There was a big boost in interest with the gas industry in the US. The idea was that they could use them at the pumping stations and have no moving parts. Efficiency is however very low in every paper on the topic i have read. I am about 6 or so months out of the loop tho.
My original interest was the other way round. Pulse tube cryo coolers. Very cool.
If you get stuck getting hold of a paper, sing out. I still have all the PDF's somewhere in my backups.