upgrading 12v peltier refrigerator to freezer

[rado28]

hello,

firstplan is simple. take some standard 12v refrigerator based on peltier cooling to have the box and upgrade it to lowest target temperature possible, until it makes sense to get small food freezer.

lets start with this example
https://www.amazon.com/K-Box-Electr...coding=UTF8&psc=1&refRID=3VNW3BS7VVDV5E7CGYQ0

based on my basics about peltier i plan to achieve this general target by simple adding more peltiers. if serial or paralel, or serial stacked. thats the first question. question - A

another question - B, how to achieve this target most effectively (least ammount of power needed from my small offgrid solar/battery power source? lets count with 2000w/day + 500w/night for 24 hrs cycle.

to achieve this i plan to order/build some water cooling and coldest place to get rid of heat. either
1 - hot water to standard radiator and fan (unknown size)
2 - closed water circuit with cooling in 200 litres barel of 10C water
3 - opened water circuit from the barel to peltier

could we get to some sizing? what peltiers, how many, what cooling sizes, pumps capacity, fan...? is there a way to have it counted first, or just try-fail approach?

 

[Harald Kapp]

if serial or paralel, or serial stacked. thats the first question. question - A

Serial or parallel makes no difference with respect to cooling if you talk about the electric side. You'll need either higher voltage at low current for a serial connection or low voltage and high current for a parallel connection. The wattage P = I × V is the same
With respect to cooling there is a difference between serial or parallel arrangement. A single peltier element can creat a max. temperature differential of about 50 K (source). Mounting the peltiers in parallel allows for extraction of more thermal energy from the goods to be cooled (aka: cool more goods in short time). Mounting the peltiers mechanically in series (hot plate of one peltier to cold plate of second peltier) alows for a higher temperature difference (2 peltier -> max. 100 K) at the cost of less extraction of thermal energy (aka: cool less goods to lower temperature).

question - B, how to achieve this target most effectively (least ammount of power needed from my small offgrid solar/battery power source?

The cooler the hot side of the peltiers, the cooler the cold side will be, too. Or with given cold side temperature: the cooler the hot side the less energy is required to achieve the target temperature on the cold side.

could we get to some sizing? what peltiers, how many, what cooling sizes, pumps capacity, fan...? is there a way to have it counted first, or just try-fail approach?

If you are into thermodynamics sure you can. Unfortunately I cannot help you here.
This calculator and also this one allow you to evaluate the required peltier size for given requirements. Google "sizing peltier cooler" for more info.

 

[Technomaniac]

Hi. I did some experiments with Peltier effect fridges about 5 years ago when I went off-grid and started with a 12v solar setup but shortly after changed to 24 volt. There were the odd 24 volt Peltier fridges found at the Junk Shop for $5 and I collected a few. Physically stacking the cells offered an improvement, the one important thing that I discovered, probably mainly because I am in Rockhampton, Australia, right on the Tropic of Capricorn, where it gets quite warm, was that the fridge needed to be powered up continuously, which I wasn't able to achieve at the time. My test subject was much cooler if it could be left on power for a couple of days, rather than being powered down at night. It obviously wasn't getting cold enough in one sunlight cycle.

 

[debe]

That is probably the most inefficient refrigeration around. Use a 12V DC Danfoss compressor system if you want efficiency or freezing. At least it will use a lot less energy.

 

[Technomaniac]

That's what I am using now. It's 12/24v with automatic switching, it is called "ICE CUBE" , uses Danfoss compressor, is chest type with two lids on top, one for freezer and the other is fridge. Approx equal sizes. I think its 80 litres,total. I could be wrong. I've had it in use for about 5 years and it's had a caning with my voltage being often low. The temp adjustment is beyond me so it runs on the default settings. There was no mention of the current it draws in the literature, and I havent measured. I have a worry with the ventilation openings, they are in inconvenient places so I have to use it in the middle of the room. I have it on my 24v solar system and if I run my AC diesel plant for any reason, the frig automatically is powered from that. I built up a low voltage cutout but still haven't tweaked it to work properly. I think with the Peltier coolers the claim was something like 30 degrees lower than ambient.