Fridge keeps its cool

[rondurrans]

See hyperlink below

http://www.newscientist.com/article/mg20927944.000-vaccine-fridge-keeps-its-cool-during-10day-power-cut.html

Ron

[beelbeebub]

"the low-power cooler is partly the result of good insulation. But it also incorporates a phase-change material to regulate the temperature"

just trying to think of an ideal phase change material that would work for a fridge,

Changes phase around 0C, ideally from liquid to solid
Non-toxic
High specific heat
Cheap
Readily available

hummmmmmmm....what could that material be?

[Justme]

They discount water / ice if you read it.

I am guessing its one of the newer freezer pack type things.

We used to send out frozen food & the packs would stay semi liquid at well below freezing & keep the frozen food frozen (thats legally frozen for food stuffs so below -18 at point of delivery, we sent them out at -24c or colder) for nearly 4 days.

[EccentricAnomaly]

Changes phase around 0C, ideally from liquid to solid
Non-toxic
High specific heat
Cheap
Readily available

Pedantically, it's not the specific heat capacity that's of interest but the latent heat of fusion.

For this application they want to keep the vaccines around normal fridge temperatures (they say 5°C).  RT 5 sounds like a possible.

[Baz]

Didn't some girl last year get a 'young scientist award' for 'inventing' the old evaporative butter cooler as a fridge for Africa.

[Ivan]

EA, I agree it's likely to be an oil/wax. Do you think the rubitherm has any significant advantage over bog standard oil/wax. I doubt it would be significant (although I haven't checked the heat latent heat of freezing for either). Alternatively, it could be a heat-of-crystallisation thing (which I think is what's in the 'gel' coolpacks, as mentioned by Justme). I think PCMs based on crystallisation energy will be cheaper, but probably not as reliable or consistent as wax.

[Ivan]

It's probably important to point out that the fridge isn't ultra-efficient, simply that by incorporating phase change materials, it takes a huge amount of energy to cool the fridge down, and once that's done, it takes a huge amount of heat-energy to warm it up again.

[EccentricAnomaly]

Do you think the rubitherm has any significant advantage over bog standard oil/wax.

No, not particularly if you can get standard oil/wax with such a tightly controlled melting point.

I wonder how easy it would be to separate at home.  E.g., cool to 7°C, filter and keep the liquid then cool that to 3°C, filter again and discard the liquid, keeping the solids.  If you only want a few kilos for an off-grid fridge it would seem better to just order stuff that somebody else has already done this to and concentrate your effort on something more useful.

[Ivan]

From what I understand, the tight melting point range is achieved by having very similar length molecules of wax. I've not looked into prices, but I think you can buy tight-range molecular weight waxes from chemical suppliers - I did find some generic waxes with well-defined molecular weight ranges from a standard chemical company. Can't remember who though (maybe Sigma or Aldrich). Yes, I think you could select it by freeze/thawing - that's freeze distillation. It's not terribly good though, as you get eutectic mixtures. I think you'd use a molecular sieve of some kind. When I did this sort of thing for a living I only worked with water-soluble chemicals, so I'm not sure how you'd do it with waxes.

[beelbeebub]

teach me to read the article fully!

I'm not sure if someone hasn't posted this before but a chest freezer can apparently make a very efficient fridge that can run off-grid.

http://www.builditsolar.com/Projects/Conservation/chest_fridge.pdf

[EccentricAnomaly]

The author of that PDF has quite a nice little web site for his house, etc: http://mtbest.net/.

Another kit of the same sort: http://screwdecaf.cx/yatc.html.  Associated blog.