Fridge Full of Water?

Most recent answer: 10/22/2007

Im having a debate with my mom about whether or not it is more efficient to fill a refrigerator with jugs of water, or just leave the empty space.
- James (age 19)
Hey James,

So I think I’m gonna have to agree with your mom. Empty space in a refrigerator is inefficient.

Imagine a giant refrigerator full of nothing but cold air and one small hunk of cheese. Let’s say that the fridge is holding the inside steady at 35 degrees. When you come along and open the door to get some cheese, the cold and warm air begin mixing together in the refrigerator. And
now that some air from the room has mixed with the cold air, the fridge is going to have to run again to pump that extra heat out of the refrigerator and back into the room. On the other hand, if all that empty space had been filled with jugs of water then there’d be no room for warm air to get inside and replace some of the cold air. So it’ll basically
be the same temperature when you close it again.

The real key here has to do with how heat moves in air. Since air isn’t very dense, it’s a poor conductor of heat. So, heat in air moves primarily by masses of warm and cold air mixing and trading places. In an empty fridge, lots of warm and cold air can change places when you open the door. In a full refrigerator heat can only enter by conduction of heat directly to some object that’s inside (i.e. the cold broccoli isn’t going anywhere, and it isn’t being replaced with warm broccoli so it’ll have to be warmed up by conduction only). And... air is a poor conductor of heat.

I do agree that it’ll take more energy to cool down the
fridge when you first fill it with water jugs. But that
only has to happen the first time. Basically, by filling
the empty space, you’re make a smaller refrigerator.


(published on 10/22/2007)

Follow-Up #1: refrigerator efficiency

Okay, then - would if be more efficient to freeze the bottles full of water before putting them in the fridge, or wouldn't it make a difference?
- Emma
To freeze the water, you'd have to cool it below the refrigerator temperature and then keep pumping heat out to freeze it. The frozen water will then soak up some heat from the refrigerator as it melts and warms to the refrigerator temperature. For any reasonably efficient refrigeration system, it takes more work to pump a given amount of heat out of something colder than something not as cold. So it would be more efficient just to put the water straight in the fridge rather than first in the freezer.

Mike W.

(published on 09/19/2009)

Follow-Up #2: making refrigerators more efficient

Question 1: So why even bother filling those jugs with water? Water's specific heat is very high and it will take a lot of energy to cool it down. Would not empty jugs (or air balloons at that point) be even more efficient?Question 2: Does not this calculation also depend on how often and for how long the door stays open vs. how long that empty space is going to stay empty?
- Jack B (age 33)
Portland OR

1) Excellent point. The empty jugs would also in effect make the refrigerator smaller, reducing the amount of cold air that escapes when  you open the door briefly.  

2) Sure. If you leave the door open a very long time the refrigerator will run full-speed all the time. Then when you close the door, it will need to run longer before getting down to its set temperature if the contents have more heat capacity. So in that very unusual sort of use, having jugs of water would actually increase the net energy use. Your empty-jug idea would work well both for normal use and for the rare case of doors being open for a long time.

Mike W.

(published on 01/14/2016)

Follow-Up #3: packing jugs in refrigerator

would filling plastic bags with packing peanuts and stuffing in refrigerator spaces work better than an empty jug?
- Irene Kanzler (age 80)

The heat capacity will scarcely be affected by the foam peanuts, since they're mostly air. They will slow down heat exchange with the outside, which could marginally reduce enrgy use if the refrigerator is opened often.

Mike W.

(published on 06/26/2018)