A difficulty with proposals for satellite solar power systems is the absence of a plausable evolutionary pathway to development of systems on the scale required. One possible pathway is discussed, where the required technologies are developed and refined on an incremental scale. The initial stages of the process are development of ground-based photovoltaic power and of beamed power systems for space use.
Starting with Peter Glaser's initial 1968 proposal , many people have discussed use of the satellite solar power system [SPS] as a means of supplying energy to the Earth to replace fossil fuel sources. The recent prominence of the "greenhouse effect" from burning of fossil fuels has again brought alternative energy sources to public attention, and the time is certainly appropriate to reexamine the SPS.
The barrier to development of SPS is social, not technological. The initial development cost for a SPS would be enormous: e.g., 102 billion [1977 dollars] for the first 5 GW unit [2,3] (possibly somewhat lower for some alternative concepts), and the construction time would be long. While it is often argued that production of solar power satellites could eventually be a profit-making commercial venture, the high initial cost and long development time presents a large barrier to commercial involvement. Thus: how can we get there from here?
The problem is that possible risks for such a large project are very large, and there is an understandable reluctance to committing enormous amounts of financial resources to a project with uncertain pay-off. The pay-off time is long, and thus fear of technological obsolescence is high. Electricity demand may be inaccurately forecast, or alternative, lower-cost generating technologies may be developed during the time required to develop and construct the system. SPS must overcome the negative experiences with large projects of the nuclear power industry, which invested heavily on long-term, large capacity projects and discovered that projected use did not materialize, while costs and environmental objections ballooned.
A significant problem with SPS is that it is inherently a large project. The optimal position is in geosynchronous earth orbit (GEO), far from the user. The physics of antennas mean that a SPS system cannot be made small, since the beam spread at target is inversely proportional to antenna size. Positioning of a SPS in low orbit is much less desirable, due to the high required antenna slew rates and the longer power-out time spent in shadow. To develop a large program such as SPS, it is necessary to find a path that involves step by step progress, with immediate pay-offs at each step, and with experience gained at each step to refine and improve the technology in evolutionary steps. This paper is an attempt to define such a path.
The first step is to demonstrate power production with ground-based solar arrays. A significant risk element for any satellite power system is the photovoltaic array. This was identified in the NRC review of SPS  as one of the most critical areas where extrapolations from current technology in terms of cost and performance were made.
Proponents of SPS often disparage the potential use of ground-based solar energy, possibly considering ground-based systems as a competitor. Nothing could be further from the truth: ground-based and satellite-based solar power are complementary technologies, and satellite-based solar power will only be economically viable if terrestrial power is.
Experience with ground-based solar power is a necessary step to shake-out the technology, define and trouble-shoot the manufacturing technologies, and move photovoltaics down the learning curve to low-cost production. Many ground sites exist in the U.S. with over 300 clear days per year.
Flat-plate photovoltaic systems will also provide significant power during overcast days. The difficulty with terrestrial solar power is that it provides power only during the daytime. Ground-based solar power is viable due to a fortuitous match between daytime peak requirements and production . Figure 1 shows a comparison of the generation profile for a photovoltaic system compared with the load profile of a utility . The curve shown is for mid-summer in southern California, when power-requirements are typically highest, and the peak loads are in the daytime (when solar is most available) due to loads imposed by air-conditioning. Utility-generated power can be usefully considered as divided into two categories, base load and peak load. The cost of generating peak power is higher than that of base power; and as long as the generation profile is primarily providing peak power the marginal benefit is high. Several analyses have shown that for generation fractions of up to about 20% of the current U.S. production, photovoltaic power generation can provide primarily peak power without cutting into baseline power. Above about 20% penetration, however, photovoltaic generation begins to displace base capacity. This is a double liability, since the power displaced is produced at low cost, while high-cost peaking power must be generated to provide a level base during the times when the solar power is unavailable.
However, 20% of the U.S. power generation capacity is a huge amount, and large amount of growth in the solar power industry is possible. An additional advantageous feature of terrestrial photovoltaics is the short construction lead-time required and the ability to add capacity in small, modular increments. As shown in the experience with the Carissa Plain field , it is possible to build a photovoltaic field in a year or two. Even the production factories to build arrays can be built relatively rapidly. This allows photovoltaic installations to avoid the uncertainty of forecasting power requirements far in advance, and also allows rapid progress down the learning curve.
As yet it is still too early to chose a technology for SPS; even the basic selection of a photovoltaic technology versus a thermal dynamic system is not clear. Among the photovoltaic technologies, many different approaches are still in consideration: the recent special issue of IEEE Transactions on Electron Devices reviewing recent advances in photovoltaic technology  had papers discussing ten different photovoltaic materials, none of which could be ruled out as a competitive choice. A significant goal of ground-based power is thus for the competition among various technologies to shake-out the lowest cost approach.
One of the leading flat-plate photovoltaic approaches is the use of thin-film photovoltaic materials such as amorphous silicon or copper indium diselenide. Coincidentally, such thin-film materials are inherently radiation tolerant and have the potential for being manufacturable on thin, light-weight substrates. Such materials could be ideal for space use . Current photovoltaic module production is about 30 MW(peak)/year. Cumulative production of several tens of Gigawatts would be required for photovoltaics to reach the technological maturity required for finalizing a SPS design. At an conservative industry growth rate of 10%/year, this is likely to take twenty years. The faster the demand for terrestrial PV grows, the more rapid the technology maturation will be.
Having gained valuable experience with solar energy, when the solar generation market share begins to saturate demand for peak power, utilities will begin to search for a solar-energy alternative that provides continuous power. At this point the SPS system should be ready to step in. SPS readiness, however, requires technology readiness of the other critical SPS element, large-scale power beaming.
The advantage of making in-space power application the initial use for power beaming is that the effective cost of power in space is considerably greater than on Earth.
There are two major opportunities for beamed power in space. The first opportunity is to replace the batteries which are required on solar powered satellites to provide power during the eclipse portion of the orbit. For low-earth orbit (LEO), the eclipse typically runs for about 35 minutes of the 90 minute orbit. A significant market is the commercially-important geosynchronous earth orbit ; for GEO the eclipse is confined to a short (70 minute) daily period centered on midnight near the vernal and autumnal equinox. A single power station might be able to provide power for several such GEO satellites. Such power stations could be satellite-to-satellite, or could also be Earth to satellite.
Such a power system, "an electric utility for space," has been discussed in some detail by Grey and Deschamps . In principle this space power utility is the nucleus of a SPS.
Providing power for a lunar base  or to roving exploration parties  on the moon might be another application of beamed power. Night power for a photovoltaic powered moon base is an important consideration ; such power could well be provided by beamed power systems. (However, it should be noted that such beamed power systems, although being studied by NASA [12,13,14], are not an element of current baseline plans for a lunar base.) The second opportunity for beamed power in space is for orbit-to-orbit transportation by electric propulsion. This has been discussed, for example, by Brown  and Faymon . Space transportation systems typically deliver payload into low orbit; raising the orbit to commercially valuable orbits such as GEO is done by an orbital transfer vehicle.
Clearly, the higher the specific impulse of the orbital transfer vehicle, the less propellant mass is required to be brought to orbit. Electrically-energized rocket engines such as the ion-thruster or magnetoplasmadynamic thruster have the advantage of extremely high specific impulse, and thus low propellant usage (or, equivalently, high payload fraction); the disadvantage is that they have correspondingly high power consumption (in fact, the power consumption is proportional to the specific impulse squared). Use of beamed power is likely to evolve from other applications demonstrating the applicability of electric propulsion to a wide variety of missions. Initial applications are for station-keeping for geosynchronous satellites; slightly further term applications may be solar-electric propulsion for planetary probes.
Since the advantage of high specific impulse is diluted if the vehicle must carry a heavy power system, electric propulsion provides a natural application for beamed power. An additional advantage of transportation use for beamed power is that continuous power is in general not required. The thrusters are used when power is available, and can be turned off when the power is unavailable.
By maintaining an aggressive policy of pursuing applications of beamed power in space, the technology of power beaming can be commercially ready by the time that photovoltaic technology has been brought to technological maturity. These two technologies will be sufficient for SPS construction, however, only if the third element is in place: large-scale space infrastructure.
Development of SPS will require a large infrastructure for space transportation and space construction. This will present a large risk element unless the transportation infrastructure is developed and tested well before commitment to a SPS. The transportation requirements will be orders of magnitude more than needed for known commercial applications such as communications satellites. A significant boost would be identification of near-term, large-scale commercial applications of space*. Pending such an as-yet unknown commercial application, however, I see little prospect for commercial space enterprise to develop transportation on the scale required.
The SPS infrastructure is thus dependent on development of the required space infrastructure by space-exploration missions conducted by the various national governments of Earth. Any of the various manned missions proposed in the near term (space station Freedom, return to the moon, manned Mars mission, "Mission to Planet Earth") could provide elements of the necessary experience. Various unmanned missions, such as planetary probes (e.g., Cassini) and exploratory missions to the smaller bodies of the solar system such as asteroids and comets (CRAF) contribute little to the transportation infrastructure needed, although they are important preliminary elements to the long term exploitation of space resources.
An aggressive planetary exploration policy has additional long-term applications to SPS. The projected cost of a SPS could be considerably reduced if extraterrestrial resources are employed in the construction . One often-discussed road to lunar resource utilization is to start with the mining and refining of lunar oxygen, the most abundant element in the Moon's crust, for use as a component of rocket fuel to support the lunar base as well as exploration missions. Once the mining and refining process is in place to produce oxygen, the next-most abundant elements, aluminum and silicon, can be refined to produce solar arrays . Such lunar- manufactured solar arrays could have many applications (figure 2): not just to support growth of manufacturing capabilities on the moon, but also in LEO, GEO, and to support planetary missions, as well as to support solar-electric inter-orbital transportation and to serve as primary power supplies for the beamed transportation systems discussed in the previous section.
Thus, with the development of the component parts of a mature photovoltaic technology, beamed power for in-space use, and a space infrastructure, the implementation of a solar power satellite consists only of integrating the pieces.
*space tourism has been suggested as one such application.
An evolutionary pathway, where each technology is developed and brought on line in a way to minimize risks, is a necessary prerequisite to development of commercial satellite solar power systems. The program suggested, large-scale photovoltaic application on Earth, beamed power in space, and construction of a space infrastructure, could lead to development of all of the elements of a satellite power system and demonstration at the appropriate large scale. This would reduce the risk associated with a project at the necessarily large scale of SPS sufficiently that it may be reasonable to consider such a project as a commercial venture.
Figures not available in electronic copy
Please see Space Power, Vol. 9, No. 4, pp. 365-371