Final Report Detailed Technical Report Systems Definition Space Based Power Conversion Systems
FINAL REPORT TECHNICAL REPORT (DPD ITEM MA-04) SYSTEMS DEFINITION SPACE-BASED POWER CONVERSION SYSTEMS Submitted to The National Aeronautics and Space Administration George C. Marshall Space Flight Center Study Contract NAS8-31628 The Boeing Aerospace Company DI 80-20309-2
CONTENTS 1 .0 INTRODUCTION AND BACKGROUND .......................................................................... 1 1.1 Introduction................................................................................................................... 1 1.2 Background................................................................................................................... 1 2 .0 PROGRAMMATICS.................................................................................................................... 7 2.1 Derivation of Satellite Energy System Program Definition ...................................... 7 2.2 Requirements ................................................................................................................9 3 .0 ALTERNATIVE POWER GENERATION APPROACHES..................................................11 3.1 Concepts Investigated ................................................................................................11 3.2 Solar Thermionic, Direct Radiation Cooled (Concept 1).........................................11 3.3 Solar Thermionic, Liquid Cooled (Concept 2).......................................................... 11 3.4 Solar Closed Brayton Cycle (Concept 3)...................................................................12 3.5 Solar Thermionic/Brayton Cycle Cascade (Concept 4)............................................. 12 3.6 Silicon Photovoltaic (Concept 5)................................................................................12 3.7 Gallium Arsenide Photovoltaic (Concept 6)...............................................................13 3.8 Nuclear Thermionic (Concept 7)...............................................................................13 3.9 Nuclear Closed Brayton Cycle (Concept 8)...............................................................14 3.10 Power Transfer System (Concept 9).......................................................................... 14 3.11 Emphasized Concepts ................................................................................................14 4 .0 SUBSYSTEMS........................................................................................................................... 15 4.1 Materials .......................................................................................................................15 4.2 Solar Concentrators......................................................................................................16 4.3 Structure.......................................................................................................................18 4.4 Cavity Solar Absorber................................................................................................. 19 4.5 Concentrator/Absorber Optimization .......................................................................20 4.6 Thermionics.................................................................................................................. 20 4.7 Solar Cells .................................................................................................................. 29 4.8 Turbomachines..............................................................................................................32 4.9 Nuclear Reactors......................................................................................................... 36 4.10 Radiators.......................................................................................................................41 4.11 Power Distribution ..................................................................................................... 60 5 .0 SYSTEM OPTIMIZATION AND CONFIGURATION DESCRIPTION ............................63 5.1 System Optimization .................................................................................................63 5.2 Solar Direct Radiation Cooled Thermionic (Concept 1) 63 5.3 Solar Liquid Cooled Thermionic (Concept 2) 66 5.4 Solar Closed Cycle Brayton (Concept 3)...................................................................68 5.5 Thermionic Brayton Cascade (Concept 4) 70 5.6 Solar Silicon Photovoltaic (Concept 5)....................................................................... 70 5.7 Solar Gallium Arsenide Photovoltaic (Concept 6) 73 5.8 Nuclear Thermionic (Concept 7)................................................................................74 5.9 Nuclear Closed Brayton Cycle (Concept 8).............................................................. 75 5.10 Solar Power Transfer (Concept 9) 77 6 .0 COST ........................................................................................................................................79 6.1 Baseline Auxiliary Systems .........................................................................................79
7 .0 COMPARISON OF CONCEPTS ............................................................................................85 7.1 Approach.......................................................................................................................85 7.2 Configuration and Mass Comparison ....................................................................... 85 7.3 Environmental Impact................................................................................................. 86 7.4 Overview.......................................................................................................................88 8 .0 SPS DEVELOPMENT............................................................................................................. 89 8.1 Developmental Goals ................................................................................................. 89 8.2 Recommended Development Program....................................................................... 89 8.3 Expanded Analysis and Ground Experiments (Part I)..............................................89 8.4 Shuttle Based Demonstrations (Part II) ................................................................... 89 8.5 Precursor System Development and Demonstration (Part III)..................................90 8.6 Operational System Development and Demonstration (Part IV)............................90
LIST OF ILLUSTRATIONS 1-1 Satellite Power Stations ............................................................................................................ 1 1-2 Receiving Antenna .................................................................................................................... 2 1-3 Solar Turbomachine Power Satellite Option............................................................................. 3 1-4 "Space Freighter" Lands............................................................................................................3 1-5 Orbital Construction Facility ...................................................................................................4 2-1 Electricity/Labor Cost Ratio ...................................................................................................7 2-2 Growth in U.S. Installed Capacity .......................................................................................... 7 2-3 U.S. Capacity Margin ................................................................................................................8 2-4 Annual Additions to Installed Capacity ..................................................................................9 3-1 Solar Thermionic Direct Radiation Cooled System.............................................................. 12 3-2 Solar Thermionic Liquid Cooled System................................................................................12 3-3 Solar Brayton Cycle System..................................................................................................... 12 3-4 Cascaded Solar Thermionic/Brayton Cycle System.............................................................. 12 3-5 Silicon Photovoltaic System............................................ 13 3-6 Gallium Arsenide Photovoltaic System.................................................................................... 13 3-7 Nuclear Thermionic System..................................................................................................... 13 3-8 Nuclear Brayton Cycle System................................................................................................. 13 4-1 Material Selection Approach ................................................................................................. 15 4-2 Material Technology Trend ..................................................................................................... 16 4-3 Faceted Concentrator .............................................................................................................. 16 4-4 Typical Reflective Facet ..........................................................................................................16 4-5 Variables in Solar Concentrator Analysis................................................................................16 4-6 Solar Concentrator Performance.............................................................................................17 4-7 Influences on Concentrator Efficiency.................................................................................... 17 4-8 Reflectivity Performance of Plastic Films................................................................................ 17 4-9 Compound Parabolic Concentrator........................................................................................ 18 4-10 Derivation of Ideal Beam Dimensions .................................................................................... 19 4-11 Typical Power Satellite Conducting Primary Structure.......................................................... 19 4-12 Cavity Solar Absorber.............................................................................................................. 19 4-13 Model for Concentrator/Absorber Optimization.................................................................. 20 4-14 Characteristics of Mass-Optimized Concentrator/Absorber Combinations ....................... 20 4-15 Thermionic Efficiency vs. Emitter Temperature .................................................................. 21 4-16 Molybdenum Work Function Plot ........................................................................................ 21 4-17 Thermionic Diode Characteristics............................................................................................ 22 4-18 Increase in Efficiency and Output Voltage with Plasma Drop.............................................22 4-19 Heat Rejection as a Function of Converter Efficiency..........................................................23 4-20 Thermal Conductivity Data..................................................................................................... 24 4-21 SPS Thermionic Converter Design ........................................................................................ 24 4-22 Isometric Cutaway of SPS Thermionic Converter...................................................................24 4-23 Multi-Foil Thermal Insulation .................................................................................................25 4-24 Thermal Conductivity Comparison ........................................................................................ 26 4-25 Multi-Foil Thermal Insulation Temperature Profile.............................................................. 26 4-26 Electrical Resistivity of Metals................................................................................................. 27 4-27 SPS Electrical Panel.................................................................................................................. 27 4-28 Solar Cell Performance Predictions ........................................................................................ 30 4-29 Fundamental Limitations May Enforce Performance Plateau .............................................30 4-30 Silicon Cell Radiation Resistance.............................................................................................31 4-31 Solar Array Buy Size Influences Costs.................................................................................... 31 4-32 Closed Brayton Cycle Schematic.............................................................................................32 4-33 Cycle Static Diagram ..............................................................................................................33
4-34 Xenon-Helium Mixture Results in Lighter and Smaller Turbomachine................................34 4-35 Specific Mass Variation with Temperature ........................................................................... 34 4-36 United States Energy Resources ............................................................................................ 36 4-37 Breeder Reactor Program Concept ........................................................................................37 4-38 Particle Bed Reactor Concept .................................................................................................38 4-39 Meteoroid Environment ......................................................................................................... 42 4-40 Meteoroid Motion ..................................................................................................................42 4-41 Resultant Interaction With Object in Earth's Orbit..............................................................42 4-42 SPS Radiators Can Be Preferentially Oriented.......................................................................42 4-43 Flux Seen By Radiator..............................................................................................................42 4-44 Meteoroid Shielding Philosophy ............................................................................................ 44 4-45 Radiator Configurations ......................................................................................................... 44 4-46 Minimum Weight Two-Sheet Aluminum Barrier .................................................................. 44 4-47 BETA Program Solves Thermal Network............................................................................... 44 4-48 Radiator Thermal Model ......................................................................................................... 45 4-49 Baseline Radiators ..................................................................................................................45 4-50 Optimum Panel Dimensions.....................................................................................................45 4-51 Radiator Heat Rejection Helium Fluid....................................................................................46 4-52 Radiator Panel Mass Helium Fluid ........................................................................................ 46 4-53 Radiator Mass Distribution (Helium) ....................................................................................46 4-54 Liquid Radiator Requires Additional Heat Exchanger..........................................................47 4-55 Optimum Panel Dimensions NaK............................................................................................ 47 4-56 Radiator Panel Arrangement—Concept No. 1 48 4-57 Radiator Panel Arrangement—Concept No. 2 48 4-58 Radiator Panel Arrangement—Concept No. 3 48 4-59 Radiator Configuration Concept ............................................................................................ 48 4-60 Original and New Radiator Configurations ...........................................................................49 4-61 Typical Feeder Path to Center Fed Headers...........................................................................49 4-62 Halo Radiator Configuration .................................................................................................49 4-63 Radiator System Solar Thermionic SPS ............................................................................... 49 4-64 Liquid Metal (NaK) Loop..........................................................................................................50 4-65 Stress Versus Creep-Haynes 188.............................................................................................50 4-66 Header or Feeder Volume, Versus Creep............................................................................... 50 4-67 Radiator System Modeling ..................................................................................................... 51 4-68 Radiator Mean Temperature..................................................................................................... 52 4-69 Effect of Radiator Parameters on Total Mass ....................................................................... 52 4-70 Effect of Power Level on Radiator Specific Mass.................................................................. 52 4-71 Radiator Fluid Temperature During Occultation...................................................................53 4-72 Radiator Welds Performed in Orbit ........................................................................................ 53 4-73 Heat Pipe Concept .................................................................................................................. 53 4-74 Heat Pipe Options .................................................................................................................. 54 4-75 Heat Pipe Fluids: Latent Heat of Vaporization .................................................................. 54 4-76 Heat Pipe Fluids: Surface Tension ........................................................................................ 54 4-77 Heat Pipe Fluids: Absolute Viscosity ....................................................................................54 4-78 Heat Pipe Fluids: Vapor Pressure............................................................................................ 54 4-79 Heat Pipe Fluids: Heat Transport Capability and Temperature Range................................55 4-80 Heat Rejection Area and Capability is Limited By Heat Pipe Length ................................56 4-81 Radiator: Pumped Manifolds/Heat Pipe/Fin........................................................................... 56 4-82 Heat Pipe/Fin Radiator Panel Concept....................................................................................56
4-83 Occultation Effects: Heat Pipe/Fin Radiator ...................................................................... 57 4-84 Radiator Mass Comparison .....................................................................................................58 4-85 Effect of Manifold Taper on Radiator Mass...........................................................................58 4-86 Heat Pipe Radiator with Tapered Manifolds........................................................................... 58 4-87 Radiator Mass Comparison ..................................................................................................... 59 4-88 Radiators for Solar Cells ......................................................................................................... 59 4-89 Microwave Efficiency Chain.....................................................................................................60 4-90 Moving Large D.C. Currents.....................................................................................................61 4-91 A.C. Versus D.C. Power Distribution ....................................................................................61 5-1 Diode Panel (Heat Pipe Side) .................................................................................................64 5-2 Power Converter Panel..............................................................................................................64 5-3 Busbar Circuit...........................................................................................................................64 5-4 Panel Masses............................................................................................................................... 64 5-5 Cavity Absorber is Formed From Panels............................................................................... 65 5-6 Thermionic SPS Module ......................................................................................................... 65 5-7 Thermionic SPS Configuration.................................................................................................65 5-8 Thermionic SPS Parameters.....................................................................................................65
DI 80-20309-2 1 .0 INTRODUCTION AND BACKGROUND 1.1 INTRODUCTION 1.1.1 The Study Effort This study was initiated on June 8, 1975 and continued until November 30, 1976. Its purpose was the investigation of potential space-located systems for the generation of electrical power for use on Earth. These systems were of three basic types: 1) Systems producing electrical power from solar energy; 2) Systems producing electrical power from nuclear reactors; 3) Systems for augmenting ground-based solar power plants by orbital sunlight reflectors. Systems 1) and 2) would utilize a microwave beam system to transmit their output to Earth. Configurations implementing these concepts were developed through an optimization process intended to yield the lowest cost for each. A complete program was developed for each concept, identifying required production rates, quantities of launches, required facilities, etc. Each program was costed in order to provide the electric power cost appropriate to each concept. 1.1.2 Contributers Mr. Walter Whitacre was contracting officer's representative at Marshall Space Flight Center. At Boeing, the study effort was directed by Daniel Gregory. Subcontractors were: the Garrett Corporation (thermal engines), directed by Mr. Anthony Pietsch, and the Thermo Electron Corporation (thermionics) directed by Dr. Peter Oettinger. Dr. J. Richard Williams of the Georgia Institute of Technology was the consultant on space-based nuclear reactors. 1.1.3 Related Efforts Studies which were underway during some portion of the study and which contributed to the data base are: I) NAS8-31308 (MSFC), "Space-Based Solar Power Conversion and Delivery Systems Study" Econ Incorporated. 2)NAS9-14323 (JSC), "Future Space Transportation Systems Analysis Study," Boeing Aerospace Company. 3)NAS3-17835 (LeRC), "Microwave Power Transmission System Studies" Raytheon/Grumman. 4)NAS9-14710 (MSFC), "Systems Concepts for STS-Derived Heavy Lift Launch Vehicle Study" Boeing/Grumman. 5) Contract NAS8-31444 (MSFC), "Payload Utilization of SEPS" Boeing/GE. 6) Contract E-(04-3)-l 111 (ERDA) "Central Receiver Solar Thermal Power System," Boeing Engineering and Construction. 1.2 BACKGROUND 1.2.1 The Space Power Concept Figure 1-1 may be used to understand the basic principle of the Satellite Power Station (SPS). A power generating system produces electric power which is converted into a narrow (total divergence angle of approximately 1/100 degree) microwave beam by the microwave transmitter. These systems are located in equatorial geosynchronous orbit and thus remain in line-of-sight of their associated microwave power receiving stations on the ground. At these stations the microwave power is converted into a form of electricity suitable for insertion into the local power network. The energy source for the SPS would be sunlight, or alternatively, nuclear reactors. Fig. 1-1. Satellite Power Stations
Fig. 1-2. Receiving Antenna The receiving stations for the SPS consist of a large number («*109) of small receiving antennas integrated in an oval array. Rectification of the received energy to direct current is accomplished by circuit elements which are integral to the antennas. Figure 1-2 shows such an array. Since the antenna may block most of the microwave energy but would be nearly transparent to sunlight, it is possible that agriculture could be accomplished beneath it. Surrounding the antenna is a buffer zone to contain those microwave "side-lobes" which are more energetic than the continuous exposure standard (assumed to be more than 10 times more stringent than the current standard which is 10 mW/cm2). These antennas could be placed relatively near demand points (note the city in the background of Figure 1-2). Figure 1-3 shows, as an example, one of the concepts studied; a solar Brayton SPS. Four power generator modules feed the circular microwave transmitter. Each power module consists of a reflector which concentrates solar energy into a cavity absorber at the focal point. The resultant high temperatures are used to energize turbomachines which turn electrical generators which power the transmitter. In this study the technical and economic practicality of these systems was investigated. While these systems produce large quantities of power (e.g., 10,000,000 kilowatts per satellite), the forecasted demands of the United States alone are sufficient to require a significant number of satellites. In the program baselined in this study, 60 satellites are made operational by the year 2016. 1.2.2 Auxiliary Systems The criterion for optimization of these systems was minimum cost per kilowatt hour of energy produced (while maintaining set standards on factors such as environmental impact). To achieve low cost
Fig. 1-3. Solar Turbomachine Power Satellite Option per kWhr, all significant elements of the program must also be appropriately low in cost. This includes not only the power generation and transmission systems, but also the systems used for space transportation and space assembly. These auxiliary systems were of necessity considered in this study although their investigation was not a primary goal. An example of an auxiliary system is the heavy lift launch vehicle ("space freighter") used to transfer SPS material to low orbit. It is shown in Figure 1-4 during the landing phase; a portion of the ascent propulsion system is used to Fig. 1-4. "Space Freighter" Lands
affect a soft landing in water. Thus the vehicle is available for reuse, contributing to the required low operational cost. Another significant auxiliary system is the orbital construction facility required to provide the necessary production rate for satellite power stations. A concept for such a station is shown in Figure 1-5. 1.2.3 Energy Overview and the SPS Ever increasing rates of consumption of the Earth's available fossil and nuclear fuel stores are characteristic of this latter half of the twentieth century. Global population is increasing, and so also is the fraction of that population which forms the energy consuming "middle class." This is true not only in the U.S., Russia, Japan, etc., but in the so called emerging nations. As a consequence, we may expect these existing global energy sources to last only to these rather approximate dates: oil, 1995 to 2005; coal, 2030 to 2080; uranium (without breeder reactors), 2020 to 2050. As they are consumed, four additional factors come into play: first their cost steadily increases as remaining quantities become more difficult to obtain (e.g., thinner coal veins). Second, their consumption releases additional pollutants to the biosphere (for example, CO2 removed from the atmosphere millenia ago by plants, which formed coal, is now being returned). Third, since energy sources are geographically concentrated (e.g., most coal in U.S., most oil in Middle East), a potential for great international tension and even war may be created as reserves dwindle. Fourth, nuclear fission involves byproduct materials which may be used for weapon production by either governments or outlaws. Thus some attention is now turning to "renewable" or "non-depletable" 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 Fig. 1-5. Orbital Construction Facility
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 towermounted 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 “sunniest” areas on Earth, and about 15 times more energy than a U.S. location with “average” weather. In geosynchronous orbit 35,786 kilometers (22,236 statute miles) above the equator, a satellite has an orbital period of 24 hours and so remains in constant line of sight to stations on the ground. Solar power satellites in such orbits would generate electric power which would be converted to microwaves and beamed to receiving stations for distribution to consumers as conventional electric power. Receiving stations in various parts of the U.S. could be associated with a number of satellites in orbit. Thus satellite systems can provide high availability “base load” power without the energy storage or backup facilities which greatly impact the cost and operational flexibility of terrestrial solar power stations. Space offers other 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 for ground solar stations. • No oxidation or corrosion. • No tidal waves, earthquakes, etc. • Far removed from demonstrators, terrorists, etc. Other potential advantages of the SPS concept include contribution to U.S. energy independence, possibility of export, reduced pollution and improved economic stability (from reduction of inflationary pressures).
2.0 PROGRAMMATICS 2.1 DERIVATION OF SATELLITE ENERGY SYSTEM PROGRAM DEFINITION The methodology used to select the system size guidelines is as follows: Background—Utilization of space-based power generation could conceivably occur as a legislated action, prompted by the resultant increase of national energy independency, reduced pollution, infinite source, etc. However, about three-fourths of our electric power currently is produced by private utilities, suggesting that economics may be a major factor influencing space-based power incorporation. Thus, market elasticity must be considered, i.e. sales will be influenced by the price of the product. Many factors have contributed to the increases in installed capacity (kW) and consumption (kWh). I) Population growth—from 1956 to 1973 the rate was 1.3% per year. The rate is predicted to decline to 0.8% in the 1973 to 1990 period. Resultant populations, millions (1)*: 1964 ................................... 192 1974 ................................... 212 1984 ................................... 231 1994 ................................... 249 2) Rising standard of living- disposable income per person has been increasing; the trend is expected to continue (1): 1964 ................................... 3248 1974 ................................... 4592 1984 ................................... 5677 1994 ................................... 7071 3) Relative reduction in electricity cost-as pointed out by Hannon (2), the cost of electricity energy has reduced relative to labor costs (electricity does not strike for higher wages). It thus seems appropriate that about 40% of our national electricity use is for process heat and industrial power while only 9% goes for lighting (3). In the following plot (Figure 2-1) from (2) the ratio of manufacturing workers hourly wage to industrial kWh cost of electricity is represented as 1.0 in 1951 on the ratio index scale. * References are given at the end of this session. Fig. 2-1. Electricity/Labor Cost Ratio ^ig. 2-2. Growth in U.S. Installed Capacity An explanation for the change in forecast is given in (1): at the end of 1973 an increase of 33,100 MW in the summer peak requirement was forecast. An increase of 43,607 MW in capacity was planned for 1974 to meet this peak, retire some obsolescent units and raise the national reserve margin to 21%. However, energy conservation (partly from recession-caused production decreases) cut the load growth, to only 15,530 MW, resulting in a generForecasts—Figure 2-2 shows trends in national installed generating capacity. Note the difference between the 1973 and 1974 forecasts. It is significant that the 1973 article in (5) was titled "Utilities Plan Expansion to Meet Record Demands" and that the 1974 title in (1) was "Slower Growth In Sales and Peaks Sparks Sharp Cut in Expansion Plans and Cost."
ating margin of 26.2%. Consequently, some of this margin can be applied to subsequent growth needs, depressing the growth curve. Figure 2-3 shows variation of this margin with time. 18% is generally considered by utilities to be desirable: the margin was 16.6% in 1969 when reductions and curtailments occurred. Fig. 2-3. U.S. Capacity Margin Table 2-1. 1968 U.S. Energy Consumption Patterns by End Use Some authors have forecast and/or recommended very low or even zero energy growth rate. Hannon (2) recommends a more labor intensive economy, i.e. one in which, in essence, human muscles perform rather than electric motors, thereby making more (lower paying) jobs. One factor is the growing labor pool resulting from population growth; if the birth rate instantly dropped to zero, the labor pool would still increase in size for two decades. A more middle-of-the-road view is that energy growth is essential to economic health. Federal Energy Administrator Zarb has recommended a 3.5% to 4.5% installed capacity growth rate for 1975 to 1985 (6). This range was plotted in Figure 2-2. The actual growth rate for 1975 was 3.0% (8). It is possible for national energy consumption to remain constant while the amount of electricity generated increases. In 1968 the U.S. Energy Consumption was as shown in Table 2-1 (from 3). In 1968, 21.2% of the energy expended went to produce electricity. The last column shows a potential of 70.7% utilization without significant changes in energy use technology; for example, electricity could be used for all process heat.
Current Predictions—Figure 2-4 shows historical (4) and forecasted (1 and 5) annual additions to U.S. installed capacity. Note that these are net additions after retirement of obsolete capacity. Actual sales are 1% to 2% greater. Again note the dramatic changes resulting from the capacity margin produced by reduced electricity consumption. The projected 1973 addition rate for the year 1990 was 64 GW (64000 MW); the 1974 projection is for 53 GW per year for 1990. Fig. 2-4. Annual Additions to Installed Capacity Figure 4 also shows the trend and forecast for the addition rate of nuclear-generated electricity. In 1973, nuclear provided 4.8% of our capacity. This was 16 years from the initial power reactor and nine years after the first "commercially competitive" reactor of 1964. In the 16 years from 1964 until 1980 nuclear energy is forecasted to grow to capture 13.6% of the electric power market. In another 15 years it will represent 30% of our capacity (but provide over 50% of the kWh) (1). It thus appears reasonable to assume early market capture rates of ^15% for SPS (assuming equivalent economics). In England, nuclear capacity was added at approximately five times the percentage rate of the United States. Should superior economics be achieved, i.e., very low costs for space-based power, the capture rate could be even higher. Other factors could also accelerate space power incorporation, such as nuclear power moratoriums or legislation which levies the full "social" costs of fossil fuel usage on the electric power customer. The current social cost for the use of coal may be 13 to 15 mills/kWh (7). 2.2 REQUIREMENTS The following requirements were applied throughout the study: 1. Provide electrical power for commercial utilization in U.S. 2. System sizes for 5 and 10 GW ground output. 3. Power source in geosynchronous orbit, microwave power transfer. 4. Program schedule highlights: 12/31/79 End of Concept Definition and Analytical Efforts, Preferred Concept Chosen, Technology Verification Plan Complete. 12/31/87 Technology Verification Activities Complete for Go-Ahead for Phase C/D on SPS, HLLV, LTV, etc. 1/1/96 Initial Operational Capability of Full Scale Power Satellite 5. Technology Level: The technology levels shall be those available for subscale (e.g., lab) demonstration five years prior to operational use. 6. Program Definition: The expansion rate of U.S. electric power generation shall be assumed to be 4.5% per year, the fraction of total capacity from satellite power shall be: a. 10 Years after IOC. 10% b. 20 Years after IOC.25% 7. Nominal life of the space power units and the ground receiving stations shall be 30 years, assuming appropriate maintenance. 8. System safety is to be such that: a. No failure mode shall cause non-program personnel to be exposed to microwave radiation flux greater than the current U.S. exposure standard of 10 mW/cm2. b. Public exposure to nuclear radiation from either system operations or failure (including reactor meltdown/vapori- zation/release) shall not exceed the current U.S. public exposure standard.
9. The system optimization criterion shall be minimum cost per kilowatt hour: both recurring and non-recurring costs shall be recovered from operational revenues. 10. Man will be utilized in space as required appropriate to the above minimum cost goal. 11. Nuclear reactors shall be of the breeder type. 12. In-space power conversion will be by thermionic diodes or closed Brayton cycle thermal engines, or by photovoltaic cells. 13. The low orbit boost vehicle shall be based on the Class 4 type from the Heavy Lift Launch Vehicle Study (NAS8-14710). 14. Launch latitude shall be assumed to be 28.5°N. 15. Radiator system metoroid resistance capability shall be such as to provide a degradation of 30% or less of the total area without repair or replacement of damaged panels, over a period of 30 years. This does not preclude such repair or replacement. 16. Program economics analyses shall be based on a 30-year investment horizon and a 7.5 percent discount rate. 17. Availability of the Space Shuttle shall be assumed. 18. Mass statements will not include a contingency factor. Requirement 6, above results in a requirement for 600 GW ground output capability by the SPS system by the year 2016. This can be supplied by either 120 5-GW units or 60 10-GW units. Requirement 15 sets an arbitrary requirement that requires 1% of the Brayton radiator systems to be repaired in each year of operation. This is not necessarily the optimum maintenance. (The optimum maintenance level would strike a proper balance between meteoroid protection mass—hence cost—and maintenance cost to yield a minimum overall cost for power.) REFERENCES 1. Electrical World, September 15, 1974. 2. Hannon, B., "Energy Conservation and the Consumer," Science, 11 July, 1975 (Vol. 189, No. 4197). 3. Hauser, L. G., "Future Trends in Energy Supply," 1974 Textile Industry Conference. 4. Moody's Public Utility Manual, 1974. 5. Electrical World, September 15, 1973. 6. "World News Beat," Electrical World, July 1, 1975. 7. Morgan, M. G., Barkovich, B. R. and Meier, A. K., "The Social Costs of Producing Electric Power from Coal: A First Order Calculation." IEEE Proceedings, Vol. 61, No. 10, October 1973. 8. Electrical World, December 1, 1976.
3.0 ALTERNATIVE POWER GENERATION APPROACHES 3.1 CONCEPTS INVESTIGATED The alternative satellite power systems shown in Table 3-1 were investigated: The last concept does not generate power in space; a mirror system in geostationary orbit would reflect sunlight to an area on Earth, potentially allowing night operation of ground solar power plants. 3.2 SOLAR THERMIONIC, DIRECT RADIATION COOLED (CONCEPT 1) In a thermionic diode, electrons are produced at the emitter (cathode) due to its elevated temperature, and travel to the lower temperature collector (anode). The circuit is completed through the load. Several processes within the emitter-collector gap tend to reduce the efficiency of power generation from the applied thermal energy. For example, the electrons in the gap tend to repel those being produced at the emitter. The diodes are mounted in the wall of the solar cavity absorber; the emitters are heated by the concentrated solar energy. By allowing the collectors to dissipate waste heat to space, the temperature differential required for operation is produced. Fins are added to the collectors to improve cooling. Table 3-1. Alternative Power Systems Individual diodes have outputs of approximately 0.8 volts, and it is not practical (due to insulation breakdown) to use series strings to produce the converter/transformer assemblies are used to provide the DC necessary to energize the transmitter, up the voltage. An AC to DC converter is used to provide the DC necessary to energize the transmitter. The solar thermionic direct radiation cooled system is shown in Figure 3-1. 3.3 SOLAR THERMIONIC, LIQUID COOLED (CONCEPT 2) In this configuration a liquid metal cooling loop is used to remove waste heat from the diode collectors. In effect, the coolant loop couples the diodes to a greater radiating area than is practical for fins directly attached to the diodes, thereby producing a lower collector temperature, a greater temperature differential across the diode and greater electrical output. Thus the diodes are more efficient, so that fewer diodes are required; however, active cooling uses power drawn from the diodes and requires a liquid metal loop with thermal radiator. Converter/transformer assemblies are used to step- up the diode output voltage. An AC to DC
Fig. 3-1. Solar Thermionic Direct Radiation Cooled System Fig. 3-3. Solar Brayton Cycle System Fig. 3-2. Solar Thermionic, Liquid-Cooled System Fig. 3-4. Cascaded Solar Thermionic/Brayton Cycle System converter is used to provide the DC necessary to energize the transmitter. The solar thermionic, actively-cooled system is shown in Figure 3-2. 3.4 SOLAR CLOSED BRAYTON CYCLE (CONCEPT 3) The Brayton cycle turbomachine provides a rotating shaft output which drives the generators. Thermal energy is added to the helium working fluid in heat exchanger tubing located within the cavity absorber. The hot gas is expanded through the turbine, providing power to turn both the compressor and generator. The recuperator exchanges energy across the loop to increase the system efficiency. Waste heat is rejected through a gas-to-liquid heat exchanger to a liquid metal cooling loop; the liquid metal pumps use power drawn from the generators. The 50,000 volt AC output of the generators is stepped-up to 382,000 volts in transformers; this high voltage facilitates on-board distribution. Stepdown occurs in the rotary transformers. An AC to DC converter is used to provide the DC required to energize the transmitter. The solar Brayton cycle system is shown in Figure 3-3. 3.5 SOLAR THERMIONIC/BRAYTON CYCLE CASCADE (CONCEPT 4) This "cascaded" system offers potentially high efficiency. All waste heat from the thermionic diodes is available to the Brayton cycle; the diodes are cooled by the helium flow in the Brayton loop. The Brayton loop is cooled by a liquid metal radiator. The DC output of the diodes is stepped-up to 50,000 volts AC in the rotary converters/trans- formers; the turbomachine generators produce 50,000 volts AC which is combined with the output of the rotary converters/transformers. An AC to DC converter is used to provide the DC required to energize the transmitter. The cascaded solar thermionic/Brayton cycle system is shown in Figure 3-4. 3.6 SILICON PHOTOVOLTAIC (CONCEPT 5) A photovoltaic, or solar, cell directly converts solar energy to electric power. Performance may be augmented, within certain limits, by concentrating
solar energy upon the cell and/or by providing cooling. Series strings of cells may be used to build to the 20,000 vdc (or 40,000 vdc), nominal, required for the microwave transmitter. Lower voltage arrays may be required if low orbit operation is required (such as for self-powered transfer) (Fig. 3-5). Fig. 3-5. Silicon Photovoltaic System The solar cells employed in this concept are the "conventional" silicon type, except they are power economics dictates that they be only approximately one-half as thick as are currently used. 3.7 GALLIUM ARSENIDE PHOTOVOLTAIC (CONCEPT 6) A photovoltaic, or solar, cell directly converts solar energy to electric power. Performance may be augmented, within certain limits, by concentrating solar energy upon the cell and/or by providing cooling. Series strings of cells may be used to build to the 20,000 vdc (or 40,000 vdc), nominal, required for the microwave transmitter. Lower voltage arrays may be required if low orbit operation is required (such as for self-powered transfer) (Fig. 3-6). The cells employed in this concept are the gallium aluminum arsenide/gallium arsenide type. This multilayer "heterojunction" cell has the apparent potential for high efficiency at elevated temperatures; it is also more radiation resistant. 3.8 NUCLEAR THERMIONIC (CONCEPT 7) The energy source in this system is nuclear; a molten salt breeder reactor (MSBR) is used. The Fig. 3-6. Gallium Arsenide Photovoltaic System salt mixture contains both fissile fuel, the energy source, and fertile fuel, which breeds to become fuel for subsequent use. The salt mixture is circulated out of the reactor core through a heat exchanger which transfers energy to a sodium loop. The sodium loop is used since there is insufficient salt flow for the diode emitter area. A small secondary salt flow is continuously passed through a fuel process system. This system removes the protactinium and wastes which would "poison" the reactor by excessive neutron capture. The fuel process system introduces fertile fuel and removes bred fuel. The MSBR is an unique breeder concept in that a single liquid fuel mixture contains both fissile and fertile fuels, and that processing of solid fuel elements is not required. The diode collectors are cooled by a liquid metal radiator loop. The low voltage DC output of the collectors is stepped-up and converted to AC by rotary converters/transformers. An AC to DC converter is used to provide the DC necessary to energize the transmitter. The nuclear thermionic system is shown in Figure 3-7. Fig. 3-7. Nuclear Thermionic System
3.9 NUCLEAR CLOSED BRAYTON CYCLE (CONCEPT 8) The energy source in this system is nuclear; a molten salt breeder reactor (MSBR) is used. The salt mixture contains both, fissile fuel, the energy source, and fertile fuel which breeds to become fuel for subsequent use. The salt mixture is circulated out of the reactor core through a heat exchanger which transfers energy to the helium loop of the Brayton turbomachines. A small secondary salt flow is continuously passed through a fuel process system. This system removes the protactinium and wastes which would "poison" the reactor by excessive neutron capture. The fuel process system introduces fertile fuel and removes bred fuel. The MSBR is an unique breeder concept in that a single fuel mixture contains both fissile and fertile fuels, and that processing of solid fuel elements is not required. The Brayton cycle turbomachine provides a rotating shaft output which drives the generators. Hot helium is expanded through the gas turbine, providing power to drive both the compressors and generators. The recuperator exchanges energy across the loop to increase efficiency. Waste heat is rejected through a gas-to-liquid heat exchanger to a liquid metal cooling loop; the liquid metal pumps use power drawn from the generators. The 50,000 volt AC output of the generators is stepped-up to 382,000 volts in transformers; this high voltage facilitates on-board distribution. Stepdown occurs in the rotary transformers. An AC to DC converter is used to provide the DC required to energize the transmitter. The nuclear Brayton cycle system is shown in Figure 3-8. Fig. 3-8. Nuclear Thermionic System 3.10 POWER TRANSFER SYSTEM (CONCEPT 9) In this concept one or more mirrors in geosynchronous orbit would reflect solar energy directly to Earth. Ground-based solar power plants would be augmented by this reflected energy, allowing night operation or increased output. 3.11 EMPHASIZED CONCEPTS By the end of the initial phase of this study, it had become evident that further investigation of concepts 2, 4, 7 and 9 was inappropriate for the reasons given in Table 3-2. Table 3-2. Evaluation of De-emphasized Systems
4.0 SUBSYSTEMS 4.1 MATERIALS Many of the material requirements of the SPS will be satisfied by the use of aluminum, magnesium and titanium alloys. However, some subsystems contain components which operate at elevated temperatures. Selection of alloys for these SPS applications is based on the temperature range involved, as shown in Figure 4-1. The tungsten/ rhenium and tantalum alloys are less well defined than the columbium and cobalt alloys. The materials identified will be used for heat exchanger tubing (e.g., within solar cavity absorbers) and for manifolds, etc., in the radiator systems. Note that the material strength shown in Figure 4-1 is the predicted 30-year creep rupture strength. Many SPS subsystems require long term confinement of pressurized gases or liquid at high temperatures, thus a fundamental problem is the long-term creep rupture at high temperatures. Table 4-1 shows additional considerations in material selection, and alloys considered as option. A trend of improvement of alloys for service above 1000K (1340°F) is shown in Figure 4-2. Iron, cobalt, columbium and nickel base systems were compared. A number of alloys having good strength properties were not considered due to their poor fabrication capabilities. While strength Fig. 4-1. Material Selection Approach rupture capabilities of the nickel and cobalt base alloys have shown only a modest advance in the past 25 years, significant improvements in thermal fatigue, oxidation resistance, and stability characteristics have been achieved. CONCLUSIONS: 1. Little or no improvement trend in the cobalt base alloys. 2. Nickel base alloys have been improving at the rate of approximately 3.4K (6.2°F) per year. Table 4-1. Material Considerations
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