The average electricity supply in economically developed countries is approximately 1 kW of generating capacity per person. Although these countries are making efforts to increase the efficiency of energy use, electricity demand continues to increase at some 2-4 % per year in most of them. That is, despite considerable progress in improving the efficiency with which energy is used in many countries, the demand for electricity continues to increase in even those countries with the highest consumption. For example, as air-conditioning becomes available, it is common for people to maintain their offices and homes at a lower temperature during hot weather than they do during cold weather. Even in Japan, where the price of electricity is several times higher than the USA, such patterns of use are clearly not deterred by the current price of electricity. Thus, although changes in behaviour that reduce energy use could in principle help to solve humans' energy problem, there is to date little evidence to suggest that this will prevent the demand for electricity from continuing to grow.

By contrast to the advanced countries, most of the world's population have electricity supplies of barely 100 W per person, or about 10 % of the rich countries' level, and they understandably wish to reach approximately the same standard of living. In recent years, the rapid economic growth achieved in Asia has given new hope to most of the world population that this is possible within a generation or two. However the world population is expected to reach some 10,000 million people by the middle of next century. Consequently, if the average level of electricity supply throughout the world is to reach even 1 kW per person, some 10,000 Gigawatts of generating capacity will be required, or roughly ten times present world-wide generating capacity. If this target is to be reached even by the end of the 21st century, and the replacement of old generating plant is included, the average rate of construction of generating capacity through the 21st century will need to be more than 100 Gigawatts of generating capacity per year.

In considering appropriate targets for future electricity supply, an interesting case can be made that world economic growth in the early post-war period had little to do with "economic policy", but was due primarily to the falling price of oil through the 1950s and 1960s. In this context it is notable that China, which has recently followed Japan and Korea in achieving impressively rapid economic growth for more than a decade, has followed a policy of maintaining a low price for electricity (1). The severe world-wide economic instability suffered ever since the advanced countries have relied entirely on discretionary economic policy, having abandoned the stability of commodity-backed money in 1971 (2), gives further support to the view that energy availability is far more important for economic growth than any putative progress in economic policy skills.

The importance of this argument in the present context is that the most effective way of reducing population growth is to raise standards of living through economic growth. Thus, if it were possible to increase the supply of electricity by several hundred GW of capacity per year in a way that was environmentally benign, this would apparently offer the best prospect of keeping the world population within sustainable limits. A target of 20,000 GW of generating capacity by the year 2050 has been proposed in this context, requiring average construction of 400 GW per year (3). Clearly the actual rate of construction of electricity generating plant will continue to grow, so an average rate of some 100 GW per year would seem a reasonable target for the first half of the 21st century. If energy supply does not grow at such a fast rate, then much of the world population will continue to live in poverty throughout the next century, and population will presumably continue to increase. Achieving such a high rate of growth as 100 GW per year or more is an enormous challenge for the electricity supply industry and related engineering industries, but deserves serious consideration.

According to current thinking, for several reasons neither fossil fuels nor nuclear energy could supply more than a fraction of such a large increase in electricity generation. That is, if fossil fuel were to supply more than a small part of this capacity it would cause severe environmental damage. Furthermore, the use of nuclear energy on even a fraction of this scale would also have severe environmental consequences and, in view of the continuing political and religious conflicts around the world, would seem likely to lead to widespread proliferaton of nuclear weapons.

By contrast, electricity generation using solar cells uses no fuel, and produces no waste products. Manufacture of solar cells depends on solid-state semiconductor technology, which is very economical in material use. In addition semiconductor technology continues to advance rapidly, and so the prospects for both improved performance and reduced costs are improving continually. However, the use of solar energy at the earth's surface suffers the well-known problem of being diffuse, intermittent and undependable. In order to provide firm electric power, very large areas of solar cells, large-scale electricity storage and long-distance power transmission would be required, making such electricity expensive.

By contrast to terrestrial solar energy, the use of solar-generated electricity delivered to Earth by microwaves from orbiting satellites, as first suggested in 1968 (4), could in principle overcome these difficulties. As proposed, the satellites would transmit microwave power to large-scale receiving antennas on low-value land or over the sea (probably in combination with terrestrial photovoltaic generation), which would deliver power directly into consumer electricity grids.

The comparison between solar-generated electricity obtained from photovoltaic systems on Earth and that delivered to Earth from orbiting satellites depends on the factors shown in Table 1. More research is required on these in order to decide whether solar power satellites ( SPS) can be competitive, as discussed further in the section on "SPS 2000" below.

Terrestrial	Space-based
> 10 times larger area	Space transportation
Large scale electricity storage	Microwave transmission
Long distance transmission	Receiving antenna ( rectenna)

A number of possible constraints on the amount of power that could be delivered by SPS have been suggested by researchers, but to date all of these seem to be avoidable.

It is commonly argued that since electricity generated by SPS would constitute a net addition of energy to the Earth, it would therefore upset the global energy balance, and in particular would add to "global warming". Although this is correct in principle, the quantities involved are far too small to be significant, particularly when 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 about 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 heating effect. It should also be noted that because of the high efficiency of rectennas in converting microwaves into DC electricity (some 90%), the heat added to the environment per unit of electric power produced by SPS is less than half that created by even the most efficient thermal power stations.

It has been suggested that the maximum possible output of SPS is limited by the limited space in geo-stationary orbit. However, this is based on the 1970s US "Reference System" design concept which comprised 60 satellites, each with an area of 50 square kilometers, in geo-stationary orbit 35,800 km from Earth (5). Since that time many alternative designs have been proposed, including the use of a range of other Earth orbits, the use of relay satellites, and siting microwave transmitting antennas on the lunar surface some 400,000 km from the Earth (6), or even at the Sun-Earth L1 libration point more than 1,000,000 km from Earth. Consequently, lack of space in orbit for power-generating satellites does not seem to pose any limit on SPS capacity.

It has been suggested that the area of "rectennas" required to receive energy on Earth would be prohibitively large. This is also based on the 1970s US studies which assumed that, in order to limit the intensity within the ionosphere, the average intensity of the microwave beams received at rectennas would be only some 60 W per square meter (5). However, by transmitting microwave beams from several different satellites to a single rectenna it seems that this limit can be avoided (7). Thus the area of rectennas required could be as much as an order of magnitude less than the 1970s studies suggested.

It has been suggested that the energy used to produce SPSs would more than outweigh the energy produced in their lifetimes. The energy balance of an SPS system was studied as part of the US DOE studies and found not to be a serious constraint. The production of modern thin-film solar cells consumes very little energy, and so the main energy requirement for SPS is that required for launching the satellite components into space. This has been studied recently and shown to be acceptable, provided that appropriate launch cost targets are reached (8).

Consequently, according to present knowledge, in a scenario in which electric power delivered from space became economically competitive, there does not seem to be any limit to the amount of electric power that could be delivered from space to Earth.

Although the construction of SPS may be technologically feasible, the commercial feasibility and desirability depend critically on the cost. Present-day space industry costs, both launch vehicles and spacecraft, are extremely high, and must be reduced by two orders of magnitude below present levels in order for SPS to be attractive. Because of the space industry's history as a government-supported activity rather than a commercial industry, such large cost reductions may be possible, but this subject is understandably controversial. However, two projects under way today offer the prospect of learning definitively, before the end of the decade, whether such targets are achievable or not.

"SPS 2000"

Proposals have been made by various researchers of development pathways towards SPS. The usual approach is through a series of space industry technology development projects, followed by projects utilising SPS technologies to supply power for various space facilities, leading in the 2020s to demonstration of power delivery to Earth.

The approach of the "SPS 2000" project currently being planned by the SPS Working Group in Japan is radically different to this. "SPS 2000" is a pilot plant project intended to demonstrate the delivery of useful electricity from space to Earth as early as the year 2000 (9). The SPS 2000 satellite is being designed, using existing technology, to deliver several MW of power to rectennas on Earth, which will provide the electricity industry with their first opportunity to gain experience of utilising energy from space (10). In order to minimize costs the satellite will operate in equatorial orbit at 1100 kilometer altitude, and will deliver power to "rectennas" (receiving antennas) at a number of sites near the equator. Power will be delivered to each rectenna for some three minutes each orbit, and will be used in combination with storage to provide continuous supply of some 100-200 kW, which is sufficiently large to be valuable for equatorial communities with inadequate supplies.

The SPS 2000 satellite is in addition being designed to have a radically lower cost than other space industry projects (11). Specifically the cost target is that it should be roughly competitive with power stations on Earth, at a cost of some 200 \ / Watt of capacity - with two provisos. The first is that this target excludes launch costs. The possibility of reducing these sharply is discussed further below. The second proviso is that the cost of the rectenna per kWh of output is also excluded. In order to provide early experience, the satellite will operate in an easily-accessible low Earth orbit and so will deliver power only intermittently. This will limit the utilization of each rectenna to a few per cent, and will necessitate power storage, neither of which will apply to a future satellite in geo-stationary orbit. In addition the microwave beam intensity will be much lower than that used in a future commercial system. However, within these provisos, the cost of the SPS 2000 satellite is sharply lower than other satellites, and the design is very different from the 1970s US studies.

Figure 2 shows the present design of the SPS 2000 satellite of the SPS Working Group (12). Using amorphous silicon photovoltaic cells for electricity generation, solid-state microwave generating modules for 2.45 GHz microwave power transmission to Earth, and passive gravity-gradient stabilization, the mass is planned to be some 12 tons / MW. According to the thinking of the SPS 2000 design team, much of the technology used to produce electricity delivered from space will be semiconductor technology. That is, in addition to the solar cells, the transmitting antenna will also be made of solid-state modules. Currently these are of lower efficiency than microwave generating tubes, but their lifetime is potentially much longer, and manufacturing cost potentially much lower.

Figure 2: "SPS 2000" pilot plant configuration (12)

Hitherto research on SPS 2000 has been performed voluntarily by researchers in research centers, universities and private industry. However, SPS research in Japan has recently been designated as a national research project (13) which will permit increased funding in the future. As a pilot plant planned to produce useful electric power on Earth, SPS 2000 is taking a very different approach to more traditional space industry approaches, and is intended to be of interest to the electric power industry, whose expertise and participation are clearly essential to the realization of SPS as an electric power source for the future.

The feasibility of electricity from space becoming commercially competitive depends critically on the cost of launch, which is extremely high today based on the expendable rockets developed from missiles during the 1950s which are still used for launch. Since the early 1980s the USA has gained experience of operating the semi-reusable space shuttle, but this has even higher launch costs than the expendable Saturn 5 rocket in the 1960s. Consequently attention has recently begun to turn to a different approach to reducing launch cost, using fully re-usable, single-stage-to-orbit ( SSTO), vertical-take-off-and-landing ( VTOL) launch vehicles, which was first proposed in the 1960s.

Currently the "Delta Clipper" SSTO VTOL is being designed by McDonnell Douglas in the USA, and the DC-X 1/3 scale test vehicle started sub-orbital flight-testing at White Sands, New Mexico this summer. If an orbital vehicle is successfully developed later this decade, it has been proposed that costs could fall as low as a few hundred dollars per kG of payload (14). If so, then the prospects for rapid development of electricity from space will be greatly accelerated. To date, despite hundreds of billions of dollars of government funding for the space industry, very little effort has been made to develop truly low-cost space transportation, and so a project having this objective seems highly desirable.

If one considers the probable pattern of development of a commercial SPS industry, it seems likely that initially all SPS components will be manufactured on Earth, and orbital operations will involve mainly orbit transfer and assembly. However, the construction of 10s of GW of SPS capacity will create a market in Earth orbit for some hundreds of thousands of tons per year of raw materials such as aluminium and silicon. In order to reduce costs these will probably come to be launched as raw materials and manufactured into SPS components in factories in orbit. As the capabilities of such a space-based manufacturing industry grows, at a certain stage raw materials may come to be supplied from extra-terrestrial sources such as the Moon, asteroids and comets, from where delivery costs could fall far below launch costs from Earth due to the low gravitational fields involved. It has been argued that technologically such developments would not be particularly difficult or expensive by comparison with modern mining projects, and that they will surely occur when there is a need for them (15). Thus, normal commercial logic suggests that the rapidly growing commercial demand for electric power on Earth could lead to the use of extra-terrestrial resources for contructing SPS units in orbit, which would in turn reduce the demand for launch from Earth.

Thus, once launch costs are sufficiently low for SPSs to be able to supply electricity at competitive prices, it seems likely that international commercial consortia will manufacture and operate SPSs, selling power profitably to electricity supply companies on Earth. Later, companies may also develop extra-terrestrial material resources, even possibly using the Moon as a site for power stations (6), all without the need for further government subsidy. In this way the space industry could become a mature commercial industry, dependent on commercial customers rather than political decisions.

A figure of some 100 GW of generating capacity per year was taken above as representative of what is ideally required through the first half of next century. It is interesting to consider some of the implications of supplying such a rapid growth of power using electricity from space. If we assume average solar cell efficiency of approximately 10%, ten square meters in orbit will produce 1.4 kW of electricity, and so ten square kilometers will produce 1.4 GW in orbit. If the microwave power transmission and reception system has an overall efficiency of 50%, which is higher than that available today using solid-state microwave generators, but less than that demonstrated in the 1970s using magnetrons, then 10 square kilometres in orbit will produce 0.7 GW at the rectenna on Earth, and 100 GW on Earth will require some 1400 square kilometers of solar arrays in orbit.

Thus the production of 100 GW of SPS capacity per year will require production of some 1400 square kilometers of solar arrays per year, or some 4 square kilometers per day. The area of transmitting antenna modules required would be approximately an order of magnitude less than this, and the total rectenna area required to receive 100 GW will be considerably less than the satellite solar arrays. Thus, as a representative figure, we assume that some 6 square kilometres of solar arrays and antenna panels would need to be produced per day. If made in strips 5 m wide, this would require some 1200 km to be produced per day, of which some 800 kilometres would be solar panels. If produced at 10 plants using 24-hour production lines, the output speed would be some 1.4 metres / second, which is not much faster than existing amorphous silicon cell production line speeds (though these are narrower today).

It may be suggested that production of tens of thousands of square kilometers of solar cells is not a realistic possibility. However, although the area of solar cells required to reach the proposed level of output is impressively large, the mass of materials involved is impressively small. 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 of 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.

Production of photovoltaic and microwave electronic circuits on such a large scale has been called "macro-electronics" (16). Being semi-conductor technology, such large scale production would be more readily automated than the manufacture of traditional thermal power stations, and the prospects for cost reduction are formidable. That is, continuing developments in the semiconductor industry, such as the development of multi-band-gap, thin-film solar cells, promise continuing improvements in performance and economy (17, 18).

As far as the transportation requirements of SPS are concerned, if launch costs fall to an acceptable level, then they too seem likely to be feasible. If a typical cargo launch vehicle has a payload to LEO of 50 tons (19), and the specific mass of an SPS is approximately 12,000 tons / GW (like the SPS 2000 satellite), construction of 100 GW of capacity per year will require 24,000 flights to LEO per year, or some 70 flights / day. This is very small by comparison with modern-day air transportation, being the traffic rate of a single small airport, and it should therefore not pose any logistics problems. Being many times greater than the flight rate of today's expendable launch industry, such a scenario offers the attractive possibility of achieving substantial economies of scale, which have not hitherto been available to the launch industry.

Overall therefore, delivery of electricity from space to Earth seems to have the potential for expansion at a rate as fast as 100 GW per year. Companies in countries which have advanced space industries and electronics (particularly "macro-electronics") industries, such as the USA, Europe, Japan, China and Korea, will have the opportunity to play major roles in developing this major new energy source. As a next step, the design of the factories needed to achieve such rates of macro-electronic production would be an interesting exercise.

To date, electricity from space has received minimal research funding - less than 0.01 % of that received by research on nuclear power. However, it appears to have the potential to provide very large scale electricity supplies that would facilitate rapid economic growth around the world without damage to the environment. It has also been suggested that electricity from space would be a particularly appropriate form of aid for economic development (20). In order to determine its true potential, research on the feasibility of electricity from space should be funded on a similar basis to other energy sources.

The aerospace industry has enormous technological capability, but with the end of cold war it is currently shrinking rapidly. Rather than being allowed to decline drastically, the industry should be encouraged to try to develop electricity from space as a major energy source for Earth. However, not having the experience of the electricity supply industry, the space industry will need guidance and encouragement in order to challenge such a new commercial field. In order to succeed in this the space industry will have to learn a new, commercial culture, in which cost reduction is the over-riding objective. For unless SPS has the potential to deliver electric power to Earth at competitive prices, it is of no interest.

Consequently, research on electricity from space for Earth should not be established as a "space project". For example it could not justify expenditure of some hundreds of billions of dollars to establish a "Moon Base" where some relevant research would be done. Expenditure used to develop SPS must be concentrated on discovering the true potential for cost reduction in the most cost-effective way. Thus, in particular, SPS 2000 should be built and operated to provide actual experience of delivering electric power from space to Earth as soon as possible, and fully reusable launch vehicles should be developed and put into airline-type operation in order to understand their true potential. In addition, from an early stage, as much work as possible should be performed by commercial companies, who necessarily continuously try to reduce costs.

Young people around the world need to be offered a challenging and optimistic vision of future technological and economic development. For this reason too it seems desirable to study the feasibility of a project with such exciting potential as SPS to a greater depth than has been done so far. Only in this way can we discover whether this project really has the potential to become a major contributor to world economic development.

1. E Osawa, 1992, China's electricity industry, Macro Review, Vol 5, No 1, pp 41-47 (in Japanese).
2. P Collins, 1985, Currency convertibility: the return to sound money, Macmillan.
3. D Criswell, 1993, " World energy requirements in the 21st century", Proceedings of 1st Wireless Power Transmission Conference, pp 285-300.
4. P Glaser, 1968, " Power from the sun: its future", Science Magazine, Vol 162, No 3856, pp 857-861.
5. US Department of Energy, 1978, " SPS Reference System Report", DOE / ER-0023.
6. D Criswell & P Harris, 1993, " An alternative solar energy source", Earth Space Review, Vol 2, No 2, pp 11-15.
7. P Collins & Gelsthorpe, 1980, " Increasing power input to a single SPS rectenna by using a pair of satellites", Electronics Letters, Vol 16, No 9, pp 311-313.
8. Y Yamagiwa & M Nagatomo, 1992, " Analytical study on space transportation cost impact on SPS by evaluation model of development of SPS", Proceedings of 11th ISAS Space Energy Symposium, pp 105-111.
9. M Nagatomo & K Itoh, 1991, "An evolutionary satellite power system for international demonstration in developing nations", Proceedings of SPS91, pp 356-363.
10. P Collins, R Tomkins and M Nagatomo, 1991, ""SPS 2000": a commercial SPS test-bed for electric utilities", Proceedings of 26th Inter-Society Energy Conversion Engineering Conference, American Nuclear Society, Vol 4, pp 99-104.
11. M Nagatomo (ed), 1993, " SPS 2000 News Letter", No 7, Supplement in English, ISAS.
12. M Nagatomo et al, 1993, " SPS 2000 Working Documents", ISAS.
13. M Nagatomo (ed), 1993, " SPS 2000 News Letter", No 8, p 1-3, ISAS.
14. W Gaubatz et al, 1992, " Single stage rocket technology", Proc. 43rd IAF Congress, IAF-92-0854.
15. D Strangway, 1979, " Moon and asteroid mines will supply raw material for space exploitation", Canadian Mining Journal, Vol 100, No 5, pp 44-52.
16. P Collins, 1993, "Benefits of electricity from space for rapidly advancing countries", Proceedings of 5th ISCOPS, AAS in press.
17. Y Kuwano, 1992, " The new era of solar cells", Kodansha (in Japanese).
18. A Suzuki et al, 1988, " SPS is the next goal of commercial solar cells", Space Power, Vol 7, No2, pp 131-143.
19. D Koelle, 1981, " SPS transportation requirements - economical and technical", Space Solar Power Review, Vol 2, pp 33-42.
20. R Leonard, 1992, " Global rural electrification: a different race initiative", Journal of Aerospace Engineering, Vol 4, No 4, pp 290-309.