Go to wikipedia and put in ‘Stirling Engine’ and see. A stirling engine is basically an external combustion engine. Patented in 1816 by a preacher, it took some years for the metallurgy science to catch up before the first practical engine could be built. Generators with Stirling engines are quite common (Onan). The working fluid can be air, helium, even hydrogen! A Stirling engine can even be made of water, sort of. Fascinating subject. Some of the big auto companies experimented with them a bit, but nothing ever caught on.
Yup, they are actually commercially available for low temperature applications. They are capable of operating at pretty low temperature differences, but they are not very efficient. They would be most useful for extracting energy from low temperature waste heat, if they can be made cost effective.
YES TO ALL
THIS IS MICRO COGENERATION FROM EUROPE
Yanmar recently came out with a small water heater and generator that utilizes a stirling engine. It runs on diesel and is designed for small boats. They had one running at a boat show I went to; very quiet and compact.
I have a thermal imaging camera that has a Indium Antimonide detector chip which must be cooled to 77 K in order to detect Mid wave (3.0-5.5 microns) infrared radiation. It therefore has a refrigeration system that cools it in about 5 minutes to that point. It uses a stirling cycle system to do this.
There are designs for making Sterling Engines sold through educational supply sources that are used in Machine Tool Technology programs. The final project is a working engine.
Hobby shops probably sell sterling engine kits too.