Thermionic Conversion Devices Research Articles

Prediction of Thermionic Energy Conversion Performance and Parametric Effects Using Genetic Algorithms to Fit Physics-Inspired Model Equations to Prototype Test Data

Abstract Thermionic converters have potential as an energy conversion technology for high-temperature space and terrestrial applications using concentrated solar, nuclear reaction, and combustion processes as the heat source. Recent studies have generated experimental performance data for narrow-gap thermionic energy conversion devices. This investigation explores the use of genetic algorithms to fit existing data with physics-inspired model equations. The resulting model equations can be used for performance prediction for system design optimization or to explore parametric effects on performance. The model equations incorporate Richardson’s law for current density, including both the saturated and Boltzmann regimes, with appropriate relations for power delivered to the external load. The transition regime is characterized using two separate models, each accounting for nonuniformity in emission surfaces and irregularities in the manufacturing process. The trained models enable performance prediction of small-gap thermionic energy conversion devices. In this study, data were fitted for two different prototype designs. The prototype test data and postulated values for the work functions and a transition regime parameter are substituted into physics-inspired model equations, yielding performance models with three adjustable constants. Optimized values of these constants are determined using a genetic algorithm to best fit the experimentally determined performance data for prototype thermionic conversion devices tested in earlier studies. This approach is demonstrated to fit the performance data to within 9%. This methodology also allows the user to back-infer the effective work function values, which were found in this study to be consistent with independent measurement.

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.

Spectral normal emittance of single crystals.

The spec t ra l normal emittances of molybdenum, tantalum. and tungsten c rys ta l s were measured normal to low Miller index planes at 2000-F, 3000'F. and 4000-F in Vacuum. The levels of spectral normal emittance obtained in this investigation were in good agrgemen: with previously published values for polished surfaces of the metals investigated. One tantalum sample recrystall ized in a manner Such that the emittance normal to a (2yl ) and a (321) plane could bp measured a t essentially the same time. ments indicated that a very small dilference in emittance did exist at approximately 0. 5 microns with the (211) plane having a higher emittance. of the ( 2 1 1 ) plane to the (100) plane of molybdenum, the (110) plane to the ( I O O l plane of tungsten, and the (110) plane to the (211) and ( 3 2 1 ) planes of tantalum showed no significant differences between planes. This se t of measureComparison of the emittance The emittance tended to decrease with successive measurement cycles because the samples became more highly polished. This phenomenon may have obscured emittance differences between planes, but the agreement between measurements for diiferent * Presently employed at Lewis Research Center , NASA planes was remarkably good. than 10 per cent in the in3irarrd region and no l a r g e r than 4 per cen t in the visible region. The results of this investigation indicate that differences in polished surfaces have a grea ter e f fec t On the Spectral normal emittance of the re f rac tory metals than the c rys ta l lographx orientation of the emitting sur face . If differences did exist, they are believed to be no larger INTRODUCTION Work performed in the las t decade' , ' has established that the thermionic emission f rom tungsten and other re f rac tory metals varies considerably with the crystallographic orientation of the emitting surface. phenomenon to improve the operating characterist ics of thermionic conversion devices. Because the efficiency of diodes is affected by the thermal emittance of the cathode, It is important to know if the thermal emittance of materials is b function of crystallographic orientation. crystallographic orientation on the thermal emittance of tungsten, tantalum. and molybdenum by measuring the spec t ra l normal emittance oi low ~ i i l e r Index planes. Recent studies have been d i rec ted toward using this The objective of this program was to empirically de te rmine the effect of 1. M. K. Wilkinson. Crystallographic Variations of F ie ld Emiss ion From Tungsten, ' ' Journal of Applied physics, XXIV (September 1953), 1203-1209. 2. E. W. Muller, 'Work Function of Tungsten Single Crys ta l Planes Measured by the Field Emission Microscope, Journal of Applied Physics, XXVI (June 1955), 732-737. 3. Single-Crystal Thermionic. Converter Studies, AD 407826 (USAF Cambridge R e s e a r c h Laboratorics. 1963).

SNAP Radioisotopic Power Systems

Auxiliary power systems deriving their energy from radionuclide decay have been under development for more than five years under Atomic Energy Commission sponsorship. Called SNAP, the program has been oriented toward satisfying the need for small, compact, lightweight and reliable power supplies for space systems. Radioisotopic power relates to the generation of heat by absorption of electromagnetic and particulate radiations emitted by a carefully sealed source of radioisotopes. The heat is then partially converted into electricity by a thermoelectric or thermionic conversion device, with the remainder rejected to the external environment. It is the intent of this paper to present a status report on SNAP radioisotopic devices and to explore some of the unique aspects of the technical problems encountered. Since these power systems have no moving parts, they lend themselves appropriately to the performance of the following types of space missions: 1. Satellites operating on equatorial orbits where the continuous power demand exceeds the storage cycle capability. 2. Satellites operating for considerable periods of time in the Van Allen radiation belts where solar cell output is significantly degraded. 3. Lunar explorations, such as NASA's Surveyor program, where storage requirements during the lunar night become excessive. 4. Venus surveys where solar energy would be excluded by the atmosphere. 5. Space probes away from the sun where the decrease in solar flux becomes significant.