Best practices for photovoltaic performance loss rate calculations

Abstract

The performance loss rate (PLR) is a vital parameter for the time-dependent assessment of photovoltaic (PV) system performance and health state. Although this metric can be calculated in a relatively straightforward manner, it is challenging to achieve accurate and reproducible results with low uncertainty. Furthermore, the temporal evolution of PV system performance is usually nonlinear, but in many cases a linear evaluation is preferred as it simplifies the assessment and it is easier to evaluate. As such, the search for a robust and reproducible calculation methodology providing reliable linear PLR values across different types of systems and conditions has been the focus of many research activities in recent years. In this paper, the determination of PV system PLR using different pipelines and approaches is critically evaluated and recommendations for best practices are given. As nonlinear PLR assessments are fairly new, there is no consent on how to calculate reliable values. Several promising nonlinear approaches have been developed recently and are presented as tools to evaluate the PV system performance in great detail. Furthermore, challenges are discussed with respect to the PLR calculation but also opportunities for differentiating individual performance losses from a generic PLR value having the potential of enabling actionable insights for maintenance.

Perspective: Performance Loss Rate in Photovoltaic Systems

“…An extensive discussion of these issues and approaches can be found in the literature. [3,4,10] For example, if a system experiences data outages for a portion of a year plagued by loss from snow-covered modules; the absence of those points would bias the SEPI high and likewise influence comparison to neighboring years. Where possible, it is advisable to carefully correct or impute (and not necessarily remove) problematic data when estimating PLR.…”

“…Existing descriptions of PLR in the literature describe PLR as quantifying all losses, both recoverable and nonrecoverable. [ 3,4 ] Examples of included loss factors are component degradation, snow loss, soiling, and outages. However, there is no formalized and accepted definition of PLR.…”

“…This is consistent with prior descriptions of PLR. [ 3,4 ] For example, if there is more snow accumulating on panels in one year than in the previous year, this would be reflected in a higher PLR for the snowier year. We also note that some loss factors can be reduced in any given year, resulting in a positive PLR for that year.…”

How Climate and Data Quality Impact Photovoltaic Performance Loss Rate Estimations

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The performance loss rate (PLR) represents both reversible (e.g., soiling) and irreversible (e.g., material degradation) losses [1,2] that can occur in a photovoltaic (PV) power plant and is an important parameter for performance modeling, monitoring, and operation and maintenance (O&M). In PV performance modeling, PLR is applied to account for the power reduction over time whereas in monitoring and O&M, it can provide a high-level indication of a power plant's health state.

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“…Although PLR has been interchangeably used with the degradation rate (Rd) in the literature, and despite the fact that their calculation approaches are similar, these two metrics are different: Rd represents only the irreversible material degradation of a component (e.g., a PV module) and is only one part of the PLR breakdown. [1] Unless all losses are individually monitored (e.g., through a soiling station, periodical module testing under artificial light to estimate degradation, etc. ), the true value of PLR (and its breakdown) is unknown when processing only field data, but it is commonly approximated by applying statistical methods on PV performance time series.…”