AC/DC Refrigerators

[Brian B-P]

Okay, I read the specs and saw the same thing: current at 24 volts is half that at 12 volts. That means that the appliance uses the same amount of power at either voltage, so there is no point in going to any effort to operate at 24V instead of 12V. It's 62 watts either way (5.2 x 12 or 2.6 x 24).

To illustrate this, let's say you are using two 12V batteries that can store 100 amp-hours of charge, and two solar panels which can produce 8 amps at 12 volts, each. You also have a refrigerator that uses 2.6 amps at 24V, or 5.2 amps at 12V.
You can connect both batteries and both panels in parallel, and have 200 amp-hours of storage and 16 amps of charging, all at 12 volts. The batteries could run the refrigerator for 38 hours (that's 200/5.2), and it would take 12 hours of full sun to recharge them (that's 200/16).
You could also connect both batteries and both panels in series, and have 100 amp-hours of storage and 8 amps of charging, all at 24 volts. The batteries could run the refrigerator for 38 hours (that's 100/2.6), and it would take 12 hours of full sun to recharge them (that's 100/8 ).

24 volt operation doesn't take any less power, so it doesn't have any advantage.

[Brian B-P]

As an aside...

Originally Posted by wayne_m

Are there 24 volt solar panels?

Yes, panels are sets of cells, and they are available with more cells in series to produce higher voltages. Also, 12 volt panels can be arranged in series just like batteries, and this is often done in large systems (not RVs) so that the power can be sent from panels to batteries with less current, and therefore lighter and cheaper wiring. Up to several hundred volts is practical.

However, it is pointless in an RV, where distances are short and the desired operating voltage is about 12V. It doesn't make the panels produce any more power.

[rbryan4]

I would think there is no real advantage to go with 24V in a small trailer. You need more battery or solar power to run and charge at 24V, and battery space and weight comes at a premium. 2 12V deep cycle batts run in series will give you 24V, but at the cost of Amp hours. Strictly speaking 24V might be better if you were going to invert all the power to AC, because the inverter has less work to do. Other than that, 12V makes more sense.

I'm not at all convinced that a compressor fridge is a viable option in the Escape, mainly because nobody has tested just how much current you could create with dual 6V batts, solar, etc, vs how much draw the compressor fridge actually has in the real world. I know they work on Motor homes, but their entire roof can be made a solar panel, and battery space is not an issue.

But, if some brave soul wants to take the plunge, I'd be waiting eagerly to see if it really works. It could be the true fix for refrigeration.

[Brian B-P]

Quite a few owners of small moulded fiberglass travel trailers use compressor-type refrigerators. A search for a suitable keyword such as "NovaKool", "Engel" or "Danfoss" (a brand of compressor used in some refrigerators of this type) in the FiberglassRV forum will produce various real-world user reports. Other than the blatantly incorrect installations in the Outback model of trailer, and perhaps in TrillumRV models as well, I don't recall difficulty with the refrigerators working as claimed, and there seems to be enough roof space on a small trailer for adequate solar panels.

[Brian B-P]

Originally Posted by rbryan4

You need more battery or solar power to run and charge at 24V...

Twice the voltage and half the current is the same power and doesn't take any more space for batteries or panels (although if comparing to a single-battery and single-panel 12V installation, 24V would mean two half-sized batteries and two half-sized panels). I agree that it's pointless, and it certainly makes problems for all the 12V stuff in the trailer (which is everything else battery-powered),

[rbryan4]

Originally Posted by Brian B-P

Twice the voltage and half the current is the same power and doesn't take any more space for batteries or panels (although if comparing to a single-battery and single-panel 12V installation, 24V would mean two half-sized batteries and two half-sized panels). I agree that it's pointless, and it certainly makes problems for all the 12V stuff in the trailer (which is everything else battery-powered),

Right. I was comparing it to the single batt/single panel. Good to know it works on smaller trailers, provided they are equipped to make enough power.

[Norseman]

This posting is all new to me but our 19 coming in late November will have a 3 rear receivers. The intent is if required the outer ones will be used to mount an aluminum Box that will hold ARB Fridge- Freezer. The 63 Qt unit draws .07- 2.3 amp hr at 90 Degrees. Can be use with either12,24 or 120 volts. The 50 Qt draws 1.35 amps at 90 degrees. The have been in use in the Australian out back for years. Will have a dedicated 12 vole drop installed into the rear so it can be hardwire in. Still trying to get a handle on their solar system, looking for more that 160 watts and may add a 120w portable. The idea of a generator is not going to happen for me.

[ericw]

I use a small 12V/120V Norcold marine fridge in my fiberglass trailer. We love it. I installed it myself and it has worked well in the desert and in the North. We would do it again.

[wayne_m]

It appears that Dometic also makes refrigerators with Danfoss compressors. You have to go the marine section of their web site.

Interesting discussions- thanks all.

[rbryan4]

Check out Thetford as well. They make several ac/dc fridges for RV. It's quite surprising that with the new compressor technology, the ac/dc fridges are far more power efficient than the 3-way fridges when running on DC.

Norcold Inc. Refrigerators

[Brian B-P]

Originally Posted by rbryan4

It's quite surprising that with the new compressor technology, the ac/dc fridges are far more power efficient than the 3-way fridges when running on DC.

I think any reasonable compressor-based refrigerator would be more efficient than an absorption-cycle unit (which has the same efficiency regardless of the heat source). As far as I know, the only reason absorption refrigeration is used in small sizes is that it will run on heat, when a mechanical drive (such as an electric motor) is not available. In an RV outside of a serviced site, it is a real advantage to be able to run on easily produced heat instead even a smaller amount of expensive-to-obtain electricity.

[rbryan4]

Originally Posted by Brian B-P

I think any reasonable compressor-based refrigerator would be more efficient than an absorption-cycle unit (which has the same efficiency regardless of the heat source). As far as I know, the only reason absorption refrigeration is used in small sizes is that it will run on heat, when a mechanical drive (such as an electric motor) is not available. In an RV outside of a serviced site, it is a real advantage to be able to run on easily produced heat instead even a smaller amount of expensive-to-obtain electricity.

All true. I was pointing out that the newer compressors are much more energy efficient than even 5 or 10 years ago. I am really thinking about the possibility of ac/dc instead of the absorption unit. We don't intend to do much dry camping, and when we do, the dual batts and solar might just give us the power we need.

[Ron in BC]

Originally Posted by Brian B-P

In an RV outside of a serviced site, it is a real advantage to be able to run on easily produced heat instead even a smaller amount of expensive-to-obtain electricity.

Although it's now not only less expensive-to-obtain electricity but easier to get electricity. In the South Pacific and before solar cells were a viable option I had something called a ""Wind Bugger" wind generator. It had a prop much larger than many other brands. The price you paid, aside from the bucks to buy it, was that it sounded like a Cessna taxiing on your deck 24/7 and often wasn't generating as much as solar cells do now.

I wonder, in 10 years if temperature extremes become the norm, will absorption type units be entirely replaced with DC compressor units?

Ron

[rbryan4]

Another good web page giving some general info on why the manufacturer is now using compressor based fridges instead of the absorption type. The idea is certainly not new, and it's becoming more and more accepted, as solar prices go down, and the fridges become more efficient:

Why Advanced RV Uses Compressor Fridges - Advanced-RV

[Brian B-P]

The absorption versus compressor question came up again in one of the Dometic 8XXX threads (New Fridge Topic: A Different Approach, post #67), and I am responding to it here...

Originally Posted by singletracker

Okay. If the compressor refers are so much better, why aren't they used instead of the Dometics?

... Is there a downside to compressor units that would explain why they are not the standard?

The compressor makes some noise, but personally I doubt that's a big issue for most people.

The significant reason that compressor-based refrigerators are not common is that - unlike absorption-type refrigerators - they cannot run on a propane flame. That has large implications to power supply, especially when camping without services, and is the fundamental reason for the dominance of absorption refrigerators in RVs of all types.

Originally Posted by singletracker

Is there a cost issue? Will the Nova Kool fit through the door?

Both types (absorption, and DC-powered compressor-based appliances optimized for mobile use) are relatively expensive compared to home refrigerators; I doubt the cost difference is the determining factor in popularity.
Both types are available in a variety of sizes, and the compressor types are probably a bit less bulky for the same usable interior volume, so I'm sure that package size is not an issue for compressor-based units.

[singletracker]

Thank you, Brian, for your response. I will keep an eye on this thread.

[grc]

In my truck camper I'm currently running an Engel 12v compressor frig, model SR70 which is a 60qt built-in unit with front door loading. I did install an extra matching 12v battery, solar charge controller and have about 54watt of folding solar panel.

Looking at the same manufacturer and the same volume units, the performance curves for the Engel 60qt SR70 and the 60qt MT60 top load unit are noticeably different. Both units use a 1 to 5 dial for temperature setting with 5 being the coldest. What is surprising is with the same ambient air temp and same temp setting the front load unit will run more often than the top load unit at steady state (door/lid is closed and not being opened and unit is down to holding temperature)

In terms of the batteries lasting to me the main question is whether or not you are also running your furnace as it has a 12v fan. The frig and furnace fan run the battery down quicker.

As far as compressor noise that was not as much of an issue as the compressor running causing other things to vibrate. Once I tracked those things down I was able to sleep with the noise. Example, my standard two burner stove has a cover with side screens and the compressor running would cause the cover/side screens to vibrate. Don't really know if the vibration issue was my particular frig or my particular installation.

Long ago the compressor frigs ran on a pure refrigerant plus oil for the compressor. Now the refrigerants are a blend and the oil is blended. The old rule of thumb used to be don't run the frig if the air temp is below 50degF due to problems getting the oil to return to the compressor. Don't know if that is still true.

Remember both propane and 12v compressor frigs absorb heat into their evaporators from inside their frig boxes and reject that heat through their condensors. The 12v compressor frig rejects that heat to the inside of the camper or trailer while the propane frig rejects that heat outside the camper or trailer.

It seems unfortunate to be faced with these issues when you just want something that works. The 12v compressor will work but you have to have enough battery and solar wattage to recharge. The trailer carries enough propane to run the propane frig but will the frig work the way it is supposed to.

[jamman]

Originally Posted by grc

...
Remember both propane and 12v compressor frigs absorb heat into their evaporators from inside their frig boxes and reject that heat through their condensors. The 12v compressor frig rejects that heat to the inside of the camper or trailer while the propane frig rejects that heat outside the camper or trailer.
...

The venting depends on the unit - many/some (?) (including the Nova Cool 6800) vent out the back. I think the Nk6800 will vent fine using the existing venting.

[rbryan4]

Originally Posted by grc

In my truck camper I'm currently running an Engel 12v compressor frig, model SR70 which is a 60qt built-in unit with front door loading. I did install an extra matching 12v battery, solar charge controller and have about 54watt of folding solar panel.

Looking at the same manufacturer and the same volume units, the performance curves for the Engel 60qt SR70 and the 60qt MT60 top load unit are noticeably different. Both units use a 1 to 5 dial for temperature setting with 5 being the coldest. What is surprising is with the same ambient air temp and same temp setting the front load unit will run more often than the top load unit at steady state (door/lid is closed and not being opened and unit is down to holding temperature)

In terms of the batteries lasting to me the main question is whether or not you are also running your furnace as it has a 12v fan. The frig and furnace fan run the battery down quicker.

As far as compressor noise that was not as much of an issue as the compressor running causing other things to vibrate. Once I tracked those things down I was able to sleep with the noise. Example, my standard two burner stove has a cover with side screens and the compressor running would cause the cover/side screens to vibrate. Don't really know if the vibration issue was my particular frig or my particular installation.

Long ago the compressor frigs ran on a pure refrigerant plus oil for the compressor. Now the refrigerants are a blend and the oil is blended. The old rule of thumb used to be don't run the frig if the air temp is below 50degF due to problems getting the oil to return to the compressor. Don't know if that is still true.

Remember both propane and 12v compressor frigs absorb heat into their evaporators from inside their frig boxes and reject that heat through their condensors. The 12v compressor frig rejects that heat to the inside of the camper or trailer while the propane frig rejects that heat outside the camper or trailer.

It seems unfortunate to be faced with these issues when you just want something that works. The 12v compressor will work but you have to have enough battery and solar wattage to recharge. The trailer carries enough propane to run the propane frig but will the frig work the way it is supposed to.

Some good points. The top load units are always more efficient than their front loading counterparts, mainly because cold air sinks. Engel doesn't make a fridge of sufficient size for the Escape trailers, at least the last time I checked. One thing about venting: compressor fridges that are "self venting" are designed to vent to the interior cabin, but they do make some that vent to the outside as well.

[grc]

On the venting I didn't see the official Nova Kool installation instructions. I did see mention for one of their big dual compressor models that talked about external ventilation.

Heat transfer is driven or governed by temperature difference. Remember when the Norcold person told Reace that if the trailer inside temperature was above 80degF (say due to 95deg outside air temp) you should run the AC unit. Knocking the inside trailer temp down to say 72 would lessen the heat transfer to the inside of the frig box. For rejecting heat from the condenser at a given temperature and given air flow you will reject more heat into 72deg air than say 95deg air.

Anyway I'd go by what Nova Kool says in their instructions.