Peltier Fridge

DeveakDeveak Solar Expert Posts: 38 ✭✭
In before a wave of "Its not efficient!"
Yes modern compressor based refrigerators are more efficient, so are dc fridges. The problem i see with both is, cost and for the AC fridges, inverters. Insulation is cheap. I have limited skills but i am pretty sure for 2-300 bucks i could build a EXTREMELY well insulated peltier fridge. If i could get the kw draw under 500 watts a day for a 5 cubic foot box i would consider it a resounding success. I found a simple 12 volt thermostat for kegerators. 15 bucks, the peltiers themselves, 5-20 bucks depending on quality and since its solid state, i would go cheap. I can replace most of the parts for pennies, service it myself and being a solid state component they have long life. Also the custom gaming computer market has a DEARTH of cooling options that fit the peltier perfectly since it heats and cools just like a cpu. The trick is finding a low power efficient cpu cooler. My question is what is to stop me from say, getting two units, running a 5 cubic foot fridge with them, using the thermostat to either turn it off or lower the voltage (makes it more efficient for most peltiers)to maintain the tempature when it gets there and building the box with 8-12 inches of foam board and mylar on the outside. Double rubber seal on the door. Yes its not a efficient at cooling per say but if it barely has to work to cool the box it lowers my daily wattage. I've seen peltiers with a temperature difference over 70 degrees. Seems a much better option and more long term than going dc or even ac compressor based fridges. A actual freezer may be an issue.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 31,085 admin
    Not usually a good idea for energy efficient applications:

    https://en.wikipedia.org/wiki/Thermoelectric_cooling
    Thermoelectric junctions are about 4 times less efficient in refrigeration applications than conventional means (they offer around 10–15% efficiency of the ideal Carnot cycle refrigerator, compared with 40–60% achieved by conventional compression cycle systems (reverse Rankine systems using compression/expansion).[6]) Due to this lower efficiency, thermoelectric cooling is generally only used in environments where the solid state nature (no moving parts, low maintenance, compact size, and orientation insensitivity) outweighs pure efficiency. Peltier (thermoelectric) cooler performance is a function of ambient temperature, hot and cold side heat exchanger (heat sink) performance, thermal load, Peltier module (thermopile) geometry, and Peltier electrical parameters.

    And only cool by ~20C/36F per stage.

    Mechanical refrigeration is still the most (electrically) energy efficient (with good insulation).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Mountain DonMountain Don Solar Expert Posts: 494 ✭✭✭
    Even if electricity was totally free, Peltier effect units have that temperature differential problem, no matter how much insulation is used. I have not seen any that cool to more than 40 F below the ambient air temperature. More would be nice....

    We have a12/24 VDC fridge in our RV (danfoss compressor). It is an energy miser. I did add an extra 2" of XPS foam to the sides, back, top and bottom. We're in NM where it is sometimes hot. When I was testing the most KwH in a 24 hour period was 480 watt/hours and that was a hot day. With a normal indoor temperature the use dropped to less than 300 watt/hours. That is for a 4.2 cu ft interior fridge. There are a few different makes and they do vary in energy consumption with how well the condensor coils are cooled.
    Northern NM, 624 watts PV, The Kid CC, GC-2 batteries @ 24 VDC, Outback VFX3524M
  • techntrektechntrek Solar Expert Posts: 1,372 ✭✭✭
    Even if electricity was totally free, Peltier effect units have that temperature differential problem, no matter how much insulation is used. I have not seen any that cool to more than 40 F below the ambient air temperature. More would be nice....

    As mentioned in the posts above, Peltiers do exist with larger differentials (the cheap coolers just use the cheaper Peltiers), and you can stack them for a larger total differential.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • DeveakDeveak Solar Expert Posts: 38 ✭✭
    Well i figure it this way, yes it is less effecient, but the alternative is something that uses AC, requires an inverter (10-15% loss of efficiency+inverter power draw 1-2%), has a pump that has a lot of moving parts, does not last as long as a peltier, can be unusable if the inverter fails and the biggest issue, most over the counter refrigerators are poorly insulated. 100 bucks can net me some massive R value. Also the possibility to run straight from a solar panel. Granted i might have a very large fridge with a small storage but it would remain cold. The lack of efficiency of the peltier becomes irrelevant if its hardly on and only has to maintain the fridge once or twice a day. extra bonus for being 100% user serviceable.
  • BB.BB. Super Moderators, Administrators Posts: 31,085 admin
    Remember that refrigerators/freezers also need to remove heat (warm food put in fridge, shopping put in fridge/freezer, making ice in freezer, etc.).

    If you don't have "enough heat removal" capacity, the "warm items will heat up the rest of the food/defrost the freezer/etc... It is not just about insulation.

    If all your food is pre-chilled (we would freeze meat/juices/etc. for trip/camping--Expecting them to thaw/keep rest of the food cool during trip, in a Peltier cooler.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • kaipo_boykaipo_boy Solar Expert Posts: 143 ✭✭
    I scrapped my huge elephant of a fridge, a 15 year old Kitchenaid which was 27 cu ft but consuming around 4kwh of power per day (side by side) due to a leaking door seal. But even with a good doorseal it would probably have used over 3kwh per day. I now have a 14.7 cu ft chest freezer, GE, straight off the showroof floor from Lowes, with a $75 digital controller off amazon regulating the temp to around 35F. My kill-a-watt meter said it consumed exactly 0.500 kwh in a 24 hour period about a week after installing. It is a chest freezer but doing duty as a fridge with the temp much higher than normal for a freezer but just right for a fridge. The insulation that a normal, modern chest freezer has comes as a bonus and adds immensely to its efficiency as it is still ac, not dc. If I were not in hawaii with such high mean temps, I"m sure it would draw much less power. The only problem is going from vertical storage to horizontal storage... but I picked this model due to its 4 large hanging baskets inside, which has eased the transition hugely. The only real problem is, its not set up to get rid of the normally occurring condensation, so once a week I have to sop it up with a towel off the bottom. I have an old chest freezer outside the kitchen door as the new chest fridge has no frozen section now (but have always had a chest freezer anyway... the kill-a-watt says the chest freezer uses 1.33kwh per 24 hour period so I've been thinking of getting a dc sundanzer for a freezer).

    edit: I looked at a dc powered danfoss compressor conversion as well prior to getting my current setup, but the numbers to do the conversion just didn't pan out in terms of the cost to do such a conversion. I'm happy I went with the ac powered freezer-turned fridge, it works wonderfully and at such minimal cost. Prior to finding this solution, I couldn't figure out how to make a battery storage system work, the amount of batteries I'd have to get was just way too high given my need to refrigerate food. Now, it's a real possibility to get rid of the grid connection entirely!
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    I had several converted freezer to fridge units over the years, each better than the last, giving the older ones to others and finally ended up with an Energy Star 8 cu ft vertical freezer. Very happy with it. I run it right on the freezing mark, when I want a drink of ice water I have ice water. And it's 24 hour consumption? Average out to 0.3 kwh/24 hour day. Wouldn't have it any other way. And if it quits? They're cheap, buy another one.
    But not to worry Deveak, I had to learn a bunch of things the hard way too. :D
  • BB.BB. Super Moderators, Administrators Posts: 31,085 admin
    Usually condensation forms higher up on the walls. You can silicon seal a "gutter" on the walls tilted down to a drain tube or freezer drain.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    BB. wrote: »
    Usually condensation forms higher up on the walls. You can silicon seal a "gutter" on the walls tilted down to a drain tube or freezer drain.

    -Bill

    That was my experience with chest type conversions. The vertical type I have now is designed using the evaporater tubes to form the racks on which the items are placed, so the condensation forms directly on the racks. If I run it at the freezing point, the condensation forms ice. When enough builds up. I place a heavy towel under each rack, turn off the power to the "fridge" and wait. About 6 or 8 hours later most of the ice has melted into the towels and it's ready to go again.
  • kaipo_boykaipo_boy Solar Expert Posts: 143 ✭✭
    I run my chest freezer turned fridge right on the line too, and ice builds up on the bottom on the bottoms of the milk crates I place on the bottom to hold items. I have to take them out and knock the ice off them, and the ice temp water that accumulates on the bottom can be sopped up with towels. I'd love to put small channels on the sides, but I'm afraid I run my temp too close to freezing most times (vegan and that low temp makes veggies last like FOREVER) and its a crapshoot whether I'll find water droplets or ice crystals on the sides. There is a definite direction to the wind in my kitchen, though, as only 1 side of the fridge walls ever shows any condensation, so I have the whole unit very slightly tilted so the water accumulates on that end only and much easier to sop up the water as it will always be found in the same corner.

    Wayne, I was thinking about that upright freezer too! glad to see someone else thought of the same thing. The price was about the same either way, but I've got 2 kids whose noses are constantly in and out of the thing, so I figured I'd save more energy by having the horizontal storage and not have the cold air spill out every time they checked the fridge.
  • BB.BB. Super Moderators, Administrators Posts: 31,085 admin
    On thing I would suggest to experiment with. Take a small "muffin"/computer fan (120 VAC or 12 VDC, etc.) and have it run when the compressor is running. See if circulating the air will help even out temperatures and prevent cold air from settling/freezing in the low spots. A small fan will not use very much energy or add much heat to the system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    BB. wrote: »
    On thing I would suggest to experiment with. Take a small "muffin"/computer fan (120 VAC or 12 VDC, etc.) and have it run when the compressor is running. See if circulating the air will help even out temperatures and prevent cold air from settling/freezing in the low spots. A small fan will not use very much energy or add much heat to the system.

    -Bill

    Bill, I've been thinking of trying that for some time. The top "shelf" is the last to cool off, (cooling starts with the middle shelf) and since the thing only runs about 5 minutes every 50 or 55 minutes, it shuts off again before the liquid refrigerant gets to the top shelf. One of these days - - - - -
    And like "kaipo-boy" says, vegies seem to last forever when the temp is set at or very near freezing. In fact. everything lasts far, far longer than in any normal fridge I've ever owned or had anything at all to do with. Not kidding. The result is something that gives results sort of halfway between a fridge and a freezer. One more reason I'd never be satisfied with a normal fridge again.
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