The Optimal Heat Sink for my Lousy Freezer

[SwordSmith]

For some reason, my freezer stops its pumps once in a while and it does not start again before I turn it off and on again. I would like to increase the time interval it takes for the freezer to heat up above 0°C where the food starts to decay faster. I therefore need a heat sink. The operating temperature of the freezer is -33°C - as low as it can go.

So the question is, all factors considered, which heat sink is the best for my freezer? I think that water is a decent start since water has a high heat capacity per volume. But the problem is obviously that ice has less than half the heat capaity per volume as does water. So if I could lower the melting temperature of water, I could improve its heat sink abilities. I am in other words interested in maximizing the required heat of heating my substance (water or brine) from -33°C to 0°C.

The relevant sizes are:
Melting point of brine (23.3 percent salt by weight, the rest is water): -21.1°C
Melting point of water: 0°C
Heat capacity of brine as liquid:
Using this formula: Cpsoln = W1 x Cps + W2 x Cpw = 0.233*0.88kJ/(kg*C)+(1-0.233)*4.19kJ(kg*C) = 3.42 kJ(kg*C)
0.88kJ/(kg*C) is the heat capacity of salt. I simply add take the weighted average of the heat capacity of water and salt and use this as heat capacity for solution.
Formula found here:
http://profmaster.blogspot.com/2009/02/heat-capacity-with-dissolved-solids.html

Heat capacity of brine as solid (ice/salt): No idea what to do here - anybody?
Heat capacity of ice: 1.80 kJ/(kg*C)
Heat of fusion from brine as solid to liquid: What do I use here - that of ice?

The formulas for the total heat between -33°C and 0°C become:
U/m=33°C*4.19kJ/(kg*°C)=
U/m=(33-21)°C*heat capacity of brine as solid + heat of fusion of brine from solid to liquid + 21°C *3.42kJ/(kg*°C)

I hope you guys cna help me with my missing constants or if I have missed anything else. The brine seems to be a much better choice than water, right?

 

[Future Bruno]

Yes, the brine solution is definitely a better choice than plain water due to its lower melting temperature and higher heat capacity. The heat capacity of brine as a solid is not easily determined as it depends on the salt concentration of the brine, but it can be approximated by taking the average of the heat capacity of the water and salt components. The heat of fusion of brine from solid to liquid can also be approximated by taking the average of the heat of fusion of water and salt components.