[Cowboy Sailer]

My dear Mr. Alan Wheeler,

The usual implementation of the Stirling Cycle as an engine does indeed have moving parts that wear against each other and require lubrication to minimize friction losses and wear. The mechanism used in the Coleman Stirling cycle is a free piston that floats the moving piston on what would be called “air bearings” except that the working fluid is Helium so I guess that they are more properly described as “Helium bearings”. This really, really lowers contact wear and friction.
This method of cooling is very different from Peltier effect cooling. The Peltier effect can only provide about 40 degrees of cooling per p-n junction pair. To get very cold temperatures one must stage them so that the heat pumped to one end of the junction is removed by a much larger next stage. Three stages are about the limit of practical use. I used a small three stage Peltier cooler to cool an infra-red detector in some space and aircraft equipment that I designed once. But I digress.
The Tropikool unit used a thermo siphon using carbon dioxide as the transfer fluid. I don’t know if the Coleman unit also uses a thermo siphon technique to transfer the heat from the cabinet to the cold end of the Stirling apparatus. If so, that may explain why they said that it had to be within 12 degrees of level.
I really appreciate allied35 telling about his experience with a Coleman Power Cooler. I also am glad that shipofools told about his Engel 35. I remember that an earlier post about refrigeration where someone ripped the guts out of an old rusted portable Engle cooler and re-installed the works in a well insulated built in compartment on his boat. That got me to thinking, “If one could use the more efficient Stirling cycle and have a very well insulated box, wow, a low energy deep freeze!” The Stirling cycle stays very efficient at sub zero temperatures unlike the Rankin cycle which looses efficiency at deep freezing temperatures in the tropics.