SATELLITE POWER SYSTEM ENGINEERING AND ECONOMIC ANALYSIS SUMMARY National Aeronautics and Space Administration NASA TM X-73344 NOVEMBER 1976 NASA TECHNICA MEMORANDUM George C. Marshall Space Flight Center Marshall Space Flight Center. Alabama 35812 AC 205 453-0034
_______________ _______________________________________________ TECHNICAL REPORT STANDARD TITLE PAGE 1. REPORT NO. NASA TM X-73344 2. GOVERNMENT ACCESSION NO. 3. RECIPIENT'S CATALOG NO. 4 TITLE AND SUBTITLE Satellite Power Systems An Engineering and Economic Analysis Summary 5. REPORT DATE November 15, 1976 6. PERFORMING ORGANIZATION CODE 7. AUTHOR(S) Program Development 8. PERFORMING ORGANIZATION REPORT ft 9. PERFORMING ORGANIZATION NAME AND ADDRESS George C. Marshall Space Flight Center Marshall Space Flight Center, Alabama 35812 10. WORK UNIT NO. 1 1. CONTRACT OR GRANT NO. 13. TYPE OF REPORT & PERIOD COVERED Technical Memorandum 12 SPONSORING AGENCY NAME AND ADDRESS National Aeronautics and Space Administration Washington, D. C. 20546 14. SPONSORING AGENCY CODE 15. SUPPLEMENTARY NOTES Prepared by Program Development ABSTRACT A system engineering and economic analysis was conducted to establish typical reference baselines for the photovoltaic, solar thermal, and nuclear Satellite Power Systems. Tentative conclusions indicate that feasibility and economic viability are characteristic of the Satellite Power System and, therefore, deserve additional study. Many key issues exist but appear resolvable with the anticipated technological advances of the next decades. EDITOR* S NOTE Use of trade names or names of manufacturers in th official endorsement of such products or manufacturers, ei National Aeronautics and Space Administration or any other Government. SPS INFORMATION SYSUM i CATALOG f (CATEGORY-ITEM) 1 This copy may not be removed | from file room | is report does not constitute an ther expressed or implied, by the agency of the United States 17, KEV WORDS 18. DISTRIBUTION STATEMENT Unclassified-Unlimited 19. SECURITY CLASSIF. (of this report! Unclassified 20. SECURITY CLASSIF. (of this page) Unclassified 21. NO. OF PAGES 280 22. PRICE NTIS MSFC - Form 3 2 9 2 (Rev December 1 9 7 2 ) For sale by National Technical Information Service, Springfield, Virginia 22151
TABLE OF CONTENTS 1.0 INTRODUCTION...................................................................................... 1-1 1.1 Background..................................................................................... 1-1 1.2 Potential Systems......................................................................... 1-3 1.3 Past Work........................................................................................ 1-6 2.0 STUDY OBJECTIVES AND GUIDELINES....................................... 2-1 3.0 DEFINITION OF SPS REQUIREMENTS.......................................... 3-1 4.0 DETERMINATION OF SPS PROGRAM ELEMENTS.................. 4-1 4.1 Satellite Power Station................................................................ 4-1 4.2 Ground Receiving and Distribution Site................................ 4-1 4.3 Manufacturing, Construction, and Maintenance Operations.............................................................................. 4-1 4.4 Space Transportation................................................................... 4-1 5.0 DEFINITION OF SPS CONCEPTS..................................................... 5-1 5.1 Solar Photovoltaic........................................................................ 5-1 5.2 Solar Thermal Concentrator................................................... 5-4 5.3 Nuclear............................................................................................ 5-7 5.4 Concept Comparison.................................................................. 5-7 6. 0 DEVELOPMENT OF SPS TRADEOFF AND EVALUATION CRITERIA AND SIMULATION PROGRAMS................................... 6-1 6.1 SPS Tradeoff and Evaluation Criteria................................... 6-1 6.2 Simulation Programs.................................................................. 6-1 7.0 SATELLITE POWER STATION ........................................................ 7-1 7.1 Photovoltaic Power Conversion System................................ 7-1 7.2 Thermal Conversion System.................................................... 7-68 7.3 Microwave Power System.......................................................... 7-85
8.0 GROUND RECEIVING AND DISTRIBUTION SITE........................ 8-1 8.1 Requirements and Analyses....................................................... 8-1 8.2 Receiving Antenna System ....................................................... 8-2 8.3 Power Conditioning, Distribution, and Utility Interfaces.............................................................................. 8-8 8.4 System Safety................................................................................. 8-9 8.5 Required Technology Advancements..................................... 8-16 9.0 RESOURCE ANALYSIS AND FUEL CONSUMPTION.................. 9-1 9.1 Ground Operations........................................................................ 9-1 9.2 Transportation Operations...................................................... 9-5 9.3 Manufacturing, Construction, and Maintenance Operations.............................................................................. 9-16 10.0 POWER MANAGEMENT....................................................................... 10-1 10.1 Requirements and Analysis..................................................... 10-1 10.2 Management Concepts and Functions.................................... 10-1 10.3 Control Model.............................................................................. 10-2 11.0 SUMMARY BASELINE DEFINITION................................................ 11-1 11.1 Photovoltaic................................................................................. 11-1 11.2 Solar Thermal.............................................................................. 11-3 11.3 Nuclear.......................................................................................... 11-3 12.0 SPS SUPPORTING PROGRAMS ........................................................ 12-1 12.1 Space Construction Base (Space Station)............................. 12-1 12.2 Transportation........................................................................... 12-5 13.0 ENVIRONMENTAL EFFECTS ASSESSMENT ................................. 13-1
14.0 PROGRAM COST AND ECONOMICS................................................ 14-1 14.1 Introduction................................................................................ 14-1 14.2 Work Breakdown Structure.................................................... 14-1 14.3 Ground Rules and Assumptions............................................. 14-3 14.4 Methodology................................................................................ 14-4 14.5 Cost Estimates........................................................................... 14-5 14.6 Economics................................................................................... 14-13 15.0 PROGRAM PLAN, TECHNOLOGY ADVANCEMENT PLAN . . 15-1 15.1 Program Plan............................................................................... 15-1 15.2 Technology Development Plan............................................... 15-3 16.0 CONCLUSIONS......................................................................................... 16-1 17.0 REFERENCES ........................................................................................ 17-1 APPENDIX A — SPACE-BASED SOLAR POWER CONVERSION AND DELIVERY SYSTEMS STUDY. A-l APPENDIX B - SYSTEMS DEFINITION - SPACE BASED POWER CONVERSION SYSTEMS.................... B-l APPENDIX C - APPLICATION OF STATION-KEPT ARRAY CONCEPTS TO SATELLITE SOLAR POWER STATION DESIGN................................ C-l APPENDIX D - FEASIBILITY STUDY OF THE SATELLITE POWER SYSTEM CONCEPT .......................... D-l
LIST OF ILLUSTRATIONS Figure Title Page 1-1. Solar power satellites ................................................................. 1-3 4-1. SPS program elements................................................................. 4-2 5-1. Solar photovoltaic SPS configuration....................................... 5-2 5-2. Solar photovoltaic SPS configuration options........................ 5-5 5-3. Solar thermal concentrator concept assembly..................... 5-6 5-4. Solar concentrator concept evolution....................................... 5-9 5-5. Nuclear thermal SPS configuration.......................................... 5-10 5-6. Nuclear Brayton cycle SPS schematic.................................... 5-11 6-1. Systems design analysis model ............................................... 6-3 6-2. SPS transportation model........................................................... 6-5 7-1. Electrical efficiency chain ........................................................ 7-2 7-2. Concentration ratio trades........................................................... 7-3 7-3. Trough configuration with concentrator radiators............... 7-6 7-4. Reflector/solar cell configuration options........................... 7-7 7-5. Solar cell description.................................................................... 7-10 7-6. Solar array structural configuration....................................... 7-16 7-7. Comparison of rectangular and elliptical planform geometry.......................................................................................... 7-17
7-8. Effect of shear stabilization cable size on bending stiffness...................................................................... 7-18 7-9. Effect of solar concentration ratio on structural cross section geometry..................................................... 7-20 7-10. Effect of structural geometry and longeron area on structural stiffness......................................... 7-21 7-11. Euler buckling loads versus longeron lengths...................... 7-22 7-12. Euler buckling loads versus beam length for selected heights................................................................................................ 7-23 7-13. Euler buckling loads versus beam truss length for selected heights........................................................ 7-23 7-14. Longitudinal girder load distribution for orbit transfer and attitude control thrust................................... 7-24 7-15. Electrical power distribution..................................................... 7-26 7-16. Individual solar array and conductor masses versus distribution losses.................................................. 7-27 7-17. Solar array plus conductor mass............................................ 7-27 7-18. Power distribution voltage trade ........................................ 7-29 7-19. SPS longitude shift due to perturbations .............................. 7-34 7-20. Electric propulsion performance and requirements .... 7-37 7-21. Relative SPS total mass versus LEO to GEO orbital transfer time........................................................... 7-38
7-22. Total initial SPS system acceleration due to orbital transfer thruster firings...................................... 7-40 7-23. Boeing 100 cm ion thruster single cathode design concept...................................................................... 7-46 7-24. Boeing 10 cm MPD thruster design concept......................... 7-47 7-25. JPL MPD thruster design concept............................................. 7-48 7-26. Electric thruster module locations.......................................... 7-49 7-27. Solar photovoltaic SPS attitude control coordinates .... 7-52 7-28. Photovoltaic concept operation attitudes.................................. 7-53 7-29. Seasonal peak gravity gradient torques.................................. 7-55 7-30. SPS study orientation comparison............................................. 7-58 7-31. Population growth of Earth satellites ..................................... 7-62 7-32. Potential collision frequency...................................................... 7-63 7-33. Optimum number of turbomachines per absorber................ 7-70 7-34. Thermionic-Brayton solar thermal concept with light pipe selective surface absorber........................ 7-72 7-35. SPS radiator location trade, tube-fin construction............. 7-76 7-36. Radiator sizing sensitivity for option 1 location ................ 7-78 7-37. Power distribution efficiency chain, thermionic- Bray ton ...................................................................... 7-80
7-38. Solar concentrator SPS rotating machinery momentum effects on uncontrolled attitude........................... 7-83 7-39.9 dB, 1 km transmit antenna..................................................... 7-86 7-40. Structural module and assembly buildup from common structural element.................................................. 7-89 7-41. Structural system thermal environment .............................. 7-91 7-42. Structural temperature versus waste heat............................. 7-93 7-43. Waste heat energy profile........................................................... 7-93 7-44. Cool-down rate in Earth1 s shadow.......................................... 7-94 7-45. Klystron and amplitron vacuum devices ............................. 7-96 7-46(a). Plot of rj/l-i) for efficiencies ranging from 8-0 to 90 percent ..................................................... 7-98 7-46(b). Klystron differential efficiency relative to an 85 percent efficient amplitron.................................................. 7-98 7-47. Microwave antenna subarray..................................................... 7-99 7-48. Microwave power transmission system phase front control concepts..................................................... 7-101 7-49. Microwave power transmission system phase front control concepts..................................................... 7-102 7-50. SPS microwave power transmission system........................ 7-104
8-1. SPS ground system concept........................................................ 8-3 8-2. 9 dB system rectenna.................................................................... 8-5 8-3. Solar power systems, 9 dB RF ground distribution pattern (1 km transmit antenna) ..................... 8-6 9-1. United States resources compared to projected demand through the year 2000 .......................... 9-4 9-2. LEO assembly and operations and maintenance of the SPS with one on-line per year for 30 years............ 9-9 9-3. GEO assembly and operations and maintenance of the SPS with one on-line per year for 30 years... 9-10 9-4. HLLV operational flow diagram................................................ 9-13 9-5. Shuttle operational flow diagram ............................................. 9-14 9-6. OTV operational flow diagrams................................................ 9-15 9-7. SPS construction schedule, one per year for 30 years . . 9-18 9-8. Array construction jig sequence ............................................. 9-19 9-9. SPS assembly................................................................................... 9-20 9-10. SPS assembly timeline................................................................. 9-21 9-11. Fabrication module design concept.......................................... 9-24 9-12. SPS assembly crew....................................................................... 9-26
10-1. SPS power management structure............................................. 10-3 10-2. Approach to power management................................................ 10-4 12-1. Typical pre-space station scenario, 1980 through 1984. . 12-2 12-2. Typical space station program option.................................... 12-3 12-3. Typical space station program option.................................... 12-4 12-4. Space station beam manufacturing .......................................... 12-5 12-5. Two-stage ballistic HLLV............................................................ 12-7 12-6. Ballistic single stage concept ................................................... 12-9 12-7. HLLV cost per flight comparison............................................ 12-10 12-8. Baseline shuttle configuration................................................... 12-12 12-9. Shuttle with liquid booster ......................................................... 12-13 12-10. Shuttle with liquid booster and reusable external tank ............................................................................ 12-13 12-11. Liquid rocket booster engine candidates ............................... 12-14 12-12. Candidate electric thrusters...................................................... 12-15 12-13. Electrically powered concepts comparison............................ 12-17 12-14. Independently powered orbit transfer systems comparison........................................................................................ 12-18 12-15. All-propulsive personnel and critical logistics orbital transfer vehicle............................................................................... 12-19
12-16. Typical aeromaneuvering orbital transfer vehicle concept...................................................................... 12-20 13-1. SPS environmental effects........................................................... 13-3 14-1. Work breakdown structure........................................................... 14-2 14-2. Generation costs............................................................................. 14-6 14-3. SPS average unit life cycle cost................................................ 14-9 14-4. Effects of solar blanket cost on SPS economics ................... 14-10 14-5. Effects of launch to LEO cost on SPS economics................ 14-11 14-6. Effects of structure cost on SPS economics......................... 14-12 14-7. Contribution of SPS to United States installed electrical capacity...................................................................... 14-14 14-8. Energy price rise relative to general prices......................... 14-15 14-9. SPS average unit generation cost............................................. 14-20 14-10. Year 1995 generation cost comparison ................................. 14-22 15-1. SPS feasibility demonstration facilities....................................... 15-5 A-l. Artist* s concept of a 5000 MW SPS................................................. A-2 A-2. Economic comparison of a 5000 MW SPS operating over the period 1995-2025 with terrestrial fossil fuel plants................................................................................................ A-3
A-3. Cumulative distribution function of net present value of an SPS unit at the initial operation date as a function of price of power at the rectenna busbar...... A-6 D-l. Reference SPS configuration...................................................... D-2 D-2. Study schedule.................................................................................... D-4
LI ST OF TABLES Table Title Page 1-1. SPS Program Schedule................................................................. 1-7 5-1. Summary Characteristics of the Photovoltaic System . . . 5-3 5-2. Summary Characteristics of the Solar Thermal Concentrator SPS..................................................... 5-8 5-3. Summary of Reference Baselines............................................. 5-12 7-1. Efficiencies of Present Solar Cells.......................................... 7-5 7-2. Solar Array Characteristics...................................................... 7-9 7-3. Key Issues for SPS Power Generation and Distribution . . 7-14 7-4. Solar Array Structural Configuration Characteristics and Requirements.................................................. 7-17 7-5. Conductor Mass and Loss............................................................ 7-31 7-6. Synchronous Orbit Perturbations............................................. 7-33 7-7. Electric Thruster Considerations............................................. 7-44 7-8. Photovoltaic Solar Power Satellite Fuel Consumptions (Z-Axis Solar Orientation)................................ 7-56 7-9. Photovoltaic SPS Orientation Comparison............................... 7-57 7-10. Electric Thruster Considerations for Photovoltaic SPS Attitude Control and Station Keeping............... 7-60 7-11. Damage Rates from Collisions................................................... 7-64 7-12. SPS Maintenance............................................................................. 7-65
7-13. SPS Crew for GEO Operations and Maintenance ............... 7-66 7-14. Summary Characteristics of Solar Thermal Concept with Spherical Reflector Facets and Thermionic- Brayton Conversion System................................ 7-74 7-15. Microwave Power System Efficiencies .................................. 7-85 7-16. Characteristics of 9 dB Microwave System............................ 7-88 8-1. Overview of Manned Participation ........................................... 8-11 8-2. Estimated Dose on Board Space Station (rem) ................... 8-14 8-3. Suggested Exposure Limits (rem) ........................................... 8-14 9-1. Material Requirement and Production..................................... 9-1 9-2. Energy Consumption for Production of SPS............................ 9-3 9-3. Description of Vehicles.................................................................. 9-8 9-4. Depot and Space Station Capacities for Photovoltaic SPS ............................................................................ 9-11 9-5. Mass Flow through Launch Sight for One Photovoltaic SPS............................................................................... 9-12 9-6. SPS Assembly Equipment and Facilities................................ 9-23 10-1. Program Requirements and Management Criteria............ 10-2 12-1. Performance Comparison of Aeromaneuvering Versus all Propulsive Orbit Transfer Vehicles......... 12-20 13-1. First Priority Environmental Effects................................... 13-2
14-1. SPS Average Unit Life Cycle Cost (Millions of 1976 Dollars).......................................................... 14-7 14-2. Design, Development, Test, and Engineering (Millions of 1976 Dollars) ................................... 14-12 14-3. Interest Rate...................................................................................... 14-16 14-4. SPS Average Unit Generation Cost (1976 mill/kWh) . . . 14-19 15-1. Development/Verification Program Outline......................... 15-2
FOREWORD The George C. Marshall Space Flight Center of the National Aeronautics and Space Administration in Huntsville, Alabama, for the past 5 years has investigated the applications of space technology and of space itself to the solution of specific terrestrial energy problems. Great progress has been made in this brief period. Our pioneer work in microwave power transmission and in solar heating and cooling of residential and commercial buildings has lead to further development and progress in these areas. As a natural extension, the collection of solar energy in space using a Satellite Power System, first postulated by Dr. P. E. Glaser, has also been under careful study and review at this Center. Substantial in-house efforts combined with numerous industrial contracts succeeded in defining critical program elements, overall system and subsystem requirements, and necessary technology advancement requirements. The main thrust of MSFC's efforts is in the overall systems engineering and integration of Satellite Power Systems including supporting systems such as transportation, space construction base, and large space structures. This summary report presents our findings to date pertaining to the unprecedented number of systems, subsystems, and operational elements with complex interrelations. The many elements of the program were broken down into manageable entities and their most sensitive parameters were defined. Numerous tradeoffs between options proceeded through increasing levels of depth in order to clearly show all areas that need concentrated technology advancement efforts. A carefully structured cost and economic analysis was carried out concurrently with the generation of technical data. With the conservative assumptions concerning cost growth and findings of these efforts, construction of the SPS could be technically and economically possible toward the end of this century.
TECHNICAL MEMORANDUM X-73344 SATELLITE POWER SYSTEMS AN ENGINEERING AND ECONOMIC ANALYSIS SUMMARY 1.0 INTRODUCTION 1.1 BACKGROUND Rapidly increasing rates of consumption of the Earth's available fossil and nuclear fuel stores are characteristic of this latter half of the 20th century. Global population is increasing, as is that fraction of the population which forms the energy consuming ''middle class." This is true not only in the United States, Russia, Japan, etc., but also in what are termed emerging nations. Asa consequence, we may expect existing global energy sources to last only to the following approximate dates: oil, 1995 to 2005; coal, 2030 to 2080; and uranium (without breeder reactors), 2020 to 2050. As these energy sources are consumed, four additional factors emerge. First, their cost steadily increases as remaining quantities become more difficult to obtain (e. g., coal veins become thinner). Second, their consumption releases additional pollutants to the biosphere (e.g., CO2 removed from the atmosphere over thousands of years by plants, which formed coal, is now being returned). Third, since energy sources are geographically concentrated (e.g., most coal reserves are in the United States, and most oil resources are in the Middle East), a potential for great international tension and possibly war may be created as reserves dwindle. Fourth, nuclear fission involves byproduct materials that may be used for weapon production by either governments or outlaws. Thus, some attention is now turning to ''renewable" or ''nondepletable" energy sources. Primary candidates for electric power appear to be nuclear breeder reactors, nuclear fusion, and solar energy. These are characterized by varying degrees of complexity, technical risk, pollution, cost, etc. Each could reduce our dependence on imports and, if adopted by other nations, serve to reduce international tensions. Solar power may be used directly for heating and cooling; it may also be used for the production of electricity. Primary concepts for electric power production on Earth are photovoltaic (solar cell arrays or ''farms") and
the thermal engine ''tower top." In the tower top concept a field of steerable mirrors (heliostats) focuses energy onto a tower-mounted heat absorber. This heat can provide steam or some other fluid to turn turbogenerators. Solar power plants on Earth suffer from the diffuse nature of solar radiation (insolation), reduction in insolation from clouds, haze, etc., the varying angle of the Sun's rays, and, of course, nightfall. A power plant located in space can receive nearly direct, unfiltered sunshine almost without interruption. For a given reception area, a space system will receive six times more energy per year than will the areas receiving the most sunlight on Earth and approximately 15 times more energy than a United States location with average weather. In geosynchronous orbit 35 786 km above the Equator, a satellite has an orbital period of 24 h and remains in constant line of sight to stations on the ground. Solar power satellites in such orbits would generate electric power that would be converted to microwaves and beamed to receiving stations for distribution to consumers as conventional electric power. Figure 1-1 shows how receiving stations in various parts of the United States could be associated with a number of satellites in orbit. Thus, satellite systems can provide high availability ''baseload" power without the energy storage or backup facilities that greatly impact the cost and operational flexibility of terrestrial solar power stations. Space also offers these advantages: • Thermal pollution from the power generation process is released in space rather than to the biosphere. • The low gravity potential permits low-mass construction of the large areas necessary to intercept the solar energy. Consequently, the total amount of resources used is less than that for ground solar stations. • There is no oxidation or corrosion. • There are no tidal waves, earthquakes, etc. • The satellite systems are far removed from demonstrators, terrorists, etc.
Figure 1-1. Solar power satellites. 1. 2 POTENTIAL SYSTEMS 1. 2.1 POWER TRANSMISSION All solar power satellites will require a microwave power transmission system (MPTS). A circular transmitter that is nominally 1 km in diameter would mount the ''tubes" which convert electric current to microwaves. This ''phased array" uses a combination of mechanical and electronic steering to direct the beam to the receiving station. Phase control is implemented by a pilot beam transmitted from the receiver. Safety interlocking could be used, whereby turning off the pilot causes the power beam to lose coherence so that it is harmlessly dispersed. The receiving antenna has a nominal width of approximately 8 km. It consists of an array of small dipole antennas with integral power rectifiers and filters. The output is processed as required for compatibility with local power grids. It may prove possible to conduct farming beneath these antennas, since only a fraction of the sunlight would be intercepted.
1. 2. 2 PHOTOVOLTAIC POWER GENERATION In this potential system solar cell arrays are connected in series and in parallel as required to provide the high voltage necessary for the transmitter. Several types of solar cells may be considered as candidates: silicon (used on current spacecraft), gallium arsenide heterojunction, and several thin film types such as cadmium sulphide. Solar cells require several advances to provide maximum benefit for power satellite use. These advances are increased efficiency of conversion of sunlight to electricity, lighter weight (i. e., thinner), increased resistance to space radiation, and, of great significance, lower cost. One approach to cost reduction is the use of solar concentrators. Mirrors of thin metal or metallized plastic film would shine additional solar energy onto the cells. This causes increased cell output, so that fewer cells per satellite are required. Concentration ratio selection must, however, take into account the heating of the cells which tends to occur, since heating reduces efficiency. Cell cooling, possibly with metal fins, is an additional possibility. A lightweight structure of limited flexibility is required to support the cells. This structure will probably also be called upon to act as a power distribution system, i. e., carry electric power from the cells to the transmitter. An attitude control system is also required to align the solar arrays to the sunlight, providing a ''base" from which to point the transmitter antenna. Since the arrays face the Sun and the transmitter must face the Earth, a rotating joint must be provided. The electric power must cross this joint on its way to the transmitter. 1. 2. 3 THERMAL ENGINE POWER GENERATION In this concept reflecting mirrors, probably in the form of a paraboloid, focus solar energy into a cavity absorber, i. e., the rays enter a hole in a sphere. This insulated sphere contains a heat exchanger assembly composed of an array of tubes. Gas (probably helium) flowing through these tubes picks up thermal energy. The gas flow expands through turbines which turn the electric generators. The turbines also turn compressors which route the gas through the remainder of the system and back to the cavity absorber to collect more energy.
This system is referred to as the ''closed Brayton cycle" and has been demonstrated for Earth-based use in sizes up to 50 MW. In a power satellite a number of engines (e. g., 50) may be used to promote redundancy. The major moving assembly of the engines, the turbocompressor, is supported on gas bearings for long life. The second law of thermodynamics permits*useful work to be removed only from a heat engine which has a temperature differential across it. Thus, a cooling system is required in the form of a radiator to reject heat to space. This cooling system and a heat exchanger (called a recuperator) are also part of the gas circuit. The radiator utilizes pumps which circulate a liquid metal that has picked up heat from the helium flow. The liquid metal passes through panels composed of tubes and fins that dissipate the heat. Meteoroid puncture of these tubes is a potential problem; however, it is possible to align the panels ''edge on" to the prevalent meteoroid direction so that the puncture rate is acceptable. An alternative radiator panel would be one composed of heat pipes in which each has its own inventory of metallic working fluid. As with the photovoltaic concept, structural and attitude control systems are required, along with the transmitter and its rotary joint. 1. 2. 4 ADDITIONAL POWER GENERATION ALTERNATIVES The thermionic converter is a potential alternative to thermal engines or solar cells. These passive devices use high temperature thermal energy to produce direct current electricity. High solar concentration ratios (over 1000) are required to achieve the necessary temperatures. Because of the high temperatures, the necessary cooling can be accomplished by fins attached to the thermionic diodes. Finally, instead of a solar energy source, it may be possible to use nuclear reactors. These could energize turbomachines to produce electric power for the microwave transmitter. Breeder reactors could be used to extend our uranium resources; ''bomb grade" fuels bred in these reactors would remain in geosynchronous orbit. 1. 2. 5 AUXILIARY SYSTEMS The space transportation of power satellites is generally considered to take place in two stages. Large reusable ''space freighters" would be used to reach low Earth orbit. Another orbit transfer system would be used on the way to geosynchronous orbit.
1.3 PAST WORK The possibility of satellite power systems (SPS) as a potential new source of terrestrial energy was first postulated, analyzed, and published by Dr. P. E. Glaser in 1968 and patented in 1973 [1-3]. The George C. Marshall Space Flight Center (MSFC) followed the evolution of Dr. Glaser's concept with great interest and proposed a major systems definition study in 1971. Consequently, MSFC, upon request by the NASA Office of Applications (OA), developed a ''Solar Power Utilization Plan" in early 1972, which was followed by a ''Twenty-Year Solar Power Development Plan" in the same year. Major systems studies were begun in 1974 and 1975, both under contract to industry and in-house by NASA [4-19]. This document is a status report of recent MSFC in-house study activities. These study efforts are a part of and follow an overall SPS program schedule, as shown in Table 1-1.
TABLE 1-1. SPS PROGRAM SCHEDULE
2.0 STUDY OBJECTIVES AND GUIDELINES The satellite power system is envisioned as a supplemental source of energy for terrestrial applications late in the 20th century. This study has postulated program options and spacecraft designs that can lead to contributions of 10 to 30 percent of the total electric energy needs of the United States beginning in the 1990's. While considering the application of the SPS for the continental United States, some consideration has been given also to how the SPS migh* be utilized as an exportable resource. Future studies should explore more detailed implications of the SPS as a national exportable resource. Early study planning indicated a need to establish reference baseline designs to provide appropriate departure points for system sensitivity studies. These reference baselines included the photovoltaic, solar thermal, and nuclear concepts with major study emphasis being directed to the photovoltaic and solar thermal conversion concepts. The summary guidelines for the study were: • 10 GW power output at rectenna/utility power interface* • 20 mW/cm2 maximum power density at rectenna center • 30 year lifetime for system operation with a reasonable repair/ refurbishment/maintenance philosophy • One additional SPS brought into operation each year* • Each satellite to be assembled in low Earth orbit and transferred to geostationary orbit* • Assumption that a space station will be available in low Earth orbit to support assembly of the SPS and that some form of space station will be available in geostationary orbit to support operations and maintenance. After the preliminary study investigation, trade studies suggested that some of the initial guidelines (denoted by asterisks) be considered for change: • Basic design parameters indicated that 10 GW satellites would nominally have two or more microwave antennas, with some trade studies indicating that smaller satellites with one microwave antenna might be more economical.
• One satellite per year may not be commensurate with developing a large contribution to energy needs late in the century. • Although the initial engineering position is based on fabrication and assembly in low Earth orbit, a better understanding of fabrication and assembly in geostationary orbit will be most desirable.
3.0 DEFINITION OF SPS REQUIREMENTS The concept of a system of space-based solar power satellites does not yield or derive its being from a conventional requirement or set of requirements. In general, the SPS is proposed as a supplemental source of electrical energy for terrestrial applications. The need is evident, but broad acceptance of SPS to satisfy the need involves the resolution of many complex issues such as economic competitiveness and environmental concerns. The total commitment to SPS is dependent upon a long term ''learn as you go" process leading to a potential application during the last decade of this century.
4.0 DETERMINATION OF SPS PROGRAM ELEMENTS An SPS responsive to the overall system requirements encompasses a large number of interrelated program elements and subelements. An approximate total of 600 of these have been recognized to date. Each one has to be considered while defining candidate SPS concepts, and each one is a part of the engineering and economic analyses. The major SPS elements discussed in the following paragraphs form the building blocks of the overall study plan and are summarized in Figure 4-1. 4.1 SATELLITE POWER STATION This element encompasses the power conversion options being studied. In addition to the photovoltaic and thermal solar energy conversion options, contractor studies are being performed on nuclear energy conversion systems (Appendix B). Each conversion option uses a rather similar microwave power conversion and transmission system as part of the satellite power station. 4. 2 GROUND RECEIVING AND DISTRIBUTION SITE This element includes the receiving antenna for the microwave beam, the utility interface with the related electric ground distribution system, the safety system related to microwave exposure protection and to the safety of maintenance and service activities, and the maintenance and service system. 4.3 MANUFACTURING, CONSTRUCTION, AND MAINTENANCE OPERATIONS This element includes ground and orbital operations and their respective systems that support the required manufacturing, construction, assembly, and maintenance activities. A special operations management activity ties together equipment and manned operations and transportation and logistics requirements. 4.4 SPACE TRANSPORTATION This element consists of five transportation systems necessary to provide operational satellite power systems: the heavy lift launch vehicle (HLLV), the personnel launch vehicle (PLV), the cargo orbital transfer vehicle (COTV) system, the personnel orbital transfer vehicle (POTV) system, and local space transportation vehicle (LSTV) systems.
Figure 4-1. SPS program elements.
5.0 DEFINITION OF SPS CONCEPTS 5.1 SOLAR PHOTOVOLTAIC The initial photovoltaic concept developed for study of the SPS is illustrated in Figure 5-1. Overall dimensions for this configuration, designed to supply 10 GW of power to the utility interface, are a maximum length of 21.05 km, a width of 9. 53 km, and a structural depth of 0. 215 km. The antenna is located at the center of the configuration to minimize distribution losses. With the selected 20 kV de distribution voltage, these losses are a greater mass factor than the nonmetallic carry-through structure at the center of the antenna. The significant mass influence of power distribution at 20 kV can be effectively eliminated by distributing power at 40 kV. The nonmetallic structure could possibly be eliminated by using the power distribution mast as the central load carrying structure. The solar array of Figure 5-1 is sized for a perpendicular attitude with respect to the Sun. This minimizes the solar array area and, in turn, the mass of the array. However, the mass of the attitude control propellant, the complexity of the rotary joint to correct for the rotation (approximately 23°) of the antenna with respect to the rectenna, and the complexity of the attitude control system to maintain solar pointing are increased. Early study results indicated that comparison of the solar perpendicular attitude to a perpendicular to orbit plane attitude results in comparable overall system masses; however, the lower complexity of the perpendicular to orbit plane attitude and later study mass estimates suggest a trend toward this attitude for future configurations. The solar array consists of trapezoidal shaped modules that are 493 m square at the top and 215 m deep. The sides of the trapezoid are typically aluminized film reflectors that provide a concentration ratio of two for the solar cells located at the base of the trapezoid. The solar cells are passively cooled. The planform shape of the photovoltaic configuration is elliptical. This shape is an efficient structural configuration with regard to the distribution of large quantities of current to the geometric center and, when analyzed for attitude control propellant consumption, results in a slightly lower mass system in comparison to an optimized rectangular configuration, because the elliptic shape is widest at the center where the structural bending moment is greatest. Pertinent descriptive data for this configuration are shown in Table 5-1 for a two antenna configuration.
Figure 5-1. Solar photovoltaic SPS configuration.
TABLE 5-1. SUMMARY CHARACTERISTICS OF THE PHOTOVOLTAIC SYSTEM
The solar array area is 88 km2, the antenna diameter is 1 km, and the system mass is approximately 117 x 106 kg, which includes a contingency of 30 percent. Alternate photovoltaic configuration concepts are shown in Figure 5-2. These concepts feature two antennas, one on each side of the SPS or at each end, and in each option the antennas are nominally 5 GW each. A side location minimizes distribution losses but causes an attitude control propellant increase, because this offsets the advantage of having the major axis of the elliptical shaped SPS perpendicular to the orbit plane for minimum attitude control propellant. Two antennas, when used to transmit 5 GW each of power to the ground, will reduce the power density to one-half as compared to transmission of this power by one antenna of the same size. This is potentially a desirable configuration change, since transmission of 10 GW of power with a 1 km diameter antenna from a geosynchronous Earth orbit at a frequency of 2. 45 GHz results in a peak power density at the center of the rectenna above 20 mW/cm2. For the transmission of 10 GW of power, two complete 5 GW power systems are lower in weight than a single power system with two antennas because of the large distribution losses for the larger solar array when distributing power at 20 kV. This conclusion could change if the distribution voltage was raised to 40 kV. In any configuration, each transmitting antenna has a companion ground rectenna. An end location of the antennas is shown by the configuration at the bottom of Figure 5-2. This antenna location does effect minimum attitude control propellant, but distribution voltage must be high to reduce distribution power loss. 5. 2 SOLAR THERMAL CONCENTRATOR A typical solar thermal concentrator configuration is shown in Figure 5-3. This configuration consists of 544 independent modules sized to minimize demonstration costs by the delivery of individual components to Earth orbit by the shuttle. The number of independent modules for solar thermal concepts which have been studied ranges from 4 to approximately 550. Recent optimization activities indicate the appropriate number of independent modules would be nearer the small end of the spectrum. AC power is distributed from the modules of Figure 5-3 to antenna locations, where the power is converted to de and transmitted to the ground. Each power module consists of a concentrator, with a concentration ratio of approximately 2000; a ''light pipe" absorber
Figure 5-2. Solar photovoltaic SPS configuration options. that minimizes thermal losses and focuses incoming solar rays to appropriate surfaces for absorption; and a thermionic conversion system that operates at high temperature (approximately 1815°C) in combination with a Brayton cycle conversion system that operates at low temperature (approximately 1038°C). Overall dimensions of this configuration for production of 10 GW of power are
Figure 5-3. Solar thermal concentrator concept assembly.
approximately 5 x 10 km for an elliptical arrangement of the power modules and a center location of the single antenna. The profile of an individual concentrator module is shown as a detail of Figure o-3. The total area of the concentrators is 322 km2, the output per module is 540 W/m2, and the total estimated mass is 221 x 106 kg. The orientation of the configuration is perpendicular to the Sun. Pertinent descriptive data for this configuration are presented in Table 5-2. An alternate configuration concept, which features end locations for the antennas, is shown in Figure 5-4. A trade study has been conducted to identify the configuration with an optimum number of modules for minimum mass and to determine the cost of the operational system. The results of this study show the optimum number of modules to be approximately 40. 5.3 NUCLEAR A typical concept of the nuclear Brayton configuration is shown in Figure 5-5. In this configuration an antenna 1 km in diameter is located between the nuclear modules and their required radiator area. A simplifying feature of the nuclear system is that occultations in orbit do not interrupt system operation. However, radiation problems make interfaces with this system complex. Also, an estimate of the mass of the nuclear system shows it to be quite heavy, approximately 300 x 106 kg. A schematic of the nuclear Brayton cycle system is shown in Figure 5-6. The concept shown utilizes a molten salt breeder reactor with continuous fuel reprocessing. The temperature limit of the molten salt limits the turbine inlet temperature and the operating efficiency of the Brayton cycle, which, in turn, affects the overall system size. The current trend is, therefore, to use a system with a gaseous fuel reprocessing system to relieve the above temperature limit constraint. 5.4 CONCEPT COMPARISON Data comparing the photovoltaic, thermal concentrator, and nuclear concepts for satellite power are shown in Table 5-3. The nuclear system, having the smaller dimensions, is the more compact of the three systems. The mass estimate for the photovoltaic system, however, shows this system to be the lighter of the three.
TABLE 5-2. SUMMARY CHARACTERISTICS OF THE SOLAR THERMAL CONCENTRATOR SPS
Figure 5-4. Solar concentrator concept evolution.
Figure 5-5. Nuclear thermal SPS configuration.
Figure 5-6. Nuclear Brayton cycle SPS schematic.
TABLE 5-3. SUMMARY OF REFERENCE BASELINES
6.0 DEVELOPMENT OF SPS TRADEOFF AND EVALUATION CRITERIA AND SIMULATION PROGRAMS 6.1 SPS TRADEOFF AND EVALUATION CRITERIA Most of the trade studies that have been conducted to date were performed to obtain an understanding of the advantages and disadvantages of various schemes rather than to eliminate options. The results of many of these studies have indicated very slight differences in one approach versus another, and with further study and increased understanding, some conclusions could actually change. In most cases where one scheme appears better, the less favorable options were not eliminated from consideration but less effort may have been devoted to their further study. Most tradeoffs were accomplished to minimize the mass and cost of the SPS program. However, there are some limiting constraints that are not as tangible as mass and cost that had to be considered. Many environmental issues had to be considered, such as operational environment, microwave beam, vehicle emissions, and other terrestrial impacts resulting from the nature and magnitude of the SPS program. For example, the power density level at which microwave beam-ionosphere interactions are expected to occur was used as an upper limit with a resulting effect on the size of the microwave transmission system. Material properties, such as thermal limitations on the microwave structure, were considered. The lifetime and reliability of materials and equipment were important parameters that had to be studied. Other factors such as the outage time for maintenance on turbomachines were considered in the tradeoffs. Factors such as program risks because of uncertainties in technological forecast were examined. The availability of resources was considered also. Tungsten was eliminated as a choice for radiators in the solar thermal concept because of the limited reserves of tungsten. Safety was also an important item for consideration. Finally, comparisons were made of the various SPS concepts such as photovoltaic, thermal, and nuclear; and, of course, the SPS program itself was compared to alternate conventional and future power systems. 6. 2 SIMULATION PROGRAMS 6. 2 1 SYSTEMS DESIGN ANALYSIS MODELS System design analysis models have been developed for both the photovoltaic and thermal SPS concepts for use in trade studies, sensitivity analyses, and to assure consistent and compatible designs. This has been
accomplished through the use of integrated computer models that simulate not only the key design parameters but also their interactions. To date most of the effort for the photovoltaic SPS has been devoted to the solar array portion, since this is the most massive and costly element of the SPS. The subsystems and related elements simulated within the model are the solar array including structures and power distribution, the microwave antenna, the rotary joint, attitude control, the rectenna, and space transportation. Figure 6-1 gives an overview of elements included in the model. The primary outputs of the photovoltaic SPS model are total mass and unit cost. Secondary outputs include detailed mass and cost statements; blanket, reflector, and planform areas; number of HLLV launches; etc. Some of the design parameters included are: concentration ratio, temperature effects including passive radiator concepts, cell and reflector characteristics, and power distribution mast/feeder line mass and efficiency losses. Since all the major elements of the SPS are included, a more optimum overall SPS design can be established. For example, from an attitude control standpoint, it is desirable for the SPS to be long and slender when oriented perpendicular to the orbital plane. However, from a power distribution standpoint, a circular concept is preferred. With this model a concept can be chosen that gives the minimum total mass or cost. This model allows the study of variations in solar cell and reflector characteristics, concentration concepts, planform configuration shapes (e.g., rectangular, diamond, or elliptical), the impact of center-mounted versus end-mounted antennas, the impact on the number of HLLV launches based on the mass of the different segments of the SPS, payload density, mass and volumetric efficiency, HLLV payload capability, shroud size, and many other elements. Trade studies that have been accomplished to date include solar array orientation trades (perpendicular to orbit plane versus perpendicular to Sun), concentration ratios, power distribution efficiency, and planform length/width trades. Sensitivity studies have been conducted which include variations in the SPS efficiency chain from the solar cell to the ground power interface, power distribution voltage and efficiency, variations in the solar cell, and reflector characteristics. Results of these and other trade and sensitivity studies are contained within this report and were used in deriving some of the specific observations included in Section 11.0, Summary Baseline Definition. Planned future studies include the investigation of alternate concentration techniques, variations in planform configurations, variations in SPS output power levels, different power distribution and structural schemes, the impact of space fabricated versus collapsible structures, etc.
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