Naked Science Forum
General Science => General Science => Topic started by: thedoc on 21/03/2012 09:19:01
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Pia Asa asked the Naked Scientists:
Hello there naked scientists,
I was wondering if you would know the answer to this question ...
All other things being equal (like external temperature and contents already frozen), is it more efficient to keep the kitchen freezer packed to its limits (making sure that the air intake does not get blocked) or does it take less energy to keep it going if it is empty? What about half full?
ie do the frozen contents somehow help to keep the whole freezer cold?
If it takes less energy to run it while half full, then should one pack it so the contents are evenly spread through the shelves or should the contents be spread more on the bottom or top shelves?
Just wondering ....
Thanks heaps for a great show!
Pia.
What do you think?
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With any contents at the required temperature, a freezer is simply a box that occasionally turns on to cool down a bit because heat seeps in through its walls. I can see a few considerations to your question.
An non-empty freezer requires a tiny bit more power because adding contents increases the surfaces over which the air passes while circulating inside. The passing of air over surfaces creates friction, which in turn creates heat, which must then be extracted from the freezer. This is also why operating a room fan (but not a window fan) without anyone in the room not only doesn't cool anyone, but it adds heat to the room by causing friction between the moving air and the objects in the room and from the heat created by the fan's motor as it runs (they can get surprisingly hot!).
However, if you actually wanted to add items to, or take items from, the freezer (even though the items going in are already frozen), then a non-empty freezer would increase efficiency because there would be less air that changes when the door opens (the colder air pouring out of the bottom of the open doorway, and the warmer room air entering into the top of the open doorway). A fuller freezer contains less air that can exchange, so there's less warmer air inside to cool down when you finally close the door.
However, if you keep the door open too long of a non-empty freezer, then the items inside start to warm, and the situation can change drastically. If open long enough, the items could get almost as warm as room temperature, and then it would take more power to cool them down than if it was just air inside.
Maybe there's more considerations, but I can't think of any at the moment.
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Presumably an empty freezer is only opened to put (warm) things in, causing work necessary to cool them. A full freezer is opposite, only the displaced air needing to be cooled as items are removed.
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The specific heat (heat capacity per unit mass) of solids and liquids is much greater than that of air. Everything else being equal, there is a greater initial cost to remove the heat from the items in a full freezer.
Once everything cools down, it won't take any more energy to maintain a full freezer at temperature than it takes to maintain an empty freezer at temperature. The heat gain through the freezer's insulation is the same in either case.
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The specific heat (heat capacity per unit mass) of solids and liquids is much greater than that of air. Everything else being equal, there is a greater initial cost to remove the heat from the items in a full freezer.
---(snip)---
Umm -- no!
The specific heat of air is greater than that of nearly everything else! It is heat capacity per unit volume that matters (and that is low for air). And the volume per unit mass is of the order of 1000 times greater for air than for solids and liquids.
specific heat capacities:
hydrogen gas: 14.5 J/K/g
helium gas: 5.2 J/K/g
liquid water: 4.2 J/K/g
air 1.0 J/K/g
aluminium 0.23 J/K/g (other metals lower)
toluene 1.1 J/K/g (other organic liquids similar)
various rocks 0.7-1.2 J/K/g
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Bummer!
Well, obviously, that's what I really meant to say [;D]
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fully stuffed!= warm air carries humidity which either needs be defrosted or insulates the cooling gizmo with ice?
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specific heat capacities:
air 1.0 J/K/g
liquid water: 4.2 J/K/g
aluminium 0.23 J/K/g (other metals lower)
various rocks 0.7-1.2 J/K/g
That is specific heat capacity per unit MASS.
You need to look at specific heat capacity per unit VOLUME.
The density of air is much less than that of your typical rocks (which I fill my freezer with), so the specific heat capacity per unit mass is not very representative.
I agree that once everything is cold, there won't be a big difference between the two situations, except for the loss of air volume when opening and closing the door. A chest freezer is supposed to also decrease the amount of cold air loss when opening the door.
If you don't have a frost-free freezer, then ice build-up on the coils is supposed to negatively impact the efficiency of the freezer. Insulating the coils with ice would mean it would take more effort to get the cold to pass through the ice to the rest of the freezer.
My guess is that one would find the temperature to be more constant in the middle of the full freezer than the empty one. Thus, the freezer might cycle fewer times for longer periods when full, and more cycles for shorter periods when empty. The average would be the same, but an overall efficiency difference would be based in part on the start/stop efficiency. Is it like a well pump that is much more efficient with fewer start/stop cycles?
Also, in the event of a power outage, keep the door closed, and the full freezer should stay cold longer.
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The specific heat (heat capacity per unit mass) of solids and liquids is much greater than that of air. Everything else being equal, there is a greater initial cost to remove the heat from the items in a full freezer.
Once everything cools down, it won't take any more energy to maintain a full freezer at temperature than it takes to maintain an empty freezer at temperature. The heat gain through the freezer's insulation is the same in either case.
More food in the freezer will make easy to freeze more new food fastlyby which it can sometimes support the food safety. Having more food may help to decrease small amount of temperature variations when opening the door oftenly, thus helps to support food safety/quality sometimes.If you have a very ineffective freezer or less insulated freezer , freezer with more food will prevent manage as much on and off.
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...Having more food may help to decrease small amount of temperature variations when opening the door oftenly,
You've mentioned the point most people miss. A full freezer minimises the amount of cold air that can move out when the door is opened, so keeping the temperature more stable.
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I'm told that, in practice, you shouldn't pack it to its limit if this will reduce the circulation of air to chill new items. Pack it as full as possible while still allowing air circulation.
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Load your food at the vents then pack the opening full with PCM, cobalt, or iron. Remove this packing when you wish to access the food. :P
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Would filling sandwich bags with air, tying knots in them and using these to fill the voids in the freezer compartments work if you were going to opening and closing the door regularly (keeping the cool air in so not allowing room temperature air to enter when the door was opened)? Or is this getting dangerously close to the kind of thing my Grand Dad used to do in his latter years?!
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https://en.wikipedia.org/wiki/Kill_A_Watt
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Yes a freezer is more efficient if kept full. Air will roll out once the door is open, but if full of fozen water, say, the less air that rolls out the less cooling needed as the frozen water will not need to be re cooled as air would.
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Definitely some factors I never thought of being discussed in here.