Degradation Rates of High-Efficiency Silicon Modules from the 7GW PV Fleet Performance Data Initiative

Silicon photovoltaic (PV) modules in the field typically lose performance over time due to small cumulative changes in their active materials and packaging. Some potential damage leading to underperformance can manifest in the cell at the macro- level including cell cracks, corrosion, and interconnect fatigue. Other cell-level changes can occur at the micro-level such as light-induced degradation (LID), potential-induced degradation (PID) and light & elevated temperature induced degradation (LeTID).<br/>Investigation of large-scale photovoltaic system performance throughout the United States is being conducted within the US Department of Energy’s-sponsored PV Fleet Performance Data Initiative. This collaboration with commercial PV system owners collects and evaluates PV field performance data, and provides reports on aggregated results. Drawing on over 1700 sites across the US and over 19,000 separate PV inverters we have collected in excess of 7.2 gigawatts (GW) of performance data, representing 6-7% of the entire US installed PV capacity. A mixture of utility-scale and large commercial systems are represented, averaging 4.1 megawatts (MW) in size and 5 years in age. The vast majority of the module technologies included are silicon, including legacy Al-BSF, monocrystalline PERC, multicrystalline PERC, N-type interdigitated back contact (IBC) and silicon heterojunction (SHJ).<br/>Initial data analysis leverages NREL’s open-source software toolkit ‘RdTools’. This automated Python-based toolkit conducts degradation rate assessment to identify continuous performance loss rates for each AC power data stream. On-site measured irradiance and temperature data is assessed for data quality prior to use in the analysis. If meteorological station data is insufficiently robust, satellite-based irradiance is substituted, which was required in a minority (&lt;25%) of cases.<br/>Preliminary results indicate that overall performance loss occurs at a median rate of -0.75%/year across the fleet. Some regional and climate dependence is identified which is statistically significant. In particular, warmer climate zones result in slightly faster performance loss than more temperate climates. The module technology type was not found to have a significant impact on performance loss rate in general, with mono-silicon modules degrading at the same rate as multi-silicon modules. Cadmium Telluride (CdTe) modules were also found to lose performance at roughly the same rate as Si modules. One interesting performance difference was found when the degradation analysis was restricted to look only at low-light conditions (&lt; 600 W/m²). In the case of PERC and SHJ module types, the annual performance loss rate under low-light conditions was nearly double that of unfiltered conditions. Conversely, Al-BSF module types showed no difference between low-light and all-light conditions. And N-type IBC modules followed the opposite trend with typical degradation around -0.6 %/yr under all-sky conditions, but an annual loss rate around 0%/yr when the analysis was restricted to below 600 W/m². Further investigation into causes of this performance trend is underway, as well as whether this is a continuous effect or restricted to early-life performance, and whether it is associated with any known mechanisms such as LeTID or degradation of the passivation layer in PERC or SHJ modules.