Fridge that uses little, if any, of your battery!

[OzeLouie]

It can be done and here is an example:

1. Sample cabinet is 90 litre capacity top opening with 75mm thick walls, 100mm base.
2. Heat load calculation for this cabinet including common use contingencies and maintaining +2C to +5C cabinet temperature in a maximum 30C ambient, indicates 433 watts per day. (By 3.41 for BTU’s)
3. Refrigeration using a 3.5cc condensing unit coupled to a 5.1 litre eutectic plate of 530 x 350 x 40mm overall.
4. Phase change capacity of 5.1 litres at 85 watts per litre, is approx. 433 watts.
5. Refrigeration would be required to run a minimum of 3.5 hours per day.
6. If *abundant power is available and the system has auto switching to consume this power during that period, then virtually no battery power is consumed for the 24 hour period. (*Abundant power is that which is available from any source like solar, gen set etc after batteries have been topped up and power in would otherwise be wasted!)
7. In this example abundant power would need to be available for at least 3.5 hours during the day otherwise some battery power would be consumed to finish the eutectic mass re-freeze.
(Heat load calculator pictured below with details, click on pic to expand).

[StuM]

> " indicates 433 watts per day"
> ...
>"85 watts per litre, is approx. 433 watts."

And incorrect units in almost every line of the attachement


You still haven't learnt the meaning of the basic electrical units I see.

Which makes any advice involving electricity dubious.

[OzeLouie]

Originally Posted by StuM

> " indicates 433 watts per day"
> ...
>"85 watts per litre, is approx. 433 watts."

And incorrect units in almost every line of the attachement


You still haven't learnt the meaning of the basic electrical units I see.

Which makes any advice involving electricity dubious.

The reference refers correctly to watts of heat.
BTW I notice you still haven't mastered spelling, so perhaps you should focus your efforts on that rather than nit picking threads of those trying to offer something of general interest.

[StuM]

Originally Posted by OzeLouie

The reference refers correctly to watts of heat.

Again, you make my point for me. "Watts of heat" is meaningless and reveals a fundamental lack of understanding of basic physical units.

Common units of heat are the Joule or the Watt hour. (I Joule = 1 Watt second)
Watt is a unit of transfer of heat. i.e. it is a rate, not a quantity. (1 Watt = 1 Joule/second)

Watt is no more a unit of heat that knot is a unit of distance.

Saying 10 watts of heat, or any other energy, s like saying a volume of 10 litres per hour.

[Boatguy30]

Btus are btus and watts are watts somunless you got a new fangled perpetual motion bru pump running in zero watts I’m not sure much is happen here

[weavis]

Can we all be nice?

Leave the moderating to the moderators. Saves a heap of bad feeling and incorrect assumptions.

if you have an issue with a post, please tell the site team and we will look at it.

[Dsanduril]

Originally Posted by OzeLouie

...433 watts per day. (By 3.41 for BTU’s)...

Regardless of whether or not you like how he presents it, Stu M is right. And I sympathize with him, this is not the first time the OP has presented this thinly veiled advertisement with bad units.

Taking just the quoted example, what does “433 watts per day” mean? Watts is an instantaneous measure of power, is the OP proposing this is the power used all day long? Another way to express 433W would be 433/12=36A@12V. A 36A load running all day (is that what the OP means by “watts per day”?) is a mighty big load on just about any boat of our size. That’s 864Ah every day, hardly seems efficient (for reference, my 105l commercial fridge uses 27Ah/day).

The OP probably means 433Wh per day (as Stu points out), which would actually make sense and equate to 36Ah@12V per day, a reasonable refrigeration load. But every time he gets called out on this issue he insists that you don’t need the time constant, and sorry, that’s just wrong.

This carries on in the table where he uses “A/H” when the correct unit is “Ah” (Amps divided by hours is quite literally the inverse of Amps times hours). We accept (sometimes not gracefully) these basic errors from our fellow boaters who may not be educated in electrical matters and have questions about their systems, and we can generally surmise from context (as we also can here) the actual intent, but it is not excusable for someone advertising their expertise. Get the units right (so we at least trust that the maths are done right) and then we can discuss the actual technical merits of the proposed system.

By way of example, I was once presented (here on CF) with an ad for Spectra watermakers that touted “13.6 Watts per gallon”. I wrote them a polite note about the missing time constant, we had a couple of back-and-forths about the details, and by the next day they had changed all of their online advertising. Print advertising was changed with the next issue.

[sailorboy1]

you guys just like to fight!

I understand 433 watts per day. That is 36AH/day for a 12V boat system, or 18AH/day for a 24V one.

But the real question for the whole thread is "so what"! Of course you can have a refrigeration system that uses little power if you put in enough insulation and live in a perfect world of never opening the lid. But after those 2 factors you are left with the heat transfer rate across the walls and the method of cooling the box doesn't change that at all!

[StuM]

Originally Posted by sailorboy1

I understand 433 watts per day. That is 36AH/day for a 12V boat system, or 18AH/day for a 24V one.

[sailorboy1]

Originally Posted by StuM

bang your head all you want

36AH each day is a pretty normal way of thinking of energy use per day. It doesn't matter if you use it all in 1 hour or spread out evenly during the 24 hours. I know how much my solar can do in a DAY. Why wouldn't I think of use the same?

You are just coming off as an ass with your constant thinking you are the unit police all the time. And if that was all you got out of my post I am right and all you want to do it is fight.

[rslifkin]

Originally Posted by sailorboy1

36AH each day is a pretty normal way of thinking of energy use per day. It doesn't matter if you use it all in 1 hour or spread out evenly during the 24 hours. I normal how much my solar can do in a DAY. Why wouldn't I think of use the same?

Agreed. Plus, a day averages out the hot, high solar heating part of the day with the cooler night, so for a fridge, it gives better power numbers. Overnight power usage will generally be lower than in the afternoon.

[Richard Kollmann]

The question I have for others to answer is, Are the Btu's of melting eutectic ice equal to the amount of Btu energy required to create eutectic ice? If answer is Yes or No explain.

[chris95040]

Imagine talking about racecar performance with someone who doesn't understand the difference between "speed" and "distance".

[Dsanduril]

Originally Posted by sailorboy1

...That is 36AH/day for a 12V boat system...

36Ah/day@12V = 433Wh/day = Average power of 18W (433Wh/day/24hr/day) = 1.5A@12V. This is the problem, 433≠18.

That could be 1.5A (18W) continuously for 24 hours, or 36A (433W) for one hour, or in the OP's case ~10A (120W) for 3.5 hours.

Watts are an instantaneous measure of power in use. Wh (or Ah with voltage appended) are a measure of energy used. You must have the time constant to talk about energy. And batteries (the initial point of the OP's post) are energy storage devices. You can extract power from them, but the entire metric of their storage is energy. Energy and power are not the same thing. The OP desires to talk about energy, but instead uses units of power. Would you buy his fridge equipment if it draws 36A (because that's what he says)? Probably not. But 10A, for 3.5 hours a day? Now we're talking. How about 1.5A but it runs all the time? That would be pretty easy to support.

[Jammer]

Originally Posted by Richard Kollmann

The question I have for others to answer is, Are the Btu's of melting eutectic ice equal to the amount of Btu energy required to create eutectic ice? If answer is Yes or No explain.

The amount of heat the melting ice absorbs is equal the amount of heat that had to be removed from it to freeze it.


The amount of electrical energy it takes to remove heat from the eutectic solution (to freeze it), dumping that heat overboard as waste heat is considerably lower. The electrical energy required is lower than the amount of heat moved by a factor of about 5:1, depending on the refrigeration system and the temperatures involved.


I hope this answers your question.