with other thermal systems and concluded that the Rankine system has a lower mass and cost potential than the Brayton cycle and other alternative thermal engines investigated. Simultaneously, the MSFC-Rockwel1 (ref. 11) investigations included the Rankine cycle concept and performed sufficient design definition to determine cost and mass estimates. The configuration selected in the study was a cascaded cesium working fluid/steam bottoming cycle with an open absorber. This arrangement proved to have significantly improved overall efficiency and the lowest specific weight. The report confirmed the Rankine cycle as the solar thermal system offering the potential for least cost. Solar Thermionics - Thermionic converters provide a potential alternative to energy conversion provided by thermal engines or solar cells. These passive devices use high temperature thermal energy to produce direct current electrical energy. In some studies, the converters were analyzed in combination with other energy systems such as the thermionic/Brayton cycle and a nuclear concept requiring thermionic conversion. Both of these systems were reported in the MSFC-Boeing study (ref.5). The latter study also looked at direct and liquid cooled radiator systems using the thermionic conversion technique. Subsequent studies confined their investigations of thermionics to the singular conversion system. The general conclusion reached in these studies,as well as more recent evaluations by Boeing and Rockwell, is that the thermionic system would be at least 50 percent heavier than other thermal cycle systems. Conversion efficiencies are relatively low (about 20 percent) with peak temperature (emitter) of 3000°F and above, The thermionic system has, therefore, not been further considered in system definition studies. Other known options have not been included in detailed evaluation for the reasons stated below: (1) Thermoelectrics - low conversion efficiency, materials resources consumption, and heat rejection considerations. (2) Magnetoplasmadynamics - rejected on grounds of problems in attaining the necessary working fluid temperature by solar heating.
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