To go slightly higher tech than a water tank, you could line the inside of the fridge with bags of phase-change material keyed to around 2.8°C. Then you program the thermostat to shut off at 1.0°C and turn on if 2.8°C or greater (and powered).
That ensures that the phase-change material is always completely frozen after a long period of being powered, and keeps the fridge at a good refrigeration temperature.
Tetradecane paraffin seems like a good starting point. It's not always easy to find phase-change data on heavier organic molecules, even plain old alkanes, but here are a few in about the right range:
The closer the branch is to the center of the chain, the lower the freezing point. It's likely one could get a decent refrigerator-liner phase-change material just by chilling liquid mixed alkanes to 4.0°C, filtering out any solids, then chilling to 1.0°C, filtering again, and keeping those solids. Those are exactly the types of molecules refiners hate. Similar isomers are already removed and reprocessed to improve the cold-temperature characteristics of diesel fuel, because otherwise they gel, and gum up cold-flow filters. Rather than cracking them all into more desirable fuel molecules, some could be diverted as fridge wax.
That ensures that the phase-change material is always completely frozen after a long period of being powered, and keeps the fridge at a good refrigeration temperature.
Tetradecane paraffin seems like a good starting point. It's not always easy to find phase-change data on heavier organic molecules, even plain old alkanes, but here are a few in about the right range:
The closer the branch is to the center of the chain, the lower the freezing point. It's likely one could get a decent refrigerator-liner phase-change material just by chilling liquid mixed alkanes to 4.0°C, filtering out any solids, then chilling to 1.0°C, filtering again, and keeping those solids. Those are exactly the types of molecules refiners hate. Similar isomers are already removed and reprocessed to improve the cold-temperature characteristics of diesel fuel, because otherwise they gel, and gum up cold-flow filters. Rather than cracking them all into more desirable fuel molecules, some could be diverted as fridge wax.