Leo G. Monford, Harmon Roberts and Tony E. Redding Urban Systems Project Office APPENDIX Section IV COMPARISON STUDY OF THERMAL ENGINE (BRAYTON CYCLE) AND PHOTOVOLTAIC SPS DESIGN CONCEPTS In an attempt to specifically identify differences and advantages between the Brayton cycle and photovoltaic methods of space-based solar energy conversion, an evaluation of existing conceptual designs was conducted and a comparability study was performed. The SPS comparability study data was compiled from reports and presentation material provided by Boeing and ADL/Raytheon/GAEC teams. A listing of these documents is included as references. The primary difference between energy conversion concepts are provided in summary form in Table IV-APP-1 under level 2.0. More detailed comparisons on a subsystem and component basis are provided. The primary thrust of the Raytheon studies is in the Microwave Power Transmission System (MPTS) which includes the transmitting antenna and ground- based rectenna for reception and rectification of the microwave power. Their concept incorporates solar cell energy conversion in the form of two large solar arrays with a centrally located power transmitting antenna (Figure IV-APP-1). Boeing uses four large faceted concentrators to focus solar energy into cavity absorbers at the focal points. The heat thus provided drives Brayton cycle engine-generators to produce electricity (Figure IV-APP-2). The Boeing effort is primarily devoted to these systems. The MPTS is essentially the same as Raytheon's with variations required by higher output power levels. Summary charts for both concepts are provided as Figures IV-APP-3 and IV-APP-4. These detail weight, efficiency, and power levels for subsystem elements as described in the reference documents and numbers as adjusted by JSC evaluators. The solar cell conversion system is characterized by low operating efficiency and low weight. Relatively, the Brayton cycle system operates with twice the efficiency and 3.5 times the weight. For about the same station size, the Boeing system produces 10 GW on the ground, compared to 5 GW for Raytheon. The primary adjustment made by JSC evaluations on the Raytheon system was an efficiency reduction from 13.3 to 10.4 for the solar array blankets. This
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