SPACE SOLAR POWER REVIEW Volume 1, Numbers 18 2, 1980 Special Inaugural Issue Official Publication of SUNSAT Energy Counc PERGAMON PRESS • New York/Oxford/Toronto/Paris/Frankfurt/Sydney
SPACE SOLAR POWER REVIEW Editor-in-Chief Dr. John Freeman Space Solar Power Research Program Rice University, P.O. Box 1892 Houston, TX 77001, USA Associate Editors Colonel Gerald P. Carr Bovay Engineers, Inc. Dr. M. Claverie Centre National de La Recherche Scientifique Dr. David Criswell Lunar and Planetary Institute Mr. Leonard David Forum for the Advancement of Students in Science and Technology Mr. Hubert P. Davis Houston, Texas Professor Alex J. Dessler Rice University Mr. Gerald W. Driggers GDA Technical Consultants Mr. Arthur M. Dula Attorney; Houston, Texas Professor Arthur A. Few Rice University Mr. I.V. Franklin British Aerospace, Dynamics Group Dr. Owen K. Garrlott National Aeronautics and Space Administration Professor Norman E. Gary University of California, Davis Dr. Peter E. Glaser Arthur D. Little Inc. Professor Chad Gordon Rice University Dean William E. Gordon Rice University Dr. Arthur Kantrowltz Cambridge, Massachusetts Mr. Richard L. Kline Grumman Aerospace Corporation Dr. Harold Liemohn Boeing Aerospace Company Dr. James W. Moyer Southern California Edison Company Professor Gerald K. O'Neill Princeton University Dr. Eckehard F. Schmidt AEG — Telefunken Professor George L. Siscoe University of California, Los Angeles Professor Harlan J. Smith University of Texas Mr. Gordon R. Woodcock Boeing Aerospace Company Dr. John Zinn Los Alamos Scientific Laboratories Editorial Office: John W. Freeman, Editor-in-Chief, Space Solar Power Research Program, Rice University, P.O. Box 1892, Houston, TX 77001, USA. Copyright © 1980 SUNSAT Energy Council
Space Solar Power Review, Vol. 1 ♦ p. 1, 1980 Printed in the USA. All rights reserved. 0191-9067/80/010001-01$2.00/0 Copyright ° 1980 SUN SAT Energy Council EDITORIAL The decision to publish the Space Solar Power Review under the auspices of the Sunsat Energy Council was taken by its Board of Directors in recognition of the increasing international interest in the solar power satellite concept and in keeping with the charter of the Council to foster the positive exploration of solar power satellites as a means of providing a nondepletable energy source for the benefit of mankind. Although solar power satellites could make significant contributions to meeting global energy demands only after the year 2000, the steps towards developing the relevant technologies and the resolution of economic, environmental, and societal issues will have to proceed in the interim. The role of the Space Solar Power Review is to serve as an archival publication where high quality contributions from authors working in many countries will record the progress made in generating power in space for use on Earth or in space. These contributions are expected to form a foundation for workers who will participate in the efforts to achieve the inevitable transition from the limited nonrenewable terrestrial resources to the inexhaustible resources available in the solar system. The success of the Space Solar Power Review will be in the hands of authors who submit their contributions for possible publication and the readers who will value the information presented in succeeding issues of the Review. The Council has high hopes for achieving the objectives of the Review under the editorship of Professor John Freeman, assisted by an outstanding Editorial Board, representative of the international technical community. Pergamon Press has been selected as the publisher of the Space Solar Power Review because of their demonstrated capabilities to produce journals of the highest caliber and to distribute such journals worldwide to institutional and individual subscribers. The Review will be available to members of the Council as part of their membership, and will also serve as a means of communication with members. The Council invites potential authors to submit the results of their creative work to the Editor-In-Chief and to join in the efforts to utilize space for the benefit of mankind. PETER E. GLASER, President Sunsat Energy Council
digitized by the Space Studies Insitute, ssi.org
FAR-REACHING SOLUTIONS DON FUQUA U. S. House of Representatives The sun has been revered and worshipped since the beginning of civilization as the great provider. Ancient civilizations built monuments and statues to it; religious cults prayed to it; and early man marked time by it. As his knowledge and understanding grew, primitive man progressed from a wanderer and food gatherer to a food grower and his dependence on the sun increased. The sun's energy also created our planet's bountiful supply of fossil fuels on which we have increasingly depended to drive an ever expanding technological society. Today we stand at a crossroads—knowing that the earth's fossil fuels are finite and will be depleted in time, but knowing also that man's ingenuity and imagination will once again find ways and means to create new sources of energy to sustain contemporary civilization and its future generations. Once more we will look to the sun as the great provider. With the aid of our technolgical expertise, man has the potential to duplicate the sun's process of fusion here on earth, as well as collect the sun's energy in space through the exciting concept of Solar Power Satellites and other space-related energy systems. Although Solar Power Satellites may not require any major scientific breakthroughs to become reality, they do, nevertheless, pose an enormous engineering endeavor in addition to some serious environmental and economic concerns. The Solar Power Satellite Research, Development and Evaluation bill, proposed by my colleague, Congressman Ronnie Flippo, has been reported out by the Committee on Science and Technology for consideration by the House of Representatives. It provides for a technology verification program that will allow us to fairly assess SPS, compare it with other technologies, and permit us to explore the critical question of microwave effect. We are moving into the new era of environmental impact verification where the public conscience demands preventive rather than remedial action. The biological effects of microwave radiation must be studied to resolve the differences of opinion that exist regarding safety standards. We must also determine how much radiation would be received off-target from these systems. Without answers of substance to these environmental questions, the proper public decision cannot be made. The concept of Solar Power Satellites also raises the major philosophical question of centralized vs decentralized energy approaches. SPS's requirement for large investments in equipment and project construction in a central location place it clearly in the category of a centralized approach. Opponents of this idea contend that our energy problem does not require a centralized approach, but rather can be met by a dispersed system of energy generation units which allow individuals maximum conRepresentative Don Fuqua (D-Fla.) is Chairman, Committee on Science and Technoloev. U.S. House of Representatives, Washington, DC 20515.
trol over energy source and use. The need of SPS for large investments of public monies before it becomes operational, makes it a particular target in this philosophical debate. However, I believe that it is not in the nation's best interest to choose between the centralized or decentralized approaches in energy, but rather to employ each where it is most efficient. A great variety of energy sources, technologies, and systems will be needed for the future. Everything from windmills and solar hot water heaters to nuclear and coal plants will help shoulder the burden. All creditable ideas for new technologies should be considered and explored. The most promising will be demonstrated to prove their feasibility, both economically and environmentally. Solar Power Satellites hold great promise for tapping the sun's unlimited energy. They too will have to meet demonstrated safety and economic requirements. The advent of space exploration has provided a new dimension in the realm of human capability. It has broken the constraints of earth-based solutions to the world's problems. In our new search to replace the world's fossil fuel resources as our energy mainstay, we will increasingly look beyond the confines of our planet to the far reaches of space in the hopes that our growing knowledge of the space environment will provide new and innovative solutions.
WELCOMING STATEMENT FOR SPACE SOLAR POWER REVIEW ROBERT O. FROSCH Administrator, National Aeronautics and Space Administration Washington, D.C. It is a pleasure to welcome the first issue of Space Solar Power Review, a journal dedicated to innovative research in the physical and social sciences related to energy retrieval and transmission systems deployed in space. For everyone in our Nation and, indeed, throughout the world, the supply of energy looms as a major concern today and for the future. One day, perhaps, the systems and concepts for supplying energy from space, which are the major focus of this new journal, may help to meet this essential need of society. The National Aeronautics and Space Administration, the organization I am privileged to lead, has many cooperative programs with the Department of Energy involving approximately 1000 NASA personnel. Our goal in these cooperative ventures has been to assure the effective use of NASA technologies and experience arising from our programs in aeronautics and space in support of national research and development needs in energy. We have been working toward this goal by two primary thrusts: terrestrial energy technology, which is the larger portion of the effort, and energy from space. The energy from space includes the Satellite Power System (SPS) as the dominant effort. It is being pursued under a joint DOE/NASA Concept Development and Evaluation Program. Should the SPS become a part of our national energy program it would involve virtually all technologies and scientific disciplines. You have recognized this in selecting the areas relevant to the journal and by the inclusion of topics related to societal acceptability and economic feasibility in the deployment of space energy systems. A journal devoted to space-based sources of energy should serve a useful function by providing a focus for collecting and disseminating relevant technical information which would otherwise be available only from a large number of publications. Any major, future SPS effort will involve practically every scientific and engineering discipline; therefore, it is critical that there be a place where all contributions of interest to SPS can be presented in a refereed format in keeping with the high standards of professional quality journals. By giving these papers a high degree of visibility to engineers, scientists and technicians directly and indirectly involved with space power systems, the Space Solar Power Review will assure itself a continued audience of interested and concerned people.
WELCOMING LETTER I have been following the development of the Satellite Power System (SPS) concept for several years as a result of my responsibilities in the Department of Energy (DOE). From the joint DOE/National Aeronautics and Space Administration (NASA) evaluation project, we are learning a great deal about SPS. Our aim is an initial assessment of the technical possibility, economic viability, and the environmental and social acceptability of the concept. As you are aware, the report will detail what we know, and equally important, what we do not know about SPS. The report will be available for governmental decision-makers and others in 1980. One cannot fail to be impressed with the enormity of the SPS concept. The dimensions and complexity of the problem—engineering, economic, environmental and social—make it particularly challenging to communicate to a broad spectrum of the public. In my judgment, your publication is a timely input to the kind of participatory technology process we at the DOE are trying to encourage by the SPS Concept Development and Evaluation Program. I welcome your contribution to this process and wish you every success. JOHN M. DEUTCH Under Secretary Department of Energy Washington, D. C. 20585
THE EARTH BENEFITS OF SOLAR POWER SATELLITES* PETER E. GLASER Vice President Arthur D. Little, Inc. Cambridge, Massachusetts Abstract — The potential of solar energy to meet global needs is surveyed with emphasis on solar energy conversion in space for use on Earth. The advantages of this approach are compared with terrestrial solar energy conversion methods. The concept of the solar power satellite (SPS) is presented and the technology options for converting solar energy in space, transmitting power, and converting it on Earth into electricity are summarized. The requirements for space transportation systems, orbital assembly, and maintenance are reviewed. Economic and institutional issues are outlined and the environmental impacts of SPS operations are highlighted. A phased SPS development program is presented and possible organizational structures to achieve the potential of this njajor option for power generation on Earth are outlined. 1. INTRODUCTION We are not masters of our destiny, and yet, the actions we take now will determine it. Our perception of the future has changed radically, as we have gained a new perspective of the limits to the availability of energy resources so dramatically brought to our attention in 1973. Intuitively, in the 60's, we still perceived the world as infinite, a perception which previously had permitted us to act almost without constraint. In the early 70's, we became conscious that this view was inappropriate. As a result, we are more uncertain about the future, and our general expectation and optimism have decreased. We have seen national economies seriously dislocated by sharply increased energy costs. The resulting energy problem has brought a consciousness to the broader public of what actually has been a reality for a long time, that energy is the key to the social development of man and essential to improving the quality of life beyond the basic activities necessary for survival. The recognition that no one energy source will, by itself, meet all future energy demands, that the search for new sources of nonrenewable fuels can only put off the day of their ultimate exhaustion, and that uncertainties in achieving the global potential of known energy conversion methods are great has led to renewed emphasis on the inexhaustible energy source represented by the sun. Solar energy could provide for virtually unlimited amounts of energy to meet all conceivable future needs. Yet, today, we are using practically no solar energy. Instead, we are burning cheap oil and gas and cheap oil and gas are limited resources. In principle, we have infinite energy in a finite world; whereas, in reality, we are using finite energy in a world that was, until recently, perceived to be infinite. Obviously, we cannot easily switch from the way we use our energy resources now to a future where we will use renewable resources. *Presented at the United Nations Institute for Training and Research (UNITAR) Conference on Longterm Energy Resources, Session on the Direct use of Solar Energy, Montreal, Canada. 4 December. 1979
TABLE 1 COMPARISON OF UNITED STATES WITH ALL OTHER USERS OF WORLD ENERGY RESOURCES (per capita - In metric tone of ooel equivalents. 1970) Numerous studies project what will happen in the next 20 years and how we can deal with our energy problem. Mostly, these studies focus on pricing, management, and allocation of available resources that must be dealt with within an existing infrastructure. However, changes which could be made within the existing infrastructure would have an impact only after 20 years or more. But, they will have to be made to bring about the transition from cheap oil and gas and other nonrenewable energy resources to the renewable energy sources which will be essential to the proper functioning of the energy economies in the future. Therefore, our time horizon must encompass a period of up to 50 years, because the conceivable impacts of renewable energy sources on society and the environment will not be visible until after that. Shifting too soon or too quickly to solar energy could strain national economies. Shifting too late or too slowly might also impose inescapable pressures on some fossil fuels, resulting in sharply escalating prices and consequent damage to these economies, as has already been experienced during 1973 and is continuing to be felt now. Huge energy supplies will have to become available if the developing countries are to approach the economic level of industrialized countries (Table 1). The future energy resource requirements of developing countries will be more than four times the total world energy production of 1970 (1). As industrial countries will remain major users of the world's energy resources, the prospect of supplying the equivalent of 30 billion metric tons of coal per year to meet the aspirations of developing countries and the resulting global environmental effects, demonstrate that solar energy would have to play an increasingly important role.
2. THE POTENTIAL OF SOLAR ENERGY The potential of solar energy as a source of power has been recognized and evaluated for more than 100 years. Each square meter of Earth's surface exposed to sunlight at noon receives the equivalent of 1 kW, or a total potential power input 100,000 times larger than the power produced in all the world's electrical generating plants together. Efforts to harness solar energy, which had accelerated during the last half of the 19th and the beginning of the 20th century as industrialization increased the global energy needs, subsided with the successful development of energy economies based on the use of oil and gas. Not until the early 1970's did the development of solar energy technology to produce power — by concentrating solar radiation to generate high temperatures to power heat engines, by direct conversion of solar radiation with photovoltaic processes, by photochemical conversion to produce fuels and by indirect methods such as biomass conversion based on the use of products of photosynthesis, wind energy conversion and ocean thermal energy conversion — again began to be seriously pursued. But these developments are being pursued regionally without — at least so far — consideration of the new, even unique role that solar energy can play as a global energy resource in the future. At times, this role has been repudiated by emphatic dismissal or ridicule or through uninformed underestimates of its global potential. The degree to which solar energy technology will be successfully applied will largely depend on its economics, which, in turn, will depend on the reduced availability of nonrenewable fuels and their future costs. Still, the successful and widespread introduction of solar energy technology will require considerable development to strike the appropriate balance among the conflicting requirements of economics, the environment, and society's needs. The political consequences of increasing solar energy use are likely to be the most far reaching and may require extensive and unprecedented international cooperation, which in conjunction with energy conservation measures could lead to a safer and more stable world than would continued reliance on non-renewable fuels, especially since most of the latter are under the political control of only a few nations. The major challenge to the application of solar energy on a global scale is the fact that it is a distributed resource with a low supply density which must serve societies with an increasing demand density. In the past, population densities were limited by the amount of food that could be produced on a given area of land. Mechanization of agriculture and resulting rural underemployment led to the movement of people to distant cities where they sought an elusive existence. But the existence they sought — and achieved — is now again beginning to elude them as the problems associated with urban population growth continue to increase. The potential of distributed solar technology would appear to be sufficient to meet the energy demands of rural populations. However, urban populations would have great difficulty in meeting their demands with this technology, even if urban centers were located in areas with exceptionally favorable supply densities from renewable resources. In the coming decades, the global population will be less likely to live in villages and single homes. Thus, it would be very optimistic to expect that distributed solar technologies would be able to meet even a low global energy demand unless societies, particularly in industrialized countries, were willing to change their life styles on an unprecedented scale. It is unrealistic to assume that modern societies would deliberately change their life styles to meet changing conditions unless forced to do so. The various solar technologies should be developed so that
they are most appropriate to meet social and economic criteria with acceptable environmental impacts and be of the greatest overall benefit to society. One of these criteria pertains to the material requirements for construction and maintenance of solar systems which are designed to generate power continuously as an alternative to power plants requiring nonrenewable fuels. Such solar systems are capital intensive because of the low flux density of solar energy compared to other energy forms and interruptions of solar energy availability by weather and diurnal variations. Furthermore, these systems must operate in a variety of natural environments and must have sufficient structural stability to ensure operation and survival under occasional extreme conditions of wind, rain, snow and earthquakes as well as continuous routine environmental conditions such as dust deposition, corrosion, seasonal temperature extremes and the effects of solar radiation. The requirement to withstand these environmental effects translates into a minimum mass density of at least 10 kg/m2, indicating that unless advanced and lightweight solar systems with extended lifetimes can be developed, enormous amounts of commodity materials would be required for the construction and maintenance of solar systems on a global scale. The challenge, therefore, is to develop solar system designs which are very low in mass and possess an extremely long lifetime. These considerations point to the advantages which can be gained in constructing a solar system in space, where the absence of gravity and environmental effects could significantly reduce the material requirements and achieve the desirable long-life characteristics for the solar energy conversion system. 3. THE SOLAR POWER SATELLITE CONCEPT In view of these considerations of the potential of solar energy to meet global requirements, solar energy conversion methods which can meet the demands of modern societies for the continuous supply of power will need to be developed. Such methods will have to have a significant global impact, be conserving of materials resources, be economically competitive with power generation methods based on the use of nonrenewable energy sources, be environmentally benign, and be acceptable to the nations of the world. One of the major options for meeting this goal is embodied in the solar power satellite (SPS) concept. It is widely acknowledged that man's conquest of space had a most profound influence on technological advances. It demonstrated that evolutionary progress need not be confined to the Earth's surface. For example, satellites for Earth observations and for communications already significantly affect the lives of the Earth's population — and the indications are that there is no limit to the uses of space technology for the benefit of society. Therefore, a logical extension of the efforts to harness the Sun was to use space technology to overcome terrestrial obstacles, such as inclement weather and the diurnal cycle, for the large-scale conversion and application of solar energy. If satellites could be used for communications and for Earth observations, then it was also logical to develop satellites that could convert solar energy and place them in Earth orbits, particularly geosynchronous orbits (GEO), where they could generate power continuously during most of the year. With their year-round conversion capability, such satellites could overcome some of the major obstacles to large-scale installation on Earth, i.e. the huge conversion area requirements and means for energy storage. Thus the demonstrated capability of industrized
alized society to develop high technology could be applied to the development of solar energy conversion methods in space on a scale which may not be possible on Earth. In the 1960's, the logical soundness of using the synergism of solar energy conversion technology and space technology led to the concept of the SPS (2). As conceived, the SPS would convert solar energy into electricity and feed it to microwave generators forming part of a planar, phased-array transmitting antenna. The antenna would precisely direct a microwave beam of very low power to one or more receiving antennas at desired locations on Earth. At the receiving antennas, the microwave energy would be safely and efficiently reconverted to electricity and then transmitted to users. An SPS system would comprise a number of satellites in GEO, each beaming power to one or more receiving antennas. Furthermore, a major advantage of solar energy conversion in space for use on Earth is that solar radiation in geosynchronous orbit (GEO) — unlike solar radiation received on Earth — is available during most of the year. Solar radiation in GEO will be interrupted by the Earth's eclipses of the Sun from 22 days before to 22 days after equinoxes for a maximum period of 72 min a day when the Earth as seen from a GEO position is near local midnight. Overall, eclipses will reduce the solar energy received in an orbital position in GEO by only about 1% of the total available during a whole year. A solar energy conversion system in GEO will collect at least four times the solar energy that would be available to it on Earth, even in favorable locations, because of interruptions caused by weather and the diurnal cycle. Therefore, terrestrial photovoltaic systems are more likely to be useful for displacement of electric utility capacity because such systems could be operated intermittently with appropriate utility- provided system storage to meet peak load demands, rather than be called upon to generate baseload power which would require storage capacity in the utility system. In contrast, the SPS will be capable of generating baseload power with either none or very limited utility-provided system storage. Furthermore, a hypothetical comparison of an SPS in GEO vs an idealized terrestrial photovoltaic system (i.e. one with no storage and retrieval losses) shows that the latter under average weather conditions, even in a geographically favored location would require an area at least twice the size of the SPS's receiving antenna site. In recognition of the potential of the SPS as a large-scale global method of supplying power to the Earth, the challenges posed by the SPS concept are being explored through feasibility studies of the technical, economic, environmental, social and international issues by the U.S. Department of Energy and NASA (3). The status of the SPS development to date has been reviewed and the issues which require resolution highlighted in a position paper issued by the American Institute of Aeronautics and Astronautics (4). As originally conceived (5), an SPS can utilize current approaches to solar energy conversion, e.g. photovoltaic and thermal-electric, and others likely to be developed in the future. Among these conversion processes, photovoltaic conversion represents a useful starting point because solar cells are already in wide use in satellites. An added incentive is the substantial progress being made in the development of low-cost, reliable photovoltaic systems and the increasing confidence in the capabilities of achieving the required production volumes (6). Because the photovoltaic process is passive, it could reduce maintenance requirements and achieve at least a 30-year or even a several hundred year operating lifetime for an SPS. Micrometeoroid impacts are projected to degrade 1% of the solar cell array area over a
Fig. 1. 5-GW SPS with photovoltaic solar energy conversion — NASA reference systems.
TABLE 2 5-GW SPS SUBSYSTEM MASS COMPARISON FOR Si AND G(Ai SOLAR ARRAYS MASS-106 kg 30-year exposure period. Because of the lesser probability of impact, larger meteoroids are less likely to affect the solar cell array. Several photovoltaic energy conversion configurations applicable to the SPS concept are being considered. For purposes of comparison and assessment an SPS reference system design has been adopted by NASA. (Figs, la and b). For these configurations, silicon, and gallium arsenide solar cells with and without concentrators could be used. For use in the SPS the solar cells will have to be highly efficient, of low mass per unit area, and radiation-resistant during transit to and in operation in GEO. They will have to be producible at rates and in volumes consistent with an SPS deployment schedule. To extend the lifetime of the solar cells, annealing methods could be utilized to eliminate or reduce the degrading effects of accumulated radiation exposure. Of interest for the SPS is the use of gallium arsenide for solar cells because at elevated temperatures their efficiency does not degrade as fast as that of the lower-
Fig. 2. Performance comparison of solar energy conversion candidates for SPS. Source: Reference 10. band-gap semiconductors; therefore, they can be used in systems utilizing concentrated solar energy. In addition, gallium arsenide solar cells are more resistant than silicon cells to radiation damage, thus promising a longer life as well as higher performance in the space environment. The mass of the solar cell arrays (Table 2) is the dominant component for both of the photovoltaic SPS reference systems. The scale of commitment of capital, material and labor to construct large-scale manufacturing facilities and to produce the solar cell arrays (7) for the SPS would have to be exceeded by several factors if an equivalent baseload power output were to be generated with photovoltaic energy conversion devices located on Earth. Gallium arsenide solar cells emerge as the most favorable (8), but when one compares the efficiency of several SPS design configurations (9) (Fig. 2), silicon solar cells are favored for near-term demonstration tests (10). 4. TECHNOLOGY OPTIONS FOR POWER TRANSMISSION TO EARTH To transmit the power generated in the SPS to Earth, there are two optional transmitting methods: • a microwave beam, or • a laser beam 4.1. Microwave power transmission system Free-space transmission of power by microwaves is not a new technology (11). In recent years, it has advanced rapidly and system efficiencies of 55%, including the
Fig. 3. Solid-state sandwich power panel. Source: Reference 12. interconversion between de power and microwave power at both terminals of the system, are being obtained. The application of new technology is projected to raise this efficiency to almost 70%. The devices which are being considered for converting de voltage to rf power at microwave frequencies in the SPS are crossfield amplifiers (Amplitrons) and linear beam devices (Klystrons). The Amplitron uses a cold platinum metal cathode operating on the principle of secondary emission to achieve a nearly infinite cathode life. With an output of 5 kW it could operate at an efficiency of 90%. The Klystron could operate at an efficiency of 80% with an output of 70 kW but will require a more complex cooling system. Microwave solid-state power transistors are being investigated as it appears feasible to combine them with solar cells in a sandwich power panel to form a resonant cavity feeding to waveguides. (See Fig. 3.) The performance of the DC-RF conversion devices which could be used with the SPS design options is shown in Table 3. Considerations of mass, costs, and efficiency at specific frequencies have led to the selection of a frequency within the industrial microwave band of 2.40 to 2.50 GHz for the SPS reference system. 4.1.1. Microwave beam transmission. The transmitting antenna for the SPS reference system is designed as a circular, planar, active phased array having a diameter of about 1 km. Micro wave power can be transferred at high efficiency when the transmitting antenna is illuminated with an amplitude distribution which is of the form (l-r2)n and when the phase front of the beam is carefully controlled at the launch point to minimize scattering losses (11, 14).
TABLE 3 DC-RF CONVERTER FEATURES Space is an ideal medium for the transmission of microwaves; a transmission efficiency of 99.6% is projected after the beam has been launched at the transmitting antenna and before it passes through the upper atmosphere. To achieve the desired high efficiency for the transmission system while minimizing the cost, the geometric relationships between the transmitting and receiving antenna (14) indicate that the transmitting antenna should be about 1 km in diameter, while the receiving antenna should be about 10 km in diameter. The power density at the receiving antenna will be a maximum at the middle and will decrease with distance from the center of the receiver. The exact size of the receiving antenna will be determined by the radius at which the collection and rectification of the power becomes marginally economical. The transmitting antenna is divided into a large number of subarrays. A closed- loop, retrodirective-array phase-front control is used with these subarrays to achieve the desired high efficiency, pointing accuracy, and safety essential for the microwave beam operation (15). In the retrodirective-array design, a reference beam is launched from the center of the receiving antenna and is received at a phase comparator at the center of each subarray and also at the reference subarray in the transmitting antenna center. 4.1.2. Microwave power reception and rectification. The receiving antenna is designed to intercept, collect, and rectify the microwave beam into a de output with high efficiency (12, 13). The de output can be designed to either interface with high-voltage de transmission networks or be converted into 60-Hz alternating current. The receiving antenna consists of an array of elements which absorb and rectify the incident microwave beam. Each element consists of a half-wave dipole, an integral low-pass filter, diode rectifier and bypass capacitor. The dipoles are dc-insulated from the ground plane and appear as rf absorbers to the incoming microwaves. The collection efficiency of the array is relatively insensitive to substantial changes in the direction of the incoming beam. Furthermore, the efficiency is independent of potentially substantial spatial variations in phase and power density of the incoming beam that could be caused by non-uniform atmospheric conditions.
The amount of microwave power received in local regions of the receiving antenna can be matched to the power-handling capability of the microwave rectifiers. The rectifiers, which are Schottky barrier diodes made from gallium arsenide material, have a power-handling capability several times that required in the SPS application. Any heat resulting from inefficient rectification in the diode and its circuit can be convected by the receiving antenna to ambient air, producing atmospheric heating which will be only twice that of suburban areas, because only 15% of the incoming microwave radiation would be lost as waste heat. The low thermal pollution entailed in this process of rectifying incoming microwave power cannot be equaled by any known thermodynamic conversion process. The receiving antenna can be designed to be 80% transparent so the surface underneath could be put to other uses. Receiving antennas would be located on land or offshore. A typical receiving antenna site will be 10x12 km. At least 100 potential land sites in the United States could be considered. Offshore receiving sites are of interest because of limited land availability near major urban centers and the proximity of these centers to coastal locations. 4.2. Laser power transmission Microwave power transmission is the present choice, based on considerations of technical feasibility, failsafe design and low flux levels, but laser power transmission is an interesting alternative because of considerable advances in laser technology over the past 10 years and the possibility of delivering power as low as 10 MW to individual receiving sites on Earth. The use of high-power lasers for laser propulsion and for power transmission between satellites is being investigated (16). These investigations include the environmental impacts of laser power transmission from space (17). Concentrated and dispersed beams generated by continuous-wave electric discharge lasers where the gas is recirculated are being considered. Gas recirculation permits removal of waste heat and minimizes consumption, thus allowing extended operation. Candidate lasants are carbon dioxide, carbon monoxide, mercury chloride, and mercury bromide, to which are added atmospheric gases as diluents. Although carbon dioxide and carbon monoxide electric discharge lasers have reached an advanced state of development, other laser concepts, including pumped free-electron lasers, are being considered. Power may be supplied to the lasers by photovoltaic conversion systems, or through their direct excitation by solar radiation. Solar optical excitation has been developed for laser communication between satellites and could be applied to advanced laser concepts for laser power transmission from the SPS to Earth. Solar energy conversion compatible with laser power transmission may be carried out in GEO or in lower orbits, particularly in a Sun-synchronous orbit. If the latter orbit is to be used, a mirror will be required in GEO to reflect the laser power to a desired receiving site on Earth. Photovoltaic cells, such as mercury-cadmium-telluride cells, could be used to convert carbon dioxide laser beam radiation into power. If successfully developed, tuned optical diodes, which are the analog of the microwave diode rectifier in the infrared portion of the spectrum, could directly convert laser radiation into power. Thermodynamic conversion relying upon the absorption of the laser radiation and the heating of a working fluid, possibly combined with a bottoming cycle, is also of interest.
Atmospheric absorption of laser radiation will be reduced when the receiving sites are located at high elevations. But even in such locations, unfavorable weather will require that the laser radiation be beamed to other receiving sites with favorable weather conditions and fed into a common transmission grid. The dimensions of a laser power receiving site, including a safety zone, will be measured in hundreds of meters vs the thousands of meters needed for a microwave power receiving antenna. Although laser power transmission is in an early stage of development and significant technology advancement will be required, there is considerable promise in a laser power transmission system for the SPS. Environmental impacts, including heating of the atmosphere and meteorological implications, are not expected to be significant, although the plasma chemistry of the upper atmosphere and induced photoreactions deserve further study. Although requirements for safety and security of laser power transmission may be adequately met, the potential for misuse of laser power transmission may be perceived as either dangerous or, under certain conditions, provocative, which could lead to political and societal opposition. 5. SPACE TRANSPORTATION SYSTEM To be commercially competitive, the SPS will require a space transportation system capable of placing large and massive payloads into synchronous orbit at low cost. The cost of transportation will have a significant impact on the economic feasibility of the SPS. The space transportation system which will be available during the early phases of SPS development for technology verification and component functional demonstration will be the space shuttle, now well along in development. Compared to the previously used expendable launch vehicles, it will not only significantly reduce the cost of launching payloads, but will also be a major step towards the development of space freighters of greatly increased payload capability — and substantially lower costs. The space freighter, which may be either a ballistic or winged reusable launch vehicle, represents an advanced space transportation system with a planned capability to place payloads ranging from 100 to 500 metric tons into LEO. The space freighter will be recoverable and repeatedly reusable. The fuel for the lower stage will be liquid oxygen and a hydrocarbon; liquid oxygen and liquid hydrogen will be used for the upper stage. Both offshore and onshore launch facilities could be developed for the space freighter. Frequent launches (e.g. ten launches per day) will necessitate maintenance and overhaul procedures similar to those employed in commercial airline operations. Personnel and cargo will be transported from LEO to GEO with chemically or electrically propelled vehicles specifically designed for this purpose. The material required for the SPS construction and assembly will be transported by a cargo orbital transfer vehicle which could be powered by ion thrusters of high specific impulse. Although the transit time to GEO would be measured in months, ion thrusters would minimize the amount of propellant to be transported to LEO. Transportation costs of ballistic or winged-launch vehicles to LEO will be about $20/kg, including amortization of the vehicle fleet investment, total operations manpower, and propellant costs (10). The total cost per flight will be about $8 million, with vehicle production and spares accounting for 40%, manpower for 35%, and propellants for 25%.
6. ORBITAL ASSEMBLY AND MAINTENANCE The absence of gravity and of the influence of forces shaping the terrestrial environment presents a unique freedom for the design of Earth-orbiting structures and provides a new dimension for the design of the structure required for the SPS, its fabrication, its assembly, and its maintenance in LEO and GEO. In GEO, the function of the structure is to define the position of sub-systems rather than support loads which under normal operating conditions are orders of magnitude less than those experienced by structures on the surface of the Earth. The structure will have to be designed to withstand loads imposed during assembly of discrete sections which may be fabricated in orbit and then joined to form continuous structural elements. The structure will therefore have to be designed to withstand both tension and compression forces which may be imposed during assembly and during operation when attitude control is required to maintain the desired relationship of the solar collectors with respect to the Sun and of the transmitting antenna with respect to the receiving antenna on Earth. The immensity of the structure alone ensures that it would undergo large dimensional changes as a result of the significant variations in temperatures that will be imposed on it during periodic eclipses. During such eclipses, temperature variations as large as 200 K could be imposed, leading to substantial temperature gradients, which, depending upon the dimensions of the structure, would cause dimensional changes of 50 -100 m if an aluminum alloy is used. Both aluminum alloys and graphite composites show promise for use as the structural materials. Graphite composites have a very small coefficient of thermal expansion compared to the aluminum alloys, but the aluminum structure could be insulated to reduce undesirable thermal effects. The contiguous structure of the SPS is of a size which does not yet exist on Earth or in space. Therefore, unique construction methods will be required to erect the structures which are used to position and support the major components such as the solar arrays to form the solar collectors and the microwave subarrays to form the transmitting antenna. The basic approaches to constructing the required large space structure are as follows (18): • Deployable systems, using elements fabricated on Earth; • Erectable systems, using elements fabricated on Earth; and • Erectable systems, using elements fabricated in space An automated machine capable of producing triangular truss shapes of desired sizes and material thicknesses in a modular configuration (19) has been constructed. This automated beam builder consists of roll-forming units which are fed with coiled strip material and automatically impart the proper shape to the individual strip, weld and fasten the individual elements, control dimensions and produce the complete structural members. The machine could produce the structural member in increments of 1.5 m at a rate of 0.5 m/min for continuous production of structural beams in space. Warehousing logistics and inventory control will be required to effectively manage the flow of material to the SPS construction facility, which will be designed to handle about 100,000 tons per year. The construction facility could be a large lightweight rectangular structure with dimensions of about 1.4x2.8 km. It would provide for launch-vehicle docking stations and 100-person crew cylindrical modules with dimensions of about 17 m diameter by 23 m long. The construction facility will be
designed to assemble the solar energy conversion system and the microwave transmission antenna. Construction costs, including transportation of the required construction crew of about 550 people and amortization of the bases, are projected to account for about 8% of the total SPS capital cost. The construction crew's primary activity would be monitoring, servicing, and repairing, with little need for extra-vehicular activities. The SPS hardware throughput in the construction facility is projected to be 15 t/h, for a construction rate of one SPS per year (10). The repetitive automated production process of space construction activities is projected to result in a productivity per crew member of 10 man-hours per ton of materials handled (the experience with terrestrial steel construction projects). To reduce the cost of space construction the production process will have to be equipment-intensive rather than labor-intensive. Thus the significant capital investments will be amortized over a number of SPSs. 7. SPS/UTILITY POWER POOL INTERFACE The large power output potential of the SPS will require careful design of the utility power pool interface to reduce the impact on the stability of a total utility system. Electrical power grids are designed to provide this stability of power supply to the user by incorporating redundant installations of reliable equipment. In addition to mechanical reliability, the reliability of the SPS will depend on generic system effects, and small variations (2-4%) caused by atmospheric absorption at the receiving antenna. Although the eclipse periods, occurring during the periods of minimum demand, are predictable outages, they are not planned outages since they are not deferrable. Thus, since they may affect the total system operation, they have to be included when calculating the forced outage availability of the SPS. The stability of the SPS will have a substantial effect on the stability of the power pool which it serves. Low-frequency fluctuations could cause the power level delivered by the SPS to the receiving antenna to vary; high-frequency fluctuations could cause line surges which might disturb the transient stability of other generators in the power pool. The magnitude of these fluctuations will have to be investigated to establish the required degree of surge protection which would be supplied by shortterm power storage (of the order of minutes) acting as a buffer. The resolution of issues inherent in the SPS utility interface will significantly influence specific design approaches and selection of technology options. A study of utility interface requirements indicated that one or more 5-GW SPSs could be installed in the utility systems of the southern states in the year 2000. The 345-kV and 500-kV transmission systems which will probably exist at that time could be readily extended to accommodate the SPSs (20). 8. SPS IMPACT CONSIDERATIONS 8.1. Economic The economic justification for proceeding with an SPS development program is based on a classical risk/decision analysis which acknowledges that it is not possible to know the cost of a technology which will not be fully developed for at least 10 years — and commercialized, i.e. produced, operated and maintained, in not less than 20 years. Justification, of course, is equally difficult to provide for other ad-
vanced energy technology projects. This justification, therefore, requires an appreciation of the competitive cost of alternative energy sources for the generation of electrical power which would be available in the same period. Any SPS development program should be timed-phased so that the “economic” purpose of each program segment will be to obtain information that will permit the decision makers to make a deliberate decision to continue the program or to terminate it — and thereby to control the overall risk. Cost-effectiveness analyses alone would be inappropriate, as they would require postulating scenarios of the future which could be extremely difficult, if not impossible. The near-term decisions regarding the SPS program should be based on resources allocated to the SPS research tasks and their priorities rather than the projected economics of the SPS in the 21st century. The benefits and cost of a development program as large as the SPS are not likely to be uniformly distributed, but are more likely to be concentrated in certain segments of society and the economies of industrialized nations. Individuals, corporations, institutions, and even entire sectors of industry will react to the cost and the benefits of the development as they perceive them. As a result of these perceptions, political pressures are likely to have a pronounced effect on the SPS development program, its schedule, and its ultimate success. In the various studies to date, the major emphasis has been given to establishing technical feasibility; only limited economic feasibility studies have been performed, primarily pertaining to system costs, development program costs, costs of terrestrial alternatives, and comparative economics of space and terrestrial power systems (21, 22). The results of these studies have shown no likely “show stoppers” which would justify abandoning the pursuit of the planned SPS programs, but have indicated technical, economic, environmental, and societal issues which require more detailed definition. For the SPS reference system which utilizes demonstratable technology, cost estimates, rough as they are, and subjected to criticism as they may be, fall within a potentially interesting range — clearly sufficient to justify not a major commitment to development and deployment of the SPS, but a continued research and technology verification program. (Advanced technology could lead to the development of an even more competitive SPS system.) The magnitude of this program has been projected at from $25 to $50 million per year during the next 3 to 5 years. Projections of SPS construction and operational costs some 30 to 50 years hence are speculative, as forecasts of future costs of an SPS system consisting of 50 or 100 satellites presume a knowledge of future technology which when contrasted with the revolutionary advances in technology during the past few decades make such projections of doubtful validity. However, these projections are often cited in an attempt to underline the magnitude of the investments for an SPS system. It should be acknowledged that introduction of any alternative advanced energy technology on a global scale will require an unprecedented level of investment over the extended time period required to make the transition from nonrenewable to renewable energy resources. 8.1.1. Cost projections. Cost projections have been an integral part of a series of system studies pertaining to the SPS. But such studies do not provide meaningful estimates of potential market penetration of the SPS, because uncertainties in forecasting prices are much larger than the cost differentials on which the choice among competing technologies will eventually be based. However, such cost studies can provide the following useful information:
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