Margaritas in the Wilderness

I am carrying a small quantity of agave nectar and 18oz of Gran Centenario Reposado tequila, two elements which will bond with lime juice to make seriously delicious molecules of margarita. The correct proportions passed down from Tommy's are 1 part fresh lime juice, 1.5 part even agave/water mixture, 2.5 part tequila, and ice. Making ice is an unsolved problem. I know chemical ice packs work by mixing water with ammonium chloride, which is available bulk in farm supply stores to treat UTIs in goats. The other option is a peltier-based chiller, or even some kind of ammonia absorption loop to use waste heat off the motorcycle exhaust. I think there was a book about someone who got tired of the bullshit of civilization, bugged out to Honduras to start a new life, and ended up trying to make his own ice in the wilderness--it was "The Mosquito Coast" by Paul Theroux. I forget how that ended.

Let's assume a styrofoam container of dimensions roughly 2"x2"x4", with a volume thus of ~8 fl. oz. and or ~240 grams by weight. Water temperature is ambient, so 25C, and desired temperature is frozen. Cooling 1g of water from 25 C to 0 C takes 25 calories. Freezing 1g of water at 0 celsius takes 80 calories (latent heat of fusion), giving a total of 105 calories. 1 watt/hr is 859 calories, so 1 watt could freeze 8.1 grams of water per hour. (Cit.)

Next we need to look at the relationship between the power used to run a chiller of some type (in watts), and the amount of heat it can move (in watts). Compressor-based refrigeration loops are the most effective, but run rather large. Peltiers are less efficient, but are very compact, taking advantage of some basic electrical properties of dissimilar metals sandwiched together in a fairly thin layer.

A peltier can move heat up to about 1 COP, meaning for every watt used it moves 1 watt of heat. Keep in mind that we are dealing with a heat pump, not an endothermic chemical reaction. The energy used by the heat pump to do its work is added to the ambient heat that's been moved on the hot side, so we ain't breakin' no laws of thermodynamics here. A 1 watt peltier at 1 COP is going to dump 2 watts of heat energy onto the hot side of the plate, which then need to be air-cooled. At larger temperature differentials between the hot and cold side peltiers become less efficient, so let's say 1 watt of input energy moves 0.5 watts of heat.

Using that number, a peltier drawing 1 watt can freeze 4 grams of water an hour. As noted above, we have 240g, so it would take 60 hours. But if we have a 60 watt rated peltier, we could freeze our 4"x4"x2" lump of water in 1 hour!