Table 5-5. Brayton SPS Mass Statement 2. The Brayton system affords an efficiency of 24.4% with a maximum temperature (turbine inlet) of 1652 K (2514°F) and a minimum gas temperature of 401 K (262°F). 3. The thermionic system has a specific mass of about 8.2 kg/kWe generated; the Brayton specific mass is about 5.0 kg/kWe generated. As a consequence all cascaded combinations of the two systems were seen to have a higher specific mass than the Brayton system alone. 5. 6 SOLAR SILICON PHOTOVOLTAIC (CONCEPT 5) Cell performance, radiation resistance, etc. are discussed in Section 4.7. A critical question is that of solar concentration ratio; if reflectors are used to augment cell performance, what should their area be relative to the cells? As solar concentration is increased, the cell area relative to that of the total SPS decreases, concentration raises the temperature of the cells so that they become less efficient, tending to increase the required area of the SPS (including reflectors). Relatively small cell areas means that thicker cover glass can be used for radiation, protection without a high mass penalty. Also relatively small cell areas (by high concentration ratios) tends towards lower total cell cost, but may be offset by increased concentrator and structure cost and mass. Because of the complexity of these interactions, an ISAIAH computer model was developed. The block diagram of this model is given in Figure 5-16. This chart shows the relationships between parametric elements which make up the model for photovoltaic system optimization. The following code is used: 5. 5 THERMIONIC BRAYTON CASCADE (CONCEPT 4) The original incentive for study of this system was as follows: l.A "topping" cycle using thermionics would allow the maximum temperatures in the SPS to occur only in the passive (non-moving) elements of the thermionic diodes. 2. After temperature reduction in the diodes, a Brayton cycle would follow, with the rotating parts consequently operating at a lower temperature. 3. The two systems combine to produce the required power. Study of the two systems, however, indicate the following factors: 1. The efficiency of the thermionic system was 24% with a maximum temperature of 1800 K (2780°F) and a collector temperature of 1000 K (1340°F).
RkJQdWJsaXNoZXIy MTU5NjU0Mg==