This doctoral dissertation discusses the performance of photovoltaic (PV) systems throughout their operational lifetime while investigating associated uncertainties. The primary focus is the correlation of climate stressors, system degradation and the appearance of failures. Assessing changes in the performance of PV systems operating under real conditions as well as detecting and understanding the root causes is important for the efficient operation of existing PV power plants and the design of new ones. Often, degradation assessments are reduced to losses on module level, but to understand the performance evolution, the system must be assessed as a whole. The main parameter studied in this thesis is the performance loss rate (PLR), which combines losses, both on module and system level, and thereby represents the overall performance decrease of operating PV systems. Until today, there is no standardized form of assessing PLR values. This work presents a critical comparison of different calculation approaches and suggests the most reliable calculation strategy. The central part of this thesis is the development of a new method for calculating multi-step performance loss rates divided into sublinear segments. This approach presents a compromise between giving a more detailed description of the performance while still being easily understandable and applicable compared to a linearized PLR (state of the art). Finally, the assessment of a systems PLR is used in the economic analysis of PV systems.
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