I'm wicked hot and it's cramping my stay-clean-with-only-one-shower-a-week game, so time to complain and make up solutions involving too much tech. AIR CONDITIONING COATS!
Okay first off: The human body is, on average, about 65% water for a typical adult male. Less for women because they've got fat everywhere, less for dehydrated sons of guns like myself, but this has to be marketable to normal people. Let's say our clientele is 150 pound dudes, at 65% water, that's 97.5 pounds of water, or 44.25 kilos, thus, 44.25 liters.
Humans are, generally speaking, 37 degrees C (No I'm not converting that to F, deal with it United States), so I'm going to estimate how much energy it would take to lower all of the water in our bodies by, say, one degree. Math o'clock.
Water takes on average about 4.2 joules to heat one gram (or CC) of it by one degree, so I'm going to assume we're going to use about that much energy to cool it. For our 44,250 grams of water, that'll be 186,000 joules. This is already looking super inviable, but let's carry on.
A joule can be expressed as a watt second, so 1 volt at 1 amp for 1 second, that's a joule of energy. Assuming we want to be cooled down in, say, 20 minutes, that's 1200 seconds, so we'd need... 155 watts. Not half bad. An 18 cubic foot refrigerator generally requires about twice that, while running, but they don't run all the time, because they're super insulated and only need to run when it gets too warm.
Now, in actuality, much more energy will be needed because refrigeration isn't very efficient. For every watt of cooling, you have to spend several more on motor friction, heat loss, other stuff. Since this is going to be a magic jacket, I'm going to presume Peltier cooling, which is a solid-state plate which has the advantage of lasting a very long time, having no moving parts, and no hazardous refrigerants. They are a miserable 10% efficient, so, the power we need is actually ten times what was listed.
Holy crap, that's 1,550 watts. This is intolerable. Where are we going to get that kind of energy? This is just ridiculous. If we use 24 volts, that's like fifty amps. The wire alone for that kind of current would weigh a ton, not to mention that most cheap peltiers are only 60w, so we'd need 26 of them.
Batteries: I went on an RC plane website and found a LiFePo4 battery that could handle 8.4 amp hours at 6.6 volts. Say we use four of them for the voltage requirements, that would still only run it for 20 minutes. Total weight of the project would have to be about 8 pounds, and there'd be losses for any kind of circulation pump, jesus. What a terrible idea.
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