present vehicles are designed to operate up to 10 years with their solar cell power source constantly collecting energy from the sun. The first use of silicon solar cells in an orbiting satellite was on Vanguard I, launched in March 1958. A one-watt radio transmitter, powered by solar cells, operated for several years before radiation damage to the cells caused the transmitter to fail. The Space Age brought solar cells from “just a curiosity” to application as a long-term power source for satellites. For space usage solar cells are highly cost- effective, since the weight to be launched per watt of continuously available power is significantly less with a power source that does not require fuel or other source of stored energy. At the same time the one-watt Vanguard solar panel was being designed, Lockheed engineers at Sunnyvale were working on the first high-power solar array for military spacecraft. As early as 1961 Lockheed Agenas were in orbit with solar arrays producing up to 1000 W. Lockheed has always been a leader in the development and use of satellite solar array power sources and very soon will flight-test the world’s largest solar array aboard the space shuttle. This paper includes a brief introduction into solar cell technology and will also discuss Lockheed’s contract with NASA to build and flight-test the array mentioned above. The paper will conclude with a brief description of our advanced development work where solar arrays producing multihundreds of kilowatts of power are envisioned. 2. SOLAR CELL TECHNOLOGY A photovoltaic cell is a solid-state semiconductor device that converts solar energy into electrical energy by using the photovoltaic effect. The most common cell used for space application is of planar geometry and is made from single-crystal silicon. Development of new techniques for concentrating sunlight and also “hardening” arrays for military missions has heightened interest in gallium arsenide (GaAs) solar cells which are discussed later. A variety of mechanical sizes and shapes with varying electrical characteristics is available, as shown in Fig. 2. Silicon cells are made from an “ingot” — a 3- to 6-in. diameter cylinder, about 12-18 in. long, of very pure silicon doped with an exact amount of boron, which makes it a “P” or positive-type silicon. Thin (8- to 12-mil) slices called wafers are cut from the cylinder, and phosphorous is diffused into the “top” surface, making an “N” or negative-type silicon. This creates a semiconductor junction across the entire cell surface area. Cells used for the important new terrestrial photovoltaics industry are usually left in this round shape; however, cells for space application are cut into square or rectangular shapes for space and weight efficiency. Metal is vacuum-deposited on both the “N” and “P” sides to provide the electrical contacts. When photons from sunlight strike the cell surface, a voltage is created across the “N”-“P” junction, hence the term “photovoltaic converter.” The electrical output characteristic is a current-voltage curve, as shown in Fig. 3. Current output is directly proportional to sun intensity and to the size of the cell. Voltage output is constant with any intensity level but is inversely proportional to temperature. Output degrades at higher temperatures. Solar arrays are made up by connecting individual cells in series to get the required operating voltage and in parallel to provide the needed current output. Systems are designed to make the solar cell operate as near as possible to the maximum power point, based on its operating temperature. Glass
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