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  • Why Eutectic.. part 2

    Extensive comparison between Eutectic Refrigeration and Cyclic Refrigeration.

    This page explains the eutectic refrigeration system and presents the results of a direct comparison trial conducted between a eutectic stainless steel system and a cyclic aluminium cold plate system, used in an identical application.
    Note: these results are of the systems and cabinet as described and therefore don’t relate to all situations, although the factors and outcomes presented are truly representative.
    1. But first:What is meant by Eutectic refrigeration:
    Eutectic refrigeration systems use a refrigeration condensing unit much like any other system. The difference is that a eutectic system involves refrigerating a storage tank that contains a solution that freezes solid to a predetermined temperature. (centigrade). The eutectic system benefits from the ‘phase change’ of a stored medium and the enormous energy storage this latent heat application provides.
    2. To Explain: 
    We all know that a cold drink with ice in it will stay cold and drinkable at a constant temperature while the ice remains, but will warm rapidly once the ice thaws.
    The ice is an example of the eutectic principle. The ice absorbs relatively huge amounts of heat from the drink, causing the drink to remain cold, while it thawed from a solid back to a liquid. (Phase changing)
    A phase change medium or eutectic solution is stored in the stainless steel eutectic tank or plate within the fridge cabinet and acts like a renewable ice block, freezing solid during the refrigeration run cycle and thawing during off periods, and all the time maintaining a constant fridge cabinet temperature just like the ice did for the cool drink!
    3. How does it work: 
    A digital thermostat probe located within the eutectic solution, causes the compressor to run when it senses that the solution has thawed. The compressor runs for a long period to refreeze the solution solid virtually storing thermal energy for cooling in advance. Hold over refrigeration, because once the mass is re-frozen it holds the cabinet at the desired temperature for many hours and often a day or so in cooler climates.
    4. So where is the benefit: 
    Phase Change occurs when we freeze a solution solid by removing its heat, or as it thaws into a liquid again while it absorbs heat.
    Other products such as a thin aluminium evaporator plate systems can also absorb and dissipate thermal energy (heat) but when a relatively large volume of solution is used as in the eutectic plate and it is allowed to freeze solid on the refrigeration run cycle, then thaw during the off cycle, a massive amount of stored thermal energy is in play.
    By comparison with a thin aluminium cold plate, the first and most obvious advantage the eutectic system has is its far greater mass and therefore thermal hold-over storage capacity. (A thin 2mm thick slice of ice will disappear much quicker than a 60mm thick block would)
    But this thermal storage advantage pales into insignificance compared to the other unique benefit that the eutectic system phase change phenomenon delivers. This is called ‘Latent heat’. It effectively multiplies the eutectic thermal storage capacity of the medium by up to 80 times (yes eighty times) providing huge hold-over periods where the unit stays off for many hours even days in cooler times.
    5. So why use a Eutectic system:
    A: Weight. The total weight of an operating eutectic system is less than half the weight of the batteries that would otherwise be required to power up a cyclic cold plate fridge unit to provide the same cooling.
    B: Power efficiency. A eutectic system will use much less power to do the same job as a cyclic system. (See test results and reasons why following below).
    C: Product temperature pull-down. Warm products added to the cabinet have the benefit of the stored eutectic mass to reduce the temperature of warm product faster, not solely relying on the refrigeration unit’s capacity at the time.
    D: Some systems can be set to refreeze the eutectic mass when power is abundant reducing or eliminating batteryconsumption.
    E: Doesn’t require a battery power supply 24 /7. Refreeze can often be when it best suits available power.
    6: Refrigeration with NO battery drain. Is it possible?
    Because a eutectic system usually only needs one continuous two to three hour run per day, (in average ambient) devices like the Ozefridge ECO2 will cause this run to occur when power is abundant eliminating the need to draw any battery power at all at other times! (Abundant power would be when batteries are fully charged and solar or whatever is being wasted, for example!)
    7: What causes a eutectic system to be so efficient?
    Firstly, given a cabinet to refrigerate, as in this test, the amount of heat to be removed (measured in watts) is exactly the same regardless of the refrigeration method used. Therefore it gets down to the efficiency of the refrigeration system’s operation. .There are two main reasons why the eutectic systems are much more power efficient.The first relates to the refrigeration systems CoP (Co-efficiency of Performance)
    To explain: COP is a factor indicating how much heat is removed relative to electrical energy consumed. A system with a COP of say 1.13 removes 1.13 watts of heat for each watt of electrical energy consumed. (Similar to miles per gallon, and the bigger the number the greater the efficiency!)
    This 1.13 COP would be typical of the cyclic fridge system as its evaporator would mostly run at -23C or colder while refrigerating.
    This 1.13 COP rate is very inefficient compared to the eutectic system which would have a much better COP of 1.95.
    Far more heat removed per watt of battery power consumed!
    (The lower the temperature a refrigeration system’s evaporator runs at, the less efficient the system is. Running a system on with a low COP is very false economy)
    Secondly, motor start up inefficiencies. All electric motors consume excess energy while providing little benefit during start up. This waste of energy is obviously far less with a eutectic system as it only starts once or twice a day instead of 20 to 60 times as for a cyclic system. This is the greatest cause of inefficiency and would be even worse if compressors didn’t have soft start motor driver modules like those in this test!8:
    8: Power consumption test method and results:
    This test was performed with a domestic 150 litre freezer (70mm walls) which had its compressor etc., removed, so basically and empty cabinet.
    Two identical Ozefridge condensing units set to operate as air cooled only.
    One condensing unit was coupled to a 400mm x 400mm aluminium Cyclic cold plate, the other was couple to an Ozefridge mid range eutectic SS plate 400mm x 330mm overall.
    During a two month period each system was operated consecutively to maintain the cabinet at between 2C and 4C controlled via the same digital thermostat. The run period for each system was in 7 to 8 day blocks each consecutively. The ambient temperature which ranged from 10C to 42C maximum was similar for both systems.
    Power consumption was recorded and resulted in the cyclic aluminium system using approx twice as much power as the eutectic system to do exactly the same job!
    Note: A similar test since in even hotter conditions saw the consumption benefit of the eutectic system reduced, but still quite significant. Also operating as a freezer will also see an efficiency advantage with Eutectic but not nearly as extreme as when operating as a refrigerator like in the test.
    The trials were done fairly and the test equipment retained at Ozefridge as is, so if anyone wishes they are welcome to inspect, by appointment.)
    9:  Other aspects of a Eutectic system:
    Eutectic plates are usually 318 grade stainless, not soft aluminium or copper therefore much less likely to be damaged / punctured, easy to clean and don’t grow ‘nasties’ than can taint/ contaminate food.
    Initial temperature pull down of a eutectic system is slower than a cyclic system. Expect first run of a DC powered eutectic system to be 3 to 4 hrs.
    A eutectic plate takes up more cabinet space than a cyclic plate.
    A eutectic system obviously costs more than a cyclic system.Pictures of the testing.
    Pictures below: The first pictures are of the 150 litre cabinet used in the test with identical air cooled Ozefridge refrigeration units. One unit refrigerates a cyclic aluminium cold plate and the other a mid size Ozefridge Eutectic cold plate.
    A monitoring, data logging station was connected to each system. (See third picture)
    The final picture is of a spread sheet colour coded to indicate which system was refrigerating the cabinet and the daily watts consumed.
    It can be seen that the cyclic system (green) benefited from ‘hold-over’ of the eutectic plate for its first days consumption for each of its run periods.
    Yet even with this advantage it still consumed twice as much power as the eutectic system. 
    Click on picture to enlarge
    This trial was conducted correctly and in average uncontrolled conditions and as the ambient rose heading into summer, so did the consumption of both systems. 
    We always could calculate the power economy that operating a eutectic system with a much higher COP would provide as that is simple indisputable maths, but calculating the loss of efficiency upon compressor start up was not feasible or accurate.
    Hence this test which even surprised us.
    We expected the cyclic system to use say 40 to 50% more but double!
    The biggest area of inefficiency is the excessive daily start ups and when we consider that the test refrigeration units had ‘soft start’ motor driver modules, and therefore more power frugal, how much worse would the cyclic systems consumption be with the old DOL modules!.
    Ozefridge manufacture both Cyclic and Eutectic systems but recommend eutectic in most cases for obvious reasons .