Using salt water to 'store cold'

[Madscientist267]

Got a little bit of a far fetched one for you guys -

In theory, according to this and this, water's lowest freezing point with a purely sodium chloride mix is about -21C or so, but the practical limit is more like -19C.

Question is, is there any benefit to using saline to lower the freezing point so as to store more 'cold'?

Put another way, does salty water have more staying power in terms of keeping the temperature of the surroundings colder for longer (due to the lowered phase change temp) than plain water?

I'd imagine that it can't absorb any more heat while it is melting since the volume of water is not changing, but the temperature would stay lower for longer, right?

I want to use the frozen saline to 'run' the refrigerator during power failures, keep things as cold as possible in coolers, etc etc.

Ideas?

Steve

[dnix71]

http://www.pcmproducts.net/Eutectic_Refrigeration_PCMs.htm  There is a pdf with their products linked from this page. Their best has over 9k BTU/FT3

The specific heat, latent heat and heat capacity are listed for each mix.

Some may be plain salt, others more exotic mixes. A sister company of ours sells specialty imprinted chocolate bars at Christmas which are shipped UPS. They are packed with gel cold packs like those above to keep them fresh.

Those particular packs are not be toxic. Some coworkers have taken them home to put in the freezer and bring to work to keep their sandwiches cold.

[Madscientist267]

That gives me some insight as to what happens, but since there are obvious shifts in composition, I can't really base a whole lot on it.

I'm getting conflicting information from elsewhere too. It seems my best approach may be with samples until I get what I'm looking for?

I suppose the answer to my original question is 'yes', but now it's a matter of figuring out the optimal mixture.

From experimentation with the freezer and a thermometer (with just fresh water ATM), I'm estimating my ideal bottom line temperature to be about 0F (-17.78C). Targeting too much colder than that really begins to show a strain on the freezer (and may not even completely freeze), too much warmer than that and it would be a waste of time.

I'm noticing from the chart in the PDF that the heat capacity drops as the ratio of 'salt' to water gets higher, while the latent heat drops only slightly. This leads me to believe that the 'holding power' at a lower temperature is worth the effort, but it 'burns' it quicker by absorbing the heat at a lower temperature.

The result being that if a given volume of pure water holds the temperature at 32F for, say 1 hour, the same volume of salt water might only be able to hold 0F for 45 minutes (made up times to illustrate the point).

Not that I would not be able to work with that, but I want to make sure... Am I on target here?

Steve

[dnix71]

The phase change of water requires a lot of energy, but the temperature it happens is not all that "cold" since what you want to keep frozen also is mostly water. Since food has a freezing point lower than pure water because it contains salt already, 0C is above the freezing point of food. Using pure water ice means the stored food is not actually frozen when the water/ice slush is at the phase change temperature.

The addition of salts to make a eutectic is to lower the temp the phase change occurs so that the solution can be supercooled, but that is a trade off. If you freeze a salt solution the freezing process changes the mix as it freezes except for very specific mixes. What is frozen is no longer the same as what remains except for a narrow range of salt concentrations.

The people above sell both eutectic and non-eutectic mixes. If you prefer supercooled slush instead of a solid brick you can probably fill the empty space in your freezer better.

Remember also, water expands about 4% when it freezes, so allowing freezing and thawing requires expansion volume. As long as the eutectic never freezes you can put more of it in the same space, plus the density of salt water is higher than plain water, so there is more cold mass in the same space as well.

[Madscientist267]

My goal though is to reach a complete freeze, just at a lower temperature.

But if I am understanding this correctly, this won't really ever happen, and will only result in a slushy mix. Which will defeat my entire purpose...

Are some salts better at this than others? One that came up amongst my travels was Ammonium Chloride. Seems to be closer suited to the job given the temps I want to work at.

Also trying to keep this as cheap as possible... LOL

Steve

[dnix71]

The eutectic stays slushy, your meat and veggies stay frozen longer. A true eutectic freezes/thaws in the same ratio.

Just go for the best cooling capacity at whatever temp your freezer can handle without damage to the compressor.

The E15 mix has a phase change at +5F and about 8600 BTU/lb cooling at the phase change. If your freezer can only cool to 0F then that's the best you can do. That mix will keep the inside of your freezer at 5F until all the eutectic has melted and then the temp will begin to rise.

[ghurd]

This is a simple solution, removing some of the fun.
Ask your vet for those frozen bag things that come in the meds.
Amazing how many come in a shipment of 'shots'.
I bet a vet office throws away 5 or 10 gallons every week.

A ~6x6x3" box of dog/cat/horse vaccine vials gets shipped in a 16~18" styrofoam cube, with a whole bunch of those ice-pack bag things.

Doctors for humans probably go through a lot too, but I have a feeling they would be less receptive to the idea.
G-

[zap]

Also trying to keep this as cheap as possible... LOL

Ammonium nitrate is pretty cheap and could be used as an "emergency cold pack"... but the ATF might be too interested?

[Madscientist267]

http://en.wikipedia.org/wiki/Texas_City_disaster

 

LOL No thanks heh

Not that I would need quite that much... But I would need a significant amount, so no, the attention would not be welcome either. 

I'm trying various mixtures of common salt to see what I can come up with.

Steve

[hydrosun]

Related to this is I put a timer on my chest freezer so it only goes on during the day when the solar panels are putting out power. There is enough mass and I added extra insulation to the outside (  I found a freezer with the coils separate from the chest walls) so the temperature varies very little. So the 400 watts per day is used while it is produced instead of at night and coming out of the batteries.  If the mass wasn't enough I guess I would be looking at your idea of a eutectic material to keep the temperature stable over a longer period. Especially if I set it up to only come on when the batteries were full instead of a general control of daytime.
 Another posting that caught my attention but I didn't respond to was about storing heat in a room from the sun. I've read of a type of sheetrock  available in Europe that includes a type of was that phase changes at above 70 F so would store more heat at a comfortable temperature. It would take any space or control to store that heat.
Chris

[Madscientist267]

Well my hypothesis was correct.

I found a good mix ratio of salt to water, and can get it to freeze completely, but it 'burns out' quicker than the distilled water does.

It was only a small scale test in pill bottles, but it definitely shows the properties of both clearly. The ratio is something to the tune of 5% salt, but this was done by eye in terms of volume, so isn't really useful just yet. I have a scale that can do .01 pounds, so I will be ramping up the amounts shortly to get an accurate measure of ratio by weight.

I also haven't checked the phase change temperature of the mix either, but it was gathering a fair amount of frost while the distilled simply was sweating.

More data will be collected on the next run, so stay tuned! 

Steve