Thermal dynamics – the best freezer blocks for your cool box

I use a passive coolbox when camping, and with careful management and a few extra bags of ice during the week it works well.

I used to use a load of the blue plastic freezer blocks, but they seem to take more space for the volume they give, and now freeze square bottles of water or ice cream tubs.

Looking in the campsite freezer the other day and everyone else is still using freezer blocks.

So I started wondering about the science of it. I presume in the freezer blocks there is some kind of gel (although it doesnt sound like it if you slosh them around, so perhaps this is variable depending on quality of them), the gel would have a greater density than water so would essentially “store more cold”, keeping your coolbox cooler, for longer.

If you extrapolate that, would it be better to “freeze” lumps of metal or stone in your freezer and use these instead, as they are even more dense than the gel?

Not sure where I’m going with this, other than I wondered!

The blocks contain a solution of water + stuff.

So the main latent heat capacity of them is from the water, so in that sense any old ice is just as efficient.

But the ‘stuff’ is antifreeze, which lowers the freezing temperature, which means they sit quite happily at -10 to -18 or so keeping whatever else is around them at 0C if needed.

So if you put icecream and ice packs in the same cool box, the ice packs will melt first keeping the icecream solid.

If you put icecream and icecubes in a cool box, the icecream would melt first. Assuming that icecream melts below zero, which it might not do, but either way the ice packs will keep it colder.

You can prove this to yourself by taking a jug if icecubes, let them sit in the room untill they just start to melt (they’ve reached OC) pour salt over them and letting them melt a bit more, and the resulting solution is significantly colder (about -18 depending on the concentration of salt in the water).

Interesting, thanks. So which is best if you dont want to keep anything frozen, you want it to act like a fridge for as long as possible?

the gel would have a greater density than water so would essentially “store more cold”, keeping your coolbox cooler, for longer.

The key thing is how much energy it takes to thaw the ice crystals, the phase change from solid to liquid is what absorbs the energy. From memory, it takes about as much energy to thaw ice as to heat water to 80 decrees C. Something like a block of steel would not be useful because there is no phase change, it just stays as a solid.

Water is very good at this and ice is easily available. Just fill your cool box with ice.

And, as described above, frozen salt water will take more energy to melt.

I have used several Yeti blocks with a bag of ice that actually started to refreeze the melt water and the burgers within. Was in a Yeti box as well mind.

Pre charging a coolbox (put a frozen 2l bottle in it the night before you leave) is a good way to keep it cooler for longer as well

And, as described above, frozen salt water will take more energy to melt.

You could Google it to check, but I don’t think the actual latent heat changes, just the temperature.

So which is best if you dont want to keep anything frozen, you want it to act like a fridge for as long as possible?

It wouldn’t matter. They’d be about the same. I suppose the differences would be:
Ice packs are basicly free and don’t require you to go to the shops to buy them, and don’t make everything wet.

But you can buy 2kg of ice at a time which would last longer than a couple of small blocks.

Although I guess there’s nothing stopping you buying 2kg of ice packs before you go either

You could Google it to check, but I don’t think the actual latent heat changes, just the temperature.

Yes, I think that’s correct, but the melting point is much lower so the temperature of the ice will be reduced. It should therefore stay cool for longer in a cool box, as long as you only have food that you can freeze solid.

A couple of 3L pop bottles. Part fill with water. Squash flattish. Freeze. Lots of ice to keep your goodies cold.

Things that will influence how long your box stays chilly for include:
Surface to volume ratio of ice
Temp of goods going into the box
Time spent open
External temp

Unless your goodies are in direct contact with the icy things they’re unlikely to freeze. Unless you have some super-amazing cool box.

Dry ice works well to keep samples chilly in transit for a few days. Though maybe not cost effective for camping.