Dear Kristian

URSI White Paper on

Solar Power Satellite (SPS) Systems


Contents

1. Executive Summary 1

2. Solar Power Satellite Systems 2

2.1 The SPS Concept 2

2.2 The Aim and Purpose of this White Paper 4

2.3 The History of SPS Research 4

2.4 A Coherent Set of Numerical Values 6

2.5 Economic Issues 7

2.6 Key SPS Technologies 8

3. SPS Radio Technologies 9

3.1 Microwave Power Transmission 9

3.2 Microwave Power Devices 10

3.3 Rectennas 10

3.4 Control and Calibration 11

4. Radio Science Influences and Effects of SPS 12

4.1 Interaction with the Ionosphere and Atmosphere 12

4.2 Compatibility with Other Radio Services and Applications 13

4.3 Microwave Power Transmission Effects on Human Health 14

5. Radio-Science Issues for Further Studies 14

6. Acknowledgements 15

7. References 16

8. Appendix 1: URSI White Papers 18

9. Appendix 2: the ten Scientific Commissions of URSI and their Terms of Reference 19

1. Executive Summary
As a consequence of an ever-increasing world-wide energy demand and of a need for a “clean” energy source, the solar power satellite (SPS) concept has been explored by scientists and engineers in the United States, Japan, and Europe. An SPS constitutes a method of generating electricity from solar energy using satellites and transporting it to the ground via electromagnetic waves. Several candidate systems have been proposed. However, so far no system has been either constructed or tested in space, and it is currently unknown when one might be.
The purpose of this URSI white paper is to provide knowledge about the SPS concept based on evidence, and an open forum for debate on the scientific, technical, and environmental aspects of the SPS concept¹.
In a typical SPS system, solar energy is collected in space by a satellite in a geostationary orbit. The solar energy is converted to direct current by solar cells, and the direct current is in turn used to power microwave generators in the gigahertz frequency (microwave) range. The generators feed a highly directive satellite-borne antenna, which beams the energy to the Earth. On the ground, a rectifying antenna (rectenna) converts the microwave energy from the satellite into direct current, which, after suitable processing, is fed to the terrestrial power grid. A typical SPS unit – with a solar panel area of about 10 km², a transmitting antenna of about 2 km in diameter, and a rectenna about 4 km in diameter – may yield an electric-power output of about 1 GW. Two critical aspects that have motivated research into SPS systems are the lack of attenuation of the solar flux by the Earth’s atmosphere, and the twenty-four-hour availability of the energy, except around midnight during the equinox periods.
Among the key technologies involved in SPS systems are microwave generation and transmission techniques, wave propagation, antennas, and measurement and calibration techniques. These radio-science issues fall within the scientific domain of the International Union of Radio Science (URSI). URSI’s ten Scientific Commissions (Appendix 2) cover a broad range of aspects involved in an SPS system, ranging from the technical aspects of microwave power generation and transmission to the effects on humans and potential interference with communications, remote-sensing, and radio-astronomy observations.
This has led URSI to organise an open forum for the debate of the radio-science aspects of SPS systems and related technical and environmental issues. The present white paper is intended to draw attention to these aspects of SPS systems. It is not URSI’s intention to advocate solar power satellites as a solution to the world’s increasing energy demands, or to dwell on areas outside of URSI’s scientific domain, such as the whole issue of the space engineering to launch, assemble, and maintain an SPS system in space, the economic justification, and public acceptance. URSI is well aware that if a practical SPS system is feasible, the realisation of such a system is far in the future. Many of the required technologies currently exist, but some of these must be substantially advanced, and others must be created.
Microwave power transmission is an important technology for SPS systems, since its overall efficiency is one of the critical factors that determines the interest in such systems from an economic standpoint. Ideally, almost all energy transmitted from the geostationary orbit should be collected by the rectifying antennas on the ground. In that respect, an overall dc-to-microwave-to-dc power efficiency in excess of 50% is needed (see Section 2.4), which requires the development of suitable microwave power devices. Accurate control of the antenna beam is essential, and measurement and calibration are important issues. Even if these technologies can be successfully developed, there remains the challenging task of combining the outputs of thousands or even millions of elements to form a focused beam. Proper safety measures have to be developed to be certain that the transmitted microwave beam remains within the rectenna’s area. Maintenance of the space systems may be very difficult and expensive in the harsh environment of a geostationary orbit. Ensuring the long-term stability of huge structures in space in the presence of solar radiation pressure and tidal forces is an unsolved problem.
The influence and effects of electromagnetic emissions from an SPS, and, in particular, the microwave power transmission, are radio-science issues that concern URSI. Atmospheric effects on the microwave beam, and linear and non-linear interactions of the microwave beam with the atmosphere, ionosphere, and space plasmas, are among the numerous issues that must be investigated and evaluated. Undesired emissions – such as harmonics, grating lobes, and sidelobes from transmitting antennas and rectennas – must be sufficiently suppressed. This is true not only to avoid wasting power, but also to avoid interference with other radio services and applications and with remote sensing and radio astronomy, in accordance with the provisions of the Radio Regulations of the International Telecommunication Union (ITU). The evaluation of possible effects on human health and the incorporation of appropriate safety measures are essential for legal operation and public acceptance of this power-generation technique.
Finally, this paper identifies specific radio-science issues requiring further studies. It is stressed that only some of these questions can be solved by laboratory work, simulations, and system analysis. Testing of elements of such large systems in space is mandatory before a possible demonstration SPS unit can be considered, and broad international consensus is likely to be required before an SPS demonstration system can be launched.

2. Solar Power Satellite Systems
2.1 The SPS Concept
A solar power satellite is a very large-area satellite in an appropriate orbit (see Section 2.6), which would function as an electric power plant in space. The satellite would consist of three main parts: a solar-energy collector, to convert solar energy into dc electric power; a dc-to-microwave converter; and a large antenna array, to beam the microwave power to the ground. For the production of 1 GW of dc power, the solar collector would need to have an area of 10 km², and would consist of either photovoltaic cells or solar thermal turbines. The dc-to-microwave converter could be realised using either a microwave-tube system or a semiconductor system, or a combination of both. For transmitting the power to the ground, frequency bands around 5.8 GHz or 2.45 GHz have been proposed, which are within the microwave radio windows of the atmosphere. The antenna array to transmit the energy to the ground would require a diameter of about 2 km at 2.45 GHz, and its beam direction would have to be controlled to an accuracy of significantly better than 300 m on the Earth, corresponding to 0.0005°, or less than 2 arc seconds (for a geostationary orbit of the satellite).
In addition to the SPS orbiter, a ground-based power-receiving site has to be constructed, consisting of a device to receive and rectify the microwave power beam, i.e. to convert it back to dc electric power. This device is called a rectenna (rectifying antenna). The dimensions of the rectenna site on the ground depend on the microwave frequency and the size of the transmitting antenna. A model system, operating at 2.45 GHz, would use a rectenna site with a diameter of 4 km and a satellite-based transmitting antenna with a diameter of 2 km (see Section 2.4). The peak microwave power-flux density at the rectenna site would then be 300 W/m², if a Gaussian power profile of the transmitted beam is assumed. The beam-intensity pattern would be nonuniform, with a higher intensity in the centre of the rectenna and a lower intensity at its periphery. For human safety requirements, the maximum-allowable microwave power level has been set to 10 W/m² in most countries, and the SPS power-flux density would be constructed to satisfy this requirement at the periphery of the rectenna. After suitable power conditioning, the electric output of the rectenna would be delivered to the power network.
The combination of an SPS in orbit and the ground-based rectenna will be called an SPS “unit” in the following. On a global scale, a very large number of 1 GW units may be necessary for a practical SPS system. More details about the SPS concept can be found in [1].
For the sake of completeness, it should be mentioned that besides microwave power transmission, laser power transmission has also very recently been suggested [1, Appendix D.9]. In such a scenario, highly concentrated solar radiation would be injected into the laser medium (direct solar pumping) and transmitted to Earth. On the ground, the laser light would be converted to electricity by photovoltaic cells. It is obvious that such a system would be fundamentally different from a “classical” SPS using microwave power transmission: In space, there would be the light-concentration system and the lasers instead of a photovoltaic-cell array and the transmitting antenna; on the ground, there would be a photovoltaic-cell array instead of the rectenna. Since the technological challenges and problems for such laser-based systems have not yet been sufficiently explored, and since many subcomponents are at a low technology-readiness level, this approach will not be treated in this white paper.

2.2 The Aim and Purpose of this White Paper
There are SPS-related issues that are highly controversial. Although several space agencies have pursued SPS studies and research (see the next section), very critical papers have been published that concluded that an SPS is impractical and will never go into operation (e.g., [2]). A more pro-SPS reply to this criticism [3] was based on the economic issues raised in [2]. Among the controversial issues is the question of the space engineering and technology that are necessary for the launch, and the assembly and the maintenance of an SPS system, all of which to a great extent are not yet possible. Other heavily debated issues are related to economic justifications (in comparison with other power sources), are related to the question of whether an SPS can provide a base-load “clean” power system on a global scale, are related to military applications, and are related to public acceptance. All of these issues are beyond URSI’s scientific domain and will therefore not be discussed in this white paper. Social issues of an SPS may perhaps be addressed by the International Council for Science (ICSU).
Instead, this white paper will focus on the radio-science aspects of an SPS. Among the key radio-science technologies involved in the SPS concept are microwave generation and transmission techniques, antennas and beam control, and the very challenging task of protecting other services to the levels required by the International Telecommunication Union (ITU). Of the various scientific organisations or unions concerned with international development and applications of these technology areas, URSI has an important role to play, because it covers most aspects of the above-mentioned radio techniques. The scientific competence of URSI’s ten Commissions (see Appendix 2) encompasses aspects of microwave power generation (Commissions B, C, and D), antennas (Commission B), calibration (Commission A) and transmission (Commissions G and H), the effects of electromagnetic emissions on humans (Commission K), the potential interference with communications (Commission C and E), remote-sensing (Commissions E and F) and radio-astronomy (Commission J) observations, and, to some extent, solar-cell technology (Commission D). Thus, URSI can provide a continuing forum for development, discussion, and debate on technical issues related to SPS systems.
In keeping with what has been said above, it is not the intention of this document to advocate an SPS as a “clean” solution to the world’s increasing power demand (as is argued, for instance, in [1]). However, it is conceivable that an SPS, and, more generally, microwave power transmission, may be used in the future for special purposes. Among such possible scenarios are bringing energy to remote areas on the globe that are difficult to otherwise access, sending energy from spacecraft to spacecraft, or providing energy to the dark side of the moon (in compliance with Recommendation ITU-R RA.479, recognising a shielded zone on the moon). Possible spin-offs from SPS-related research have been considered elsewhere [1, Section 3.6].
A number of the issues related to radio science that are addressed here are also of relevance to the process that the International Telecommunication Union (ITU) has initiated towards an ITU-R Recommendation and/or Report on wireless power transmissions, to be completed by 2010 at the latest [4].
It should be stressed that an SPS is not imminent. Many changes in technology can be expected before an SPS is launched. Major technological problems still have to be solved, even before a demonstration project could be realised. On the other hand, the radio-science aspects of an SPS encompass many interesting scientific, engineering, and technological challenges. To identify, to describe, and to discuss these items is the main aim of this white paper.

2.3 The History of SPS Research
The first concept of an SPS system was proposed by P. Glaser in 1968 [5], after a series of experiments on microwave power transmission [6a, 6b]. Following this article, the United States conducted an extensive feasibility study in 1978-1980. The feasibility study was a joint effort of NASA (National Aeronautics and Space Administration) and the Department of Energy. A reference model was proposed in 1979, known as the NASA/DoE reference model (Figure 1, [7]). Research on an SPS was suspended in the US in 1980, due to high estimated costs. Given a pre-set policy to re-evaluate the SPS concept after an appropriate time interval, the Fresh-Look-SPS concepts were published in 1977 as an improved SPS reference system. This included the “Sun Tower” SPS concept (Figure 1, [8]). This is a constellation of medium-scale, gravity-gradient-stabilised, microwave-transmitting space solar power systems. Each satellite resembles a large Earth-pointing sunflower, in which the face of the flower is the transmitting array, and the “leaves” on the stalk are solar collectors. The Sun Tower is assumed to transmit at 5.8 GHz from either a low Earth orbit or a geostationary orbit, and to operate sun-synchronously at a transmitted microwave power level of about 200 MW. NASA stated that due to its extensive modularity, the low-Earth-orbit concept entails the use of relatively small individual system components, which could be developed at a moderate price, ground-tested in existing facilities, and could be demonstrated in a flight environment during a sub-scale test.
An SPS system using mirrors for sunlight concentration on the solar cells, the Integrated Symmetrical Concentrator, was also proposed. It uses 24 or 36 plane mirrors of 500 m diameter for a concentration factor of two or four (Figure 1, [9]).



Figure 1. An artist’s impressions of various current SPS models: NASA/DoE SPS Reference Model (top left), Sun Tower (NASA, top centre) [8], Integrated Symmetrical Concentrator (top right) [9], JAXA 2003 Free Flyer Model (middle left) [18], Tethered-SPS (USEF, middle right) [19], and Sail Tower (ESA, bottom) [10].

The European Space Agency (ESA) proposed a Sail Tower SPS (Figure 1, [10]), the design of which is similar to that of NASA’s Sun Tower SPS. However, the Sail Tower SPS uses thin-film technology, and an innovative deployment mechanism developed for 150 m × 150 m solar sails. The power generated in the sail modules is transmitted through the central tether to the antenna, where microwaves at 2.45 GHz are generated in mass-produced inexpensive magnetrons. The energy emitted would be 400 MW. In 2003, the Advanced Concepts Team (ACT) of ESA initiated a three-phased, multiyear program related to solar power from space (including laser power-transmission concepts) [11]. In addition, a European Network on Solar Power from Space was established. It provides a forum for all relevant and interested European players in the field of SPS, including industry, academia, and institutions.

Japanese scientists and engineers started their SPS research in the early 1980s. They conducted a series of microwave power-transmission experiments, such as the world’s first rocket experiment with powerful microwave transmission in the ionosphere [12, 13], experiments on the ground [14], computer simulations [15], theoretical investigations [16], and system studies for a demonstration experiment [17]. After a conceptual study phase, two Japanese organisations have recently proposed their own models. JAXA (Japan Aerospace Exploration Agency) proposed an SPS 5.8 GHz/1 GW model (Figure 1, [18]), which is different from the NASA/DoE model. It is based on a formation flight of a rotating mirror system and an integrated panel, composed of a photovoltaic-cell surface on one side and a phased microwave-array antenna on the other side. Formation-flying mirrors are used to eliminate the need for rotary joints. The Institute for Unmanned Space Experiment Free Flyer (USEF) proposed a simpler model (Figure 1, [19]). The USEF model is a tethered SPS, which is composed of an integrated panel similar to JAXA’s, but suspended by multi-tether wires emanating from a bus system above the panel.
The leading group in Japan in basic SPS-related research is based at Kyoto University. Many projects on microwave power transmissions have been conducted, and several important papers have been published (e.g., [12-16]). To a large extent, this white paper is based on an extensive review of SPS issues prepared by an URSI Inter-Commission Working Group [1] under the leadership of the Kyoto group.
International collaboration was established at a Japan-US SPS workshop [20], an International Conference on SPS and Microwave Power Transmission [21], by the International Astronautical Congress (IAC) Space Power Committee, and by an URSI Inter-Commission Working Group.
More details about the different proposed models are available in [1].

2.4 A Coherent Set of Numerical Values
A set of typical numerical values was extracted from the various concepts of SPS mentioned in the previous section. This set forms the basis of the discussion in this white paper.
Assuming that an SPS unit will generate 1 GW effective power on the ground, the characteristic efficiencies are summarised in Table 1. The figures are given for a 2.45-GHz unit; corresponding values for a 5.8-GHz unit are not fundamentally different. Therefore, in order to generate 1 GW at the ground, one needs to collect about 14 GW in space. Since the solar radiation power flux is equal to 1.37 kW/m², one needs a solar-panel area of approximately 10 km². The transmitted RF power is GW. Taking into account the RF collection efficiency of 87%, the RF power received at the ground level is GW. The efficiency of the microwave power transmission (dc-microwave-dc) is the product of the efficiencies given in lines 2-4 of Table 1, i.e. 54%. (Actually, 54.18% was demonstrated and certified in a NASA laboratory test [22]).
Table 1. The efficiencies for SPS processes (for 2.45 GHz).

Quantity	Efficiency	Reference
Solar-power-to-dc-power efficiency	13%	[1, Section 2.4.1.2]
dc-power-to-RF-power efficiency	78%	[1, Section 2.4.1.2]
RF collection efficiency	87%	[1, Section 2.4.1.2]
RF-power-to-dc-power (rectenna)	80%	[1, Section 2.3.6,1] (average of 70% and 90%).
Total efficiency	7%