A number of systems and subsystems have been suggested for collecting solar energy in space and transmitting electrical power to Earth. Each has certain benefits and drawbacks. OTA reviewed each of the major proposed systems and chose four representative concepts to carry through the assessment process. 1. The reference system (12,13). Large satellites in geostationary orbit (GEO) would convert solar energy collected on vast photovoltaic panels (55 km2) to electricity and use klystrons mounted on 1-km diameter antennas to convert it to microwave power. The resultant microwaves would then be beamed to receiving antennas on Earth and converted to either d.c. or a.c. power suitable for distribution in the terrestrial grid. In order to have a fixed technical basis for feasibility studies, DOE and NASA developed two slightly different versions of a “reference” satellite power system: One used solar panels composed of silicon photovoltaic cells, the other of gallium arsenide. The reference system represented a design based on the information available at the time, but it was not intended to be the last word in systems definition. 2. The solid-state sandwich design (14). In this configuration, microwave power would be supplied directly by solid-state devices “sandwiched” on the opposite side of the photovoltaic panel. The resultant antenna would be as large as the photovoltaic panels, making possible relatively smaller Earth-side receiving antennas. 3. The LEO-based laser design (15). Chosen because it represents a different basing configuration, as well as because it offers a radically different transmission mode, the power satellite would revolve in a sun synchronous low earth orbit (LEO). Power generated by photovoltaic cells and then converted to infrared laser beams or generated directly by a solar-pumped laser would be beamed to relay mirrors in GEO and thence to Earth. Infrared receivers on Earth would convert the resulting power to electricity for the grid. Because the diameter of the antennas is directly proportional to the wavelength of the transmitted energy, the laser option allows a small size for the space-based sending mirrors as well as for the terrestrial receivers. 4. The SOLARES mirror design (16). This option requires by far the smallest amount of mass in orbit. This alternative envisions using a number of large mirrors in LEO to reflect sunlight to Earth-bound stations where solar thermal or photovoltaic converters would transform solar energy to electricity for the grid. For convenience, we divided the assessment into four major areas: [1] technical alternatives and economics, |2] issues arising in the public arena, [3] institutional and international questions, and |4] the programmatic context, i.e., the place of SPS within our national energy and space programs. In addition to generating a number of working papers on the above topics, OTA convened three workshops: • SPS Technical Options and Costs. The major task of the workshop was to assess the DOE/NASA reference system from a technical perspective and to study alternatives to it. It discussed the key uncertainties of each major system or subsystem which has been suggested in the literature and chose four generic systems for further evaluation in later workshops. • SPS Public Opinion Issues. Participants with experience in analyzing and responding to a variety of public interests and concerns met to identify the major issues which could affect the public perceptions of SPS. The workshop sought a range of viewpoints from participants who have a sense of the issues, and of the political players and public attitudes involved.
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