Free piston stirling engine, emergency generator, 2.1 W electricity

Hello Stirling friends, welcome back in my roof chamber. After some optimizations I can demonstrate a 2 W electricity generating stirling engine. Not pressurized engine. Displacer volume increased by 40%, but the improvement in power output was only 5 to 7%. I don't plan to improve this engine further. What do you think is the efficiency of such an engine?

Видео Free piston stirling engine, emergency generator, 2.1 W electricity канала dude007