7 '.. X'X< ' <^*A A*‘r< i- — < A '>'A -''\ A SURVEY OF SATELLITE POWER STATIONS PRC SYSTEMS SCIENCES COMPANY A Company of Planning Research Corporation
A SURVEY OF SATELLITE POWER STATIONS PRC R-1844 September 1976 Prepared for the Task Group on Satellite Power Stations Energy Research and Development Administration Under Contract E(49-18)-2071 Prepared by Charles E. Bloomquist PRC SYSTEMS SCIENCES COMPANY 7600 Old Springhouse RoadOMcLean. Virginia 22101 0(703) 893-1800 1100 Glendon Avenue □ Los Angeles, California 90024 □ (213) 473-1163
This report was prepared to document the background studies and to incorporate the data and information gathered by the support contractor for the ERDA Task Group on Satellite Power Stations. This report is the sole work of the support contractor and does not necessarily represent the views of the ERDA Task Group or of the U.S. Energy Research and Development Administration.
FOREWORD The Energy Research and Development Administration established on 15 March 1976 the Task Group on Satellite Power Stations to recommend to the ERDA Administrator the appropriate role, if any, of satellite power station research and development in ERDA's overall program. The Task Group requested the support of PRC Systems Sciences Company to consist of (1) consolidating the relevant documentation, (2) providing summary descriptions of the major systems and subsystems, (3) delineating the status of key technologies, (4) preparing brief summaries of the key economic and environmental issues, and (5) assisting the Task Group in preparation of specified documentation. This report documents these supporting efforts. The study was conducted under the direction of Charles E. Bloomquist. Other members of the study team were Winifred C. Graham, Allan D. Kotin, and Lloyd L. Philipson. Eloise E. Bean provided daily liaison with the ERDA Task Group and encouragement to the support contractor study team. Technical direction for the support work was provided by Ira L. Helms of the ERDA Task Group. Complete membership in the ERDA Task Group is given in Section XII of this report.
Glossary AEC Atomic Energy Commission AMO Air Mass Zero CdS Cadmium Sulfide CFA Crossed field amplifier COMSAT Communications Satellite Corporation DDT&E Design, Development, Test and Evaluation DSI Development Sciences, Inc. EFG Edge-defined film-fed growth EIR Environmental Impact Report ERDA Energy Research and Development Administration EVA Extra-vehicular activity GaAs Gallium Arsenide GEO Geosynchronous orbit HLLV Heavy Lift Launch Vehicle IOC Initial Operating Capability JPL Jet Propulsion Laboratory JSC Johnson Space Center LEO Low Earth Orbit LeRC Lewis Research Center LMFBR Liquid Metal Fast Breeder Reactor LWR Light Water Reactor MSBR Molten Salt Breeder Reactor MSFC Marshall Space Flight Center NaK Sodium-potassium coolant NASA National Aeronautics and Space Administration NSF National Science Foundation OTV Orbital Transfer Vehicle PRS Power Relay Satellite RD&D Research, Development, and Demonstration RFI Radio Frequency Interference Si Silicon SPS Satellite Power Station SSPS Satellite Solar Power Station
TABLE OF CONTENTS Page Foreword............................................................. 1V Glossary ............................................................. v I. INTRODUCTION................................................... 1 II. NASA PROGRAM AND SYSTEM CONCEPT .............................. 5 III. BRIEF SATELLITE POWER STATION CONCEPT DESCRIPTION ........... 13 IV. TECHNOLOGY STATUS AND REQUIREMENTS............................ 47 V. ECONOMIC ANALYSIS ............................................. 85 VI. ENERGY BALANCE....................................................133 VII. ENVIRONMENTAL EFFECTS ......................................... 155 VIII. KEY ISSUES........................................................169 IX. THE O'NEILL CONCEPT............................................. 209 X. ENERGY STORAGE TECHNOLOGY .................................... 231 XI. DEVELOPMENT TIME FOR NEW SYSTEMS.................................255 XII. TASK GROUP ON SATELLITE POWER STATIONS.......................... 269 REFERENCES..............................................................271 BIBLIOGRAPHY ......................................................... 279
LIST OF EXHIBITS Number Page 1 Satellite Solar Power Station (SSPS)................. 15 2 Solar Photovoltaic Satellite Configuration........... 16 3 Detail of Solar Collector Array ..................... 18 4 SSPS Major Components ................................ 20 5 SSPS Efficiency Chain ................................ 21 9 6 Unit Cost for a 5,000 MW SSPS ($1974 x 10 ).......... 23 7 SSPS Development Program Phasing...................... 25 8 Satellite Solar Thermal System........................ 27 9 Solar Thermal Satellite .............................. 31 10 Solar Cavity Absorber ................................ 32 11 Cost Comparisons, Solar Thermal Approaches........... 34 12 Program Schedule, Solar Thermal Approaches........... 36 13 Orbiting Nuclear Reactor Powerplant ................ 37 14 Molten Salt Breeder Reactor Concept ............... 39 15 The Power Relay Satellite............................ 42 16 SSPS Technology Summary.............................. 49 17 SPS Transportation Systems............................ 53 18 Microwave Conversion Devices.......................... 68 19 Microwave Power Beam - Idealized...................... 71 20 Material Technology Trend ............................ 74 21 Typical Requirements/Characteristics of Energy Conversion Approaches ............................... 64 22 Conceptual Cost Model of SSPS........................ 87 23 Example Unit Costs and Assumptions in Their Derivation............................................ 89
24 Example Impacts of Alternative Discount Rates on Unit Energy Costs.................................. 91 25 Payback Analysis of SSPS Development Programs (r=7.5%).............................................. 94 26 SSPS Unit Energy Costs Required to Recover Costs Under Alternative Discount and DDT&E Assumptions. . . 9^ 27 Unit Cost of Energy as a Function of Discount Rate and DDT&E at Full Buildout of SSPS (109 5-GW Satellites). 99 28 Major Components of Capital Cost for a 10 GW Satellite (Excluding DDT&E) (Millions $1974 or $1975) ........ 101 29 Example Solar Array Total and Unit Costs............. 103 30 Estimates of Competitive Generation Costs by Various Sources.............................................. 1°8 31 Estimate of Baseload Energy Generation Costs for SSPS (Photovoltaic) Using TRW Methodology (Per kW) .... Hl 32 Baseload Plant Comparisons........................... H2 33 Busbar Energy Costs for Three Alternative Systems ($1975).............................................. HO 34 Comparative Energy Costs (Busbar) for Alternative Systems (1975)........................................ H® 35 Alternative Estimates of LMFBR Busbar Energy Costs (1974-1975$).......................................... i24 36 Estimate of Sales Revenue for Fissile Plutonium for the LMFBR............................................ 126 37 Alternative Estimates of Terrestrial Solar (Central Receiver Thermal) Busbar Energy Costs ............... I29 38 Hypothetical Allocation Over Time of Alternative Estimates of LMFBR Program Costs..................... I29 39 Preliminary Terrestrial Solar R&D Estimate: (Central Receiver Plant) ...................................... I39 40 Hypothetical Growth Patterns of Generating Capacity for Alternative Power Sources to Reach 445 GWe by 2020 (in GWe).............................................. i32
41 External Energy Subsidies for Selected Alternative Power Plants.......................................... 136 42 Summary - Comparison of Energy Subsidies for Electric Systems...................................... 137 43 Energy "Payback" Periods After Operating Energy Subsidies for Alternative Generation Systems (Per 1,000 BTU Output).................................... 140 44 Energy Budgets for SSPS.............................. 142 45 Capital Energies for SSPS by Major Component......... 145 46 Operating Energies for SSPS by Major Component. . . . 147 47 Energy Cost of Goods and Services (Btu/$ Final Output................................................ 150 48 SSPS Materials Required Versus Potential Demand and Available Resources .................................. 152 49 Environmental Impact Areas........................... 156 50 Distribution of Microwave Power Density from the Beam Center.......................................... 163 51 Microwave Radiation Exposure Limits and SPS Beam Values at Various Locations ......................... 164 52 Worker Radiation Dose................................ 167 53 Solar Photovoltaic Array Cost Projection.............. I80 54 Operating Costs...................................... 195 55 Capital Costs........................................ 195 56 Environmental Impact Areas............................ 202 57 Microwave Power Density Limitations (Typical Ground Power Density Pattern............................... 206 58 Program Cost Estimates................................ 218 59 Comparison of O'Neill Concept with Alternative Power Systems.............................................. 223
60 Non-Thermal Storage Survey............................ 232 61 Non-Thermal Storage Systems Capital Cost............. 233 62 Capital Cost Uncertainty - 6 Hours Storage........... 234 63 Non-Thermal Storage Efficiency - 6 Hour Storage ... 235 64 Summary Thermal Heat Storage.......................... 236 65 Projected Characteristics and Status of Energy Storage Systems (EPRI, 1974)......................... 238 66 Criteria and Ratings for Storage Technologies .... 241 67 Criteria for Storage Technologies - Technical and Economic Aspects (Costs in 1970 Dollars)............ 243 68 Decision Process...................................... 256 69 Development Time for Ten Innovations of Various Types................................................ 258 70 Development Time for a Number of Technological Innovations.......................................... 260 71 Chronology of the Discovery and Development of Nuclear Power Generation...................................... 262 72 Chronology of the Discovery and Development of the Automobile............................................ 263 73 Chronology of the Discovery and Development of the Airplane.............................................. 264 74 Chronology of Discovery and Development in Television Broadcasting.......................................... 265 75 Chronology of the Discovery and Development of Electronic Computers.................................. 266 76 Chronology of the Discovery and Development of Numerically Controlled Machine Tools................. 267
I. INTRODUCTION Over the past several years there has been a growing interest in the possibility of using space and space technology to provide significant quantities of energy for utilization on earth. This interest has been made manifest in a number of studies conducted largely, and appropriately, under the auspices of the National Aeronautics and Space Administration. Attention has centered about a satellite power station concept in which electrical energy from solar or nuclear power plants is generated in earth orbit and then transmitted to the ground by microwave beams. To the extent that this approach can effectively contribute to the solution of the energy problem, it is of interest to the Energy Research and Development Administration and should become a part of ERDA's national energy research, development and demonstration program. The Task Group on Satellite Power Stations was formed within ERDA to investigate this potential energy source and to make recommendations to the Administrator regarding future program options. This report was prepared to assist the Task Group in discharging that responsibility. It is basically a review and summary of available documentation as listed in the bibliography. In some instances, previously published results were extended by independent analyses to more clearly illuminate the underlying issues. These analyses were conducted primarily in the areas of economics and energy balance. The overall organization and content of this report is described in the following paragraphs . The basic thrust of the NASA program related to satellite power stations is described in Section II. The major satellite power station concepts themselves are summarized in Section III. This section also includes a description of related orbital reflective systems and supporting terrestrial facilities. Section IV deals with the technological status and requirements of six SPS developmental areas. It is recognized by most researchers that a successful SPS program will require (1) new launch and orbit transfer
vehicles, (2) breakthrough in SPS solar power components, particularly solar cells, (3) resolution of the nuclear power question, (4) further study and experimental work in the microwave transmission of power, (5) a considerable advance in current capabilities for the fabrication and assembly of large space structures, and (6) substantial technological development to achieve the necessary power conversion processes and devices. In determining SPS feasibility the economic question is almost certainly the pivotal one. It is reasonably clear that the technological problems can be solved, given sufficient resources; it is not so clear that these problems can be solved economically. Section V discusses the economic issues and presents a coherent economic analysis of the SPS concept. Section VI, Energy Balance, calculates the energy requirements of an SPS system, compares the results with the energy requirements of several alternative systems, and summarizes the basic material resources required for a nominal SPS program. The environmental inpact associated with the SPS can be considered under three headings: (1) social impacts, (2) environmental pollution, and (3) microwave transmission. Social inpacts include radio frequency interference with communications systems, land use for microwave receiving and launch facilities, and the general issues of public safety. Environmental pollution stems from the massive manufacturing processes and the extremely large number of launch vehicles required. The microwave transmission issue revolves primarily about the acceptable levels of such radiation that can be permitted. Section VII contains a detailed summary of the potential SPS environmental inpact. There are basically three key issues in the undertaking of an SPS program. These issues, which are discussed at some length in Sections IV through VII of this report are: (1) technology, (2) economics— including the question of energy balance, and (3) environmental impact. Section VIII, Key Issues, highlights several of the more pressing problems that must be solved in these three areas. Three SPS-related topics are then treated in Sections IX, X, and XI. These are, respectively, the O'Neill concept characterized by space
colonization and the utilization of lunar materials for manufacturing an SPS, energy storage technology, and the development time required for new high technology systems. Section XII lists the membership in the Satellite Power Station Task Group.
II. NASA PROGRAM AND SYSTEM CONCEPTS There are several ways in which space technology can respond to society's energy needs. Of primary interest here is the possibility that electric power can be generated in space and then transmitted to antennas on the earth by microwave radio beams, thereafter to be distributed by conventional surface systems. An alternate possibility might be to use orbiting microwave reflectors to relay power from a remote earth-based generator to a spot conveniently close to a demand center. Power in space could be generated by nuclear reactors, safely removed from the earth's biosphere, or it could be generated by using the energy of the sun itself, an inexhaustible source and from the point of view of a geosynchronous satellite available essentially 24 hours a day. The Office of Energy Programs at NASA Headquarters and several of the NASA Centers have conducted, and are pursuing, a number of efforts related to a satellite power system both in terms of system concept definitions and supporting studies. This section of the report does not attempt to cover all relevant activities nor does it attempt to place a program structure on the essentially diverse but increasingly coordinated NASA activities that have been undertaken. The difficulty of such effort is indicated, for example, by the fact that the Marshall Space Flight Center currently recognizes the following SPS elements: • 13 Programs • 48 Projects • 124 Systems • 122 Subsystems • 233 Activities and Operations • 29 Facilities This section does have two specific aims. The first is to give an indication of the major activities and general thrust of the NASA efforts related to the SPS. The second aim is to briefly describe the major system concepts for an SPS that have thus far been developed.
1. System Concepts Each of the major proposed systems can be readily partitioned into three subsystems: (1) the orbital subsystem, (2) the microwave transmission subsystem, and (3) the terrestrial receiving/rectifying subsystem. The latter two subsystems are essentially identical for any of the proposed orbital subsystems. The Power Relay Satellite concept also requires a terrestrial transmitter. The orbital subsystem, then, determines the concept. There are seven basic conceits: Photovoltaic Solar Thermal (Closed Brayton Cycle) Solar Thermal (Thermionic Conversion) Solar Thermal (Cascaded System) Nuclear (Closed Brayton Cycle) Nuclear (Thermionic Conversion) Power Relay Satellite Each of these is very briefly defined in subsection a, below; sub- I sections b and c deal with the microwave and rectenna subsystems. More complete descriptions of all concepts and subsystems are contained in Section III. a. Orbital Systems It is the basic function of the orbital system to generate sufficient microwave power to provide 5 (or 10) GW at the terrestrial busbar after all losses due to transmission and conversion. The Power Relay Satellite, of course, must reflect rather than generate this amount of power. (1) The Photovoltaic Concept This concept directly converts solar radiation to electricity by means of two extremely large arrays of silicon solar cells. The usual design consists of rectangular arrays about 2.7 mi x 3.2 mi (4.3 km x 5.2 km) separated by the microwave transmitter. Mirrors are used to concentrate the solar radiation onto the cells to increase the power output. The solar collector panels are supported by both
nonconducting and conducting structures which carry the power to microwave generators for transmittal to the terrestrial rectenna. A reaction control system is required to keep the satellite in the appropriate orbit and to assure that the solar collector panels point towards the sun while the microwave antenna is directed towards the receiving antenna on earth. Rotary joints to permit rotation of the microwave transmitting antenna are the only continuously active components in an otherwise passive satellite. (2) The Solar Thermal Concept This concept uses huge concentrating mirrors to raise the temperature of a cavity absorber which in turn is used to drive an energy converter. There are generally four modules, each consisting of a hexagonal dish-like reflector (achieving concentration ratios up to 2000) and a cavity absorber. One module also carries the microwave generator and transmitter. The four modules are connected together in a row whose overall length is upwards of 10.5 miles (16.9 km) depending on conversion method. The module (and satellite) width is upwards of 2.4 miles (3.9 km). The heat is converted to electricity by one of three methods: (a) Thermionic Conversion In this scheme, thermionic 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 provided. Fins are added to the collectors to improve cooling. In one variation to this scheme, a liquid metal cooling loop is added to further lower collector temperatures and hence improve the conversion efficiency. Rotary convertors step-up the diode output voltage to achieve the level required by the microwave transmission. (b) Closed Brayton Cycle Conversion In this scheme, Brayton turbomachines are mounted on the outside wall of the solar cavity absorber. Thermal energy is added to the helium working fluid in heat exchanger tubing located within the
cavity absorber. Waste heat is rejected through a gas-to-liquid heat exchanger to a liquid metal cooling loop. The ac output is then distributed to the microwave generator for transmission. (c) Thermionic/Brayton Cycle Conversion In this "cascaded" system the waste heat from the thermionic diodes is used as input to the Brayton cycle turbomachine. The diodes are cooled by the helium flow and the Brayton loop is cooled by a liquid metal radiator. The output voltages from the two systems are reconciled and combined for distribution to the microwave subsystem. (3) The Nuclear Concept This concept, rather than using solar radiation as its primary source, uses an orbiting molten salt breeder reactor (MSBR). Sixteen 1 GW reactors will be combined in geosynchronous orbit to provide the necessary power. The heat produced by the reactors will be converted to electricity by two methods: (a) Thermionic Conversion In the thermionic system each generating module is located in the center of a 2300 ft (700 m) square radiating panel. Eight panels are joined linearly and connected to the 0.6 mi. (1 km) transmitting antenna between the fourth and fifth modules. The other eight panels are similarly assembled and oriented 90° from the first set to minimize the mutual view factor. Distributing the panels in this fashion increases the distance over which electrical distribution occurs, but the resultant mass penalty is less than the radiator manifold penalty which would occur if the module were clustered together. The most recent Boeing studies (Ref. A17) indicate that nuclear thermionic conversion is not feasible with 1985 technology and the MSBR, since the temperature differential which can be obtained across the diodes is not great enough for efficient operation. (b) Closed Brayton Cycle Conversion In the closed Brayton cycle system, the salt mixture is circulated out of the reactor core through a heat exchanger which
transfers energy to the helium loop which, in turn, drives the Brayton turbomachines. As in the solar system, waste heat is rejected through a gas-to-liguid heat exchanger to a liquid metal cooling loop. The mechanical configuration is not clear for this system, but its ac output will be provided to the microwave generator for transmission to the ground. (4) The Power Relay Satellite This concept consists of an on-orbit reflector which receives microwave beams from earth-based power sources and reflects them back to earth near a power distribution point. The relay function is generally seen to be passive and to consist of a primary 0.6 mi. (1 km) structure that is built up of 354 ft. x 354 ft. x 66 ft. (108 m x 108 m x 20 m) deep bays. A substructure would provide support for the microwave reflector. The ground transmitting antenna would be a phased array with wave guides and convertors similar to the orbital antenna in the power generating satellites but larger. The receiving antenna would be essentially the same. The feasibility of the PRS significantly depends on large-scale, highly efficient transmission of energy by microwaves, as well as very high precision reflectors and attitude control. It is not currently seen to be competitive with ground systems for distances less than about 3500 miles (5600 km). Furthermore, the high power density of the up-beam in the earth's atmosphere may violate reasonable safety standards. 2. Materials Materials are the basic building blocks of all space vehicles and several materials improvements are of particular interest to the SPS. These include weight reduction of structural materials, high temperature materials (particularly inportant for nuclear and solar thermal conversion processes), lightweight and durable reflecting surfaces and large cost reductions in solar cells. Temperatures encountered in the heat exchanger tubing used in the nuclear or solar thermal systems are on the order of 1200°F - 1600°F (925K - 1150K) which combined with weight restrictions, a capability to withstand mechanical stresses, and reasonable
fabrication capabilities severely limits the possible materials choices. While there have been some improvements in existing alloys, the introduction of a new alloy type, e.g., columbium-based B-66, would be more likely to resolve the problems in this area. Reflective surfaces in space degrade from the meteoroid flux encountered and from proton irradiation. The SPS reflective surfaces are usually assumed to be metalized plastic film (probably aluminized Kapton). The meteoroid problem is not considered to be significant (less than 5 percent degradation from this source over 30 years). Tests to date on degradation due to proton irradiation are quite inconclusive but potentially very severe. The only reported results (Ref. A12) indicate that an irradiation dose rate 900 times that expected at geosynchronous orbit caused reflectivity to degrade from 0.92 to 0.59 in 78 hours (900 x 78 hours = 8 years). Work is continuing in this area. 3. Processes The operational details of many of the processes required in successful implementation of an SPS are as yet unavailable, and in some cases are not even defined. Two areas that have been the subject of some study are microwave transmission of power and conversion of heat to electricity by the thermionic process. Raytheon and JPL, in particular, have dealt with the problem of microwave power transmission; Section IV summarizes these efforts. The thermionic conversion process is theoretically desirable since it is essentially a passive process utilizing large numbers of identical thermionic diodes and, therefore, has a good reliability and producibility potential. The main problem is the relatively low efficiency of these devices, coupled with the high temperature differential required between the diode emitter and collector. At an emitter tenperature of 2780°F (1800K) and a collector temperature of 1160°F (900K), which is nearly ideal and difficult to realize in space, thermionic converter efficiency is currently only about 15 percent. This figure is projected to reach 36 percent by 1995 but consi durable efforts will have to be expended to reach this goal.
4. Transportation Propulsion systems are expensive to develop and they are usually the major component of a vehicle system and, considering fuel, comprise the majority of the weight at launch. Therefore, propulsion technology improvements can substantially improve performance and reduce cost. Most researchers in the SPS area consider that a successful program will require a new generation of launch vehicles, including a heavy lift launch vehicle (HLLV) which can place on the order of 500,000 lbs (225,000 kg) into low earth orbit. This compares to a shuttle capability of 30 tons (27,000 kg). This problem is currently under study at the Johnson Space Center and at various contractors. It is discussed more fully in Section IV of this report. It is also generally agreed that the SPS would be assembled (at least partially) in low earth orbit and then transferred to a geosynchronous orbit using some new orbital transfer vehicle (OTV). Ion propulsion is an often mentioned option for this stage although cryogenic tugs do offer one alternative. NASA (Ref. A6) has outlined a rather detailed list of tasks for the development of the OTV and Section IV of this report also deals with the development of a new transfer stage. 5. Large Space Structures All investigators agree that considerable advances are needed in the fabrication and assembly of large space structures prior to development of the first operational SPS. Section IV, which contains a rather detailed discussion of this area, lists 14 requirements/considerations that must be addressed. The Johnson Space Center sponsored a study conducted by Martin-Marietta which dealt in detail with the orbital assembly and maintenance of large space structures. Many other studies dealing with the design and construction of these systems have been completed or are in progress. Both Marshall Space Flight Center and the Johnson Space Center have proposed alternate designs to the configuration proposed by A.D. Little, et al., for the photovoltaic system based primarily on structural considerations.
6. Economics ECON, Incorporated, under contract to the Marshall Space Flight Center, and with the assistance of Arthur D. Little, Grumman and Raytheon, has recently completed a rather comprehensive economic evaluation of an SPS based on the photovoltaic concept and a power relay satellite (Ref. All). JPL (Refs. A7, A8, A15) is also conducting comparative assessments of orbital and terrestrial center power systems. All investigators acknowledge that the economic question is the pivotal one in determining SPS feasibility. It is reasonably clear that the technological problems can be solved, given enough money; it is not so clear that these problems can be solved economically. Section V discusses the key economic issues and contains a comprehensive economic analysis. 7. Environmental Impact The primary environmental impacts associated with the SPS that have been addressed thus far can be grouped into the three areas of social impact, environmental pollution, and microwave transmission. The social impacts include such things as radio frequency interference with communication systems, land use for microwave receiving and launch facility, and the general issues of public safety associated with the SPS, particularly if nuclear power generation is used. Environmental pollution stems from the massive manufacturing processes and extremely large number of launch vehicles required. The microwave transmission issue revolves primarily about the acceptable levels of such radiation that can be permitted for humans and other animal life although there may be some secondary climatological effects, such as inosphereic heating. Most concept studies deal with the environmental issue to one extent or another and the microwave transmission has been investigated in some detail. A detailed treatment of SPS environmental effects is presented in Section VII.
III. BRIEF SATELLITE POWER STATION CONCEPT DESCRIPTION In 1968, Dr. Peter Glaser of Arthur D. Little, Inc., proposed a solar photovoltaic satellite for the generation of power to be used on earth. This concept has been developed and refined under a series of studies sponsored by NASA's Lewis Research Center and Marshall Space Flight Center and largely conducted by a team comprised of Arthur D. Little, Grumman, Raytheon, Spectrolab, and ECON. The Boeing Company has proposed a solar thermal conversion system which is an outgrowth of their work for ERDA and the Electric Power Research Institute on land-based solar energy systems. Boeing has been assisted in its studies, which have been expanded to include nuclear power generation and reflecting systems, by AiResearch and the Georgia Institute of Technology. Krafft A. Ehricke of Rockwell International first suggested the power relay satellite (PRS) as well as nuclear power satellites and large orbiting mirrors to reflect sunlight onto the night side of the earth at selected locations. All these concepts and several others have also been studied independently to various depths by the Johnson Space Center, the Marshall Space Flight Center, the Ames Research Center, the Jet Propulsion Laboratory, and NASA Headquarters, resulting at the present time in essentially four satellite system concepts which may be identified as follows: (1) Solar Photovoltaic (2) Solar Thermal (3) Nuclear (4) Power Relay The first concept is generally referred to as the satellite solar power system (SSPS), the next two are often called Powersat and include not only the two power sources but several conversion methods as well. The final concept is referred to as the Power Relay Satellite (PRS). These concepts as well as the related Reflective Systems and Supporting Terrestrial Systems are described in the following six subsections. The objective of these descriptions is to summarize current NASA work in this area. Their presentation here implies no endorsement by ERDA.
1. Solar Photovoltaic The solar photovoltaic approach is perhaps the most widely discussed means for utilizing space in the satisfaction of the nation's energy requirement. It is based on the use of solar cells to convert sunlight directly to electricity in orbit with the resultant energy beamed to earth via microwaves. Exhibit 1 is a highly simplified representation of this concept. The satellite will be located in geosynchronous orbit and the ground station at any of a large number of suitable locations on earth. The ground station is discussed in subsection 6 of this section and microwave transmission is treated in Section IV. a. Physical Configuration Although several alternate satellite configurations have been proposed and are currently under consideration, the one shown in Exhibit 2 has evolved from a number of studies over several years to become * essentially the baseline configuration. The two solar collector panels are designed to provide an output of about 8500 MW which results in an effective power output at the ground receiving antenna bus bar of about 5000 MW. A 328-foot (100-meter) diameter central mast and stiffened carried-through structure running through the assembly provide structural integrity. A microwave transmitting antenna is located between the two solar collector panels. The solar collector panels are arranged to face the sun continuously while the microwave antenna will rotate once a day with respect to the solar collector in synchronous orbit. * These studies have, for the most part, been led by Dr. Peter E. Glaser of Arthur D. Little, Inc., and include contributions from Grumman, Spectralab-Raytheon, and Econ. This concept description relies heavily on the resultant study documentation fully cited as References A2, A3, All, Al3, A14, some popularizations cited as Dl, E12, Fl, and a statement by Dr. Glaser before the Subcommittee on Space Science and Applications and the Subcommittee on Energy Research, Development and Demonstration on 20 February 1976 entitled ''Development of the Satellite Solar Power Station."
EXHIBIT 1. SATELLITE SOLAR POWER STATION (SSPS)
EXHIBIT 2. SOLAR PHOTOVOLTAIC SATELLITE CONFIGURATION
The solar collector panels are supported by nonconducting as well as conducting structures which carry the power to the microwave generators via the central masts. Dielectric materials are used for the continuous support structure which is transparent to the microwave beam. Rotary joints are provided at the perimeter of the central mast to allow rotation of the microwave transmitting antenna. These joints are the only major continuously active components in an otherwise passive satellite. The solar cells will actually be based on the "roll out" blanket design of current technology but will incorporate improved fabrication techniques, a substantial reduction in cell thickness, and the use of solar concentrators to reduce weight from the currently attainable 30 Ib/kW (14 kg/kW) to about 3 Ib/kW (1.4 kg/kW). Solar concentrators with Kapton film mirrors coated to reflect solar radiation onto the solar cells and to filter undesirable portions of the solar spectrum are designed to reduce the area requirements for the solar cells and their weight and cost. Exhibit 3 indicates the arrangement of the solar cell arrays and concentrating reflectors. A concentration factor (n) of 2 will reduce the efficiency of an 18% silicon solar cell to about 14% at the operating temperature when heat rejecting coatings are used for the solar cell array. The exposure of the arrays to the space environment is projected to result in a logarithmic degradation of silicon solar cells, with a 6% loss of the original efficiency after the first five years. Micrometeroids are projected to impact 1% of the solar cells during a 30-year operational lifetime. A reaction control system based on the use of ion engines (Argon is the candidate propellant) will be required to keep the satellite in the appropriate orbit and to assure that the solar collector panels point towards the sun to within one degree, while the microwave antenna is directed towards the receiving antenna on earth. To achieve the desired stationkeeping and attitude control, about 100,000 lb (45,000 kg) of propellant will be required per year, depending upon specific orbital characteristics .
EXHIBIT 3. DETAIL OF SOLAR COLLECTOR ARRAY
The major components of the Satellite Solar Power System (SSPS) are shown in Exhibit 4 together with some of their more pertinent characteristics and overall mass estimates for the 5000 MW system. b. Efficiency Exhibit 5 indicates the efficiency goals for the various steps in the process. The solar energy conversion efficiency of 14 percent assumes a solar concentration factor of 2 and initial solar cell efficiences of 18 percent. The end-to-end efficiency is just under 8.5 percent. Efficiency of all steps other than the solar cell collection is projected to be about 60 percent. c. Transportation The SSPS will require a space transportation system capable of placing a large mass of payload into synchronous orbit at the lowest possible cost. The cost of transportation, assembly and maintenance will have the most significant impact on the economic feasibility of the SSPS. It is highly likely that a two-stage transportation system will evolve, which will carry payloads first to low-earth orbit and subsequently deliver partially assembled components to synchronous orbit or possibly to an intermediate orbital altitude for final assembly and deployment. The space transportation systems being considered are primarily an extension of existing systems. The potential systems, starting with the space shuttle now under development, vary from the use of a modified space shuttle to the development of a fully reusable liquid oxygen/liquid hydrogen heavy-lift launch vehicle (HLLV) with a potential 500,000 lb (225,000 kg) payload capability to low-earth orbit. The current shuttle or its modifications can be used for SSPS technology verification and flight demonstration and for transporting elements of the prototype SSPS into low-earth orbit. The cost for such a system capable of lifting payloads up to 160,000 lb (73,000 Kg) to low-earth orbit, is projected to be $100-200/lb. The heavy-lift launch vehicle is expected to reduce payload costs to between $20 and $60 per pound for delivery to low-earth orbit. The large mass of payloads will require about 80 to 100 flights for each SSPS assembled in synchronous orbit when an advanced space transportation
Components SOLAR ARRAYS Solar Cell Blankets: • Solar Cells - 2 mil (50 micron) Silicon, 18% Eff. • Radiation Shield - Teflon • Metal Interconnects • Substrate - Teflon & Kapton Film Laminate Weight: 0.06 Ib/ft (0.0282 gm/cm ) Solar Concentrators: 1/2 mil (12.5 micron) Kapton With Aluminized Coating Weight: 0.004 lb/ft2 (0.002 gm/cm ) Busses Including Mast: Aluminum, Transmitting 40,000 Volts Used as Structural Members and Optimized for Maximum Efficiency. Non Conductive Structure: Aluminum, Utilizing a Basic Building Element. Designed for Stiffness & Control Requirements. TRANSMITTING ANTENNA Microwave Antenna Waveguides: 4 Layers Graphite Epoxy, Metalized Internally. 0.002 inch (0.005 cm) per Layer. Microwave Generators: Outputs 4.5 kW Each Weight: 0.5 lb(0.227 kg) Radiators Pyrolytic Graphic Discs 0.72 lb (0.32 kg) CONTROL SYSTEM ROTARY JOINT TOTAL SYSTEM EXHIBIT 4. SSPS MAJOR COMPONENTS
Projected System Efficiency = 60% X Solar Collection Efficiency EXHIBIT 5. SSPS EFFICIENCY CHAIN
system based on heavy-lift launch vehicles is used. The achievement of low-cost space transportation will be essential to the commercial success of the SSPS. d. Assembly The large number of components, most of them performing an identical function, and the role of man in assembling these components pose the requirement for careful evaluation of the methods of assembly, the packaging of components, assembly rates, and maintenance support facilities. There are two basic approaches to assembly: (1) Remote assembly using ground controlled teleoperators . (2) Assembly of components delivered to synchronous orbit by an assembly crew operating from a space station support base as part of extravehicular activities. It is highly likely that a combination of both manned operations and teleoperators will evolve, where man's most important function will be to exercise control over the assembly process. e. Maintenance With any major system such as the SSPS, the design criteria, choices of materials, and data on component life and the expected operating conditions will determine reliability. Redundancy of components through the use of large numbers of identical components, for example, solar cells, will tend to reduce maintenance requirements. The cost of performing repairs has to be evaluated and compared with the option of delaying repairs and accepting the potential loss of revenue while achieving operational lifetimes consistent with cost analyses of the SSPS operation. The goal will be to evolve maintenance-free designs, particularly for the solar cell blankets and the microwave generator subsystems. f. Cost An operational 5000 MW SSPS would cost about $7.6 billion (Exhibit 6) , or about $1500/kW. The largest cost element is space
9 EXHIBIT 6. UNIT COST FOR A 5,000 MW SSPS ($1974 x 10 )
transportation. Improvements in SSPS efficiency, particularly at the receiving antennas, and reduction in weight are both very significant factors in achieving and further reducing the projected unit cost. For an operational life of about 30 years the cost of power at the bus bar would be 27 mills/kWh. The expected life cycle revenues will be about $35 billion for each SSPS, while operating costs will be $140 million per year (or $4.2 billion over a 30-year life cycle). The revenues from a series of SSPSs will be used to offset the development program costs - $20 billion for the development of SSPS technology and another $24 billion for the development of the space transportation system and related technology. The SSPS development program costs could be repaid if 60 SSPSs were operational by the year 2014, assuming that alternative system generation costs average 35 mills/kWh. This number of operational SSPSs will provide at least 10 percent of incremental installed generation capacity in the United States. A larger number of operational SSPSs would be required to repay development program costs if alternative system generation costs were less than 35 mills/kWh between 1995 and 2014. g. Development Program The SSPS development program can be divided into three phases, as shown in Exhibit 7. The critical technology developments will be concerned with: • Improvements in photovoltaic conversion - including fabrication of solar cells; • The design of solar arrays and the performance analysis of large structures; • Techniques for manufacturing and assembling components in orbit; • System stability and control; • Microwave generation and transmission - including the development of DC-to-RF converters and filters, the development
EXHIBIT 7. SSPS DEVELOPMENT PROGRAM PHASING
of waveguide materials, phase front control, control of the transmitting antenna attitude, control or suppression of radio frequency interference; • Mechanical systems - including rotary joints, slip rings, motor drives, and switch gear; and • Stationkeeping systems - including ion engine development. At the end of the first phase, the subsystems and system functions would be verified in an orbiting test facility which may take the form of a space station. In parallel with the definition of the SSPS technology, the development, production and operation of the space transportation system for materials, equipment, and personnel from launch through deployment for the specified mission orbit would proceed. The development of the space transportation systems would include the development of the second-generation space shuttle, transfer vehicles between low-earth orbit and synchronous orbit, orbital propellant storage, and maneuvering vehicles to transport equipment, materials and propellants to the vicinity of the assembly site. The development of the space transportation system would coincide with the assembly of a large prototype SSPS which should be operated long enough to provide data and experience to guide the design of the full scale operational unit of about 5000 MW. After the successful completion of the second phase of the SSPS development program, the emphasis would shift to mass production, to provide at least 100 units by the year 2025. This development program is geared to achieve commercialization of the SSPS a few years before the year 2000 so that this option for the large-scale use of solar energy can play an increasingly important role in the generation of power on a world-wide scale in the 21st century. 2. Solar Thermal The primary alternative to the photovoltaic concept is the solar thermal system. Exhibit 8 is a highly simplified representation of this
EXHIBIT 8. SATELLITE SOLAR THERMAL SYSTEM
concept. Except for the conversion of sunlight to electricity, this system is very similar to the solar photovoltaic concept and has similar microwave, transportation, and support considerations. a. Power Conversion Methods Three power conversion methods, with one method having two * approaches, have been studied. The first method is the closed Brayton cycle system which utilizes rotating machinery. The second method is the thermionic system, one approach for which is passive except for coolant pumps, if required, while the other approach involves active cooling. The third method is a cascaded system employing thermionic devices and the Brayton cycle in series, i.e., the devices are cooled by the Brayton cycle, with each extracting a portion of the solar energy available. A brief description of each of these possibilities is given in the following four subsections. (1) Solar Thermionic, Direct Radiation Cooled 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. 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 20,000 volts required by the transmitter. Therefore, rotary converters are used to step up the voltage. * The basic information for this subsection has been obtained from Reference A17.
(2) Solar Thermionic, Actively Cooled 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 radiators. Rotary converters are again used to step-up the diode output voltage. (3) Solar Brayton Cycle 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 the generator. A 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 since this high voltage facilitates on-board distribution. Step-down to the 20,000 volts required by the transmitter occurs in rotary transformers. (4) Solar Thermionic/Brayton Cycle 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 de output of the diodes is stepped-up to 50,000 volts ac in the rotary converters; the turbomachine generators produce 50,000 volts ac which is combined with the output of the rotary converters.
b. Physical Configuration The system is generally composed of power generating modules consisting of solar concentrators and cavity absorbers, and the transmitter antenna as shown in Exhibit 9. These modules are individually assembled in low orbit and self-powered to high orbit; one module carries the antenna. The long (roll) axis panels are mounted on two cavity absorber support arms so as to move edge-on to the predominant meteoroid flux. The panels are inclined 11.8 degrees relative to the perpendicular to the concentrator to reduce solar absorption and shadowing of the concentrator. The solar concentrators each consist of a framework supporting 10,000 individual reflector facets. The cavity absorbers each mount 12 closed Brayton cycle turbomachine sets although there is some indication that only 4 sets of higher capacity would be used. In thermionic configurations, the diode panels are mounted directly on the cavity as shown in Exhibit 10. A five GW ground output version of this system would employ only two concentrator modules. Each of the 10,000 individual reflector facets incorporates active mirror control to maintain focusing in spite of disturbing forces due to thermal and gravity loads, aging and assembling inaccuracies. Each facet is made of metallized plastic film (aluminized Kapton) and is tensioned to form a plane surface. The cavity absorber receives the solar energy flux which for the most part is absorbed into the cavity walls. This is because multiple reflections must in general take place before the reflection back out the aperture can occur. Once absorbed, the energy is available for removal by the energy converter (Brayton cycle or thermionic devices). c. Efficiency The Brayton cycle efficiency is currently on the order of 45 percent although efficiencies up to 66 percent are projected for the 1990 time frame, although Reference A4 indicates optimum in-space efficiencies less than 40 percent. Thermionic converters are less efficient with current technology providing only about ten to fifteen percent, a figure that is projected to rise to as much as 36 percent by 1995.
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