Concentrator/Ref1ector The concentrators used in the reference system are Kapton 12.5 uM thick with a highly reflective layer of aluminum 10 pM thick. It was assumed that the reflectivity of the concentrator will be 0.85. Thus, to achieve an actual concentration ratio of 2X, a geometric ratio on the order of 2.15X will be required. Reference System Performance Reference system performance as shown in Table IV-B-l-b-1 is based on the use of an average 16% efficient (30aC) silicon solar cell. The predicted steady state temperature of 100°C under a concentration ratio of 2X as selected for the reference system. At this temperature the basic cell efficiency is predicted to be approximately 11%. With buildup and other system losses the realized performance will be approximately 10.3%. Each cell will have an output voltage at maximum power on the order of 0.32 volts and a 4 cm x 4 cm cell will have a current output of approximately 1.4 amps/cell (in 2X concentrated system) that will yield approximately 240 watts/M2 of active blanket area. Weights (Blanket and Concentrator) Table IV-B-l-b-1 shows a breakdown of blanket and concentrator weights for the reference system on a per unit and system basis. Also shown are the impacts of cell and cover thickness variances. From this table it is seen that the reference weight of approximately 0.4 Kg/M2 for the blanket is not the absolute minimum achievable should a thinner solar cell be available in the SPS time frame. In this study it is considered that the minimum achievable solar cell blanket weight is something in excess of 0.31 Kg/M2 and that a reasonable maximum would approach 0.5 Kg/M2. The concentrator material previously described will have a weight/unit area (actual area) on the order of 0.04 Kg/M2. Degradation Two major causes of solar blanket degradation are expected — radiation damage and damage due to thermal cycling. The radiation environment associated with geosynchronous orbits is not too severe. Thermal cycling degradation will possibly be greater than radiation unless a blanket system design can be devised which accommodates the severe thermal cycles. In flexible substrate arrays of the type to be used in the SPS the thermal time constants will be very short. It is conceivable that the blanket will swing from its steady state temperature of 100°C to a low level (-100°C) in approximately one minute. Under these conditions tremendous stresses can be generated in the interconnect, cell, weld, and cover interfaces. If, through design, these can be dealt with, the total expected degradation of the array should be on the order of that currently experienced by satellites in geosynchronous orbits; i.e., approximately 6% in first five years and probably 1%/year thereafter. (31% in 30 years.)
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