This document discusses the impact of non-uniform dust accumulation on photovoltaic (PV) module performance and different methods for measuring soiling losses. It notes that dust often builds up non-uniformly at module edges and that measuring changes in short-circuit current (Isc) alone can over or under-estimate true power losses compared to measuring changes in maximum power (Pmax). An experiment is described that tested these effects on crystalline silicon PV modules with simulated soiling patterns, finding that Isc measurements severely misrepresented power losses in non-uniform soiling cases. The document recommends measuring both current and power when evaluating soiling impacts and using a system that captures module-specific effects through on-site power measurements of an

1. From: F. Brill, “EnviroPolitics Blog: PSEG building solar farms--and not just in
New Jersey,” 16-Nov-2012. [Online].
From: E. Lorenzo, R. Moretón, and I. Luque, “Dust effects on PV array
performance: in-field observations with non-uniform patterns,”
Progress in Photovoltaics: Research and Applications, 2013.
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SoilingRatio
Edge Cells Soiling Level
SRIsc (12 cells)
SRPmax (12 cells)
SRPmax (6 cells)
SRIsc (6 cells)
From: Gostein, Littmann, Caron, Dunn, Proceedings of the 39th IEEE Photovoltaic
Specialists Conference (PVSC), Tampa, FL, June 2013.
Accurately Measuring PV Soiling Losses
Using Module Power Measurements
M. Gostein and B. Stueve, Atonometrics, Inc.
Non-uniform PV soiling can’t be measured by short-circuit current alone.
• In some conditions, soiling
may accumulate non-
uniformly on PV modules
• Wind, rain, condensation,
temperature, and gravity all
affect soiling distribution
• Often, soiling is
concentrated at module
edges
• “Soiling Ratio” (SR) metric
can be based on losses in
measured current (Isc) or
power (Pmax)
• Isc measurement simpler,
but only approximates true
power loss in case of uniform
soiling
• Experiment shows Isc
measurements can severely
over- or under-estimate true
power loss for non-uniform
soiling on c-Si PV modules
• Soiling measurement: use
“soiled” module and “clean”
control
• “Clean” control: may use
automatically cleaned
reference cell
• For “soiled module” use
actual PV module
representative of site, to
capture module-specific
effects and non-uniformities
• Should measure both
module current and power
Example of edge soiling
on utility-scale tracking system:
Examples on ground and
roof-mount systems:
Non-Uniform PV Soiling
Pmax vs. Isc as Soiling Metric
Soiling Measurement System
Soiling Underestimated
ResultsExperiment with Simulated Soiling
72-cell c-Si module, two orientations
Soiling Ratio Metrics
Explanation: Module has 3 internal bypass diodes. Shaded 12-cell
string on long edge is bypassed by internal diode, so Isc is not
limited by shading; but shading 6 cells across all strings results in
the shaded cells controlling and limiting Isc.
Soiled Reference Module
Washed
Ref Cell
Measurement
Electronics
Wash
Unit
Stand-alone system Utility plant monitoring system
Washed
Ref Cell
Measurement
Electronics
Wash
Unit
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