Fig. 2. Performance comparison of solar energy conversion candidates for SPS. Source: Reference 10. band-gap semiconductors; therefore, they can be used in systems utilizing concentrated solar energy. In addition, gallium arsenide solar cells are more resistant than silicon cells to radiation damage, thus promising a longer life as well as higher performance in the space environment. The mass of the solar cell arrays (Table 2) is the dominant component for both of the photovoltaic SPS reference systems. The scale of commitment of capital, material and labor to construct large-scale manufacturing facilities and to produce the solar cell arrays (7) for the SPS would have to be exceeded by several factors if an equivalent baseload power output were to be generated with photovoltaic energy conversion devices located on Earth. Gallium arsenide solar cells emerge as the most favorable (8), but when one compares the efficiency of several SPS design configurations (9) (Fig. 2), silicon solar cells are favored for near-term demonstration tests (10). 4. TECHNOLOGY OPTIONS FOR POWER TRANSMISSION TO EARTH To transmit the power generated in the SPS to Earth, there are two optional transmitting methods: • a microwave beam, or • a laser beam 4.1. Microwave power transmission system Free-space transmission of power by microwaves is not a new technology (11). In recent years, it has advanced rapidly and system efficiencies of 55%, including the
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