Lake Atitlan, Guatemala
Technology, Kullfallet 

How to stabilize the temperature of a freezer

A common way to stabilize freezer temperature is to insert some bottles of water, which will turn to ice. This will increase the thermal mass of the freezer, so the temperature will be more even over time.

With freezer stabilization, you can extend the time the goods in your freezer will be cold in the case of a power out. This can also be useful for a hand carried freeze box.

My freezer uses a lot of power, which puts stress on my solar powered batteries, so an automatic timer turns off the fridge in the night and on again in the morning, so it can catch up and cool down the goods again.

It is important that the temperature doesn't rise too high during the night though, and here is where the stabilization comes into play. The more thermal mass a material has, the more heat (or cold) it can store, acting like an accumulator. In other words, the more thermal mass that is inside the freezer, the longer it will take to raise the temperature inside, when the freezer is turned off.

One simple way to do this is to fill PET bottles with water and put them in the freezer. Water has a specific heat capacity of 4.19 Joules per gram for each degrees Celsius. In other words, to cool down 1 kg of water (1 liter) from 20 C to -15 C (a 35 C difference), you have to remove 147 kJ of heat energy from the water. 

However, this is not the whole story, because the water also solidifies into ice. This process is called a phase change, and requires that additional energy is removed. The enthalpy of fusion for water is 335 J per gram. So for our example of 1 kg of water, an additional 335 kJ of energy has to be removed, in total 482 kJ. The cooling effect of the PET bottle is 70% due to the thawing of the ice.

A phase-change material is a material that exploits this effect, to provide heat or cold for extended periods of time, because of the large amount of energy that is released or absorbed when a material freezes/melts.

The most common phase-change material is water. Many other materials have been engineered to deliver heat/cold by manipulating the temperature of melting/freezing to fit specific applications.

Improving the phase-change material

When my freezer is turned off, the temperature inside immediately starts rising at a slow steady pace, until it reaches 0 C. The bottles of frozen water now require a large amount of heat energy to thaw, so the temperature stays at 0 C for a long time.

This can be improved by lowering the freezing point of the water to for example -14 C. This will not change the overall cooling effect of the ice, but the freezer will stay at this much lower temperature for a long time while the ice thaws, which is better for my stored goods.

There is a very simple way to do this: add common salt (sodium chloride) to the water. The trick is to know how much salt to add, because different salt solutions have different freezing points. A -14 C freezing point is a 18% salt solution, by weight. This can be hard to measure up on a scale, so an easy way to make this solution is to first make a fully saturated salt solution, and then dilute it by volume.

Heat a pot of water and pour salt into it. Keep pouring and stirring until no more salt can be dissolved. Some grains should rest at the bottom. Let the pot cool off and salt crystals should form on the surface, as a proof that the solution is saturated. Skim them off.

You now have a fully saturated salt solution where 26% by weight is salt and the density is 1.197 kg/liter. You can now mix it by volume according to your needs:

3 parts saturated solution, 1 part pure water: 1.148 kg/L, 20% solution, -16.5 C
2 parts saturated solution, 1 part pure water: 1.132 kg/L, 18% solution, -14 C
1 part saturated solution, 1 part pure water: 1.099 kg/L, 14% solution, -10 C
1 part saturated solution, 2 parts pure water: 1.066 kg/L, 11% solution, -6 C

For example if you want a PET bottle with brine that freezes at -14 C, you mix 2 glasses of the fully saturated salt solution with 1 glass of pure water, and pour the mix into the bottle. Once this bottle is frozen, it will stay at -14 C for a very long time while the brine thaws.

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