainly can't hurt.

 

[NascentOxygen]

Why start at 0c? Why not start at -40c or -50c? I realize most of the heat is absorbed during melting but starting colder certainly can't hurt.

No harm in replacing heat stroke by frostbite...