- d.c. power generation - pilot beam generation • Grid interface • Rectenna structure - land, offshore • Launch sites and control • Logistics. SOLAR ARRAY BLANKET A solar array comprises a large number of interconnected solar cells mounted on a thin flexible substrate, usually reinforced kapton, to form a blanket. The individual cells have a thin bonded cover glass to protect the cell from radiation damage, and the interconnection system is either welded or soldered depending on the design environment. Power generation on long-life spacecraft has been almost exclusively the province of the silicon solar cell. Many photovoltaic materials have been examined but only silicon and more recently gallium aluminium arsenide (GaAlAs) have emerged as realistic candidates. The basic solar cell consists of a thin (250 /am) slice of processed silicon measuring up to 5 x 5 cm. The generated power is nearly proportional to the intensity of the incident sunlight and the active cell area. To form an array, cells are series connected to provide rows to give the required voltage and these rows are connected in parallel to provide current. The contemporary silicon cell has a conversion efficiency of about 14% at 25°. In orbit the cells usually work at higher temperatures which reduces efficiency by a small amount. Silicon cells are also subject to performance degradation due to proton and electron bombardment while in orbit, although the cover glasses help to reduce this effect. The foregoing technology has been adequate for state-of-the-art solar arrays, but it is labour intensive and costly when powers greater than 10 kW are being considered. It is important to note the large number of solar cells that are required by conventional space programmes, for example the Intelsat IV and Comstar programme required over one-half million cells for 16 satellites. The present total industrial output is about 200 kW per year which is equivalent to about 2.5 million cells. How then should the technology studies be categorised to meet the enormous challenge of the SPS array requirements. The studies fall into three main categories: • The establishing and understanding of the photovoltaic processes and devices • Design and development improvements in their application • Production methods leading to high volume production and low cost/watt. In the case of solar array blankets the first two categories have been well researched and developed, but the high volume/low cost production technology is an emerging and challenging field, and applies equally to silicon and GaAlAs cells. For the Reference System which uses silicon cells, the solar array area is about 50 km2 and the selected cell size is 7.44 by 6.55 cm and 50 /am thick with a 75 /xm cerium doped cover slide. The total number of solar cells per SPS is about 1010 which should be compared with the present capacity of all types of cell of about 2.5 x 108. The magnitude of the production technology improvement needed is clear. At present only single crystal silicon is being considered for the reference design, but there is ample evidence that the newer forms of silicon such as amorphous, polycrystalline, and continuous ribbon have much potential from a high-volume production standpoint, although at present their efficiencies are comparatively low.
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