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Efficient Refrigerator DIY
#21
Interesting relevant thread

http://forum.expeditionportal.com/showth...p?t=188346
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#22
Very interesting read, Would there be any benefit to using copper pipe in place of PVC? I can only come up with them being more space conscious as the walls are thinner and the endcaps not so bulky. I guess the same heat transfer would happen in both just slightly slower in PVC? Maybe the copper pipe would be beneficial on the freezing stage????

This is deep into the weeds and the physics is a bit beyond me but would a DIY polar tube with a slight vacuum drawn on it create a situation where the water freezes at a slightly higher temp but as it "thaws" it sublimate directly to gas? Would that phase change be even more endothermic than the phase change from solid to liquid? I suppose that only a limited amount of sublimation would occur before the pressure reached a point inside the tube that just normal melting would occur.... just a crazy late night though.
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#23
This looks like a cool product that could be used to make any size ice pack and already has ratios provided for those of us who don't want to try and recall 9th grade chemistry class.

https://koolergel.com/

Would be cool if the OP replaced his baggy of water with koolergel and tested setup again. Hint Hint! Smile
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#24
Yes copper would transfer heat better coming and going, but super pricey.

Would be interesting to compare meltpoints of that "gel" mix with seawater.

And raises the fact seawater can be diluted, it does contain a LOT of salt.

If, you have your little freezer with you, no need to waste so much storage space on massive long-lasting sizes, just rotate 1" ones daily.

Also may allow running the freeze cycle only during sun hours with panels going.
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scottorious (12-20-2017)
#25
It looks like lowest freezing point that you can reach with salt dissolved in water would be -6 degrees F. What I found online is that koolergel says it will freeze between 0 and -10 degrees F. So unless there is another positive to the "gel" it might be easiest to just make up a salt water solution of 27% salt by weight.
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#26
That's a **lot** saltier than seawater.

You do actually want your solution to freeze, maximum eutectic energy absorption happens at the phase change from solid to liquid.

If you make it too salty, some freezers won't go down that low, especially in hot ambient.

For example some don't get below +10°F in 90° weather.

The ideal solution would have a freezing point that is a bit higher than the worst case for **your** freezer.
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#27
(12-20-2017, 03:13 AM)scottorious Wrote: Would be cool if the OP replaced his baggy of water with koolergel and tested setup again. Hint Hint! Smile

With the Koolergel, like with salt, the freezing point can be made very low.  I want a fridge.  If I am keeping lettuce and a tomato I don't want it frozen.  Just the regular freezing point of water is cold enough.  A few degrees colder wouldn't freeze produce but it would not be an advantage.  

When 1 pound of water melts it absorbs 144 BTUs.  From what I can tell with the help of Google, adding salt doesn't change that.  With salt it does the absorbing at a lower temperature but it is still 144 BTUs per pound.  That would be ignoring the weight of the salt.  If you add .27 pounds of salt to 1 pound of water the mix would be 1.27 pounds.  If the references Google found are correct that 1.27 pounds would still absorb 144 BTUs when melting.  

Since I have spilled once I am convinced that it could happen again.  I expect that the idiot will be more careful in the future.  I am still a slow learner but in this case I know that salt water is bad to spill in a steel van and it can be avoided by not using it.
You cannot bring about prosperity by discouraging thrift. 1 of Ten Cannots, Rev. William John Henry Boetcker, 1916.
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#28
Flexible polyethylene bags make good thermal contact with the bottom of the freezer compartment.  They conform to the irregular surface of the refrigerant tubes.  There is a large area that conducts the heat out of the water.  Rigid containers like polar tubes don't have that.  The larger diameter caps on the ends will keep the middle part of the tube elevated above the bottom of the freezer compartment.  The air gap will insulate.  The spots where the end caps touch the evaporator tubes have a very small area.  

With a low thermal conductivity between the freezing water in a polar tube and the refrigerant the temperature of the refrigerant will be colder.  That makes a higher pressure differential in the compressor resulting in higher consumption of electricity.  Even with the colder freezer compartment the heat in the water will not flow from the water to the freon nearly as quickly without conduction.  Convection through air is not as effective as conduction.  The run time of the compressor will be longer resulting in higher consumption of electricity.  

Thermal conductivity, k, is reciprocal of thermal resistance, r-value, k = 1/r
401   Copper
205   Aluminum
43    Steel, Carbon 1%
2.18  Ice (0C, 32F)
0.58  Water
0.33  Polyethylene, LDPE
0.19  PVC
0.04  Glass wool insulation
0.024 Air

Normally a cup of water in the freezer will freeze at the top first.  With the poly bags in contact with the evaporator tubes of the freezer compartment ice forms first at the bottom.  Initially there is no ice, eventually there is 2 inches so on average there is an inch of ice.  The thin layer, 2 mils (.002 inch) of poly (k=.33) and then an inch of ice (k=2.18) will conduct the heat from the freezing boundary between the ice and water to the freon quite effectively.  

The key to making this fridge efficient is minimizing the compressor run time.  The compressor runs at about 3500 RPM due to the AC being 60 Hz, 3600 cycles per minute.  Reducing the RPM is how the 12 volt fridges get low current use,  A 35 Hz inverter would make the RPM about 2000.  The inverter I got for $20 is 60 Hz.  There is no magic 270 ohm resistor on the circuit board to remove to slow it down.  Good thermal conductivity and 5 pounds of water lets the compressor pump out a day's worth of BTUs in 1 to 2 hours.
You cannot bring about prosperity by discouraging thrift. 1 of Ten Cannots, Rev. William John Henry Boetcker, 1916.
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#29
Yes I agree I would not carry salt water if there was a high risk of spillage.

The purpose of the lower temp is the longer "lifespan" of the tube/block before the cooler gets to say the 45° swapping point.

As I said even after the melting point the tube itself will remain below 32° for quite a long time.

Perhaps the ideal is to have a mix of tubes with different strength solutions, so the phase change events keep happening at different times, maybe even days apart.
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#30
(12-20-2017, 07:35 AM)John61CT Wrote: If, you have your little freezer with you, no need to waste so much storage space on massive long-lasting sizes, just rotate 1" ones daily.
I presume that by rotate you don't mean turn them 90 degrees but rather take them out of the freezer and put them in the fridge.  I only have one cold box.  The Haier 1.7 cubic foot fridge is it.  The water bags go in the freezer compartment of the fridge.
You cannot bring about prosperity by discouraging thrift. 1 of Ten Cannots, Rev. William John Henry Boetcker, 1916.
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