Test to compare Eutectic Refrigeration and Cyclic Refrigeration.
In the past Ozefridge also manufactured a Cyclic Aluminium plate system, however complaints about excess power consumption and more importantly, system failure due to aluminium cold-plate and aluminium tubing gas leaks, caused us concern.
These gas leaks were commonly caused by corrosion or puncturing of the soft aluminium and often required the entire system be replaced.
This trial was set up to evaluate the worth of continuing to manufacture and sell these soft aluminium cold plate systems.
This page explains the eutectic refrigeration system and presents the results of a direct comparison trial conducted between a stainless steel eutectic plate system and a cyclic aluminium cold plate system, used in an identical situation.
The outcomes presented indicate the different power consumption rate between the two system types and explains why.
Further testing has confirmed that while different applications and type of use provided varying results they were all generally similar to this particular test.
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 tank inside the fridge cabinet that contains a solution that freezes solid to a pre-set temperature.
The eutectic system then benefits from the ‘phase change’ of a stored medium and the enormous thermal energy storage this latent heat application provides. This is similar to how a lump of ice keeps a drink cold until the ice eventually melts.
2. To Explain:
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 the ice thawed from a solid back to a liquid. (Phase changing)
A phase changeable eutectic solution is stored in the stainless steel eutectic tank or plate within the fridge cabinet and acts like a large 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 then runs for a long period (usually 2 to 5 hours) 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 up to 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 and fragile 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. Thermal energy storage verses batteries! 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. (Like having money in the bank!)
D: Some systems can be set to refreeze the eutectic mass when power is abundant reducing or eliminating battery drain.
E: Doesn’t require a battery power supply 24 /7. Refreeze can often be when it best suits available power like when solar has topped up batteries and is now otherwise wasted. Reducing demand on batteries greatly increases their life.
6: Refrigeration with NO battery drain. Is it possible?
Because a eutectic system usually only needs one continuous two to five hour run per day, (in average ambient) devices like the ECO2 will cause this run period to occur when power is abundant eliminating the need to draw battery power at other times! (Abundant power would be whenever batteries are fully charged and solar etc., is being wasted, for example!)
So in situations where the system is correctly engineered there can be sufficient thermal energy stored when power is abundant to hold-over the cabinet without drawing from the battery, or in other situations eliminate the need for additional battery storage.
7: The two reasons a eutectic system uses far less power to do the same job: (COP differences and excessive start ups)
But first, 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. .
The first reason why eutectic systems are much more power efficient relates to each refrigeration systems CoP rating. (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 means it removes 1.13 watts of cabinet heat for each watt of your battery electrical energy used. (Much like MPG, miles per gallon, and the bigger the number the greater the efficiency!)
A COP of 1.13 would be typical of the normal cyclic fridge system as its evaporator would mostly run at approximately -23C while refrigerating. This 1.13 COP rate is very inefficient compared to the eutectic system's 1.90 COP.
The difference in evaporator temperature and therefore COP is because a eutectic plate is absorbing cabinet heat 100% of the time whether the compressor is running or not, while the cyclic plate only removes hear while the unit is running. So assume the cyclic runs a 33% duty cycle, it has to run at a much lower temperature to remove cabinet heat during that limited 33% run period.
Secondly, motor start up inefficiencies. All electric motors consume excess energy yet provide no 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 30 to 60 times as for a cyclic system. Energy is wasted getting the compressor rotating and getting the pressures to allow refrigeration. 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!
As a manufacturer of both types of marine systems we are always testing and compiling data. The following is an outline of a test we did and presented here as it may interest especially cruisers.
8: Power consumption test method and results:
This test was performed using a domestic 150 litre freezer (70mm walls) which had its original AC compressor etc., removed, so basically an empty cabinet. (Chosen because of its similarity size and insulation wise to a typical marine cabinet.)
Two identical 12VDC condensing units set to operate as air cooled only were employed.
One condensing unit was coupled to a 400mm x 400mm aluminium Cyclic cold plate, the other was coupled to a 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 1C 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 and was similar for both systems. This was in a non controlled environment, spring time heading into summer.
Power consumption was recorded and resulted in the eutectic system using approx 50% less power than the cyclic 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 systems 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.
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. Stainless is an approved material for food storage areas.
A eutectic plate is constantly absorbing cabinet heat, not just when the compressor runs.
Initial temperature pull down of a eutectic system is slower than a cyclic system. Expect the first run of a DC powered eutectic system in a hot cabinet 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.
This trial was conducted correctly and in average uncontrolled conditions so as to replicate a typical environment 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 the need for this test, the results of which even surprised us.
We expected the eutectic system to use say 20% to 25% less power than the cyclic system due to the better COP factor, but 50% lower was surprising. In an average of typical scenarios we believe the eutectic system would use at least 30% less power.
The biggest area of cyclic systems inefficiency is obviously the excessive daily start ups and when we consider that the test refrigeration units both 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!
Cyclic systems have some benefits such as faster initial cabinet cool down which suits occasional users (weekenders) and those where power consumption is not an issue.
Since this in house testing Ozefidge no longer manufacture any systems using aluminium evaporators or aluminium refrigerant tubing.
We offer a custom made ss cyclic plate service to suit odd shaped cabinets and a ss forced air cyclic system for rapid cooling, but only with stainless steel and copper refrigerant galleries.
Subsequent trials have confirmed these initial findings. Albeit differences in consumption were far less when operating as a freezer and less significant in some other situations such as fully or part loaded cabinet, and in differing environments.
Ozefridge manufactured both Aluminium Cyclic and Eutectic system types at the time of these trials but now recommend Eutectic or Stainless Steel Cyclic systems only for obvious reasons.
This trial equipment remains intact and can be checked and tested by anyone wishing to inspect.
<< System testing area.
The cabinet used for the test . >>>
Below: Energy consumption logged.