the output voltage of the cell as the remaining principal factor for improvement. The highest open-circuit voltage obtained has been 0.63 volts. A 0.7 volt output at open circuit should be possible, and simple theory suggests that it can be obtained by increasing the base doping level. However, increased doping decreases bulk resistivity, and according to Fischer and Pschunder, puts the photovoltaic process into the Auger regime where carrier lifetime is reduced, decreasing quantum yield. If the cell open circuit voltage can be raised to 0.664 volts from the 0.59 volts now obtained in the nonreflecting cells, and no other efficiencyinfluencing factors are degraded, then an 18 percent solar-cell efficiency is possible. Further improvement in silicon cells would require such phenomena as multiple charge-carrier generation by blue and violet photons, for which there is as yet no theoretical basis. 4.7.2.3 Radiation Although the geosynchronous orbit location of SPS is high enough to escape the intense particle flux of the lower belt, trapped particles (primarily electrons) are still present. Solar flare particles can penetrate the geomagnetic field. Approximately three flare cycles can occur in the 30 year life of an SPS. If all solar arrays were of the roll-up type, flare prediction could be used to indicate when roll-up was to be used. However, since all satellites would be affected almost simultaneously, this would not seem to be appropriate for SPS ("all the lights would go out"). Cell degradation versus time in orbit is shown for a range of cover thickness. By comparing the dashed and solid lines the effects of flares can be seen. Radiation resistance of solar cells from protons and electrons as a function of solar cell and coverglass thickness will be predicted up to 1990. In the last 9 years the solar cell degradation resistance has not changed, however the solar cell output at beginning-of-life has increased so that now more power is available from cells after irradiation than was available several years ago. 4.8 TURBOMACHINES AND ASSOCIATED HEAT EXCHANGERS 4.8.1 Brayton Cycle Principal elements of the closed Brayton cycle system are shown in Figure 4-32 in schematic form. The maximum gas temperature occurs just as the flow exits the heat exchanger tubing in the cavity heat absorber. The flow is then routed to Fig. 4-32. Closed Brayton Cycle Schematic
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