radio frequency energy, and a Ikm-diameter, phased-array transmitting antenna to beam microwave energy to Earth at 2.45 GHz. On Earth, a 75 km2 rectifying antenna would receive the microwave energy, convert it back to electrical energy, and insert it into a terrestrial utility grid. Each system would provide 5 gigawatts of output on the ground, which is approximately the capacity of conventional nuclear power plants (in France Paluel Nuclear power plants = 4 X 1,300 = 5,200 MW). The reference concept required the development of a launch system which had a single mission delivery capability of about 400 tons to low Earth orbit (LEO). This payload of satellite components, building materials, construction equipment, and expendable supplies would be delivered to a LEO staging base. A transfer system using electric propulsion with ion engines would take the cargo from LEO to GEO. Once at GEO, the transfer vehicle would dock with a 5000-ton construction base and unload its cargo. The base would have a crew of about 400 people who could build an SPS in about 6 months. Following the checkout of the satellite system, the base would be moved on to another point in GEO where another satellite would be built. The analysis carried out in this study was based on the construction of two solar power satellites a year for 30 years. In such a plan, the heavy-lift requirement from Earth to LEO would be eight 400-ton cargo shipments per week, or about 400 per year. Personnel would shuttle back and forth from the Earth about 32 times each year on ships capable of carrying 75 to 80 passengers one-way. The crew at the LEO station would consist of about 135 people, with an individual stay time of about 3 months. In the 1979-1981 timeframe, capital cost estimates for the SPS concepts ranged from $1,200 per kilowatt to $7,000 per kilowatt, with most of the variance due to the cost of the photovoltaic cells and construction time. A capital cost of $2,000 per kilowatt, along with other operating assumptions, resulted in estimates that terrestrial electricity would cost 5.5 cents per kilowatt hour, which at the time was considered very competitive with alternative energy sources. Technological Developments New elements lead us to believe that future space power systems could be very different from and much more efficient than those developed within the framework of the 1979-1981 reference project. Power Generation and Conversion Photovoltaic Conversion Progress made since 1975 in the fields of photovoltaic cells has allowed the inlaboratory efficiency of monocrystalline silicon cells to be raised from 13% to 16%. It is expected that GaAs-based photocells using thin cascade multispectral layers will currently have an efficiency increase from 30% to 40% under concentration. In XnzTZ’z? /2,?<'Zin ZZ'ZZC’ 7lzn>z>lzvn»i/»tf
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