Fridge Size

[Redbearded]

Hi All,

So I'm just a couch commander on this stuff so far, but I'm puzzling stuff out so that can change

I think I have decided that I am going to use a chest freezer converted to a fridge for my food storage, and I have no idea what size I should go with. Power will not be an issue, but the depth from the back wall may be. Any thoughts on what size to go with? Smallest looks to be 5 cubic feet, and they go up from there.

I saw one that was a dual zone one that looked interesting conceptually (set one up for a freezer and one a fridge maybe?) it was 9 cubic feet (this may be too big, I have no idea).
Link to dual zone the reviews kinda suck, but I like the idea. I like that it is only 2ft wall depth.


I'm thinking about putting a cutting board top above it, that has a mechanism to be able to open the freezer while not disturbing the stuff on the cutting board (still at the conceptual stage, maybe gas strut assisted), and having the sink next to it.

So, thoughts on sizing?

 

[Almost There]

For one person that will be absolutely huge!

My 65 Qt Whynter unit set at 0 F for freezer use is almost always full to the brim but then I have enough food for me to eat for over a month without going grocery shopping (ice cream being the exception). Full to the brim means that it's more economical to run too!

I had a deep freeze in my apartment that was the same size as the one you have in mind. It was rarely full - only when I had a big turkey as well as a full sized ham in there. It did come in handy for when I was blanching and freezing summer veggies straight from the farm market for use in the winter. Now I live with farm fresh veggies year round because I migrate...I don't need to put up my own for the winter.


I buy 'family' size packages of meat and baggie individual portions before freezing so I can take out one of anything. It also comes in handy when I have last minute dinner guests - simply grab a second of whatever.

From what I've seen so far of your plans, you really are still in a sticks and bricks mode of thinking and are only putting wheels under it.

Keep in mind that every extra pound of 'stuff' you carry is going to cost you, not only in initial set up but each and every day. It will cost you in the amount of solar you have to install, the price/size of the battery charger you'll need, the size, cost and operating expense of the generator you'll end up needing. The weight of the bigger generator and battery system added to the weight of all the extra gear you're carrying will result in higher operating costs for the larger vehicle/trailer that you'll need. Higher insurance expense, higher repair costs.

Just because you  have the budget for a bigger set up doesn't necessarily mean that you should. 

Most of us who choose to go on the road because we love the freedom have also learned that part of being free is the concept of 'less is more'.

 

[tx2sturgis]

...Power will not be an issue...

Or, maybe it will be.

I looked at the spec sheet and that unit is rated at 338 kwh per year.

That's approaching one kwh per day.

With solar as the source of power, that's a lot!

 

[Redbearded]

The 1kwh/day is if it's run as a freezer, the plan is to convert to a fridge with a bit of reengineering (see pdf below). From what I have read I can cut the usage dramatically (he found that usage was 37kWh/year...). As a daily average that is like 100w/day with more in the summer and less in the winter (exactly at the power from solar would be), and even if it's dramatically wrong which I don't think it will be, given his background, it's still almost worth it to spend $100-200 on a freezer and try it. Worst case is I sell the freezer for a small loss and buy an Engel or something else.
View attachment chest_fridge.pdf

 

[jimindenver]

3 or 3 1/2 ft is the smallest I know of.

 

[tx2sturgis]

This idea has been around for decades. Many off-grid homes use a converted chest freezer as a chest refrigerator by converting, supplementing, or modifying the thermostat.

It works, but if it were me, I would pick the most efficient and simple, non-electronic unit you can find as a starting point.

 

[wagoneer]

Saying power is not an issue is Neye-eve ( I can't spell cause I am a dropout) it's not 12 volt. seriously consider this.

 

[Redbearded]

Just learned that the engel and other compact fridges that everyone raves about being so economical are all using a Danfoss compressor. The reason why it seems is that even though they are all hooked up DC the danfoss controller needs 3 phase AC to be soft start and variable speed. Link It also seems that it is possible to convert, by changing out the compressor, most 120V fridges to 12V link. But even when you do that the motor is still running on 3-phase AC.

 

[SondraRose]

Just learned that the engel and other compact fridges that everyone raves about being so economical are all using a Danfoss compressor. The reason why it seems is that even though they are all hooked up DC the danfoss controller needs 3 phase AC to be soft start and variable speed. Link It also seems that it is possible to convert, by changing out the compressor, most 120V fridges to 12V link. But even when you do that the motor is still running on 3-phase AC.

If I recall correctly, Engel uses a different compressor, than the Danfoss. Called a "swing" compressor.  Excellent for solar due to its low start up amperage.  The 40 qt. one we are buying pulls .7 to 2.7 amps.

http://www.roadtrucker.com/engel/engel-12-volt-fridge-freezer-technical.htm

 

[tx2sturgis]

There are a lot of consumer devices that convert DC to a voltage and/or waveform internally to match what is needed by individual components.

TVs, laptops, smartphones, radios, clocks, LED lamps, battery chargers, the list goes on and on. 

In the case of the Danfoss, and presumably some others, is that the 3 phase AC waveform used to control the compressor is not the same as the single phase 120 v AC from normal household AC single phase power.

 

[Redbearded]

Thanks for the link to the swing compressor!

I had heard of them, but I hadn't really realized how they worked... I think the name swing compressor was throwing me off as I kept picturing something you know, swinging...
From wikidifference "[font=Roboto, arial, sans-serif]As verbs the difference between [/font][font=Roboto, arial, sans-serif]swing[/font][font=Roboto, arial, sans-serif] and [/font][font=Roboto, arial, sans-serif]oscillate[/font][font=Roboto, arial, sans-serif] is that [/font][font=Roboto, arial, sans-serif]swing[/font][font=Roboto, arial, sans-serif] is to rotate about an off-centre fixed point while [/font][font=Roboto, arial, sans-serif]oscillate[/font][font=Roboto, arial, sans-serif] is to [/font][font=Roboto, arial, sans-serif]swing[/font][font=Roboto, arial, sans-serif] back and forth,"[/font]

It looks like it is a linear motor that moves the piston up and down between a set of springs, very clever! It also looks like LG uses one in some of their fridges, and in their chest freezers in other parts of the world...

Link to linear compressor study

Link to LG linear compressor press release

LG linear compressor teardown (video)

Danfoss vs Sawafuji (used in Engel) thread
Looks like the "Inverter Linear compressor" is what LG calls their tech and they are using it in at least some of their stand up fridges now.

 

[John61CT]

> The 1kwh/day is if it's run as a freezer, the plan is to convert to a fridge with a bit of reengineering (see pdf below). From what I have read I can cut the usage dramatically (he found that usage was 37kWh/year...). 

The units you want to work with, for apples to apples, are AH per 24 hours at 12V.

> As a daily average that is like 100w/day with more in the summer and less in the winter

Watts per day has no meaning. Watt-hours per day does make sense, divide by volts to get AH.

But 8AH per day is a gross underestimate for a mains unit.


All the 12V compressor fridge/freezers are ballpark same energy efficiency, that is **very**.

Many threads report as low as 10-20AH per day, obviously higher used as a freezer and/or in high ambient temps.

Size of the unit is not very significant, nor is chest vs front-opening.

Insulation is most significant, then comes adding ventilation over the hot condenser / electronics areas.


No unit designed for shore power comes close to these in energy efficiency. But many "science projects" do run them off an inverter, some with a 12V thermostat controlling the inverter.

None of the credible report come close to the 12V ones, but yes they are much cheaper up front.

Fine if you have a big bank and lots of extra energy incoming every day to spare.

Otherwise 12V compressor fridge is IMO worth saving up for, can sometimes get good deals $400-600 new, or of course much less on local Craig's. Plus no money wasted on an inverter.

 

[highdesertranger]

Engel's use a Swafuji swing compressor,

http://www.sawafuji.co.jp/english/tech/shindou.html
also
http://www.fridgefreezersite.com/sawafuji-compressor/

highdesertranger

 

[Redbearded]

Correct me if I'm wrong, but my understanding on one of the reasons why the Danfoss fridges are more efficient is because they can run at various speeds, and don't have to overcome the starting/inrush current due to the variable speeds and 3 phase that a normal compressor does. As to the Linear (swing) compressor,  one benefit is that they get to avoid the startup current as well, but they do run at the natural frequency of the spring system, so no variable speed.

Also watts/day was a shorthand of taking the 37 kWh per year figure which is really 

37,000 Wh     1 year      1 day
----------- x  ----------- x ---------- = 4.2 Watts while running (again approx). It made sense to describe the usage in per day for me 
 1 year        365 days     24 h       

because if it runs all day the fridge will draw out 100 Watt*h (4.2 x 24 h) in a day is the cycle that my battery bank is going to be going through. 

I can see where I got the units confused and forgot the hours. <facepalm>

I am trying to think of it from the standpoint of the battery and what I will have to replace, powerwise. I still feel weird thinking of batteries and talking about amp hours because they are only describing the amount of charge the battery holds and not the work it can do. As soon as you start mixing different voltage systems in the conversation goes downhill fast. It seems to make more sense to just think of the power capacity of the bank and how much you need to replace on a daily basis, especially for the daily solar cycle.

I like your thinking on the insulation, I agree it is one of the main factors. Which is why the freezers have thicker insulation than a normal fridge.

There are 2 factors why the 12V fridges are more efficient, first better compressor design as I have already discussed and second better insulation quality, but not a lot thicker than a normal fridge (it would be interesting to measure it to see how thick it is though).

I would think that a household freezer would have if not better insulation, certainly more of it compared to a portable cooler due to less issues with space and weight. More insulation leads to better thermal efficiency. The downfall is the less efficient compressor and the need to overcome startup current, hence needing a larger inverter that is more electrically costly than one sized for the running power.

So if you run a more thermally efficient freezer with a less efficient compressor at a higher temperature than it's design (using the compressor less) I would think that would offset to some degree (again, dependent on operating conditions).

I will note that the LG freezers sold in Africa using a linear compressor (at least I think they are, info is kinda sketchy) have the ability to maintain for 5 days without power... no batteries, and under perfect conditions, obviously.

Also to the 12v fridges, the ones using the Danfoss compressors, they are taking the 12v and turning it into 3 phase AC power to operate their motors. So there is some conversion going on in most all of them anyways. 

My thinking is that if I use a relay to trigger the inverter, and maybe back it up with a starting capacitor I can overcome most of the inefficiencies of the normal compressor.

I do need to do some more research though, so if there is anything you see missing let me know! I'd rather get it right the first time

I was thinking about seeing if I could get a LG compressor and hook it up in place of the regular one, it could be interesting, lol.

 

[John61CT]

No, most portable units' compressors are just on/off, consumption is the result of how many minutes on per hour. The variable speed ones must be more expensive.

Converting a mains unit will be an interesting science project, but IMO you can't overcome their relative inefficiency.

Thus only saves $ if you're planning on overbuying solar capacity anyway.

_____
AH is battery storage capacity, but also when "AH per day" is how you budget load devices' usage.

So if you use a total of 70 AH per day, and want three days' storage, with lead you need a 420AH bank, LFP it goes down to say 250AH.

Only use wH if systems other than 12V are involved.

 

[Spaceman Spiff]

Just learned that the engel and other compact fridges that everyone raves about being so economical are all using a Danfoss compressor. The reason why it seems is that even though they are all hooked up DC the danfoss controller needs 3 phase AC to be soft start and variable speed. Link It also seems that it is possible to convert, by changing out the compressor, most 120V fridges to 12V link. But even when you do that the motor is still running on 3-phase AC.

The Sawafugi compressor also converts to AC.  From their technical report:

  "For this reason, to compress the refrigerant effectively, the swing motor synchronizes the frequency of input current with the specific frequency of oscillating system consisting of the piston, coil and spring so that the resonance occurs. . . .  In order to obtain the maximum efficiency using the resonance phenomenon of a spring, it is necessary to carry out the precise frequency control."
http://www.sawafuji.co.jp/english/tech/shindou.html

 . . . The variable speed ones must be more expensive.

According to Sternwake, when he modified his Vertifrigo, it was just changing a resistor on the control board.  It was just being able to adjust the running speed of the compressor.


A reciprocating motor (piston compressor) will always take more energy than a rotating motor (rotary compressor).  It takes more energy to accelerate the piston on each stroke than it does to keep a armature rotating.  This is also why a rotating compressor has a startup surge (it has to accelerate the armature to operating speed) and the piston compressor doesn't (it actually has a startup surge each stroke of the piston).

Reciprocating motors also create more vibration than a properly balanced rotating motor.

 

[tx2sturgis]

All of this refrigeration via solar power is a bit like a pendulum swinging one way or the other:

On the one hand, you can build a more powerful solar array and then use a less efficient refrigerator,

Or,

You can conserve on a solar array and then spend the money you saved on a more efficient, and more expensive, refrigerator.

Either way, to keep a good quantity of food cold in the summer heat, it will ding your credit card pretty hard.

 

[jimindenver]

Nice thing about over building the solar to handle a load is that you can do a lot more with the power than just run the fridge.

 

[highdesertranger]

also refrigerators or freezers designed for household use are meant to be installed in a climate controlled atmosphere. also they are not designed to be moved around or for the vibration of vehicle travel. they are also designed to be level when operated. highdesertranger

 

[Weight]

Do not misunderstand the electrical function of the danflos or other 12 volt dc brush-less motors. Yes they use a phased current, but they are much more, much, efficient than a 120 volt compressor run from an inverter. There is no connection with any mains supplied multi-phased current, it is a different beast.