'Saving Energy' in the Freezer Compartment

evolving_doors

Ok so on foot of a conversation that''s a bit urban mythish I was wondering what the Termo experts on here thought!

Basically I was told that if you had an empty freezer that it would use a lot of energy. To counteract this he suggested filling the freezer with a tonne of wrapped up newspaper or some such filler to 'keep in the cool' (for want of a better expression!).

My thinking was that it would take more energy to cool a something (newspapers) than a nothing (air!)

Who's right?

Labarbapostiza

Armelodie wrote: »

My thinking was that it would take more energy to cool a something (newspapers) than a nothing (air!)

Air is not nothing. A vacuum is nothing. Air is gas - oxygen, nitrogen, etc.

A fridge works by pumping heat out of the freezer - the energy expended is in the pumping. It takes as much energy for the pump to cool a kilo of air from room temperature to freezing, as it does to cool a kilo of water to freezing. When you open your fridge, if there was a kilo of frozen water, would it unfreeze the instant you opened the door, or would it take much longer.

Solids, cool and heat up slower. Air will keep bleeding in and out of your freezer - unless you've got a super airtight lock. The less air, in your freezer the cheaper to run it will be. A well packed freezer, in a cold room, will use very little electricity.

SOL

Labarbapostiza wrote: »

A fridge works by pumping heat out of the freezer - the energy expended is in the pumping. It takes as much energy for the pump to cool a kilo of air from room temperature to freezing, as it does to cool a kilo of water to freezing. When you open your fridge, if there was a kilo of frozen water, would it unfreeze the instant you opened the door, or would it take much longer.

This is entirely wrong and or misleading!
Even ignoring phase transitions the amount of energy it takes to cool air by one degree is 2-4 thousand times less than the amount of energy required to cool an equivalent volume of water! You are never going to have more than a few hundred grams of air in the entire freezer however if you filled it with water you could have 100's of kilos depending on your freezers capacity, so making a kilo for kilo comparison of air and water is not really relevant. Even still water has a higher specific heat capacity by a factor of 4 not to mention the freezing transition which takes about 80 times the energy required to heat/cool by one degree!

http://en.wikipedia.org/wiki/Heat_capacity


As for the myth, well if you freezer contains nothing but air it will convect the heat inside the freezer quite well so that any warmer spots inside the freezer will more effectively heat up the entire freezer. If you fill you freezer with a material like crumpled newspaper or Styrofoam it will block the convection and possible help your freezer stay cooler longer. Of course it will cost more energy to get your freezer to temperature but it may cost less energy to maintain it at temperature.

As to whether the effect would even be noticeable, I have no idea.

Labarbapostiza

SOL wrote: »

This is entirely wrong and or misleading!

Okay, no need to shout. I was thinking in terms of energy, and thermal equilibrium, and disregarding a few things.

As for the myth, well if you freezer contains nothing but air it will convect the heat inside the freezer quite well so that any warmer spots inside the freezer will more effectively heat up the entire freezer. If you fill you freezer with a material like crumpled newspaper or Styrofoam it will block the convection and possible help your freezer stay cooler longer.

But you need gaseous convection to carry heat away from the solids.

But the more space you have, the more room for warm gas entering from outside then freezer.

Of course it will cost more energy to get your freezer to temperature but it
may cost less energy to maintain it at temperature.

The energy used by a freezer is the cost of pumping energy out of the freezer. Solids will lose their energy slower, but this also applies in reverse. Gases in the freezer will heat much quicker - infrared from outside will enter. Meaning the pump cycle has to be activated more often, than if the freezer was mostly solid.

As to whether the effect would even be noticeable, I have no idea.

In my experience it is noticeable.

evolving_doors

Thanks folks, not sure if I'm any wiser though. I get the feeling that this 'myth' is an offshoot from the 'litre bottle in the cistern' notion.

One thing I forgot to ask was about how 'smart' a fridge is. Maybe there's the same input of cold air regardless of what's in the fridge! I,e does a fridge freezer respond to the temp fluctuations and act accordingly ?

Labarbapostiza

Armelodie wrote: »

One thing I forgot to ask was about how 'smart' a fridge is. Maybe there's the same input of cold air regardless of what's in the fridge! I,e does a fridge freezer respond to the temp fluctuations and act accordingly ?

I'm not going to explain fully how it works but the freezer pumps heat/warmth out of the freezer. The electricity to run the pump is what costs.

The pump is switched on by a thermometer somewhere in the fridge - when the temperature begins to rise, the pump comes on and pumps the heat out, dropping the temperature. And then the pump switches off after the temperature falls.

Having the fridge packed to save energy doesn't sound like a myth.

evolving_doors

Labarbapostiza wrote: »

I'm not going to explain fully how it works but the freezer pumps heat/warmth out of the freezer. The electricity to run the pump is what costs.

The pump is switched on by a thermometer somewhere in the fridge - when the temperature begins to rise, the pump comes on and pumps the heat out, dropping the temperature. And then the pump switches off after the temperature falls.

Having the fridge packed to save energy doesn't sound like a myth.

Ok thanks for clearing that up.

Leaving aside the process of initially getting the substance to equilibrium with the fridge's desired temperature, does the fridge have to keep kicking in to keep it there?

I presume it does so periodically anyway, but does it kick in more often when theres more stuff in there?

Labarbapostiza

Armelodie wrote: »

Leaving aside the process of initially getting the substance to equilibrium with the fridge's desired temperature, does the fridge have to keep kicking in to keep it there?

The fridge has to keep kicking in to keep cool.

I presume it does so periodically anyway, but does it kick in more often when theres more stuff in there?

The only way to be really sure is to do an experiment. You could always email your fridge manufacturer, they would have done an experiment.

Capt'n Midnight

If you have a chest freezer it doesn't matter so much.

Otherwise what's happening is when you open the door is cold air being heavier sinks out the door. Filling the freezer just means less cold air can flow out each time.

evolving_doors

Capt'n Midnight wrote: »

If you have a chest freezer it doesn't matter so much.

Otherwise what's happening is when you open the door is cold air being heavier sinks out the door. Filling the freezer just means less cold air can flow out each time.

Leaving aside the initial opening and closing, just thinking about the difference between a full and empty freezer, which freezer would be less of a draw on electricity?