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Old 05-15-2012, 02:16 AM   #1
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Default Upping frig efficiency

Well, at least, maybe, kinda.

We run a compressor frig so we don't have to worry about level, or switching modes, or not having it cool properly, etc. Of course, this means we use more power (usually 25-40 AH/day), so it would be nice to improve efficiency.

To attempt to improve the consistency of the temp in the frig, and improve the efficiency, I added a small computer fan, that blows right on the freezer compartment that also is the evaporator. It is wired off the thermostat positive and runs continuously.



It moves about 25 cfm and draws a miniscule .062 amps (measured).

To get good airflow around the freezer compartment, I had to space out the drip tray a bit at the back to hold it away from the rear wall.



We also leave the freezer door open as far as we can (had been doing that for a long time) to help get the cold air out of the freezer where the thermostat bulb is.



At startup, it cooled down in about 1/3 the normal time, but then would not shut off because the freezer was too warm to trip the thermostat off. It was at 29* instead of the normal 3*. I adjusted the range on the thermostat and tried again, and again, and again until I got the right range on it. We have it set up to run 6 different compressor speeds, and it seems to work well at 3rd speed, so far anyway. We now have the range on the thermostat that gives us 38/45* to 25/33* at an outside temp of about 70*. With the frig empty, it is extremely stable temp wise once it stabilizes. It is only running about 5-10% duty cycle, which is about 1/2 of what it did before.

This all makes sense, as when I went looking for data, the best I could find said that you could get about 3% better efficiency for every 1 degree C warmer that you have for the evaporator temperature.

We don't need or use the freezer for anything much, so loosing that efficiency to keep the freezer at 0* seemed a bit counterproductive.

We have a 4 day trip coming up this weekend, so we will see how works with the frig full, driving and parked, and it is supposed to be in the 80s F outside, so it should be a good test.

It sure would be nice to be able to use 10-20% less power for the frig and have it be more uniform temp.
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Old 05-15-2012, 06:57 PM   #2
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Default Re: Upping frig efficiency

This is good info, as we are considering switching over to a compressor fridge. Our 3 way is too inconsistent on temps. It easily warms up above "safe" food temps.

I will be stuck with getting the model designed to be a direct replacement for our current 3-way. If it is not as efficient as it could be, your info will be a big help.

let us know how the fridge works, after you upcoming trip.

Safe Travels
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Old 05-15-2012, 08:10 PM   #3
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Default Re: Upping frig efficiency

Quote:
Originally Posted by Photog
This is good info, as we are considering switching over to a compressor fridge. Our 3 way is too inconsistent on temps. It easily warms up above "safe" food temps.

I will be stuck with getting the model designed to be a direct replacement for our current 3-way. If it is not as efficient as it could be, your info will be a big help.

let us know how the fridge works, after you upcoming trip.

Safe Travels
Which one is designed to be a direct replacement? When we looked, we were kind of on our own with sizing. As it turned out, we got the same cubic feet, but gained 5" of storage under the new frig, which now holds the Chevy jack and 100' of extra power cord.
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Old 05-15-2012, 08:56 PM   #4
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Default Re: Upping frig efficiency

I believe this is the replacement for the current fridge. It fits in the same opening, and adds 2 cubic feet to the fridge volume. It makes better use of the space that was previously used for the evap-system.

http://www.novakool.com/products/single ... rf5810.htm
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Old 05-16-2012, 01:26 AM   #5
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Default Re: Upping frig efficiency

Quote:
Originally Posted by Photog
I believe this is the replacement for the current fridge. It fits in the same opening, and adds 2 cubic feet to the fridge volume. It makes better use of the space that was previously used for the evap-system.
http://www.novakool.com/products/single ... rf5810.htm
Great looking unit & specs. But, I didn't see a hint about pricing?/! So I guess if I have to ask...Well!
As I've said before, I like the aspect of not having to level to the fridge.
Maybe if we ever get our lift/suspension sorted-out we can use the R/T & look into a comp. fridge.
Overland Expo. is this weekend at Flagstaff. But Arizona is BURNING & Will get worse.
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Old 05-16-2012, 01:38 AM   #6
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Default Re: Upping frig efficiency

Quote:
Originally Posted by Photog
I believe this is the replacement for the current fridge. It fits in the same opening, and adds 2 cubic feet to the fridge volume. It makes better use of the space that was previously used for the evap-system.

http://www.novakool.com/products/single ... rf5810.htm
I couldn't find any power usage specs on it, but they did say that the compressor cooling fan option wasn't available for it, which might reduce efficiency. It appears that the compressor location on that one precludes the fan. I do like that the condenser coils are in back, which will work better than ours does in a Roadtrek. It does use the Danfoss compressor, and has the instructions for making the compressor variable speed, like we have done.

If possible, you may want to consider buying a frig that is a bit smaller than the opening, so you can add some insulation to it. We gained over 20% in efficiency by adding foil faced urethane insulation board to ours (.5-1.5 inches all around the sides and the bottom stuffed with fiberglass batt). We got that info from the boater folks, who routinely insulate them beyond factory amounts.
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Old 05-17-2012, 12:21 AM   #7
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Default Re: Upping frig efficiency

Maybe we should start a new thread, and move all this chatter over there.
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Novakool R5810/R5812 replacement for the 3-Way Dometic in the Class B.

Price = $1300

Power Draw = less than 30 watts (2.5 A/H), when running. Most solar chargers easily cover this draw, during the day.

DC only or AC/DC

Can be ordered with or without a freezer.

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Old 05-17-2012, 12:44 AM   #8
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Default Re: Upping frig efficiency

Thanks Brian; very good info. as usual. I'll keep it in mind for updating our R/T.
Safe Travels & ADVentures. Ric.
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Old 05-18-2012, 08:53 PM   #9
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Default Re: Upping frig efficiency

We just got back from our "abbreviated" trip with the internal frig fan. Unfortunately, DW had a semi-serious intestinal issue that made us cut it short, after one day, but the drive distance was the same.

With the fan, the frig, as expected, changed temps a bit when full of food, compared to empty. The freezer and frig temps got further apart, and ti took longer for the frig to react to open door warming. It did still work pretty well, however. The biggest issue we had probably had to do with the extreme wind we encountered, at 30-40 mph. The Roadtrek has the in and out air for the frig very close together on the drivers side of the van, and high wind can severely affect the airflow for the frig compressor and condenser. All in all still worked OK.

I left the food in it when we got home an into the garage, so I can test what the various compressor speeds will do for the temp reactions. In the past, I have found that slower speeds made the temps closer together, which would be good, so I will try and see how it goes. Once I get the temp reactions as I want them, I will put on the Watts-up to see the actual energy usage.

The wind issue points out that I may still need a booster exhaust fan on the compressor discharge side to keep good airflow. The absorption frigs don't have as much issue with this as the generate much more heat and create a chimney effect to keep the air moving better.
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Old 05-26-2012, 05:03 PM   #10
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Default Re: Upping frig efficiency

We got in a week of pretty good testing, and found a few interesting things. What has helped the most so far, is to remove the drip tray from under the freezer. With just the fan in place, we got around 17 AH/day usage in the 70* garage and the frig at 43* (freezer ran about 30*). This bettered the best test we ever got on the bench in free air of 17.5 AH, and was even more of an improvement over the 18+ we got when mounted in the van. I then took out the drip tray from under the freezer, and with the frig at 43*, the freezer ran 39-41*, and the 24 hour use was down to a little over 14 AH. Since the little fan inside does use some power, I decided to shut it off and see what happened with the drip tray out. Surprisingly, the frig and freezer temps did not change, but the usage took a big drop to only 11.4 AH in 24 hours. It makes sense, as the fan used 1.44 AH, and then you have to remove that heat also. This is a 1/3 reduction in power use, so we are pretty happy with it, and since we very rarely, if ever, used the freezer, we now have a bit more frig space at the new temps. I just need to figure out how the catch any condensate without messing up the improvements. Very surprised that we don't need the fan.

I now have temporarily hooked up a 180mm computer fan on the outside of frig vent, under the window, and blocked the other areas of the vent, to see how well that will improve the flow through the compressor and coils. On our frig, the compressor is in the upper right rear corner, with the fan blowing toward the front over it, and the coils are on the right side, also getting some of the fan air. I built a duct from the lower frig vent to feed the compressor fan when I put in the frig, because it would just loop the air and not cool very well. With all the top vent blocked except where the new fan is blowing out, all the fan air should be helping the compressor fan and moving lots more air up the duct and over the compressor. It is running right now, with no other changes, to see how the frig reacts. I don't expect to gain anywhere near enough efficiency improvement to cover the power the fan uses (.05-.39 amps), but the goal of this fan is to keep good flow through the compressor in difficult conditions, like if it is very hot outside, or if high winds are messing up the airflow. You can feel a big difference in airflow, with the new fan on. The fan is variable speed, and it would run it only when the compressor is on, if all works out right. It would mount behind the upper vent a ways, blowing out. Hopefully, I would be able to baffle it enough to get the flow out well, with hurting the gravity flow.

If we get lucky, we may get quite a bit more off-grid time available, and also get better cooling in the really tough situations, which is where most of these small frigs, both absorption and compressor, have problems staying cool. We will see how it shakes out.
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