Solar Thermionic-Thermoelectric Generator (ST²G): Concept, Materials Engineering, and Prototype Demonstration

The thermionic-thermoelectric solid-state technology, characterized by solar-to-electric conversion efficiency feasibly >40%, is comprehensively proposed and discussed for conversion of concentrating solar power. For the first time, the related solar generator prototype is designed and fabricated by developing advanced materials functionalized for the specific application, such as thermally resistant hafnium carbide-based radiation absorbers, surface-textured at the nanoscale to obtain a solar absorptance >90%, and chemical vapor deposition diamond films, acting as low-work-function (2.06 eV) thermionic emitters. Commercial thermoelectric generators and encapsulation vacuum components complete the prototype. The conversion efficiency is here evaluated under outdoor concentrated sunlight, demonstrating thermionic stage output power of 130 mW at 756 °C, combined to the maximum thermoelectric output power of 290 mW. The related solar-to-electric conversion efficiency is found to be 0.4%, but, once the net thermal flux fed to the conversion stages is considered, a thermal-to-electric efficiency of 6% is revealed. Factors affecting the performance of the present prototype are analyzed and discussed, as well as a strategy to rapidly overcome limitations, in order to prepare an efficient and highly competitive solid-state conversion alternative for future concentrating solar plants.