SPACE POWER An International Journal on Systems, Technology, Economics, Environment and Policy Formerly Space Solar Power Review Volume 6, Number 4, 1986 Papers Presented at the 1985 International Astronautical Federation (IAF ’85) PERGAMON PRESS New York / Oxford / Beijing / Frankfurt / Sao Paulo / Sydney / Tokyo / Toronto
SPACE POWER An International Journal on Systems, Technology, Economics, Environment and Policy Published under the auspices of the SUNSAT Energy Council Editor-in-Chief Dr. John W. Freeman Space Solar Power Research Program Rice University, P.O. Box 1892 Houston, TX 77251, USA Associate Editors Dr. Eleanor A. Blakely Lawrence Berkeley Laboratory Dr. William C. Brown Raytheon Company Colonel Gerald P. Carr University of Texas Monsieur Lucien des Champs Electricite de France Mr. Philip K. Chapman Arthur D. Little, Inc. Dr. David Criswell California Space Institute Mr. Hubert P. Davis Raytheon Company Mr. Gerald W. Driggers, President Combustion Engineering Mr. Arthur M. Dula Attorney: Houston, Texas Mr. I.V. Franklin British Aerospace, Dynamics Group Professor Norman E. Gary University of California, Davis Dr. Peter E. Glaser Arthur D. Little, Inc. Dr. Arthur Kantrowitz Dartmouth College Mr. Richard L. Kline Grumman Aerospace Corporation Gregg Maryniak Space Studies Institute Johannes Ortner Austrian Solar and Space Agency Makoto Nagatomo Institute of Space and Astronautical Science Dr. Klaus Schroeder Rockwell International Professor Harlan J. Smith University of Texas Mr. Gordon R. Woodcock Boeing Aerospace Company Editorial Assistant: Diana White Editorial Office: John W. Freeman, Editor-in-Chief, Space Solar Power Research Program, Rice University, P.O. Box 1892, Houston, TX 77251, USA.
PEACEFUL SPACE AND GLOBAL PROBLEMS OF MANKIND The following four papers were presented at 1AF ’85 The 36th Congress of The International Astronautical Federation Stockholm, Sweden October 7-12, 1985 The cooperation of Luigi G. Napolitano is appreciated. Dr. John W. Freeman Editor-in-Chief
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0883-6272/86 $3.00 + .00 Copyright 1986 SUNSAT Energy Council ENVIRONMENTAL IMPLICATIONS OF THE SOLAR POWER SATELLITE CONCEPT Or. Peter E. Glaser, Vice President Arthur 0. Little, Inc. Acorn Park, Cambridge, MA. 02140 ABSTRACT The accelerating pace of space missions, the growth of the space industrial infrastructure, and the evolution of humanity beyond the surface of the Earth will require assessments of the environmental impacts of a wide range of space activities. The solar power satellite (SPS) was selected as representative of the capability to develop large-scale spaqe systems and to illustrate the environmental implications involved because the size and scope of the SPS concept assessment were unprecedented in the history of space project evaluations. The key environmental issues addressed include effects on human health and safety, on the ecosystems, and on astronomy. Resource requirements are surveyed and legal issues are discussed, indicating the need to evolve policies, international agreements, and a consensus regarding the future course of SPS development. KEYWORDS Solar power satellite; environmental implications, microwave exposure effects; nonmicrowave effects; atmospheric heating effects; effects on ecosystems; geostationary orbit allocations; effects on astronomy; resource requirments; legal issues. INTRODUCTION The human footprint made on the lunar surface on July 20, 1969, signaled the crossing of an evolutionary threshold. It demonstrated that the limitless energy and material resources of space and the unique environment beyond the Earth's surface are accessible and can be the arena for human activities. In the intervening years since then, the process of building a space industrial infrastructure has begun. Expendable and reusable launch vehicles are forming part of a growing space transportation capability that is making it possible to engage in routine manned operations in orbit. Industry is assessing business opportunities that range from specific commercial products manufactured in space to services provided for spacecraft operators and users to open up markets both in space and on Earth. Potentially, space commerce could be as significant in the 21st century as aviation, electronics, computers, and communications are in the 20th century. As the space industrial infrastructure continues to expand, it will lead to a spectrum of large-scale space missions such as advanced scientific and industrial facilities in low-Earth orbit (LEO), scientific and communications facilities in geosynchronous orbit (GEO), a permanent base on the Moon, a manned expedition to Mars, and the mining of resources on asteroids and other planets, as well as obtaining power from space for use on Earth utilizing Solar Power Satellites (SPS). These missions will require detailed evaluation of technical feasibility, economic viability, environmental implications, and societal consequences. For most advanced space missions, however, environmental impact assessments have not been seriously considered. The exception to this has been the solar power satellite (SPS) concept which is representative of the emerging capability to develop a large-scale space system for converting solar energy to meet the requirements of the global population in the 21st century. The SPS concept has been the subject of studies and assessments in many countries, and a growing literature base attests to the continuing interest in its potential. The fact that its environmental implications must be assessed, evaluated, and quantified was recognized when it was first proposed. While the SPS concept would avoid most of the environmental impacts of terrestrial energy conversion options, it would result in environmental effects that could be quantified only after additional research was performed. A key aspect of the NASA and U.S. Department of Energy (DOE) investigations of the SPS was the environmental implications assessments performed as part of the SPS Concept Development and Evaluation Program (CDEP) (DOE, 1980a). The CDEP utilizes the SPS reference system (DOE, 1980b) as a basis for these assessments. The SPS reference system does not represent a preferred engineering approach or design. However, it was characterized in sufficient detail to identify potential constraints in the SPS concept that could make it infeasible and to determine if measures could be taken to mitigate them. 1 Space Solar Power Review, published under the auspices of the Sunsat Energy Council by Pergamon Press.
The scope and size of the environmental implications assessment of the SPS concept were unprecedented in the history of energy-option evaluations. It also created a precedent for future assessments of major space initiatives. Although different space technologies may be required for other space missions with varying operational requirements and criteria, the SPS concept assessment can serve as a model for evaluations of other large-scale space projects. ASSESSMENT OF SPS ENVIRONMENTAL IMPLICATIONS The objectives of the environmental implications assessment of the SPS concept were: o To identify environmental issues associated with the SPS reference system; o To prepare an assessment based on existing data; 0 To suggest mitigating strategies for guiding the design of the SPS and for p lanning long-range research to reduce the uncertainties identified during the assessment. Although the potential consequences of SPS technology were assessed, they could not all be quantified because of the lack of data on environmental effects or uncertainties associated with the performance of space technologies developed only to a conceptual stage. The key environmental issues that were the subject of the CDEP assessment included: o Microwave exposure effects on health and ecosystems; o Nonmicrowave effects on health and ecosystems; o Effects on the atmosphere; o Atmospheric heating effects; and, o Electromagnetic compatibility. The results obtained from the SPS assessment are summarized in the following sections. MICROWAVE EXPOSURE EFFECTS Microwave radiation falls in the band of frequencies from 30 MHz to 300 GHz. This radiation does not have sufficient energy to ionize biological molecules, but instead agitates them. When microwave radiation impinges on skin tissue, it may be reflected or absorbed or may pass completely through it. The specific effect will depend on the frequency of the radiation and on the orientation, composition, and thickness of the tissue. At the frequency of 2.54 GHz selected for the SPS reference system, absorption will take place primarily in skin tissues. 2 Microwave power densities at a receiving antenna site range from 23 mW/cm at the antenna center, to 1 mW/cmz at the antenna edge, to 0.1 mW/cnr at the antenna site exclusion boundary. If 60 receiving antennas in the continental United States were spaced^an average of 300 km apart, the minimum microwave power density at any point would be about 10”H mW/cm . At present, 1% of the U.S. population is potentially exposed to microwave power densities of 10" mW/cirr as a result of radar, television ^ignals^ and appliances. The U.S. population is experiencing a median exposure value of about 10” mW/cin for a time-averaged microwave power density. The immaturity of the theoretical understanding of microwave effects and the complexity of experimental conditions and biological systems contribute greatly to the difficulty in quantifying microwave biological risks. Exposure conditions will depend on operating frequency, polarization of the microwaves, and laboratory chamber characteristics. Any object introduced into the microwave radiation field tends to distort the field pattern in an unpredictable manner. The field strength measured by a probe at a particular location will change when an experimental subject replaces the probe, making environmental monitoring, personal dosimetry, and depth-dosimetry di fficult. International standards on safe exposure for microwave radiation levels have not yet been developed. World opinion on the choice of levels ranges over four orders of magnitude, and dffferent countries are applying different criteria. The United States and Western Europe have adopted 10 mW/cm^ ^s a guide for both public and occupational continuous exposures. Canada adopted a limit of 1 mW/cm for exposure-of the public, while the Soviet Union and Eastern European countries allow only 0.001 mW/cnr for long-term public exposure. Occupational exposure standards are about an order of magnitude higher than those for the public. Also, the United States and Western European exposure standards are primarily guided by risk avoidance of thermal biological effects, while the Eastern European countries have considered nonthermal effects of microwave radiation. There have been considerable controversy and skepticism about the health hazards of prolonged low-intensity microwave exposures and the validity of the observations. A theoretical understanding of possible nonthermal mechanisms and a quantitative assessment of scientific evidence are essential before conclusions can be arrived at regarding the potential biological implications of the SPS microwave power transmission system and the effects of microwave radiation from an increasing number of industrial, scientific, medical, and domestic microwave radiation sources. Occupational exposure to microwave radiation would occur at the receiving antenna on Earth (up to 23 mW/cmz) and at the transmitting antenna in GEO (about 2200 mW/crrT). Thus, occupational exposure to the SPS microwave power densities must be controlled both in space and on Earth. Protection
would be required for workers on the upper surface of the receiving antenna. Workers in supporting on-site facilities could be protected by architectural shielding. In GEO, transmitting equipment may have to be turned off during maintenance. Shielding provided to space workers against the GEO space environment would also serve as microwave protection for some tasks. The limit of protection that can be achieved could affect the choice of system parameters and could guide system design and operational strategies. There are no definitive data available to assess whether the microwave radiation associated with SPS operations might be harmful to ecosystems. Although information is available on the effects on specific animals or plant species in controlled laboratory environments, the data are insufficient for making an informed judgment on environmental effects on ecosystems in general. Honeybees have been studied thoroughly, and considerable data have also been obtained on birds since free-flying species could be exposed to the maximum power density at the receiving antenna site. The preliminary data obtained indicate that effects observed at peak SPS microwave exposure levels do not change behavior characteristics or affect survival. Extensive research is required to support a quantitative assessment of microwave effects on human health and ecosystems. International standards of microwave exposure will be required to guide the design of an SPS microwave transmission system, to estimate the environmental effects of the SPS system, and to ensure that the SPS will meet all applicable international standards for public and occupational exposures and compatibility with ecosystems. NONMICROWAVE HEALTH AND ECOLOGICAL EFFECTS The scope of SPS activities would greatly exceed the extent of other space activities considered to date. Construction of the SPS would require extracting material resources, shipping those resources to factories for processing and manufacturing, transporting finished products to a launch site, and launching the product to space for orbital assembly. Large areas of land would be needed for the construction of the SPS receiving antenna. Space transportation vehicles would have to be built and transported to the launch site, and factories and housing would have to be provided and various energy sources utilized during construction of the SPS and for the launch. Most of these activities would be conventional processes normally associated with mining, manufacturing, and transportation. Their environmental consequences -- and potential SPS-related impact -- can be assessed on the basis of experience with these related activities. Even if the SPS is not placed into operation, similar environmental impacts might occur because other energy conversion systems might be implemented to meet future demands. Terrestrial support activities and their effects on workers in terms of occupational illnesses and injuries would be governed by industrial safety measures at accepted levels. The principal risk will be to space workers during launch, space travel, and LEO and GEO operations. Growing rapidly is the scientific and engineering database for LEO operations regarding suitable protection for workers to live and work in space safely and to enjoy good health after returning to Earth. Many of the conditions required for the construction, assembly, and operation of the SPS in terms of medical safety and occupational criteria are already the subject of research. The medical effects include substantial acceleration and deceleration forces during launch and return to Earth, living and working in a weightless environment, and potential hazards of space radiation. The significant data accumulated for manned operations for extended periods under microgravity conditions in LEO can be acceptable for crews having a broad range of physiological characteristics. There is no substantial evidence that unpreventable or noncorrective adverse effects will be experienced by SPS space workers in LEO, and that if potentially adverse effects are identified in the future, ameliorating measures can be developed to avoid them. Of primary concern will be the exposure of space workers to ionizing radiations during transfer from LEO to GEO and during operations in GEO. The ionizing radiation environment is characterized by fluxes of electrons, protons, neutrons, and atomic nuclei. In LEO, electrons and protons are trapped by the Earth's magnetic field in the Van Allen belt. Unpredictable solar radiation resulting from solar events (solar storms) can endanger crews during the transfer from LEO to GEO. In GEO, trapped electrons, trapped protons, galactic cosmic rays, and solar events contribute to the radiation environment. Galactic cosmic rays originate outside the solar system and are made up of protons, atomic nuclei, electrons, and high-energy heavy ions (HZE). The biological effects of HZE are not well understood and could produce impacts of an entirely different character than other types of ionizing radiations. Solar events are not predictable and temporarily greatly increase the radiation in GEO. Preliminary calculations made for HZE and other types of ionizing radiation for SPS workers in GEO indicate that radiation exposure might exceed current limits recommended by National and International Commissions on Radiation Protection. The risk from ionizing radiations tn space could be reduced through carefully designed shielding of the space vehicles and the working and living modules through solar storm shelters. Concepts to limit such radiation exposure include a layered metal alloy shielding, bulk material shielding, and creation of magnetic or electric fields. Monitoring systems will be necessary to obtain comprehensive, immediate accounts of radiation conditions in places occupied by space workers. Personal dosimeters will also be required because of differences in exposures among individuals performing different tasks under varying conditions and work schedules.
Research on the effects of ionizing radiation in space is essential to the establishment of the requirements for developing space construction and operational missions for a variety of projects in GEO. This research will provide generic information and will strongly influence specific SPS construction and maintenance scenarios. It is likely that most SPS construction activities will take place in LEO where radiation is less severe and more predictable. Assembly in GEO could be performed by automated equipment and controlled from an LEO space station or terrestrial location. EFFECTS ON ATMOSPHERE The Earth's atmosphere from the ground to GEO would be affected by the construction launches and operation of the SPS. Climatic effects caused by waste heat released at the receiving antenna’site would be small and comparable to the heat release in suburban areas. The absorption of microwave in the troposphere will increase during heavy rain storms, but would have only a marginal effect on local conditions. The most important SPS-related effects on the atmosphere are associated with inadvertent weather modifications and air-quality degradation by launch vehicles, e.g., frequent launches of space shuttles and heavy lift launch vehicles (HLLV). Ground clouds could be formed as a result of the large thermal energy injected into the atmosphere during a launch. Weather modifications will depend strongly on meteorological conditions, the type of launch vehicle, and the launch location. HLLV launches could affect convection patterns, alter cloud populations, and induce precipitation. Air-quality impacts of HLLV launches are predicted to be small, and the increase in the acidity from combustion product would not be great enough to cause environmental effects. The induced changes in the globally averaged ozone layer and the effects of nitrogen oxide releases are expected to be minimal. The ratified nature of the upper atmosphere makes it susceptible to disturbances by external sources of mass and energy that may be deposited during SPS-related launches, depending on the launch frequency. Significant growth of the space industrial infrastructure related to other space missions will provide the information that would be essential to define mitigating measures and select alternative systems for use in SPS construction and operation. The projected atmospheric effects include: o Atmospheric modifications caused by launch vehicle exhaust effluents and reentry products; o Ionospheric heating produced by microwave power transmission; o Increase of water content and alteration of the natural hydrogen cycle above 80-km altitude; o Formation of clouds at mid-latitudes near 85 km altitude; and o Effects on the magnetosphere caused by space vehicle exhaust effluents discharged when traversing from LEO to GEO. Except for microwave heating of the ionosphere, similar effects will be associated with space systems supporting a variety of projected space missions. Possible mitigating strategies for preventing atmospheric effects of transportation to LEO include the selection of the most appropriate orbit insertion technique and fuels that are least likely to interact with atmospheric constituents and exhibit long-term residence. Potential alternatives to chemical propulsion for expendable launch vehicles include laser propulsion and electromagnetic launchers, especially for commodity materials that would constitute a major portion of SPS payloads. Another strategy is to select vehicle trajectories that minimize possible effects in critical atmospheric regions. Ion thrusters will most likely be utilized for electrical propulsion to transport payloads from LEO to GEO and to control the position of the SPS, its solar array, and microwave transmitting antenna. Ion thrusters will inject ions into the plasmasphere and magnetosphere. The ion density there is very low, and the motion of ions is dominated by the Earth's magnetic field. The composition and dynamics of the magnetosphere are complex and not completely understood. SPS-related launch and orbital transfer vehicles and attitude and position control systems may induce effects that include: o Van Allen belt radiation enhancement; o Generation of electric currents in the ionosphere; o Modified auroral response to solar activity; o Satellite communication interference; o Enhanced air glow that may interfere with remote sensing systems on GEO satel1ites; and o Potential changes in weather and climates. As information on exhaust effluent effects on the magnetosphere from other space missions becomes available, alternative space transportation strategies could be selected and ameliorative measures developed to resolve uncertainties surrounding atmospheric effects of the SPS. Microwave heating of the ionosphere as a result of beam transmission could increase ambient energy levels and temperatures of the electrons comprising the D and E regions. Heating of regions of the ionosphere could lead to interference with telecommunication systems. Experiments on the effects of microwave beam heating of the ionosphere have indicated that at a peak level of 23 mW/cm , the
microwave beam would not adversely affect the performance of telecommunication systems and that possibly the beam power density could be increased. Because of equipment limitations, these experiments deposited power in the lower ionosphere comparable to the microwave beam power density. Modified and expanded facilities would be required to stimulate heating of the upper ionosphere, verify the frequency-scaling theories, and establish the effects of the microwave beam on the upper atmosphere. The effects of a system consisting of many SPSs on telecommunication systems and those operating at frequencies higher than 30 MHz will have to be analyzed in more detail to establish the response to ionospheric heating and to indicate permissible peak power densities in the microwave beam. Although the SPS reference system was based on the use of microwave power transmission, some consideration was given to SPS laser systems. The most significant potential environmental effects for laser systems appear to be local meteorological changes and beam spreading as a result of tropospheric heating. The tropospheric heating will result from energy absorption by aerosols and molecules. Scattering from molecules and by absorption and scattering from aerosols will be greatest at short wavelengths and would be most significant for visible wavelength lasers. Aerosol effects will become important for infrared lasers only under hazy and overcast conditions. Laser wavelengths that have high atmospheric transmittance would be less likely to suffer from thermal blooming. The severity of the thermal blooming would be the function of several parameters, including the intensity of the laser, wind velocity, atmospheric density, absorption, and altitude. Thermal blooming could degrade and spread the beam, which would be less critical for the space-to-Earth SPS beam than for Earth-to-space transmission. It is unlikely that global climate changes could result from tropospheric heating since the absorption of laser energy would be less than the typical natural variations of the atmosphere. Ionospheric heating would also be negligible because the interactions would be confined to the laser beam volume. The findings on the effects on the atmosphere of SPS construction and operation are based upon transportation and power transmission characteristics associated with the SPS reference system. Since these characteristics were not defined precisely, and because the nature of the upper atmosphere is not completely understood, uncertainties remain. Many of these uncertainties are expected to be resolvable as information is obtained from other space missions. GEOSTATIONARY ORBIT ALLOCATION GEO and other orbits are accessible to all countries in accordance with the 1967 Space Treaty that states: "Outer space...is not subject to national appropriation by claims of sovereignty, by means of use, or occupation, or by any other means." The SPS along with other satellites and platforms would occupy positions in GEO. The spacing between adjacent SPSs in GEO will be determined by station-keeping requirements and capabilities and by electromagnetic compatibility. Spacing requirements will be influenced by microwave radiation frequency, its harmonics and noise, and off-axis antenna gain. If adjacent satellites are for telecommunications, spacing requirements will also be influenced by the susceptibility of such satellites to system noises and interferences that would be influenced by their off-axis antenna gain, filtering, and shielding. Reduction of the electromagnetic radiation from an SPS in the direction of an adjacent satellite and the ability of such a satellite to reject unwanted electromagnetic radiation will permit closer spacing. The U.N. Committee on the Peaceful Uses of Outer Space, the International Telecommunications Union, and the World Administrative Radio Conferences will have jurisdiction over the allocation of SPS frequencies and GEO positions to ensure that GEO will be of maximum benefit to all users. This requirement, operative under existing space laws, is imposed not only on the SPS, but on any other satellite or platform operating in GEO. EFFECTS ON ASTRONOMY The SPS and other large objects in Earth orbit have the potential to interfere with astronomical observations if light is reflected from their surfaces. Optical astronomy will be affected if an increase in night sky brightness results in a proportional reduction in the effective aperture of an optical telescope for the study of faint light sources. Suitable materials and surface finishes and design approaches could be selected to reduce reflected light from SPS surfaces such as solar arrays and transmitting antennas. Radio astronomy will be affected if undesirable microwave radiation from an SPS interferes with or damages sensitive radio astronomy receivers or if re-radiation from a receiving antenna on Earth interferes with radio astronomy observations. Emissions from man-made sources in allocated radio astronomy bands of the frequency spectrum are constrained by international treaty. To avoid interferences, microwave transmission equipment would have to be designed to comply with existing regulations. Potential interferences that could not be resolved by including such design features in the microwave transmission system or by satisfying receiving antenna siting criteria may be amenable to technical and functional solutions applied to radiotelescopes, including long baseline interferometry and signal cancellation techniques. By the time an SPS system is placed into operation, it may also be possible to construct radiotelescopes on the far side of the Moon, where they would be shielded from SPS and terrestrially produced electromagnetic radiation interferences.
ELECTROMAGNETIC COMPATIBILITY The SPS must be designed and operated to satisfy established national and international regulations for the use of the electromagnetic spectrum. The SPS has the potential to produce interference because the amount of microwave power transmitted from an SPS to Earth is unprecedented and the size of the microwave beam would be very large at the Earth's surface. The SPS could interfere with military systems, public communications, radar, aircraft communications, public utility, transportation systems communications, other satellites; and radio astronomy. For example, the SPS field intensity would be 1 volt per meter at a distance of 30 km from the center of a receiving antenna site. Communication systems that generally operate with a received signal strength of several microvolts per meter would receive sizable signals from the SPS, even at distances of about 100 km. The interference potential of the SPS microwave beam would not be especially unusual except in the extent of the geographic area affected. High-power radar systems produce interference of similar electomagnetic intensities, but over limited areas. Prevention of SPS interference by direct energy coupling to any class of equipment would be part of the engineering deisgn of the transmitting and receiving antenna. This will reduce undesirable emissions at frequencies other than the operating frequency by constraining the size and shape of the transmitted microwave beam and its side lobes. Appropriate siting of receiving antenna will require a trade-off between the desire to locate antennas near energy load centers and the need to avoid interference with the large number of other users of the radio spectrum. For example, a minor change in location could substantially reduce the impact on national defense facilities without increasing interference with civil systems. Also to be considered are interferences with other satellites. Satellites in orbits lower than GEO, including those in transit to GEO, may pass through the microwave beam, causing interferences with satellite systems or potential damage to sensors. The duration of the encounter of satellites with the beam can be up to 2 seconds in LEO and up to 4 seconds during transport to GEO. It may be possible to choose a satellite orbit altitude and phase so that encounters with a known microwave beam location occur very rarely — if at all. RESOURCE REQUIREMENTS Indirect environmental impact of the SPS could result from physical resource requirements including land use, materials availability, and energy utilization. Studies showed that there are many suitable locations for receiving antenna sites in the United States. Site selection will be influenced by the desire to avoid migratory bird fly-ways, undesirable topography, proximity to defense installations, and population centers. The size and intensity of use of the contiguous land area necessary for a receiving antenna site and site construction will require that environmental impact be established for each specific site. Secondary uses of such sites, for example, for agricultural purposes or for terrestrial solar energy conversion systems, will need to be assessed. Locating receiving antennas off-shore may be attractive for major population centers near the seacoast, not only because of the possible proximity, but also because floating off-shore structures may be competitive with land-based structures. Floating structures would also provide opportunities for extensive mariculture; for example, one offthore antenna would produce from 5 to 10% of the fish products used by the United States. An analysis of the material requirements for the construction of the SPS indicated that no insurmountable material supply problems are evident in terms of world and domestic supply and potential manufacturing capacity. Over one half of the materials for the SPS reference system are readily available, but there are potential supply constraints on tungsten, silver, and gallium. An industrial infrastructure to fabricate SPS components (e.g., ion thrusters, dipole rectifiers, microwave generators, and graphite composites) will be adequate; however, facilities for the mass production of solar cell arrays will be required. Net energy analysis comparing alternative energy technologies in terms of energy produced by each system per unit of energy required indicated that when fuel is excluded, the energy ratio for the SPS reference system is marginally favorable with respect to other energy production methods. When fuel is included, the SPS energy ratio is very favorable, with the energy payback period projected to be about one year. LEGAL ISSUES The 1967 Space Treaty, Article VII, stipulates that each state is "internationally liable for damages" to others caused by activities in space. The 1973 “Convention on International Liabilities for Damages Caused by Space Objects" amplifies these responsibilities. The existing space law implies that if the global or local environment is damaged through SPS system operation, the SPS owners might face law suits or other forms of grievance procedures. Even if operation of an SPS system had no other effect than that caused by a nation making use of the power supplied to it, the design of a globaly marketable SPS system to meet widely varying national standards could add significantly to its cost. Furthermore, the possiblity of law suits could make insurance expensive or impossible to procure, unless the development, construction.
operation, and monitoring of the SPS system would be undertaken within the framework of international agreements. Such agreements would also assure the peaceful use of the SPS. The U.N. Committee on the Peaceful Uses of Outer Space, the International Telecommunication Union, and the Committee on Space Research of the International Council of Scientific Unions are examples of the organizations that could evolve policies for organizations to develop and operate an SPS system. The principles embodied in international agreements will require a sense of participation for all nations that could benefit from the operations of the SPS systems and a consensus regarding the future course of SPS development. SUMMARY The environmental implications of the SPS illustrate the close coupling between technological and broadly based societal requirements that include environmental effects on human health and safety and ecosystems. These implications are broadly applicable to future large-scale projects that will be an integral component of the evolving space industrial infrastructure and capabilities. The global implications of large space projects imply that the international institutions, numerous government agencies, industrial organizations, and public interest groups involved in these projects will change during their duration. The involved project participants will need to develop management techniques and arrive at a consensus for actions that will be of the widest possible benefits to all participants. In fact, the development of effective management techniques pioneered during the Apollo Program and evolving for the space station program may prove to be crucial to the successful utilization of the inexhaustible space material and energy resources to meet human needs and to provide the opportunity for continued evolution and progress of civilization. REFERENCES U.S. Department of Energy (1980a). Program Assessment Report, Statement of Findings. D0E/ER/0085. NTIS, Springfield, VA 22161. U.S. Department of Energy (1980b). Satellite Power System Concept Development and Evaluation Program -- System Definition Technical Assessment Report. D0E/ER/10035-03. NTIS, SpringfieId, VA 22161. ’
0883-6272/86 $3.00 + .00 Copyright ' 1986 SUNSAT Energy Council A NEW APPROACH TO OPTIMUM SIZING AND IN-ORBIT UTILIZATION OF SPACECRAFT PHOTOVOLTAIC POWER SYSTEMS M. S. Imamura and B. H. Khoshaim Midwest Research Institute/SOLERAS P.O. Box 5927, Riyadh, Saudi Arabia ABSTRACT - This paper presents a systems approach to optimization of the size and orbital life of photovoltaic systems via minimizing the nighttime energy demand while maximizing the daytime energy consumption. The Day-Night Management of Load (DANMOE) strategy calls for sizing the system to a pre-selected day/night average load power ratio and operating the spacecraft in orbit within the day and night capacity capability, rather than the conventional single orbital average power capability. Examples for the Space Station and the telecommunication satellites show that the reduction in their specific masses can be substantial using any of the photovoltaic system technologies. The DANMOE scheme may also be used effectively to extend the life of batteries on currently orbiting satellites, and hence prolong their lifetime. The paper also discusses other benefits at the spacecraft level and the method of implementing the DANMOE approach. INTRODUCTION The photovoltaic power system wi 11 . continue to be the principal source of electric power for a vast majority of future spacecraft missions. Two key issues that have confronted all spacecraft in the past are the high specific mass of the photovoltaic system and the battery life limitation. The battery life compounds the mass problem because lowering the depth of discharge increases the battery life but also increases its specific mass. Historically, the mass of the power system for the geosynchronous orbit satellites (e.g., telecommunications) has been 15 to 25 percent of the total spacecraft mass. About a third of this mass (5 to 8 percent) is in the solar array and another third in the batteries. Some of the recent 3-axis stabilized spacecraft have resorted to planar foldout sun-oriented arrays which resulted in a lower specific mass. Even so, there is a severe size restriction on the solar arrays for the GEO satellites becuase of the mass-volume limitations of the present launch vehicles (STS/IUS or PAM, Titan 3D/IUS, and Ariane). In a 1983 forecast of commerical telecommunication satellites for the free world [1], Comsat General Corporation predicted that over 280 satellites will be launched between the year 1985 and 2000. Thus, the reduction of the photovoltaic power system will continue to be a key issue for the telecommunication satellites. The orbit duration on low earth orbit vehicles is in the order of 1.5 hours and is much shorter than the GEO (24 hours). Thus, the energy and cycle-life demands for the LEO are more severe on the energy storage system. For this reason, the specific masses of the array and batteries and the battery life problems are higher for the LEO spacecraft. The best example of the future LEO Spacecraft is the Space Station, recently initiated by the United States with other countries participating. The current prediction is that the power level, meaning the orbital average bus power capability, will increase from 75 kW on the first fully operational configuration in the early 1990 to 150 kW in late 1990 [2] . Fig. 1 illustrates the possible orbital configurations for the above power levels using standardized flexible solar array wing designs. Based on the "heaviest" photovoltaic system (rigid Si array/NiCd battery) with a 290 kg/kW specific mass estimated by NASA [2], the initial 75-kW station would require a power system mass of 21,750 kg (47,850 lbs). Several Shuttle flights are needed to assemble this initial stage and many more for the final 150-kW configuration. The reduction of its specific mass, therefore, is one of the key issues and drivers in the Space Station Program. SIZE AND LIFE OPTIMIZATION VIA LOAD MANAGEMENT Two ways to reduce the specific mass and life of any photovoltaic system can be classified as hardware and non-hardware approach. In the hardware (or design) approach, the designer selects and utilizes the best mix of mass-efficient reliable components and minimizes the energy loss factors such as component efficiencies. The degree to which the system specific mass and battery life can be optimized here is dependent largely on the available technology or hardware. The non-hardware method is essentially a "systems" approach because it resorts to operational strategies to achieve the desired objective. There is one "system" approach that has never been implemented in the past for the specific purpose of reducing the overall mass of the photovol-
taic system and/or increasing the battery life. For simplicity, we will refer to this load control strategy as the "DANMOE" (Day and Night Management of Energy) technique. The DANMOE method is the principal basis for the sizing and orbital life optimization concept presented in this paper. The basic scenario behind this approach is to run the spacecraft in orbit such that electrical loads during the night portion of each orbit are reduced and the daytime energy consumption is correspondingly increased. As will be shown later, the average bus power during an orbit stays at least equal to the case when the day and night loads are equal. To determine the required solar array during the spacecraft design phase, day and night loads are to be defined for the maximum eclipse orbit, but the night load is to be minimized and the day load maximized. The advantages of doing the night to day load shifting are generally known to the power system designers, but no attempts have been made in past spacecraft. The main reasons for this are due to a combination of the following: (1) the use of a single orbit load value (the orbit average power) is easy, and it is a general practice for solar array sizing, (2) the electrical loads for many spacecraft, especially the low-power ones, stay relatively constant once the spacecraft is deployed and all loads are turned on, (3) power switching (on/off) are avoided for reliability reasons, (4) insufficient knowledge of the accuracy of load power and duty cycle, (5) too complicated for on-board or ground implementation, (6) insufficient technical justifications at the spacecraft level, and (7) on-orbit operation and optimization needs, especially for a housekeeping subsystem, usually end up at the bottom of the priority list. The DANMOE scheme, although inherently simple in concept, is not exactly easy to mechanize and perhaps not even practical on low-power spacecraft. Spacecraft system designers as well as the flight controllers and the crew may generally find it difficult to accept the idea of a power subsystem having a maximum capacity for eclipse periods and another for the daylight periods rather than the conventional one orbital average capability. However, the future high power space vehicles, the Space Station in particular, cannot afford the luxury of an unlimited energy source, and should resort to an operational strategy to reduce the weight and cost of the power system. The first manned Space Station, the SKYLAB program, has taught us the following vital lessons [4], for a manned spacecraft: (1) load management is not only desirable but technically mandatory to accomplish certain experiments and to accommodate various emergency and contingency conditions, (2) there was a large amount of reserve power available during the actual orbital operation that was not utilized simply because the subsystem and spacecraft designers did not plan for its use, and (3) mission operations objectives and requirements, specifically the load management area, were not considered and emphasized during the initial design phase. The two basic requirements to effectively implement the DANMOE strategy are as follows: 1. Operate the photovoltaic system in orbit to achieve the following criteria: Maintain a positive bus power margin based on the orbital day and night average power capabilities and the corresponding average power demand for day and night durations. Avoid battery discharge more than the pre-selected depth of discharge limits. Satisfy the basic mission and spacecraft operational constraints. Minimize electrical loads during eclipse periods by reduction of loads or shifting them to the adjacent or succeeding daytime period. 2. Size the solar array and batteries to a pre-defined orbit average bus power level and day to night load ratio, with the night load defined for the maximum eclipse orbit but minimized and the day load maximized.
To accomplish item (1) above, an analytical tool for rapid and accurate determination of power demand and capabilities is quite essential even for a low-power spacecraft. The data base for this computer program would include mission sequence and crew activity timeline, load priority and classification of all user loads in terms of: (1) load criticality by various mission phases, (2) load type like essential, non-essential, and emergency, (3) load operating requirement as day only, night only, or both day and night, (4) duty cycle, and (5) power requirements at several operating bus voltages. It is also important to initiate a system-level document covering the above information and maintain it through the life of the program. As the design matures, all specification type power values must be replaced by actual performance data. The required analysis, planning, and validation of the load profile compatibility with the power system capability must always be accomplished a priori by the ground personnel. But future high-power spacecraft with adequate computaional capability like' the Space Station would eventually want the above analytical tool onboard the spacecraft for an automated implementation of the DANMOE requirements. By load shifting and/or eclipse load reduction, the battery mass can be effectively decreased roughly in proportion to the nighttime electrical energy displaced. This will reduce the solar array size, but the average bus power for the entire orbit will not decrease as a result of both solar array and battery size reductions, as will be discussed later. Table 1 summarizes the potential benefits of the DANMOE strategy at the power system and spacecraft levels. PHOTOVOLTAIC SYSTEM SIZING The DANMOE strategy and its principal benefits can be best illustrated by the use of the sizing equations. The following sections discuss the general relationships between array power, day and night power, and orbit average bus power, and between the total specific mass and the above terms that appear in the energy balance equation. The Space Station and telecommunication satellites are cited as examples for the LEO and GEO applications, respectively. However, the DANMOE technique is useful in general to any application that uses photovoltaic/battery combination. Energy Balance Equation The energy balance equation for a given power subsystem design is the principal basis for sizing of solar arrays and batteries. Energy balance simply means the energy developed by the solar array is equal to the sum of the load energies required during the orbital daytime and nighttime. To develop the sizing relationships, the direct energy transfer configuration depicted in Fig. 2 was selected. It is representative of a high power and highly efficient photovoltaic power system. Because of redundancy, component commonality, size availability, and a host of other considerations, a multi-kW photovoltaic power system for the Space Station class or direct broadcast communication satellites is expected to be comprised of multiple power channels or modules that can be easily paralleled electrically at the main output bus. Further, each of these modules is expected to range between a few kW to tens of kW, and to be designed to achieve high efficiency and capable of operating independently of each other, except at the main bus. Also, the Space Station may utilize several isolated power sources, each providing a separate set of captive
loads. Nevertheless, for the purpose of this paper, any type or arrangement of a photovoltaic power system would suffice because the main idea behind this paper is not to optimize the specific design but to illustrate an entirely different strategy in load equipment operation that gives rise to a new concept in sizing of the key elements of the photovoltaic power system, namely, the solar cell arrays and the batteries. Based on the power flow diagram of Fig. 3, the basic energy balance equation using average quantities of power and component efficiencies, is:
The required array power in Equation (2) can be expressed in terms of orbit average power, P , and day/night load ratio, o, by introducing the following definition of (see Fig. 4): Using the values of the various parameters of Equation (6) as listed in Table 2, the effects of increasing the day/night ratio on the ratio of the required array power to orbit average power are shown in Fig. 5 for LEO and GEO applications. This shows that the solar array size reduces as the day/night load ratio is increased. Note that the reduction in Pq./P^ ratio is greater for the LEO spacecraft, which indicates that the LEO benefits more from tne DANMOE scheme than the GEO.
The battery size is dependent upon the nighttime energy demand and the depth of discharge (DOD) as given in the following relationship (based on Fig. 3): Where is the rated watt-hours, and C_ is the allowable depth of discharge (in fraction of rated capacity) selected for the particular design. The rated ampere-hours for the batteries and the number of batteries can then be determined using C and battery interconnection configuration . Specific Mass The total specific mass of the power system is defined herein as the sum of the specific masses of the individual components and those of the thermal control system elements (mostly the radiators) to dissipate the waste heat. Mathematically, it is the ratio of the total system mass to the orbit-average bus power capability (P^) which is given by Equation (4). The total specific mass of the photovoltaic system is as follows: Note that each term inside the brackets in Equation (9) represents the mass of the solar array, battery, power conditioning/distribution, and radiators for thermal dissipation in batteries and regulator. Letting PnT = oP , and replacing Pn and P_. from Equation (5) and Equation (2) in Equation (9), W becomes: Equation (10) shows the specific mass of a photovoltaic system as a function of day/night load power ratio (o), the individual component specific mass; and other terms in the energy balance equation. The last three fractions in the bracket in Equation (10) represent the masses of thermal dissipation radiators required by the batteries and regulators. The specific masses of several candidate photovoltaic systems were plotted as a function of the day/night load ratio using the values of parameters listed in Tables 2 and 3. Figs. 6 and 7 show the results for the Space Station and the telecommunication satellites, respectively. These plots indicate that the most significant reduction in the specific mass is in the range of o between one and four.
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