Version 29.Nov. 2005
DRAFT
URSI White Paper on Solar Power Satellites (SPS)
Solar Power Satellite (Artist’s Conception)
October 2005
Content
I.
II.
Executive Summary
Background of SPS Research an Development
II.1
II.2
II.3
II.4
III.
Solar Power Satellite Systems
III.1
III.2
III.3
III.4
III.5
III.6
III.7
IV.
SPS concept
SPS as CO2-free energy source
Comparison with terrestrial photovoltaics
Economic
Typical parameters for a SPS system
SPS key technologies
SPS research: past and present
SPS Radio Technologies
IV.1
IV.2
IV.3
IV.4
IV.5
V.
Scope of life in the present century
Energy demand in the future
Kyoto Protocol and global warming
Sustainable energy resources
Microwave power transmission
Microwave power devices
Rectennas
Measurements and calibration
Spin-off technologies
Influence and Effects of SPS
V. 1 Interaction with space and atmosphere
V.2 Compatibility with other radio services and applications
V.3 MPT on human health an bio-effects
VI.
Pros and Cons of SPS Systems
Acknowledgements
References
2
I.
EXECUTIVE SUMMARY
Background
Recent increases in population and the efforts to increase the quality of life have put a stress
on global resources and have been accompanied by major environmental problems. A prime
example is global warming caused by the burning of fossil fuels in both the developed and
rapidly developing countries. As an effort toward the solution of this problem, the Kyoto
Protocol was signed in 1998. The protocol came into effect in February of 2005 and requires
most of the developed nations to substantially reduce greenhouse gas emissions. This must
take effect between 2008 and 2012.
The Solar Power Satellite (SPS) concept offers the potential of an electrical power source that
is available 24 hours a day and is essentially free of CO2 emission during its operation phase.
Scientists and engineers in the United States, Japan and Europe have conducted research on
that concept, proposing a range of systems. In a typical SPS system the solar energy is
collected in space by a satellite at a geostationary orbit (lower orbits are possible as well). It
converts it to electrical energy which is transmitted to the ground at Gigahertz frequencies
(microwaves). The emitted energy is typically of the order of 1 GW. In such a system, the
solar energy is converted to direct currents by solar cells, and the direct currents in turn are
used to power microwave generators. The generators feed a highly directive satellite-borne
antenna which beams the energy to the ground. The system would use a phased-array
transmitting antenna with a large number of antenna elements and a diameter of the order of 12 km. On the ground a rectifying antenna (rectenna) converts the microwave energy from the
satellite to direct current, which after suitable processing, is fed to the terrestrial power grid.
Solar Power Satellite radio technologies and URSI
Some key technologies involved in the system will be solar cell technology, microwave
generation and transmission techniques, and antennas. Techniques for interconnection to the
terrestrial power grid will also need development. Of the various scientific organisations or
unions concerned with international development and application of this science, the
International Union for Radio Science (URSI) is the appropriate organisation covering the
techniques involved in the Solar Power Satellite systems. URSI’s ten commissions1 cover the
broad range of aspects involved in SPS systems ranging from technical aspects of microwave
power generation and transmission to the effects on humans and the potential interference with
communications, remote sensing and radio astronomy observations. URSI-affiliated scientists
and engineers will develop the techniques that would contribute to system feasibility. In
addition URSI can provide a forum for debate of issues on SPS systems.
Microwave power transmission is a key technology for SPS systems, since its overall
efficiency and the satellite weight and cost will be critical factors in determining feasibility.
Ideally, almost all energy transmitted from the geostationary orbit should be collected by the
rectifying antennas on the ground. An overall DC to microwave to DC power efficiency in
excess of 50% is needed. The DC to microwave conversion efficiency of the microwave
power transmitter should be approximately 80%, so the development of semiconductor or
tube-based microwave sources is required. On the ground the rectenna array which converts
the microwave energy to direct current must again be highly efficient. Accurate control of
antenna beams is essential, and measurement and calibration are issues well covered by URSI.
3
It is foreseen that techniques developed for this application will be valuable in other
specialized and niche applications.
Influence and effects of Solar Power Satellites
The influence and effects of Microwave Power Transmission and Solar Power Satellites fall
into the research domain of URSI scientists. Atmospheric effects from and on the microwave
beam, and linear and nonlinear interactions of the microwave beam with the ionosphere and
space plasmas are subjects to be investigated and evaluated. Undesired emissions such as
harmonics, grating lobes, and sidelobes from transmitting antennas and rectennas must be
suppressed sufficiently to avoid interference with other radio services and applications, in
accordance with the provisions of the Radio Regulations of the International
Telecommunication Union (ITU). The evaluation of possible effects on human health and
appropriate safety measures are essential for public acceptance of this energy technique.
Concluding remarks
This paper presents the general technique of power generation by means of solar power
satellites. It places the technique in the context of other sustainable energy sources such as
terrestrial solar energy, biomass energy, wind and nuclear energy. Previous research in the
area is reviewed. The required technologies are given including areas where development is
required. A brief discussion of the interaction with space and the atmosphere, on compatibility
with communications, remote sensing and radio astronomy and the effects on human health
are presented. Finally the most important arguments in favour and against an SPS-system are
summarised. The issue is sufficiently promising that further investigation is warranted, with
emphases on scientific, technological, economical, environmental and societal points of view.
4
II. Background of SPS Research and Development
II. 1 Scope of human life in the present century
Improving living conditions often causes environmental problems. The population increase in
this century will accelerate, hence making the problems worse.
Mankind has recently enhanced its living standards and increased its population in an
explosive way. In fact, human population has quadrupled and primary power consumption
increased 16-fold 2 during the 20th century. The consumption of energy, food, and material
resources are predicted to increase 2.5-fold in the coming 50 years. As a result of our efforts to
improve our life, we are being confronted in this 21st century with serious global issues
threatening the safety of our lives and even the existence of the human race. Major threatening
issues are global warming, environmental degradation, and the rapid decrease of the fossil fuel
reservoir. Since both the living standards and the population of developing countries are
increasing continuously, the demand on energy will increase several fold in the next 50 years
compared to the present.
II.2 Energy demand in the future
Greenhouse gas emissions should be reduced
At present, fossil fuels such as oil, coal and natural gas satisfy an important part of our energy
needs. However, fossil fuels have two serious drawbacks which prevent them from being used
as a long term primary power source. Because fossil fuel is a finite resource, it will be
exhausted in a not too distant future if consumption is continued at the present or at an even
higher rate (as predicted). The other drawback is the production of carbon dioxide, a
greenhouse gas, which is assumed to cause global warming. The fossil fuel greenhouse theory
has become more credible as observations accumulate and as we understand better the links
between fossil fuel burning, climate change, and environmental impacts3. Atmospheric CO2
has increased from 275 parts per million (ppm) before the industrial era began to 379 ppm in
March 2004. Some scientists suggest that it will pass 550 ppm sometime during this century.
Climate models and paleo-climatic data indicate that 550 ppm, if sustained, could eventually
produce global warming comparable in magnitude, but opposite in sign, to the global cooling
of the last Ice Age4.
II. 3 Kyoto Protocol and global warming
Most of the developed nations are urged to substantially reduce greenhouse gas emission from
2008 to 2012.
To avert the threat of global warming, many countries came to agreement in Kyoto, Japan, in
December 1997 to assent to the “Kyoto Protocol to the United Nations Framework
Convention on Climate Change (UNFCCC)” 5 . It was opened for signature on March 16,
1998, and closed on March 15, 1999. The Kyoto Protocol came into force on February 16,
2005 after ratification by Russia on November 18, 2004. The US has indicated its intention not
to ratify it. The Kyoto Protocol calls for a reduction by 5% to 8% of the greenhouse gas
emission by the developed nations below the levels of 1990 in the period from 2008 to 2012.
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II. 4 Sustainable and renewable energy resources
CO2-free renewable energy sources will play an important key role, but most of them cannot
be used as a base load power.
To ensure for the well being of future generations, we need to develop science and technology
for a sustainable society. Such science and technology can be called Green Science and
Technology. Methods to stabilise or reduce the emission of carbon dioxide are key elements in
Green Science and Technology and require the development of primary energy sources that do
not emit carbon dioxide to the atmosphere. Such renewable energy technologies include
biomass, solar thermal, photovoltaic, wind, hydropower, ocean thermal, geothermal, and tidal
power generation. However, most of the above suffer from low power densities, and they
cannot provide a continuous base load power supply for human activities, unless suitable
energy storage facilities are developed. Nuclear energy is, in addition to the difficulties in
handling the waste, not generally regarded as a renewable source of energy.
Another renewable power source is Solar Power from Space. Solar Power from Space was
proposed several decades ago as a feasible solution that satisfies the demand of sustainable,
CO2-clean energy and that is usable as a base load power supply.
III.
Solar Power Satellites (SPS)
III.1 SPS concept
SPS collect solar power at a geostationary orbit (GEO) and transmit this power to the ground
by microwaves. This system works continuously.
The concept of the SPS is very simple. It is a gigantic satellite in a GEO designed as an
electric power plant in space. It consists 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. The solar collector can consist of either
photovoltaic cells or solar thermal turbines. The second part of the SPS can be either realised
as a microwave tube system or as a semiconductor system. It may also be a combination of
both. The largest part is an extensive antenna array. The required beam control accuracy is less
than 0.0005 of a degree.
In addition to the SPS orbiter there is a ground power receiving site consisting of a device to
receive and rectify the microwave power beam. This device is called a rectenna (rectifying
antenna). The rectenna system converts the microwave power back to DC electric power that
is delivered to the power network. In some applications (or situations), the electricity obtained
can be converted to another source of energy such as hydrogen.
The SPS system can produce electricity with a higher efficiency per unit ground area than a
photovoltaic system on the ground (see III.3).
III.2 SPS as a CO2-free energy source
The SPS system is an inexhaustible, clean and almost CO2-free energy source for the future.
Presently, there is no comparable project for powerful and clean energy.
The CO2 emission from fossil fuel comes from the burning of fuel, whereas the CO2 emission
in case of SPS originates from the use of energy to build it. An inexhaustible and clean power
6
source is to be developed for sustainable economic growth with a sufficient suppression of
CO2 emissions. Terrestrial photovoltaics, wind, geothermal, and other natural resources
depend on environmental conditions, which makes them less reliable. To overcome this
disadvantage, they require storage technology and/or broad geographical distribution for base
load capability. The most important and serious issues of nuclear power generation are safety
concerns, non-proliferation and radioactive waste.
III.3 Comparison with terrestrial photovoltaics
Compared with a terrestrial photovoltaic system, the SPS has an advantage as a base-load
power source because of its nearly-continuous availability and higher total electric power
output.
One may compare the output power from a space-based solar power system with that from a
terrestrial photovoltaic array with the same area as the SPS rectenna. The SPS system has the
advantage of producing electricity with considerable higher efficiency compared with a
photovoltaic system in a sunny location on the ground. Since SPS is placed in space, e.g. in a
GEO, there is no atmospheric absorption, and therefore the solar input power density is about
30% higher compared to the ground solar power density. In addition it is available
continuously over 24 hours (except for a maximum of 70 minutes during 42 days around the
equinox), and is not affected by weather conditions. Therefore, the long-term average of the
solar power deliverable per unit area on the ground is approximately 2.5 times higher from an
SPS than from the corresponding terrestrial photovoltaic system. The nearly continuous
availability and high power output means that the SPS can be used as a base-load power
source, without the storage units required by terrestrial systems for base load applications.
III.4 Economics
Innovative technologies, especially radio wave technologies, need to be developed and
improved continuously in order to reduce the costs of SPS. In particular microwave power
transmission (MPT) efficiency, which dominates the total SPS costs, needs to be enhanced6.
Furthermore, it should be assessed whether or not cost is the sole criterion for the
development of the SPS.
There are four technological challenges in the standard SPS scenario: photovoltaic module
costs, MPT efficiency, mass per peak kW of the solar modules and the transmission system,
and launch costs. MPT efficiency and space module weight per kW are purely technical
issues. The target is an efficiency of about 50 % for the total MPT DC-microwave-DC
conversion, a launch cost of $15/kg and 1 kg/kW for the space module. The SPS cost estimate
is based on these target assumptions, which lead to an estimated power generation cost of the
SPS of approximately 0.1-0.2 dollar per kWh7. Innovative radio wave technologies have to be
developed because the improvement of MPT is the most effective way to reduce the cost of
SPS. Whether cost is the most important reason to possibly abandon SPS development should
also be discussed. It may be necessary to pay a higher price for the development of a clean
new energy source for the sustainability of our society.
III.5 Typical parameters of an SPS system
The dimensions and efficiencies of a 1 GW power generator at 5.8 GHz for a typical SPS
system8 are presented.
7
The area of the solar cell panel is approximately 10 km2 for the production of 2 GW of DC
power with a solar cell conversion efficiency of 15%. The output of the photovoltaic cell panel
is converted to microwave energy, transmitted to the ground-based rectenna system, and
converted back to DC currents. An overall MPT efficiency from DC (output from the solar
panel) to DC (output from the rectenna system) of approximately 50% is assumed.
There is some freedom in the design of the aperture of the microwave transmitting antenna
array. A typical dimension of this array would be a diameter of 1 km.
The dimension of the rectenna site on the ground depends on the microwave frequency and the
transmitting antenna size. A typical size of the rectenna site would be a diameter of 4 km for a
transmitting antenna of 1 km diameter emitting power at 5.8 GHz. The peak microwave power
density at the rectenna site is then 27 mW/cm2 if a Gaussian power profile of the transmitted
beam is assumed. The beam intensity pattern is non-uniform, with a higher intensity in the
center of the rectenna and a lower intensity at its periphery. It is noted that the human safety
requirement for microwave power is set to 1 mW/cm2 in most countries. The SPS power
density satisfies this requirement at the periphery of the rectenna.
III.6 SPS key technologies
Key technologies for the SPS are: launch and transportation, solar cells, thermal control,
MPT, microwave generators, beam control, rectennas and ground network.
The first key technology is the infrastructure to launch, assemble and transport the SPS
system. First, a reusable launch vehicle is needed for the transportation of heavy materials at
reasonably low cost to a low Earth orbit where assembly work will be conducted. At such an
orbit, cell degradation and debris impact are serious problems that can be mitigated by
constructing the SPS in a short period of time. Then an orbital transfer vehicle is needed to lift
the SPS from the low Earth orbit to the final GEO. These two rocket technologies are essential
for the realization of the SPS system.
The key element in the DC power generation of the system are the solar cells. Thin-membrane
(amorphous) silicon solar cells are expected to be the most suitable type for the SPS system
because they have good performance for a given weight and because of the conservation of
natural resources, although their conversion efficiency is not high. Two types of power
generation systems have been studied: (a) a massive light-concentration type and (b) a super
light-weight thin-membrane type 9 . An increase of the total power conversion efficiency is
greatly desired.
Thermal design and control of the SPS are also of importance. One method of thermal control
of the generator is the blockage of infrared radiation from the sun, either by effective
reflection or by band elimination filters for infrared radiation.
The main parameters of the microwave power transmission (MPT) system from the SPS are
the frequency, the diameter of transmitting antenna, the output power (beamed to Earth), the
maximum power density, and the antenna spacing. In comparison with the NASA reference
system10, the 5.8 GHz system can operate with lower power from each antenna element. In
addition to the system parameters described above, the weight is also of importance.
For the microwave generators, many possibilities have been proposed, such as microwave
vacuum tubes, semiconductor transmitters, and combinations of both technologies. These
types of generators are compared with respect to their efficiency, output power, weight and
emitted harmonics. A DC-to-radio frequency conversion efficiency for microwave vacuum
tubes can be as high as 65 to 75 %. In case of semiconductor transmitters the best achievable
8
efficiency is 40%. Wide bandgap devices such as those using GaN have significant power
outputs, in particular at the microwave frequencies of 2.4 and 5.8 GHz. In MPT technology,
the reduction of the weight per unit of generated power is also of importance to ensure a
reasonable cost performance.
Another important issue of the microwave antenna is the high precision of the control of the
beam direction. Accurate beam control is necessary for several reasons: to maximize the
energy transfer to the Earth and to limit radiation in undesired directions to avoid adverse
effects on existing telecommunication systems. The center of the microwave beam should be
confined to a region within 0.0005 degrees of the center of the rectenna. To meet this stringent
requirement, several ideas have been proposed, such as a retro-directive system which uses a
pilot signal emitted from the rectenna site at Earth. The beam control accuracy of the SPSMPT system can be achieved using a very large number of power transmitting antenna
elements. In order to realize 0.0005 degree beam control accuracy, the SPS-MPT system must
suppress the total phase errors over the antenna array to a few degrees. Technologies
achieving these goals are presently under study 11 . It is noted that the beam collection
efficiency is as important as the beam control accuracy. The beam collection efficiency
depends on the power lost in sidelobes and grating lobes.
The rectenna receives the microwave power from the SPS and converts it to DC electricity. It
is composed of an RF antenna, a low-pass filter and a rectifier. A low pass filter is necessary
to suppress the microwave radiation that is generated by nonlinearities in the rectifier. Most
rectifiers use Schottky diodes. Various rectenna schemes have been proposed. The maximum
conversion efficiencies achieved so far are 91.4% at 2.45 GHz 12 and 82% at 5.8 GHz 13 .
However, it is noted that the actual rectenna efficiency depends on various factors, such as the
microwave input power intensity and the load impedance.
A commercially feasible SPS should produce power of the order of 1 GW and hence would
deliver significant electric power to power grids. No significant problems are expected in the
connection of the SPS power to terrestrial power grids. SPS systems provide predictable and
stable power similar to nuclear power plants or large hydroelectric power plants. Therefore an
SPS system provides CO2-clean “base-load” power.
III.7 SPS research: past and present
The United States, Japan, and Europe have conducted research on SPS, proposing various
models.
The first concept of an SPS was proposed by P. Glaser in 196814 after a series of experiments
on Microwave Power Transmission (MPT) 15 . Following this article, the United States
conducted an extensive feasibility study in 1978-1980. The feasibility study was a joint effort
of NASA and the Department of Energy. They proposed a reference model, known as the
NASA/DOE reference model, in 197916. Research on SPS was suspended in the US in 1980
due to its high estimated costs. Given a pre-set policy to re-evaluate SPS after an appropriate
time interval, in 1997 the Fresh-Look-SPS concepts were published as an improved SPS
reference system. The “Sun Tower” SPS Concept is one of the new proposed concepts17. It 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 an initial orbit of 1000 km and operate sunsynchronous at a transmitted microwave power level of about 200 MW. This LEO-concept,
9
owing to its extensive modularity, entails the use of relatively small individual system
components that can be developed at a moderate price, can be ground-tested in existing
facilities, and can be demonstrated in a flight environment during a sub-scale test. An
Integrated Symmetrical Concentrator has also been proposed18.
Artist’s impressions of various current SPS models: NASA/DOE SPS Reference Model (top left), Sun
Tower (NASA, top center)19, Integrated Symmetrical Concentrator (top right)18, JAXA 2003 Free Flyer
Model (middle left)20, Tethered-SPS (USEF, middle right)21, and Sail Tower (ESA, bottom)22.
Japanese scientists and engineers started their SPS research in the early 1980’s. They
conducted a series of MPT experiments such as the world’s first rocket experiment in the
ionosphere 23 , 24 , experiments on the ground 25 , computer simulations 26 and theoretical
investigations 27 . After a conceptual study phase, two Japanese organizations have recently
proposed their own models. JAXA (Japan Aerospace Exploration Agency) proposed an SPS
5.8 GHz-1GW model11. The JAXA model 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
10
Unmanned Space Experiment Free Flyer (USEF) proposed a simpler model 28 . 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 European Space Agency (ESA) proposed a Sail Tower SPS29. The Sail Tower design is
similar to NASA’s Sun Tower SPS, but uses thin film technology and an innovative
deployment mechanism developed for 150 m x 150 m solar sails. The power generated in the
sail modules is transmitted through the central tether to the antenna, where microwaves of 2.45
GHz are generated in mass-produced inexpensive magnetrons. The energy emitted is 400
MW.
International collaboration was initialised as a Japan-US SPS workshop 30 , an International
Conference on SPS and MPT 31 , by the International Astronautical Congress (IAC) Space
Power Committee, and by the URSI inter-commission working group.
IV.
SPS Radio Technologies
IV.1 Microwave Power Transmission (MPT)
Microwave power transmission is a key technology for an SPS system. Basically almost all the
energy transmitted from GEO could be collected by the rectenna system on the ground. The
overall DC-to-Microwave-to-DC efficiency is expected to be approximately 50%.
Wireless communication uses radio waves as a carrier of information. In the MPT system,
however, radio waves are used as a carrier of energy. The energy-carrying microwaves are in
principle monochromatic waves without any modulation. The MPT uses three or four orders
of magnitude higher power densities than wireless communication systems.
Efficiency is very important for the MPT system. Efficiency includes DC-to-radio frequency
(RF) conversion, RF to DC conversion, and beam collecting efficiencies. Conversion
efficiencies higher than 80% for both RF-DC and DC-RF conversions are necessary to make
the cost of the SPS system reasonable. If the apertures of the transmitter and receiver antennas
are sufficiently large, a beam control efficiency of almost 100% can be achieved.
Power loss during propagation over even tens of thousands of kilometers will be less than 1%.
As mentioned, the aperture of a transmitting antenna array of a 1 GW SPS system has a typical
diameter of 1 to 2 km. The average microwave power density at the array of the SPS will then
be 100 mW/cm2 on the surface of the transmitting antenna. For the SPS system, a phased
antenna array is used in order to obtain a high efficiency beam collection under the condition
of fluctuating SPS attitudes. Depending on the transmitting frequency of the MPT, e.g. 2.45
GHz or 5.8 GHz, the number of antenna elements per square meter is of the order of 100 or
400, respectively, and the total element number is of the order of billions. Such a large phased
array has up to now not been manufactured nor constructed. Hence, there exist many
challenging engineering targets, such as, phased arrays with an RF-DC conversion efficiency
higher than 80%, a phase-shifting system with very low RMS (root mean square) errors for
accurate beam control, phase synchronization over billions of elements, and very low cost
mass production of these elements.
IV.2 Microwave power devices
The microwave devices for the SPS power transmitters will be either semiconductor devices or
microwave tubes. A hybrid system is another solution.
11
High efficiency power transmitters with low harmonics and low-loss phase shifters need to be
developed. Manufacturability should be one of the important topics in the implementation of
particular technologies for the MPT. Since the SPS requires huge investments even in the
electronic parts, availability of particular materials and manufacturability need to be
examined.
From a manufacturing point of view, recent semiconductor technologies could be useful for
SPS. However, their reliability in space needs to be investigated. One of the recently
developed microwave technologies is the “Active Integrated Antenna” technique. It has many
features applicable to SPS such as a thin structure in the power transmitting section.
In contrast to semiconductor technologies, a microwave tube has higher efficiency, lower cost,
and a smaller power-weight ratio (kg/kW) even if one includes the power source, the DC-DC
converter, the cooling system, and all the other elements to drive the system. Some of the SPS
concepts are based on a microwave power transmitter with microwave tubes such as klystrons
and magnetrons. For example, a new concept of a microwave transmitter called a phasecontrolled magnetron, which satisfies both the requirement of high efficiency and beam
controllability, has been developed32. A hybrid tube-semiconductor system is also a possible
solution currently under investigation.
IV.3 Rectennas
The ground segment is composed of the rectenna array and the connection to the power grid.
The efficiency of the rectenna should be high enough to avoid undesired loss. Also higher
harmonics radiated from rectennas should be reduced to avoid interference to existing
communication networks, remote sensing and radio astronomy.
The rectenna array is an important and interesting radio technology. The rectenna is composed
of a rectifier and an antenna for which high efficiency is essential. It is noted that the
efficiency depends on the input power, and the input power density is not constant over the
entire rectenna site for SPS. Therefore, we need to develop rectennas that maintain high
efficiency under various input power conditions. Recently, a low power (only 100 µW or less)
high efficiency rectenna system has been started to be developed for the perimeter of the
rectenna site. Connection of the rectenna output to the existing power grid is another issue of
importance. The method to connect the rectenna elements can be either serial or parallel.
Studies and experiments have also been performed for a hybrid technique33.
IV.4 Measurement and calibration
Measurement and calibration are needed for SPS and MPT because the SPS-MPT system
requires accurate beam control with an extraordinarily large phased array. Development of
new methods of antenna measurement and calibration is mandatory for the SPS.
Measurement and calibration are important issues for SPS and MPT because the SPS-MPT
system requires accurate beam control with a large phased array. Space is a harsh environment
with large temperature gradients, solar wind and ionizing radiation.
Such physical conditions can be simulated in a laboratory environment only on a relatively
limited scale. The testing of large antennas presents not only the usual difficulty of making
accurate RF measurements over a substantial aperture, but also the unusual problems of
devising tests that can accurately predict the performance of the antenna under the harsh
mechanical and thermal conditions in the space environment. Therefore new methods of
measurement and calibration have to be developed. Microwave measurements and calibrations
12
are necessary for the evaluation of power, interference, and spurious emissions from the SPS
and rectennas.
The antenna is expected to be so large that it cannot be tested in its entirety on the ground.
Computer simulations can give close predictions of the antenna performance in terms of gain,
beam-width and near sidelobes. However, the antenna can only be accurately tested once in
orbit. Antenna measurement and calibration techniques have to be developed before the SPS
can be realised.
IV.5 Spin-off technologies
MPT technologies are essential not only for the SPS, but also for novel terrestrial
applications. Wireless power transmission has advantages compared to conventional power
line transmission for several applications.
MPT technologies are not only applicable to the SPS, but also to terrestrial applications. There
are some advantages of wireless power transmission over conventional power transmission
using stationary conducting lines. One application is MPT for moving entities, e.g. a fuel-free
airplane, a fuel-free electric ground vehicle, a moving robot in a limited area. One cannot
apply conventional power line transmission for the power supply to the moving object.
Various experiments were carried out34,35,36.
Another application of MPT is ground-to-ground power transmission to a distant place where
wired power distribution networks are either unavailable or very poorly distributed. The merits
of the ground-to-ground MPT are a quick installation and an easy disassembly because there
are no lines between the transmitter and the receivers. Emergency power supply to disaster
affected areas could be easily provided with MPT.
The most recently proposed MPT application is the “Ubiquitous Power Source” or “Wireless
Power Source”. In a “Ubiquitous Power Space”, where power is fed via microwaves, one can
extract electric power from weak microwaves anywhere and at any time. Laboratory
experiments have already been carried out in a shielded room37. The concept of the Ubiquitous
Power Source uses microwaves in a very similar way as communication systems.
V.
Influence and Effects of SPS
V.1 Interaction with space and atmosphere
Atmospheric effects by and to the microwave beam, as well as linear and nonlinear
interactions with the ionosphere and space plasmas should be evaluated theoretically,
experimentally and by computer simulations. Another issue is the interaction of ion beams
ejected from electric propulsion for SPS construction to the space environment.
Very few groups have worked on the effects of microwaves on the atmosphere. Studies
presently available refer to potential effects via the heating of ionospheric electrons or via
ionization of the air. The expertise is limited, but it exists. However, at a time where the
observation of transient luminous events (sprites, blue jets, elves, etc.) in the upper atmosphere
pose basic questions on the electrical processes in the Earth environment, new studies are
needed on all phenomena that may influence the atmospheric electrical conductivity and thus
the global electric circuit.
13
Test microwave injections from a sounding rocket have been carried out in Japan 19. Although
Ohmic heating effects were not observed, plasma waves were excited by the ejected
microwaves. There have been several theoretical predictions that microwaves at high power
may produce plasma instabilities in the ionosphere. The SPS microwave power density may be
high enough to cause such effects.
Some effects of powerful microwaves on the atmosphere have been studied both theoretically
and experimentally38. In particular, theoretical and experimental studies have been carried out
to study the effects of ozone-destroying pollutants in the troposphere and to create an artificial
ozone layer in the stratosphere. The ideas involve artificial ionisation of the air by high power
electromagnetic waves. The field strength and intensity necessary for this are much higher
than the values that will be used by the SPS. Therefore, such effects on the atmosphere are not
expected.
In the process of the SPS construction, an enormous amount of materials will have to be
transferred from low Earth orbit to the GEO by electric propulsion, in which accelerated ions
are ejected from ion engines. The interaction of the heavy ions with the surrounding plasma
could change the electromagnetic environment of the ionosphere/magnetosphere 39 . A
quantitative evaluation of these plasma processes is needed.
V.2 Compatibility with other radio services and applications
Undesired emissions of the MPT beams, such as grating lobes, sidelobes, and spurious and
out-of-band emissions, must be suppressed sufficiently to avoid interference with other radio
services and applications, in accordance with the provisions of the ITU-R Radio Regulations.
Most SPS systems are assumed to use frequency bands around 2.5 GHz or 5.8 GHz, which are
allocated in the ITU-R Radio Regulations to a number of radio services and are also
designated for ISM (Industry, Science and Medical) applications. Undesired emissions40, such
as carrier noise, harmonics, and spurious and out-of-band emissions of the MPT beams must
be suppressed sufficiently to avoid interference to other radio services and applications, in
accordance with the regulatory provisions of the ITU-R Radio Regulations. The bandwidth of
SPS emissions is quite narrow, as an essentially monochromatic wave without modulation will
be used.
In addition all possible measures should be taken to avoid contamination and disturbance of
radio astronomical and passive Earth remote sensing measurements. Even if received
indirectly from reflections, harmonics, spurious and out-of-band emissions generated by MPT
beams of the SPS could degrade substantially the performance of these systems.
V.3 MPT on human health and bio-effects
The evaluation of possible effects of MPT on human health has been extensively studied and
its safety measures are essential for the public acceptance of SPS systems.
A variety of environmental considerations and safety-related factors should continue to
receive consideration because of public concerns about radio wave exposure 41. The power
density is projected to be 1 mW/cm2 at the perimeter of the rectenna site. Beyond the
perimeter of the rectenna, the potential exposure would be well below the currently
permissible level to the general public42.
Above the rectenna, where the power density may be as high as 25 mW/cm 2, research has
shown that some birds exhibit evidence of detection of the microwave radiation. This suggests
that migratory birds, flying above the rectenna, might suffer disruption in their flying paths.
14
To assure environmental safety and health, the proposed limit for the center of beam power for
microwave transmission should not be exceeded, should be under controlled conditions, and
should be monitored continuously by tightly tuned phased array techniques and by automatic
beam focusing.
VI.
Pros and Cons of SPS Systems
The most important arguments in favour and against SPS systems are summarised below.
Pros of SPS

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SPS is one of the cleanest base load powers, which does not emit CO2, and so may be
considered as a possible substitute for fossil fuel and as a possible remedy for global
warming.
Among the sustainable energy sources (wind energy, solar power, geo-thermal power
sources, etc.) SPS is one of the few which is available continuously 24 hours a day at a
high level.
The amount of energy transmitted from space to Earth in potential SPS systems is too
weak (five orders of magnitude less than the total input of the solar radiation reaching
the Earth surface) to contribute to the global warming.
Except perhaps at the SPS beam centre, the exposure level of the microwave density at
the perimeter of the SPS receiving rectenna can be made less than the safety level fixed
by international norms.
The operations can be made safe by a precise control of the high power beam achieved
with a pilot signal from the Earth.
Considerable spin-off can be expected from SPS-related research and development.
SPS technology would create an entirely new satellite construction and launch
industry.
Cons of SPS

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There are still uncertainties on the magnitude of the energy crisis and on the
cost/benefit analysis, compared to other sources of power, to undertake such a huge
investment.
There are still uncertainties on potential effects of the SPS power beam on the
environment (magnetosphere, ionosphere, atmosphere, etc.).
Too much energy (and CO2 production) would be required to build, launch and
transport the huge number of SPS which would be required to satisfy the worldwide
energy demand. In order to cover 20% of the projected demand about one thousand 1
GW SPS systems are needed.
Congestion at the GEO and interference with communication satellites are noteworthy
concerns.
To place SPS into orbit is a complex operation ( (i) launch of subsystems to a low
Earth orbit, if possible by a reusable launch vehicle, (ii) assembling and checking of
15
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the subsystems, then (iii) transportation of the full system to a GEO, if possible by an
electric propulsion orbital transfer vehicle), which would require long term and costly
research and development activities.
SPS could be turned into a weapon.
Space debris might damage an SPS, and could also be generated by an SPS.
SPS electromagnetic emissions at the microwave frequency and at other frequencies
(harmonics of the microwave frequency, unexpected and harmful radiation resulting
from malfunctions) may impact telecommunications and remote sensing systems as
well as radio astronomy studies. Constraints imposed by the relevant International
Telecommunication Unions (ITU) regulations might result in more costly systems.
Around the rectenna, disturbances and security problems may arise with : (i) biological
systems: human beings, flying birds, insects and plants, etc. (ii) flying vehicles such as
airplanes, (iii) other electric/electronic equipment and telecommunication networks.
According to present knowledge, elements of answers to the cons may be provided. They rely
mainly on:
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preliminary technical studies (see previous sections) and development of innovative
technologies, especially radio wave technologies,
SPS designs strictly defined and controlled,
the respect of international norms for the power density of the microwave beam,
the respect of the International Telecommunication Union (ITU) radio regulations,
the use of reusable launch vehicles and of an electric propulsion orbital transfer
vehicle,
a one year period to recover the energy spent for construction.
SPS is in the situation of any new technology which has to face the “Precautionary principle”.
Further studies are needed: (i) to demonstrate the technical feasibility, (ii) to better identify the
environmental and economical risks, (iii) to define key parameters to monitor at each
development phase.
Acknowledgements
This White Paper is accompanied by an extensive and detailed report on SPS prepared by the
URSI inter-Commission Working Group on SPS (SPSICWG) called “Supporting Document
of the URSI White Paper on Solar Power Satellite Systems”. The URSI Board of Officers is
indebted to the members of this Working Group:
Hiroshi Matsumoto (Japan), Kyoto University, Kyoto, Japan, (Chair)
Andrew C. Marvin (UK), The University of York, York, UK, (Co-Chair Commission A)
Yahya Rahmat-Samii (USA), University of California, Los Angeles, California, USA, (CoChair Commission B)
16
Takashi Ohira (Japan), ATR Adaptive Communications Research Laboratories, Kyoto, Japan,
(Co-Chair Commission C)
Tatsuo Itoh (USA), University of California, Los Angeles, California, USA, (Co-Chair
Commission D)
Zen Kawasaki (Japan), Osaka University, Osaka, Japan, (Co-Chair Commission E)
Steven C. Reising (USA), Colorado State University, Fort Collins, Colorado, USA, (Co-Chair
Commission F)
Michael T. Rietveld (Germany), EISCAT, Ramfjordbotn, Norway, (Co-Chair Commission G)
Kozo Hashimoto (Japan), Kyoto University, Kyoto, Japan, (Co-Chair Commission H, and
Secterary of SPSICWG)
Michael M. Davis (USA), SETI Institute, California, USA, (Co-Chair Commission J)
James Lin (USA), University of Illinois at Chicago, Chicago, Illinois, USA, (Co-Chair
Commission K)
Furthermore the contributions from the following colleagues are gratefully acknowledged: Q.
Balzano, P. Degauque, R.M. Dickinson, D. Emerson, K. Hughes, M. Inoue, H. Kawasaki, S.
Kawasaki, G. Maeda, J. Mankins, T. Mitani, M. Mori, H. Nagayama, M. Oda, M. Ohishi, Y.
Omura, D. Preble, Y. Saito, S. Sasaki, R. Schillizzi, N. Shinohara, L. Summerer, N. Suzuki,
T. Takano, M. Taki, K. Tanaka, A.R. Thompson, M. Utashima, W. van Driel.
17
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