2 x 10^-8 and 0.85 x 10^-10 cm^2/day have been predicted, corresponding to useful lifetime of an optical surface of between one year and 200 years, with a 'best estimate' of ten years, which we use here." Consequently, the Boeing reflector design consists of three layers of aluminized plastic film, the first two layers of which are to be discarded after ten years each of use. It is felt that the area of reflector subsystem design, with particular emphasis on long-term degradation, is one of the more critical areas in the entire thermal engine concept, and very little knowledge has been accumulated in this regard to date. This data is urgently needed. Additionally, Boeing uses (based on a 32 ft^ facet experiment) a reduction in efficiency of 2% due to edge scalloping (to reduce surface waviness), 2% for gaps between facets, and 1% for pointing error and facet failures. These values are shown in Table IV-B-l-c-3. Without further experimentation it is difficult to argue with these numbers, except to say that facet failures alone could account for more than a 1% loss, hence the reflector should be overdesigned from an area standpoint to account for some preplanned number of facet failures (higher than 1%) over the 30-year lifetime of the system. A final point on the reflector is that much work is still required to determine (1) the range of control required for each facet due not only to relative movement between the solar vector and the main axis of the reflector, but also due to relative movement between the reflector and the absorber because of structural stiffness (or lack of), and (2) required pointing accuracy of the reflector. Another most critical technology area in the system design is the thermal absorber, due to the fact that it is the highest-temperature component of the system. Some technology is available for small space units and larger terrestrial units, but much work remains to be done. Design optimization of very large absorbers on the order of the SPS has just begun at Boeing, but analyses must be refined to include the combined effects on system efficiency of such factors as geometrical configuration, absorption qualities, surface radiation, etc. Until such data becomes available, the assumed Boeing overall thermal efficiency of 0.87 cannot be disputed. The technology of the Brayton thermal engine is the most extensive and well-developed of all components of the electrical conversion system. Literally tens of thousands of hours of operation have been achieved on entire Brayton systems as well as individual components, these systems including large terrestrial Brayton systems for the utilities industry. The choice of the Brayton cycle, it is felt, over the Rankine and other conversion methods for this application is a wise one. As discussed earlier, Rankine cycle technology trails that of the Brayton, and due to the ever-present corrosion problem in high-temperature liquid metal systems, inherent limitations constrain Rankine cycle systems for this application. Cycle efficiencies depend primarily on turbine inlet temperature and compressor/turbine efficiencies since other parameters such as heat rejection temperature and pressure ratio are dictated by weight and other considerations. The Boeing Brayton system baselines a 1900F
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