The potential for microwave power beams transmitted to terrestrial receiving antennas from large solar power stations ( SPS)in space to become a major source of clean electric power for people on Earth in the future is a very suitable theme for the Third United Nations Conference on the Peaceful Uses of Outer Space, UNISPACE 3. During the years since UNISPACE 2, recognition of the likely extremely high costs of permitting "greenhouse gas" emissions to grow indefinitely has spread worldwide. Consequently, although world oil prices are currently at a historic low in real terms, contrary to some predictions made during the 1970s, the need to curb emissions of CO2 from fossil fuel combustion is growing increasingly urgent, and is already the subject of global negotiations.

Over the past 30 years a great deal of experimental work has been done that shows the technical potential of SPS as a new electric power source. A great deal of analytical work has also shown the benefits to world economic growth that will arise from expansion of a range of space-based commercial activities which the successful development of power supply from space will generate. Furthermore, microwave power beams from space could become a particularly attractive source of power for many developing countries, as described below.

However, before clean power can be delivered from space to Earth by commercial satellite power stations, a pilot plant system is unquestionably needed. All new energy sources need to be demonstrated in practice before electricity supply companies can be expected to invest in them. No matter how advanced theoretical analyses and laboratory demonstrations may be, the first actual implementation of a new engineering system always reveals new aspects and new problems that were not foreseen in theoretical and experimental studies. As a result, the first demonstration system is not profitable - and commercial investors are famously wary of investing in technological development. As a new electric power system, this is particularly true of a system of which much of the technology is outside the experience of electricity companies, being space-based.

It is in fact a weakness of the SPS system, from the point of view of obtaining government support, that it cuts across the responsibilities of several different government departments, including energy, telecommunications, technology, environment, industry and foreign policy, as discussed in (1). As a consequence the total amount of government support for SPS research has not yet even reached 1/1000 of that given to other new energy technologies. For example, governments have given (and continue to give) subsidies to various different nuclear power generation technologies of hundreds of billions of US$.

It would therefore be only a continuation of normal electricity industry practice for governments also to fund a solar power satellite ( SPS) pilot plant, which need not be commercially viable. The operation of such an SPS pilot plant will provide engineers with a test-bed that they can use for a wide range of experiments under different operating conditions, including load following, response to transients, reliability, electromagnetic noise and others, as described in (2). It will also be used to test the design, manufacture, assembly, operation and reliability of a range of different sub-system and component designs.

Over and above these uses for engineering tests, an SPS pilot plant could demonstrate to members of the electricity industry, investors, manufacturers, governments and the media that the SPS concept is now technically mature, environmentally benign, potentially profitable - and popular with users and the general public. This would be useful because, unfortunately, more than 30 years after its inception, the SPS concept is still little known outside professional circles. For example, very few public exhibitions concerning energy, such as those held at science museums, discuss energy from space - although they always have a section relating to nuclear fusion, which has received many tens of billions of dollars of government funding without becoming a realistic energy option.

An SPS pilot plant should be designed also to provide data on economic aspects as well as engineering matters. In this case, on the basis of operating experience, electricity companies and energy policy makers should be able to judge the future economic promise of SPS systems.

By providing electricity supply to ordinary users, a pilot plant system can also be used for the important purpose of market priming. Even if standard figures-of-merit such as the satellite's specific output in KW/Kg and specific cost in $/W are not yet low enough to attract commercial investment, provided that engineering progress is expected to meet the necessary targets in due course, subsidising operations of an early system has great value from the point of encouraging rapid market penetration of subsequent commercial or near-commercial systems.

This is a well-known form of policy support, and is currently used, for example, by the Japanese Ministry of International Trade and Industry (MITI) to grow the market for roof-mounted domestic photovoltaic power generation systems. These are not yet economically competitive with commercial electricity supply but they are confidently expected to become so within a few years due to continuing advances in semiconductor technology, and as the market grows and further economies of scale and accumulated experience are achieved. The same logic is a strong additional reason for using an SPS pilot plant to deliver useful electricity supplies to users in the general population.

For simple physical reasons, an SPS demonstrator satellite in low orbit is economically much more attractive than one in geo-stationary orbit. For example, in order to focus a beam of microwave energy onto a terrestrial antenna of a given diameter, a transmitting antenna in geo-stationary orbit ( GEO) would need to be nearly 36 times greater in diameter, and so 1000 times larger in area, than a transmitting antenna at 1000 km altitude.

Low Earth orbits of course have the disadvantage that the satellite will not remain fixed above a single receiver, but will move continuously around the Earth. Thus, in order to provide a continuous power supply to users on Earth it will be necessary to include electric power storage in the ground segment. Among the range of low Earth orbits, those directly above the equator have the important operational advantage that a satellite can deliver to the same receiving antenna on every orbital revolution, that is every 90 minutes or so. This is very different from orbits inclined to the equator, from most of which it is possible to deliver to the same antenna no more than once per day at best.

2 An Equatorial SPS Pilot Plant Project

A study on a low Equatorial orbit SPS pilot plant system has been under way in Japan since the late 1980s (3). A guideline for the study was developed in order to focus researchers' work on designing a system suitable for use by the terrestrial electricity industry, which included the following 6 requirements, as described in detail in (4 & 5).

Basic Requirements

2.1

The first construction will be started no later than 2000.

2.2

Commercial and versatile technologies will be used for this system.

2.3

The electricity generated by the SPS 2000 shall be commercially competitive with existing small scale utility electricity (NB this excludes launch costs).

2.4

The SPS 2000 satellite will be placed on a low altitude equatorial orbit by commercial launch systems.

2.5

Prospective customers for the electric power utility service are residents in the equatorial zone.

2.6

Design a basic model with 10MW microwave power allowing for system growth in the future.

Based on these requirements, a system design was developed, shown in Figure 1, which came to be named "SPS 2000". The basic configuration is a light aluminium frame in the shape of a triangular prism some 300 metres in main dimensions, carrying some 18 hectares of amorphous silicon solar cells in a saddle-back configuration so that active sun-pointing is not required. A 130 metre square phased-array transmitting antenna is connected to the satellite's lower Earth-pointing face, and delivers some 10 MW of 2.45 GHz power in a beam with a cross-section some 2 km in diameter at the Earth's surface. The beam can be steered +/- 30 degrees west and east, and so can deliver power for 200 seconds to rectennas within 3 degrees latitude of the equator. With storage this should allow the delivery of up to a few hundred KW of continuous power.

Figure 1: "SPS 2000" solar power satellite pilot plant design

As the SPS 2000 study project progressed, by 1994 it came to be felt that it was timely to start to plan the use of the system in operation, which requires collaborative research with users in countries around the equator. In order to maximise the many benefits of operating such a pilot plant, it was decided that power receiving antennas, known as 'rectennas', should be sited in as many different countries as possible.

From the system design, rectennas need to be sited within 3 degrees, or about 300 km of the equator. As it happens, many countries in this category are at a relatively early stage of economic development, and there are many tens of millions of people living without any electric power supply within a few degrees of the equator.

For this field research, Professor Hideo Matsuoka of Teikyo Heisei University (formerly of Tokyo University), Professor Makoto Nagatomo of the Institute for Space and Astronautical Science, and Professor Patrick Collins of Azabu University have since 1994 been performing Field Research on the siting of rectennas for an equatorial SPS pilot plant "SPS 2000". This has involved visiting a series of countries around the equator, establishing professional relationships with researchers, meeting government representatives with a range of responsibilities, and making provisional agreements to participate in the project by hosting a rectenna and collaborating in the system operation and data collection.

There are a number of constraints on the selection of suitable sites for rectennas, including the need to be separated by some 1200 km west-east for maximum transmission time, and the need to be near an adequate user population, as described in (6, 7 & 8). The results of these field research visits have been described in a series of reports, which discuss candidate rectenna sites that were visited during the field trips (9). A number of case studies of potential rectenna sites are currently under preparation.

Country	Site	Terrain	Use of power
Tanzania	Namanga	bush	border village supply
Papua New Guinea	Manus island	tropical forest	high school + villages
Brazil	Alcantara + ?	forest	link to village grid
Indonesia	Halmahera	coastal plain	villages
Ecuador	Galapagos (?)	open fields	local grid
Maldives	Gan atoll	shallow lagoon	villages
Malaysia	UTM campus	hillside	research
	Sarawak state	tropical forest	villages
Colombia	Tuquerres	agricultural land	native villages
Kiribati	Tarawa atoll	tidal lagoon	villages
	Phoenix islands	lagoon	villages
Nauru	over mining spoil		link to local grid

Potential sites are also under consideration in Gabon, Peru, Somalia and Jarvis island (USA), though these may not all be possible for various reasons.

The planning, construction, operation and monitoring of several of these rectennas may involve collaboration with researchers in neighbouring countries, such as Kenya and Uganda in the case of Tanzania, Sri Lanka in the case of Maldives, Singapore and Thailand in the case of Malaysia, and others. There is obvious potential for equatorial countries to benefit by collaborating over their participation in the SPS 2000 project, and from contacts with interested researchers there is also a possibility of technical cooperation with researchers in Ghana and India.

There will also be a need for discussion with all countries over which the satellite will pass, not only those which participate by hosting a rectenna. These countries could also participate by monitoring the satellite on its passes overhead.

Finally, the United Nations could clearly play an important role in overseeing the initial stages of what could grow into a major new global energy source. Because the project is inherently international it will require the achievement of international agreements on a number of issues concerning international law and space law - and it will particularly involve developing countries (5). This is because the greatest growth in energy demand in future is expected to be in currently less-developed countries. Moreover, it is notable that SPS operators looking in towards the Earth from geo-stationary orbit will sell their power supplies to the global market - and many developing countries will have an economic advantage in purchasing power supplies offered by orbiting satellites.

This is because the price that rectenna operators will offer for supplies of microwave 'fuel' will be based on the difference between the cost of other means of generation and the cost of their rectenna. Both land and labour are low-cost in developing countries, and so rectennas in these countries will have much lower costs than rectennas in more advanced countries where land and labour are much more expensive. But existing means of electricity generation in developing countries are expensive, since prices are to a large extent set by global markets. Consequently in many of these countries rectenna operators will be able to offer higher prices for microwave power supplies from space than companies operating rectennas in more economically developed countries. This is an additional incentive for planning a pilot plant system which involves developing countries as users.

To date, researchers in both Russia and France have participated in work towards realising the "SPS 2000" project, and interest has also been expressed by researchers in a number of other countries. Space agencies in the leading countries are currently preparing and assembling an international space station ( ISS), which is due to be completed within a few years. However, beyond that there are no major new projects in space for which there is significant political support for government investment, in which case space agencies' budgets may be expected to decline substantially.

By contrast, the public is widely concerned about global environmental problems and the prospects for energy supply during the next century, and they support government expenditure aimed at resolving these problems. International cooperation on an SPS pilot plant as a policy initiative aimed at the related problems of energy, environment and economic development can therefore be expected to receive strong popular support. In view of the problem referred to above that SPS falls between the responsibilities of different government departments, it is potentially in the hands of the space community to propose and prepare such a project to follow ISS.

The major engineering problem identified to date in the SPS 2000 study is the planned automated assembly in orbit. It is not possible to adequately test the deployment of such a large structure - taller than the Eiffel tower - in the gravity field on Earth, since it is designed for a micro-gravity environment and so will be much too flimsy to support its own weight. Consequently the deployment system's reliability cannot be estimated accurately, and so it would be very risky to try to realise the project if there was no possibility of crewed intervention: in the event of a single unrecoverable mishap during assembly the entire project could be wasted.

Hence, the possibility of crewed intervention during the assembly process, even if only periodically, would add greatly to the perceived reliability of the system, and hence to its expected value. Until a new generation of crewed reusable launch vehicles becomes available, the only possibilities for crewed intervention are by US astronauts and/or Russian cosmonauts. However, neither US nor Russian launch vehicles can reach orbits above the equator from their existing launch sites.

If international cooperation to realise an SPS pilot plant in the near future expanded to include the possibility of assistance by crews in orbit, the remaining problems of orbital assembly should be soluble. As discussed above, there are good reasons for aiming to deliver power to sites near the equator. Several possible ways of doing this are worth analysing and comparing in detail.

One possible means of using US and Russian crewed space flight operations capabilities would be to establish a new equatorial launch site for a space shuttle and/or Proton and Soyuz launch vehicles. The Alcantara launch site near the town of Sao Luis in northern Brazil is an attractive candidate since it is the closest launch site to the equator, and the Brazilian government has stated its keenness for international cooperation in its utilisation.

This option would probably involve launching the satellite modules into a low assembly orbit at a few hundred km altitude, and then to raise the satellite's altitude to the planned 1100 km using ion-engines self-powered by the satellite's several MW of solar panels. This would substantiallyu increase the satellite's overall complexity and cost; however, self-propelled orbit-raising is a feature of many SPS concepts, and so a pilot plant test of this operation could be of considerable value in itself.

Another means of using crewed intervention during the assembly of an SPS pilot plant would be to launch the satellite components from existing launch sites to an inclined orbit convenient for crewed intervention, assemble and check them out, and then to use self-powered ion-engines both for orbit-raising to 1100 km altitude ,and for orbit-transfer to the originally planned equatorial orbit.

A third possibility would be to launch the satellite into inclined orbit from existing sites as in 4.2, and to use a non-equatorial orbit for the satellite operation. If the satellite had no self-propulsion capability it would be constrained to remain in the initial orbit at low altitude reachable by existing crewed vehicles. This would have several disadvantages, including subjecting the satellite to considerable damage from debris (which is many times more numerous at lower altitudes than at 1100 km); shortening the length of time during which it could deliver power continuously to any receiver; and shortening its lifetime due to faster orbital decay. Thus the use of self-powered ion-engines must be considered desirable in enabling the orbital altitude to be raised.

It is notable, however, that if an ion engine orbit-raising system is used, then changing the orbit inclination is also possible, widening the choice of orbits. The possibility of orbital transfer would be limited primarily by the length of time allowed for orbit-transfer, since ion engines' propellant mass will be only a small fraction of the overall satellite mass. Thus, unless the satellite was planned to remain in the assembly orbit during operation, there would be no need not to use an equatorial orbit.

Selection of a non-equatorial orbit for satellite operation would need to be done in parallel with selecting a number of planned rectenna sites and a transmission schedule to deliver a given amount of energy to each rectenna. Power could also be transmitted to rectennas on the equator, though much less frequently than from an equatorial orbit - less than once per day for most orbits. Hence the average amount of power delivered per rectenna would be significantly reduced below current SPS 2000 plans, unless the satellite output was increased. On the positive side, use of an inclined orbit could enable some power to be delivered to rectennas in a wider range of countries than an equatorial satellite.

In the case of a non-equatorial orbit, it would be necessary also to ensure that the amount of energy to be delivered would be considered sufficient and sufficiently frequent to be judged worthwhile from various different viewpoints. One of the important potential benefits of a successful SPS pilot plant will be to obtain good media coverage. However, it must be remembered that publicity "cuts both ways": the mass media are very unforgiving, and enjoy criticising and belittling projects that they judge to be misguided (for whatever reason). Consequently, if the energy transmissions came to be considered merely "token" amounts by the electricity power industry and the media, the resulting poor publicity could be very damaging to the growth of public support for SPS in general.

An important result of all three of these approaches is that using crew-tended assembly in low orbit would change the overall system design constraints significantly from those used in establishing the initial requirements for the SPS 2000 project. Consequently a newly optimised system design might be significantly different from the current SPS 2000 system.

When should such a pilot plant be built? The SPS project has lacked substantial support for several decades, and it would benefit greatly from the construction of a successful pilot plant as soon as possible. In view of the lack of any other agreed objective for government space agencies after the completion of the ISS in 2002, such a pilot plant could realistically be planned to be assembled within a few years after that.

The possibility that future launch vehicles developed in the coming years will be cheaper than existing vehicles is not sufficient reason to wait. Even using existing launch vehicles, the project need cost no more than a few $billion, which is only a small fraction of the budgets spent on past candidate energy sources such as plutonium-fueled "fast-breeder" nuclear-reactors - until research on them was scrapped in all the leading countries of USA, Russia, Germany, Britain and France.

As mentioned in section 2 above, the SPS 2000 project is planned to lead on to a range of further developments, in the same way that other energy generation systems progress through a sequence of progressively more advanced generations of technology. This should remain an important objective of any international SPS pilot plant system. Once there is a family of rectennas operating on Earth, their operators will be in a position to purchase microwave "fuel" from other power satellites, as discussed in (2), since a single satellite could deliver power to each for only a small percentage of the time. This will create the opportunity for other groups to profit from this 'market priming' by building more advanced and economical satellites to sell additional power to the same users.

In negotiating with companies planning successive new satellites, existing rectenna operators will play an important role in establishing specifications for the beams they will accept, and laying the groundwork for future international standards for this new industry, as described in (10).

Even if the first SPS pilot plant system is successful, it is not certain today that the next satellites could be commercially profitable. There are many possibilities whereby governments could provide partial subsidies to a second generation of satellites if this was considered desirable in order to help the new space power industry to grow. However, the major objective of SPS research is to develop an environmentally benign new power source that will be economically competitive with other sources, and government involvement must not be permitted to hinder the achievement of this over-riding objective.

1. P Collins, 1997, "Widening the Base of SPS", Equatorial Times No 5, pp 6-7; also downloadable from www.spacefuture.com
2. P Collins, R Tomkins and M Nagatomo, 1991, "SPS 2000: a commercial SPS test-bed for electric utilities+2000%3A+a+commercial+SPS+test-bed+for+electric+utilities">", Proceedings of Inter-Society Energy Conversion Engineering Conference, American Nuclear Society, pp 99-104; also downloadable from www.spacefuture.com
3. M Nagatomo and K Itoh, 1991, "An Evolutionary Satellite Power System for International Demonstration in Developing Nations", Proceedings of SPS91, pp 356-363; also downloadable from www.spacefuture.com
4. M Nagatomo et al, 1994, "Conceptual Study of A Solar Power Satellite, SPS 2000+2000">", Proceedings of ISTS, Paper No. ISTS-94-e-04; also downloadable from www.spacefuture.com
5. M Nagatomo, 1996, "An Approach to Develop Space Solar Power as a New Energy System for Developing Countries", Solar Energy, Vol. 56, No 1, pp 111-118; also downloadable from www.spacefuture.com
6. H Matsuoka and P Collins, 1996, "Equatorial Cooperation for SPS 2000 rectennas+2000+rectennas">", Proceedings of ISAP '96, pp 421-4; also downloadable from www.spacefuture.com
7. P Collins, 1998, "SPS 2000 and its International Importance+2000+and+its+International+Importance">", Space Energy and Transportation, Vol 1, No 4, pp 279-287.
8. P Collins, 1996, "SPS 2000 and its Internationalisation+2000+and+its+Internationalisation">", Proceedings of Space 1996, ASCE, pp 269-79; also downloadable from www.spacefuture.com
9. H Matsuoka et al, 1994-99, " Field Research for Solar Power Satellite Energy Receiving Stations", Matsuoka Laboratory Working Papers 1-10, Tokyo University / Teikyo Heisei University.
10. H Matsuoka, 1999, "Global Environmental Issues and Space Solar Power Generation: Promoting the SPS 2000 Project in Japan+2000+Project+in+Japan">", Technology In Society, Vol 21, No 1, pp 1-17.