I have very limited space in the car for batteries, two of the three 26ah batteries shoehorned under the seat, so I did some tests on the 28L Waeco going in the car so it would use the least amount of power possible. 28L is the largest fridge I could fit without taking over.
I heated a bedroom in the house for about 3 days to 35°C with three heaters (pretty horrendous electricity consumption) & tested the fridge with different levels of insulation. I also installed a very efficient Noctua fan in the fridge from an old computer. Put a baffle around the fan as well, as the fridge design was recirculating hot air. Added velcro as hot air was traveling up between the fridge & cover reducing thermal efficiency. With foam sleeping mats added efficiency is better than 30mm of XPS foam but I didn't have accurate figures.
28 litre fridge per hour, ambient 35°C, fridge set at +2°C 0.0140 kW/h The uncovered fridge. 0.0132 kW/h Standard cover. 0.0116 kW/h 30mm XPS foam. 0.0082 kW/h 60mm XPS foam.
So cover up your fridge, except vent, & keep it out of the sun. If you have to run it in a locked car leave the windows open a few mm. I can get my fridge to 45°C below ambient temperature & it still does not use that much electricity. A well packed & covered fridge you can turn off for a day if you have limited battery capacity without too much increase in temperature. Throw your sleeping bags over it during the day if not running the fridge & or keep the vents clear.
I have stuffed a minimum of 2 thickness of 10mm sleeping mats between the fridge & cover & on the lid. The fridge cover is a bit tight but the extra insulation helps a lot. Macpac sell grey sleeping mats which match really well. The rear handle had to be removed to fit the fridge in the car so I filled the gaps with more foam. Last photo is a custom aluminium & stainless steel bracket to reduce height as the original bracket was too high. I did use the feet that came with the original bracket & machined them down to reduce height.
A wet towel seals and cools . I often turn our portable fridge off overnight on cloudy days . On start up in morning . It doesnt run very long before it turns off, down temp ., Often place wet towel over the top seal . Figure if there is any leaks or place insulation is leaking it would be there . I wrapt the inbuilt compressor fridge with floor insulation. Only approx 8mm thick . But it Atleast puts another boundary layer from outside heat . Both can be turned off for quite some time .
I heartily commend your engineering approach to the testing.
For consideration of whether insulation reduces energy consumption of a fridge... take it to the extreme and consider if the fridge were in a vacuum... how much energy would it then use?
The more insulation the better.
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"I beseech you in the bowels of Christ think it possible you may be mistaken"
Oliver Cromwell, 3rd August 1650 - in a letter to the General Assembly of the Kirk of Scotland
A vacuum would be best & you can get vacuum panels but you would still have issues at the corners. Also one slight misjudgement on assembling the fridge & the vacuum is gone. They also loose there vacuum over the years.
Next option is Aerogel insulation but that has moisture issues which reduce its thermal rating. So it's back to basic insulation but use as much as you can add, & keep the fridge out of the sun & put a white cover over it as a minimum as the fridge manufactures seem to have a fetish with dark colours!
Bag colour is a chicken and egg situation: black is the most efficient colour (by around 5%) for transmission and absorption of heat energy - note that heatsinks are always black.
So if the fridge will rarely be in sunlight then black is best however if it is in sunlight for a decent time each day then white would be best.
In my bush camps my fridge, in black cover, lives in the back of the 4WD and usually receives some sunshine during the day. If I remember I put an old white sheet over it, keeping the vents clear, during that time.
__________________
"I beseech you in the bowels of Christ think it possible you may be mistaken"
Oliver Cromwell, 3rd August 1650 - in a letter to the General Assembly of the Kirk of Scotland
I have replaced the original 80mm fan in the fridge with a Noctua 120mm fan.
Now have replaced the Noctua 120mm fan with another Noctua fan which not only has a higher volume it has better pressure.
Now I can feel the air being pushed through the fridge to a greater degree even with a dust filter. It also barely used any more power to the first one I installed & less than the original fan as Noctua fans are really efficient. Well worth spending the money to save energy.
Anderson plug on the fridge & a filter, it's surprising the amount of dust that would otherwise cover the condenser. I still blow out the fridge with airgun after every trip & it is quite amazing the amount of dust in the fridge covering the condenser & everything else.
I have had a temporary piece of aluminium folded over the divide in the fridge to get the butter department cooler when running the fridge as a fridge. I have replaced it with a neater job! Works well, there is some sail cloth repair tape on the back of the aluminium so the evaporator coils don't get damaged. I used a 1.5mm thick baking tray (4028574815277) as the aluminium is a bit softer, (unfortunately not anodized but the heatsink itself has more value than being anodized) but not too soft, hopefully 1200 alloy which would be nice, which would have better thermal properties having less alloying elements, but nevertheless easier to bend by hand than 5005 alloy off the shelf sheet.
I can pull the heatsink out for cleaning both sides. Have been testing it for a week now and it works really well, wish I had upgraded this modification earlier. The tail edge of the heatsink gets considerably cool. Even a bit cooler than I was expecting, which now makes this area actually useful when running the freezer as a fridge.
If you notice there are 2 thicknesses of 10 foam sleeping mat stuffed down inside the fridge cover for additional insulation, there are 3 on the opening end, the compressor end does not need it.
Fold the front bottom edge of the butter compartment first, rear bottom corner of butter compartment second, then rear top edge of divide third & top front edge of the divide last.
Any heatsink over this divide will be better than nothing, even a couple of aluminium disposable baking trays will help make the butter compartment a bit cooler!
The above post was a cut & paste. These modifications were done quite awhile ago & the fridge has worked really well. Couldn't be happier. Also tried to reduce AM interference as well, but with mildly moderate success.
The fridge has been on 4000km of corrugated roads & every night we sleep the fridge is moved to the passenger seat. That's about 290 times now & 290 time back to the boot.
This test was done before I installed the larger thermal bridge (piece of aluminium sheet) into the butter area. This would now get very cold, my guess at least around -5°C if I did the same test again.
A torture test to see how long it takes to freeze to 10.5 kg of ice bricks to -22°C (lowest fridge setting. On previous tests thermometers I stuck in the fridge showed -24°C). Ambient over the days was 26°C morning & night, including ice bricks at the start. I also put a thermometer in the centre of the ice bricks as this area is slower to freeze.
24 Hours: fridge -11, centre of ice -2
36 hours: fridge -16, centre of ice -10
44 hours: fridge -20, centre of ice -16
46 hours: fridge -22, centre of ice -19
The fridge started cycling at 46 hours.
The bottom of the fridge looses more cold than the sides so I think I will be battling to get the centre of the ice to -22°C until I can come up with thicker insulation & mounting mechanism.
The back of the fridge near the internal light only got down to 0.2°C but the ice brick closer to the freezing area was frozen solid.
About half the time it was running off my old 80Ah car hattery (still in good condition) which was around 12.4 volts with a 1 amp charger trickle charging. Fridge current was 3.1 amps. So 1.768 kWh for the 46 hour test. I could hear the compressor running a touch faster on AC but not a lot in it, maybe maximum 10% faster.
Exhaustair from compressor was about 30°C (this was with the slower Noctua fan, not the last one I installed with higher capacity & pressure, & I have added a little bit more insulation).
