This paper presents a reliability study of two solar-energy systems using air as the heat-transfer medium. Failure modes and effects analysis are used to examine the controls of the systems for three operating modes. Principal components are identified for each mode, and a fault tree and reliability block diagram are constructed to structure the fault or failed sequence. Established failure rates are assigned to each component, and an exponential failure distribution is assumed. For components operating on demand at a given average frequency, the adjusted demand and normal operating failure rates are combined to reflect the total contribution. Representative operational times and component frequencies are found for each operating mode. A fault-tree computer code is used to determine minimal outsets to the top event, remove redundant events, and perform necessary calculations. The computer code obtains results at each level (component, cutset, and tree) using repairable and nonrepairable models.

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