You are reading content from Scuttlebutt
@juul

Free Piston Sterling Coolers

Ever since one of my friends showed me his Coleman Stirling Power Cooler I've been fascinated by Stirling engines and Stirling coolers. For those who don't know about the Coleman Stirling Cooler, it was a camping cooler briefly sold by Coleman which used a very high efficiency 12v Stirling engine rather than the more common Peltier elements. Stirling coolers are more efficient than both peltiers and compressors, and require less maintenance than compressors. They also have the unique property that they can get down to cryogenic temperatures in a single stage (whereas compressors need at least two stages). This is interesting to me for long term biological strain storage, but it has many other uses. Stick the cold-head of a high-powered Stirling cooler in a thermos, run it for a few hours, and you have liquid nitrogen. You could use this with freeze distillation to make your own off-grid Argon for TIG welding, though I doubt the Coleman model's Stirling engine is powerful enough.

The Coleman briefly retailed at about $800 and apparently didn't sell enough units, as it was almost immediately discontinued with the remaining stock liquidated at $300 per unit. They are now almost impossible to find used.

I did some research and it looks like the Stirling engines used are manufactured by Twinbird. In fact Twinbird now sells camping coolers that look remarkably similar to the Coleman. I called them up a couple of years ago and no-one spoke english, but they did reply to an email after a while and gave me a quote for $1000 + shipping for their -40 C Stirling engine. So a bit more than I wanted to spend but not outrageous compared to the original $800 for the whole Coleman camping unit.

Getting to the point: Something went down in North Carolina this week (likely a biomedical company closing up shop) that dropped a bunch of these Twinbird camping coolers on Ebay. They are rebranded as "Envirocooler" and sold as mobile vaccine storage fridges but you can see the Twinbird "F | P | S | C" square logo on the front. Go on Ebay right now and you'll see several sellers, with prices down to ~$280 including shipping. I ordered two myself. These are very good as solar fridges, as their power consumption when the engine runs is 45 watts max (and it of course only runs some portion of the time), it runs off 12 volts DC directly, is fairly quiet and has no significant startup power spike. Be warned that it looks like these are meant for 4 C operation only so if you plan to run them as a freezer then you might have to hack the electronics control unit. I believe they are somewhat heavier than most camping coolers so you might want to take this into account if you're on a boat.

My friend has been using his Coleman 24/7 for over three years as his primary fridge and has only good things to say. Get 'em while they're hot!

@juul

I should mention that one of the sellers is giving a significantly reduced price on 12 or more units, so please speak up in the thread with how many you want and if we reach 10 then I will get the last two and negotiate the price. This only works if you're willing to pick them up in the bay area.

@joeyh
Voted this
@joeyh

I suppose you'd only need to hack the temp sensor, which seems like a potentially easy hack. (Eg, relocate to outside the case.)

@neftaly

I'd be keen on one from a group order, if everything but the electronics/engine were removed!

@nanomonkey

Count me in on two...maybe more if need be. I wonder if you can run them in reverse (off a heat source) and produce energy.

@Trigger Warning
Voted this
@juul

@neftaly Hah, I think that can be arranged.

@Dominic

@juul what do I search for to find these?

User has chosen not to be hosted publicly
User has chosen not to be hosted publicly
User has chosen not to be hosted publicly
@elavoie
Voted this
@juul

@Dominic envirocooler

@Wao
Voted this
@sean
Voted this
@sean

Incredible -- I've been on the lookout for one of these coolers. Thanks for the tip!

@robin

I only just saw this, am very interested. If possible I'd like to get one, could you let me know if the group purchase went ahead.

Cheers

@Sam Smith

This is really cool! Good find! I just got on SSB, and it's cool to find folks talking about this kind of stuff. I've already got an ammonia absorption chiller in my bus that pulls 60W, but I'm tempted to buy one of these anyway just because they're so damn useful. I did some digging and found the data sheet for the actual stirling cooler module these things use. According the manual it uses 2-8VAC 80HZ power to actually run the cooler, and the rated min cold side temp is -50C. That's not cold enough to make liquid nitrogen or argon, but it IS just cold enough to hit the triple point of water for small-scale freeze drying. If you were to put one of these in a vacuum insulated hydroflask, with some food like chili or stew in it, get it down to -50C, and then pull a fairly hard vacuum on it and let it come back to temp slowly, you should be able to produce one-off freeze dried meals. Freezing drying is awesome bc its preserves texture and nutrient content much better than dehydration. With an O2 absorber, most freeze dried camping meals are labeled to be stable for 30 years (!!!) and could likely viable for much longer than that. Something like this could probably only do one serving at a time, but it could also be solar powered and much more compact than any other freeze drying system available. Thanks for posting this! manual_SC-TC04_1702.pdf

@Rivet

Just installed a wood stove in my bus. Now, I need to pipe things into it to take up as much heat as I can. The first obvious one is water. I was also considering trying to get an ammonia fridge going with the fire, but one of these would work just about the same, yeah?

@Rivet
Voted this
@Sam Smith
Voted this
@nanomonkey

Stirling Cooler Teardown

Last night @juul and I tore down one of his medical stirling coolers. Both of us had purchased a couple off of ebay and wanted to see if we could modify them to go lower than the 4 degC that they are set to.

Long story short, we removed the outside enclosure and a layer of insulation around the cooling head and found the unit had two thermistors and a connector marked "heater". One thermistor went to the hot end of the Stirling engine and the other went inside of the container with the heat pipes. My suspicion is that the heater is actually used to control over swing and keep the contents from actually going too far below 4 C and thus freezing.

We looked up the specs for the thermistor and concluded that we could put a potentiometer in parallel with it to adjust it's output and convince the control board that it has not in fact reached it's desired temp. A snip here and a twist of wire there and we had done just that. We set up two thermocouples to read the actual cooling end's temp and the temperature inside the unit and found that we could get the chamber to get below zero with our simple modification.

Here in the photo you can see the heat cold end covered in frost along with the heat pipes going into the cooling chamber.

IMG_3073.JPG

Join Scuttlebutt now