You have two fridges, one empty, and one full of food.
AFTER they both reach the same temperature (ignoring the time it takes the full fridge to cool its contents), does the full fridge use more power than the empty one to maintain its coldness?
I don’t think so. I think the energy requirements would be pretty close to equal.
Possible reasons the full fridge would take more power: part of the power has to be used to circulate cold air, and the air would run into more obstacles in the full fridge. The things, I don’t think the fan would actually stay on any longer in the obstacle-ridden fridge, it would just be a little less windy in there.
Possible reasons the empty fridge would use more power: there is more heat capacity in the full fridge, that is, more stuff to absorb small changes in temperature. Like the way a cooler full of water will resist heating in the sun faster than a cooler full of styrofoam. This would make the inevitable heating, due to the fridge being in a relatively warm environment (mine’s right next to a gas stove with a standing pilot light, pretty darn inefficient), faster in the case of the empty fridge.
All of these bets are off if you due take into account the amount of time it takes to chill a room-temperature food item, or (worse yet) a lasagna hot out of the oven.
My momma always said that full fridges (and freezers too) take less electricity.
How to say it – the cold stuff already there will help cool the new stuff?
I’m pretty sure I read this somewhere too. Only they said it better.
I think for normal uses, an emptier refrigerator takes a little more power. The reason? Because when you open the door, cold air escapes, but cold food doesn’t escape. The empty space will largely be replaced by warm air each time you open the door. More warm air, more work.
Let’s go the extreme to illustrate why it takes less energy to maintain a refigerator that is full:
We have a walk-in freezer at work. let’s say the shelves were not-so filled, i.e. a lot of empty space. every time the door is opened, the warm air replaces some of the cool air. When you close it, the freezer very nearly returns to its original temperature, partly because of the food in there, which is at optimum cold temperature. If the freezer were opened a lot, or for an extended period, the thermostat would kick in and the temperature would recover, partly due to the heat exchange of the freezer unit, partly because of the convection of the cold food items already there.
Now it gets interesting (provided you are not asleep yet)
the less cold mass in the freezer (frozen food) the harder the unit must work to recover and maintain temperature, because it must make more of the difference on its own, where if the freezer were chock full of goodies, they’d give a significant amount of aid to the unit to chill everything off again.
This principle–I['m sure it has a scientific name–came into valuable practical use last year…when my freezer went on the fritz…but due to the fact that everything was already frozen, and it was brimming full, the temperature never got above freezing even after 1 1/2 days without working.
The converse is true…which is why saunas usually have lava rocks on the heating unit to help maintain the steaminess we’ve all grown to know and love.
For the sake of argument, imagine the doors are never opened (so nothing is added or taken out of the fridge, be it food or air).
Would the energy requirements be the same?
Given the permanently closed freezer, I would speculate that a full freezer would take slightly more energy that an empty one due to the thermal radiation of the items inside.
Reread the OP, Undead. He means after the freezer reaches steady state.
undead dude… IF the items were JUST recently placed in the freezer… if everything if frozen and down to the freezers lowest temp both freezers should use the same amount of energy whether a permenently closed freezer is full or not.
The wisest man I ever knew taught me something I never forgot. And although I never forgot it, I never quite memorized it either. So what I’m left with is the memory of having learned
something very wise that I can’t quite remember. -George Carlin
beaten by a minute… if only I had just gotten to the point sooner… haha
I know that he meant stable state, but let me rethink what I said slightly. I do think that radiation is a likely cause for a difference, and I would still guess that a full freezer or refrigerator would take more energy, but the issue would be absorption of radiation from the outside, since the radiation coming in would be greater than the radiation coming out.
Doesn’t it seem like solid objects would absorb more of this radiation than would empty air?
Refrigerators cool by circulating the freezer air. I’ve heard my brother-in-law, who runs an appliance store, advise customers to keep their freezers full. Apparently this helps keep everything cold. Think surface area. He advises keeping a bag of ice in the freezer if you don’t have enough stuff to freeze.
Just a note:
Most everything that follows has already been said. I’m just providing some quantitative values for reference.
For the following I’m going to use strictly English measurements. This is because insulation R factors have units of BTU/hr/sq. ft/delta F. That is, BTU per hour per square foot per degree difference Fahrenheit.
Deep freezes I’ve seen are 6 ft long, 3 ft wide and 3 ft deep. That is 90 square feet of surface area. Assuming an indoor temperature of 75 F., and a freezer temperature of 20 F, the BTU/hr requirements for this can be computed based on the R factor of the insulation and the number of air exchanges per hour. Let’s assume that 2-inch foil faced polyisocyanurate foam is used at the insulation. This material has an R factor of 14.20. The BTU/hr requirements for this deep freeze would be:
Ereq=(Area)(Delta T)/R
Ereq=9055/14.20
Ereq=349 BTU/Hr
Now this result does not take air volume exchanges into account. There’s a simple equation that computes BTU cooling requirements based on air exchanges per hour. It’s:
Ereq=(number of air exchanges/hr)*(air volume)0.018(delta T)
For an empty freezer and one exchange an hour this works out to be: 53 BTU/hr
For a half full freezer (by volume), one exchanges it’s: 27 BTU/hr
So fuller freezer gains less heat when the door opens, so it takes less energy to return the air to the correct temperature.
::SPECULATION, in place of a mathematical treatment::
Freezer contents have their own heat capacities, and as such the more contents, the more energy required to reduce their temperature to the desired value. Likewise, the contents would help buffer the temperature to some degree thus providing a more stable temperature, and maybe reducing the number of cooling cycles per day. This assumes that there is some sort of hysteresis in the physical thermostat.
As for radiation: I’m not sure about it. There are some nagging black-box radiation results swirling around in my head. Primarily “A good emitter is a good absorber,” but I can’t seem to either apply or dismiss it. Since that cabinet is usually steel, and insulation typically has some sort of foil on it. It may be possible that the effects of radiation are negligible, or at least non-significant. I’m thinking along the lines of the radiation (as photons) being trapped inside until absorbed. You know, just like how a stack of razor is absolutely black when observed on edge.
"For the sake of argument, imagine the doors are never opened (so nothing is added or taken out of the fridge, be it food or air).
“Would the energy requirements be the same?”
The only reason any energy is necessary to cool the closed fridge is because of the difference in temperature between inside and outside of the fridge. Everyone has heard their fridge run in the night when it is not opened, or in the morning before opening. That’s because it is working to maintain the temperature difference, even while no one opens the door.
Air has relatively little mass compared to whatever items are enclosed in the fridge. The greater the total cold mass - meaning the total mass of the fridge plus its contents - the greater the “loss” of cold by heat exchange. (Instead of loss of cold, the physicist might call this heat absorption by the fridge.) So all other things equal, the more full the closed fridge is, the more energy it needs.
Concerning the other aspect of the real-life problem, the physics of gases is extremely complicated mathematically, and the number of factors that come into play in the simple act of opening a fridge door makes for an extremely complicated physics/mathematics problem. The mass of the air exchange when the door is open is fairly minimal given both the small mass of air, as well as the inertia of air. Furthermore, the greater the mass of cold objects in the fridge, the greater the contact with warm air entering the fridge when the door is open. These objects will absorb more heat if there are more of them.
Further complicating the problem are:
- ambient temperature
- frequency and length of door opening
- the temperatures and mass of objects being removed or returned or placed in the fridge
These three “complications” certainly have the potential for a much greater effect than the original issue raised.
Otherwise, if items are placed in the fridge simply to increase volume to reduce air flow, their mass will also come into play. IF air movement is seen as the primary factor of loss of cold, then an EMPTY water bottle would be more effective than a full one, because it would lose less cold to air movement than a full bottle which has greater mass.
There is one reason for having “cold ballast” in a fridge (frozen water bottles, for example), especially in the freezer. That is because new items placed in the freezer will get cold more rapidly, since the surrounding cold mass will be greater. (This doesn’t theoretically change energy consumption, just speed of energy use. The water bottles, after passing some of their cold to the new objects, will then need energy to return them to their previous cold temperature.)
Even the difference in temperature between the inside and the outside isn’t the reason you need energy, though it’s close. The reason you need to spend energy is that there’s constantly some heat leaking in, and you need to pump that much heat back out. The amount of leakage is proportional to the temperature difference, and also depends on the insulation, but it does not depend at all on the contents, and so the energy required should also not depend on the contents.
This, except that freezers are the best place to keep zombies.
I also want to add that in the real world, a refrigerator that is never opened or closed is a rather useless energy-consuming space-taker to keep in your kitchen.
I’ve heard this question many, many times in my life, long before the OP was posted, and the one who poses the question always throws in “But, what it you never opened the door or took anything in or out?” If you are not going to ever open the door or take anything in or out, why even have the refrigerator in the first place?
Refrigerator technology has changed in the last 20 years, so that might affect the answer. 
Is this thread the winner for the oldest revived zombie? I don’t recall an 18+ year gap between posts before.
I would think a full refrigerator would run more shorter cycles as the compressor can only cool the circulating air. Being as there is less air it would rapidly cool to below the t-stat off pont then pick up the heat from the mass inside rapidly and warm, needing to restart the compressor.
My guess is there’s very little difference in how much energy it takes to run a full fridge or an empty one. But there’s a big difference in the temperature stability inside.
If you open a nearly-empty fridge, all the cool air slumps out, and is replaced by warm air, heating your single carton of orange juice.
It may not take a ton of energy for the fridge to bring all that low-heat-holding air back down to cool, but your orange juice is sitting next to warm air until it does.
Whereas if you have a full fridge and open it, not much air is replaced, and the thermal mass of all the error in the fridge quickly brings it’s temp down to nearly the temp it was before.
Now, the fridge has to work to cool all that thermal mass again, even though it’s a small temperature change. So I don’t think you save much money.
But none of your food warmed up enough to matter. And that’s a win.
I think that’s why we are often advised to keep the fridge full. Because it works better, not because it’s cheaper.
Free/natural convection occurs inside a refrigerator due to buoyancy forces and temperature gradients. Heat is able to get into the refrigerator due to these convection currents via “convective heat transfer.” In other words, air inside the refrigerator that is able to freely move around tends to make the refrigerator less efficient; stagnate air improves efficiency. Blocking these convection currents will reduce convective heat transfer and thus make the refrigerator more efficient. A full refrigerator does a better job at blocking convection currents, and will therefore be more efficient vs. if it were empty.