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Summary

Degradation Losses (DC Applied) model the time-dependent reduction in PV system output due to module aging and performance decline. PlantPredict offers five degradation models—None, Linear DC, Non-Linear DC, Linear AC, and Stepped AC—differing in where the loss is applied (DC power upstream of the vs. AC power downstream of the inverter) and how the rate evolves over time (constant, per-year schedule, or annual steps). This page documents the two DC-applied models and the optional DC-applied model. For AC-applied degradation, see Degradation Losses (AC Applied).

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Inputs

NameSymbolUnitsDescription
DC Power InputPDC,inP_{DC,in}WDC power before degradation
Energization Datet0t_0datetimeBlock energization date (system commissioning)
Linear Degradation Raterdegr_{deg}%/yearAnnual degradation rate
Non-Linear Degradation Rates[r0,r1,...,rn][r_0, r_1, ..., r_n]%/yearPer-year degradation rates starting at year 0
LeTID Annual Rates[l0,l1,...,ln][l_0, l_1, ..., l_n]%/yearPer-year LeTID rates starting at year 0

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Outputs

NameSymbolUnitsDescription
Degraded DC PowerPDC,outP_{DC,out}WDC power after degradation
Degradation LossLdegL_{deg}WPower loss due to degradation
LeTID LossLLeTIDL_{LeTID}WPower loss due to LeTID

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Detailed Description

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Application Point

When Linear DC or Non-Linear DC degradation is selected, degradation is applied to DC power upstream of the inverter, after DC field power calculation and before the inverter determines its operating point. Because the inverter sees the already-degraded DC power, DC-applied degradation can affect the behavior: a system that clips in early years may stop clipping as modules degrade. AC-applied degradation models, by contrast, are applied after inverter conversion and do not affect clipping behavior. Before the , the system is not yet commissioned, so all models set PDC,out=0P_{DC,out} = 0.

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None (No Degradation)

When the degradation model is set to None: Udeg=0U_{deg} = 0 PDC,out=PDC,inP_{DC,out} = P_{DC,in}

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Linear DC Degradation

Linear degradation applies a constant annual rate rdegr_{deg} over the system lifetime. Degradation accumulates from the energization date tonset=t0t_{onset} = t_0 when First Year Degradation is enabled, or from tonset=t0+1t_{onset} = t_0 + 1 year when disabled. The degradation coefficient is: Udeg=rdeg⋅ΔtU_{deg} = r_{deg} \cdot \Delta t where Δt=max⁡(t−tonset, 0)\Delta t = \max(t - t_{onset},\, 0) is the elapsed time expressed as a fractional number of years (using an 8760-hour year). The degraded power is: PDC,out=(1−Udeg)×PDC,inP_{DC,out} = (1 - U_{deg}) \times P_{DC,in} Ldeg=PDC,in−PDC,out=Udeg×PDC,inL_{deg} = P_{DC,in} - P_{DC,out} = U_{deg} \times P_{DC,in}

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Non-Linear DC Degradation

Non-linear degradation specifies a separate rate rir_i for each year ii of operation, always starting from the energization date (no delayed onset option). When Use Leap Years is disabled, elapsed time Δt\Delta t is normalized to a 365-day year by subtracting leap days; when enabled, leap days are included. The cumulative degradation sums all complete years plus a pro-rated portion of the current year: Udeg=∑i=0⌊Δt⌋−1ri+(Δt−⌊Δt⌋+nleap365)×r⌊Δt⌋U_{deg} = \sum_{i=0}^{\lfloor \Delta t \rfloor - 1} r_i + \left( \Delta t - \lfloor \Delta t \rfloor + \frac{n_{leap}}{365} \right) \times r_{\lfloor \Delta t \rfloor} where Δt\Delta t is in fractional years (as defined above), ⌊Δt⌋\lfloor \Delta t \rfloor is the number of complete years elapsed, nleapn_{leap} is the count of February 29th occurrences between t0t_0 and tt, and Δt−⌊Δt⌋\Delta t - \lfloor \Delta t \rfloor is the fractional part of the current year. The first term sums the rates of all complete years; the second term pro-rates the current year’s rate. Example: t0t_0 = January 1, 2027 00:00 and tt = March 15, 2033 00:00. There are 2265 elapsed days, with nleap=2n_{leap} = 2 (February 29 in 2028 and 2032). Then Δt=(2265−2)/365=6.2\Delta t = (2265 - 2)/365 = 6.2, so ⌊Δt⌋=6\lfloor \Delta t \rfloor = 6 and the pro-rated fraction is 0.2+2/365≈0.2050.2 + 2/365 \approx 0.205. The cumulative degradation is Udeg=r0+r1+r2+r3+r4+r5+0.205×r6U_{deg} = r_0 + r_1 + r_2 + r_3 + r_4 + r_5 + 0.205 \times r_6. PDC,out=(1−Udeg)×PDC,inP_{DC,out} = (1 - U_{deg}) \times P_{DC,in} Ldeg=PDC,in−PDC,out=Udeg×PDC,inL_{deg} = P_{DC,in} - P_{DC,out} = U_{deg} \times P_{DC,in}

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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. Unlike conventional degradation, LeTID is partially reversible—modules typically degrade over the first few years of operation, then partially recover (Repins et al., 2020). Per-year rates lil_i can therefore be negative in later years to capture this recovery. LeTID losses are reported separately from primary degradation and are applied at the same level (DC for Linear DC/Non-Linear DC, AC for Linear AC/Stepped AC). The algorithm uses the same cumulative approach as Non-Linear DC degradation: ULeTID=∑i=0⌊Δt⌋−1li+(Δt−⌊Δt⌋+nleap365)×l⌊Δt⌋U_{LeTID} = \sum_{i=0}^{\lfloor \Delta t \rfloor - 1} l_i + \left( \Delta t - \lfloor \Delta t \rfloor + \frac{n_{leap}}{365} \right) \times l_{\lfloor \Delta t \rfloor} where Δt\Delta t, ⌊Δt⌋\lfloor \Delta t \rfloor, and nleapn_{leap} follow the same definitions as for Non-Linear DC degradation. LLeTID=ULeTID×PDC,inL_{LeTID} = U_{LeTID} \times P_{DC,in} When both primary degradation and LeTID are active, their losses are additive: PDC,out=PDC,in−Ldeg−LLeTIDP_{DC,out} = P_{DC,in} - L_{deg} - L_{LeTID}

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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.
  • Repins, I., et al. (2020). Light and elevated temperature induced degradation (LeTID) in a utility-scale photovoltaic system. IEEE Journal of Photovoltaics, 10(4), 1084–1092.
  • Kersten, F., Engelhart, P., 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.