Reliability of Solar Arrays

Possible failure modes of solar arrays are discussed briefly. An open circuit due to a mechanical break in the circuit is considered to be the predominant failure mode. The effect of various combinations of open-circuited cells on current output (at a fixed terminal voltage) from a series-parallel interconnected array of solar cells is examined using a typical solar cell I-V characteristic. It is found that for certain practical types of interconnected arrays, the degradation of the modular components of the array is most likely to result in specific current output values from a module rather than in a continuous distribution of output values. A formula for the calculation of the probability of occurrence of modules with a specific current output value is derived. The multinomial probability distribution than allows the approximate calculation of the probability that the current output of the entire array is greater than some minimum required value. Finally, upper and lower bounds to the reliability of any interconnected array are derived.