Recent progress in thermionic conversion devices and outlook toward future developments

In the past five years, rapid progress has been achieved in thermionic conversion devices. The lifetime of experimental devices has been extended from a few hundred hours to over ten thousand hours. Overall efficiencies of over 14% have been measured on several devices at temperatures consistent with practical heat sources. Reproducibility of devices has been improved greatly as evidenced by the construction and testing of several large batches of similar converters. Hardware devices intended to fit the envelopes and restraints of specific systems have been designed and are now under development. Preliminary results achieved during these developmental efforts are quite promising. Although recent progress has been encouraging, much work remains to make thermionic conversion devices useful as real system components. Converter designs must be matched better to overall power system parameters and heat sources. The reproducibility of converters must be investigated in more detail to determine the present state of the art and to formulate requirements for improved processing techniques. More life-testing under well simulated environments must be done on hardware devices designed to fit system needs. This paper will review the progress achieved in experimental and hardware devices. The problems that were solved in achieving this progress will be discussed. A description will also be presented of the problems remaining to achieve practical thermionic hardware.