Reliability Terminology and Formulae for Photovoltaic Power Systems

Photovoltaic (PV) solar energy systems operate in a unique environment compared with most electronic and electrical power generation systems. The input stimulus, solar radiation, is both highly variable and noncontrollable. This presents a certain amount of difficulty in measuring and reporting reliability, maintainability, and availability characteristics. Many of the terms, definitions, and performance indices employed in other fields do not directly apply and need to be revised (tailored) to the PV technology. This paper proposes reliability and availability terms, definitions, performance indices, and mathematical expressions. The rationale for their relevance in photovoltaic solar energy technology and applications is given. There is a need to provide uniform terminology and formulae, for effective communication. Such communication promotes the orderly development of a uniform and effective reliability methodology for PV systems and components. The material draws extensively from ANSI/IEEE Standard-762, which defines reliability, availability, and productivity terms for electric power generation systems. This paper extends the standard terminology to include PV power systems. The unique characteristics of the PV array and the variability of input energy are described. The language will grow as PV technology and associated reliability and maintenance methodology are further developed.