Dr. R Bryan Erb

The problem is that current practices for supplying the earth with energy are not sustainable in the long run. The great bulk of our energy comes from fossil fuels of which there is a finite supply and the use of which is having adverse effects on human and planetary well-being.

Nonetheless, world demand for energy is growing rapidly. Since greater supplies from our present sources are made available only at increasing cost and, most often, with even greater environmental impact, we must seek new sources that will be sustainable, economical, and which will not exacerbate environmental damage.

World energy use* has been growing rapidly, especially during the last 100 years, and is currently running about 12 terrawatt years per year** (Figure 1). Per capita use varies from over 6 kilowatt years per year in the highly industrialized countries to less than 1 kilowatt year per year in many developing countries.

Most of this energy, currently around 88%, comes from the combustion of fossil fuels (Figure 2).

Energy use is the product of two factors:

1. population, which is growing steadily; and
2. per capita consumption, which is largely driven by economic activity

Population studies indicate that there will be 10 billion of us by the year 2050, whereas today we number around 5.4 billion*. Thus, on a population basis alone, making no provision for economic growth, energy consumption could be nearly doubled by the middle of the next century.

Projected rates of consumption vary widely depending on the assumptions made and the region or country being considered. Typical projections are shown in Table 1, and these projections are used to depict graphically an envelope of possible energy-use futures (Figure 3).

The lowest projections are from a study (3) which postulates very significant and successful conservation efforts in the industrialized countries that cut present per capita consumption in half. Furthermore, it assumes the rapid introduction of highly efficient technologies in developing countries as their economic activity increases. This scenario is very optimistic.

Basically, the energy problem is that, by the middle of the 21st century, some 20 to 30 TW will be necessary. That is, we need to increase the present supply by a factor of two or three, or by at least an additional 10 TW. Is this feasible?

Earth can certainly supply more energy. However, we have been living on an energy legacy from the past and have already used much of that which is easily obtained. We now face problems of availability, environmental damage, and cost. The next part of the paper will examine the ability of each of the present earth-based sources to meet the needs of the mid 21st century with regard to these considerations.

A detailed treatment of energy costs is beyond the scope of this paper but it is clear that continued reliance on fossil fuels will result in increased energy costs in the future. For example, as the easily-found oil is used up, other sources such as deep offshore oil, tar sands, heavy crudes, and shales will be exploited, however, only at substantially higher costs.

The ability of current sources of energy to serve us adequately in the future is one concern. The effect on the well-being of the planet which continued use of some of these sources will have is another. The feasibility of current sources to provide the additional energy needed by the middle of the next century and the limitations associated with such sources are given in Table 2.

The rather pessimistic view presented in Table 2 can correctly be criticized for assuming that each source would have to bear the whole burden of the additional supply. Alternatively, one could ask "what mix of sources could be invoked to meet the need?" To pursue this question one must estimate the extent to which fossil fuel can continue to be used* without permanent damage to the planet and then examine other contributions.

Two approaches to this question are illustrative. Hoffert and his colleagues (7) have simulated scenarios in which the use of fossil fuels is curtailed enough to limit warming during the next century to 0.5 deg. C. To accomplish this, a 1% per year decrease in fossil fuel use would have to be implemented immediately. Another approach by Dessus and Pharabod (5) looks at options which would limit CO2 production from fossil fuel to a level the earth is believed capable of accommodating. Their scenario allows a slight rise in fossil fuel use (mostly natural gas) during the early years of the next century followed by a steady decline to about 2/3 of the present levels by 2100. These "allowable" fossil fuel futures are illustrated (on the same scale as that for the historic use) in Figure 4.

The likely extent of the problem can now be determined by comparing the total energy need and the portion which can be supplied by fossil fuels if we accept that the use of these fuels must be strictly limited. A gap is projected (Figure 5) as the difference between this author's estimate of a best-case scenario which might come about and the higher "allowable" shown in the previous figure.

Fossil fuels, then, will continue to provide part of our energy needs. Furthemore, nuclear and hydroelectric power sources can and will contribute. However, we will still need new sources of clean energy to fill a gap on the order of 15 TW by the year 2050.

What are the alternative terrestrial sources that might meet the demands of the next century? Many options have been suggested but most have significant limitations. Table 3 summarizes this author's assessment of the contributions which might come from various sources. Another assessment, interpreted from reference 5, is shown in Figure 6.

If solar power captured on the earth is a possibility, given all the problems that clouds and the day-night cycle impose, then perhaps solar power captured elsewhere is even better. And, if nuclear fusion is a possibility with earth-based fuels, perhaps alternative fuels from space would offer advantages. These thoughts lead to the consideration of power sources in space.

Space-based power can be obtained in at least two ways:

• capture solar power directly and beam it to earth.
• obtain fuel (for fusion) from extraterrestrial bodies.

A recent study (8) has assessed three leading possibilities, two direct approaches in which pure energy is imported, and one indirect approach in which fuel is imported for use on earth.

The Solar Power Satellite ( SPS)

The Solar Power Satellite concept was proposed years ago by Peter Glaser. It involves large collectors in orbit around the earth capturing solar power which is then converted into microwave form, beamed to the earth, and converted back to useable electrical power at rectifying antennas. A great amount of detail on this concept has been developed (9). Studies of the economics of such systems show promise, at least when lunar materials are used in construction of the satellites.

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.

Most of the work on fusion energy to date has focussed on Deuterium - Tritium reactions. There are other altematives and an attractive one uses Deuterium and Helium-3. The advantage with D-3He is that the primary reaction produces almost no neutrons, greatly reducing the damage to the reactor walls. Reactor walls can thus serve for decades, instead of for a year or two. Furthermore, the severity of the contamination is much lower, and this can simplify the disposal problems when the reactor reaches the end of its useful life.

While the D-3He reaction has been researched somewhat, it was only when the presence of 3He on the moon was noted that serious consideration was given to this source of power. Clearly mining on the moon and transporting large quantities of 3He to earth is not a trivial matter. Nor is the commercial use of such a fuel by any means assured. However, it is an interesting option, perhaps for the latter half of the next century when a substantial infrastructure is in place for operations on the moon.

It is not clear which of the possible space-based systems can best provide energy for the earth in the next centuiy. However, each appears to offer a very large and essentially pollution-free source. Detailed studies, technology development activities, and demonstration projects should make the path clearer over the next few years.

Note: A concept that uses space-based equipment to transmit power generated on the earth from the location of production to the location of need has been proposed by Angelini (10). He notes that much hydroelectric power remains to be developed but that promising locations are far from the prospective consumers. And he proposes the intercontinental transmission of power using orbiting microwave reflectors. Researchers in India also suggest this approach as one way of transferring Himalyan hydroelectric power to the more densely populated areas of the country.

Interest in space-based power has gone well beyond theoretical studies and there is a substantial experimental base on which to build. This section will describe: previous work, projects currently planned, studies for demonstration systems, technology activities, and studies for operational systems.

One element of the technology needed for space-based power is in actual use, specifically, photovoltaic (PV) arrays for satellite power systems. This is a very welldeveloped technology.

The critical technology of microwave power beaming has already been demonstrated in limited ways. Two important tests were:

• Microwave power beaming between ground stations which was demonstrated (11) at power levels of 30 KW. Overall efficiency was 54%.
• The flight of an unmanned aircraft (12) using power beamed by microwave from the ground (Figure 7).
  
  Figure 7. SHARP Stationary High Altitude Relay Platform
• Japan has conducted a rocket test (13) called MINIX (Microwave Ionospheric Nonlinear Interaction eXperiment) to study the interaction between microwave power beams and space plasma.

Several projects are underway to demonstrate further various aspects of space power technology. These include:

This is one of Japan's projects for the International Space Year (14). Sponsored by the Institute of Space and Astronautical Science, it will build on the 1983 MINIX test and is planned to operate ata power level just under one kilowatt. Like MINIX, this is a rocket experiment in which a daughter section separates from the mother section and provides the receiving element of the system. A microwave beam at a power just under one KW will be transmitted using a phased-array antenna deployed in a cruciform shape. In addition to control of the power beaming, an objective of the test is to investigate further beamplasma interactions.

Figure 9. SPS 2000 Demonstration Space Shuttle Experiment

The SPS 2000 is another Japanese project, planned for launch by the end of the century (15). Features of SPS 2000 include:

• Low equatorial orbit (1000 kilometers)
• 3 kilometer diameter beam footprint
• A power level starting at 300 kilowatts with growth to one megawatt
• Potential for evolution to 10 megawatt
• Ground facilities in developing countries

NASA and one of their Centers for Space Commercialization (at Texas A & M University) is planning to fly a Shuttle-based power beaming experiment within the next few years.

Many organizations are doing concept studies of space power systems and their related technologies. Several are doing technology development work as well. Representative efforts include:

Known as GSEK (Global Solar Energy Concept), this project (16) would demonstrate on-orbit assembly

of large, flexible structures and explore biological and atmospheric interaction aspects of power beaming.

The system would:

• Operate at one megawatt
• Be launched on Arianne in 2005 to low earth orbit
• Be assembled by the Hermes crew
• Later be boosted to geosynchronous orbit using electric propulsion

This proposal would promote the use of energy from space for developing countries to help them achieve sustainable economic growth (17). The notion is to seek mainly private funds and to use simplified approaches for an early, low-cost demonstration at the 100 kilowatt level that would enhance the marketing of larger operational systems.

These proposals include building a small-scale solar power satellite (250 - 500 kilowatts) to serve, initially, remote areas in the arctic (18, 19) and includes technology development for:

• Light concentrators and thin-film optical reflectors
• Orbital radioreflectcrs
• Combined phototransforming and microwave emitting systems
• Phased antenna grids and receiving aerials
• Highly efficient lasers
• Energy accumulators of low specific mass

Several concepts have been put forward to build space power systems to serve satellites as a profitable step in building coward space-based power for earth. This would enable initial installations to serve those users who are used to paying $800 per kilowatt hour (rather than those paying $0.10 per kilowatt hour.) A typical example is the study by Toussaint (20) that includes the following steps:

• Space-to-space power-beaming tests in low orbit
• Building a "Powersat" which would serve a variety of satellites with power, providing the total need, an augmentation, or a backup
• Building a megawatt-class plant to demonstrate transmission of power to earth
• Placing a 5-megawatt "pre-operational" plant in geosynchronous orbit

There is substantial experimental activity including:

• Lasers forpower beaming
• Cooling techniques for magnetrons
• Photovoltaic collectors of greater efficiency (25%)
• Stacked solar cells to capture multiple wavelengths (efficiencies over 30%)
• Self-annealing cells which will be stable in space
• Solar dynamic systems
• High frequency microwave generators (higher than 140 gigahertz)
• Alternatives to rectennae
• Designs of high-frequency rectennae (35 gigahertz)
• Large deployable structures for space

Finally, there are some concept studies that address the longer term:

• Glaser proposes stepwise construction according to what he calls a "Terracing Strategy", (Figure 12)
• An ESA study proposes building a European Space Station, viewed as an essential step leading to the moon and supporting, among others, the strategic goal of lunar-based space industrialization. This in turn would lead to space power for earth, either from an SPS built using lunar materials or from 3He mined on the moon.
• America's Space Exploration Initiative suggests an "Energy to Earth Waypoint" as a step in the moon Mars exploration. This waypoint is a set of lunar surface activities that would provide energy to the earth either by mining 3He or by collecting and beaming solar energy to earth.

Basically, some good work has been done in developing the concept and the technology for providing space-based power to the earth. More is in progress or planned. Nevertheless, the possibilities are not well known outside a very narrow community. What should be done next?

The aerospace community should take the lead in an international effort to develop space-based solutions for the earth's energy needs. The strategy for this effort will include: promoting awareness, sharpening the feasibility studies, building a constituency, developing a technical plan, developing an organization and a business plan, building test beds and carrying out demonstrations, and implementing the system(s).

The aerospace community should lead in coming to understand the energy problem and the role aerospace can play in its solution. All of us should communicate this understanding to those with whom we come in contact so that space-based solutions become part of the common vocabulary and subject to wide discussion.

Individuals and organizations should continue to carry out detailed systems studies to assess the feasibility and to estimate the likely costs of the various space-based systems and alternative sources. It will be important to project:

• Over what time frame it will be economical to develop and use such systems
• What technologies will be needed and how these can be developed and proven
• When these technologies can be brought to the appropriate state of readiness

As awareness increases, support should be sought from various communities to promote the concept of clean energy from space.

Various interests should find this concept appealing for very practical reasons:

• Consumers because of potentially cheaper power
• Environmentalists because it can limit the use of fossil fuels
• Electric utilities because it can serve for the indefinite future
• Constructors and developers as an opportunity to build rectennae
• The space community as an opportunity to develop major lunar and orbital infrastructure
• Industrialized countries that provide aid to developing countries as an opportunity to foster economic growth by providing the space segment
• Developing countries as a path to leapfrog the dependency previous growth paths had on fossil fuels

The societal implications of space-based power are enormous. Some will be seen to provide clear advantages e.g., those that will stem from affordable power for developing countries that have, historically, been resource-poor. Others may be viewed as drawbacks unless we are wise and very effective in garnering public acceptance of the new technology. For instance:

• The transmission of microwave power through the atmosphere will be alarming to some. The ability to point and control the beam must be convincingly established and effects on the atmosphere and possibly on earth life must be accurately described (21).
• If 3He turns out to be practical, some of the concerns presently voiced about conventional nuclear facilities are likely to be carried over to fusion systems. Hopefully, we will be able to learn from the experience of the nuclear fission industry and acquaint people with the relative safety of fusion

In any event, the hazards of space-based systems must be carefully compared and honestly contrasted with the hazards of continuing our present modes of supplying power.

If the analytical studies continue to show promise and no technological show-stoppers are identified, the next step would be to develop an organization and a business plan to implement space-based power systems. Certainly demonstrations can be done on a regional basis, but any comprehensive use of space-based power will be a major international effort. There are previous large international ventures that could provide examples for organizing and implementing such an effort, e.g., major engineering works like the Channel Tunnel and international ventures such as Intelsat. It is important that all sectors, private, governmental, academic, and special interest groups, share in the planning and execution of the venture.

We should support the presently-planned demonstrations and assess what further testing is necessary. It will be important to address not only the technical and engineering issues, but also the development of enough information to allow practical designs at modest scales and accurate cost estimates for operational systems. The track record of estimating costs for radically new power systems is very poor.

The results of the previous steps clarify the path along which space-based energy systems should be developed. The details of the implementation cannot be prescribed at this early stage but it is believed that such a venture will be within the capability of the technical community by the early decades of the next century. The investment required will also be a challenge but is not unreasonable in the light of current expenditures to provide energy, e.g., 10% of GNP for electricity in the USA.

This paper has examined the aspects of space-based power for the earth, the significance, the need and possible solutions and finds that

• Present energy-supply practices cannot continue indefinitely
• Economic development and a growing population will, by the middle of the next century, combine to require at least twice the energy we use today
• Conservation, improved efficiency, and use of earthbased renewable sources are all important, but are unlikely to yield an adequate supply
• Importation of energy from space could make up the shortfall in an environmentally-benign way
• Initial studies and technology demonstrations have shown that space-based energy supplies are feasible with technology that is now or soon can be available
• A plausible strategy to obtain a sustainable energy supply for the earth has been described
• Implementation of space-based energy systems over the next several decades is within the capability of the world's technological community
• The organization, management, and the garnering of public support for such a venture are probably greater challenges than the technical aspects

The conclusion is that space-based power can provide the necessary new source of clean energy for earth in the next century to loosen our dependence on fossil fuels. This will need to be done in conjunction with improving energy use and expanding the use of terrestrial renewable sources as appropriate for each country or region. An international effort will be needed for any space-based system and many segments of the world's business and technical communities must be involved.

A significant part of the enterprise presents an opportunity for the aerospace community which is the logical leader for the near-term work. We need to promote this concept as an idea whose time has come.

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