CHAPTER 1

INTRODUCTION

A solar power satellite, or SPS or Powersat, as originally proposed would be a

satellite built in high Earth orbit that uses microwave power transmission to

beam solar power to a very large antenna on Earth. Advantages of placing the

solar collectors in space include the unobstructed view of the Sun, unaffected

by the day/night cycle, weather, or seasons. It is a renewable energy source,

zero emission after putting the solar cells in orbit, and only generates waste as a

product of manufacture and maintenance. However, the costs of construction

are very high, and SPS will not be able to compete with conventional sources

(at current energy prices) unless at least one of the following conditions is met:

Sufficiently low launch costs can be achieved

A determination (by governments, industry ...) is made that the

disadvantages of fossil fuel use are so large they must be substantially

replaced.

Conventional energy costs increase sufficiently to provoke serious

search for alternative energy

Fig. 1.1 SPSFig. 1.1 SPS

1

CHAPTER 2

HISTORY

The SPS concept was first described in November 1968. At first it was

regarded as impractical due to the lack of a workable method of sending power

collected down to the Earth's surface. This changed in 1973 when Peter Glaser

was granted U.S. patent number 3,781,647 for his method of transmitting

power over long distances (eg, from an SPS to the Earth's surface) using

microwaves from a very large (up to one square kilometer) antenna on the

satellite to a much larger one on the ground, which came to be known as a

rectenna.

Glaser then worked at Arthur D. Little, Inc., as a vice-president. NASA became

interested and signed a contract with ADL to lead four other companies in a

broader study in 1974. They found that, while the concept had several major

problems -- chiefly the expense of putting the required materials in orbit and

the lack of experience on projects of this scale in space, it showed enough

promise to merit further investigation and research.

During the period from 1978 - 1981 the US Congress authorized DOE and

NASA to jointly investigate. They organized the Satellite Power System

Concept Development and Evaluation Program. The study remains the most

extensive performed to date. Several reports were published addressing various

issues, together investigating most of the possible problems with such an

engineering project.

Fig. 2.1 First Model of SPS

2

CHAPTER 3

DEVELOPMENTS IN SOLAR POWER SATELLITE

There have been a number of important changes in the external context for

consideration of space solar power during the past 15-20 years. The most

important is the increasing demand for energy globally and the resulting

increasing concern regarding carbon combustion, CO2 emissions and global

climate change, discussed below. As a result, there is a major priority being

place on the development of renewable energy sources.

Another important change has occurred at the US national policy level. US

National Space Policy now calls for NASA to make significant investments in

technology (not a particular vehicle) to drive the costs of ETO transportation

down dramatically. This is, of course, an absolute requirement of space solar

power. This policy is, of course, independent of any SSP-related considerations

and thus need not be "charged" against the cost of developing SSPtechnology.

Also, a variety of other key technical advances have been made involving

many key technological areas and diverse new systems concepts. Although

systems-level validation of key technologies, such as power conversion and

large-scale wireless power transmission (WPT) have not occurred, component-

level progress has been great.

There are fundamentally new opportunities for partnerships compared to the

environment of 20 years ago. Strong opportunities exist now for international

teaming and resultant support. Recently, SSP activities have occurred in Japan,

Canada, Europe, and Russia. For example, the Japanese have conducted a wide

variety of experiments, studies and technological research related to space solar

power during the past 10 years, including a particular SSP study entitled: "SPS

2000".

3

Finally, there is a new paradigm for the relationship between governments and

industries, for example with NASA's role in research and development to

reduce risk and to seek government mission applications -but not to actually

develop operational systems.

As a result of these and other factors, in 1995 NASA's Advanced Concepts

Office determined that the time was appropriate to revisit the subject of space

solar power.

4

CHAPTER 4

FUTURE ENERGY NEEDS

In the richer countries, electricity supply is of the order of 1 KW per person,

though some countries use substantially more. If this is taken as a target for

electricity supplies, China alone will need more than 1000 GW - some 10 times

its current generation capacity. A world population of 10 billion people, which

is expected to be reached by the middle of the 21st century, will need some

10,000 GW of generating capacity. Thus, depending on the plant lifetime, up to

several times this amount will need to be installed during the next century.

Consequently, in order for most of the world population to have a reasonable

standard of living, energy sources will be required that are capable of

expansion at an average rate of more than 100 GW per year through the next

century.

With normal economic growth, the actual rate will grow through the century by

an order ofmagnitude from less than this figure to several times more, but 100

GW per year is representative of the rate of construction required to solve

humans' energy problems.

In recent years the ever-increasing scale of human industrial activities has

started to threaten the global environment. Consequently the quality of life on

Earth in the future will increasingly depend on using energy sources that are

more environmentally benign than those used during the development of the

older industrialized nations. Some researchers claim that a "fully industrialized

world" is unachievable, based on the view that the energy supplies that would

be necessary, the rate of raw material utilization that would be involved, and

the accompanying environmental damage would not be sustainable. In a similar

vein, long-term projections of world energy supply have been published that

5

foresee average energy utilization per person in the poorer countries 100 years

from now which is only a fraction of that in the rich countries.

However, these projections do not seem acceptable as images of the future for

poor countries, and particularly not for those growing rapidly. Now that it has

been shown that sustained rapid progress is possible it seems likely that such

growth will become increasingly common around the world. A major benefit of

this is that economic development and rising living standards are the most

effective means of reducing population growth. It can also be argued that a

major cause of the economic growth in the advanced countries during the

period after WW2 was the low and falling price of oil during the 1950s and

1960s. For these reasons the development of environmentally benign energy

supplies that are capable of rapid expansion to a very large scale is of

enormous importance. It will also be of enormous commercial value.

Construction of 100 GW of electricity generating capacity per year at some 200

\ Watt will represent a market of some 20 trillion \ per year. Revenues from this

capacity will grow at several trillion \ per year, and there will also be a growing

maintenance and refurbishment market.

Of course such rapid expansion of energy supplies will require capital

investment on a very large scale. And in their high rate of personal and

corporate saving the fast-growing Asian countries also seem to be a better

model for developing countries than the rich countries of Europe and America.

Although average incomes in the latter are much higher than in Asia, the

proportion of income that is consumed is also much higher, and savings are

correspondingly lower.

In order to power the expected economic growth, humans will need all of the

energy sources to which they have access; the use of coal, oil, gas, wood fuel

6

and nuclear energy will continue for many years to come.However, for reasons

that are well known, it is also clear that none of these sources is capable of

providing sustainable energy supplies on the scale required to provide all

humans with an acceptable standard of living in an environmentally benign

manner.

Fig. 4.1 Solar PannelsFig. 4.1 Solar Pannels

7

CHAPTER 5

SOLAR POWER SATELLITES

Microwave energy transmitted from space to Earth apparently has the potential

to provide environmentally clean electric power on a very large scale, and with

the potential for very rapid growth. In the 1970s the US Department of Energy  

considered a system of solar power satellites (SPS) of 300 GW capacity,

suitable for supplying the USA. This work was very valuable in clarifying both

the potential of SPS, and the research that needs to be done. However, a system

of 300 GW is far too small from the point of view of world economic

development, since it represents no more than 3% of the 10,000 GW of

capacity that is needed in the next century.

During the 1980s most SPS research was performed in Japan. Currently the

major project is the "SPS 2000" project to demonstrate the actual transmission

of 10 MW of power from space to Earth using near-term technology. SPS

2000 satellite configuration. In order to be demonstrated in the near future, the

satellite will operate in 1100 km altitude low-Earth equatorial orbit, from

where it will deliver power to receiving antennas (rectennas) within a few

degrees of the equator.

In the following the feasibility of solar power satellites is not considered; it is

assumed that transmission of electric power to Earth from space is in principle

an attractive energy source, capable of supplying continuous electric power to

Earth in an environmentally benign way, on an effectively unlimited scale. The

following is a brief review of some of the implications of usingSPS to increase

electricity output capacity on Earth by 100 GW per year.

8

It has been suggested that if humans import such large amounts of power, they

will alter the energy balance of the Earth, and in particular will add to "global

warming". However, any such effect would be small compared to the heating

effect of adding carbon dioxide to the atmosphere. The solar energy intercepted

by the Earth is some 180 million GW, of which only approximately half, or

some 100 million GW, is absorbed, due to the reflection of sunlight from the

Earth. Humans' total electricity production today is of the order of 1000 GW,

which is therefore some 0.001% of the solar energy absorbed by the Earth. If

this increases by a factor of 10, it will still be only of the order of 0.01% of the

Earth's insolation, which is too small to have a significant global warming

effect. It is also notable that because of the high efficiency of rectennas (some

90%), the heat added to the environment by SPS is less than half that created

by even the most efficient thermal power stations.

9

CHAPTER6

MACRO ELECTRONICS

Many designs of SPS are still competing, but for simplicity we follow the

design of the SPS 2000 satellite of the SPS Working Group. This uses

amorphous silicon photovoltaic cells for electricity generation, and 2.45 GHz

microwave power transmission to Earth, using solid-state microwave

generating modules. We assume average solar cell efficiency of approximately

10%, which is higher than that currently available, on the grounds that the

invention of multi-band-gap solar cells renders the probability of achieving

efficiencies even higher than this within a few decades reasonably high. On this

assumption ten square meters in orbit will produce 1.4 kW of electricity, and so

ten square kilometers will produce 1.4 GW in orbit. We assume that the

microwave power transmission and reception system has an overall efficiency

of 50%. Thus 10 square kilometers in orbit will produce 0.7 GW at

the rectenna on Earth, and so 100 GW on Earth will require some 1400 square

kilometers of solar arrays in orbit.

Production of SPS electronic components on such a scale might be called

"Macro-Electronics". Being semi-conductor technology, it would be more

readily automated than the manufacture of traditional thermal power stations. If

the specific mass of SPSs is approximately 12 tons / MW, which is the target

for SPS 2000, annual production of satellite parts for 100 GW output will have

a mass of 1,200,000 tons. This is only a few percent of the mass handled by the

automobile industry, and so is clearly not in itself difficult to achieve. The

design of the factories, materials supply and transportation systems needed to

10

achieve such rates of macro-electronic production would be an interesting

exercise.

CHAPTER 7

SPS STRUCTURE

The structural components of solar power satellites will be simpler than the

electronic components, but very large numbers of units will be required,

sufficient to support 1400 square kilometers of solar panels per year.

Production and assembly of these will comprise many repetitive operations,

and so will be very suitable for robotic construction systems. Thus, if SPS can

supply power at competitive prices, construction will provide engineering

companies with demand for light-weight structures on the scale of WW2

aircraft manufacturing, when, for the only time, tens of thousands of aircraft

were produced per year. Another relevant precedent is the post-WW2

experience of production of the "Liberty ships" built rapidly in the USA to

carry "Marshall Aid" to Europe. Using a new welded design the first ship

reputedly took some two years to build, while the 200th ship took less than two

weeks. Compared to these two examples, SPS production will be simpler,

involving much larger numbers of fewer different components, and will

therefore have the potential to reach cost levels little above the cost of the

materials used.

Today perhaps only car manufacturers and electronic consumer-product

makers have experience of such large scale manufacturing. SPS units will

therefore probably be built by consortia of these companies and construction

companies, who have experience of managing the production of such large and

spatially complex structures. Modern-day aerospace companies, with their

11

tradition of small production runs of high-cost, hand-built products will face

fierce competition if they are to have a share of this new commercial business.

Fig. 7.1 Working of SPS

12

CHAPTER 8

PRACTICAL APPROACHES TO SPACE SOLAR POWER

RESEARCH

In addition to having realistic targets, space solar power research should be

practical, not theoretical. Assuming that we were to prepare a business plan for

space solar power research, I would like to emphasize familiarity as the most

important feature to be required.

The first is familiar technology available from the industries concerned. It will

be most favorable for industry to participate in the project, but even otherwise,

the necessary technology should be available on a commercial basis. Space

technology is not attractive to commercial industries, because it is too

expensive and its markets are too small.

The second is familiar size of activities which may be represented by ordinary

research and development projects of industry and governments. Research on a

new energy system is the first opportunity for future customers and developers

to be familiarized with it. It is important to make it attractive and beneficial to

potential partners even if some risks are involved.

The third is the promise of low cost space transportation. Space is unknown

territory for ordinary people. Even highly educated people believe that space is

only for adventurers and not for business. The existing commercial launch

business is desperate in this respect, but fortunately development

of SSTO (single-stage-to-orbit) vehicles has been started in the United States,

and transportation cost is expected to be reduced to an acceptable level.

13

8.1. SPS 2000 study

The SPS 2000 study is a design study conducted by researchers of the Solar

Power Satellite Working Group led by the Institute of Space and

Astronautically Science of Japan. The main purpose of the study was to

identify study subjects in each special field through developing a straw man

concept of SPS. The study was intended to familiarize them with solar power

satellites, and the concept was intended to be practical in economy and realistic

in technology. The two substantial features of the economic aspect at the earlier

stage are a lower power level and a shorter transmission distance. A lower

power level is practical for reducing the risk of marketing the produced power.

A shorter transmission distance means firstly reduction in the size of antennas,

especially of the microwave transmission antenna onboard the space power

station, and secondly lower altitude of orbit for the solar power station to be

transported by rockets, and to be constructed. In the study of SPS 2000, the

solar power station was assumed to be built at an altitude of 1100 km above the

equator. Different from the Reference System, it moves eastwards with a

period of about 90 mm. Accordingly, the users of this power system have to get

the microwaves beamed to their receiving facility during the power station

passes over the rectenna. The microwave power is only 10 MW. More detail on

the concept of SPS 2000 was reported by Nagatomoetal (1994).

14

Fig 8.1 SPS 2000 

The SPS 2000 study was conducted on a voluntary basis with support by the

organizations to which researchers belong. After the conceptual study ended

with submission of the final report on a preliminary design of a proposed

system, the researchers are continuing experimental studies on individual

technical fields. Some of the results have been applied for assembling a

demonstration model displayed at ISAS in Sagamihara in July 1994 and later at

the exhibition on energies for today and tomorrow held from December 1994 at

the Stella Matutina Museum in Reunion, France.

An interesting feature of the SPS 2000 study is that because this study has not

been limited to the initial framework of study, that is to design a straw man

solar power satellite, it is escalating to research on the rectenna sites. A

1994FY Ministry of Education, Science and Culture Research Grant was

awarded for these studies.

8.2 The key issue of ground receiving station: Economy

Among many studies on SPS systems, there are relatively few papers on

subjects relating to rectennas. Most of the papers discussed the ground

receiving station as a part of the total system in terms of radio power

transmission, or focused on electric technology to be used for microwave

power rectification. In the case of the U.S. CDEP, the SPS was assumed to be

the U.S. national electric power system. The study on the rectenna was

technical concerning processing the high power microwaves from space for

utility power with the same standard of large power plants presently operated

in industrially developed countries. Accordingly, little attention was paid to the

15

commercial aspect of the rectennas which were assumed to be given the huge

market of electric power together with nation-wide grid networks.

The study for CDEP is not applicable to rectennas of the SPS 2000 concept,

whose power level is much smaller and premature in its quality of electricity.

The microwave power available for reception by a rectenna at one time is only

10 MW for about 5 mm, or averaged power of 300 kW if storage is provided.

By the standards of industrialized nations, this small and intermittent power

supply would seem to limit the practicability of this system as an electric

power system. However, from the standpoint of solar energy development, for

example, solar photovoltaic cells, 300 kW will be large enough to be designed

as a regional electric power system. Even several-kilowatt power systems are

being tested and verified for practical use of electricity. It should be also noted

that the first commercial power station built by Edison in the last century had a

similar power generation capacity.

Current research on the rectenna has revealed a new area of study demand

for SPS in developing countries. Energy in most such countries depends on

wood and solar heat energy, which can be supplied by even such a

small SPS as SPS 2000. In many places in Asia, small electric machines can

free women and children from labor carrying water for their living. There is a

demand, but we are still concerned about the practicality of SPS from the users'

viewpoint, for example, construction and maintenance.

The construction and maintenance work required for the rectenna and relating

facilities have to be compatible with workers' skill at the sites. No special high

technology is used for critical parts of the rectenna and related equipment. All

the system should be planned and managed locally and autonomously, as is

done for ordinary power system installations. One of the technical outputs of

the SPS 2000 study is a hand-built rectenna unit. The total cost for the

16

materials and electric parts is less than twenty Japanese Yen (about twenty U.S.

cents), which is mainly for the diode.

CHAPTER 9

ADVANTAGES

The SBSP concept is attractive because space has several major advantages

over the Earth's surface for the collection of solar power.

There is no air in space, so the collecting surfaces could receive much

more intense sunlight, unobstructed by weather.

A satellite could be illuminated over 99% of the time, and be in Earth's

shadow on only 75 minutes per night at the spring and fall equinoxes.

Relatively quick redirecting of power directly to areas that need it most.

Higher collection rate: In space, transmission of solar energy is

unaffected by the filtering effects of atmospheric gasses. Consequently,

collection in orbit is approximately 144% of the maximum attainable on

Earth's surface.

Longer collection period: Orbiting satellites can be exposed to a

consistently high degree of solar radiation, generally for 24 hours per

day, whereas surface panels can collect for 12 hours per day at most.

Elimination of weather concerns, since the collecting satellite would

reside well outside of any atmospheric gasses, cloud cover, wind, and

other weather events.

Elimination of plant and wildlife interference.

Redirectable power transmission: A collecting satellite could possibly

direct power on demand to different surface locations based on

geographical baseload or peak load power needs.

17

CHAPTER 10

DISADVANTAGES

The SBSP concept also has a number of problems.

The space environment is hostile; panels suffer about 10 times the

degradation they would on Earth. System lifetimes on the order of a

decade would be expected, which makes it difficult to produce enough

power to be economical.

Space debris are a major hazard to large objects in space, and all large

structures such as SBSP systems have been mentioned as a potential

sources of orbital debris.

The broadcast frequency of the microwave downlink (if used) would

require isolating the SBSP systems away from other satellites. GEO

space is already well used and it is considered unlikely the ITU would

allow an SPS to be launched.

18

Fig. 10.1 Receiver of SPS

CHAPTER 11

THE LUNAR POWER SYSTEM (LPS)

In the Lunar Power System approach, collectors are located on the surface of

the moon. The power is beamed to the earth as with the SPS. Mirrors in orbit

about the moon would direct sunlight to the collectors during the lunar night

and microwave reflectors in earth orbit would serve those ground stations that

are not in sight of the moon at any given time. Again, the economics look

promising.

19

CHAPTER 12

IMPORTANCE OF SPS

The reason for governments to do this nuclear research, even though it's

extremely expensive - and unpopular is because they see no alternative. They

foresee fossil fuels running out in a few decades, and no other potentially large-

scale sources of electricity.

The reason for not doing similar research on SPS is because SPS doesn't have

enough credibility - because no one will say what an SPS is, other than a kind

of dream for 50 years from now. Such small-scale, expensive experiments have

nothing to do with electricity supply.

However, if there was a realistic plan to build and test an SPS, which would

deliver useful solar-generated electricity from space to Earth, within a

reasonable budget and a reasonable time-scale, there must be a good chance

that it would be funded. But there is no such plan. In nearly 30 years the

"SPS community" has never produced such a plan! There are visions of

enormous systems far in the future, and plans for little bits of research here and

there, but nothing in between. (Please remember: a vague proposal isn't a plan.

In order for a project to be implemented it's necessary to produce a detailed

engineering plan.)

20

CHAPTER 13

REALITY OF NEW ENERGY DEVELOPMENT

Economy and environment are the key words of the new energy situation. As

well known by the global warming issue, recent energy development is

concerned not only with energy to support economic growth but also with

environment protection. In the New Earth 21 Action Program proposed by the

Japanese government to the international community, space solar power

generation is categorized together with nuclear fusion as a candidate for

development as a future energy technology until 2040. It is also stated that

"The lasting solution to global warming thus requires an undertaking within a

framework of international cooperation, not only among the developed

countries but also developing countries (Hashimoto, 1991)".

Although it is not clear if the general scheme of the New Earth 21 Action

Program  was accepted by other governments, the view is encouraging to space

solar power research, and suggests that the general plan of research for space

solar power should be prepared on a realistic basis of current energy

developments rather than as a theoretical option for future space programs.

From this viewpoint, I summarize two aspects of realism required for planning

space solar power research and development as follows.

13.1 Sustainable energy

To meet the final goal of providing sustainable energy for future growth and

protection of the environment, the design and technology for space solar power

21

should be evaluated by the criteria of availability of resources, energy economy

(payback time) and waste production such as carbon-dioxide through all the

processes required for production of SPS. Power from space should be

competitive with other energy sources in this respect.

For example, we should consider long-term factors of solar cells to be used

for SPS Solar cells are still a big target of sustainable energy for which industry

is undertaking research and development to increase the various performances

and production. It should be noted that the present annual production in the

world is several 10 MW, and only a fraction of 5 GW would be the maximum

capacity of gallium arsenide (Ga-As) solar cells produced by all resources

available on the earth. Thus, the Ga-As Reference System of 60 5 GW solar

power stations for the United States early next century is found to be based on

unrealistic assumptions.

Generally production costs broadly reflect energy consumption for industrial

products, and terrestrial use solar cells are now reaching the level to compete

with other energy systems on this factor. The energy payback time is defined as

the time required for a power system to recover the total energy used for its

production. In the case of terrestrial solar panels, the payback time including

the supporting structure is estimated to be less than 30 yr. These efforts for

terrestrial solar cells are acknowledged as a firm basis to support efforts for

space solar power, because the most important advantage of space solar power

over terrestrial solar power is in the fact that for a similar solar cell panel

approximately one order of magnitude more solar energy is available in space

than on the earth. Therefore, if solar cells are used for SPS, SPS will be

designed as a variation of a solar power station on earth in terms of solar cell

technology.

13.2 Economy

22

Economy is the fundamental motivation for people to accept a new system in

society. When a new system like a space solar power system is introduced in

society, finance is the key factor. In the past the space solar power system was

a power system which was a large collection of electric power systems to be

deployed over a huge territory from the earth to space. As a result, the project

was predicted to be so big that it was unrealistic for interested parties, except

governments, to invest in it.

The concept of microwave "fuel" (Collins, 1991) is unique, as it broke down

the traditional SPS into the relation of suppliers and buyers of microwave

"fuel". Using this concept, space solar power stations sell microwave power to

any unspecified rectenna, so that commercial relations can be established

between them. This concept not only gives a new assessment method of this

new power system, but it is also expected to establish definite goals for

technology development for orbital power stations and for ground power

receiving stations independently.

Transportation to and from space has been considered to be the main obstacle

to development of massive systems like space solar power stations in space. In

principle, this can be improved technically by designing reusable space

vehicles to be operated similarly to present-day airlines. For such a space

transportation system, the businesses building and operating space power

stations will be major customers. The economy of microwave "fuel" will be

important in assessing this respect as well.

23

CHAPTER 14

CONCLUSIONS

1. There are many concepts of space solar power systems that have been

proposed for space solar energy to be used for humankind. However, most of

them were theoretical and not evaluated on the basis of becoming practical

power systems. The SPS 2000 study was made on practical assumptions and

has indicated a realistic approach to space solar power research which can be

interpreted as follows:

2. To facilitate research on this power system as a future energy source to

compete with other sustainable energy candidates, it is necessary to consider

the space solar power system as a variation of solar power systems now

under research and development for terrestrial use.

3. The advantage of space solar power over terrestrial solar systems is one order

of magnitude larger solar power in space than on the earth. The disadvantage

is the high cost of transportation of the required facilities to space. Even if

reusable space transportation systems under development realize lower costs,

the advantage over terrestrial systems is expected to be marginal. A cost

target is therefore mandatory for engineering space solar power stations. The

microwave "fuel" concept can be applied to this case too.

It is practical to apply the concept of microwave "fuel" as the interface between

space power suppliers and buyers, as utility power suppliers and consumers are

related to each other by the standard of commercial electric power. Considering

that a properly selected microwave frequency makes it possible for users to

24

plan and even build their rectennas, I strongly recommend the use of 2.45 GHz

as a standard for wireless power transmission.

REFERENCES

http://www.spacefuture.com/power/sps2000.shtml

http://www.spacefuture.com/archive/power_from_space_for_the_next_century.shtml

http://www.spacefuture.com/archive/

an_approach_to_develop_space_solar_power_as_a_new_energy_system_for_develo

ping_countries.shtml

http://www.spacefuture.com/archive/

a_fresh_look_at_space_solar_power_new_architectures_concepts_and_technologies.

shtml

http://www.spacefuture.com/archive/

solar_power_satellites_an_idea_whose_time_has_come.shtml

http://www.spacefuture.com/archive/

benefits_of_electricity_from_space_for_rapidly_advancing_countries.shtml

25