DIY deep freezer from mini fridge?

[Avery Ashley]

I have a cheap mini fridge (like $120) that can consistently stay at -10c in it's small freezer compartment. Storage for the majority of reagents seems to be around -20c. Any ideas on how we could convert a regular mini fridge into a deep freezer? Any ideas on how to get that last -10c?

That way we could buy in larger bulk, which usually means cheaper. 

My roommate has suggested getting a tiny desktop fridge and putting it inside the mini fridge. This sounds ridiculous, but he swears it'll work. Any other ideas before I try that out?

[ByoWired]

Have you considered making a Peltier cooler you could place inside your fridge?  If your room mate's "desktop" fridge is based on the Peltier thermoelectric effect, then perhaps it would work, so long as the thermostats and so forth were tweaked.  It won't be very efficient, electrically speaking, as the Peltier will dump 10x or 20x more heat into the bigger fridge than it takes from the smaller fridge, but it could probably get you down to that final -20C, methinks.

On Thursday, January 24, 2013 6:02:08 PM UTC-5, Avery Ashley wrote:

... Any ideas on how to get that last -10c?...

[Avery Ashley]

I've never even heard of them before now. So I can just buy one like in the link below, hook it to power and stick it in the fridge to get that last -10?

http://www.amazon.com/TEC1-12705-Thermoelectric-Peltier-Cooler-40mm/dp/B002UQKEU8/ref=pd_sxp_f_pt

[ByoWired]

Well, not exactly.  For one thing, you'll need to read some on how to design this sort of system.  When you apply electricity to a Peltier, one side will get cooler but the other side will get not only the heat from the cold side but also heat from the inefficiency of the thermoelectric effect.  Think of the Peltier as a heat pump that can cool one side of itself but in doing so heats up the other side by a factor of maybe 20x.  It is absolutely essential that you ascertain that the heat being dumped on the hot side gets carried away somehow - usually with the help of a cooling fan, or in some cases even cooling fluid.  You'll notice that Peltiers have a heat max rate.  How big of a Peltier you use will depend on the size of the volume you want to cool down and the rate of heat leaking back into that volume through its insulated walls, etc.  You'll need to consider the heat transfer rates of the volume you'll be placing inside your fridge.  If you have a tiny volume, say the size of a thimble, then you might be able to keep your temperatures very, very low.  But if you've got a volume the size of a loaf of bread, then heat might be leaking into your cooled volume faster than the Peltier can suck it out.  So there are many things to consider - insulation of the walls, convection, etc.  There are tons of DIY Peltier device gizmos on the internet, so I'm confident if you poke around, you might even find something applicable to exactly what you're doing.  

On Thursday, January 24, 2013 7:52:51 PM UTC-5, Avery Ashley wrote:

I've never even heard of them before now. So I can just buy one like in the link below, hook it to power and stick it in the fridge to get that last -10?...

[Avery louie]

I do not know for certain, but there is not much of a difference between -10 and -20.  You might be fine at -10c anyways.

--A

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[ByoWired]

One word of caution: Peltiers are somewhat unusual in that they suck up a lot of current even at low voltage.   Don't be lulled into the 12 Volt rating as being easy to deal with, since such a device might require 12 volts but at 2 amps, etc.  

On Thursday, January 24, 2013 7:52:51 PM UTC-5, Avery Ashley wrote:

I've never even heard of them before now. So I can just buy one like in the link below, hook it to power and stick it in the fridge to get that last -10?...

[Nathan McCorkle]

On Thu, Jan 24, 2013 at 5:29 PM, ByoWired <byow...@gmail.com> wrote:
>
> One word of caution: Peltiers are somewhat unusual in that they suck up a
> lot of current even at low voltage. Don't be lulled into the 12 Volt
> rating as being easy to deal with, since such a device might require 12
> volts but at 2 amps, etc.

I recently bought on amazon.com a 12V 15A power supply for around $15

[Avery louie]

ATX (or other) computer power supplies can be used to power them as well.

--A

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[Patrik D'haeseleer]

The problem with Peltiers for this particular application is not so much being able to drive them, but that you're essentially putting a hefty heater inside your fridge, whihc is obviously not designed to be treated such.

I don't know whether this is feasible, but you might want to consider retrofitting a mini fridge with cooling elements from a large-scale freezer, so you'll have enough cooling capacity for the Peltier freezer inside the mini fridge.

Patrik

[Nathan McCorkle]

On Tue, Jan 29, 2013 at 12:26 AM, Patrik D'haeseleer <pat...@gmail.com> wrote:
> The problem with Peltiers for this particular application is not so much
> being able to drive them, but that you're essentially putting a hefty heater
> inside your fridge, whihc is obviously not designed to be treated such.

Is there someone in a cold part of the world who also has a peltier
that can measure how cold the cold side gets if you put it outside
when it's already 0 C?

It's -3 C in New York City right now... Sebastian do you have a
peltier and a thermometer?


--
-Nathan

[Cathal Garvey (Phone)]

Most peltiers are made of lots and lots of semiconductor junctions. Most non-milspec semiconductor equipment is only rated for use within 10/20 degrees of water's liquid phase, so it's possible peltiers/drivers/sensors wouldn't survive the cold.

Sent from my Android phone with K-9 Mail. Please excuse my brevity.

[Patrik D'haeseleer]

Looks like typical Peltiers are rated for a minimum temperature of -40 to -50 C, so they can definitely go well below freezing. And I would assume you can find ones that are specifically intended for cooling and are rated for lower than that.

One problem is that the efficiency of a Peltier drops at low temperatures - and they're really not all that efficient even at room temperature. So you wind up having to dissipate lots more waste heat on the hot side than you're sucking out of the cold side. 

[Sebastian S. Cocioba]

Yes I do, its small and I'll run it off a 9v. I got a touchless laser thermometer that is pretty damn accurate and a traceable vwr metal probe thermometer. I,ll let u know asap.

Sebastian S Cocioba
CEO & Founder
New York Botanics, LLC

Sent via Mobile E-Mail

[Patrik D'haeseleer]

Not sure just dangling a Peltier element in sub-freezing air will tell us anything useful. The cold end will be directly exposed to ambient air, not to mention to the heat given off by the hot end.

If you really want to see what is possibly, I would embed the cold end in a block of styrofoam, and attach a big heatsink on the hot end.

[Avery louie]

I looked into getting a -80 setup for a while.  Peltiers are expensive, tricky to run, and fickle without control electronics.

One option may be to get a liquid nitrogen dewar (just a vacuum insulated vessel) and fill it with dry ice, which is almost exactly -80C.

You could do it in a small thermos as a test.  Just remember to let the sublimated CO2 escape and have it be well ventilated- you dont want CO2 poisoning or explosions from pressure building up.

--A

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[Cathal Garvey]

CO2 is considered low risk for poisoning or asphyxiation as your body
readily detects elevated CO2.. in fact, it is elevated CO2 that gives
you that feeling that you need to gasp for more air. That's why nitrogen
or argon are considered hazardous, as you can breathe them without
realising you'll asphyxiate; you keep exhaling CO2 and inhaling no O2..

So, while CO2 is a significant explosion hazard if kept in a sealed
vessel, it's fairly safe from a poisoning angle.. provided you can walk
away from it and get some fresh air.

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[Avery louie]

That said, a CO2 alarm is pretty cheap- if it is a home lab other people in the house or building may not be aware of the risks.

--A