USB Fridge

October 7th, 2010 by Potato

Wayfare got me this cute little USB-powered Peltier cooler single-can “fridge” for our anniversary (aside: thanks to Amazon, I got her a big pile of late).

The idea is neat: using the power from your computer’s USB hub, you can cool a plate down to a respectably cool temperature, and use that to cool your pop. Then, encase the whole thing in retro-styled plastic. It’s a great gift idea for me, as I’ve been known to drink a half dozen or more caffeine-laced beverages through the course of a stressful work day. Because I’m cheap, I keep a case of Coke bought from the grocery store under my desk rather than go down to the vending machine or caf to spend waste $2.25 a pop. However, I then have to go to various lengths to make them cold, such as stealing ice from various hospital ice dispensers (the caf used to have a free ice dispenser for this purpose, but they took it out during the last renovation and never put it back in), or putting my Coke outside on the window ledge in the winter. After all: a One that is not cold, is scarcely a One at all.

Unfortunately, the cooler doesn’t look to be quite powerful enough to get the job done. After several hours on the plate, my Coke was barely any cooler than the ones that had been sitting on my desk at room temperature. However, the plate itself does get nicely cool, so I figured it maybe just needed more time to bring the temperature of the Coke down. However, even after sitting there overnight, the Coke is at best “not warm”. The air inside the fridge is also not particularly cold.

So, time to hack!

I can immediately identify two issues with the design. The first is that though the plate gets cold, only a small ring of aluminum from the can actually contacts the plate to transfer heat. So, I grabbed a handful of copper wool sitting around to see if I could increase the surface area for conduction.

The other issue is insulation: the little plastic fridge isn’t insulated. I figured it would at least limit convection and so should work better than the similar pop chillers that consist of just the cooler plate alone… however, remember in thermodynamics there is no free lunch. Though the top half of the plate gets cold, the bottom half gets hot, and in the system as a whole there is a net increase in heat. There’s no insulation break in the plastic around the heat dissipating area on the bottom and the chilled area on the top — the plastic may be serving as a route for the heat to get back up to the can, working against the chiller’s job. Though the fridge is very nearly perfectly sized for a single can, there is a tiny bit of wiggle room for insulation. Though I do actually have some syrofoam here, it’s way to messy to try to cut down to the right size, so I’m going to start by testing some bunched up tissues.

The Coke chiller with my crappy mods.

Unfortunately, even after leaving it on with the new mods in place overnight, my Coke was still no cooler than before. The copper wool turns out to not be a particularly good thermal conductor, despite being made of copper (I guess it’s all those air pockets). So, I grabbed some aluminum foil and tried to pack the bottom hollow of the Coke can to get better conduction, but still no joy. Then I did a bit more reading:

The Wikipedia entry on the thermoelectric coolers mentions that these USB drink chillers may not be very useful, providing milliWatts of effective cooling. For a quick calculation, the specific heat of water is about 4.2 J/°C/g. So, it would take 4.2 * 355 = 1.5 kJ of energy to lower the temperature of my pop by 1 °C. If 0.1 W of effective cooling were getting into the can, it would take over 4 hours to lower the temperature by 1 °C. Yikes!

Now I know why it says “keep your drink cold” on the box and not “cool your drink down” — it looks like the chiller is only powerful enough to slow down the warming up of an already cold drink. The answer may be more power!!!!!11one!111!! I’m pretty sure there are some DC power supplies not being used around here, though I’ll need to check with some of the more electrically-intelligent people around here if feeding more power to this thing could blow it up (or even help at all).

Other oddities: even though it only takes power from the USB (it doesn’t seem to try to load any drivers or work with the software in any way), it has system requirements, including 100 MB of free disk space…

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