4) A simple solid radiator system is insufficient to reject the required 3.8 kW of heat from each converter. Hexagonally shaped, sodium-filled fin heat pipes with reinforced 1 mm-thick nickel casing are adequate. In order to reduce mass requirements, these heat pipes must be open to the atmosphere during satellite launch, and subsequently filled with sodium and sealed in orbit. The NiO heat-radiating surface per converter is 0.1 m~ in area. 5) MULTI-FOIL thermal insulation is excellent for shielding the electrical leads and heat pipes from the intracavity solar flux. The mass of such insulation is negligible with respect to the masses of other converter components. 6) The total mass per converter, comprised of the emitter and collector, the heat pipe radiator, thermal insulation, and copper leads is 6.48 kg, or 6.28 kg per kWe net electrical output power. Dominant, and approximately equal, contributions come from the heat pipe and the leads at 2.67 kg and 2.84 kg, respectively. Complete parallel, as well as series, converter electrical connections, to enhance operational reliability, would substantially increase lead mass. More extensive analyses may indicate that the 1-mm casing thickness of the heat pipe and, consequently, its mass can be reduced. It must be emphasized that the results of this brief study are intended to furnish sufficient data for evaluating the possibility of using thermionic energy conversion in Satellite Solar Power Stations. Additional analyses are necessary to provide detailed converter and heat pipe designs, determine interunit busbar configurations and masses, establish tradeoffs between operational reliability and busbar mass penalties, and identify constructional problems for on-earth or in-orbit converter, heat pipe, and panel assembly. REFERENCES 1 ) R. F. Honig and D. A. Kramer. Vapor Pressure Data for the Solid and Liquid Elements, RCA Laboratories Report PTR-2720, April 21, 1969. 2)G. N. Hatsopoulos and E. P. Gyftopoulos, Thermionic Energy Conversion-Vol. I: Processes and Devices. The MIT Press, Cambridge (1973). 3)E. M. Sparrow, L. V. Albers and E. R. G. Eckert, "Thermal Radiation Characteristics of Cylindrical Enclosures," ASME, Journal of Heat Transfer. 61-SA-23 (1961). 4)P. D. Dunn and D. A. Reay, Heat Pipes, Pergamon Press (1976). F. N. Huffman, Preliminary Design Review Document: Application of MULTI-FOIL Insulation to the Brayton Isotope Power System and Conceptual Design of MULTI-FOIL Insulation for the Flight System, P-399625, Thermo Electron Corporation, June 11, 1976. 5)D. L. Gregory, Private Communication, October 25, 1976. 6) D. L. Gregory, Space-Based Power Conversion and Power Relay Systems Study, Fourth Monthly Progress Report, NAS8-31628, Boeing Aerospace Company, November 14, 1975. 7)D. L. Gregory, Space-Based Power Conversion and Power Relay Systems: Preliminary Analysis of Alternate Systems, Final Performance Review, NAS-31628, Boeing Aerospace Company, April 14, 1976. 4.7 SOLAR CELLS 4.7.1 Requirements The challenges imposed on solar cells by the SPS concept include: l)The cells must be light, i.e., thin. Current practice in spacecraft arrays is the use of cells having a thickness of 250 gM to 500 /zM (10 mils to 20 mils). SPS studies have focused on cells of 100 /zM (4 mils), or thinner. 2) The cells must be radiation resistant. Electrons and low energy protons trapped by the Earth's magnetic field tend to cause a steady deterioration. High energy protons associated with solar flares occur aperiodically and may cause additional damage. Cell damage is manifested by a reduction in cell efficiency. Cell radiation protection is usually provided in front (the sun side) by cover glasses; backside protection is by the cell substrate (mounting system). Thin solar cells are usually less radiation resistant than thicker
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