Putting containers of water in freezer

quartzz

I've seen a few sources which say to put containers of water in empty freezers, because it means the freezer has to produce less cold air, to keep the inside cold (because some of the volume has "something" in it). I'm thinking....surely energy can't be "magically" gained, and cooling it down initially would "negate" any cost savings during normal operation -- or somehow it would still take as much energy, you're just expending it somewhere else, rather than an empty freezer cavity. Any ideas, suggestions, or Brian Cox's/Jim Al-Khalili's around?

Section62

quartzz said:

I've seen a few sources which say to put containers of water in empty freezers, because it means the freezer has to produce less cold air, to keep the inside cold (because some of the volume has "something" in it). I'm thinking....surely energy can't be "magically" gained, and cooling it down initially would "negate" any cost savings during normal operation -- or somehow it would still take as much energy, you're just expending it somewhere else, rather than an empty freezer cavity. Any ideas, suggestions, or Brian Cox's/Jim Al-Khalili's around?

If buying a smaller freezer isn't an option then filling some of the space with airtight empty containers might help.  If the issue is cold air coming out of the freezer when the door is opened and needing the replacement warm air being cooled down then trapping the cold air so it can't escape so easily would be a potential solution.  You are right that containers of water would require energy to freeze them, and whether or not the container has anything in it doesn't make any difference to whether that volume of cold air can escape when the door is opened.  The only way in which the water would really help is if you have a long power cut - the frozen water will help to keep the freezer cabinet colder for longer, in the same way ice packs keep coolboxes colder.

CliveOfIndia

You're not "magically" making energy, just making more efficient use of the energy the freezer is consuming.  In very simple terms, it takes a lot more energy to keep air cold than it does to keep a more solid object cold.  Fill your freezer with "things" rather than air spaces, it has less work to do so consumes less energy.  Plus, every time you open the door, there's less cold air coming out and less warm air going in, which then has to be cooled down again.

It's not a new idea - even when I was a kid, the advice was to stick loads of loaves of bread in the freezer if it had a lot of empty space ( assuming you would eventually eat the bread, of course! )

quartzz

Clive - I understand the principle of keeping air cold rather than a solid object (or maybe I don't). But surely the energy required to freeze the object in the first place, would "compensate" for "running" savings later?

Or. Is the "freezer environment" such that the extra energy required for the initial freeze, would be compensated for later, given that the compressor etc etc is an active component, which adjusts itself on the fly (I'm taking "opening the door" actions as being relatively insignificant)

Without getting too Brian Cox, I guess the question is, do items inside the freezer act as a "passive" item (where the freezer does the work), or an "active" item (where the frozen object actually has some effect on the freezing effect of the......freezer)

If this is over thinking it, just think £15 per year running cost saving. I actually have a kill-a-watt gadget, I might see if it has a "cumulative daily energy" setting as well as "current (no pun intended) usage".

Section - tbh I will use most of the freezer during normal use, but occasionally I'll be out of full stocks of frozen food. wrt empty containers -- isn't that just essentially having to cool more air?

p00hsticks

CliveOfIndia said:

It's not a new idea - even when I was a kid, the advice was to stick loads of loaves of bread in the freezer if it had a lot of empty space ( assuming you would eventually eat the bread, of course! )

Yes, we still use cheap sliced bread sometimes to bulk out the freezer if it's getting a bit empty - freeze it in four slices together and it defrosts really quickly when needed - even straight in the toaster.

Section62

quartzz said:

...
Without getting too Brian Cox, I guess the question is, do items inside the freezer act as a "passive" item (where the freezer does the work), or an "active" item (where the frozen object actually has some effect on the freezing effect of the......freezer)

If this is over thinking it, just think £15 per year running cost saving. I actually have a kill-a-watt gadget, I might see if it has a "cumulative daily energy" setting as well as "current (no pun intended) usage".

The issue you've presented is way simpler than that.  If you open the door of a freezer you allow some of the cold air to escape, to be replaced by warmer air.  The freezer will consume energy to cool that air down.  The larger the volume of free air contained in the freezer cabinet, the greater the potential swapping of cold air for warmer air.  Reducing the volume of free air by keeping the freezer fuller reduces the potential for cold air to escape, because there is less of it.

The bread 'trick' works because it fills volume in the freezer - bread has no magical properties.

quartzz said:

Section - tbh I will use most of the freezer during normal use, but occasionally I'll be out of full stocks of frozen food. wrt empty containers -- isn't that just essentially having to cool more air?

No.  If the container is (more or less) airtight then the air needs to be cooled down once, then stays trapped in the container.  Less energy is needed to cool a container of air than the same size container of food or water.

Air isn't the problem here - it is the cold air escaping being replaced with warmer air which needs to be reduced.  Hence the advice in a power cut to avoid opening freezer doors in order to keep the food frozen for longer.

quartzz

yeah. first part. that makes sense. the containers are literally acting like a cold-battery, which will last a few seconds or hours. again the second, the sealed container of air is acting like a battery. again that "less energy" part is what I questioned for the whole "energy required to cool" process

it isn't about magical properties as such, that was just a phrase....[the cooling required to set things up (injecting cold into the object), negates the cooling saved while "cruising" (when the now-cooled object is "giving off" cold -- or at least, not absorbing it any more)]

but "cold batteries" which store cold when you open the door--makes sense

grumpy_codger

CliveOfIndia said:

In very simple terms, it takes a lot more energy to keep air cold than it does to keep a more solid object cold. 

Is it a fact, not an urban myth? I don't see any physics behind this.

The only real difference between a full and an empty freezer is that that the compressor has to kick in more frequently, but for shorter times. This saves the compressor, not the energy.

Having said that, cold air leaks out when you open a freezer. The more full the freezer is, the smaller volume of air can leak out and the smaller volume of warm air the freezer has to cool after it's closed. So adding any containers (full or empty) can save some energy, not necessarily "a lot".

quartzz

penny drops. in theory, with a couple of plastic containers which hopefully wont crack with water turning into ice, does this not mean, once frozen, you could have the freezer switched on for less time, because the ice blocks will help? or....is that getting into perpetual motion territory, and it evens out in the end

"in theory you have inserted energy into the freezer (put the containers of water in there)"

Section62

grumpy_codger said:

CliveOfIndia said:

In very simple terms, it takes a lot more energy to keep air cold than it does to keep a more solid object cold. 

Is it a fact, not an urban myth? I don't see any physics behind this.

The fact is the opposite - the specific heat of water is about 4 times that of air.

grumpy_codger said:

The only real difference between a full and an empty freezer is that that the compressor has to kick in more frequently, but for shorter times. This saves the compressor, not the energy.

If the freezer door is a perfect seal and stays shut then the 'cold' loss through the cabinet will be more or less the same whether the freezer is full or half empty.  So the amount of work done by the compressor should be about the same in either case.

Opening the door is what makes a difference.  Assuming no contents are added or taken from the freezer, the result of opening the door is that some of the previously cold air will be replaced by some warmer air.  The compressor then has to run to cool that air (and contents if the air has warmed them up as well) back down to the set temperature.  It follows that the more warm air that gets in, the more work needs to be done by the compressor to get everything cold again.

Section62

quartzz said:

penny drops. in theory, with a couple of plastic containers which hopefully wont crack with water turning into ice, does this not mean, once frozen, you could have the freezer switched on for less time, because the ice blocks will help? or....is that getting into perpetual motion territory, and it evens out in the end

"in theory you have inserted energy into the freezer (put the containers of water in there)"

Yes, it is just that - because the argument forgets that a whole load of energy was required to freeze the water in the first place.

If the ice blocks have been called on to 'help' by giving up some of their cold, then the compressor will need to run some more to bring them back down to the set temperature again.