Lowering freezer energy consumption

[NickE]

Is it possible to calculate how much energy would be saved if you reduced the running temperature of a freezer from the normal -18°C to say -10°C? Assume that there is no change to the refrigeration system hardware, purely a thermostat setting. Also, assuming that the products stored in that freezer would survive the experience!

 

[KenGr]

To be exact could be very complicated. However you can closely approximate it very simply. If you assume the freezer is leak tight and seldom opened, the heat loss is largely due to conduction. At room temperature convection or radiation effects won't make a difference. Conduction is proportional to the surface area and the temperature differential. Assuming the room is about 20C, in one case you have a differential of 30C and in the other case a differential of 38C. Therefore, by raising the temperature you would use only about 79% as much energy.

 

[glugglug]

Alternatively, you could lower the temperature of the room...

 

[davee]

isnt there a formula w=mct-losses or something

 

[KenGr]

The equation for conduction is very simple:

The heat loss rate Q/t = [(kA(Thot-Tcold))/d] where Q/t is heat transfer rate (BTU/hr), k is the heat transfer (conduction) coefficient, A is the surface area, T is the temperature and d is the thickness of the conducting material. For compound materials you can solve simultaneous equations or calculate homogenized coefficients. As you can see, the only variable in the problem you posed is the temperature inside the freezer.

Freezers have constant speed, constant condition compression refrigeration cycles so they operate at essentially constant efficiency and cycle on and off. Therefore, the energy used is directly related to the heat transferred into the freezer.

Convection and radiation equations are very complicated but don't really enter into this problem. The flow around the freezer is so slow and the surface temperature so close to ambient we would usually approximate convective effects by a "surface conduction coefficient".