sponsorship for various space power supply requirements, and under DOE/SERI sponsorship for high efficiency terrestial concentrator cell applications. This technology should be extended with specific orientation for the SPS, which includes the added specification of a limited substrate or encapsulant mass. Other advance concepts which should also be investigated include split-spectrum systems, thermophotovoltaic converters, and combined thermal and photovoltaic systems. There are a variety of solar thermal options that have been investigated as possible alternatives to photovoltaic conversion. The advantages of solar thermal systems include insensitivity to radiation effects, simpler power conditioning, and the potential for high efficiency. Concern has been expressed that the current overwhelming emphasis on the photovoltaic approach could lead to insufficient examination of promising solar thermal options. Energy conversion systems considered by the solar thermal working group headed by Abraham Hertzberg included Breyton, potassium Rankine, cesium/steam combined cycles, organic Rankine, and thermionic. Applicable concentrators include parabolic dishes, compound parabolic concentrators, faceted reflectors, cassegranian concentrators, low concentration planar reflectors, and inflated structures. Both heat pipe and tube-fin radiators were considered. It was concluded that all the possible concepts require substantial advances in technology in order for the goals set for SPS to be achieved. Because of this, none of the concepts has such low risk that it can be relied upon to the exclusion of the others. Extremely high reliability can be achieved with solar thermal systems by the use of frictionless bearings (gas bearings) for rotating equipment and a large number of redundant power generation modules.
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