JSC-11568 INITIAL TECHNICAL, ENVIRONMENTAL, AND ECONOMIC EVALUATION OF SPACE SOLAR POWER CONCEPTS VOLUME I - SUMMARY AUGUST 31, 1976 National Aeronautics and Space Administration LYNDON B. JOHNSON SPACE CENTER Houston, Texas
INITIAL TECHNICAL, ENVIRONMENTAL, AND ECONOMIC EVALUATION OF SPACE SOLAR POWER CONCEPTS VOLUME I - SUMMARY VOLUME II - DETAILED REPORT
CONTENTS I INTRODUCTION ................................................................................. 1-1 II CONCLUSIONS................................................................................. H-l III PROGRAM REQUIREMENTS .................................................................... III-l A. Projected Energy Demand ....................................................... III-l B. Implementation Scenarios ....................................................... III-l IV POWER STATION.............................................................................. IV-1 A. System Analysis.................................................................... IV-2 1. Efficiencies.................................................................... IV-2 2. MPTS/MRCS Analysis .......................................................... IV-2 3. Orbit Considerations ...................................................... IV-5 4. Configurations................................................................. IV-9 5. Mass Properties............................................................. IV-11 B. Solar Energy Collection System .............................................. IV-15 1. Solar Array.................................................................... IV-15 2. Power Distribution .......................................................... IV-17 3. Structure....................................................................... IV-18 4. Attitude and Orbit Control ............................................. IV-19 5. Instrumentation, Control and Communications ..... IV-21 6. Maintenance Station ....................................................... IV-21 - C. Microwave Power Transmission System .................................... IV-22 1. Antenna Array............................................................... IV-22 2. Microwave Generators ..................................................... IV-22 3. Subarrays...................................................................... IV-22 4. Phase Control ............................................................... IV-23 5. Pointing Control ............................................................ IV-23 6. Power Distribution ........................................................ IV-25 7. Structure...................................................................... IV-26 8. Rotary Joint.................................................................. IV-26 9. Thermal Control ............................................................ IV-28 D. Microwave Reception and Conversion System ....................... IV-28 1. Rectenna......................................................................... IV-28 2. Grid Interface............................................................... IV-29
E. Operations.............................................................................. IV-29 F. Unit Costs.............................................................................. IV-31 V SPS CONSTRUCTION AND MAINTENANCE SYSTEM ...................................... V-l A. Systems Requirements and Analysis ......................................... V-l B. Construction Base.................................................................... V-5 1. Construction and Manufacturing Facility ....................... V-6 2. Orbital Construction and Support Equipment ................. V-8 3. Logistics Facility ......................................................... V-8 4. Integration Management Facility ..................................... V-8 5. Crew Habitability Facilities ........................................ V-9 C. Construction Operations .......................................................... V-9 VI SPACE TRANSPORTATION SYSTEMS ..................................... VI-1 A. Systems Requirements and Analysis ......................................... VI-1 B. Heavy Lift Launch Vehicle...................................................... VI-2 C. Personnel and Priority Cargo Launch Vehicle............................................................................. VI-8 D. Cargo Orbital Transfer Vehicle ............................................. VI-8 E. Personnel Orbital Transfer Vehicle ...................................... VI-12 F. A Summary of Projected Transportation System Characteristics.................................................. VI-17 VII INTEGRATED OPERATIONS ................................................................... VII-1 A. Systems Requirements and Analysis .......................................... VII-1 B. Program Model........................................................................... VII-1 C. Mission Management Concept ................................................... VII-3 D. Mission Management Functions ................................................ VII-3 1. Program Headquarters Mission Control ........................... VII-3 2. Launch and Recovery Control ........................................... VII-12 3. LEO Operational Base Control ........................................... VII-13 4. GEO Operational Base Control ........................................... VII-13 5. SPS (Individual Unit) Ground Control ........................... VII-14
E. Key Considerations and Areas for Further Investigation ................................................................... VII-14 1. Prelaunch, Launch, and Recovery Operations ................. VII-14 2. Space Manufacturing and Construction Options ............. VII-15 3. Operational Space Control Operations ........................... VII-15 4. Simulation and Training Operations .................... VII-15 5. Safety in SPS Operations ............................................... VII-16 VIII ENVIRONMENTAL CONSIDERATIONS .............................. . ................. VIII-1 A. Methodology ............................................................................ VIII-1 B. Environmental Questions ........................................................ VIII-1 C. Comparisons with Conventional Systems ................................. VIII-2 IX MANUFACTURING CAPACITY, NATURAL RESOURCES, TRANSPORTATION, AND ENERGY CONSIDERATIONS ................................................. IX-1 A. Requirements......................................................................... IX-1 B. Manufacturing Capacity ........................................................ IX-1 C. Natural Resources .................................................................. IX-1 D. Surface Transportation ........................................................ IX-1 E. Energy Payback...................................................................... IX-3 X PROGRAM DEVELOPMENT PLAN ............................................................. X-l A. Program Phasing...................................................................... X-l B. System Definition and Exploration Technology Phase .... X-l C. Technology Advancement Phase .............................................. X-6 D. System Development ............................................................... X-7 E. Program Costs......................................................................... X-7 XI PROGRAM COST AND ECONOMIC ANALYSIS............................................. XI-1 A. Methodology............................................................................. XI-1 B. SPS Costs................................................................................ XI-1
C. Comparison with Conventional and Other Advanced Systems ............................................................ XI-4 D. Summary Remarks...................................................................... XI-10
TABLES IV-1 A SUMMARY OF MICROWAVE SYSTEM(S) PARAMETERS ........................ IV-6 IV-2 SUMMARY OF UNIT MASSES.......................................................... IV-12 IV-3 MASS PROPERTIES SUMMARY.......................................................... IV-14 IV-4 COST ESTIMATING RELATIONSHIPS ................................................ IV-32 V -l ORBITAL CONSTRUCTION EQUIPMENT REQUIREMENTS ........................ V-ll V -2 COLUMN/CABLE CONFIGURATION TYPICAL MAN LOADING .................. V-l2 V -3 TRUSS CONFIGURATION TYPICAL MAN LOADING (GEO CONSTRUCTION).................................................. V-l 3 V -4 TYPICAL PEAK STAFFING (LEO) FOR ANTENNA SUBARRAY FABRICATION..................................... V-l4 V I-1 HLLV CANDIDATE ENGINE CHARACTERISTICS .................................. VI-3 V I-2 HLLV CANDIDATE CONFIGURATION CHARACTERISTICS ..................... VI-3 V I-3 HLLV COST ESTIMATES................................................................ VI-6 V I-4 OTV CANDIDATE THRUSTER CHARACTERISTICS .............................. VI-1O V I-5 HLLV RANGE OF PROTECTED ESTIMATES......................................... VI-18 V I-6 PLV RANGE OF PROJECTED ESTIMATES......................................... VI-18 V I-7 COTV RANGE OF PROJECTED ESTIMATES......................................... VI-19 V I-8 POTV RANGE OF PROJECTED ESTIMATES......................................... VI-19 V I-9 RELATIVE TRANSPORTATION COSTS FOR SEVERAL SPS CONFIGURATIONS AND CONSTRUCTION LOCATIONS ... VI-20 V II-1 SPS PROGRAM MODEL INPUTS, ASSUMPTIONS, AND GUIDELINES..................................................... VII-4 V I1-2 PROGRAM MODEL SUMMARY FOR "COLUMN/CABLE" SPS IN GEO..................................................... VI1-6 V II-3 PROGRAM MODEL SUMMARY FOR "TRUSS" SPS IN GEO..................... VII-8 V II-4 PROGRAM MODEL SUMMARY FOR "TRUSS" SPS IN LEO ..................... VII-1O
VIII-1 ENVIRONMENTAL COMPARISON OF 10-GW POWERPLANT OPERATIONS................ ... .................................. VI11-3 IX-1 NATURAL RESOURCE DEMANDS OF A 10-GW SPS COMPARED TO NATIONAL AND WORLD DEMANDS IN THE YEAR 2000 . IX-2 IX-2 ENERGY PAYBACK OF A 10-GW SPS............................................ IX-2 XI-1 DDT&E COST ESTIMATE SUMMARY ................................................ XI-5 XI-2 SUMMARY OF COST ESTIMATES AND RELEVANT PARAMETERS FOR 10-GW SPS................ .............................. XI-5
1-1 Study task structure............................................................ 1-4 III-l Projections of U.S. electrical energy requirements and possible SPS implementation scenarios .... III-2 IV-1 SPS functional description .................................................. IV-1 IV-2 Estimated efficiencies of the various steps in the collection, conversion, and transmission process .... IV-3 IV-3 Output power limits ............................................................. IV-4 IV-4 Power density at rectenna ................................................... IV-7 IV-5 Eclipse geometry .................................................................... IV-8 IV-6 Example configurations (a) Column/cable............................................................ IV-10 (b) Truss......................................................................... IV-10 IV-7 Solar power satellite total mass ......................................... IV-13 IV-8 Solar concentrator ................................................................ IV-16 IV-9 Solar cell blanket................................................................ IV-17 IV-10 Gravity gradient torques (a) Short period............................................................ IV-19 (b) Long period............................................................... IV-19 IV-11 Counterweight location .......................................................... IV-20 IV-12 Antenna element.................................................................... IV-24 IV-13 Typical antenna distribution system ..................................... IV-25 IV-14 Antenna primary structure ................................................... IV-26 IV-15 Ball joint and drive concept................................................ IV-27 IV-16 Rectenna construction .......................................................... IV-29 IV-17 Variation in ground de power output ..................................... IV-30 IV-18 Photovoltaic array cost projection ...................................... IV-33 FIGURES
V -1 Beam builder machine concept ................................................... V-l V -2 Typical SPS construction sequence - column/cable (POP) . . . V-2 V -3 Partial construction of column/cable configuration .............. V-3 V -4 Concept for solar cel 1/concentrator deployment.............. V-4 V -5 Construction base concept for truss configuration ............ V-5 V -6 Typical SPS construction sequence - truss configuration. . . v-6 V -7 Construction base concept for column/cable configuration ...................................................... V-7 V -8 Operational schematic of construction base (column/cable) ..................................................... V-10 V I-1 Modified single-stage-to-orbit launch vehicle ....................... VI-4 V I-2 Two-stage winged launch vehicle ............................................ VI-5 V I-3 Two-stage winged launch vehicle ............................................ VI-7 V I-4 Personnel and priority cargo launch vehicle .......................... VI-10 V I-5 LEO-to-GEO transfer time as a function of thrust/weight ..................................................... VI-11 V I-6 Propellant burden contribution for high-thrust 02/H2 COTV............................................................ VI-13 V I-7 Cargo orbital transfer vehicle (COTVq) characteristics .................................................. VI-14 V I-8 Crew module concept................................................................ VI-15 V I-9 Crew rotation passenger module . ......................................... VI-16 V I-10 Personnel orbital transfer vehicle (POTV|_) characteristics.................................... <>.... vi-17 V II-1 SPS mission scenario................................................................ VII-2 V II-2 SPS mission management concept ................................................ VII-12
X-l Space solar power projected program phasing ....................... X-1 X-2 System definition and exploratory technology phase activities......................................................... X-2 X-3 The relative merits of space solar power and other systems - coal, nuclear, solar terrestrial .... X-5 X-4 Significant test activities, initial phase, July 1976 to July 1978 (partial listing) ....................... X-5 X-5 Technology advancement phase . . ......................................... X-6 XI-1 Cost equations....................................................................... XI-3 XI-2 SPS cost parametrics........................................................ . XI-6 XI-3 Conventional and advanced power generation system costs................................................................... XI-7 XI-4 Terrestrial solar power ......................................................... XI-8 XI-5 The 5-GW solar power tower concept with electrolysis cell/fuel cell energy storage ........................... XI-9 XI-6 The 5-GW solar photovoltaic-fuel cell/electrolysis cell system................ ... ..................................... XI-9 XI-7 Terrestrial solar photovoltaic power cost for 5-GW plant......................................................... XI-10 XI-8 Terrestrial solar thermal power cost for 5-GW power tower concept..................................................... XI-11 XI-9 Land area requirements for 5-GW plant..................................... XI-12 XI-10 Satellite cost breakdown...................................................... XI-13
I. INTRODUCTION The requirements for energy in the U.S. and the world will continue to increase to support a growing population and to improve the quality of life for that population. Projections indicate the U.S. requirements will grow by a factor of 2 to 3 between now and the year 2000. The manner in which we will meet this requirement is not clear. Oil and gas are expected to be depleted within decades. Fuel for the present class of nuclear reactor systems will also be depleted in the same time frame. The breeder reactor system, when successfully developed, will greatly extend the natural fuel resource but presents continuing safety and environmental concerns, not the least of which is the disposal of nuclear waste as it accumulates from large-scale nuclear energy production. Fusion reactor systems also have potential, but these require significant scientific advances. Coal resources appear sufficient for several hundreds of years. The environmental concerns associated with mining coal, and the subsequent problems or costs in reducing air pollution to an acceptable level during its use, are well known. The logistics of a greatly expanded coal industry is also a significant although not unsolvable consideration. In view of the problems or concerns related to obtaining the required energy from oil, gas, nuclear, and coal sources, the Nation is actively pursuing alternate sources of energy for the future. Solar energy is an obvious candidate for consideration. Solar energy is inexhaustible and clean, and the increasing costs of other sources will make solar energy more attractive in the future. The use of solar energy collected on the Earth has several basic limitations, however, which will tend to inhibit its widespread use. At any given location on the Earth, a solar collector will be limited by such factors as the day-night cycle, cloud cover, and atmospheric attenuation. The day-night cycle, particularly, requires the use of expensive storage capacity or limits the solar application by requiring additional power sources. A concept has been presented ("Power from the Sun: Its Future," Dr. Peter E. Glaser, Amer. Assn. Advan. Sci., Vol. 162, Nov. 22, 1968, pp. 857-861) that is intended to alleviate limitations associated with the collection of solar energy on Earth. This concept involves placing large solar power satellites in geosynchronous orbit and beaming microwave energy down to collection stations on the Earth. Some of the advantages of this concept are that the satellite is in near-continuous sunlight that is not attenuated by the atmosphere, no electrical storage facilities are required, the land use requirement is reduced by a factor of 5 to 10, and the ground power output can be located near the user rather than in desert-type regions.
The space concept, while having advantages, also introduces new requirements. These include the need for transportation of the power station into space and the transmission of power from space to Earth by microwave radiation. Several studies conducted in the past few years have been directed toward exploring the feasibility of this concept. The results of these studies have generally been favorable, while reflecting a need for significant technological advancement if the concept is to be economically competitive with ground-based systems. Critical areas were identified during the course of these studies and research and development programs have begun to be formulated to investigate these areas. A particular effort was conducted at the NASA Lyndon B. Johnson Space Center (JSC) during the summer of 1975 to evaluate the need and feasibility of a Space Solar Power Development Laboratory. The study was done in support of the NASA "Outlook for Space" study and was documented in JSC-09991. Possible requirements for a development laboratory or "pilot plant" type solar power satellite were evaluated and the technical feasibility of such a plant was established. In view of past study results, the 6-week study, and the conclusions of the "Outlook for Space" study, it was decided to implement at JSC a more detailed study of the Space Solar Power Concept. This document (Volume I) presents a summary of the results of that study. Volume II contains the detailed studies on which the summary was based. The study was conducted between September 1975 and June 1976, by JSC personnel. The general objectives of Solar Power Satellite (SPS) studies include: 1. Establishment of realistic technical and economic design criteria and requirements for a full-scale SPS. 2. Definition of technology development and flight-test programs necessary to achieve the optimum SPS design. 3. Comparison of the SPS with other energy generation options to establish the relative economic, environmental, and social advantages/ disadvantages of the SPS concept. These objectives are quite broad and definitive answers will require a number of years of study augmented by technology efforts in a number of areas. Nevertheless, the present study provides further insight into a number of aspects of the concept and provides a point of departure for further work. This summary (Vol. I) presents a number of preliminary conclusions and a synopsis of the more detailed studies that are presented in Volume II.
Certain programmatic guidelines were chosen to initiate the study and bound the study effort. 1. Program plans and technology projections will be developed based on deployment of the first operational SPS as early as 1990. 2. The capability will be provided as early as 1995 to deploy two to four SPS1 s per year. 3. Dedicated transportation systems will be developed and optimized specifically for use in deploying and operating the SPS network. 4. Materials used in fabricating and operating an SPS will be obtained only from the Earth. 5. The SPS will be deployed in appropriate geosynchronous orbits only. 6. The lifetime of an SPS will nominally be 30 years, although liberal refurbishment/replacement of parts may be assumed. 7. The SPS will be designed in a manner to optimize participation of man in its fabrication, assembly, and operation. 8. Availability of scarce resources will be a major consideration in projecting technologies to be used in fabricating the SPS network. 9. Energy as well as economic payback will be assessed in determining the SPS development strategy. 10. Aspects of social and environmental impact will be assessed. 11. Assembly fabrication strategies for SPS will be developed such as to minimize overall costs. The first two guidelines were modified slightly as the study progressed in that various scenarios were defined and evaluated. Available resources defined the scope and depth of the study. For example, the study was primarily limited to consideration of the photovoltaic concept for solar energy collection and conversion, although a rather thorough review of past system studies involving the use of the thermal energy conversion concept was accomplished (Vol. II). Similarly, the more detailed design studies were limited to consideration of silicon solar cells. Given these restrictions, a range of power station sizes and weights was determined based on conservative and optimistic estimates of collection, conversion, transmission, and receiving efficiencies. Analyses and/or design studies were conducted for each element of the systems to varying degrees. These studies included several satellite configurations, construction concepts, crew requirements, alternate microwave generator concepts, rotary joint designs, attitude and control concepts, and structural designs. Several program scenarios were developed that defined the number and schedule of space power satellites required to provide varying percentages of the Nation's energy needs in the 1995-2025 period.
Satellite weights were then coupled with the number and schedules of satellites required to define a range of transportation requirements. These requirements were used to guide the study of various transportation elements and to estimate integrated transportation requirements such as fleet size. Transportation elements for which specific studies were conducted included multistage winged and ballistic heavy lift launch vehicles, a variety of orbital transfer vehicle thrusters, and personnel launch and transfer vehicle designs. In a similar manner, the satellite and transportation system characteristics, number, and schedule were used as a basis to estimate the cost of design, development, test, and evaluation (DDT&E), total program, and mills per kilowatt hour. Preliminary estimates are also provided of natural resource requirements and pollutants emitted from processing and launch operations. Estimates of energy payback are also presented. Figure 1-1 presents the task structure that was used in the study effort. The present report (Vol. I) and Volume II are also organized according to this task structure. Figure 1-1.- Study task structure.
II. CONCLUSIONS The scope and complexity of the satellite power concept coupled with the limited depth of the present study would make it inappropriate to draw absolute conclusions. However, the SPS concept appears to be technically feasible in that no design or operational problems were encountered that did not appear amenable to solution. The economic viability of the system appears promising but is obviously dependent upon a combination of technology advancement and/or the costs of competitive sources. Within the limitations of the study and based on a variety of assumptions and/or estimates, the following preliminary conclusions are presented. 1. The maximum power output of an individual microwave transmission link to Earth is about 5 GW and the transmitting antenna diameter is about 1 km, based on the following assumptions: a. An operating frequency of 2.45 GHz 2 b. A maximum allowable power density at the ionosphere of 23 mW/ciri c. A maximum allowable antenna waveguide temperature of 485 K 2 resulting in a power density at the antenna of 21 kW/m d. A 1O-dB Gaussian taper of the microwave beam 2. The estimated mass of a 10-GW SPS (incorporating solar energy converters sufficient for two 5-GW microwave power transmission systems) 6 6 is between 47 x 10 and 124 x 10 kg, based on the following assumptions: a. Silicon cell arrays with an efficiency of 15 to 17 percent at 30° C and a concentration ratio of 2 b. An overall system conversion and transmission efficiency range of 4.2 to 8.0 percent c. A weight growth of 50 percent over present estimates 2 The resulting solar array areas ranged from 96 to 183 km . 3. The silicon solar cell arrays make up well over half the weight and cost of the satellite. Consequently, additional effort on solar arrays offers the most potential for overall system improvement, particularly with respect to new approaches that could result in significant weight reduction. 4. Considerations of the structure indicated that minimum weight can be achieved if design loads are limited to those encountered on orbit and after construction. If this is done, the structure can be held to a very small percentage (~5 percent) of the SPS weight. The major factor in design will not be weight but the development of techniques for automated on-orbit construction and for conducting large electrical currents.
5. Development of automated construction techniques is complex and requires a great deal of further effort. A preliminary task evaluation based on a conceptual construction technique suggests that as many as 600 personnel may be required in space to construct an SPS in 1 year, with minor variations expected in personnel required due to configuration and construction location. Placing and supporting these personnel in orbit is a relatively small factor in the overall transportation requirement. 6. Past studies have indicated an apparent performance advantage of constructing, assembling, or deploying all or a portion of the solar arrays in low-Earth orbit and then utilizing solar energy with electric thrusters to propel the system or major elements thereof to geosynchronous orbit. The conclusion of the present study is that this area needs further study with full consideration given to the following factors: a. Degradation of the exposed solar arrays during transit b. Protection of unused arrays during transit c. Earth shadowing during portions of transit possibly requiring nonsolar propulsion d. Docking and assembly of large SPS sections at geosynchronous orbit and resulting impact on structural design e. Relative simplicity of chemical stages for transfer of "containerized" packages to geosynchronous orbit f. Radiation conditions at geosynchronous orbit 7. The SPS in equatorial orbit will be eclipsed both by the Earth and by other satellites. These eclipses result in as many as three brief (up to 75 min) power outages per day for two 6-week periods per year, although less than 1 percent of the available energy is lost. The SPS/grid system must be designed to accommodate these outages. 8. Conceptual designs and characteristics were developed for two- stage winged and ballistic heavy lift launch vehicles of varying payload capability. Although the ballistic systems are much smaller and lighter, recovery and reusability will be key issues in establishing the desired configuration. 9. Heavy lift launch vehicle design considerations established hydrocarbon fuel rather than hydrogen as the choice for first-stage propellant because of its greater energy density. 10. Considerations of I and confidence in technical development of candidate electric engines indicate that the MPD arcjet engine appears to be the best choice for self-powered orbital tranfer. These engines are also suitable for subsequent use as thrusters for the SPS attitude control system. 11. The high launch rates required indicate that launch window and related operational considerations may become significant factors. Launch latitudes near the Equator greatly expand the launch window and offer performance advantages.
12. Based on varying assumptions as to performance, construction, location, orbital transfer modes, and reusability, achievable transportation costs to geosynchronous orbit are estimated to range from $75 to $300/kg. The major contributor to the total transportation costs for a given program was the cost of transporting the necessary material to low-Earth orbit. 13. The cost of producing electricity from solar power satellites as described herein is estimated to be in the range of 29 to 115 mills/ kWh. This range of estimates is based on the following assumptions: a. An implementation of 112 10-GW satellites over a 30-year period b. A range of satellite weights and transportation costs as indicated earlier c. A design, development, test, and evaluation (DDT&E) cost amortized over the 30-year implementation period d. A space hardware repair/replacement rate of 1 percent annually e. A plant factor of 92 percent allowing for eclipses and maintenance time f. A return on capital investment of 15 percent 14. The cost of producing electricity with conventional (nuclear and fossil) plants is predicted to be in the range of 15 to 30 mills/kWh in the 1995 time period, depending upon the cost, fuel, and type of powerplant. The cost of producing electricity with potential ground-based powerplant concepts (ground solar, geothermal, wind) is estimated to be from 28 to 121 mills/kWh. 15. The introduction of SPS in lieu of meeting an equivalent portion of the Nation's energy needs with new nuclear and coal-burning electrical powerplants will result in significant reduction in emissions (particulates, NO , SO , and nuclear waste). X X 16. The microwave power density at the edge of the rectenna 2 (1 mW/cm ) is about one-tenth of the present U.S. standard for human exposure. The system is fail-safe in that the beam would be dispersed to harmless intensity levels should the microwave beam pointing control fail. 17. Implementation of SPS on a large scale would create an increased demand for resources such as aluminum and rocket propellant gases (hydrogen and argon). Also, production capacity would have to be substantially increased in the areas of solar cells and reduction of arsenic from oxides (for the manufacture of gallium arsenide diodes). However, there does not appear to be any critical shortages of resources for SPS construction based on world reserves.
III. PROGRAM REQUIREMENTS A. Projected Energy Demand Projections of the Nation's electrical energy demand have been made by the Federal Power Commission (FPC), the Energy Research and Development Administration (ERDA), and other Federal agencies and private organizations. Figure III-l shows the FPC and ERDA projections for electrical energy demand through 1990 and 2000, respectively. The FPC projection was presented in the 1970 Federal Power Survey report, Volume I. The ERDA projection (presented in ERDA-48, Volume 1, June 1975) involves six different scenarios that are encompassed by the shaded area of figure III-l. The highest electricity generation scenario is based on intensive electrification and it has a 4.4 percent/yr growth rate in the year 2000. The lowest electricity generation scenario is based on improved efficiencies in end use and it has a 1.4 percent/yr growth rate in the year 2000. The FPC projection, which is higher than any of the ERDA projections, has an annual growth rate of 6.0 percent/yr in 1990. The FPC projection has been extrapolated to the year 2025 at the 6.0-percent growth rate in order to provide a reference for the development of solar power system implementation scenarios. B. Implementation Scenarios Effective use of space solar power implies an implementation program that will produce a significant portion of the future electrical program demand. Therefore, scenarios of SPS implementation rates were developed that would provide 25 percent of the new capacity by 2015 (scenario A), 50 percent of the new capacity by 2010 (scenario B), and all of the new capacity by 2005 (scenario C), in relation to the extrapolated FPC projection. Scenario B was used as an illustrative example by which to examine the SPS in terms of its program requirements and resulting economic analysis. This scenario results in providing a significant quantity of the total electrical energy by 2025. The SPS installed capacity by 2025 would be 1120 GW or about 30 percent of the FPC extrapolated projection. If the power output of each SPS is 10 GW (as described in sec. IV), implementation of Scenario B results in a total of 112 satellites in orbit by 2025. The construction rate varies from one per year initially (1995) to seven per year during the last 3 years of the 30-year period.
Figure III-l.- Projections of U.S. electrical energy requirements and possible SPS implementation scenarios.
IV. POWER STATION The power station of the SPS consists of a Solar Energy Collection System (SECS), which converts solar energy into electricity; a Microwave Power Transmission System (MPTS), which converts the electricity into microwave energy and transmits it to Earth; and a Microwave Reception and Conversion System (MRCS), which converts the microwave energy into electricity suitable for interface with a distribution grid. These elements of the power station are depicted in figure IV-1. The purpose of this part of the study was to explore the factors involved in the design of the power station. This involved evaluating the power output of individual satellites, methods and efficiencies of energy conversion and transmission, requirements and design approaches to system elements, weights of equipment and material in orbit, and the orbital characteristics of the satellites. Figure IV-1.- SPS functional description.
Several configuration approaches were considered and two typical examples were studied in some detail for the purpose of defining ranges of weight, cost, and construction approaches. These two examples are referred to as the "column/cable" and "truss" configurations. A. System Analysis 1. Efficiencies The energy collection, conversion, and transmission process involves a number of steps, each having an associated efficiency. An initial task of the study was to estimate these efficiencies. Three estimates were made of the efficiency of each step, including a minimum efficiency that could be achieved with virtual certainty, a probably achievable (nominal) efficiency, and the best, or maximum, efficiency that might be achieved. These estimates are presented in figure IV-2. The estimated overall efficiencies from incident sunlight to de output were 4.2, 5.4, and 8 percent for the "minimum," "probable," and "maximum" cases, respectively. The estimated efficiencies of the system excluding photovoltaic conversion of sunlight to electrical energy were 41, 52, and 69 percent, respectively. These estimated efficiencies were used for collector sizing and weight estimates. Revised efficiency estimates indicated that the "probable" achievable (nominal) efficiency was more appropriately 58 percent than 52 percent. The efficiencies of the various steps resulting in this revised "probable" estimate are also presented in figure IV-2. 2. MPTS/MRCS Analysis An analysis was conducted to determine the appropriate size of the power station, defined in terms of the de output power at the rectenna and the overall microwave system(s) parameters. Two specific constraints were identified that would limit the maximum power output. These constraints were maximum allowable power densities of 21 kW/m at the transmitting antenna and 23 mW/cm at the ionosphere. The former is the result of the thermal limitations of the aluminum waveguides. The latter is the result of a theoretical analysis (ref. Meltz) which indicates that nonlinear interactions between the beam and the ionosphere will not exist below this level. Given a system frequency (2.45 GHz) and the estimated efficiencies of steps in the transmission process, the two aforementioned constraints can be related to de output power and transmitting antenna diameter. These relationships are illustrated in figure IV-3. It can be seen from the figure that the maximum power output that does not exceed the constraints is 5 GW, achieved with a transmitting antenna diameter of 1 km. Accordingly, a 5-GW de output power at the rectenna and a 1-km antenna diameter have been used as nominal, or reference, values throughout the study.
Figure IV-2.- Estimated efficiencies of the various steps in the collection, conversion, and transmission process.
Figure IV-3.- Output power limits. A microwave frequency of 2.45 GHz was selected for study purposes. This frequency is at the center of a 100-MHz band reserved for industrial, scientific, and medical use, so that interference with communications will be minimized. Atmospheric attenuation is also low at this frequency. A higher frequency, such as 3 GHz, offers higher gain for the same antenna diameter and should be considered, but would cause substantial interference with present users of this band. The mainbeam pattern and sidelobe characteristics of the antenna will vary with the power density taper over the antenna. Increasing the amount of taper produces a lower boresight density, a wider main- lobe, and lower sidelobes. For a given rectenna radius, the collection efficiency increases with the amount of taper. A 10-dB taper has been adopted for this study. For a no-error/no-failure condition, this gives a 90-percent collection efficiency at a rectenna radius of 4300 m. An ideal continuous taper would be too complex to be practical and was replaced in this study by a 10-step approximation that gives virtually the same performance.
The transmitting antenna consists of a number of subarrays, each of which is phase controlled as a unit. Increasing the size of individual subarrays reduces the number of receivers and phasing electronics required, and therefore the cost of the phasing control system. Decreasing the size of the subarrays reduces thermal distortion and the probable need for active positioning to compensate for misalinement. Subarray sizes of 4, 10, and 18 m (square) were studied. The 10-m size was selected as a reference, because it required less phase-control equipment than the 4-m size while not needing the active mechanical alinement of the 18-m size. A summary of the microwave system(s) parameters is presented in table IV-1. These parameters were utilized in the calculation of the power density distribution across the rectenna, which is presented in figure IV-4. Power densities of 23 mW/cm2 and 1 mW/cm? exist at the center and edge (5 km) of the rectenna, respectively. The latter density corresponds to one-tenth of the current U.S. standard for allowable human exposure to microwave radiation. 3. Orbit Considerations There are three orbit perturbations of importance. The Earth's equatorial ellipticity, solar and lunar gravity gradients, and solar radiation pressure result in satellite movement that must be assessed and possibly counteracted. The equatorial ellipticity causes a drift in longitude centered about either longitude 120° W or 60° E. Such a drift is unacceptable in view of an expected large number of satellites in this orbit and the need to maintain a proper relationship between the satellite and the receiving antenna. The velocity increment required to counteract this drift, however, is less than 1 m/s/yr. Solar and lunar gravity gradients cause an initial inclination of zero to grow to about 15° in 27 years. Nonzero inclinations require larger rectennas (approximately 10 to 30 percent for 7.3° inclination). Zero inclination can be maintained with a velocity increment of 46 m/s/yr; this appears to be a reasonable price. Solar radiation pressure produces an eccentricity in the orbit. To maintain the eccentricity at zero requires a velocity increment of a few hundred m/s/yr. The problems associated with a slightly eccentric orbit, primarily a moderate departure from constant velocity antenna rotation and a small (on the order of +1 percent) variation in rectenna output, do not appear to warrant the expenditure.
TABLE IV-1.- A SUMMARY OF MICROWAVE SYSTEM(S) PARAMETERS
Figure IV-4.- Power density at rectenna.
The SPS, in synchronous equatorial orbit, will be eclipsed by the Earth daily for about 43 days at the spring equinox and 44 days at the fall equinox (fig. IV-5). The maximum duration is about 75 minutes. The eclipse is total and occurs at about local midnight. Because the maximum dimension of a typical SPS is 6 to 7 percent of the width of the penumbra, the illumination gradient is slight. Total power loss is slightly less than 1 percent of total annual output. The close spacing (about 0.5° of longitude) that results from a large number of satellites (112 located to serve the United States) will cause the satellites to eclipse each other twice a day, at about 6 a.m. and 6 p.m., for about 2 weeks, at the equinoxes. This eclipse is shorter and is not total, but will cause almost complete microwave power loss for as long as 15 minutes. The penumbra is much narrower than the satellite dimensions, so that illumination gradients are steep. Differential thermal expansion must therefore be accounted for in the system design. Figure IV-5.- Eclipse geometry.
Power loss is less than 0.1 percent of annual output; however, these eclipse conditions must be considered in integrating satellite-generated power with surface systems. 4. Configurations In an attempt to minimize structural weights, the concept illustrated in figure IV-6(a) (column/cable) was developed utilizing compression columns and supporting cables (see "SECS Structure," sec. IV-B-3). With this configuration, the transmitting antenna has to be mounted on the north or south end of the solar array to be able to view the Earth continuously. However, microwave recoil from the antenna (about 5 lb) causes a constant disturbing torque, and the offset in the center of mass also produces a solar radiation pressure torque. To eliminate these disturbances, the solar array area was doubled and an antenna was mounted on each end. The resulting configuration is essentially two 5-GW satellites sharing a commmon structure. For a given total power requirement, this approach has an additional advantage in that the number of satellites is halved and consequently the distance between satellites doubled. This simplifies traffic control and maintenance and reduces the impact of eclipse by other satellites. This configurational approach did result in a very low structural weight, as will be seen in the subsequent presentation of mass properties; however, it should also be noted that the SECS structural weight is not a large percentage of the satellite total weight, ranging from 1 percent (minimum, column/cable) to 6 percent (maximum, truss) for the cases considered. The column/cable configuration has the potential disadvantage of being incompatible with a mission mode that involves construction of the satellite, or modules thereof, at low-Earth orbit, which then provides solar energy to propel the satellite to geosynchronous orbit. A second configuration was also considered in some depth (fig. IV-6(b)). It is referred to as the "truss" configuration. Like the column/cable, it has two 5-GW antennas and a "double-size" solar array, although a single 5-GW system with central antenna is also possible. It is built up as a three-dimensional truss and may be easier to construct in geosynchronous orbit than the column/cable configuration. It can also be built in modules at low-Earth orbit. Several other configurations were considered briefly; they were adaptations of the two concepts described previously and did not appear to offer any overriding advantages. The sizes of the configurations as presented in figure IV-6 are related to "probable," or nominal, efficiencies of conversion and transmission as discussed in section IV-A-1. The relationship between efficiency, array size, and mass will be presented in the next section.
Figure IV-6.- Example configurations.
5. Mass Properties To determine the range of weights of the satellite, the following process was used. The three estimates of system efficiencies presented in figure IV-2 were used to define the solar array area necessary to provide 10-GW power output to two rectennas (5 GW each) for each estimate. The resulting areas were as follows. increased 4.3 percent to compensate for solar angle-of- incidence losses (see sec. IV-B-IV). For each subelement of the satellite, a "minimum," "nominal," and "maximum" unit weight was estimated. In this case, the minimum and maximum terms have the inverse meaning of that applied to the efficiency estimates. For example, the minimum weight is the best that might be achieved, whereas the maximum can be achieved with virtual certainty. Table IV-2 sunwarizes the minimum, nominal, and maximum unit weight estimates for the various subelements of the satellite. If the three subelement estimates are applied to each of the three array sizes and the two different configurations, a total of 18 (3 by 3 by 2) weight estimates are obtained. The resulting range, or envelope, of weights is presented in figure IV-7. The satellite mass is seen to be in a range between 47 000 and 124 000 metric tons. Note that this weight is associated with a satellite that provides 10 GW of power to two rectennas via two 1-km transmitting antennas. A satellite weight breakdown for 6 of the 18 estimates is presented in table IV-3. The six estimates presented are identified by symbols in figure IV-7. The breakdowns indicate the significance of the solar cell blankets to the total weight, approaching 50 percent in all cases. The SECS structure, on the other hand, is not a major contribution to the total, varying from 1 to 6 percent of the total for cases presented. The microwave generators contribute approximately 15 percent of the total weight. Note that the klystron was assumed for all estimates. Experience has shown that the total mass invariably grows during the course of any aerospace program, the amount depending on the degree of technology advancement involved. Fifty percent growth from the initial concept weight can reasonably be expected for a program of this nature. Accordingly, the totals obtained by summing the estimates of subelements have been increased by 50 percent. This weight growth has been included in the weights presented in figure IV-7.
TABLE IV-2.- SUMMARY OF UNIT MASSES
Figure IV-7.- Solar power satellite total mass.
TABLE IV-3.- MASS PROPERTIES SUMMARY
B. Solar Energy Collection System The SECS includes the necessary elements for the collection and conversion of sunlight to electrical power, the distribution of that power to the antenna interface, structural loads, and attitude and orbit control. A preliminary analysis indicated that the most promising conversion systems from the standpoint of current state of development were the photovoltaic silicon solar cell and the thermodynamic Brayton cycle. It was also recognized that more efficient and advanced systems might be required to establish SPS viability. For the purpose of the immediate study, however, systems effort was concentrated on the photovoltaic silicon solar cell approach to provide a departure point for comparative evaluation with other approaches in future studies. 1. Solar Array Silicon solar cells have been developed and utilized in spacecraft for a number of years. More recently, under the impetus of proposed terrestrial use, an intensive effort has been initiated to improve the efficiency and reduce the cost of silicon cells. Typical characteristics of space operational solar arrays and those projected to result from the present development efforts for Earth use are as follows. For the purposes of the present study, it has been estimated that efficiencies of 15 to 17 percent at 30° C are achievable within the projected SPS time frame. Cost and weight of the total solar array can be reduced by concentrating the sunlight so that the entire area need not be covered with solar cells. Accordingly, a parametric study of performance as a function of concentration ratio for both silicon (Si) and gallium arsenide (GaAs) cells was performed. It was found that GaAs becomes cost competitive only above concentration ratios of 4 to 6. At these ratios, the solar array requires relatively complex structure and must be oriented toward the Sun more accurately to avoid excessive losses. Silicon cells were used, at a concentration ratio of 2, as a reference for the current study. At this ratio, a simple trough can be used (fig. IV-8). Nominal conversion efficiency, including losses within the cell blanket, is estimated at 10.3 percent for the 100° C cell temperature expected with 2:1 concentration. Cell degradation due to radiation damage and thermal cycling is expected to be a total of 6 percent for the first 5 years and 1 percent/yr thereafter.
Figure IV-8.- Solar concentrator. A typical solar cell blanket, used as a reference design, is shown in figure IV-9. The electrical connections between cells (copper or aluminum) are sandwiched between two layers of Kapton and welded to the cell through holes in the upper layer of Kapton. The cells are covered with a plastic such as FEP Teflon. Cell thickness is 2 0.1 mm (4 mil). Total blanket weight is estimated at 0.31 to 0.46 kg/m . Concentrators are 12.5 mm (0.5 mil) with a thin aluminum coating. Their 2 weight is approximately 0.04 kg/m . Considerable development will be required in cell manufacture and blanket assembly. The present technique of growing a singlecrystal ingot about 3 in. in diameter, sawing it into disks, cutting the disks into square blanks, and lapping and polishing to make a cell, cannot hope to meet cost or quantity requirements of the program. Work has been done on growing silicon in thin sheets, but crystal defects, which reduce efficiency, are numerous.
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