The SPS designs mentioned above have masses of about 50,000 metric tons and generate 5 GW (gigawatts, or thousands of megawatts) of electricity. Recently, therefore, another strategy for reducing SPS launch costs has been considered. Work at the NASA Lewis Research Center has shown that it may be possible to use thin-film solar cells deposited on a lightweight substrate along with solid state microwave transmitters. Since the entire area of the substrate can be covered with microwave transmitters as well as solar cells, large effective transmitting antenna diameters become feasible. Due to the physics of power beaming, the larger the transmitting antenna, the less the microwave power beam will spread as it reaches the Earth. Since the rectifying antenna ( rectenna) at the Earth's surface can now be made correspondingly smaller, the SPS need not supply as much power as a conventional SPS in order to be economical. (Indeed, it ought not to supply as much power, or else the microwave beam will become too intense.) With smaller SPS's having a lower mass per kilowatt of power generated, the system becomes easier to build and finance.

Current research at New York University, supported by SSI, aims to achieve the best of both approaches to launch cost reduction by using thin films and lightweight substrates built from lunar materials. The first step in the study was to consider two lightweight SPS designs suggested by Geoffrey Landis and Ronald Cull of the NASA Lewis Research Center: the "bicycle wheel" and the "inflatable sphere." The bicycle wheel SPS would consist of a disk-shaped solar cell/transmitter array stretched out over spokes, with a pole running through its center. Additional stiffness would be provided by guy wires running from the ends of the pole to the rim of the array. Since the array cannot directly face the Earth and the sun simultaneously, there will typically be about a 30% loss in power. This can be made up for by using a mirror that orbits along with the SPS. The mirror would track the sun and reflect light to the SPS, which would face the rectenna site on the Earth. Although this would increase the available power, and, perhaps more importantly, assure a more constant supply of power, the mirror would weigh about as much as the SPS, there by increasing the total mass required to supply a given amount of power by about 40%. In the inflatable sphere concept, the solar cell/transmitter array would be a giant balloon supported by a low-pressure gas. The mass of this gas would be quite small compared to that of the array. However, some of this advantage may be lost due to the fact that the array itself has four times as much actual area as cross-sectional area (due to the fact that the surface area of a sphere is four times its cross sectional area). It would thus have four times the mass of the equivalent bicycle wheel array, not counting the support structure of the latter.

In order to design a lightweight SPS, it must be kept in mind that if the entire array area serves as both solar collector and microwave transmitter, then as the array is increased in size, more power is being collected and squeezed into a tighter beam. Thus, a limit on beam intensity will set a limit on the size of the SPS. In this study, a peak beam intensity of 30mW/cm2 was assumed (the intensity near the edge of the beam is, of course, much lower). This is the same as in the Space Research Associates study mentioned earlier, and is not significantly different from the NASA/DoE figure of 23 mW/cm2. It is also only three times the US safety standard for human exposure to microwaves. Thus, as seen in other SPS studies, the microwave power beam is no death ray.

Another important parameter to consider is the frequency of the power beam. For a given transmitting antenna size, the higher the frequency, the narrower and more intense the beam. Thus, for high frequencies, the array must be kept small in order to avoid too concentrated a beam. For lower frequencies, a larger array is needed in order to keep the beam from spreading out too much as well as to supply enough power to make economical use of the land area at the rectenna site.

NASA researchers have investigated several different thin-film photovoltaic materials, such as cadmium sulfide (CdS), copper sulfide (Cu2S), copper indium diselenide (CuInSe2), and amorphous silicon (a-Si). The latter compares favorably with the other materials in terms of the efficiency at which it converts solar energy into electricity. All of these materials are radiation-tolerant, thereby eliminating the need for a protective cover glass. They are also low in mass, and therefore, inexpensive for the quantities that will be needed. Amorphous silicon is at a slight disadvantage in terms of light degradation (10 to 15% after two years, compared to, say, CuInSe2, which has none), but it is believed that this can be improved upon. Because of its favorable characteristics, and the fact that it is the only thin-film photovoltaic material available on the Moon, amorphous silicon served as the basis for this study for both the terrestrial and lunar SPS. An efficiency of 11.5% was used, based on NASA projections of achievable efficiencies for the 1990's. The material considered for the substrate for an SPS built of terrestrial materials was Kapton polyimide. This is the same material used to make rectenna substrates. Steel foil was the substrate of choice for an SPS built of lunar materials. The thicknesses considered were based on NASA figures for advanced substrates, and were 7 microns for Kapton and 7.5 microns for steel foil.

A bicycle wheel SPS using thin-film technology will have a diameter of just over 4 kilometers for a power beam frequency of 2.45 gigahertz (this is the frequency of the NASA/DoE reference design). It will supply about 450 megawatts of power to consumers. The mass of the solar cell/transmitter array (including the substrate, but not the support structure) is just over 200 metric tons if Kapton is used for the substrate (terrestrial materials), and just under 800 tons if steel foil is used (lunar materials). The effect of increasing the frequency was also considered. A 10 GHz power beam yields a bicycle wheel SPS that has half the diameter, and thus one-fourth of the array mass and power level as the 2.45 GHz design; i.e., roughly 2 km, 50 tons (terrestrial) or 190 tons (lunar), and 110 MW. A bicycle wheel with a mirror, or an inflatable sphere will be 8.5% smaller in diameter and supply 19% more power than a conventional bicycle wheel, due to th e elimination of the tracking loss. Higher frequencies will yield even smaller, more easily constructed SPS's, but the amount of power for their size will be lower, since higher frequencies are subject to rain and air attenuation, and solid state microwave transmitters are less efficient at higher frequencies.

Preliminary research in the design of the support structures indicates that they can be built from lightweight materials available both on the Earth and on the Moon. A likely terrestrial material is a graphite organic matrix. A likely lunar material is a glass/glass laminate. Both materials are light in weight, with the graphite organic matrix being somewhat lighter than the glass/glass laminate. These two materials have opposite coefficients of thermal expansion. They can be combined to yield a material with zero coefficient of thermal expansion. Only a small amount of glass needs to be added to the graphite to achieve this, making this a feasible approach for a terrestrial SPS, but not for a lunar SPS, since graphite is not available on the Moon. Thus, a lunar SPS supported by a glass/glass laminate structure may have a problem with thermal expansion. However, the design of the support structure is presently in its early stage, so it is not yet known if this is a serious problem.

Dr. Peter Glaser, the originator of the SPS concept, has called for a "terraced" approach to SPS technology development. In this approach, smaller projects would pave the way toward a full-scale geostationary SPS. One possible small or mid-sized project would be an SPS in low Earth orbit beaming power to a rectenna near the equator for a small portion of each orbit. (One such project is the Japanese SPS 2000 program.) Using thin-film technology, an SPS in low Earth orbit may be light enough to be launched by a single Space Shuttle mission. For a bicycle wheel SPS orbiting at an altitude of 1200 kilometers, beaming power at a frequency of 10 GHz, the diameter would be 340 meters, the power available to consumers would be 3 MW, and the array mass would be an amazingly low 1430 kilograms (though the support structure would increase the mass). The full capacity of such an SPS can be exploited if a series of SPS's and equatorial rectennas are spaced such as to enable a given rectenna to lock onto the next SPS after the previous one has disappeared from view. The previous SPS would then lock onto another rectenna, further east.

If support structures can be designed which weigh about as much as a photovoltaic array built from terrestrial materials, then overall specific power levels of about 800 to 1000 watts per kilogram may be possible for a terrestrial SPS. Since the NASA/DoE reference SPS had a specific power level of about 100 watts per kilogram, the potential improvement is considerable. In addition to the improvement in specific power, a thin-film SPS would be smaller in overall size than the NASA/DoE reference design, thus making each SPS a more easily financed step in the growth of a national or international power system. A thin-film SPS built from lunar materials would lose some (but not all) of the specific power advantage; however, the lowering of launch costs may make this worthwhile, if a lunar infrastructure can be constructed and operated economically.

To date, thin-film solar cells have been produced in relatively small modules at manufacturing volumes far below that required for SPS construction. The substrates commonly used are not lightweight. Research in depositing thin film solar c ells on lightweight substrates is only just beginning. However, the promise of thin-film technology, combined with future world energy needs, suggests that it is worthwhile to develop manufacturing technologies which would allow thin-film solar cells to be deposited on lightweight substrates and produced in large quantities.

Seth D Potter was a Research Scientist in Physics at New York University when this article was written. He is currently an engineer at The Boeing Company in Seal Beach, California, USA, and serves on the Board of Directors of the National Space Society Education Chapter.