Water in an empty fridge.

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frankwolf
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Post by frankwolf »

Hello Jacob,
What is the reason for your recommendation to fill empty space in a fridge with water? (6.2.5 Lights and electric, p. 143)

So - as the waters heat capacity is higher, than that of air it saves more coldness. But why should it lose less heat than air does?

My explanation was, that hot and warm air - as it is a gas - diffuses more easily into each other when the fridge is opened. Beyond that it may also diffuses more easily if it is closed. Thus the difference in temperature is more easily equalized.

Water a higher heat conductivity than air. However water as liquid is a lot less susceptible to convection.

What is the physical explanation?
Thanks in advance
Frank


orinoco
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Post by orinoco »

I think the explanation is much simpler. Cold air will readily flow out of the fridge the moment the door is opened being heavier than the warmer air outside, the water in the container will not.


dragoncar
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Post by dragoncar »

I think there are a number of factors involved:
1 - Air flow: As orinoco says, cold air will dump right out of your fridge when you open the door because it is heavier than room temperature air. This could be alleviated by, for instance, filling empty space with air-filled jugs. Thus, sort of like putting water jugs in your toilet tank, you effectively reduce the size of your refrigerator. When you open the door, only a small fraction of air will spill out, requiring less energy to replace.
2 - Heat capacity: Instead of filling the jugs with air, you could fill them with water. Water has a higher heat capacity than air and will therefore act as a buffer to the temperature in the fridge. I'm not entirely convinced this is anywhere near as important as 1, above. I say I'm not convinced, because I think refrigeration equipment is most efficient when it is running continuously (as opposed to starting and stopping -- think a small air conditioner running 100% vs. a larger air conditioner that has to cycle on and off).
3 - Heat conductivity: I'm not sure this really matters much. Assuming you are deciding between air-filled and water-filled jugs, thermal conductivity can work both ways. When you open your fridge, the water jugs will transfer energy quicker than would air, losing more heat to the outside world. But when you close the door again, that energy is being transferred to the warm air in the fridge, so it could cool that air faster. I think either way, however, your compressor would already have started running. The main benefit of water would be if it could prevent the compressor from having to run at all.


jacob
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Post by jacob »

Point 2 of what dragoncar said, which bears on point 3. I don't think decreasing the amount of air in the fridge with something that can't flow out is that important.
Typically a cooler will be either on or off. In a control circuit you'd have to somehow translate this binary decision into trying to be as close to the dialed in temperature as possible. Obviously you can't switch the motor on and off twice a minute. This would be very inefficient. The water thus acts as a buffer (of heat capacity) to make sure that the compressor runs longer at a time but less often thus increasing its efficiency (point 2). If the water was cooler than the thermostat setting and the door was opened, the water would also help cool the new warm air preventing the compressor from running just to heat the air (point 3).
Of far greater efficiency would be to simply get a smaller fridge so you wouldn't have to do this.


AlexK
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Post by AlexK »

Of course if it's winter time and you heat your house with electricity, leave the door open as much as you'd like because the refrigerator is a 100% efficient electric heater :)


ishidav
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Post by ishidav »

Or get a heat pump (in a moderate climate) for greater than 100% efficiency.


George the other one
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Post by George the other one »

Here in Wisconsin we use cases (30) or cubes (24) of beer (And if you're curious a typical case here is a between 5 and 18 bucks... just in case you're from a state that has high alcohol prices). :-D


McTrex
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Post by McTrex »

The thing I'm always wondering about regarding this topic...wouldn't you lose the energy you're trying to save by having to cool this water? (Not if you're getting it from the pond behind your house in mid-winter in a cold climate of course, I get that...)


frankwolf
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Post by frankwolf »

In fact our fridge(*) is already pretty small. I simply wanted to understand which I still don't do, to be honest.
(*) I think our expense in terms of energy is 130-160 kwh per year. But I would like to try to go without a fridge at least for some weeks just to experience how that is and to see how I am going along with it. Going totally without a fridge thereafter would - in my case (in terms of people I live with) - require certainly a lot of persuasion effort ;) but lets see.
So, the energy saving is due to avoiding a cycle of ons and offs of the compressor or is it due to waters heat capacity - which I would doubt by now. Even though water is actually able to store more caloric energy, why should it also loose less of it in terms of energy compared to air - which should (could) be the main point.

So, is it this effect just about a continously running fridge?

Another thing is, that the water has to be cooled down in the first place which requires energy that has to be saved later on.

Somewhere the notions "reversible" an "irreversible" process in the context of the carnot process resonate in my mind, but I cannot remember quite why.

Maybe I missed the point somewhere.


dragoncar
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Post by dragoncar »

Your confusion is understandable because you've heard a few different theories. I'm not actually sure an empirical study has been done.
Water won't lose less energy than air, it will lose more - because it has a higher thermal conductivity. However, because it is in a container, it will not fall out of the fridge. Jacob and I disagree on the importance of the "not falling out" point.
Jacob ascribes more importance to the buffering qualities of water. However, when considering a container of water vs. a container of air, you are correct that the same amount of heat would need to be removed from the fridge. The difference is that water will act as a buffer.
For example, lets say your empty fridge is set to 35 degrees. When you open the door, all the air falls out and is replaced by 75 degree air. If you have containers of water in there, the cold water will cool the 75 degree air to 40 degrees, and at the same time the water will warm to 40 degrees (I made these numbers up... you could calculate the exact numbers based on the volumes and heat capacities of air and water). Then, the fridge will need to cool the 40 degree water and air back to 35 degrees.
On the other hand, if you have containers of air, the temperature will first equalize at a higher temperature, say 65 degrees. This is because of the lower heat capacity of your cool air containers and additionally the equalization will be slower compared to the compressor because of the lower thermal conductivity of the air containers. Then, the fridge will need to cool the 65 degree air back to 35 degrees. Although the fridge is cooling a greater number of degrees, the heat capacity of air is lower than water, so the same amount of energy is required to return the system to 35 degrees. The more I think about it, the less I buy the premise that the compressor would cycle differently to remove the same amount of energy from the system.
As I see it, the advantage of water containers is that your fridge returns to a lower temperature faster because the water is helping cool the air. This is very important for preventing bacterial growth while the refrigerator brings the temperature back down. But this doesn't save you any electricity, because a fridge with water instead of air will gain at least the same amount of energy when the door is opened, and that energy must removed again. As far as I'm concerned, the primary saver of electricity is the effective reduction in the size of your fridge by blocking off sections with containers (of water, air, or whatever else).
Think about the corner case of a completely full fridge (it can be full of water, air containers, or anything else). You open the door, but no air falls out, because the fridge is completely full. The only losses are heat radiation and conduction/convection with outside air. I think these will be minimal. Furthermore, MORE energy will be lost to conduction/convection with water because of the higher thermal conductivity.
The other corner case is an empty fridge. You open the door and all your cold air flows out and is replaced by warm air. This is a much bigger loss, although it doesn't take that much work to cool the low-heat-capacity air.
Maybe I'm also missing the point, but as far as I can reason, water in itself doesn't reduce the energy needs of the system. Energy use is reduced by reducing the amount of cold air lost upon opening the door (and thus reducing the amount of heat added to the system). The reason you want water containers instead of air containers is because it will help keep your food cool while the fridge brings the total energy in the system back down to the original point (preventing bacterial growth in the "danger zone").


Kevin NC
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Post by Kevin NC »

A couple of years ago,my refrigerator was running almost constantly.The empty space was about 75 percent.By taking a couple of gallon milk containers,filling them up with water and placing more items in the refrigerator,it cut the running time in half.I didn't read about it on the internet,it just made sense to me,so I tried it and it works!I was getting ready to buy a new one,glad I didn't!


chilly
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Post by chilly »

@dragoncar's point #2 is really primary contributor.
Think of the outer walls of the refrigerator as defining a fixed control volume.
At any given temperature difference from inside to outside of that control volume, heat will enter the fridge at a certain rate, regardless of contents. Similarly, a certain amount of heat will enter when the door opens (this will vary a bit between empty and full, but a few gallons of water won't make a difference to this).
An empty fridge has a volumetric heat capacity of N (joules per degree... ie. requires N joules to change the temperature of the system by 1 degree).
A fridge with 3 gallons of water has a capacity of (N + M) j/degree. M is *MUCH* greater than N since one gallon of water holds 4,000 times more heat than a gallon of air.
So regardless of where the heat is coming from, the overall temperature of that control volume will always change much less with the water present. That means the empty refrigerator will trigger the thermostat more frequently because the temp varies more rapidly.
In an ideal system, this wouldn't actually affect the energy consumption... just help with wear and tear on the fridge due to on/off cycles. In reality, there is also some efficiency lost during the initial spool up of the cooling system.... this is really the only $$$ savings you will see. I'm not sure how much this really is - but whatever your'e saving on your bill, it's from this and this alone.
Also, yes you do need to initially cool the water, but that's one-time, and it's inconsequential in the long run.
I'm not seeing it now, but thought someone had a link to someone using a chest freezer instead of a fridge, and claiming 10% of the electricity costs. In principal, it's a good idea, and a freezer is probably better insulated too. The 10% is most likely due to the insulation (or just not accurate)... the outflow from door opening will not save you 90%!


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