I saw this thread and got really interested in the stirling concept. I have acquired a 90 degree v air compressor and am in the process of converting it to solar. I plan to use R134A as the working gas, 65 psi @ room temp and 400 psi @ 200 degrees. The Alpha configuration is simple to convert. I'm machining a new style of heat exchanger for it, one that'll give me 48" of heat exchanger in a package 3" long. which will be a serpentine layout. We'll see how well it works. In any case, its an interesting learning experience.