The power bus interconnecting the major segments of the solar cell arrays, which will have to carry several hundred thousand amperes, must be designed to minimize magnetic field interactions. This can be done by suitable arrangement of the power distribution circuits. High-voltage switching circuits will have to be developed to control sections of the solar cell array for maintenance and operational purposes, and to protect the solar cell arrays when they enter and leave the Earth's shadow. The system also must provide the capability of switching off all power by open-circuiting solar cells instantaneously in the event of system malfunction. Because the SSPS system provides no energy storage, it will be safer than conversion systems that rely on thermodynamic power. d. Effects of the Space Environment The state of the art of solar cells is now at a level where lifetimes of 10 years are achievable. For example, the effective life expectancy of the Intelsat IV satellite is eight years. But the operations of the solar cell arrays will be influenced by the space environment. One influencing factor in space will be solar radiation. Solar radiation damage will cause a logarithmic decay of sok.r-cell effectiveness. However, improvements in radiation-resistant solar cells are expected to result in a 30-year minimum operational lifetime for the SSPS, after which normal SSPS effectiveness can be restored by adding a small area of new solar cells. Thus, there will be no absolute time limit on elfective SSPS operation. Another aspect of the space environment that will influence SSPS operations is the impact of micrometeoroids. In synchronous orbit, the SSPS is expected to suffer a 1% loss of solar cells, based on the probability of damage-causing impacts by micrometeoroids during a 30-year period. The benign nature of the space environment and the absence of significant gravitational forces, however, permit the design of solar collector arrays which have a minimum material mass. In addition, their performance would be much more predictable than that of an Earth-based solar energy conversion device because of th. absence of the vagaries of the Earth environment. Microwave Power Generation, Transmission, and Rectification. — The power generated by the SSPS in synchronous orbit must be transmitted to a receiving antenna on the surface of the Earth and then rectified. The power must be in a form suitable for efficient transmission in large amounts across long distances with minimum losses and without affecting the ionosphere and atmosphere. The power flux densities received on Earth must also be at levels which will not produce undesirable environmental or biological effects. Finally, the power must be in a form that can be converted, transmitted, and rectified with very high efficiency by known devices. All these conditions can best be met by a beam link in the microwave part of the spectrum. In this part of the spectrum a desirable frequency can be selected, e.g., about 3.3 GHz, and induced radio frequency interference limited so that an appropriate internationally agreed upon frequency could be assigned to an SSPS.
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