Freezer "ballast" and eutectic solutions question

[Zanshin]

I use 1L plastic bottles filled with water in both my fridge and freezer as "ballast" when these are only partially filled. The bottles in the freezer only get filled to 3/4 full because of water's 9% expansion when freezing. This helps keep temperatures relatively constant.

If I were to mix a brine solution of 18% NaCl that would have a freezing point of -14°C and use that in my freezer rather than pure water (which freezes at 0°C) and if my freezer thermostat were set to -15°C, then wouldn't this solution be more effective than mere water?

The fridge compressor could chug away at cooling the eutectic solution and it would take energy to convert it from liquid phase to solid phase at about -14°C, then when the temperature goes below that and the compressor kicks off, the freezer could absorb more heat from the outside before reaching -13°C again and triggering the compressor to switch back on; thus increasing the cycle time and reducing compressor starts/stops over time.

Has anyone tried anything along these lines yet? I know that the commercial packs don't use water for their eutectic solutions which are geared to very specific temperatures; I assume that this is because of the expansion properties of water and resulting mechanical wear-and-tear on whatever enclosure they choose.

[sfnz]

errr if you look outside Einstein you'll eventually notice your boat is suspended in.............a brine solution...use that as that is what most eutectic solutions are doing their level (and expensive best) to imitate, oh and you needn't bother with worry over expansion etc just fill plastic bottles up with salt water and freeze they accommodate the expansion no probs, what is effective is a 5 litre oil pack of salt water frozen as it lasts an age in the fridge ....so..... freeze your empties and then drain em and bin em when you're done.....

[StuM]

Originally Posted by sfnz

errr if you look outside Einstein you'll eventually notice your boat is suspended in.............a brine solution...use that as that is what most eutectic solutions are doing their level (and expensive best) to imitate,

Err, if you look at the concentration of that brine solution and its freezing point you'll find it is about -2°C which is a long way from Zanshin's desired -14°C from a 18% solution.

[Alan Mighty]

How about a 30% solution of propylene glycol (30% PG, 70% H20). Should have a freezing point of between -14 and -16C.

See: http://www.engineeringtoolbox.com/pr...col-d_363.html

[MarkJ]

You don't use less energy. The freezer will just cycle less.

The other point is you don't need -18C for a freezer.

There is no scientific basis for -18C. It was just a convenient number to make an industry standard 0 Fahrenheit.

The only thing that really wants to be cold is ice cream as the sugar and air make it spongy warmer than -10 C.

Meat only needs -4C to last 6 months.

My freezer is set at -8 C and when Piggy Boi decides on an ice cream diet buster I drop it to -12.

In the tropics those degrees are very, very power hungry... just as I am getting


Mark

[StuM]

Originally Posted by MarkJ

You don't use less energy. The freezer will just cycle less.

I think the OP knows this. That's why he said:
"thus increasing the cycle time and reducing compressor starts/stops over time."

[SV THIRD DAY]

Read this prior Cruising Forum thread:

http://www.cruisersforum.com/forums/...er-157081.html

This horse has been pretty much beat in the above thread:

Summary:
Heat in = BTUs needed to remove the head to maintain a desired internal box temp. Adding water bottles to your freezer won't change your daily power usage. In fact...it could make it worse if you are wasting BTUs to do the liquid/solid phase change. It's one of those cruiser/dock myths that makes the rounds on the chat room every so often.

[senormechanico]

I'm no expert, but suspect there is some overall efficiency loss when the compressor starts more often.
It takes about half a minute for the pressure to build and get cooling to happen.
Therefore, longer cycles at lower compressor speeds are better.
That's the reason for the smart speed regulators as well.

I would think it's similar to short tacking going upwind generates a small loss during each tack. The reasons for that of course, are better wind or current conditions, but a fridge doesn't have those to deal with.

[sailorboy1]

Originally Posted by MarkJ

You don't use less energy. The freezer will just cycle less.

In the tropics those degrees are very, very power hungry... just as I am getting


Mark

That's the truth! The only way to use less power on the frig is to reduce the heat transfer by either adding insulation, opening the lid less, or decreasing the temperature differential between the outside air and inside the system. I turned up my freezer temperature 2 degrees and the system used a lot less power and stopped icing up as much.

[donradcliffe]

Originally Posted by SV THIRD DAY

. Adding water bottles to your freezer won't change your daily power usage. In fact...it could make it worse if you are wasting BTUs to do the liquid/solid phase change.

Hate to bust your myth, but adding water bottles does reduce your power usage--they take up free air space, which reduces the introduction of warm air each time you open the door. You also don't waste any BTUs in the liquid/solid phase change--you get every one of them back in the solid/liquid phase change.

Use of brine or other eutectic solutions in the bottles is the same principle as putting in a holding plate, except that the heat transfer to and from the brine in the bottles will be less.

[SV THIRD DAY]

Originally Posted by donradcliffe

Hate to bust your myth, but adding water bottles does reduce your power usage--they take up free air space, which reduces the introduction of warm air each time you open the door. You also don't waste any BTUs in the liquid/solid phase change--you get every one of them back in the solid/liquid phase change.

Use of brine or other eutectic solutions in the bottles is the same principle as putting in a holding plate, except that the heat transfer to and from the brine in the bottles will be less.

What Don is asking me to do is believe his non-lab tested ideas over the hard data/lab testing that Technautics had been doing since 1968 with a Scientist with a advanced degree in thermodynamics that worked for JPL.
Having a fully instrumented test box really helps you weed through the facts from fiction. So here we go again with this myth that just won't die.

The false comparison of a bottle of water or even a smartly designed concentration of brine or glycol eutectic solution sitting in the bottom of your box to a properly designed holding plate leads many people astray. The problem with this approach is that you have zero control on the phase change of the liquid.

A properly designed eutectic 12v holding plate set up is not turning the compressor on and off based on the box temp, but rather on the holding plate temperature so that the eutectic solution is NEVER allows to phase change from a solid back to a liquid. Now, an engine driven holding plate concept/set-up IS different than a 12v engineering approach...which leads to more confusion and myth.

You never want your eutectic solution to defrost and phase change from a solid to a liquid on a 12v system. Because you will spend energy (A LOT) just to complete the phase change without a corresponding temperature change. This doesn't really matter on engine driven holding plates because you have such a huge over powering and BTU capacity in the compressor, this is not the case of a 12v system.

On a front opening refrigerator, sure the bottles of water take up volume, and prevent cold air from spilling out and being replaced with warm moist air, BUT you would be much better off by putting in an empty plastic jug than trying to make a parasitic holding plate out of a bottle of water.

[MarkJ]

On some cruising boat we have excess electricy generated during the day by solar. Many good setups are at 100% by midday.
So wouldn't using 'excess' power in the early afternoon make an overall benefit?

Mark

[SV THIRD DAY]

Originally Posted by MarkJ

On some cruising boat we have excess electricy generated during the day by solar. Many good setups are at 100% by midday.
So wouldn't using 'excess' power in the early afternoon make an overall benefit?

Mark

If you can keep your compressor from kicking on at night to refreeze the water bottles yes...otherwise you just "spent" the power you "saved" at 3AM when you are running completely off your battery bank. In fact, there is an advanced thermostat for holding plate systems that will indeed freeze down the holding plate to a lower temperature when it senses the batteries are in fully charged mode (as evidenced by a voltage of 13.7v or higher). This takes advantage of the excess power during the day to set-up a longer "off time" between cycles.

At the heart of the argument here is the control of lack of control of when you will be refreezing those water bottles. If during the day...with excess power...sure...bingo...you are a winner. But what happens at night without the benefit of the excess power? This situation is actually where we measured the "water bottle parasitic" approach to use MORE daily power that without...with it getting worse the more water bottles you put into the box and tried to freeze down.

All of these little details is also why this Myth is so powerful because one guy can put such a large amount of water bottles into his box with such good insulation that his do NOT phase change over the night and they are kept frozen during the day. Or they phase change, but he has so much solar that his unit can run all day long and refreeze them back without really taking power away from his bank. Or a guy with a front opening Box says how much it works to fill the air void space in his box with empty milk jugs, but the guy getting the advice has a top opening box where the cold air does not spill out. Advice without details behind it is dangerous.OR...OR...OR... So he tells all his dock and internet friends that his water bottle trick works. And then it becomes gospel truth, even when a guy that builds eutectic holding plate systems for a living is on the end of the dock waving his arms saying...yes but....wait a second...what about situation A, B and C.

Because there are so many factors involved I try to boil it down to the bottom line which is, in a side by side box with same insulation will the water bottles help with your end of the day power usage...Usually No. But there are always the exceptions to the data test condition.
This bottom line issue may not matter to a guy that runs his genset all day, or has 1300W of Solar (like me), or has 6" of new insulation.

[Zanshin]

I didn't want to go into too much detail in the initial post, but here goes:
a) I used -15 as an example temperature since that is used as the ice-cream "Gold Standard"; in reality I would want the phase change at a higher temperature.
b) Why should I use a chemical when it is easy to use salt and water for the solution?
c) I have a digital thermometer and small computer (Arduino) attached to this system and has been stated in this thread I would indeed use power surplus during the day to force the phase change to solid and could then program the processor to let the freezer warm up to just above the phase change temperature until there is once again power to spare.
d) According to the literature supplied by Danfoss and other compressor manufacturers, there is a significant energy loss on restarting the compressors and they recommended running for as long as possible at lower compressor speeds when it comes to total energy efficiency. Unfortunately the typical boat setup only has an analog on/off thermostat system which doesn't lend itself easily to controlling compressor speeds (although some smart speed control systems try to do a pretty good job).
e) With the little microprocessor, it is easy to measure when the phase change takes place, since the electrical conductivity of a brackish solution changes significantly upon freezing; so measurement and control of the phase-change temperature can be done precisely.

[Zanshin]

Don stated

You also don't waste any BTUs in the liquid/solid phase change--you get every one of them back in the solid/liquid phase change.

to which SV Third Day responded

Originally Posted by SV THIRD DAY

What Don is asking me to do is believe his non-lab tested ideas over the hard data/lab testing that Technautics had been doing since 1968 with a Scientist with a advanced degree in thermodynamics that worked for JPL...

I hate to have to disagree with him and his scientist, but I'm a firm believer in the first law of thermodynamics and Conservation of Energy. Entropy notwithstanding, I'll side with Don on this one.