re: solar powered Sterling engines - adventures in Sterling-engine-based concentrating solar power systems from a few years ago.
http://en.wikipedia.org/wiki/Stirling_Energy_SystemI thought Sterling Energy Systems had a great set up, with tracking parabolic solar collectors. For a while, they were a hot commodity in the solar business. The simplicity, reliability, and falling costs of passive, non-tracking PV eventually did them in.
Thermal engines, whether they are internal combustion, or external combustion/heat source heat-engines, are complex mechanical machines. Add tracking complexities, heat sinks, extreme high heat materials, etc....small scale Sterling engines just won't pencil out compared to basic semi-conductor-based PV. Simplicity = trumps theoretical efficiency.