Eutectics and Refrigeration

[cuttersailor]

Just a message to try and clarify the seemingly ongoing debate over what's involved in refrigeration on boats.

First of all, the word eutectic is used to describe the joining of more than one substance and have it change from solid to liquid (state) at a temperature that is consistent throughout.

In other words a good example is solder. Solder can be a mix of Lead, Tin, Silver and various other stuff. The precise mix of these substances enables a melting point that is not actually the melting point of any one of the elements that make up the compound of solder. It's an observed physical property that's been seen to work and probably can't be calculated without the "The Big Bang Theory" . And it should do it consistently. At the same temperature every time. The root of Eutectic is from the Greek Eu meaning the same.

Anyway, the story here is refrigeration.

The good reason for Eutectic/Holding plates is based on the fact they, in of themselves, require to get good and cold before the thermostat tells the compressor to shut down. In other words it's like freezing a bunch of bottles of water and sticking them your icebox because they're already pretty darn cold. And, as a result, keep everything else cold as well.

A good Eutectic/Holding tank system requires that the cold cycle of the refrigeration continues until the Eutectic/Holding tank is properly cold before it's shut down and it's done while the system--solar, engine, wind, shore power--is available and doesn't cycle constantly like a normal fridge does. In other words "Make hay while the sun shines" . "Save the batteries" is the key. So what you're really doing is freezing a good old block of ice when the power is available. Otherwise it's just a normal fridge and gets cold when it needs to and keeps cycling.

So that's it, in all of it's simplicity---AND-

A disclaimer. I think that the use of the word "Eutectic" is a a bit of a stretch. The solution used to hold the cold in the evaporator is usually antifreeze. It does have a eutectic property and I suppose that's a good marketing reason but as far as I'm concerned they are still "Holding Plates". But just the same, the principal is correct.

Just remember,

USE THE POWER WHEN IT'S AVAILABLE.

cutter

[colemj]

Originally Posted by cuttersailor

"Make hay while the sun shines" . "Save the batteries" is the key. So what you're really doing is freezing a good old block of ice when the power is available.

…...
Just remember,

USE THE POWER WHEN IT'S AVAILABLE.

This is only true if one has a surplus of power that would be otherwise "thrown away". If one has only enough power to just charge the batteries and operate the boat, then it does not matter when the compressor is used to freeze a holding plate - there is no advantage to any certain time of operation.

In other words, if the boat's usage all total is, say, 200Ahr, and you only have 200Ahr of charging each day, then there is no advantage of running the compressor at any specific time.

If you have 300Ahr of charging each day, then there is an advantage to running the compressor while the charging source is active.

Mark

[senormechanico]

Mark,
You forgot the Peukert exponent (battery charge/discharge efficiency).
Better to use the juice as it gets produced rather than putting it into a battery and later removing it at loss of around 15% for lead acid batteries.
That's one reason to LOVE LiFePo4 cells, as the Peukert exponent is on the order of 1% not 15%.
It dramatically changed our boat for the better.

[cuttersailor]

I agree entirely,

That's why I said in my original post that you can only justify a holding system if you have "bulk" power available. That's when the holding system can exploit the available surplus energy. In other words have a compressor that will continue to charge when extra current is available when otherwise normal systems will continue cycling.

cutter

[Sandero]

I built an engine drive system with a "freezer" plate. When we runnthe engine and energize the compressor coil the refer cycle cools the plate down and the longer you run it the colder it gets... I think until it reaches some design(temperature) for the plate acting as a "freezer"... ie a block of super cold ice. It warms and then the refer system cools it back down. If you don't run it long enough it does not get very cold and works like some not so cold ice in the box. If we have milk or OJ next to the plate and the refer is running a decent amount of time it turns to icey slurry. Unlike ice.. it doesn't melt... but you still have condensation to deal with.

[cuttersailor]

Originally Posted by senormechanico

Mark,
You forgot the Peukert exponent (battery charge/discharge efficiency).
Better to use the juice as it gets produced rather than putting it into a battery and later removing it at loss of around 15% for lead acid batteries.
That's one reason to LOVE LiFePo4 cells, as the Peukert exponent is on the order of 1% not 15%.
It dramatically changed our boat for the better.

Hi Senor

I know this is off topic but are LiFePo4 cells?

Thanks
mark

[cuttersailor]

Sandero,

This unfortunately is a condition that applies to all cold systems that don't have enough heat removal and retention properties to suit the condition.

All I can suggest is since you have an engine driven system and are probably reluctant to run it more than you'd like, is to explore the possibility of insulating the box further. However, bear this in mind. There is nothing magic about this stuff. That's what I was trying to convey in my original post.

It has to get cold (by running the engine long enough) and you need to be able to keep it cold (read good icebox).

How you do that is a function of having an efficient refrigeration unit and a well insulated box that's not opened frequently.

At this point in time I know of no magic answer.

mark

[Oceanride007]

One very big (In my experience) of a Holding Plate system, (When either large array of solar, Gen Set is the ability to reduce the Battery AH Drain over night. Hopefully long enough till recharge in next morning, anyway, peaceful night for off watch, no compressor or Gen run.

This has the wonderful advantage of reducing the draw down of the house battery. thus improving battery life cycle.

So to be clear on this if Holding plates are properly sized in that there is still some frozen (Eutectic) fluid by morning. State of Charge (SOC) of battery will be higher than that of a non eutectic boat, given the same SOC night before.

Haven't worked out, the cost comparison but Eutectic tanks last many Battery life cycles, weigh less than the equivalent AH capacity of Battery.

Not much use if Eutectic tanks and Solar undersized. If you got GenSet for Watermaker or AC or Hotplates, its a no brainer.

[cuttersailor]

Oceanride,

You are correct. And that's the real key. Holding plates are nothing more than batteries that store cold as opposed to batteries that store electrical energy. They are lighter and generally have a more efficient State of Charge than electrical batteries. Too bad that they usually cost more---HaHaHa.

mark

[Oceanride007]

Cuttersailor

I just did a back of envelope calculation.
Assumption Compressor efficiency 61% from:
Index of /ndejong/ME 210Refrigeration%20Systems.ppt

Latent Heat info from:
Latent Heat of Melting of some common Materials

Freezing from water at 0 deg C to frozen Eutectic fluid is equivalent to 126 Ah of Battery Power that would have been required to give same Heat conversion by traditional Refrig.

I reckon a decent 10lt tank would cost Au$150 and would last 20 years. against that a Couple of 126 Ah batteries need 2 so that SOC not reduced below 50% rough guess Au$150 each replaced every 5 years. (Say $1200)

I reckon the money is looking pretty good for Eutectic.

Of course you got to have the space for the tank and you need more solar or Gen. But planning for it makes cents.