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Freezer/fridge cold battery

I've been learning about freezer to fridge conversions, done for efficiency. Can get very efficient, but they still need to pull power from the battery bank when it's not sunny. I want as small a battery bank as possible, so am thinking about making a cold battery.

The cold battery would be a energy efficient chest freezer, run off an inverter, with the temp turned down as low as possible, and full of jugs of water to freeze (and some frozen food, although there's a risk it will thaw occasionally). It would be the diversion load of the solar charge controller, so would only be powered on when the battery bank is full and the solar power would otherwise be thrown away.

The fridge component is simply an insulated box, perhaps a small dead chest freezer but it could even be a simple cooler. It is physically connected to the cold battery by some form of vent that can be opened and closed, to keep the fridge's temperature cold enough, but not freezing.

One way would be to elevate the cold battery above the fridge, and have a vent connecting them. When the vent was open, warmer air from the fridge would rise to the battery, and cool air from the battery would fill the fridge. A vent control device would wake up every hour or so and check if it needs to adjust the vent.

Or, they could be side-by-side with a fan at the top to move the warm air from fridge to battery, and a bottom vent to draw cold air into the fridge. If the fan kept its vent mostly blocked when not running, a vent control device might not be needed.

The vent control or fan would be the only part of the system that would consume any battery power, and should use much less than a compressor.

There's some simularities to a regular fridge which after all has a freezer compartment, a vent, and a fan. Except I'll bet the cold battery needs to be bigger than a fridge's freezer to store enough ice to get through a cloudy warm week.

Seems like this would be fun to play with building, though a bit unweidly.


Have you looked into how holding plates work? They use a container of antifreeze around the elements to retain temp and "buffer" it. You actually want to stop stuff freezing if possible, as there is a temperature spike when a liquid transitions to a solid (energy gets dumped out).

I think you could get everything you wanted with a fridge using holding plates, and some beefed-up insulation - "crystal kitty litter" is available very cheaply (at least in NZ) and is nothing more than re-branded Aerogel. A bit of that and some polyurethane foam to seal it, and you're set.


Crystal Kitty Cat Litter is silica crystals, where as Aerogel is a silica gel that has been dehydrated through supercritical drying. The cat litter will not be as insulating as aerogel. Although you could possibly make aerogel from the cat litter if you have a pressure chamber with the proper temperature control, or better yet a CO2 supercritical extraction unit. Probably easier to just buy the stuff.


I was not aware of holding plates. They seem expensive, as marine stuff is.

But.. Containers filled with antifreeze should be cheap to DIY. Just be sure to avoid it ever leaking into food! Ice packs for coolers work on the same principle and are inexpensive and probably use something less toxic than regular antifreeze.

Liked this in #solarpunk

You could also use alcohol, like cheap vodka or everclear, which has a freezing point of -114 C, so you could still pump it around. In an emergency situation you could also use the alcohol as fuel, calories and for sterilizing wounds.

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Ooh, pumping subzero alcohol between 2 chest freezers seems really fun. Also, it would be a hilarious liquor store buy.

Not a design for anyone with alcoholisim in the family though. I suppose something could be added to make the cheap vodka even more undrinkable.


Brine seems more practical. Freezes well below zero (-6 F), much cheaper, and no chance of it reacting badly with the pump.

@Trigger Warning

Auto stores sell "low tox" anti freeze, which is mostly propylene glycol and sodium nitrite (which you will recognize as junk food ingredients for candy and hot dogs). There are additional chemicals for corrosion resistance, etc, which make it nastier - but still low tox.

You could maybe make your own propylene glycol antifreeze - available from candy making supply stores.


Ar you worried about dissolving the seals? You could always used methyl alcohol if you are worried about alcoholics, although that would be worse for dissolving seals.

If you mix glycerine (food safe) with water at a 66% concentration by weight you can get a solution that will go down to -46.1 C. And it's safe to eat.

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