2.1 SILICON PLANAR 2.1.1 Introduction The silicon planar concept uses a large, modular array of silicon solar cells facing the sun. The sunlight is not concentrated nor is any attempt made to alter the spectrum. The cells are cooled by radiation from both surfaces. The cells are protected from harmful particle radiation by a thick window in front and a thick substrate behind. A modular box-frame structure maintains the shape of the solar array. Because the silicon planar concept does not require concentrated sunlight, it has an advantage over all other power conversion systems in that the SPS does not have to be precisely oriented to face the sun. Rather than being oriented perpendicular to the ecliptic plane, the SPS can be oriented perpendicular to the plane of its orbit. This results in reduced stiffness and control requirements, so the structure and attitude control systems are less massive than for other conversion systems. Several other studies have extensively reviewed the silicon planar option(l - 4). Most details presented here are based on the Boeing reference design(l,2). Lunar material substitutions for the cell covers, substrate, and cell interconnects are based on the studies by General Dynamics(3) and MIT(4,p3.5). Cell panels and the panel support system have been redesigned for better efficiency and reduced non-lunar mass. Silicon solar cells have been used for power production on satellites for many years. A 1984 Space Shuttle flight demonstrated a large solar array which is similar to the proposed SPS array. The vast amount of research that has been associated with silicon cells in space has resulted in a comparatively low technological risk for the silicon planar option. However, the useful lifetime of silicon cells in the GEO environment is questionable because repeated effective annealing silicon cells has not been demonstrated. High temperature silicon cells being developed now may solve this problem.(11) If repeated annealing cannot be effective, some redesign of the silicon SPS will be needed. Two possible changes are to use thicker cover glass and substrates, or to use a larger array area. Using a larger array is preferred, since doubling array size gives the same end of life (EOL) performance as increasing cover glass thickness by a factor of ten.(12) 2.1.2 Design Description The individual solar cells are 50 micron wafers of single crystal silicon fitted with aluminum contacts. These cells are connected into an 18 x 14 cell array which is placed between two plates of silicon dioxide glass to form a panel (Fig. 2.1-2). The glass serves to protect the cell from radiation damage as well as provide structural support. The 75 micron front plate is the optical cover and the 50 micron back plate is the cell substrate (Fig. 2.1-1). The optical cover is grooved to refract light around the grid fingers, increasing the effective efficiency by approximately 10X(2,p56).
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