Question for the physics gurus

[Homeslice]

Does it take more energy to run a completely full refrigerator/freezer, compared to a nearly empty one? Or does the "coldness" stored inside the food help keep everything cool longer, thus saving power?

[Tmall]

The only way I could see this working is this...

Heat goes from a high to low concentration. The more frozen things in there, the greater the ability for heat to be transferred.

I could see this cooling down the hotter object more quickly, but the heat still needs to be removed from the compartment.

I'm no thermo expert, but we did do a few weeks on heat transfer.

[Adeptus_Minor]

First, a few assumptions:

1. No defroster
2. No kids opening the lid for a popsicle
3. No items in the full freezer that can generate heat
4. All items in the full freezer are already at the desired temperature when the experiment begins.
5. There are no air leaks once the door is closed.

Now, as Nick pointed out (I know you're Tmall now, but you'll always be NickName to me), heat moves from higher concentration to lower concentration.
Heat passes more quickly through solids and liquids than through gases.
The only way for heat to get into the freezer is through the walls.
If you have less in the freezer and there's an air space between the contents and the inner walls of the freezer, then heat conduction into the freezer should be slower than if you have solids against the inner walls of the freezer.

However, we all know kids want their damn popsicles, and a freezer with more in it should be more resistant to temperature changes when the door is opened and shut because the solids are there as opposed to just cold air which can be easily displaced. This assumes that the door isn't open long enough for significant heat to be transferred into the solid contents.

That's just my thinking... :idk:

[derf]

Its all about air volume. The more stuff in the fridge, less air volume, less air for the fridge to cool off.

Assuming that the door isn't left open excessively each time it is opened it takes less energy for the fridge to replace the cold air that escaped thus using less power.

[Homeslice]

Hmm I never thought of it in terms of air volume. But I did know they recommend keeping jugs of frozen water in your freezer in case the power goes out, it will help keep your food cold longer.

[Rider]

I have no idea and I don't really care. My refrigerator keeps my stuff cold, that's all that matters. The difference in energy costs between a full or empty refrigerator would be pennies a month...not enough to be concerned about.

However, I do know that the outside temperature plays a part. I used to keep a refrigerator in my garage for beer and soda and large freezer items. In the winter when it would be below freezing outside, the food in the freezer section would begin to thaw and the stuff in the refrigerator compartment would freeze. Why, I have no idea, but I no longer keep my refrigerator in the garage. It's now in my basement and it works great all year.

[Tmall]

You guys are forgetting that energy can't be created or destroyed. Only manipulated.

The heat in the fridge is the amount that must come out. Only the cooling rate would change. Not the work being done.

[fasternyou929]

You guys are forgetting that energy can't be created or destroyed. Only manipulated.

The heat in the fridge is the amount that must come out. Only the cooling rate would change. Not the work being done.

I would think you're right.

A fridge/freezer should have the same efficiency whether full or empty once all items stored in them are at the same temperature as the air.

The only way to make a fridge/freezer less efficient is to put something in it that it needs to cool down.

The only way to make a fridge/freezer more efficient is to improve its insulation, seals, or cooling system.

[101lifts2]

I would think you're right.

A fridge/freezer should have the same efficiency whether full or empty once all items stored in them are at the same temperature as the air.

The only way to make a fridge/freezer less efficient is to put something in it that it needs to cool down.

The only way to make a fridge/freezer more efficient is to improve its insulation, seals, or cooling system.


Yes...non-living items do not create or reject heat in a closed system (refrig). So...once the items reach the same temp as what the cooling system is giving off, its the same energy to maintain the heat empty or with food .

The only reason they tell you to keep water in there is that the frozen water will then accpet heat once the power goes out. It has nothing to do with energy.

Thermodynamics 101.

101

[EpyonXero]

Once the items in the freezer are at a stable temperature the freezer will use less energy than an empty freezer because air loses heat faster than most food items.

Putting jugs of water in an empty or almost empty freezer will cause the freezer to do more work in order to cool the water but if the power were to go out the freezer would remain colder longer because it takes more energy to heat water than air. However, if your freezer is full adding water isnt necessary because the food will keep each other cool.

[fasternyou929]

Once the items in the freezer are at a stable temperature the freezer will use less energy than an empty freezer because air loses heat faster than most food items.

Did you notice you contradicted yourself there? If air loses heat faster than food items, air (an empty freezer) would be the most efficient scenario.

Putting jugs of water in an empty or almost empty freezer will cause the freezer to do more work in order to cool the water if the power were to go out the freezer would remain colder longer because it takes more energy to heat water than air. However, if your freezer is full adding water isnt necessary because the food will keep each other cool.If a package of ground beef is chilled to 0F and placed in a freezer where the air temp is 0F, how do you suppose ground beef is going to generate heat? Remember, in thermodynamics there's no such thing as "cold", just loss or addition of heat. For food to make the freezer less effective it would need to generate heat.

Food, water, etc. can not "keep each other cool".

[Adeptus_Minor]

If a package of ground beef is chilled to 0F and placed in a freezer where the air temp is 0F, how do you suppose ground beef is going to generate heat? Remember, in thermodynamics there's no such thing as "cold", just loss or addition of heat. For food to make the freezer less effective it would need to generate heat.


You know, I did have this idea for endothermic food.
Most food, when metabolized, is used to generate heat... but what about a food that actually cooled you off when you ate it?

Bah, it'd probably make you fat or cause cancer anyway. :rolleyes:

[Homeslice]

Food, water, etc. can not "keep each other cool".

Except if the power goes out :shrug:

Or if you are buying groceries and put the frozen food together so that it doesn't thaw while you're driving home :lol:

[101lifts2]

Once the items in the freezer are at a stable temperature the freezer will use less energy than an empty freezer because air loses heat faster than most food items.....

Its a non-living item meaning it will not produce chemical reactions causing heat. Once the items are at the same temperature as the freezer they will remain. Same goes with the air. The energy to maintain the temp is the same unless the frig has huge leaks in the weatherstripping.

[fasternyou929]

Except if the power goes out :shrug:still not true unless you open the freezer. But that scenario is not the original question and has zero impact on a freezer's efficiency.

[Homeslice]

still not true unless you open the freezer.

Even if you don't, solids and liquids are more dense than gases, with more molecules per sq inch........Therefore they can act as a better heat absorber, to protect against invading heat once the power goes out.

I got the tip about putting jugs of water in the freezer from the power company, I think they know what's up :shrug:

[EpyonXero]

Did you notice you contradicted yourself there? If air loses heat faster than food items, air (an empty freezer) would be the most efficient scenario.

If a package of ground beef is chilled to 0F and placed in a freezer where the air temp is 0F, how do you suppose ground beef is going to generate heat? Remember, in thermodynamics there's no such thing as "cold", just loss or addition of heat. For food to make the freezer less effective it would need to generate heat.

Food, water, etc. can not "keep each other cool".

(sigh)

Its not a contradiction at all. If your goal is to make a freezer cold as fast as possible then an empty one would be the most efficient because, as I said, air loses heat quickly. But, if you want your freezer to stay cold when the power goes out and run more efficiently then a full freezer is better. The property that makes air fast to cool also makes it quick to lose heat though the freezer walls.

I dont know why you think I said anything would generate heat. Im talking about putting food or a jug of water, which obviously isnt frozen yet, into a freezer. Obviously adding a jug of liquid water to a freezer will raise the temperature of the system and cause the freezer to work to cool it until it reaches whatever the temperature the thermostat is set at. The freezer wants a constant temperature, anytime you alter that it causes the freezer to do more work.

The more mass in a system that needs to be heated the longer it will take for that system to increase in temperature for a given amount of heat transfer. This is why food and water in a full freezer can keep each other cool. If that wasnt the case, then nobody would bother putting ice on things that are already frozen.

[fasternyou929]

(sigh)

Its not a contradiction at all. If your goal is to make a freezer cold as fast as possible then an empty one would be the most efficient because, as I said, air loses heat quickly. But, if you want your freezer to stay cold when the power goes out and run more efficiently then a full freezer is better. The property that makes air fast to cool also makes it quick to lose heat though the freezer walls.
Sigh is right.

You keep bringing up "when the power goes out" and for the last time, you're right. I agree with you. Spot on!

I was talking strictly about the efficiency of a closed system, and it's not affected by adding or subtracing food. Unless, like you said, you put in food/liquid that the system needs to work to cool.

[EpyonXero]

Sigh is right.

You keep bringing up "when the power goes out" and for the last time, you're right. I agree with you. Spot on!

I was talking strictly about the efficiency of a closed system, and it's not affected by adding or subtracing food. Unless, like you said, you put in food/liquid that the system needs to work to cool.

Look, my response was to the original question which is about a real freezer and not a closed system.

[askmrjesus]

Food, water, etc. can not "keep each other cool".

Bullshit!

It happens all time.

Cantaloupe: I'm going to fuck me some bacon!

Eggs: Back off the bacon, melon boy!

Mayo: brumgble!

Ketchup: What?

Mayo: BRUMBGLE!

Ketchup: You're fucking high.

Potatoes: Don't make me come outta this drawer, I will fuck you up.

Sour cream: Yeah!

JC

[Homeslice]

So let's review:

1) If your freezer is brand new and you plug it in for the first time, it will take more power to bring a full freezer down to specified temperature than it would for an empty one.

2) If the freezer is already at the desired temp, and the door is never opened, then it doesn't matter if it's empty or full, its power consumption will be the same. However, if the door is opened, or if the seals leak, then an empty freezer will consume more power.

3) If the power goes out, then a full freezer will stay cold longer than an empty one.

[askmrjesus]

So let's review:

1) If your freezer is brand new and you plug it in for the first time, it will take more power to bring a full freezer down to specified temperature than it would for an empty one.

Not true. If you fill your new freezer with stuff that's already frozen, it would take less time.

2) If the freezer is already at the desired temp, and the door is never opened, then it doesn't matter if it's empty or full, its power consumption will be the same. However, if the door is opened, or if the seals leak, then an empty freezer will consume more power.

Also not true. If the freezer is empty, it's doesn't need any power.

3) If the power goes out, then a full freezer will stay cold longer than an empty one.

True, but who cares? If there is nothing in the freezer, what difference does it make? :lol:

JC

[Homeslice]

ermkay

[askmrjesus]

ermkay

ermkay is right.

Do you sit around all weekend, wondering whats happens in your freezer when the light goes off? :lol:

Dude, go on a date.

JC

[anthonyk]

Since the specific heat of water ice is about twice that of air (and frozen food usually has a bunch of water in it), an empty freezer will change temp more quickly than a full one given the same amount of heat transferred. So I guess the power company knows what's up.

But yeah, operating on a day-today basis? I'm pretty sure it's a wash.