I'd have to do the calculations but wouldn't you be better off with a solution that just barely froze at freezer temps? Because of latent heat of melting? i.e. you want it to freeze--just at as low a temperature as possible given the freezer temperature.

Your basic point makes sense to me though of course thawing isn't even.

ADDED:

Doing the calculation. It takes 334 joules to melt 1g of water vs. about 4 joules to raise that same gram of liquid water 1 degree. Freezer temp is supposed to be about -18 degrees Celsius.

So the latent heat of melting is a bigger deal than a few degrees of raising liquid temperature. So you should have ice packs maybe somewhere in the -10 degree range which is what I assume commercial ice packs do.

Completely saturated salty water freezes right around -6F or -21C. And most home freezers are set to right around that for energy conservation reasons, but max out even lower. So you could use any briny water and it will phase change before your food melts.

I'm surprised a 12v based deep freezer that runs off a reasonably sized solar panel is not more popular/affordable. With the right amount of thermal mass, and good insulation, and a small battery you could greatly extend how long your food stays cold. There are a lot of people who live in places where snowy weather or high winds and loss of power is an eventuality. In an emergency losing your food is a bigger concern than just the cost of the food.

If you're going to the trouble of doing solar it might be better to just do a small system with batteries to power other things as well like this:

https://fridge0.branchable.com/

Or go a bit higher end and put a larger solar and battery instalation with an inverter that can power the whole house. As solar panels and lithium batteries come down in cost that's becoming increasingly attractive for rural areas where the grid is flaky the best of times.

12V/120V freezers do exist and are marketed towards RV, camping, and off grid markets. Disadvantage is they tend to be smaller, cost more. But they are very efficient. Where a residential freezer might draw a couple of hundred watts dual voltage fridges draw 25-50 watts. And automatically switch between 12VDC and 120VAC. So they can be connected to both shore power and a battery at the same time and it 'just works'

There are also propane fridges/freezers but they are also smaller. (Though at the point where you have a large propane tank, you could arguably just as well have a backup generator.)

My impression is the big advantage of absorption fridges is they will run off a tank of propane for a long time. But Downside they are very slow to cool down and running off AC/DC they aren't very efficient.

But yeah cheapest thing to do is get a furnace transfer switch and a generator. That seems to be how rural people in the upper midwest avoid freezing to death when the power goes out in the winter.

Specific heat capacity:

---

Water (liquid) - 4.80 J/g.K

Water (solid) - 2.108 J/g.K (transition 334 J/g)

Brine (liquid, 300g/L) - 2.48 J/g.K

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So 1 gram of water (s) going from -18C to 0C (liquid) is 373 J.

1 gram of brine (l) going from -18C to 0C is 44.64 J.

Seems like ice is a better choice? The phase transition requires a huge amount of energy.

Brine is 70% more dense than water, so if you do it by volume, one mL of brine would be 76 J, so better, but still only ~1/5 that of pure water.

>Seems like ice is a better choice? The phase transition requires a huge amount of energy.

Why are you excluding the phase transition energy of the brine? Presumably you can calibrate the salinity so it freezes at around say... -15C, and set your freezer to -18C. That way it's absorbing heat more heat when it's colder, which is helpful if you want to keep your food frozen. The latent heat of water isn't of use to you if all your meat is already thawed.

Because OP mentioned saturated brine and a freezing point of -18C.

That's the point! The brine is frozen at -19c and liquid at - 17c which is well below the freezing point of everything else in the freezer.

Or you can have normal water which is frozen at -1c and liquid at 1c. Your pick.

The handy thing about the brine is that its absorbs heat at a temperature below that which everything else thaws. So as the brine thaws it keeps your freezer at -18c which means everything in it is still frozen.

But if the brine is not frozen at the working temperature of your freezer, it's completely ineffective compared to water.

Of course, you need to choose a salt % that gives a melting point just above the temperature you want to keep everything else, so that the salt water is normally frozen.

I mentioned -18C because that is the recommended minimum temperature for freezers such that some foods can last almost indefinitely; certain chemical reactions are completely shut down.

This is an interesting idea, would be great to see this tried with graphs.

Why don’t they do this with the ice blocks you get for keeping food cold on camping trips?

Vodka.

Isn't the freezing temp of vodka too low? I've kept vodka in the freezer, and it never actually freezes. In order to get the benefit described during a power outage, you need something that will be frozen at normal freezer temps, but will melt before the food starts thawing.

The latent heat of alcohol is also only about 1/3 that of water per unit mass, so not a very dense way of storing energy, even if you were to freeze it.

Adding salt to water reduces the latent heat density too (pure water is hard to beat there), but not as much as adding alcohol does.