Solar cells produced today are reliable. We have learned a lot since the Bell Laboratories developed the first solar cell more than 20 years ago. Solar cells have been used on such spacecraft as Sky Lab, and as the panels designed for Sky Lab show, the technology for producing large areas of solar cells is available. We have learned how to make thin solar cells and laboratory tests have shown that we can obtain high efficiencies. Today this Nation has the ERDA national photovoltaic conversion program, the objective of which is to develop within 10 years methods of mass producing solar cells so that costs will be 50 times lower than today. Solar cells incorporated in a satellite, such as the SSPS to produce 5,000MW, will require a large area. In a sense, it is unusual to consider such a large object in space—about 4 kilometers by 11 kilometers with a transmitting antenna 1 kilometer in diameter. In order to reduce the area of solar cells, we concentrate solar energy onto these cells with lightweight plastic film reflectors. The mass of this satellite—about 40 million pounds—is huge by any present-day standard. However, on a per-kilowatt basis, it is only about 8 pounds, which is a remarkably low unit mass for any energy production method on Earth, and thus the SSPS is resource conserving. We could transmit the power from the SSPS to earth by either of two methods: Lasers or microwaves. Both use the electromagnetic spectrum. In my view, lasers are not suitable, because of their low conversion efficiencies and because laser beams are absorbed by clouds. On the other hand microwaves can travel through the earth's atmosphere without more than 2 or 3 percent being absorbed, even in moderate rainstorms. Industry understands how to produce microwave devices. It is a large industry, producing devices in the United States, Europe, and Japan. The devices are efficient, they can be mass produced, and they are available now. We believe that we can design the SSPS microwave system to meet the most stringent international safety standards and not expose populations to microwaves. Thus, today there are many types of microwave devices which are articles of commerce. This microwave generator is mass produced. In space we can dispense with the glass enclosure and replace the permanent magnet with samarium-cobalt, indicating the type of device we would expect to use in the SSPS transmitting antenna. This is the view from earth of the transmitting antenna, showing several microwave generators shown. These devices are about 90 percent efficient—we can reject the 10 percent of waste heat into space by means of radiators. The transmitting antenna is based on the phased-array principle, which has been known and explored over the past 15 years. An example of the status of technology is the phased-array transmitting antenna five stories high, which Raytheon has constructed in Alaska. The elements have been proven to work, indicating that we understand very well how to make these kinds of antennas work today. The microwave beam is formed in the transmitting antenna and sent back to earth, where it is intercepted at the receiving-antenna
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