Why not simply a one-way 110V fridge?

[markopolo]

Originally Posted by cruising7388

Can you disconnect all your batteries and still get the fridge to work, and if so, in what modes?

No batteries needed to run this fridge in 110V or Gas modes. It functions as described by Avanti. This type of fridge does not have a re-igniter. Fridges with a re-igniter would require 12V.

[booster]

Originally Posted by Mtsailor

George,
Sorry, I didn't keep the raw data, just the notes I made of the end results. The test of the Nova-Kool was at an ambient temperature of 73 degrees ave, and the Magic Chef was at 70 degrees, so that would have favored the Magic Chef slightly, but I doubt that would have been significant. Bottom line is that for similar size refrigerators operating at the same internal temperature, with similar ambient temperatures, the DC refrigerator used 40 amps from the battery in 24 hours, while the AC refrigerator operating through an inverter used 34 amps from the battery.

I think that there may be several reasons for the surprising results. Many devices are inherently more efficient in higher voltage versions. Inverters for example are typically more efficient in 24 and 48 volt versions than 12 volt versions of the same inverter. I've noticed the same thing in some motors and fluorescent lights. I suspect that this is because resistance loses are less at higher voltage, not just in the wiring, but in the device itself.

Inverter efficiency can be very misleading. If an inverter were 100% efficient, you would expect that to get 1 amp at 120 volts (120 watts) out would require 10 amps of 12 volts in, or 11 amps with 90% efficiency (modern inverters are often more like 95%). Surprisingly, it's not unusual to get that 120 watts out with less that 10 amps in. That would appear to mean more than 100% efficiency which is, of course, impossible. What is going on is that what is important to inverters is watts, not amps. A fully charged battery is more than 12volts, so it takes fewer than 10 amps to get 120 watts. With solar panels charging, it could be 13 or 14volts or even higher. At a float voltage of 13.4 volts, the inverter would need to use just 9 amps for 120 watts in (if 100% efficient). With 95% efficiency, current in would still be less than 10 amps, making it look like more than 100% efficiency. In actual use, I've often found this effect to often be even more dramatic, particularly with inductive loads (like a refrigerator). Perhaps some inverters have circuitry to help them handle inductive loads better.

I just looked up a few of he Magic Chef dorm frigs, which have energy star rating stickers so pretty easy to do some calcs. They give yearly watt-hours of energy used, so you can go backwards to amp-hours if you want to. Watt hours would be more accurate to use both places but we only have AH to compare to. A 4.4cf Magic Chef would have averaged about 50AH per day if battery voltage was good at 12.4v or so, when calced off the energy star test numbers, so in the same ballpark as the testing showed. These are very different power use numbers than I saw over 5 years ago when we got our compressor frig, when dorm frigs were using in the range of 3X the power of the Danfoss models. Our 3.1cf Isotherm uses between 17 and about 40ah per day depending on temps. At 70* it would be in the low/mid 20's ah range most of the time. How often the door is opened and how much mass is in it make quite a difference.


Did you happen to monitor the frig temp inside during the tests?


The inverter efficiency is surprising, and it would be interesting to know the make and model to see what the spec on it show. We see specs on inverters that talk about 40 milliamps of idle current, but even the best ones still seem to lose about 10% in their most efficient range. It would be fun to run a test with watt meters on both sides of the inverter so the voltage variations were taken into account.

[avanti]

Originally Posted by booster

How often the door is opened and how much mass is in it make quite a difference.

[You have baited me into discussing one of my favorite topics.]

I am skeptical about these claims.

Obviously, both of these things (opening the door and differing mass of contents) has to have SOME effect, but I believe that they both are very small.

Why would opening the door matter? Well, it is because you are "letting cold out". But how, exactly? The main one is that you are briefly removing the insulation provided by the door, thus allowing radiative and convective heating from the ambient environment. So, it is if the fridge were slightly less-well insulated. This effect is real, but it would seem to be pretty small, since you are only (briefly) removing the insulation from one of the six surfaces (the door), and at that it is the one that is typically least-well insulated to begin with.

Yes, I know, there is also the "lost air" idea, in which big billows of cold air fall down onto the floor, to be replaced with hot air from the top. This effect is provably negligible. The reason is that the specific heat of air (even moist air) is tiny, when compared to the contents of the fridge. So, almost all of the "cold" is being held by the food in the fridge, not in the air. As a practical matter, the "lost air" theory is almost entirely a myth.

As for the "how much stuff is in the fridge" issue, this does matter, but only for a narrow reason. In the steady state, it takes the same amount of energy to keep a fridge at a given temperature, no matter whether it is full or empty. The work done is determined by the amount of energy that leaks into the fridge and that is not affected by the contents (except perhaps a tiny bit by internal convection interacting with variations in insulation). What mass DOES affect is the cycle time. The thermal mass of the food provides a reservoir of "cold", so the fridge will cycle on and off much more often (but run more briefly each time) with an empty fridge. Since it takes extra power to start a compressor compared to simply running one, this does lead to a small increase power consumption with an empty fridge.

These effects are real, but they both are small. Also, any efficiency advantage of a chest fridge over an upright is due to better insulation, not to "saved air".

[booster]

This is one of those discussions that can go on forever, but what I have seen in most of the actually tested literature is very basic.


The thermal mass benefit is to reduce cycling and compressor start up inefficiencies as you state. Surprisingly, or not, is that just changing the hysteresis on the thermostat to be wider can have even more good effect for the same reason.


The "lost air" of an open door is not the largest heat loss by any stretch, as air has low heat capacity. The tests I saw attributed most of the heat loss of opening the door to induced airflow from the temperature difference which causes warmer, moister, air to hit the cold surfaces. You have the aforementioned heat in the air itself to get rid of, but the real culprit appears to be the condensing of the moisture in that air to liquid, which requires it to give up it's heat of vaporization, which is very large for water. This condensation will go on until all the moisture in the air is condensed out down to the dewpoint of the air, even after the door is shut. It would be continuous while the door is open. This would explain why most of us see a much larger warming in the frig when the door is open longer, as the condensation will drop heat very quickly into the frig and it's contents since the air is continually moving over it from the temp difference convection. This doesn't happen in chest frigs because you don't get the airflow because the cold air stays in place, so yes, a door and chest frig will be equally efficient if the door seals well so no airflow, and the door isn't opened.


A good test, that I saw done a long time ago, was to shut off and open a chest freezer and an upright freezer side by side. Both were full of water bottles that were frozen. The upright was totally liquid in the bottles in less than 1/2 the time of the chest freezer. All the bottles were on racks, so could see airflow if there was any.

[booster]

Back to the mini frig stuff.


I looked at a whole bunch of the newest models, and for similar sizes they nearly all use the same energy star rated power. This would say they are all likely to be using the same compressor setups.


This certainly can change the evaluation a lot, as the cost in extra power is probably very much lower, or not there, compared to just a few years ago.


Hopefully, we will be able to see some more testing on the minifrigs is some decide to try them, as well as some more detail of actual inverter losses of various types.


One thing that may be an influence is that many of the 12v frigs have built in fans like our Isotherm, or optional fans like the Novakool, or have had fans added to install to remove hot exhaust air, so if the minifrig needed the same, that power would need to be included in the comparison.

[Mtsailor]

Booster,
"Did you happen to monitor the frig temp inside during the tests?"
Yes, as stated, they were both adjusted to the same temperature, but I don't remember what it was. Probably 40.

"it would be interesting to know the make and model to see what the spec on it show"
The inverter used in the test was a Trace 2512 rated as 92% efficient. The higher voltage versions are rated 95%. Idle current is rated as 1 amp (it is actually a bit less).

"It would be fun to run a test with watt meters on both sides of the inverter so the voltage variations were taken into account."
That is exactly what I did. Measuring both watts and amps directly on the AC side and measuring amps and volts (and calculating watts) on the CD side. and have done the same thing on numerous other devices as well. As expected, comparing watts in and out, there is always loss due to inverter efficiency. However, since it is battery discharge we are interested in, it is DC amps we are concerned with, and when the volts are above 12v, the DC amps are less for the same watts. This is usually the case for me where I am using the Magic Chef (in my home) because I have 1500 watts of solar power, and am almost always somewhere between 12.2 volts (night) and 13.5 volts or higher (daytime with sun).

I admit that in a RV, the Magic Chef probably wouldn't be quite as efficient because there probably wouldn't be the excess voltage and the inverter standby loss would be a factor. In my case (and my testing) the standby loss wasn't included because since the inverter runs my home, it is always out of standby because of other loads. I just measured the change in DC current between on and off of the item being tested. Bottom line still is that the power requirements between AC and DC refrigeration are pretty similar. Other considerations like location of condenser coils, price, reliability, need for an inverter, etc are probably more important.

A note regarding energy star ratings: For the rating, refrigerators are tested in an ambient temperature of 90 degrees, so in most installations, actual power usage will be less. At home, I now use a 17 cu ft size GE refrigerator rather than the old Servel propane. Energy Star rating says power consumption is 1 KWH per day. According to a Kilo-Watt meter, it's actual use averaged .7 KWH per day over a 1 month period. More than likely, in most installations the Magic Chef will also use lass than rated.

When I started shopping for an electric refrigerator to replace the old Servel, I first looked at the full size 12 volt refrigerators sold to the alternative energy market. Most used Danfoss compressors, and the rated energy use was usually considerably higher than similar sized AC refrigerators in spite of the fact that they typically have thicker insulation and use a less stringent rating system than Energy Star. For whatever reasons, it appears to me that 110 volt compressors must be capable of higher efficiencies than low voltage DC compressors. I speculated on why in my earlier post, but really don't know the reason. If anyone does, I'd love to hear why.

[avanti]

Originally Posted by booster

The tests I saw attributed most of the heat loss of opening the door to induced airflow from the temperature difference which causes warmer, moister, air to hit the cold surfaces. You have the aforementioned heat in the air itself to get rid of, but the real culprit appears to be the condensing of the moisture in that air to liquid, which requires it to give up it's heat of vaporization, which is very large for water. This condensation will go on until all the moisture in the air is condensed out down to the dewpoint of the air, even after the door is shut.

Nah.
The problem with all such arguments is that the size of any such effect is limited by the percentage of water held in humid air. The heat capacity of humid air is a function of absolute humidity. At 77 degrees F, the absolute humidity of air is 1.94%. Regardless of fancy state-change arguments, the bottom line WRT temperature effects reduces to heat-carrying capacity. Even at 100% humidity, the capacity of air to affect temperature in short periods of time is very small.

A good test, that I saw done a long time ago, was to shut off and open a chest freezer and an upright freezer side by side. Both were full of water bottles that were frozen. The upright was totally liquid in the bottles in less than 1/2 the time of the chest freezer. All the bottles were on racks, so could see airflow if there was any.

This is true. It is just a demonstration of convective heat loss. Moving air will certainly melt things faster than still air. It is just that in the time periods that we are talking about for opening and closing doors, it is not very relevant. If you are going to leave the door open for a long time, then you should get a chest fridge. Otherwise, quality of insulation is all that matters much.

[booster]

I think it is very possible that the technology of the 110v compressors has surpassed the now relatively old tech of the Danfoss which has been around a long time. Energy Star rules are very "inspiring" I think. The Danfoss is also a wide range DC voltage setup, with variable compressor speed capability, so the are wasting some energy on electronics. I saw the 90* ambient for the testing specs, but didn't see what internal temp they use, just low, medium, high, which was weird. We normally run at 37/38* which does use noticeably more energy than 40/41* in our Isotherm.

From what it looks like, and depending on what starting current is, the 110v setup could be built with a 100-200 watt PSW inverter built in. We have a Samlex 100 watt that rates at .5 amp idling but is really .2-.3, and the parasitic does not seem to add on to the load so when running not wasted, which surprised me. Of course the units would likely have to be hardened for mobile use rather than stationery.

[avanti]

I had a similar though that the Danfoss units, great though they are, are getting kind of long in the tooth. Meanwhile, as booster says, the residential units have been getting a lot of pressure to improve.

I don't think there is any intrinsic reason why a 12VDC compressor couldn't be engineered to be just as efficient as a 120VAC model, avoiding the inverter losses. Whether anybody is interested in paying for the R&D is another matter. I also agree that a dedicated "just right" inverter could be very efficient.

[booster]

Yep, I don't think we will see Danfoss changing a bunch of things just to break even on energy use, or even get small gains.


I do think it might be possible that a startup would just take the 110v setup, harden it for mobile and put on the aforementioned "just right" inverter. It would be very inexpensive way to get into the mobile refrigeration business, I would think.


One big question would be how much condenser area and airflow the 110v units need to run efficiently. That could pose problems in many RVs.

[dicktill]

Originally Posted by gregmchugh

We replaced the 3 way in our 08 Roadtrek RS Adventurous with the R4500 and never regretted it, actually kept things at a constant temperature in the summer...

Thanks Greg. Any photos to share? Did you use the existing 12vdc wiring? Any other comments on the installation process?

Regards, Dick

[dicktill]

Originally Posted by Saldar

The nova kool 4500 is the fridge that's always been on the Travato 59K. I like mine a lot. It's vast for 2 people. My only problem is that it's easy to knock the temp knob and turn it into a giant freezer. Yes, it can be a giant freezer. I put electrical tape lines on the knob so it's easier to monitor the setting.

Can you post a photo of the fridge and the problem temp knob please?

Thanks, Dick

[GeorgeRa]

I have a GE small fridge in the garage, I think it is about 4.5 CF, specified current is 0.89A, measured 0.88A. At 50% duty cycle and at 90% inverter efficiency it would be around 114 Ah per day at 12.4VDC. That is at least twice as hungry as my Isotherm 3 CF fridge - granted it is bigger.

Since I installed the Isotherm Smart Controller on my fridge it is very quiet. I can't tell if it operates in a continuously variable speed or multiple speeds but can tell that is sound level varies to some degree. I changed fan as well it is practically inaudible.

[booster]

Originally Posted by GeorgeRa

I have a GE small fridge in the garage, I think it is about 4.5 CF, specified current is 0.89A, measured 0.88A. At 50% duty cycle and at 90% inverter efficiency it would be around 114 Ah per day at 12.4VDC. That is at least twice as hungry as my Isotherm 3 CF fridge - granted it is bigger.

Since I installed the Isotherm Smart Controller on my fridge it is very quiet. I can't tell if it operates in a continuously variable speed or multiple speeds but can tell that is sound level varies to some degree. I changed fan as well it is practically inaudible.

Interesting that you mention the Isotherm Smart Controller as I was just looking at them the other day. When we got our frig quite a while ago, all you had for a choice was mechanical or the ASU coldplate automatic unit. The ASU is really slick, but it had a fixed frig temp of 41*F which we felt was too warm, so we went with the mechanical. I have replaced the thermostat twice, as it seems to not like the moisture that condenses in it when in humid weather and in and out of the frig a lot, combined with frost building up over the sensor bulb. I have since moved the bulb off the evaporator and it is much better, but pretty big temp swing on/off. I thought the Smart Controller would do some good, and it might, but I like to have ours running on the same compressor speed all the time for the most part. I made a rotary switch with resistors to make ours speed selectable, and have never really found a need to go over the slowest speed, which is most efficient. In stock form the frig ran on speed 3, I think, so was less efficient than it could have been, so I can see how the Smart Controller could help on a stock setup.


I currently am looking at just getting the Frigiboat digital thermostat without speed control. The big selling point is the settable hysteresis and the fact that it hooks up to just 12v and the thermostat connections on the compressor. This should make it so I can leave the wiring for the mechanical thermostat in place in case the electronic one dies. I would then just disconnect the wires at the electronic control panel and plug in the wires for the mechanical one in the box. No need to pull the frig to switch them.

[GeorgeRa]

Originally Posted by booster

Interesting that you mention the Isotherm Smart Controller as I was just looking at them the other day. When we got our frig quite a while ago, all you had for a choice was mechanical or the ASU coldplate automatic unit. The ASU is really slick, but it had a fixed frig temp of 41*F which we felt was too warm, so we went with the mechanical. I have replaced the thermostat twice, as it seems to not like the moisture that condenses in it when in humid weather and in and out of the frig a lot, combined with frost building up over the sensor bulb. I have since moved the bulb off the evaporator and it is much better, but pretty big temp swing on/off. I thought the Smart Controller would do some good, and it might, but I like to have ours running on the same compressor speed all the time for the most part. I made a rotary switch with resistors to make ours speed selectable, and have never really found a need to go over the slowest speed, which is most efficient. In stock form the frig ran on speed 3, I think, so was less efficient than it could have been, so I can see how the Smart Controller could help on a stock setup.


I currently am looking at just getting the Frigiboat digital thermostat without speed control. The big selling point is the settable hysteresis and the fact that it hooks up to just 12v and the thermostat connections on the compressor. This should make it so I can leave the wiring for the mechanical thermostat in place in case the electronic one dies. I would then just disconnect the wires at the electronic control panel and plug in the wires for the mechanical one in the box. No need to pull the frig to switch them.

Nice unit, it has a display which is good. It is about the same price as the Smart Controller.

[gregmchugh]

Originally Posted by dicktill

Thanks Greg. Any photos to share? Did you use the existing 12vdc wiring? Any other comments on the installation process?

Regards, Dick

No photos handy and I am not near it at the moment. Had the installation done by the marine refrigeration company where I bought it. They had do some mods to the cabinetry to get it in but nothing major and it fits fine. They used the existing 12v and 120v power connections and capped the propane line.

[Trulymadly]

Originally Posted by PinIN

For the OP,
I have a $120 3.5 cuft freezer chest, 120v, 95w. For $20 I installed a temp controller that turns the inverter on and off, maintains 32°-40°, uses about 1.5 amps during the on cycle.
2x100 panels, 2xGC2 FLA, no external charger.
Parked on a semi shaded driveway.
Poorly ventilated cargo van, internal temp 75°-95°
Been running for 2 weeks, no problems.

We opted for a 110v freezer/fridge since we mostly go to plug in sites, but would like to dry camp in the future without an ice chest. My question is do you have a dedicated inverter for the fridge or are the other electronics such as lights, etc somehow tied to this inverter as well? I’m a newbie so I’m still learning this stuff! Very interested in your setup.

[PinIN]

Originally Posted by Trulymadly

We opted for a 110v freezer/fridge since we mostly go to plug in sites, but would like to dry camp in the future without an ice chest. My question is do you have a dedicated inverter for the fridge or are the other electronics such as lights, etc somehow tied to this inverter as well? I’m a newbie so I’m still learning this stuff! Very interested in your setup.

I only have one, 2k PSW inverter, if I need to microwave or heat water I just turn it on and off as needed, making sure the temp controller and inverter are still in sync.

The hard part was figuring out how to get the freezer to control the inverter's momentary switch.

DC loads are fans and lights, nothing fancy.

Later on I may get an appropriate sized dedicated inverter.

[GallenH]

I have another thread that I started about problems with an old Dometic 3-way in a 1997 PleasureWay. Some comments there and many here on compressor fridges. Given that my PW has a single group 24 cabin battery period (no inverter), what specifically would one need to upgrade in the electrical system to be able to run a compressor fridge plus all of the usual (lights, pump, heater fan, etc.) other items? I do have one 100w portable solar panel that I use to top up my cabin battery during the day.

Thanks.........

[SteveJ]

Originally Posted by GallenH

I have another thread that I started about problems with an old Dometic 3-way in a 1997 PleasureWay. Some comments there and many here on compressor fridges. Given that my PW has a single group 24 cabin battery period (no inverter), what specifically would one need to upgrade in the electrical system to be able to run a compressor fridge plus all of the usual (lights, pump, heater fan, etc.) other items? I do have one 100w portable solar panel that I use to top up my cabin battery during the day.

Thanks.........

One more battery and another 100 watts of solar should do the trick. If your controller is only 10 amps you will have to upgrade to at least 20.

IIRC, you can figure on 30 to 40 amp hours per day for the fridge.

Bigger batteries would be better but two group 24 s would be minimum, I should think.

I do not have a compressor fridge, just going by what has been posted.

I have an one year old Dometic AC/LP fridge that has been working flawlessly. 95 in the coach upon return from all day motorcycling and the fridge is at 30 to 34 degrees. It is the bigger one that goes in the RT 200 Versatile.

On TDY near Mammoth Lakes CA, moving tomorrow.