Shadow Catcher wrote:A eutectic system still uses a compressor but incorporates a tank filled with liquid that continually freezes and then defrosts. As the liquid freezes in the tank, its cooling energy is stored. This stored cooling ability is then released as the tank defrosts drawing heat from objects in the cool box. When the tank has almost defrosted the compressor starts again to freeze the liquids and repeat the process. This process is called a Eutectic phase.
Refrigeration systems are more efficient when heat rapidly saturates the evaporator, and as the eutectic evaporator is in a liquid it can rapidly absorb heat and lower temperatures. By comparison aluminum plate evaporators cooling the air run at much less efficient temperatures where the refrigeration systems coefficient factor is very low. (Put a hot can of drink in ice water and another on the shelf of a fridge at the same temperature. You will be drinking the one from the ice water ten times sooner!) So not only is the system operating in much more efficient conditions, it does the job so much quicker and with the eutectic hold-over, runs much less often. An aluminum evaporator plate system will typically cycle say 10 minutes on 20 minutes off. (Two cycles per hour or 48 cycles per day while the Eutectic system runs once per day or twice in hot conditions.)
Having seen and used "holdover" systems on boats a lot of the above is not consistant with my understanding and experience with marine use of holdover systems. The reason holdover plates (containers filled with a eutectic solution which in marine refrigeration systems is a solution of salt water or antifreeze that freezes below 32 degrees f) is that you only have a limited time each day (when you run your engine) to make cold. Commonly an automotive type air conditioning compressor is used, with a
HUGE capacity for a short time to make a block of frozen solution that melts over the course of the day to keep the icebox cool. Given that the latent heat of fusion is used (melting a solid into a liquid absorbs lots of heat) a short freezing cycle can keep the icebox cold all day. Typically 30 minutes or so of run time a day. The key to this is the huge capacity of the compressor and refrigeration system to remove a whole days worth of heat infiltration into the icebox in a short time and the ability to store "cold" in a frozen solution.
Any compressor system will have to remove the heat that infitrates an ice box over time. A small DC compressor will cycle on and off all day and get the job done a little bit at a time. A "holdover system" will cycle once a day, doing the job quickly, and storing the cold in a frozen solution to be utilized all day long. In the end, if the iceboxes are similar, they will be required to remove the same amount of heat during the course of a day. The soda can example above is kind of the same idea. It's going to take so much energy/cold to cool the soda can from warm to frosty cold. If you surround the can with air, the compressor will cool the air, which cools the soda and it will take a bit of time. The compressor will cycle on and off to keep the air cool as it absorbs the heat from the soda. If you surround it in ice water (which conducts heat about 25 times better/faster than air) you can turn off the compressor, and it will cool the soda down faster because the soda is sitting in ice water and the melting ice will consume the heat. When the soda is cooled, you will have less ice, but the remaining ice will still have capacity to keep the icebox cool as heat infiltrates. In the end, however, each system (small compressor long accumulated run time versus holdover system with a huge compressor and system and a single short run time) will require roughly the same amount of energy to cool the soda.
The primary reason to use holdover systems is to minimize the compressor run time and the entire system is designed to produce a lot of BTUs of cooling power quickly, make a block of "ice" (no matter what the ice is made of), and spread the use of this cooling power over the lenght of a day. Large compressors are used for a short period of time. Using a small compressor to make a block of ice inside the icebox will not, in the long run, improve the efficiency of a system running on battery power or shore power.
Not sure if what I'm saying is clear, but I can't see any advantage in using a small DC or AC compressor to make a block of ice, unless power for the compressor is going to be unavailable for long periods of time requiring the cooling power of the ice to keep things cool in between compressor runs. The system will have to remove the same number of BTU with or without the ice.
If I'm missing something, please let me know.
A number of people are reporting good results with the small DC compressor powered units with batteries/solar/converters/tow vehicle charging. Energy use is proportional to the temperature differerence between the inside (of the cooler) and outside enviornment. If your icebox is 40F inside, the temperature outside averages 70F you'll use X amout of amp/hours of juice. If the temperature outside is 100F, you'll use about 2X amp/hours or more (efficiency will drop at higher temps). You, can't, however, argue with success!
From what I've seen, a lot of full size RVs seem to use three way systems. Propane, AC, and DC.
Cheers,
Gus
The opinions in this post are my own. My comments are directed to those that might like an alternative approach to those already espoused.There is the right way,the wrong way,the USMC way, your way, my way, and the highway.
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by Jeffmo63 » Thu Jun 03, 2010 6:10 pm 
. I have played with the idea of constructing an eutetic refrigerator using one of the Dan Foss compressors and a marine Igloo cooler. A eutectic system still uses a compressor but incorporates a tank filled with liquid that continually freezes and then defrosts. As the liquid freezes in the tank, its cooling energy is stored. This stored cooling ability is then released as the tank defrosts drawing heat from objects in the cool box. When the tank has almost defrosted the compressor starts again to freeze the liquids and repeat the process. This process is called a Eutectic phase.
