Member Since: 29 Nov 2018
Location: Home
Posts: 34
Brilliant! Many thanks for the quick and detailed reply!
1st Sep 2023 11:24 am
Lightwater
Member Since: 21 Oct 2018
Location: Sydney NB
Posts: 97
Peltier vs compressor fridge comparison.
You do not want to leave a cooler on as it will flatten your battery very quickly. This is a test I did comparing a 28L Waeco fridge (a few years ago) with a peltier (last year) using a CPU Noctua heatsink, which is about as efficient at removing heat as one can get.
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I bought a 68W/8.5amp 40 x 40mm peltier. I have some Noctua heat pipe CPU heatsinks for 130 watt CPUs. They are large heatsinks with 120mm fan. Some I have 2 fans, push pull setup. They would be a lot larger than in any peltier esky.
So I thought I could use my 28L fridge & place on top a thick extruded styrene foam lid, I have offcuts. Using the same fridge carcass with all the same additional insulation will give a reasonably good comparison of compressor vs peltier, with the same 10.5 litres of water.
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Click image to enlarge
This is a torture test freezing 10.5kg of water, so not a realistic real life situation, but it is to show the limits of peltier vs compressor as best as one can reasonably do.
At 42 hours I stuck a Noctua fan in the fridge with resistor outside for a low speed fan inside the fridge to gently move air. It is adding 0.5 watt of heat but I feel at this stage there is an advantage of having an efficient fan in the fridge for cooling.
The evaporator plate is not as cold due to the fan but the 10.5kg of water seems to be getting cooler
a bit quicker.
I will go a bit longer to either freeze the water or to the point where the diminishing returns are not worthwhile.
I have decided to stop at 3 days because the fridge is not getting any colder than 3.5°C. This has consumed 7kWh (not including heating the room to keep it at about 25°C ambient temperature, similar to the same compressor test), about 4 times the energy of the compressor which reaches -22°C & quicker.
The compressor fridge gets the centre of the ice to -2°C in a day. So you are looking at about 8% or less of the energy consumption with a compressor fridge. My fridge only uses typically 3.8 amps when the compressor is running. The peltier uses twice that continuously.
As a small fridge & precooling contents & if you have enough battery resources or 240v these fridges are ok. Also very light weight. But if you need something beyond these limitations get a compressor fridge.
My compressor fridge uses 15AH per 24 hours for 5°C setting.
I noticed how hot & how quickly the heatsink got without fans, within a minute, & then it is completely useless. The peltier is creating about 100 watts of heat plus the heat it is moving to the outside of about 50 watts, so that is about 150 watts of heat that has to be dissipated. If your fridge is exhausting warm air the heatsink & or fan is not good enough. At least stick another fan on the outside to move a higher volume of air through the heatsink.
The exhaust air on this setup was only 3°C above ambient which I think is pretty good. So if your fridge has a small heatsink & or small fan you will have a much higher temperature above ambient so the fridge will not get as cold. You need to get the exhaust air as close to ambient as possible to get the evaporator side as cold as possible.
The evaporator plate is -7.3°C at roughly an average distance from the peltier to the edge of the edge, width length average.
One would need to calculate the optimal size evaporator plate in relation to the peltier for the temperature which one would need to achieve in the fridge. You can get an extremely cold temperature, but the surface area of the evaporator is so small that you don't have any "volume" to cool a reasonable mass. So the evaporator area needs to be larger, but then obviously not as cold.
I think this size plate is a reasonable area for the task, probably leaning more to guessing, but the condenser heatsink should have at least 2.5 times the rate of that of the evaporator heatsink.
The condenser heatsink has 0.52m² & the evaporator heatsink (exposed side) has a surface area of 0.048m² including edges. So roughly 10.8333... times the surface area of the evaporator heatsink. Factoring the 3°C exhaust fan increase in temperature, I think that to squeeze out any more efficiency will provide a single digit percentage gain if lucky.
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Polished the aluminium & added thermal grease.
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(The above notes are a shortened version of the test I did. Basically, don't leave a peltier cooler running or you will have a flat battery)
A further note the compressor fridge at the coolest part of the evaporator gets down to -26°C.
The fridge is basically a permanent fixture in our car with custom built aluminium & stainless quick release bracket.
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Click image to enlarge
A few more fridge ideas:
An aluminium cooking tray (a bit softer to hand bend) to get this dead end part of the fridge cooler. Initially I used layers of aluminium foil which worked quite well.
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On the back of the aluminium up against the evaporator I stuck on some sailcloth repair tape so the evaporator wouldn't get scratched, so thermally not quite as good. A small compromise, but it works brilliantly.
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& better fan in fridge, Noctua 120mm industrial fan, actually pretty quiet, to some degree due to the acoustic insulation I added to the fridge.
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