The cavity interior insulation selected was "MultiFoil," developed by the Thermo Electron Corporation, which consists of a number of layers of thin refractory metal foils spaced in vacuum by oxide particles. The oxide is selected on the basis of low thermal conductivity and foil compatibility. Multifoil provides a very high thermal impedance with minimum mass. 4.5 CONCENTRATOR/ABSORBER OPTIMIZATION An optimization was conducted to find the values of primary solar concentrator and cavity absorber parameters which would (in combination) yield the minimum total mass for a given power removed from cavity at a given temperature. The optimization was conducted with the ISAIAH (Integrated Sensitivity and Interactions Analysis, Heuristic) program using a model as depicted in Figure 4-13. Optimization results are given in Figure 4-14 fora concentrator area at 1.5 x 10^ m^ (1.61 x 10^ ft^). The power removed per unit mass is seen to decrease with increasing wall temperature. The optimum value for the geometric concentration ratio (GCR) increases with wall temperature. The GCR optimum is 2260 for the 1620K (2456° (2456°F) of the Brayton system and 2450 for the 1800K (2780°F) of the thermionic system. Not shown in Figure 4-14 are optimum values for the number of reflector facets per concentrator. For the Brayton system the number is 16,800; for the thermionic, 17,500. _ Fig. 4-14. Characteristics of Mass-Optimized Concentrator/Absorber Combinations 4.6 THERMIONICS 4.6.1 Background This section was provided by the Thermo Electron Corporation (TECO): Chronologically, improvements in thermionic energy conversion, as measured by the barrier index (collector work function plus inter-electrode plasma voltage losses), occurred first with the addition of small amounts of oxygen into the diodes, and subsequently with the use of tungsten oxide collectors. Currently, a number of other semi-conducting oxides are showing even better potential for thermionic loss reductions. In Figure 4-15, we see these losses correlated to thermionic efficiency and emitter temperature. For an emitter temperature of 1800K and a barrier index of 2.1, the thermal-to-electrical conversion efficiency is approximately 15 percent. Laboratory converters are being constructed that, for short generating periods, have demonstrated barrier indices as low as 1.9. From the historical development, and the Fig. 4-13. Model for Concentrator/Absorber Optimization The parameter to be optimized is number 19 "Power per kilogram'" The legend explains the types of parameters used; for example, parameter 17 is a table expressing the power loss per square meter of cavity wall as a function of wall temperature and the mass per square meter of insulation added to the wall. Parameter 14, the "power removed" is that which is available to do useful work. Parameter 1 is the total mass of the solar concentrator and the cavity absorber. Parameter 12, the reradiation per square meter of aperture area is based on an effective wall interior emissivity of 0.9. The solar concentrator performance determination uses the data shown in Figure 4-7. Parameters 13, 14, 16 and 18 were automatically varied to obtain minimum values of parameter 19 over a range of values of parameter 19.
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