Upgrade and present limitations of solar thermionic-thermoelectric technology up to 1000 K

The thermal and electrical performance of the innovative solar thermionic-thermoelectric generator (STG) has been carefully analyzed by using a high-flux solar simulator. Specific technological solutions have been integrated with respect to the first prototype for the operational testing at the moderate temperature range of 700-1000 K. The influence of spacers with different thickness has been studied to quantify the reduction of output current due to space-charge effect, whereas a BaF-based coating has been deposited on the anode to make its work function (~2.1 eV) approach the cathode one, on which a hydrogen-terminated nitrogen doped nano-diamond film acts as emitter (work function of ~2.0 eV). The engineered ceramic absorber has demonstrated to be a reliable selective solar absorber with a high solar-to-thermal efficiency of 84% for solar concentration ratio of 200 suns, extrapolated to 83% at 500 suns. As expected, the thermionic energy converter still provides a small converted power. However, the minimum inter-electrode spacing used (50 ?m) and the engineered anode allow to better figure out the present limitations in the electrical power generation and how to surpass them in the near future.