Was wondering why the GVCS did not cover Stirling engines. Their explaination is probably the easiest answer (and a concise summary of the state of the art)

Elsewhere, Stirling engine enthusiasts are working on this [project]. The Stirling engine is an efficient energy generating technology that has been replicated innumerable times and ways over the past two centuries, but nothing has penetrated the market in any significant way yet. The thrust, therefore, of an open sourcing project should be to identify and develop a variation that will result in a marketable design of widespread applicability.

Update: Stirling Engine Generates 1 Watt

Stirling Engine driving high-power LED