Question on 12 volt refer (updated with some answers)

[bdosborn]

I inadvertently did a thermal mass experiment of my own when I threw a 12 pack of Fat Tire in the RV fridge:

The fridge temp went from38F to 48F, I guess they don't keep the beer coolers that cold at Tipsys. It took around 6 hours to get the temp back down to 38. I guess I'll be pre-chilling the beer in the house refrigerator before packing the camper. The trouble with this thermal mass approach is the mass keeps diminishing in direct proportion to the outside temperature.

I think I'll call it the izhedrunk enigma.

Bruce

[eamarquardt]

The trouble with this thermal mass approach is the mass keeps diminishing in direct proportion to the outside temperature.

I think I'll call it the izhedrunk enigma. ;)

Bruce

For the reasons cited above, beer is not used in marine holdover plates as the eutectic solution. Salt water is best (in terms of cooling power, but is corrosive), antifreeze (in spite of its attendant consequences if consumed) is the most common, and beer is in last place (again for the reasons stated above).

Cheers,

Gus

[Karl]

Folks living off grid with +12 or +24 VDC Fridges sometimes use thermal mass to 'coast' the Fridge through power outages, or to reduce Compressor cycling. Some use cases of Beer; some use Concrete Pavers in the bottom of the Fridge; others get even more creative.

My own vision is to put a 'bit' of mass in the bottom of our future CT Fridge or a horizontal 5 day Cooler. This will get us through cloudy days, when boondocking off of PV Panels. It will let us 'charge up' this mass on good Solar days, or from the TV while driving.

Let’s see if I understand some of this that you and others have mentioned in this thread.

Adding mass to an electronic cooler helps keep the internal temperature from fluctuating as much as it might otherwise do without mass. Thus, mass can equate to fewer on and off cycles of the cooler’s compressor although the amount of energy used to maintain the cooler’s temperature over time would be roughly the same as without the mass since mass itself doesn’t have an actual cooling affect, of course. An advantage of using mass is that you are more likely to maintain the needed temperature of your cooler during times that you don’t have sufficient power (from solar or otherwise).

So, if you were to determine, for example, that you could spare enough space in your electronic compressor cooler for, say, 216 cubic inches of mass, would you use:

A: 1 six-inch cube of mass (surface area of 216 square inches)…or…
B: 8 three-inch cubes of mass laid out apart from each other (surface area of 432 square inches)…or…
C: 216 one-inch cubes of mass laid out apart from each other (surface area of 1,296 square inches)?

If we were talking about keeping a regular cooler cold using a 216 cubic inches of ice, I assume you would want to you one 6-inch cube of ice if you wanted to maximum the time or, if you wanted to cool things down fast, you would use the same volume of ice in smaller pieces, such as 3 inches or even 1 inch, right? But, for an electronic compressor cooler, do you want to have a fast transfer of heat or a slow transfer? Should I assume that the answer depend on the interval of time in-between charging the batteries (having solar power)?

Two last questions. If expensive were not an issue, what material would you use for mass? Assuming that the answer you just gave was outrageously expensive for most people, what reasonably-priced material would you use for mass?

Sorry for all the questions but I never thought of putting "mass" in a cooler to reduce compressor cycling for times that I might be off the grid without sufficient battery power. Interesting idea to think about.

Thanks!

[eamarquardt]

You ask a lotta questions!

Folks living off grid with +12 or +24 VDC Fridges sometimes use thermal mass to 'coast' the Fridge through power outages, or to reduce Compressor cycling. Some use cases of Beer; some use Concrete Pavers in the bottom of the Fridge; others get even more creative.

My own vision is to put a 'bit' of mass in the bottom of our future CT Fridge or a horizontal 5 day Cooler. This will get us through cloudy days, when boondocking off of PV Panels. It will let us 'charge up' this mass on good Solar days, or from the TV while driving.

Let’s see if I understand some of this that you and others have mentioned in this thread.

Adding mass to an electronic cooler helps keep the internal temperature from fluctuating as much as it might otherwise do without mass. Don't think so. The temperature fluctuation will be a function of your thermostat, your system's ability to cool, the temperature of what you put in the box, etc. Adding thermal mass will just lenghten the cycle times. Thus, mass can equate to fewer on and off cycles of the cooler’s compressor although the amount of energy used to maintain the cooler’s temperature over time would be roughly the same as without the mass since mass itself doesn’t have an actual cooling affect, of course. An advantage of using mass is that you are more likely to maintain the needed temperature of your cooler during times that you don’t have sufficient power (from solar or otherwise).

Yes to all of the above.

So, if you were to determine, for example, that you could spare enough space in your electronic compressor cooler for, say, 216 cubic inches of mass, would you use:

A: 1 six-inch cube of mass (surface area of 216 square inches)…or…
B: 8 three-inch cubes of mass laid out apart from each other (surface area of 432 square inches)…or…
C: 216 one-inch cubes of mass laid out apart from each other (surface area of 1,296 square inches)?

You'd like to maximize the amount of surface area to facilitate heat transfer so the answer is "C".

If we were talking about keeping a regular cooler cold using a 216 cubic inches of ice, I assume you would want to you one 6-inch cube of ice if you wanted to maximum the time or yes, if you wanted to cool things down fast, you would use the same volume of ice in smaller pieces, such as 3 inches or even 1 inch, rightyes ? But, for an electronic compressor cooler, do you want to have a fast transfer of heat or a slow transfer? I'd guess you'd want a fast transfer when cooling down the "thermal mass" with the compressor running and the release of the "cold" would be as required to absorb the heat infiltrating into the box which would be slower than the cool down under most circumstances. I'd guess the cycle time compressor on to compressor off (depending on the design of the cooler and the temperature difference between the inside of the cooler and the outside) might be in the neighborhood of one to three. Adding thermal mass will not alter the cycle ratio for a given temperature difference over time but will lenghten the on time and the off time. Should I assume that the answer depend on the interval of time in-between charging the batteries (having solar power)? I'm not sure what the question is.

Two last questions. If expensive were not an issue, what material would you use for mass? Assuming that the answer you just gave was outrageously expensive for most people, what reasonably-priced material would you use for mass? I think that this is your "lucky day". According to the chart here: http://en.wikipedia.org/wiki/Heat_capacity it looks like water is just about as good as/or better than anything as far as having a high specific heat goes!

One other thought: There is a big difference in the energy it takes to heat a gram of water one degree and the amout of energy it takes to melt a gram of ice. If you use a salt water solution blended to freeze right between the on and off temps of your thermostat in your freezer you can take advantage of the "latent heat of fusion" and get a lot longer period of time between cycles as long as you can freeze the salt water fully and thaw it fully (meaning probabaly more and smaller containers of salt water).Sorry for all the questions but I never thought of putting "mass" in a cooler to reduce compressor cycling for times that I might be off the grid without sufficient battery power. Interesting idea to think about.

Thanks!

I've tried to give the "simple" answers. There are a lot of factors involved and the only REAL way to know what will happen (short of really complicated computer models) is to "do it" and see what happens. My thoughts, should however, be in generally the "right direction".

Cheers,

Gus

[Karl]

Gus,

That was exceptionally informative. Thank you!!!

Regarding water being the ultimate in terms of having a high specific heat, do I need to splurge on expensive bottled water or will tap water do???

Regarding your answer of water for mass and your answer of C for maximizing the surface area to facilitate heat transfer, sounds like I was following reasonable scientific theory (without knowing it) with my non-electric cooler when I had water from melted ice filling up the bottom few inches of the cooler.

Seriously, though, I appreciate your time explaining all that! You have been very helpful.

Thanks again!

[eamarquardt]

I was following reasonable scientific theory (without knowing it) with my non-electric cooler when I had water from melted ice filling up the bottom few inches of the cooler.

Almost! Water conducts heat about 25 times better than air. So, if you have water in the bottom of your cooler you are going a better job of conducting heat accross the "interface" of your cooler's interior and the cooler's exterior. In the design of boat iceboxes (where my experience comes from) it is common to have a drain with a trap in it. This let's the water escape while the water trap keeps cold air from flowing out. Ice should be kept as high in the box as possible and out of standing water to make it last as long as possible. If you want to use water as thermal mass, you should bottle it. It has been scientifically proven that water/ice from Hell is better than other types of water/ice. It is, however very rare, expensive, and hard to get!

Water doesn't have much cooling power. It takes only one BTU to raise the temperature of one pound of ice one degree F. It takes 144 BTU to melt one pound of ice into one pound of water keeping the temperature at 32F. Simply put, ice has 144 times more cooling power than the same amount of water. Cool!

Cheers,

Gus

[Engineer Guy]

Gus ~

Thanks for the intriguing ideas w/supporting Math.

At the leisure of fellow Forum-ites, read here here about some off grid ideas I've reviewed over the years to keep the Brain ticking, and for my intrinsically-warm House design. 'Home Power' Magazine, linked below, was a great resource for tried & true House design concepts. Physics 101 fundamentals apply regardless of House or CT square footage.

http://www.google.com/search?sourceid=c ... rigeration

One validated Solar House idea I would expropriate would be a combo of fast/slow storage/release Mass. Dark-colored Tubes or drums of water tastefully integrated into a Stone or Tile-floored Home Greenhouse is one such 'combo' idea that's been wrung out over the years. A Fridge adaptation might be a combo of ice - hey, in Pretzel containers perhaps! - along with ~1" high, Saran-wrapped Patio Pavers, etc.. Some very creative off grid House Geeks have tried all sorts of interesting Fridge tests.

You betcha, internal Cooler space tradeoffs for mass would have to be 'reasonable' and casually tested. Energy exchange doesn't happen 'for free'. The trick would be to store any 'excess' energy in mass on very sunny days, or from the TV Alternator while towing. 5 day Cooler internal temp 'sine wave' excursions and run times on a plot would simply lengthen between Compressor cycles; they wouldn't change energy input needs fundamentally. Alternate eutectic storage media certainly would be 'fun' to experiment with. There's Gel Pads and interesting 'stuff' from Industry ripe for adaptation. 'Safe' Glycol used in Solar Heating exchange is another commercial possibility.

http://www.google.com/search?sourceid=c ... ing+glycol

Blowing chilly night air through a Cooler with just a small Fan would provide 'pre-cooling' on the cheap down to temp setpoint 'x' if the interconnected Fan and Compressor did not run simultaneously. These Controls can be very simple and mechanical from the Grainger's Catalog; not Pentium-based!

Sunfrost and Sundanzer have super-insulated enclosures to fundamentally shift the paradigm of off grid energy consumption. Mantra: you never have to replace energy you never lose. I envision my 5 day Cooler in a high R Foam 'well' to shift heat loss/heat ingress. Room temp items from the store would cause faster Cooler heat rise and energy transfer/consumption until cooled. 'Sufficient' Batteries are simply another energy storage 'mass' [though chemical and more complex than Pavers] when off grid.

The positive role of internal mass is acknowledged in the advice that a conventional Chest Freezer runs more efficiently when more full of food. Surface area-related heat transfer is confirmed by the Cooler lifetime of block ice vs. cube ice. Mess-free water, or other liquids, in flat containers is a given.

http://www.sunfrost.com/refrigerators_main.html

http://www.sunfrost.com/extreme_efficiency.html

[eamarquardt]

Yup, what he said! Money spent on insuluation is well spent. You don't have to get rid of heat that doesn't get in! There are NO SILVER BULLETS as even Congress can't amend the laws of nature/physics/thermodynamics although I've heard they have considered trying! As mentioned earlier, water/ice is just about as good as it gets for the "bang for the buck/volume/weight". The reason to use glycol or other solutions is they freeze at colder temperatures than water so you can keep your icebox colder than with straight ice. You can make salt/water solutions (function better than glycol) to get below 32 F but they are corrosive particularly if you have different types of metal in contact with them but salt water in plastic containers is a win/win!

Understanding how "the world rotates" is fun and it's great to get the "mostest for the leastest". I'm not a fanatic (no real energy saving technology employed) but I'll bet my home energy bills are hard to beat just because I don't use a lot for heating and cooling due to the weather here near the ocean in So. Cal and I keep most of the lights off. My wife's heat requirements, however, are a bit more demanding than mine but she's gone most of the time! I do, occasionally, trip an 85 amp 240 volt breaker with my welder, ha!

Cheers,

Gus

[kstephenson]

After reading the last 3 pages of information. I ve come to the conclusion......I M LOST.....haha
After trying to read all the posts on this subject. I m unable to remember what the point was. I guess what I m trying to say is.....what happen to trying to keep things simple? I believe thats why most of us have gotten into this hobby. Simple Tiny Trailers..... I m sure some of you may have engineering degrees...or maybe think you do......but most of us don't.
Hope I didnt step on any toes here.....if I did,.,,I m sorry....but I had to say something.

[eamarquardt]

After reading the last 3 pages of information. I ve come to the conclusion......I M LOST.....haha
After trying to read all the posts on this subject. I m unable to remember what the point was. I guess what I m trying to say is.....what happen to trying to keep things simple? I believe thats why most of us have gotten into this hobby. Simple Tiny Trailers..... I m sure some of you may have engineering degrees...or maybe think you do......but most of us don't.
Hope I didnt step on any toes here.....if I did,.,,I m sorry....but I had to say something.

It's all about getting the mostest for the leastest! But if you don't need much, you can ignore most/all of it! Your choice!

Cheers,

Gus

[Karl]

I guess what I m trying to say is.....what happen to trying to keep things simple?
Hope I didnt step on any toes here.....if I did,.,,I m sorry....but I had to say something.

Toes are still intact.

Simple is a relative term. For me personally, my camping life is simpler by having my 86-quart Edgestar compressor cooler act as a refrigerator and my 40-quart Waeco compressor cooler act as a freezer and then not worrying about anything until I get home. They work perfectly according to what I want and need. If I want some well-frozen ice cream by the camp fire at night, I can have it. If I want my favorite beverage chilled to 33 degrees, I can have it. For others, however, a non-electronic cooler with ice (not loose ice, of course, but rather properly frozen in containers!!!) is what makes things simple. And either way (ice or electronic) is perfectly fine, of course. To each their own, right???

While I know this thread got far off of the original subject, I think that the discussion about mass was fascinating regardless of the type of coolers we use.

[Engineer Guy]

Intact toes here... Some folks hold up their Galley hatch with poles and bungee cords. Others like gas-filled struts w/Party Lights. And A/C; and DVD Players; and Microwaves. As the old song goes, 'you say toe may toe; I say toe mah toe'. 'Simple' is in the mind of the TD beholder, eh?

I'm definitely into the K.I.S.S. approach to off grid Food Preservation. So, Karl's approach is one I hope to emulate. Make ice very efficiently; store ice very efficiently. Way back when, the advent of refrigerated Railroad Cars using ice totally changed Food distribution from regional to national, and home meal shopping schedules from daily to bi-weekly or so. Any approach that lets a Camper not fret ice replacement w/o soaking Food over a weekend, or a week, is an updated sort of extended Food storage solution circa 2010.

5 day Cooler Manfs have figured out the bennies of more insulation. SIP Manfs did too. A pal has built 2 Houses w/SIPs, and they are sweet and simple. Such construction is an energy consumption game changer at >9,000' in the Rockies, or when adapted to a TD/CT potentially roasting in Quartzsite AZ. Bdosborn's homebrew SIPs in another Thread were brilliant!

Mass in either Houses or Coolers is about as simple as it gets; this is why I like that approach. Replace Karl's Home Utility Provider with a TD or CT Battery [or Propane Tank] in concept, and the reduced energy consumption/increased creature comfort advantages still ring true.

See a nice overview of +12 VDC Fridge fundamentals here, buried in a Google Link above:

http://www.backwoodshome.com/articles2/yago102.html

[kstephenson]

I m glad all toes are still present. I guess what I was trying to say was it seem like things were way off the orginal subject......kind of turned into a science lesson.

Happy Camping

[eamarquardt]

I m glad all toes are still present. I guess what I was trying to say was it seem like things were way off the orginal subject......kind of turned into a science lesson.

Happy Camping

Toes intact and fingers too! Sometimes there is more to the question than meets the eye. Many ways to keep things cool. Lots of misinformation gets posted and taken for "gospel". Oringinal question was about using 12 volt reefers to keep things cool. Science is cool as it explains how everything (except human emotions) works. We didn't discuss sex, religion, or politics so it all seemed pretty germain to the orginal topic to me (but my standards are a bit looser than some, ha).

Me, I'm gonna use a flame/resistance heater to keep my stuff cool. Only ask how if you really want to know! It's not that complicated but unintuitive when you first think about it but it is cool.

Cheers,

Gus

[astrotrailer]

We did our first 5 day warm weather trip with the Engel 45 quart fridge freezer. I kept the Engle
set at 10 degrees and rotated 2 liter water bottles from the Engel to a 24 quart cooler that kept the
beer, soda and water bottles cool. We also kept food in the cooler that we didn't want frozen. As rotated
the water bottles daily back to the freezer so there were also 3-4 frozen bottles keeping the cooler cool.
The freezer had no problem keeping at 10 degrees. I kept popcycles in the freezer and they were good all
through the trip. The freezer could go colder but I found that 10 degrees was good enough to keep
freezing the water bottles and keeping the bag of ice for drinks cold.