My latest engine is a mostly 3D printed Stirling engine. Although I'm not running it on solar power, it could be easily made to do so. The design is open source so you can get all the drawings, CAD files, and other information to make one. The information starts here:
Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.