Question for the physics gurus

Homeslice · 07-18-2009, 03:55 PM

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 · 07-18-2009, 04:44 PM

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 · 07-18-2009, 04:59 PM

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...

derf · 07-18-2009, 05:09 PM

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 · 07-18-2009, 07:09 PM

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 · 07-18-2009, 07:28 PM

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 · 07-18-2009, 07:29 PM

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 · 07-19-2009, 05:36 AM

Quote:

Originally Posted by 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.

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 · 07-19-2009, 10:17 AM

Quote:

Originally Posted by fasternyou929

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 · 07-19-2009, 12:35 PM

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.

07-18-2009, 03:55 PM	#1
Homeslice Elitist Join Date: Nov 2008 Location: SF Bay Area Moto: Gix 750 Posts: 11,351	Question for the physics gurus 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? Last edited by Homeslice; 07-18-2009 at 03:57 PM..

07-18-2009, 04:59 PM	#3
Adeptus_Minor Hopster Join Date: Nov 2008 Location: Austin, TX Moto: 2009 Buell 1125R Posts: 4,743	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... __________________ “Well, obviously before; after was all gendarmes and dick stitches.”

07-18-2009, 05:09 PM	#4
derf token jewboy Join Date: Nov 2008 Moto: CBR 900, KLR ugly ass duckling, Gas Man Posts: 10,799	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. __________________

07-18-2009, 04:44 PM	#2
Tmall Aspiring Rapper Join Date: Nov 2008 Location: Halifax, NS Moto: '12 CB1000R Posts: 3,569	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.

07-18-2009, 07:09 PM	#5
Homeslice Elitist Join Date: Nov 2008 Location: SF Bay Area Moto: Gix 750 Posts: 11,351	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.

07-18-2009, 07:28 PM	#6
Rider Moto GP Star Join Date: Feb 2008 Posts: 12,156	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.

07-18-2009, 07:29 PM	#7
Tmall Aspiring Rapper Join Date: Nov 2008 Location: Halifax, NS Moto: '12 CB1000R Posts: 3,569	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.

07-19-2009, 12:35 PM	#10
EpyonXero AMA Supersport Join Date: Nov 2008 Location: Redneck Riviera, FL Moto: 2003 VFR800f6 Posts: 2,531	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.