Refrigeration - a question on thermal mass/load

[woodPixel]

I'm asking here as there are a LOT of very smart people from diverse backgrounds....


With my fridge (or anyone's, really) IS IT better, or worse, to load it up with a thermal mass to help it keep cold?


For specifics - lets pretend I put ten 2-litre milk containers of water in there. Fridge is set to 2°C.

What for? - IF I should open and close the door ten times a day, does that thermal mass of refrigerated water "hold" the cold into the fridge better? i.e. does the fridge use less energy to stay at the set temperature? Or is it, as I suspect, exactly the same?


I was thinking of this for a while as I saw a post on Reddit where as a "life hack" (Christ I hate that term) the poster suggested freezing bottles of water in case the power should cut out. They suggested that the fridge would remain colder for longer, hopefully saving your milk if one should loose power for a day or two.

Obviously, during a major power-outage, one doesn't expect cold beer. Opening the door 20 times to retrieve them won't help any.... but it had me thinking....

IF I should store a lot of water in the fridge, for quenching giant thirsts, would this confer some benefit to long term power saving?

Further, I have friends whose fridges are empty. Lucky to have a desiccated carrot and a bottle of vodka in them. Being completely empty, if one should open the door, ALL of the cold air would WHOOSH out, so when the door is closed the whole thing needs to chug back into life to re-cool all that air.

IF that same fridge had a mass of water, that water would impart some of its thermal capacitance into the surrounding air, re-cooling it faster, therefore saving the fridge from ever firing up?

Now, I know that it would take a lot more energy to get that water cold in the first instance*. BUT, the fridge is only going to radiate the same quanta of energy away, empty or 100% full... after all its the outside surface and the effectiveness of the insulation that matters.

...
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So, any engineers who might know how to answer this?



* unless I use bottled water "from outside" in Canberras winter and put it in the fridge at 2°C to start with!

[BobL]

Our "beer" fridge is always kept full.

When I say "beer" it only has about a 6-pack worth of beer in it
The remainder consists of about 2 dozen bottles of wine for SWMBO, a few cool drinks (for guests) and the rest is water - about 8-10 L. There are also various chemicals like silicones and glues and 4L of Tannic acid.

The freezer in that fridge is kept full of dog bones and dog roo meat.

It only gets opened on average maybe 2-3 times a day. I keep meaning to do some real electrical measurements but I have yet to get around to it.

Interestingly answers on the web usually say there will be either no savings or marginal savings for the fuller fridge depending on how often the door is open.
It also depends on what's in the fridge as (slowly) decomposing food does generate some heat. Sealed bottles of cans or bottles obviously don't

[Bohdan]

Every time that you open the door the cold air leaves. When you close the door the frig then has to cool the new warm air.

The amount of power used relates to the amount of air that has to be cooled. If the frig is full less air leaves and so less power is used to to run it.

The cold water stored in the frig cools the air quicker, maybe without starting the frig, but ultimately the water has to be recooled.

So the net result is the more air that is lost the more power is used. The contents of the frig only use power the first time that they are cooled and after that, if the door is not opened, the power used is to make up for the insulation losses of the frig.


So there is no power benefit from storing water in a frig unless it is to keep it full.

PS beer works just as well.

[chemfish]

Thermal mass reduces the speed things want to change temperature.
Having more in a fridge shouldn't really change the amount of energy the fridge uses, poor seals or insulation will. A loaded fridge might cycle on less often but will have to run longer to reduce the temperature of the items in the fridge. Depending on factors there may be small efficiency gains with not having to cycle the compressor on/off so often, I wouldn't bet on it being significant though.
Having high thermal mass will reduce the speed things warm up in a fridge that's got no power though. Essentially if your energy input is the same it takes 10 times more energy to increase the temperature of 10 litres of water than 1 litre of water. Note I say energy, not time, it's not necessarily that simple as heat transfer is also effected by the difference in temperature and other factors.
I wouldn't bet on a freezer keeping things properly frozen either, it may take longer for a loaded freezer to completely defrost but that won't stop the outer layers of your frozen product from defrosting while the center stays frozen. Someone once left one of the large freezers at work off over a weekend, several thousand liters of frozen product didn't stop the outer layers of everything defrosting.

[rrich]

I am not a refrigeration engineer. The only thing about thermal dynamics that I know is from casual conversations.

I've been told that cold does not move but rather heat moves toward cold. First law ?

If a fridge is 'loaded' there are more places that the heat has to penetrate to make the fridge work harder. If you move a calorie into a fridge, the cooling system has to remove that calorie of heat. A calorie (Not the dietary Calorie) is the amount of heat required to raise one ml of water one degree C. Food Calories (Capital C) are the equivalent of 1000 calories.

So in a full fridge there are multiple places to absorb the heat. In a skinny fridge it is relatively easier for heat to warm the air within the fridge and require less heat to change the temperature.

[Skew ChiDAMN!!]

To my mind, the thermal mass would be irrelevant apart from, as Chemfish points out, slowing down how quickly things warm up as any given energy input is spread across more mass. It'd still take the same amount of power to cool things down again though.

Where it'd make a difference, in a frequently opened 'upright' frig, is as Bohdan pointed out. A fuller frig means less cold air being replaced with warm air... and frankly putting empty sealed containers would work just as well here as full ones.

Although the beer option sounds good to me...

[woodPixel]

So, the consensus seems to be that I'm to load the fridge to bursting with beer.

I'll discuss that with SWMBO....

[rrich]

So, the consensus seems to be that I'm to load the fridge to bursting with beer.

I'll discuss that with SWMBO....

Tell her it will save money on the power bill.

[chemfish]

The down side is to make it work you can't take the beer out and drink it. This is the basic definition of "living hell".

[justonething]

It only saves money if you dont drink the beer.