Degradation Losses (AC Applied)

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Summary

Degradation Losses (AC Applied) model the time-dependent reduction in PV system output due to module aging and performance decline. When Linear AC or Stepped AC degradation models are selected, PlantPredict applies degradation to AC power after inverter conversion at the array level. This page documents the AC-applied degradation algorithms. For DC-applied degradation (Linear DC and Non-Linear DC), see Degradation Losses (DC Applied).

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Inputs

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Outputs

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Detailed Description

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Application Point

When Linear AC or Stepped AC degradation is selected, degradation is applied to AC power at the array level, after summing inverter outputs and before auxiliary loads (DAS, cooling, tracker motors) and transformer losses. This differs from DC-applied degradation models which are applied before inverter conversion.

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None, Linear DC, and Non-Linear DC (Brief Description)

When None, Linear DC, or Non-Linear DC degradation models are selected, no AC-level degradation is applied. The AC power from inverters passes through unchanged to subsequent AC loss calculations.

• None: No degradation applied at any level
• Linear DC: Constant annual rate applied to DC power before inverter (see Degradation Losses (DC Applied))
• Non-Linear DC: Variable annual rates applied to DC power before inverter (see Degradation Losses (DC Applied))

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Linear AC Degradation

Linear AC degradation applies a constant annual rate to AC power output, with degradation accumulating continuously over time. Delayed Onset Calculation: If First Year Degradation is disabled: $$t_{delayed} = t_0 + 1 \text{ year}$$ If First Year Degradation is enabled: $$t_{delayed} = t_0$$ Time Elapsed: $$\Delta t = \frac{(t - t_{delayed})}{8760 \times 60} \text{ (in years, from minutes)}$$ If $t < t_{delayed}$, then $\Delta t = 0$. Degradation Coefficient: $$U_{deg} = r_{deg} \times \Delta t$$ Degraded Power: $$P_{AC,out} = (1 - U_{deg}) \times P_{AC,in}$$ $$L_{deg} = P_{AC,in} - P_{AC,out}$$

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Stepped AC Degradation

Stepped AC degradation applies degradation in discrete annual increments rather than continuously. The degradation coefficient increases in steps at annual intervals. Delayed Onset Calculation: If First Year Degradation is disabled: $$t_{delayed} = t_0 + 1 \text{ year}$$ If First Year Degradation is enabled: $$t_{delayed} = t_0$$ Step Period: $$T_{step} = 8760 \text{ hours}$$ Hours Elapsed: $$\Delta h = (t - t_{delayed}) \times 24$$ where the result is the total elapsed time in hours. If $t < t_{delayed}$, then $\Delta h = 0$. Degradation Coefficient (Version 5+): $$U_{deg} = r_{deg} \times \left\lceil \frac{\Delta h}{T_{step}} \right\rceil$$ Degradation Coefficient (Versions 3-4): $$U_{deg} = r_{deg} \times \left\lfloor \frac{\Delta h}{T_{step}} \right\rfloor$$ The ceiling function (Version 5+) causes the degradation step to apply at the beginning of each year, while the floor function (Versions 3-4) applies the step at the end of each year. Degraded Power: $$P_{AC,out} = (1 - U_{deg}) \times P_{AC,in}$$ $$L_{deg} = P_{AC,in} - P_{AC,out}$$

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Light and Elevated Temperature Induced Degradation (LeTID)

LeTID is an additional degradation mechanism that can be enabled independently of the primary degradation model. When Linear AC or Stepped AC degradation is selected, LeTID losses are also applied at the AC level. Both AC degradation and LeTID losses are calculated independently from the original inverter AC power output ($P_{AC,in}$). The losses are then subtracted together from the degraded power to determine transformer input power. LeTID Calculation: The LeTID algorithm uses cumulative annual rates with leap day handling: $$n_{days} = \lfloor t - t_0 \rfloor + 1$$ where $\lfloor t - t_0 \rfloor$ is the integer number of days between timestamps. $$n_{leap} = \text{count of February 29th in } [t_0, t]$$ $$y = \frac{\Delta t_{min} - (n_{leap} \times 1440)}{8760 \times 60}$$ where $\Delta t_{min}$ is the total elapsed time in minutes from $t_0$ to $t$. $$U_{LeTID} = \sum_{i=0}^{\lfloor y \rfloor - 1} l_i + \left( (y \mod 1) + \frac{n_{leap}}{365} \right) \times l_{\lfloor y \rfloor}$$ LeTID Loss: $$L_{LeTID} = U_{LeTID} \times P_{AC,in}$$

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First Year Degradation Setting

The First Year Degradation setting controls whether degradation begins immediately at energization or is delayed by one year:

• Enabled (On): Degradation accumulation begins at the energization date ($t_0$)
• Disabled (Off): Degradation accumulation begins one year after energization ($t_0 + 1$ year)

This setting applies to Linear AC and Stepped AC models. The delayed onset provides flexibility for modeling warranty conditions where first-year degradation may be excluded from performance guarantees.

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Use Leap Years Setting

The Use Leap Years setting controls how elapsed time is calculated for degradation:

• Enabled (On): Leap days (February 29th) are included in elapsed time calculations
• Disabled (Off): Leap days are excluded, using a standard 365-day year (8760 hours)

When disabled, the algorithm counts leap day occurrences in the period and adjusts the elapsed time calculation accordingly.

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AC Loss Sequence

When AC-applied degradation is selected, the array-level loss sequence is:

1. Sum inverter AC power outputs ($P_{AC,in}$)
2. Apply AC degradation (Linear AC or Stepped AC) → $P_{AC,out}$
3. Calculate LeTID loss from original input power (if enabled) → $L_{LeTID}$
4. Subtract auxiliary loads (DAS, cooling, tracker motors)
5. Apply MV transformer losses
6. Apply AC collection losses

The power into the transformer is calculated as: $$P_{transformer,in} = P_{AC,out} - L_{LeTID} - L_{DAS} - L_{cooling} - L_{tracker}$$

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References

• Jordan, D. C., & Kurtz, S. R. (2013). Photovoltaic degradation rates—an analytical review. Progress in Photovoltaics: Research and Applications, 21(1), 12-29.
• Jordan, D. C., Silverman, T. J., Wohlgemuth, J. H., Kurtz, S. R., & VanSant, K. T. (2017). Photovoltaic failure and degradation modes. Progress in Photovoltaics: Research and Applications, 25(4), 318-326.
• Kersten, F., Engelhart, P., Ber, H. C., et al. (2015). Degradation of multicrystalline silicon solar cells and modules after illumination at elevated temperature. Solar Energy Materials and Solar Cells, 142, 83-86.