The ISAS Solar Power Satellite Working Group organized in 1987 has been engaged in four projects of research activities, one of which is the conceptual design study of an SPS strawman later designated as SPS 2000. The primary objective of the study was to gain practical knowledge about problem areas of research for SPS systems. A preparatory study was conducted by a special team in the Working Group for one year (1). Later, more researchers and groups were invited from industry and other universities to participate in the first phase design activities. Consequently, forty eight members were registered in the Task Team.

The first phase study for the conceptual design was completed in July 1993, and a report was published and submitted to the Working Group recently (2). This paper, based on the report, will consist of three parts; the first one is concerned with the basic design requirements to meet the primary study objective, the second one is a summary of general concept of the SPS 2000, and the third one is considerations on the study result. Relating to this general survey paper, four papers are to be presented on the technical detail of the SPS 2000 system in this Symposium, 19th ISTS.

The motivation of the conceptual design of SPS2000 was to find an answer to a question; what is a solar power satellite. This question is primitive for specialists of space engineering, but frequently asked by specialists of terrestrial power industry. Only an available answer was the gigantic U.S. Reference System, which has been well known as a fantastic idea of future space program. Obviously, those who asked the question expected to get an answer different from the Reference System. Thus, it has been the primary objective of this study to present as an answer a realistic and practical concept of SPS.

Considering that most of the Task Team members engaged in space oriented works were not familiar with power systems for terrestrial use, and that the SPS Reference System impressed them very much, we had to establish a guideline for design work of this study. For this purpose it was emphasized that the strawman solar power system should not be a space system for space infrastructure but a part of electric utility system to serve for terrestrial use. Accordingly, the basic requirements for this study are based on practice of terrestrial utility power system. For researchers to be engaged in this study to reflect this view in their design work, the following six requirements were specified.

Requirement 1: The first construction will be started no later than 2000. This requirement is to give realistic background of the study, that is, technology option, finance and economy of power utility, international relations and the global environmental situations. Possibility that can be predicted to be incorporated into the system design has to take place in a decade. In the past experience, two decades are too long for probable future prediction, and three decades practically implies infinite future. Relating to this requirement, the microwave power level of SPS2000 has been assumed to be 10 MW.

Requirement 2: Commercial and versatile technologies will be used for this system. The present space technology is a typical high value-added technology which only communication industry affords. Considering the large scale of utility power systems, we cannot use such expensive technology in a large quantity for solar power satellites. Some say mass production of electric parts would reduce the cost per unit. However, mass production of GaAs semiconductor used for hand-carry telephones could not reduce the price so much as expected. Implications of this requirement are that more design efforts will be required for application of lower performance technology than existing space-use hardware.

Requirement 3: The electricity generated by the SPS 2000 shall be commercially competitive with existing small scale utility electricity. Although it is generally accepted that cost of solar power satellites has to be lower than existing space systems, cost saving efforts used for space project management are not enough to be applied for solar power satellite business. To give more definite cost targets of subsystems of the satellite, the following assumptions were used;

1. Transportation cost will be treated separately from the satellite cost, since these two costs will be determined independently from each other in future.

2. Electricity generated by the solar power satellite will be sold in a form of microwave power transmitted from the satellites to the customers who build and own rectennas, to simplify the sales interface (3).

3. Price of microwave electricity is ten Yen/kW-hour.

Table 1 is an example to show the relation between a loan condition and cost targets of three major subsystems of solar power satellite.

There are discussions on additional value of solar power satellites because of their substantial merit of no production of carbon dioxide in the atmosphere. These discussions are concerned with a global scale of economy and environment, so the issue of carbon dioxide will not be considered for this study, although it will be potentially in favor of SPS.

Requirement 4: The SPS 2000 satellite will be placed on a low altitude equatorial orbit by commercial launch systems. In the CDEP, dedicated space transportation systems were supposed to be a part of the SPS program. Thus, development of gigantic space launchers like HLLV featured the SPS Reference System. On the other hand, there are several commercial launch vehicles capable of carrying the satellite of this study, although they are very expensive for SPS business. It is widely believed that rocket vehicles require a large amount of energy which result in the high price of space transportation that is the main reason to explain difficulty of building solar power satellites. However, current efforts in the field of SSTO like Delta Clipper seems to promise to reduce the cost to several percents of the present launch cost. The SPS business will not be needed to be engaged in development of launch vehicles, but will buy the service. Thus, in this study, the Ariane V and Proton have been assumed as carrier vehicles in order to provide requirements for payload accommodation of preassembled hardware of the strawman SPS.

Requirement 5: Prospective customers for the electric power utility service are residents in the equatorial zone. Adoption of a low earth equatorial orbit makes it possible to supply microwave power to many rectennas, over which the solar power satellite passes. People living in the equatorial zone will take advantage of the small amount of electricity supplied by SPS2000, even if it would be too small for industrialized nations to use as their electric power systems. It is assumed that the rectennas will be built and operated by electric utility companies which buy microwave from the satellite. It is important for the satellite design to know practical demand for this type of electric power.

Requirement 6: Design a basic model with 10MW microwave power allowing for system growth in the future. Once SPS 2000 demonstrates technical and economic feasibility of power from space, larger and more stable systems such as the Reference System will be expected to follow. The power level of 10 MW is not necessarily the optimum size of SPS power station, and the satellites need not remain in low earth orbits. In this respect, the design of SPS 2000 should consider evolution to make a larger system from the first operating model.

A 1100km altitude equatorial orbit will be used. This choice minimizes the transportation cost and the distance of power transmission from space. Various parameters depending on the altitude were analyzed. Among them, Fig. 1 shows the relation between power reception duration at a rectenna on the equator and the orbital altitude.

Microwaves of 2.45 GHz frequency are used to transmit power from the satellite to the rectenna. The system power is defined by the microwave power transmitted from the satellite, not by the power received on earth. Figure 2 shows a schematic of microwaves beam control and rectenna location.

Fig. 1. Orbital altitude vs. power reception time at a rectenna on the equator.

System elements of the satellite are:

100 Bus
200 Structure and Assembly
300 Power Generation and Power Line
400 Spacetenna
500 Robotics

A general view of the system is given by Fig. 3 for the satellite. The general shape of the satellite looks like a saddle back roof. The roof is formed by solar panels and the spacetenna is built on the bottom plane to transmit microwaves to the ground.

Rectenna

The ground segment of the power system is rectennas. A typical rectenna is shown in Fig. 4 . Different types of rectennas are assumed to be designed to meet different regional requirements. The following system elements are specified for rectennas:

1100 Rectenna Control
1200 Rectenna Construction

Fig. 4. A rectenna of the SPS 2000 system.

To simplify the attitude control system, especially to avoid needing a reaction control system, the geometry of the satellite at each phase of construction is designed for the gravity gradient force to stabilize the attitude. Figure 5 is a Kr-Ky diagram used for evaluation of the final configuration of SPS 2000 satellite changing the parameter l, the length of the top roof beam, keeping the power generation constant. The stable region is indicated by shadow.

Fig. 5. Kr-Ky diagram to depict SPS 2000 stability at different values of parameter l (4)

The first orbital assembly work will be begun with deployment of a working platform as shown by Fig. 6. To satisfy the requirement for attitude stabilization, a scheme for managing the configurations of the satellite during assembly in orbit has been proposed to design the construction procedure of the SPS satellite. Figure 7 shows an example of deployment of the first module carried by the first launch. The stability of the partially assembled SPS has been discussed for every configuration of the module attachment.

Thermal environment has not been studied in detail. Characteristics of the electrostatic field and electric current distribution on the satellite have been studied to protect the system against predicted hazards.

The total system is shown in Fig. 8.

Solar Cell: The following baseline data used for Solar Cell Unit is based on the current performance of ground-use a-Si solar cells and their possible evolution in the near future. Further detail of solar cells under test will be presented in a separate paper (6).

Conversion Efficiency	15	%
Unit Weight	0.22	kg/m2
Specific Power	950	Watt/kg
Thickness	0.2	mm

Array Module: A subarray is composed of 12 solar cell units. The array module, composed of 110 subarrays, is a mechanical element for assembly. Each array module generates 180A at 1 kV. The weight of the array module is 270 kg per each module. Forty five array modules are assembled in each wing; northeast, southeast, northwest, and southwest.

Power Collection and Distribution: The Wing Summing Bus Line(321) collects the electric power from the array modules. Each bus line has hot and return bus cables. The bus lines are insulated copper plates 1 mm thick. They get wider as they approach the center of the SPS2000 satellite to keep the joule loss per surface area constant. The Wing Summing Bus Lines are connected to the Central Bus Lines(322) which are interfaced with the spacetenna system. The Central Bus Lines are insulated copper plates 0.7 mm thick by 100 mm wide. The Bus Lines are mechanically attached to the truss pipes using insulated adapters. The power loss in the bus lines is 7 % in total. The total weight of the power lines is approximately 11,000 kg.

Power Generation: Power generation of SPS2000 changes with the local time, depending on the sun angle of the arrays and the cell temperature. The diurnal and seasonal variation of the generation power have been calculated.

Power transmission from the satellite to a rectenna is made by 2.45 GHz micro-wave beam emitted from the spacetenna, the antenna onboard the satellite, provided with retrodirective beam control capability. Using the principle similar to that of the U.S. Reference System, electrical and mechanical design of this system is simpler by employing a square shape and a single power level. Detailed design of the spacetenna will be shown in separate papers (7, 8). This makes the microwave beam broad, and results in relatively inefficient power transmission and an increase in microwave exposure outside the rectennas. However in this case, the microwave power level is much lower than in the case of the Reference System, and well below international safety standards.

The beaming angle as large as 60 degrees of this case makes this requirement more important than in the case of the Reference System.

Spacetenna Design: Antenna characteristics are shown by Table 2.

The microwave power distribution on the earth surface is shown by Fig. 9.

Rectenna and Electricity Supply

Rectenna Technology: For SPS2000 two basic rectenna designs have been considered to date, the high-efficiency "wire mesh reflector" supported on a rigid frame above the ground, and the low-cost "magic carpet" which could be pegged to the ground. Power collection, conditioning and energy storage will be provided according to customers' requirement.

Rectenna system: SPS2000 rectenna systems may be developed for different purposes, such as a small-scale, low-cost system; a full-size maximum-output system; a system intended to be developed later into a commercial system. At least one SPS2000 rectenna site will be used as an SPS operation research center. Rectennas may deliver power into an existing grid, or operate independently.

Rectenna site conditions: To deliver power for the maximum length of time, rectennas will be at least 1200 km apart. Rectenna construction and operation will have environmental and economic impacts which will need to be analyzed for each site. Figure 10 shows equatorial zones where electricity of SPS2000 can be received.

Rectenna construction engineering: Rectenna construction will depend on the terrain. Possibilities include over forest, over desert, over the sea, or on hills. Preliminary case studies are being performed of different possible means of using the rectenna output. These include water-pumping; charging electric vehicles; supplying energy to a bio-reactor system; supporting an airport complex; and providing part of the output of an artificial "energy island".

Depth of the conceptual study varied by subsystems or technical areas of the system. Achievements of the study have been schematically shown by Fig. 11, in which comparative estimation of solved and unsolved is made for each critical work field depicted by a bar with key words featuring individual issues. For this estimation, the basic requirement given initially are used as criteria, although extent of satisfaction of each can not be quantified. It should be noted that nontechnical requirements, especially economic requirements affected the result significantly.

Solar cells: The most critical issue of SPS2000 design to satisfy the basic requirements is availability of the key electric technologies, such as a large number of solar cells and a large number of high efficiency and low cost semiconductors for the spacetenna. However, solar cells developed for terrestrial use seem to be usable in space, according to our preliminary study (6). On the other hand, the application of the current space technology will be difficult both in the required reduction of cost and in mass production.

Phased array antenna: The key issue is to minimize the power level of "grating lobes " which are unavoidable for a phased array antenna. The number of antenna elements is increased while each element transmits less power in order to satisfy this requirement. Semiconductor devices are chosen for the antenna elements. However, the efficiency is not high enough to achieve high power density at the antenna surface using the present-day technology. Low cost and mass production will be realized with application of terrestrial use technology as in the case of solar cells.

Power collection: The power line to feed power generated by solar cells and to the spacetenna has been found a more serious problem than expected, since the mass of the power line has been found to dominate that of the main structure. As far as a simple d.c. system connecting the output power of solar cells in series and parallel is used, the situation will not be improved. A superconducting power line or high frequency a.c. system is expected to solve this problem technically as well as economically.

In addition to lack of the basic electric parts technology, inexperience of some areas of design has been found to be a critical problem. As described in the following, construction in space is a typical example of engineering area which is not ready to initiate basic design of such a large system in a near-earth orbit.

Automated assembly: The satellite structure and the antenna was not designed so much in detail as other subsystems. One reason is that these subsystems significantly depends on the design of interacting subsystems, such as solar panel and spacetenna including their construction procedures. Especially, fantastic ideas of space robotics have been expected to solve all the difficult problems that might accompany every construction and assembly procedures in space. Later, the functions actually required for automated construction of the satellite have been found to be similar to those of industrial robots for terrestrial manufacturing and operation. The key performance factors of such a robot or automated construction machine are low electrical power consumption and high speed operation. In this respect, the conceptual study is very preliminary.

Construction method under gravity gradient force: If construction and assembly are made slowly in near-earth space, the general construction procedure has to take account of difference of orbital motions of structure elements to be assembled. The same effect appears as gravity gradient force on a solid body. The SPS 2000 employs gravity gradient force to take attitude reference toward earth. However, the conventional space construction featured by weightlessness cannot be used even for determining a basic order of construction of a large system in space, like conventional space robots could not give solution for this problem.

Different from terrestrial engineering project, design for solar power satellites requires more knowledge based on practice and experiment. Typical examples of the basic knowledge that require space experiments are design verification of the general geometrical shape to stabilization by gravity gradient force and prediction of performance of hardware to be used under microgravity. Microwave power transmission and large scale solar arrays in space are generally considered to be difficult. It is important to analyze the problems in more detail so that the most effective experimental approach can be taken. These technical and engineering researches should consider publicity of SPS and support efforts towards better understanding this new energy system. The "Microwave Country" is proposed for demonstration of safety as well as verification of rectenna technology.

The SPS2000 conceptual study of the ISAS SPS Working Group was intended to provide the researchers having interest in power from space with ideas of practical problem areas of an early phase of research. A set of basic requirements were defined and used for design work of the SPS 2000 concept from standpoints of economic and technical feasibility of power utility based on solar power satellites. As a result, a unique concept of SPS has been developed. It was found that technology was not mature in the key electrical parts such as solar cells and semiconductors, and engineering for automatic construction and assembly methods was not ready. In this respect, this study has given researchers a remarkable insight into uncertain future of development of power from space.

The authors acknowledge the following contributors for the original Japanese edition of the SPS 2000 Concept:

P Collins, T Fukuoka, S Fukuzawa, Y Hashimoto, H Hirayama, M Horiuchi, I Igarashi, S Inoue, K Itoh, M Iwabuchi, H Kuninaka, M Mori, H Nagano, T Nagase, T Nishimura, T Saito, Y Sakai, T Sogawa, F Sugimura and M Tanaka.

1. M Nagatomo and K Itoh, 1991, "An evolutionary satellite power system for international demonstration in developing nations", SPS 91 Power from Space
2. SPS 2000 Task Team, July 1993, " SPS 2000 Project Concept - A Strawman SPS System", S2-T1-X, Preliminary (in Japanese, an English summary is available)
3. P Collins and R Tomkins, 1991, " A method for utilities to assess the SPS commercially", SPS 91 Power from Space
4. G M Lerner, 1978, " Spacecraft Attitude Determination and Control", ed. by Wertz J. R. KluwerAcademic Publishers, p.612
5. S Fukuzawa, M Nagatomo and V J Modi, 1994, " On the Constructional Methodology and Dynamical Formulation for the Proposed Solar Power Satellite SPS 2000", ISTS 94-e-06
6. S Sasaki, A Ushirokawa, Y Morita and M Tanaka, 1994, " Investigation of Solar Cells for Solar Power Satellite SPS 2000", ISTS 94-e-29
7. K Itoh, M Omiya and Y Naruo, 1994, " Design and Construction of Spacetenna of Solar Power Satellite, SPS 2000", ISTS 94-e-05
8. K Itoh, M Ohmiya and Y Ogawa, 1994, " A Phased Array Antenna for Spacetenna of SPS 2000", ISTS 94-e-07