Calculation of Optimally Reliable Solar Cell Arrays

Present state-of-the-art emphasis has been placed on the use of silicon solar cells interconnected in series-parallel groups to form a solar array providing basic power for long lifetime spacecraft (perhaps greater than 3 months). To assure that sufficient power will be available to operate equipments during the specified mission time, a reasonable margin must be designed into an array to compensate the degradation of power output due to catastrophic failures and environmental degradation effects. The degree to which these effects can he compensated becomes atradeoff between weight, cost, and satellite lifetime - the time during which adequate power will be available to operate selected equipments. In this paper, the physics of solar cells, their operation, degradation due to radiation, and particularly random failure events are recounted. A generalized solar array configuration is presented and the effects of individual solar cell failures (in a random distribution) on the output are considered. A probabilistic technique for determining the distribution of failures is derived and the expected current output in a multiredundant solar cell configuration described. Some partical limitations are discussed throughout the paper and the analytical results applied to a sample array configuration.