Changing my mind about fridge power source

[ACamper]

Originally Posted by Rovobay

So, I have salvaged a fridge from a camper where I got a ton of parts to repurpose in my bus. The fridge is an item of concern for me. I will be doing mostly boondocking and not connected to shore power. This is why I wanted a 2 way fridge that can run off of propane and 110v power. That being said. I am scare that my salvaged fridge will break and I will need to replace it with something that new that will cost an arm and a leg. I am trying to convince myself to go with a smaller "apartment" fridge that runs off of 110v. some of the reasons I am trying to convince myself of this are many. first, I will not need to cut a vent in the roof to vent the heat and fumes which I would love to avoid. Second, also I would not need to install an access panel on the back of the fridge for maintenance purposes.

This is where I am at right now. I was thinking about a "3-way" fridge but changed my mind due to the complications you mentioned.

Now I am leaning towards a DC fridge with a good amount of energy saving features. I want to cut out the inverter and find a fridge with some good insulation.

I hate running generators very long so I am going to take the leap and get solar. As of now I have a generator and the bus engine and it would take too much generator for me.

Originally Posted by Rovobay

My main concern would be securing it down while we are in motion.... can you all provide me with some options? also keeping the doors closed while in motion is another issue as RV fridges have a locking latch.

It is hard to make suggestions with out seeing where it is going. I would secure it from the bottom and top.
Possibly a 2x2 frame around the bottom and both sides at the top screwed into the wall framing.

At the top, the back of the fridge might have some extra metal casing to attach some brackets to, same with the bottom. Really depends on the fridge and design some casings might not be study enough to safely handle weight. Also where the skin wraps around on the back might be sturdy enough to put a longer screw and bracket.

Not sure about the door, maybe some strong adhesive and a latch ? Super magnet ?

[Rock-N-Ruth]

Originally Posted by Danjo

It’s a challenge. I met an RV parts dealer that had everything under the sun except used refrigerators. When asked he said they’re never any good, referring to the Dometic 3 way fridges. We had propane ones in all the RVs growing up and they all performed marginally, mostly not working, but then freezing up solid. Pilot light would go out a lot and then stuff would get warm. It’s been a while, so maybe they have gotten better.

Running a gennie to charge the batteries to run the fridge sounds like a bummer. I hate generators with a passion.

If you have battery, why not put a couple panels on the roof?

It’s expensive I know.

Doesn't have to be expensive to ad a couple of panels. Search Craigslist in the areas surrounding you. I found a seller in Phoenix selling new old stock for .30 per watt. A small solar controller and a couple of 300 plus watt panels can be had for less than 300 bucks.

[Rock-N-Ruth]

Originally Posted by JackE

Thanks for the answer, but....

Just to be clear, I know what an inverter is, I know what an inverter does, I know it wastes as much power as it uses.

I'm not planning on running my entire bus off one measly battery and an inverter. I am also not going to spend $10K on a monster solar array and huge battery system.

I plan on being an old retired guy hopping from RV park to RV park with an occasional relatives house in-between. On the road, seeing the amazing sights our beautiful country has to offer. I expect to be plugged in, charging the batteries, most nights. Most likely running the generator most days while running down the road to power the A/C, while also charging the batteries.

My question was simply, will this small inverter with a built in 20 amp solar charger and up to 300 watts of solar be a good idea to keep the batteries charged up and the fridge running for those infrequent times the batteries aren't being maintained by the generator or shore power. ie. Hiking up a trail to see some wildlife at a state park while we are not in the bus and the fridge isn't being opened so most likely not running very often.

Jack, it depends on how often your fridge cycles. And the efficiency of your inverter. Modified sine wave inveters may run your fridge compressor, but slower and hotter. (Experience) Ruth and I have been off grid sporadically since 2009 and continuously since 2015. Keeping our food cold was our first challenge. The first big purchase for us after trying the 2000 watt modified sine wave inverter on 600w of solar was a small pure sine wave inverter. It added nearly half as much run time and the fridge got colder faster.
Can you do it with 300w solar? Sure. As long as you get enough sun, your fridge is well ventilated, you don't open it often, and you pre chill big items before you put them in. You might find times when you will have to augment your charging as well. Consider a small chest type deep freeze, converted to fridge with a controller you can get on Amazon for 25 bucks.
When you open the deep freeze most of the heavier cold air stays in. Our 130 dollar deep freeze only runs about 180 watts to start an 90 to run. When we we doing the fridge conversion with it it only ran several times a day for 15 minutes or so.
If you are interested on how to do it, hit me up and I'll find some time to write a whole list on it.

[john61ct]

Not a major point, but the whole "cold air falling out" point against vertical doors is largely a red herring.

Air has very little thermal mass.

Keeping the cubic space full of high thermal mass contents (water bottles as the rest is consumed) is a much more significant factor.

However, the real key to energy consumption is the R-value of insulation, 4-6" thick being much better than what you usually get.

[JackE]

Originally Posted by john61ct

Not a major point, but the whole "cold air falling out" point against vertical doors is largely a red herring.

Air has very little thermal mass.

Keeping the cubic space full of high thermal mass contents (water bottles as the rest is consumed) is a much more significant factor.

However, the real key to energy consumption is the R-value of insulation, 4-6" thick being much better than what you usually get.

Can you explain why when I'm standing by the fridge and the kids open it I feel a cool breeze float across my feet? Warmer air rises, cooler air falls. I'm not saying it's a significant problem, but I have watched the cold condensed air in a humid environment turn to a cloud and float across the floor when the upright freezer is opened.

I completely agree keeping the fridge or freezer full does make for less run time for the compressor.

[john61ct]

As I said, air has no significant thermal mass.

So the quantity of cold "negative energy" lost by emptying out that small amount of air

is insignificant compared to the huge amount of it stored in the heavy (thermally massive) contents of a well packed fridge.

So the "recovery time" is very short, with a higher energy usage percentage duty-cycle of the compressor running more, no matter which door style,

as opposed for example to replacing a cold or frozen 2 liter bottle of water with a room temperature one

which can take many hours to return to the much lower equilibrium / maintenance-only duty cycle.

I am sure there is a more scientifically accurate, jargon filled way to explain it, but that's the best I can do.

[musigenesis]

Originally Posted by JackE

Can you explain why when I'm standing by the fridge and the kids open it I feel a cool breeze float across my feet? Warmer air rises, cooler air falls. I'm not saying it's a significant problem, but I have watched the cold condensed air in a humid environment turn to a cloud and float across the floor when the upright freezer is opened.

I completely agree keeping the fridge or freezer full does make for less run time for the compressor.

Opening the door does allow cold air to be displaced by warmer air. It just doesn't matter much because the energy contained in this warmer air is very small compared to the energy needed to raise the temperatures of the still-cold items inside. So when the door closes, the stuff inside quickly absorbs that extra bit of heat from the air without increasing in temperature a significant amount (since the stuff inside has much more thermal mass than does the air).

[john61ct]

yep, nicely phrased

[Ronnie]

Every time someone mentions all the cold air going out, I think of the thermal mass of air compared to the mass of the food items, and the air is just a very small part.

So I looked up the mass, one cubic foot of air is .074 lbs. Water is 62lbs per cubic foot.

[john61ct]

Yes regular mass, as in weight does correlate to some extent with **thermal** mass.

But the latter is specifically the ability of a material to absorb and store heat energy.

Water works very well, and we have to carry a certain amount anyway.

[Mudhutwarrior]

Its not just air but humidity and the most efficient reefers and freezers are top opening. Been like that since refrigeration came into being and before that ice mass refrigeration.
When cold air falls out of the open door it has to be replaced and guess what with?
There is a reason freezers have door heaters. Floor mounted and top opening freezers need no door heaters. That seriously affects efficiency.


I read a lot of people who may know the word Science but that doesn't count in the real world.

[Danjo]

I have a chest fridge/freezer. They are more space-efficient than an upright because food gets stacked up so you can fit more stuff compared to fitting stuff on a shelf where there’s empty space above the stored items and the shelf above. I have noticed that I can fit an awful lot of stuff in 4 cubic feet compared to a dorm fridge. So if you think you need 5 cu ft of dorm fridge maybe that is really only 3 feet, resulting in a far smaller space to refrigerate. This can also be a pain in the butt since you may need to dig for items at the bottom. Get one that has stackable baskets so you can get to the beer at the bottom.

[john61ct]

I absolutely agree that mass market manufacturers may very well make their chest style units more energy efficient than their front-opening ones.

R-rating / thickness of insulation being the number one factor.

But that in no way implies that that specific design factor contributes significantly.

Correlation does not imply causation.

You can make your own custom box, and add a 12V compressor kit, and thus choose whatever layout and openings design you like.

The truly scientific way to learn how efficient a particular unit is, is to set it up running on 12Vdc, and actually measure the Ah drawn over 24hrs under various temperature differentials.