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!