Cooling airflow calculation

Question

I want to unplug my refrigerator in the winter, and cool it down by taking air from outside. The idea is to lead cool air from the outside through a pipe into the fridge, then let hot air out of the fridge with another pipe to the outside, powered by a small fan.

By looking at the electric power consumption, I estimate that the heat transfer through the walls of the fridge is approx 50 W, when the fridge temp is +5 C and room temp is +20 C.

How can I calculate the needed airflow from outside (L/min) if I want to maintain +5 C when it is -10 C outside?

Answer 1

Having a fixed airflow will not be very appropriate for the things stored in the fridge. First of all, you can't be certain what the external temperature will be.

It would be preferable to use a thermostat to turn on and off the fan.

Even better would be to regulate the speed of the fan again with a thermostat and some sort of PI control. The reason is that if you have a very high speed on the fan, the cold air at -10C is going to freeze the first things that are in the fridge (if they are fruits and veg they will be ruined). Regulating the speed (and even better using some sort of diffuser) will make the fan rotate at much lower rpms (and therefore air velocity).

Another point is that it might be preferable to use a different box altogether (i.e. don't drill holes into your fridge).

Answer 2

Let's assume you 50 W is the correct figure. Then you need to solve $50 W = C_{air}*(T_{out}-T_{in})*\dot{m}$ with thermal capacity at constant pressure C and mass flow rate $\dot{m}$ (thisgives you your volumetric flowrate.). The tricky part is $T_{out}$ - this will be between the temeprature of the stuff in your fridge (around 5°C) and the incoming air (-5°C). I don't think this can be calculated with reasonable accuracy and effort.

If you really want to go ahead with this project, follow NMech's idea and be aware that part of your fridge might freeze in the -5°C air.