Why are freezerless refrigerators more efficient and just out of curiosity, about how much energy can I save per month per cubic feet by going all-refrigerator?

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    $\begingroup$ Your question would be quite a bit stronger if you provided some links backing your assumptions. Likewise, there's an underlying assumption behind your question that is quite likely false. Namely, that there is a linear correlation between efficiency of freezers refrigerators and their capacity. $\endgroup$ – user16 Sep 25 '15 at 2:17
  • $\begingroup$ @GlenH7 I am not making that assumption. I am expecting an answer of the type: "A typical 15 cubic foot freezerless refrigerator such as the Miele’s K1801 will use roughly 20% less energy than a dual freezer-refrigerator of the same size" $\endgroup$ – Wallace Park Sep 25 '15 at 2:42
  • $\begingroup$ Calculating these values basing on theoretical assumptions about power consumption, thermal conductivity and temperature gradients would be very difficult and probably extremely inaccurate. This data is always best to obtain experimentally - instead of aiming at "reducing power usage by 20%", engineers will apply better, less conductive materials, better engine and pump with less friction, hydraulics that transfer heat with less obstructions, more emissive radiators, and when it's done they measure how much the prototype's efficiency improved relative to original. $\endgroup$ – SF. Sep 25 '15 at 11:24

Heat flux is directly proportional to gradient of temperature - in other words, if you have an imperfect insulation wall, it will pass the more heat the higher the temperature difference between its two sides.

Say, your room temperature is 22°C. If the freezerless refrigerator internal temperature is 4°C, and the freezer temperature is -5°C, that's 18°C and 27°C difference between the inside and outside respectively. With the same insulation, the latter will leak heat inside faster than the former, necessitating the pump to run more frequently and remove it for a longer time.

The heat flux is linearly proportional to temperature gradient:

$$ \overrightarrow{q} = - k {\nabla} T $$

The remaining features - energy usage per joule of heat transmitted etc remain (almost) linear as well. Therefore the efficiency change between a refrigerator and a freezer of the same volume, size and construction is directly proportional to the difference between the two gradients. For the above example, 27/18 = 1.5, the freezer will use about 150% of similar refrigerator's energy. But that largely depends on the thermostat settings, ambient temperature, construction, insulation used (the freezer is likely to have it thicker) and so on. If you add a joint refrigerator/freezer into the mix, this becomes quite tricky as the temperature gradient changes throughout the volume. Given a ballpark figure of freezer part occupying 20% of the volume, and assuming the changed geometry doesn't influence it, I can give you a rough estimate of 10% energy savings when using a pure refrigerator vs one of the same volume and parameters, but with a freezer part.

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Speaking about efficiencies in fridges is misleading. You typically use the concept Coefficient of Performance (COP): The heat removed by the heat pump devided by the input in mechanical work. The higher you COP, the less energy input you need to remove a given amount of heat from your fridge.

The higher the temrpeature difference the heat pump has to bridge, the lower the COP. So if you have a freezer, you need to cool further below 0°C, this gives you a higher temp diff to the roomthe fridge is standing in. Hence a lower COP, and more energy needed. So far for thermodynamics. Other factors affecting the actual energy uptake are the insulation, how you operate your freezer, efficiency of the pump, etc.

For the second part of your question, I don't think a comparison between 'typical' freezers will help you much. Look up actual energy demands of actual freezers and make your comparison.

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