High "TEC" Refrigerator Revisited

Credit

First of all, I'd like to give credit where credit is due. I got the idea and most of the how-to for this article from a very excellent article by Kenton Chun. "High 'TEC' Refrigerator" appeared in the August 1999 (Vol. 20 No. 8) issue of Nuts & Volts magazine. If you want to review it, look for back issues at your local library, or visit their website to see how to order back issues. $link:http://www.nutsvolts.com/$

The Theory

Figure 1 shows a ThermoElectric Cooling (TEC) device, which is basically a solid state heat pump. The actual working part is inside the styrofoam insulation in the middle. The two big metal things on the top and bottom are the heatsinks. Back around 1834, a guy named Jean Charles Athanase Peltier discovered that when he ran current through two dissimilar metals, one metal would get hot while the other grew cooler. The metals being such good conductors of heat, however, meant that he couldn't get much of a temperature differential. It would take until modern day, with the advent of semiconductors (which conduct electricity but not heat), to come up with a practical TEC device, which some still call peltiers after the original discoverer. A modern TEC device can create cold side temperatures as much as 40F below ambient. This particular TEC is rated at 13-15V at 4A and was ordered from Jameco, part #147942. I'm told that this particular item is no longer available, not surprising as I think it was pulled out of some junked equipment they'd received for salvage, but similar items become available all the time. At the time of this writing, several thermoelectric cooler chips are available, though none with huge heatsinks like the one shown. $link:http://www.jameco.com/$

I had orginally purchased this TEC as a cooling device for my computer. After finding it simply impossible to unbolt the heatsinks, I decided it would not be possible to attach it to my CPU. Using it to cool air being sucked into the case didn't have much of an effect. Then I started reading about condensation problems, so I scrapped that project.

Attempt #1

Figure 2 shows my first attempt at creating my own refrigerator. I used the cheapest styrofoam "ice chest" I could find. I cut a hole in the lid and mounted the TEC unit using wood scraps and cable ties. I used the taller heatsink as hot side, and the shorter but broader one as cold side. For power, I used a 12V cigarette lighter plug off a ceramic auxillary car heater. I held all the wiring together with wire nuts. As you can tell, I don't put a whole lot of effort into making these "proof of concept" designs pretty. However, it did work! After a few hours, the hot side heatsink got too hot to touch, the cold side heatsink deliciously cold and was covered in condensation, and the air inside the ice chest was noticably cooler than ambient. I didn't have any type of thermometer handy, so I can't give exact temperatures.

TEC's aren't 100% efficient, so in addition to the heat it's pumping from the cold side to the hot, it also generates quite a bit of heat itself, hence the huge hot side heatsink. Since the hot side heatsink was getting too hot to touch, I added a cooling fan. This was a Sunon 120mm 12V fan with a finger guard, both borrowed from a Card Cooler XT. $link:http://www.cardcooler.com/$

With the hot side fan in place, the cold side heatsink quickly became cool enough to ice over, which I tried to show in Figure 3 but the picture didn't come out too well. Taking the cooler on a test run at work, I discovered that once the cold side heatsink iced up, it didn't do a very good job at cooling the contents of the cooler. Mr. Chun suggested either a larger cold side heatsink or a cold side fan. I couldn't get the heatsinks unbolted, so I decided to go with the fan for the next revision.

Brain Cramp #1

I also decided to try to power my homemade refrigerator with 120V AC, so I could also use it in motel rooms. Totally forgetting the power requirements involved, I dug through my junk drawer for a 12V wall wart. Er, I mean one of those little plastic-covered AC adapters the plugs into the wall outlet. I found that I had two, both rated for 500mA. Each seemed to power the TEC, but only one of them would spin the fans. So I wired them in parallel (not recommened) and ran into town (also not recommened) to find a more suitable portable cooler. I returned to the smell of melting plastic. That's when I checked the specifications on the TEC and found that it draws a nominal 4 Amps. My two wall warts together barely supplied 1 Amp, so no wonder they melted down. Back to the drawing board...

Attempt #2

Next step was to use the insulated cooler I purchased for $8, as shown in figure 4. More by accident than design, I stumbled across the same 8 quart Coleman cooler than Mr. Chun used. $link:http://www.coleman.com/$

I briefly considered mounting the TEC unit in the side or back of the cooler. I figured the food and drinks would get cooler if directly in contact with the cold side heatsink. But then I figured the one drink can resting against the heatsink would freeze, and the rest would be warm. Plus, it would make it harder to put a fan on the cold side heatsink to keep it from frosting up. I finally decided to do as Mr. Chun suggested and mount the TEC unit in the lid.

Cutting through the lid of a Coleman cooler is not as easy as it sounds. I was able to drill some starter holes easily enough, but after that things went downhill. Using the Dremel was slow going, because the cutting wheel would melt the plastic and then bog down. I finally ended up marking a more or less straight line, scoring it repeatedly with a small utility knife, then "sawing" through with a larger knife.

I marked and cut a little on the inside of what I thought I needed, because we all know how hard it is to make a hole smaller after you cut it too large. I whittled a bit until the hole was larger enough to admit the hot side heatsink. I had intended just a test fitting, but once the TEC's heatsink and styrofoam spacer got through the hole, they didn't want to come back out. So much for my idea of filling the inside of the cooler lid (mostly air) with additional insulation. My cutting wasn't as straight as it could have been, but the gaps were pretty small. I sealed the hot side heatsink to the lid using the trusty old hot glue gun. Yes, silicon caulk or something like that would have been better, but the glue gun was handy, glue's a good insulator, and with a 120mm fan mounted on the heatsink, it shouldn't ever get hot enough to melt the glue.

I drilled a couple of small holes in the hot side heatsink and mounted the 120mm fan with cable ties. The cold side heatsink already had holes, so I mounted an 80mm fan on it as well. I had intended to feed the wires for the 80mm fan through the lid next to the styrofoam spacer, but the unexpectedly tight fit made that impossible. Instead, I drilled a small hole through both layers of the lid, fed the wire, and then sealed up the holes with a little help from the hot glue gun. If you look closely, you might see this in figures 4 and 5.

Brain Cramps #2 and #3

I tested the new design and discovered it worked much better, bringing some canned drinks down to drinkable temperatures in just a few hours. By this point I had used my homemade refrigerator for several hours on my truck's battery, apparently without draining it much at all seeing how promptly it cranked whenever I tried it. I decided to leave the refrigerator running overnight to see how cold the drinks would get. The next morning, after finding a pair of jumper cables, I decided that overnight was too long for the battery.

My next stroke of genius was going to a flea market and finding a Radio Shack 13.8V regulated power supply, as shown in figure 6. This is catalog number 22-504, rated for 3 Amp, and supposedly has a temperature protection circuit to prevent overheating. (Yes, I keep a Radio Shack catalog in my truck with me. Want to make something of it?) $link:http://www.radioshack.com/$

After seeing it power a CB radio the guy was trying to sell, I talked him down to $20 and went to try it out. It powered the refrigerator for about an hour before starting to smell like burning dust. The back of the case was too hot to touch. I had hoped at the 3 Amp supply could handle the 4 Amp load, since Tandy is fairly conservative about some of their ratings. I found that by only running it a half hour at a time, with an equal cool down time, it worked well enough. I took it apart and blew the dust out of it, and also checked out where the heat was coming from. The transformer put out a little heat, but the really big source was the heatsink on the rectifier unit. I have some ideas on how to fix this, but that's the subject of a future article. In the mean time, I still needed a workable "leave it on overnight" solution.

120V AC Solution

During an earlier trip to a thrift store, I'd found several old computer power supplies. I didn't have a use for them then so I didn't pay them much attention, but now I decided they deserved a second look. I passed over some newer ATX-style power supplies, even though they looked to be in excellent condition. These supposedly require some sort of "Motherboard OK" signal to power up properly, and I don't know how to fake that. I picked through the AT-style power supplies and found several with original factory stickers describing, among other things, how many amps they put out on the 12V and 5V rails. I found two rated for 6A at 12V, and took the cleaner one for $3. This is the one pictured in figure 7.

After blowing the dust out of it, I found which two wires to short to power it up. I added a molex connector to my refrigerator and plugged it into the power supply. It ran for several hours without getting hot, smelling funny, or the fan stalling. I put in several canned drinks and let the whole setup run overnight out in the garage. The next morning, I had ice cold drinks, a cool power supply, and a big grin on my face.

Conclusion

With this high "TEC" refrigerator, I was able to drive several pounds of frozen steaks halfway across the state, yet have them arrive still frozen enough to drive nails with. Using the 3 Amp supply sparingly, I was able to keep drinks and sandwich meats cold during a 3 day hotel stay, with only a single trip to the ice machine. Using the 6 Amp supply, I can keep things cool for as long as I want. And what did this cost me? $28 for the TEC, $8 for the cooler, $3 for the 6 Amp power supply, and some old fans, wires, and connectors. That's about $40 plus a trip to the junk drawer, versus $80-$120 for the ones I'd seen in catalogs at the time.

Updates and Stuff for 2005

Most of this article was written in early 2001. Since then, I've used the cooler on several occasions, but nowhere near as often as I thought I would. I changed workshifts and am no longer bringing in my lunch everyday, and have also cut back on canned drinks during road trips. More hotels I've been staying at have been providing free ice. A friend gave me an old mini-fridge for the game room. I just haven't had that much use for my creation.

The Radio Shack power supply comes in handy for recharging my flashlight and powering small auto projects, but I still haven't gotten around to adding a cooling fan to see if it will power the cooler or any other high-draw appliance for any length of time.

Thanks to everyone who has taken the time to write in and tell me how much they liked the project.