I know you folks count on May Shrink Or Fade to raise, and answer, the tough, controversial questions that most people can't be bothered to think of
, let alone think about
. And I am not going to let you down. And by the time you finish reading this post, most of you are going to be saying to yourselves "O... M... G. That guy really is whacko." No matter. This must be written.
So, for years now I have been pottering up & down the coast of the Smallish State in sailboats with iceboxes. In case you are too young to remember how an icebox works, it's fairly simple: you put ice in the box, and then you put things that you want to stay cold in the box, and for a while the things stay cold. Eventually the ice melts and you need to put in more or the cold things get warm again. Luckily, though, the Smallish State is relatively cold to begin with, especially the parts of it that are below the waterline of a sailboat, so 20 or so pounds of ice can hold you nicely for week. (Also, you can chill a beer just by tying a line around it and dangling it 10 or so feet under water.)
Nonetheless, the perennial sailboat ice-user starts to ask himself some questions. Questions such as:
1) Is all ice created equal? Isn't it likely that one ice-vendors' freezer is colder than another's, thus providing colder ice? Cold ice will obviously not melt as fast as warmer ice, right? So how much better is colder ice than warmer ice?
2) Is it worth buying ice at all? How much does it cost to make ice in the freezer at home, compared to buying it?
3) As the ice melts, the cold water drains out of the icebox. But you could keep the ice instead in a container within the icebox, and thus hold on to the ice-cold melt water. That would be sort of a pain-- but would it be beneficial, or not?
I have given these questions a lot of ill-deserved thought. Here's what I've come up with for answers:
1) A: All ice is not created equal, but the differences are trivial.
Discussion: The thermal value of ice, I came to realize, lies not in its coldness-- it lies in its solidity. Raising the temperature of liquid water absorbs relatively little heat: one calorie per gram per degree (centigrade). Raising the temperature of ice absorbs even less heat: half a calorie per gram per degree. But melting 0 degree ice to 0 degree water absorbs a great deal of heat: 80 calories per gram. So, the temperature the ice starts at is relatively irrelevant. It's the final melting moment that does most of the cooling work. In fact, to get just 10% more cooling than about-to-melt 0C ice, you have to get the ice wicked cold first: -16C (3F) So, it's not worth looking for cold ice. Just make sure it's solid.
2) A: It costs very little to make ice. If you have the freezer space, you should home-freeze.
Discussion: A 5lb block of ice sells for, what, $2.50? And we learned above that turning 5lbs of water into ice absorbs 181,136 calories*, or 181 kcal. So to freeze 5lbs of water into ice, you need to cool your freezer by the same amount. 181 kcal is the same as 0.21 killowatt hours. Making a wild guess that a freezer is about 33% efficient, you would use 0.63 kwh of electricity to make a block of ice. And doing a little division with my latest electricity bill shows I'm paying about $0.16 per kwh. So that works out to roughly $0.10 per block. Such a deal!
3) A: Keep the meltwater if you're expecting to run out of ice. Drain it if you aren't.
Discussion: Ice-cold water has the potential to absorb some additional heat as it warms to room temperature-- about 41 kcal for 5 lbs of water ("room temperature" around here being 65F). But what's really going on in the ice box? Theoretically, once things have stabilized, and as long as there is any solid ice in the box, everything in there exists at an equilibrium: the ice, the meltwater, and the food all sit at 0C. Heat gradually seeps in through the sides of the box; the ice gradually melts to absorb it. The meltwater is pretty much a passive zero-degree bystander-- it is neither absorbs heat from the food, nor from the ice, both of which are also at 0C. So at this phase, keeping the meltwater in the icebox is neither useful nor deleterious. However, once the last shred of ice melts, the story changes: now the temperature of the icebox will start to rise above 0C. The heat which creeps in will be evenly divided by the fridge contents, all rising in temperature together. And here is where having a large mass of cold water will be briefly useful, because it absorb some heat, will slow the overall temperature rise, and will help keep your food a little less warm, a little bit longer.
Next up for pointless thermodynamic consideration: the old question of whether you should empty your bladder to stay warmer when you're winter camping.
[* 1 block = 5lbs = (5lb) / (2.2lb/kg) *1000 = 2,274 grams. For melting ice, (80cal/g) * (2,274g/block) = 181,136cal/block.]