SOLAR POWER FROM SATELLITES HEARINGS BEFORE THE SUBCOMMITTEE ON AEROSPACE TECHNOLOGY AND NATIONAL NEEDS OF THE COMMITTEE ON AERONAUTICAL AND SPACE SCIENCES UNITED STATES SENATE NINETY-FOURTH CONGRESS SECOND SESSION JANUARY 19 AND 21, 1976 Printed for the use of the Committee on Aeronautical and Snace Sciences U.S. GOVERNMENT PRINTING OFFICE WASHINGTON : 1976 66-608 O
COMMITTEE ON AERONAUTICAL AND SPACE SCIENCES FRANK E. MOSS, Utah, Chairman STUART SYMINGTON, Missouri JOHN C. STENNIS, Mississippi HOWARD W. CANNON, Nevada WENDELL H. FORD, Kentucky DALE BUMPERS, Arkansas BARRY GOLDWATER, Arizona PETE V. DOMENICI, New Mexico PAUL LAXALT, Nevada JAKE GARN, Utah Gilbert W. Keyes, Staff Director James T. Bruce, Professional Staff Member James J. Gehrig, Professional Staff Member Craig M. Peterson, Chief Clerk/Counsel Joseph L. Platt, Assistant Chief Clerk William A. Shumann, Professional Staff Member Craig Voorhees, Professional Staff Member Dr. Glen P. Wilson, Professional Staff Member Charles F. Lombard, Minority Counsel Earl D. Eisenhower, Professional Staff Member, Minority Subcommittee on Aerospace Technology and National Needs WENDELL H. FORD, Kentucky, Chairman JOHN C. STENNIS, Mississippi HOWARD W. CANNON, Nevada PAUL LAXALT, Nevada JAKE GARN, Utah James T. Bruce, Counsel
CONTENTS Monday, January 19, 1976: Pa^e Opening statement, Senator Wendell H. Ford, chairman---------------- 1 Testimony of Dr. Peter Glaser, vice president and head of engineering sciences, Arthur D. Little, Inc------------------------------------------- 2 Biography of Dr. Peter Glaser___________________________ 2 Prepared statement of Dr. Peter Glaser____________________ 8 Testimony of Richard W. Taylor, vice president, Boeing Aerospace Co.; accompanied by Ralph Nansen and Glenn L. Keister_______ 42 Biography of Richard W. Taylor_________________________ 42 Prepared statement of Richard W. Taylor__________________ 48 Supplemental statement of Richard W. Taylor---------------------- 52 Testimony of Dr. Gerard K. O'Neill, professor, Princeton University, Princeton, N.J________________________________________ 102 Biography of Dr. Gerard K. O'Neill_______________________ Prepared statement of Dr. Gerard K. O'Neill_______________ 109 Testimony of G. Harry Stine, Phoenix, Ariz____________________ 140 Biography of G. Harry Stine_____________________________ 139 Wednesday, January 21, 1976: Opening statement, Senator Wendell H. Ford, chairman---------------- 149 Testimony of Klaus P. Heiss, president, ECON, Inc., Princeton, N.J., and Edward Greenblat, ECON, Inc., accompanied by Thomas Kelly, vice president, Grumman Aerospace Corp______________ 150 Biography of Klaus P. Heiss_____________________________ Biography of Dr. Edward J. Greenblat_____________________ 152 Prepared statement of Dr. Edward J. Greenblat_____________ 155 Testimony of Dr. John M. Teem, Assistant Administrator for Solar, Geothermal, and Advanced Energy Systems, ERDA; accompanied by Richard Blieden, Assistant Director for Solar Electric Applications, ERDA_____________________________ 192 Biography of Dr. John M. Teem__________________________ 191 Testimony of Dr. George M. Low, Deputy Administrator, NASA, and Dr. William B. Lenoir, Scientist/Astronaut, Johnson Space Center, Houston, Tex.; accompanied by R. D. Ginter, Acting Assistant Administrator, Office of Energy Programs, NASA, and Christopher C. Kraft, Director, Johnson Space Center________ 206 Biography of Dr. William B. Lenoir (Ph. D.)_______________ 205 Prepared statement of Dr. William B. Lenoir_______________ 219
SOLAR POWER FROM SATELLITES MONDAY, JANUARY 19, 1976 U.S. Senate, Subcommittee on Aerospace Technology and National Needs of the Committee on Aeronautical and Space Sciences, Washington, D.C. The subcommittee met, pursuant to call, in room 235 of the Russell Senate Office Building, at 9:34 a.m., Wendell H. Ford, chairman, presiding. Present : Senators Ford and Goldwater. Also present: James T. Bruce, counsel to the subcommittee; Amy Bondurant, Senator Ford's staff; and the following staff of the full committee: William A. Shumann, James J. Gehrig, Craig Voorhees, Glen P. Wilson, and .Joseph L. Platt, professional staff members: David Haun, research assistant; Patricia Robinson, clerical assistant; Charles M. Lombard, counsel for the minority; Mary Fay, minority clerical assistant. OPENING STATEMENT OF THE CHAIRMAN Senator Ford. The subcommittee is meeting to consider concepts involving advanced aerospace technology that might help satisfy one of our greatest national needs—future sources of energy. Specif- icallv, we will be looking at ways to collect solar power in space with satellites and to beam that power down to Earth to supplement our other sources of electricity. In addition, we will look at novel ways, to say the least, to construct those satellites. The witnesses today will be Dr. Peter Glaser of Arthur D. Little, Inc.; Richard W. Taylor and Ralph Nansen of the Boeing Aerospace Co.; Prof. Gerard K. O'Neill of Princeton University; and G. Harry Stine, an engineer and author from Phoenix, Ariz. For those of you who have always thought of space as an eternal void, you are in for a revelation today, I think. These witnesses envision a time when space will be the scene of bustling activity. They foresee the construction of gigantic facilities that will provide the world with energy, which is an essential and increasingly costly part of our industrial society. The questions will be: Is all this feasible? What will it cost? When will it come? These are the questions we will consider today. So, if I might say, hold onto your seats, we are going to step into the future. And the first step will be made by Dr. Glaser.
Dr. Glaser, will you come forward? We are delighted to have you this morning, doctor, and if you will proceed with your statement. If you want to condense it, that will be fine—anything you might have for the record will be printed. [Biography of Dr. Peter Glaser follows:] Biography of Peter E. Glaser, Vice President and Head of Engineering Sciences, Arthur D. Little, Inc. Dr. Glaser, has directed a number of advanced engineering development projects in thermodynamics, space and lunar science instrumentation, and the utilization of solar energy. He has published and spoken widely on the potential of solar energy to meet future energy demands. Dr. Glaser received his undergraduate training in mechanical engineering at Leeds College of Technology, and Charles University, Prague. He obtained his M.S. and Ph.D. degrees in mechanical engineering from Columbia University in 1955. Since joining the staff in 1955, he has directed research on: methods of generating high temperatures, including the construction of solar and arc imaging furnaces, thermal insulation systems, and properties of postulated lunar surface materials. He was responsible for the development of scientific experiments for all Apollo lunar landing missions, including measurements of the heat flow from the lunar surface, lunar gravity and the earth-moon distance. He is directing projects on the feasibility of a satellite solar power station, solar climate control systems for buildings, and photovoltaic energy conversion. Dr. Glaser is a past President of the International Solar Energy Society and is currently serving as Editor-in-Chief of the Society's Journal. He is a member of Committees of the National Academy of Sciences, the American Association for the Advancement of Science, the New York Academy of Sciences, the American Institute of Aeronautics and Astronautics, American Society of Mechanical Engineers, the American Society of Heating, Refrigeration and Air Conditioning Engineers, the Society of Automotive Engineers, American Ordnance Association and Sigma Xi. He is the 1974 recipient of the Carl F. Kayan medal awarded by Columbia University. He has over sixty publications, books and patents in the fields of solar energy applications, thermal insulation, thermal properties measurements, thermal imaging techniques, lunar surface characteristics, extraterrestrial resource utilization, and technology transfer. TESTIMONY OF DR. PETER GLASER. VICE PRESIDENT AND HEAD OF ENGINEERING. ARTHUR D. LITTLE. INC.. CAMBRIDGE. MASS. Dr. Glaser. Good morning. Mr. Chairman, I am greatly honored to be invited to present my testimony at these hearings before your subcommittee. When I presented the concept of the satellite solar power station to the Committee on Aeronautical and Space Sciences in October of 1973, I recommended that the concept receive serious consideration as an alternative energy production method. In the intervening years, considerable work has been carried out on the technical and economic feasibility of this concept, particularly by the industry team of Grumman Aerospace Corp., Raytheon Co., and Spectro Lab Inc., associated with my company, Arthur D. Little, Inc. My proposal has now been recognized as a credible alternative, as various presentations at these hearings will disclose. With your permission, Mr. Chairman, I would like to place my written testimony in the record and supplement it with comments pertaining to the technical and economic feasibility and program objectives of the development of the solar power station concept. Senator Ford. Your statement will be included in the record. You may proceed with other comments.
Dr. Glaser. I will show some slides to help the flow of the discussion. Let us first examine the total amount of solar energy reaching the Earth. Perhaps the most important consideration is that solar energy is a diffuse form of energy which, in order to be converted on Earth, requires large areas to be covered with capital-intensive devices. At best we receive around midday 1 kilowatt per square meter, or about 1 horsepower per square yard. Furthermore, we have to contend with weather and we have to accept the night-and-day variation which prevails on our Earth. Therefore, if we wish to use solar energy on a large, worldwide scale, I propose that we seek another way of doing it—that is by using a satellite solar power station in synchronous orbit where sunlight is available to us nearly 24 hours a day. The objective of the satellite solar power station, or SSPS for short, is that it be able to provide power on Earth on a large scale and be cost-competitive, resource conserving, compatible with the environment and of benefit to society. Three technical considerations are involved: We have to learn how to convert the vast amount of solar energy available in space into electricity, how to beam the electricity back to Earth and then how to convert the beam to useful power on Earth. One possible way of doing this is the concept that the team of Arthur D. Little, Raytheon, Grumman, and Spectro Lab has been studying, that is the conversion of solar energy directly into electricity by a photovoltaic process, using solar cells. As illustrated, the satellite functions by using solar cells to convert the solar energy into electricity, conducting the electricity to microwave generators—which form part of a transmitting antenna—where it is converted into microwaves which are then beamed back to Earth where they can be effectively and safely reconverted directly into electricity. We are considering a large satellite because of the geometric requirements of a transmitting antenna capable of directing a low-density beam to the receiving antenna back on Earth. A typical output of power on the ground is about 5,000 megawatts, therefore, for our studies we developed baseline SSPS design with this power output. Now, why should we actually go into space? Why should we consider this kind of large project? The primary advantages of power from space are that even if we were to place solar energy conversion devices in a region with copious sunshine, we would need 6 to 10 times the area on Earth. Furthermore we have a very favorable operating environment in space; we have more options for siting the receiving antenna on land or offshore; we have the potential with one satellite to generate very substantial power for use on Earth. And finally, such a project allows us to industrialize space, building on the first step that this Nation is taking through its commitment to the Space Shuttle. A number of other methods are also feasible for converting solar energy for use on Earth. You will be hearing more about the thermalelectric conversion, for example—I will be discussing, therefore, primarily photovoltaic energy conversion. There are reasons why solar cells are of interest—they have been widely used in the space program, they can be improved to have an adequate efficiency to do the job as we foresee it, they can be mass produced, they operate passively in their operation, and they allow the design of the SSPS to be reasonably flexible.
Solar cells produced today are reliable. We have learned a lot since the Bell Laboratories developed the first solar cell more than 20 years ago. Solar cells have been used on such spacecraft as Sky Lab, and as the panels designed for Sky Lab show, the technology for producing large areas of solar cells is available. We have learned how to make thin solar cells and laboratory tests have shown that we can obtain high efficiencies. Today this Nation has the ERDA national photovoltaic conversion program, the objective of which is to develop within 10 years methods of mass producing solar cells so that costs will be 50 times lower than today. Solar cells incorporated in a satellite, such as the SSPS to produce 5,000MW, will require a large area. In a sense, it is unusual to consider such a large object in space—about 4 kilometers by 11 kilometers with a transmitting antenna 1 kilometer in diameter. In order to reduce the area of solar cells, we concentrate solar energy onto these cells with lightweight plastic film reflectors. The mass of this satellite—about 40 million pounds—is huge by any present-day standard. However, on a per-kilowatt basis, it is only about 8 pounds, which is a remarkably low unit mass for any energy production method on Earth, and thus the SSPS is resource conserving. We could transmit the power from the SSPS to earth by either of two methods: Lasers or microwaves. Both use the electromagnetic spectrum. In my view, lasers are not suitable, because of their low conversion efficiencies and because laser beams are absorbed by clouds. On the other hand microwaves can travel through the earth's atmosphere without more than 2 or 3 percent being absorbed, even in moderate rainstorms. Industry understands how to produce microwave devices. It is a large industry, producing devices in the United States, Europe, and Japan. The devices are efficient, they can be mass produced, and they are available now. We believe that we can design the SSPS microwave system to meet the most stringent international safety standards and not expose populations to microwaves. Thus, today there are many types of microwave devices which are articles of commerce. This microwave generator is mass produced. In space we can dispense with the glass enclosure and replace the permanent magnet with samarium-cobalt, indicating the type of device we would expect to use in the SSPS transmitting antenna. This is the view from earth of the transmitting antenna, showing several microwave generators shown. These devices are about 90 percent efficient—we can reject the 10 percent of waste heat into space by means of radiators. The transmitting antenna is based on the phased-array principle, which has been known and explored over the past 15 years. An example of the status of technology is the phased-array transmitting antenna five stories high, which Raytheon has constructed in Alaska. The elements have been proven to work, indicating that we understand very well how to make these kinds of antennas work today. The microwave beam is formed in the transmitting antenna and sent back to earth, where it is intercepted at the receiving-antenna
site. There are several possible shapes for this beam. An ideal Gaussian distribution is the least attractive, because it has a relatively high microwave power density at its center. But even at this density, the beam has a lower power density than sunlight. Therefore, this beam cannot cause damage, and can be well controlled. At the edges of the antenna site we can meet the most stringent international standards for continuous exposre to microwaves. We have already learned how to direct microwave beams to earth with microwave interferometers, as used in the ATS-F spacecraft. Thus, the beam can be accurately directed from the SSPS in synchronous orbit back to earth. In addition, in our concept, we use a low power control signal transmitted from the center of the receiving antenna towards the satellite, which forces the microwave beam to travel down the signal, thus eliminating any possibility of beaming to the wrong target. Should anything happen and that signal not be received, the microwave beam will demodulate, because we no longer effectively control the phase front, and the beam reading the earth would be at communication signal levels, such as that received from present satellites. The receiving antenna is stationary. It always looks toward the same spot in synchronous orbit, where the satellite is located. The device which allows us to convert microwaves directly into electricity is a dipole rectifier. This is one of the early models which Mr. Brown of the Raytheon Co., has devised. There are the kind' of dipole rectifiers which could be produced in huge quantities, as would be required for the receiving antenna. This summer a very significant experiment was carried out at Goldstone, Calif., under the direction of the Jet Propulsion Laboratory. In this experimentat the site of the Venus antenna—we mounted a portion of the receiving antenna on top of a tower 1 mill' distant from the transmitting antenna. There are the dipole rectifier elements in the receiving antenna. The 86-foot-diameter dish transmitting antenna then sent a microwave beam across a 1-mile distance. Thirty kilowatts of microwave power were received at the receiving antenna and converted directly into electricity. Lamps lighting up just below the antenna indicate that electricity was actually being produced. The conversion of the microwave beam directly into electricity was achieved with an 82-percent efficiency, which is a major achievement for this part of the SSPS system. The satellites will have to be deployed in orbit. There are transportation systems which can be developed for this purpose. This Nation already is embarked on the development of the Space Shuttle. A modified Space Shuttle is adequate to let us pursue the technology verification steps as well as to place a prototype SSPS into orbit. Heavy-lift launch vehicles which are being studied by NASA are the preferred second-generation system, because they could have lift capacities in excess of 400,000 pounds to low Earth orbit. If the SSPS were just a matter of developing the technology, we would be very confident that this project can start now. But we know that we have to access the SSPS on the basis of several criteria to decide whether the SSPS is indeed beneficial to society beyond just being technically or even economically feasible.
The SSPS is of worldwide interest. Thus, whatever we shall decide to do, we have to realize that many other nations will ask questions. What we have attempted to do so far is to assess the environmental impacts and understand some of the socioeconomic questions which will need to be answered. If we examine the capital costs of the SSPS, we find that the total cost of $7.6 billion for a 5,000-megawatt satellite can be broken down into various of its components..The major cost element is for space transportation. However, even with transportation, we project the unit cost to be $1,500 per kilowatt, which is the competitive range of most other generating systems in the time frame of the 1990's when this satellite would be operational. A utility could supply SSPS produced power at the bus bar for 27 mills per kilowatt-hour, if we assume a 30-year operational life. We actually expect the satellites to last longer than 30 years. We have compared these costs in mills per kilowatt-hours—and here we are using 1974 dollars—'to the cost of coal, oil and terrestrial solar systems. Nuclear power is not shown, since it is hard to project costs which have recently increased at such rapid rates. We have assumed for example, that coal will increase in noninflationary terms, either at 2.6 or 5 percent yearly increments from 1995 to 2025 while oil will increase at an increment of 0.7 percent with a 5 percent yearly increment being outside the range of economic interest. I do not believe that oil for power-generating purposes will be significant much beyond the turn of the century. Thus we project that at 27 mills per kilowatt-hour, the SSPS can be competitive with coal-burning plants, and it certainly is competitive with terrestrial solar plants. The solar system cost projects, ranging from 35 to 65 mills per kilowatt-hour, have been produced by JPL in a study carried out for NASA. We have compared the capital costs of terrestrial solar plants, using photovoltaic conversion, with the costs of the SSPS for a number of solar cell efficiencies—48 percent and 10 percent—with and without energy storage. In all cases, we find that the cost of power produced by the satellite tends to be very competitive—in fact, lower when the cost of the solar cell per square meter is the basis for the comparison. The reason we are not now using solar cells on earth is, of course, their high cost, which is expected to be reduced as the national photovoltaics conversion program begins to achieve lower costs. The SSPS development program can be divided into three phases. The first phase is concerned with development of the technology and its verification—for example, in a space station or other means that could test the various functions of the components. These tests should be completed in the mid-1980's, allowing us then to take the next step to construct a prototype SSPS so that we can then be assured how such a device would operate on a reasonably large scale, and be ready to proceed with commercial SSPS construction. As far as the near-term aspects are concerned, we have identified a development program over the next 4 or 5 years to improve our knowledge of photovoltaic conversion, particularly the fabrication of solar cells into large arrays, the analysis of large structures, the techniques for manufacturing and assembling components in orbit, system for providing stability and control, the generation and transmission of
microwaves about the performance of mechanical systems—such as rotary joints required for the antenna rotating with respect to the solar collectors—stationkeeping, and performance of socioeconomic and environmental assessments. After the successful completion of the prototype construction during the second phase, the emphasis should shift to mass producing the SSPS so that there could be at least 100 units operational by the year 2025. If I look at where we are today, and where I started back in 1968 when I first proposed the concept of the SSPS for solar energy or conversion in orbit, I am greatly encouraged by the tremendous progress that has already been made. What is required now is to proceed with the near-term development program for the SSPS so that this option can be protected and so that future decisions regarding the implementation of the full-scale development program can be based on factual data. NASA and industry have the capability to undertake the development program. The commitment to this program within the technical community is growing. The public is greatly interested. With the support of Congress and other elements of the Federal Government, this option could be established as one of the major initiatives to meet future national and world needs for energy. But even beyond this, the satellite solar power station represents an opportunity to enter not only a new era of energy resource development, but, in a broader sense, it represents a first step toward the industrialization of space and the extension of civilization beyond the confines of the Earth's surface. Thank you very much, Mr. Chairman. [The prepared statement of Dr. Peter E. Glaser follows:]
DEVELOPMENT OF THE SATELLITE SOLAR POWER STATION Testimony of Dr. Peter E. Glaser Vice President Engineering Sciences Arthur D. Little, Inc. Cambridge, Massachusetts at the Hearings of the Subcommittee on Aerospace Technology and National Needs of the Committee on Aeronautical and Space Sciences United States Senate January 19, 1976 Arthur D Little, Inc
THE POTENTIAL OF SOLAR ENERGY Today, the application of solar energy is recognized as a promising alternative to meeting future energy demands, since the Earth receives prodigious quantities of solar energy (1.7 x 101 4 kW are intercepted by the Earth). However, it, is a widely distributed resource; one square meter of the Earth's surface exposed to direct sunlight receives the energy equivalent of only one kilowatt. Moreover, this energy is not easily convertible and certainly is not “free.” Thus, while solar energy is abundant enough to provide self-sufficiency and clean enough to satisfy the most ardent environmentalists, methods must be found for converting it efficiently and economically into useful forms. One important drawback to the large-scale application of terrestrially based solar-energy conversion is the interruptions of solar radiation during periods of inclement weather or at night. These interruptions lead to a requirement for substantial energy storage capacity. Another drawback to the large-scale application of terrestrially based solar-generated power is that it will be economical in only a few locations. Consequently, terrestrial solar systems probably will be useful only in meeting peak demands. These obstacles can be overcome when the solar energy conversion system is placed in synchronous orbit around the Earth where solar energy is nearly constant 24 hours a day.1 Synchronous orbits are utilized today by communications satellites, and are ideally suited for the large-scale conversion of solar energy in a satellite solar power station (SSPS), see Figure 1,* as presented in hearings before the Committee on Aeronautical and Space Sciences, October 31, 1973. The SSPS has the potential to provide an economically viable and environmentally and socially acceptable option for power generation on a scale substantial enough to meet a significant portion of future world energy demands. The concept of the SSPS is based on the extension of existing technology and on the successful start of the development of an effective space transportation system, as represented by the space shuttle. The SSPS could use solar cells to convert solar energy to electricity on a nearly continuous basis. The electricity would be fed to microwave generators incorporated in a transmitting antenna in the SSPS. The antenna would direct a microwave beam to a receiving antenna located in direct line of sight on Earth, and there the microwave energy would be reconverted safely and efficiently to electricity. Additional SSPS systems can be established to deliver power almost anywhere on Earth. ‘This baseline design 2 represents the present stage of evolution (see Figure 2), which began with a planar array of solar cells. This baseline design utilizes silicon solar cells in combination with solar reflectors to convert solar energy into electricity, and was evolved by the team of Arthur D. Little, Inc., Gruman Aerospace Corporation, Raytheon Company, and Spectrolab, Inc..
FIGURE 1 DESIGN CONCEPT FOR A SATELLITE SOLAR POWER STATION BASED ON PHOTOVOLTAIC CONVERSION The advantages resulting from solar energy conversion with the SSPS in synchronous orbit are as follows: 1) The amount of solar energy available in synchronous orbit ranges from 6 to 15 times that available in areas receiving copious sunshine on Earth. 2) The solar energy in orbit is available nearly continuously except for short periods around the equinoxes, at which time the satellite will be shadowed by the Earth for a maximum of 72 minutes a day. Averaged over a year, this shadowing results in only a 1% reduction of the energy that would be available if the SSPS were continuously exposed to sunlight. Furthermore, the shadowing will occur near midnight at the receiving antenna site, when power demands are lowest. Therefore, energy storage is unlikely to be required.
3) Synchronous orbit represents a favorable operational environment for the SSPS because zero gravity conditions and the absence of wind and rain and other natural environmental effects permit the deployment of large-area structures with minimal weight. Hence there is a marked reduction in the materials used per unit of delivered power. In addition, the space vacuum permits the operation of microwave generators and other components without the evacuated enclosures required on Earth. Moreover, because the SSPS in synchronous orbit is stationary with respect to a desired location on Earth, the microwave beam can be directed to most receiving antenna sites in the vicinity of major power users. These sites can be established on low-value land or offshore. Furthermore, because the receiving antenna is transparent to solar radiation and permits rain to reach the land below it, opportunities for multiple land use are provided. 4) The environmental effects of the SSPS and the associated space transportation system are projected to be within acceptable limits. First, all waste heat associated with solar energy conversion and microwave generation can be rejected to space. Second, no waste products are generated. Finally, the microwave beam densities can be designed to meet international standards.
FIGURE 2 CONFIGURATION EVOLUTION
TECHNOLOGY ALTERNATIVES SOLAR ENERGY CONVERSION As originally conceived, the SSPS can utilize a number of approaches to solar energy conversion3 — thermionic, thermal electric, photovoltaic conversion, and others likely to be developed in the future. Among these conversion processes, photovoltaic energy conversion was chosen as a starting point because solar cells represent a demonstrated technology as a result of widespread use in the space program. Solar cells are used widely in space power supply systems whereas earlier efforts based on solar thermal and nuclear power were not as successful. In addition ERDA's National Photovoltaic Program has as its objectives to develop low-cost reliable photovoltaic systems and to stimulate the creation of a viable, industrial, and commercial capability. The photovoltaic process, since it is a passive one, could reduce maintenance requirements and lead to increased reliability during the desired 30-year operational lifetime of the SSPS. Present communication satellites (e.g., INTELSAT IV) already have a projected lifetime of 10 years. Because the space environment is benign compared to the terrestrial environment it should be possible to extend the lifetime of solar cells beyond 30 years by processes such as annealing, or recycling in a space manufacturing facility. Solar thermal conversion is of interest primarily because machinery operating on thermodynamic cycles; e.g., the Brayton Cycle, has had a long and successful history in terrestrial applications. Furthermore, the development of orbital solar power plants could be based on the development of optical focussing systems and central receivers for solar thermal plants which could be adapted for use in the SSPS.4 Novel techniques to achieve geometric perfection desired for the solar concentrators through active mirror-shaping controls are being investigated. If successful, these efforts will result in the large concentration factors required to achieve elevated temperatures for high thermodynamic efficiency. Progress in gas-bearing technology holds promise that the reliability of rotating machinery could be extended beyond the few thousand hours associated with most terrestrial applications of rotating machinery. Rejection of waste heat through active systems, including radiators, still poses a major challenge because of the mass required for a large-area space radiator and for operational reliability. Thermionic energy conversion, which has been investigated in the space program but not yet applied, may be useful as part of a topping cycle in combination with thermal conversion systems. Considerable technical and economic analyses will be required to establish which of the potential approaches for solar energy conversion will be optimal. However, the fact that there are several promising approaches to meet the
requirements of the SSPS indicates a high likelihood that appropriate technology will be developed. POWER TRANSMISSION TO EARTH There are several approaches for transmitting the power generated in the SSPS to Earth. Of these, the microwave method uses state-of-the-art or achievable technology to obtain high efficiency in generation, transmission, and rectification. Moreover, it promises to satisfy environmental requirements and safety considerations? Microwave transmission and rectification technology is based on demonstrated results from commercial use and developments to meet military requirements. Mass production of more than one million microwave devices serving an annual market of half a billion dollars in the United States alone, is indicative of the commercialization of the technology. The transmission of power from orbit to Earth by laser, although receiving considerable attention, is not the preferred choice because of the low efficiencies associated with the conversion of electricity into laser power and the reconversion of laser power into electricity. In addition, the absorption of laser beams in the atmosphere, and by clouds, would reduce the overall efficiency of power transmission to an unattractive level. The possibility of concentrating sunlight with mirrors placed in synchronous orbit to overcome the diurnal variation of solar energy on Earth has also been explored. Such an approach is unattractive because of the large area of concentrating mirrors that would be required in orbit to achieve a reasonable concentration factor at a location on Earth and because of the losses from absorption in the atmosphere.
SSPS DESIGN CONSIDERATIONS SOLAR CELLS Substantial data is available on the performance of silicon solar cells. Present silicon solar cells are about 200 microns thick and efficiencies of 15% have been achieved and further increases in efficiency are considered feasible, even with reduced thicknesses. The present technology uses silicon solar cells mounted on rigid substrates with cover glasses bonded to the solar cell to achieve radiation shielding. Advanced technology based on the “roll out” blanket design which exhibits weight-to-power ratios of about 30 Ib/kW, have been fabricated. With improved fabrication techniques, reductions in thickness to less than 100 microns, and use of solar concentrators, solar cell array weights of about 3 Ib/kW are projected to be achievable in 10 years. These projections are based on reasonable improvements for single-crystal silicon solar cells and successful achievement of the goals of the National Photovoltaic Conversion Program being conducted by ERDA. The low solar array weights would be achieved through the use of plastic film to replace the cover glass, and are based on successful laboratory development of such solar cell blankets. Solar concentrators with Kapton film mirrors coated to reflect solar radiation onto solar cells and to filter undesirable portions of the solar spectrum are designed to reduce the area requirements for solar cells and their weight and cost. Figure 3 indicates the arrangements of solar cell arrays and concentrating reflectors. FIGURE 3 DETAIL OF SOLAR COLLECTOR ARRAY
A concentration factor of 2 will reduce the efficiency of an 18% silicon solar cell to about 14% at the operating temperature when heat rejecting coatings are used for the solar cell array. Increases in solar cell thickness and lower efficiencies will be reflected in increased capital cost because more material will have to be transported into orbit. The exposure to the space environment is projected to result in logarithmic degradation of silicon solar cells with a 6% loss of the original efficiency after the first five years. Micrometeroid impacts are projected to affect 1% of the solar cells during a 30-year operational lifetime. Recent progress in gallium arsenide solar cells has renewed interest in their use in the SSPS. The advantages of gallium arsenide solar cells are the higher efficiencies that have been reported at high concentration ratios and the lower susceptibility to degradation in the space environment. In addition, gallium arsenide solar cells may be produced at about one-tenth the thickness of silicon solar cells. Thus gallium arsenide deserves attention as an alternative solar cell material. As a result of studies over several years, the design for the SSPS based on the silicon solar cell array configuration shown in Figure 4 has evolved. The two solar collector panels are designed to provide a power output of about 8500 MW which results in an effective power output at the receiving antenna bus bar of about 5000 MW. A 100-meter diameter central mast and stiffened carried-through structure running through the assembly provide structural integrity. A microwave transmitting antenna is located between the two solar collector panels. The solar collector panels are arranged to face the sun continuously while the microwave antenna will rotate once a day with respect to the solar collector in synchronous orbit. The solar collector panels are supported by both nonconducting and conducting structures which carry the power to the microwave generators via the central masts. Rotary joints are provided at the perimeter of the central mast to allow rotation of the microwave transmitting antenna. Dielectric materials are used for the continuous support structure which is transparent to the microwave beam. These joints are the only major continuously active components in an otherwise passive satellite. Analyses of structural stiffness indicate that conventional, analytical techniques and structural design techniques are applicable to the SSPS. The structure of the SSPS will be subjected to thermal stresses and distortions induced by thermal gradients during the eclipses of the SSPS by the Earth's shadow for a short period before and after the equinoxes. The dwell time in the Earth's shadow will reach a maximum of 72 minutes during this period. Because the structure is so large, the thermal exposure cycle could cause it to oscillate; this possibility has to be evaluated in terms of fatigue effects which could shorten
service life. Although it is possible to select structural design approaches which will minimize the effect of such thermal exposure, more detailed evaluations of these effects are warranted. FIGURE 4 SSPS BASELINE CONFIGURATION The large structure required for the SSPS will be subjected to orbital perturbations, see Figure 5, of which the gravity gradient will be the most significant. A reaction control system based on the use of ion engines (Argon is one candidate propellant) will be required to keep the SSPS in the appropriate orbit and to assure that the solar collector panels point towards the sun within one degree, while the microwave antenna is directed towards the receiving antenna on Earth. To achieve the desired stationkeeping and attitude control for the SSPS about 100,0001b of propellant will be required per year, depending upon specific orbital characteristics. The mass of the SSPS, assuming that 5000 MW are delivered to the bus bar on Earth, is:
FIGURES ORBIT PERTURBATIONS The weight of about 8 Ib/KW for the orbiting portion of the SSPS, is remarkably low compared to that of terrestrial systems and is indicative of the advantages of placing the solar energy conversion system in synchronous orbit. MICROWAVE POWER TRANSMISSION SYSTEM Figure 6 shows the functional blocks of the microwave power transmission system designed to transmit the electrical power generated by the solar energy conversion system to a receiving antenna on Earth, and the associated efficiency goal.
FIGURE 6 EFFICIENCY CHAIN Microwave Power Generation The device which is being considered for converting DC to RF power at microwave frequencies is a cross-field amplifier (Amplitron). The Amplitron uses a platinum metal cathode operating on the principle of secondary emission to achieve a nearly infinite cathode life. The DC voltage required for the Amplitron is 20kV. A Samarium Cobalt magnet provides low specific weight compared to that of conventional permanent magnets utilized in other microwave devices and makes it feasible to use such devices in the SSPS. Radiating surfaces, which will be operating at 300 to 400°F, are designed to reject waste heat, representing about 10% of the input power, to space. Pyrolitic graphite, because of its low density and high emittance, is being considered for the space radiators. A movable magnetic shunt is incorporated in the Amplitron to regulate the output if the input current fluctuates. The movable magnet shunt is the only element subject to wear. Considerations of specific weight, costs, and efficiency at specific frequencies have led to the selection of a frequency near 2.45 GHz, which falls within the
industrial microwave band of 2.40 to 2.50 GHz. The output power level of the Amplitron at 2.45 GHz exhibits a near optimum value when the output is about 5 kW.6 Microwave Beam Transmission Space is an ideal medium for the transmission of microwaves; an efficiency of 99.6% is projected to be achieved after the beam has been launched at the transmitting antenna and before it passes through the upper atmosphere. Over the transmission distance of 23,500 miles, the curvature of the phase front of the beam will be very small; nevertheless, the front must be controlled with high precision to achieve high efficiency. To achieve the desired high efficiency for the transmission system, the geometric relationships between the transmitting and receiving antenna indicate that the transmitting antenna should be about 0.8 km in diameter, while the receiving antenna should be about 10 km in diameter (depending on latitude). The large size of the transmitting antenna is required to achieve a reasonable power density within the microwave beam at the receiving antenna for efficient conversion of microwaves into DC. To reduce the dimensions of the transmitting antenna, the microwave amplitudes can be tapered from the center to the edge over the range of 5 to 10 db. The advantage of transmitting-antenna amplitude taper, as opposed to uniform illumination, is that it reduces the intensity at the center of the beam to less than 50 MW/cm2 . To achieve the desired control of the phase front in the transmitting antenna, 18-x 18-meter subarrays are arranged into sectors to provide the required center-to-edge amplitude taper. Using a large number of small subarrays reduces the effect of attitude-control inaccuracies. Phase control electronics are provided for each subarray to compensate for subarray distortions which may be induced by thermal effects. The phased array wave guide approach is used for the subarrays to achieve very high efficiency, low-frequency cut-off, and reduced RFI effect. Figure 7 shows the microwave generators and space radiators installed in a typical subarray. A closed-loop phase-front control is used to achieve the desired high efficiency and safety essential for the microwave beam operation. A command and adaptive phase-front control concept is utilized. The reference beam launched from the center of the receiving antenna is sensed at each subarray and at the reference subarray in the transmitting antenna center. The central subarray transmits the reference signals to the subarrays over calibrated coaxial cables. The difference in phase between these signals which, for example, may result from the displacement of a subarray from the nominal reference plane because of thermal distortions of the structure, corrects the phase of the transmitted beam at the
displaced subarray so that the required beam front is launched toward the receiving antenna. For a subarray of 18 meters by 18 meters, the maximum radiated power will be about 7 MW. FIGURE 7 MICROWAVE GENERATORS AND SPACE RADIATORS INSTALLED IN SUBARRAY Aluminum has been selected for the wave guides, which have an overall thickness of about 0.5 mm. The subarray is divided into five meter segments to limit the aluminum wave guide deflection over the 5-meter distance, which results in a beam power loss of less than 1% for the subarray. The receiving antenna is designed to intercept, collect, and rectify the microwave beam into a DC output as efficiently as possible. The DC output will be designed to interface with either high-voltage DC transmission networks or to be converted into 60 Hz AC. The receiving antenna consists of an array of halfwave dipole antennas which rectify the incident microwave beam. Each dipole has an integral, low-pass filter, diode rectifier and RF bypass capacitor. The dipoles are DC insulated from the ground plane and appear as RF absorbers to the incoming microwaves.
In principle, efficiency may approach 100% because the receiving antenna element and the microwave radiation are coherent and polarized in an orderly manner. Hence, the effective conduction cycle of the diode rectifier circuit and the reactive energy storage combine to produce a very high efficiency. (Up to 87% conversion efficiency has been achieved in the laboratory.) A receiving antenna based on the principle of halfwave dipole rectification is fixed and does not have to be pointed precisely at the transmitting antenna; thus, its mechanical tolerances do not need to be severe. Furthermore, the density distribution of incoming microwave radiation need not be matched to the radiation pattern of the receiving antenna; therefore, a distorted incoming wavelength caused by non-uniform atmospheric conditions across the antenna does not reduce efficiency. 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. Any heat resulting from inefficient rectification in the diode circuit can be convected by the receiving antenna to ambient air, producing atmospheric heating which will be less than that over urban areas, because only about 15% of the incoming microwave radiation would be lost as waste heat. The low thermal pollution achievable by this process of rectification cannot be equaled by any known thermodynamic conversion process. The rectifying elements in the receiving antenna can be exposed to local weather conditions. The receiving antenna can be designed to be 80% transparent so the land underneath the antenna could be put to other uses. In the summer of 1975, tests of a 24-square-meter array of microwave rectifier elements were conducted at the NASA Venus antenna site at Goldstone, California, to demonstrate the effective performance of dipole rectification.7 The transmitting antenna, which consisted of an 86-foot-diameter dish antenna, was located about one mile from the receiving elements. At a radiated frequency of 2388 MHz, incident peak RF intensities up to 170mW/cm2 have yielded up to 30.4 kW of DC out power. An average conversion efficiency of 82% was obtained at the receiving arrays under these conditions. SSPS TRANSPORTATION, ASSEMBLY, AND MAINTENANCE The SSPS will require a space transportation system capable of placing a large mass of payload into synchronous orbit at the lowest possible cost. The cost of transportation, assembly and maintenance will have the most significant impact on the economic feasibility of the SSPS. Several approaches to achieving this objective are being investigated. It is highly likely that a two-stage transportation system will evolve, which will carry payloads first to low-earth orbit and
subsequently deliver partially assembled components to synchronous orbit or possibly to an intermediate orbital altitude for final assembly and deployment. Transportation The space transportation systems which are being considered are primarily an extension of existing systems. The potential systems, starting with the space shuttle now under development, vary from the use of a modified space shuttle to the development of a fully reusable liquid oxygen/liquid hydrogen heavy-lift launch vehicle with a potential 400,000-lb payload capability to deliver to low-earth orbit. The current shuttle or its modification can be used for SSPS technology verification and flight demonstration and for transporting elements of the prototype SSPS into low-earth orbit. The cost for such a system capable of lifting payloads up to 160,0001b to low-earth orbit, is projected to be $100-200/lb. The heavy-lift launch vehicle is expected to reduce payload costs to between $20 and $60 per pound for delivery to low-earth orbit. The large mass of payloads will require about 60 flights for each SSPS assembled in synchronous orbit when an advanced space transportation system based on heavy-lift launch vehicles is used. Ion propulsion, using solar power sources, could be used to transport a completely assembled SSPS from low-earth orbit to synchronous orbit. There is the option to transport to an intermediate orbit at 7,000 nautical miles. Chemically powered stages would transport payloads from low-earth orbit to this intermediate orbit, which lies outside the Van Allen Belt and ion propulsion would transport the assembled SSPS to synchronous orbit. The cost for each flight will be strongly influenced by the feasibility of using ion propulsion for the orbit-to-orbit transportation and by the ability to reuse most of the components of the space transportation system for a large number of successive flights. Challenges inherent in the development of a low-cost, heavy-lift space transportation system are being explored.8 The achievement of low-cost space transportation will be essential to the commercial success of the SSPS. Assembly The large number of components, most of them performing the identical function, and the role of man in assembling these components require that the methods of assembly, packaging of components, assembly rates, and maintenance and repair support facilities required during the assembly and subsequent operational phases be carefully evaluated. There are two basic approaches to assembly: (1) Remote assembly using ground controlled tele-operators.
(2) Assembly of components delivered to synchronous orbit by an assembly crew operating from a space station support base as part of extravehicular activities. Assembly by tele-operators can be expected to be less costly than assembly by work crews based in synchronous orbit, which would require a space station for support of their operations. The choice between manned or tele-operator assembly will depend on the cost effectiveness of either approach. Tele-operators using remote assembly techniques should achieve assembly rates in excess of about 10 lb an hour, and would appear to be more effective than manned assembly, which would have to achieve rates of about 20 lb an hour to justify the cost of space stations and recycling of crews. It is highly likely that a combination of both manned operations and tele-operators will evolve, where man's most important function will be to exercise control over the assembly process. Maintenance With any major system such as the SSPS, the design criteria, materials choices, and data on component life and the expected operating conditions will determine reliability. Redundancy of components through the use of large numbers of identical components, for example, solar cells, will tend to reduce maintenance requirements. The cost of performing repairs has to be evaluated and compared with the option of delaying repairs and accepting the potential loss of revenue while achieving operational lifetimes consistent with cost analyses of the SSPS operation. The goal will be to evolve maintenance-free designs, particularly for the solar cell blankets and the microwave generator subsystems. SPACE-BASED MANUFACTURE The SSPS provides a unique opportunity to evolve manufacturing methods which are particularly suited for operations in space. The space transportation system, which will evolve to transport payloads from Earth to synchronous orbit, will have a substantially greater lift capability than even a space shuttle. However, this lift capability can be utilized appropriately only when the payload is designed to be weight limited rather than volume limited. Prefabricated beams (required, for example, for the microwave transmitting antenna structure) transported to orbit for subsequent deployment may be less desirable because packaging densities of about 5 Ib/cubic foot are typical for folded and compressed designs for deployable structures. Thus available volume probably will be the limiting factor rather than lifting capacity if components are prefabricated on Earth. The required high load factor could be achieved if fabrication and assembly were performed in orbit from appropriately prepared flat-rolled stock from which girders and other mechanical components could be produced by automated