Estimated solar setup to run 3 way fridge

[noteven]

I have a vintage Alaskan camper with a Dometic 3 way fridge that was never set up for propane operation. It would be a fair bit of work to cut in and install the double vent system needed for top up and top down propane operation. 

It works fine on AC on shore power and DC on the road.

I have a large set of cables behind an isolator back to the camper deep cycle batteries from the truck. 

136 amp alternator on the truck.

I believe the current draw is 10amp on 12v. 

Is it more efficient to operate this unit by inverter on 110v if I want to set up solar? 

Any other information is appreciated...

[DesertMiner]

Solar charges your 12 vdc battery system. No need for an inverter to power the 12vdc fridge.  Do an energy assessment of you total 12vdc needs and you will be in a good position to determine your battery  requirements. How much solar you may need to supplement your other available changing methods can be then determined by battery capacity. Some say you should have 1.5 - 2 watts of solar power for every available AH of battery capacity.  There are lots of other variables in the mix but this should get you started

 

 

[Al F]

I would go with the inverter and run the fridge on 120V with or without the solar.

The reason for this is when running on 12V the heater element in the fridge works fine, when you are driving, because the voltage is around 13.5V.  When running off of the battery only, your voltage (at the very best) will be 12.6V with a fully charged battery.  However as soon as you add the 10amp load the voltage will drop to 12.5 to 12.4.  Then after an hour or three the normal discharge of the battery voltage will drop another 0.1-0.3V.  All this combined will reduce the heat to the boiler, making the fridge less efficient, allowing the inside temp to rise.  Turn on a couple of lights, or power your laptop or a propane furnace blower and you reduce the voltage even more.

Yes, using the inverter, you will loose some efficiency, but at least the heater element will be at the proper voltage for the heater element. This is more likely to keep the fridge cold. (or as cold as these fridges get)

Now, about how much battery you will need.  10amps (and I doubt operating on 120V will reduce the amps pulled from the battery) running for 24 hours is 240AH.  Even if the heater element is only on for half the time, that is 120AH.  120AH is the max you can realistically take from a pair of 6V golf cart batteries.  However that doesn't leave any capacity for lights, water pump, computers, etc.

Even if you put 200 watts of solar on your roof, you won't be able to keep up the batteries full charged with this kind of power draw.  After a couple of days you are going to have pretty much dead batteries.  You may be able to keep up if you idle your truck for a couple of hours each day. 

You might survive if you have 4 golf cart batteries and 400watts of solar.  However a day or two of cloudy weather and you are fighting to keep your batteries full for night time use.

[noteven]

Good info thanks.

I know it’s not cool pardon the pun but I have survived 2yrs full time using propane to power the absorption fridges in my 2 trailers when boondocking- maybe I should look at doing the venting project... I would pipe fuel to the unit and test it before cutting holes...

the rest of the camper is very low power demand... as in direct vent LP heater with no fans/circuit board, LED lights etc. windows that open for ventilation. It’s a “camper”....

I do have a 300hp diesel generator under the hood a the truck- but I have been learning how batteries charge etc is why I’m looking at solar...

This rig is more for travel and exploration vs sitting.

[Al F]

We too have survived for the last 15 years of extensive travel in an RV, 6 to 10 months a year, plus 4.5 years full time with the absorption fridges.  Since we spend lots of our time dry camping or boondocking, an elect fridge is not something we wanted to fool with.

As long as you drive and charge the batteries at least every other day, and add about 200 watts of solar you may be able to just use the elect part of the fridge. 

About RV electrical, battery charging and solar here are a couple of links with basic info:

http://www.marxrv.com/12volt/12volt.htm

http://www.marxrv.com/12volt/12volta.htm

You may already know most of the info in the website, but it never hurts to review the info.

There are links to other websites I can provide with extensive details on inverters, batteries & solar.  These are more for long term RV'ing with TV's, computers, microwaves, coffee makers, etc.

[oldjohnt]

noteven good question I think I'm pretty well in Al's camp on this one. I have used 3 way fridges in the past and provided I ALREADY HAD an Inverter set up of adequate size and a sufficient battery bank and charging system/capacity, Id operate on 120 VAC versus DC but prefer LP whenever possible to conserve electricity. SURE when driving a good alternator and battery can provide 10 amps of extra DC to power the fridge on DC and SURE there may be 10% Inverter losses, but if I already was using an Inverter anyway I just used 120 VAC to run the fridge PERSONAL CHOICE EITHER WILL OF COURSE WORK . I agree if you pulled say 10 stored energy battery amps for say 12 hours of a day and weren't replenishing them, that's 120 battery amp hours meaning you need a minimum of 240 lead acid battery amp hour capacity. Now, if you used solar charging and had say 200 watts of panels (use say 15.87 amps at 13 charging volts) and perhaps 6 good sun hours per day (6 x 15.87 =  95 amp hours) that's gonna crowd your system.  A good MINIMUM estimate I agree with Al would be 400 solar watts and 450 battery amp hours (like say four Trojan T-105's).  I run an extra 120 VAC dorm aux fridge 24/7 and a CPAP all night off an Inverter and "got by" dry camping with 450 battery amp hours and 470 solar watts  but now do soooooooooooo much better with 950 soar watts as I may camp under a total shade canopy or it might  rain days at a time  yayyyyyyyyyyyyyyyyy  

NOTE still any solar is good solar and NONE of the above figures are accurate but ONLY an approximation so don't anyone have a calf, I'm ONLY saying for dry camping Id prefer 400 solar watts and 450 battery amp hours to power a fridge that drew 10 DC amps !!!!!!!!!!!! 

 

John T

[noteven]

I’m doing some comparisons with high efficiency compressor units. A Nova Kool R3800 DC or AC/DC will install where the 3 way is now, with room for insulation. 

It’s draw when running is 2.2 amps on 12v and 1.1 on 24v and not much on 110v ac I expect.

So my dumb question of the day is can I supply the fridge with 24v DC while the rest of the camper is supplied with 12v DC from 4 - 6volt or 2 - 12volt battery bank? 

[Yarome]

3 hours ago, noteven said:

...can I supply the fridge with 24v DC while the rest of the camper is supplied with 12v DC from 4 - 6volt or 2 - 12volt battery bank? 

Sure. Just need a 12v to 24v step up converter. Maybe $20-$30... however... power wise you're not gaining any savings by stepping up to 24v. You'll actually loose more in the converter than just running it on 12v. That being said, I don't know it as fact, but you "may" get better cooling on 24v than 12v (at least a question I would ask), but it would be at a cost.

For semi/serious boondocking... IMO, the only options that make sense are either a high effeciency 120v residential or LP. LP being the most self sustainable off your battery bank and limited solar. After more in-depth research and discussions on the DC compressor refers... there doesn't seem to be any advantage over modern day, low energy, residentials when you consider the lower initial cost, better overall cooling and available refer space factors.

I choose to stay with LP, but mine is also modded and I don't stay in ambient temps that might challenge most absorption refers.

[noteven]

I think step one is to remove the 3 way and bench test it on LPG - once I see in the cabinets the venting and gas supply install project may not be that involved. 

I may have more questions 🙃

[Al F]

7 hours ago, noteven said:

I think step one is to remove the 3 way and bench test it on LPG - once I see in the cabinets the venting and gas supply install project may not be that involved. 

I may have more questions 🙃

Did the cost of the Nova Kool help you to look at installing the venting for LPG again? 😀

[ALLOY]

An abortion fridge works the best when on propane because the flame produces more BTUs (evaporates more liquid) than the electric elements do.

If you want a top quality 12V or 110V fridge take a look at these.

http://www.engelaustralia.com.au/products_category.asp?pid=11

 

 

[Kirk W]

57 minutes ago, ALLOY said:

An abortion fridge works the best

 

I'm thinking that you meant to say absorption?   😏

[Yarome]

5 hours ago, Al F said:

Did the cost of the Nova Kool help you to look at installing the venting for LPG again? 😀

🤣 Amen.

If it's feasible to vent properly (running an LP line should be "cake"), even by fashioning and adding an external vent/chimney box,  I think you would be far and long much happier with the LP solution for longer/more frequent boondocking. 

For those with expanded solar and the "backbone" (battery bank) to back it up... or who are able and willing to do regular genset runs... a small residential is more than doable.

That being said, as a solo traveler my LP refer is more than adequate in size to sustain me for extended stays. If that wasn't the case then a residential would certainly be my very next option.

[noteven]

Ya the Nova Kools and Dometics etc cost a lot of beer and pizza 😀

For gas the Alaskan needs a two vent wall penetration set up added. Or something similar to provide exterior venting. An efficient 12v fan might suffice. 

Electric only like mine has a vent high in the back of the fridge enclosure that vents to the room and out a cracked open window in electric top down in travel mode.  Yes it makes heat in the camper.  On gas models this vent has a slide closure to divert to the wall vents in top up mode. If that makes sense...

The fridge works great in the two electric modes. My truck supplies 14.6v down the dedicated #4 supply cables to the camper connection. 

I bet there is a control that would sense the flame start, and power up a vent fan at the same time for positive venting in warm weather?

I can carry lots of LPG that is not a problem.

A nice bit of solar would run a vent system...

[ALLOY]

12 hours ago, noteven said:

😀

 

I bet there is a control that would sense the flame start, and power up a vent fan at the same time for positive venting in warm weather?

 

 

An ARP with the fan option will start a fan.

https://www.arprv.com/

[ALLOY]

On 9/6/2018 at 3:13 PM, Kirk Wood said:

 

I'm thinking that you meant to say absorption?   😏

Yup : ) 

[noteven]

Thanks Alloy - my 3 way fridge is of the piezo lighter generation no circuit boards...I’ll contact ARP 

[Kirk W]

25 minutes ago, noteven said:

Thanks Alloy - my 3 way fridge is of the piezo lighter generation no circuit boards...I’ll contact ARP 

I have one of those ARP systems with the fan and it has served us very well for 4 years now. I know that Mark Nemeth is also running one. I would not use an RV type refrigerator without one. 

[ALLOY]

On 9/7/2018 at 6:45 PM, Kirk Wood said:

I have one of those ARP systems with the fan and it has served us very well for 4 years now. I know that Mark Nemeth is also running one. I would not use an RV type refrigerator without one. 

X2.  The latest model is ARP4.0. 

[bigbillsd]

X3 on the ARP.   I use its blower fan to suck air from outside and blow it up toward the condenser coil.   Its not used for venting CO from the propane flame, I was a bit unclear to me what you wanted to vent so I added that.   

[Kirk W]

9 hours ago, bigbillsd said:

I use its blower fan to suck air from outside and blow it up toward the condenser coil. 

I take it that your fan is at the bottom and blowing into the back of the RV. On ours the fan is at the top and blows warm air out. With ours the ARP controls the fan also and it doesn't run except when internal temperature rises which can be important if you use the refrigerator in cold weather as they must be warm enough for the contents of the boiler to get up to the vaporization point.

[bigbillsd]

Yes,  I have mine installed at the bottom blowing up and the temp is set for around 130 degrees to turn on. We have a roof mounted vent so I didn't think I needed one above the coil.  I did build in a 45 degree airfoil just above the bottom vent as there was what looked like a 2x4 with the 4" horizontal about 5" above the opening. I used some thing aluminum flashing to help ease the airflow upward.  

That little blower fan ran a lot this summer once we moved east of the Cascades.  But the 12 yo Norcold 1200 kept the ice cream hard the whole trip.  A much better result then years sojourn when the ice cream was soft most of the summer.   

[noteven]

17 hours ago, bigbillsd said:

X3 on the ARP.   I use its blower fan to suck air from outside and blow it up toward the condenser coil.   Its not used for venting CO from the propane flame, I was a bit unclear to me what you wanted to vent so I added that.   

Hi bigbillsd- the venting project is to vent flue gas from the LP flame. Right now the fridge is AC/DC electric only. 

[noteven]

Follow up - I got my hands on the manual for this fridge. There is a design minimum distance for upper and lower sidewall ventilation of the enclosure with certain baffling installed. The location of the fridge now does not meet these requirements. 

I don’t think I’m  willing to depend on power venting alone to handle flue gas in LP mode.

Time to research Danfoss compressor units again. My current fridge could be installed in the enclosed trailer which is sometimes taken along as a garage.