Fig. 4-15. Thermionic Efficiency Versus Emitter Temperature current status of thermionic conversion, it is projected that the plasma arc drop will be sufficiently reduced to lower the barrier index to 1.8 by the year 1985 and to 1.6 by 1995. Such reductions represent gains in efficiency to 21 percent and 24 percent, respectively. Since silicon solar cells, presently about 13 percent efficient, can theoretically attain only 22 percent efficiency, and practically may never exceed 18 percent, these projected gains for thermionic converters are quite significant in their potential application to the SPS. 4.6.2 Converter Characteristics The proper choice of operating conditions, materials, and design configuration is important in providing thermionic converters that are efficient, low in mass, and reliable for long periods of operation. High performance, stable output has, in fact, been maintained in a converter for over 40,000 hours, at which time, although the converter was still fully operational, the program was terminated. One mode of failure in a converter occurs when a monolayer of the emitter is evaporated onto a different material collector, thereby altering its properties. This evaporation limits the useful emitter materials and their operating temperatures unless similar materials are used for both emitter and collector. Although tungsten would provide the lowest vapor pressure emitter material, its availability is limited. Boeing has specified that Thermo Electron investigate the potential of molybdenum. If operated at 1800K (2780°F) for 30 years, molybdenum would dispense 0.6 mm (0.024 inch) of material onto the collector (Ref. 1). However, efficient radiation cooling of the collector requires a high collector temperature and, therefore, precludes the use of very low work function collector materials (to prohibit excessive electron back emission from collector to emitter). Consequently, molybdenum is found to be an attractive collector material as well, and emitter evaporation will therefore not affect the efficiency of the converter. To reduce weight and cost, nickel was chosen for the collector structure and radiator material. Deposition of molybdenum on the nickel electrode during converter operation will quickly produce a molybdenum coated collector for essentially the entire lifetime of the converter. Constant and uniform close spacing of the electrodes should be maintained because of the straight-line deposition from emitter to collector. Laboratory converters are usually fitted with cesium reservoirs to provide cesium vapor in order to reduce electrode surface work functions, as well as to provide space charge neutralization in the interelectrode region. The effect of cesium on molybdenum electrodes is shown in Figure 4-16 where T is the electrode temperature and Tr the reservoir temperature of cesium. Fig. 4-16. Molybdenum Work Function Plot Determination of the output characteristics of a converter is detailed in Ref. 2. Mathematically the efficiency, ri, is related to the output power density, P, and the heat supply rate, q, by where S is the active area of the planar emitter and collector. The output power density is a function of converter current, J, and voltage where V is the output voltage of the electrodes and Va the voltage drop across the electrical leads connecting the converter to the load. The heat supply rate can be expressed as
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