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Summary

The NOCT-SAM model estimates cell temperature from a module’s , typically determined according to , applying corrections for irradiance level, module-level optical losses, electrical conversion efficiency, and conductive and convective heat dissipation. This model is based on the System Advisor Model (SAM) implementation (Gilman et al., 2018).

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Inputs

NameSymbolUnitsDescription
Front POA IrradianceGPOA,frontG_{POA,front}W/m²Front POA irradiance after transposition, before optical adjustments (from irradiance calculation)
Effective Front POA IrradianceGPOA,front,effG_{POA,front,eff}W/m²Front-side POA irradiance after optical adjustments, before DC system losses (from irradiance calculation)
Ambient Air TemperatureTaT_a°CAmbient air temperature
Wind Speedvwv_wm/sWind velocity
Module STC EfficiencyηSTC\eta_{STC}—Module efficiency at standard test conditions
Nominal Operating Cell TempNOCTNOCT°CNominal operating cell temperature, as defined by IEC 61215
Transmittance-Absorptanceτα\tau\alpha—Fraction of incident light absorbed by the PV cell at normal incidence
Cell-to-Module Temp DifferenceΔTc−m\Delta T_{c-m}°CTemperature difference between cell and module surface
Reference IrradianceGrefG_{ref}W/m²Reference irradiance for cell-to-module temperature difference (typically 1000 W/m²)

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Outputs

NameSymbolUnitsDescription
Cell TemperatureTcT_c°COperating temperature of PV cells
Surface TemperatureTmT_m°CBack-of-module surface temperature

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Detailed Description

The NOCT-SAM model takes as its starting point the Nominal Operating Cell Temperature (NOCT), measured under the standardized conditions defined in IEC 61215, and adjusts it to estimate cell temperature under actual operating conditions. Unlike the Heat Balance and Sandia models, which use only the effective front-side POA irradiance (GPOA,front,effG_{POA,front,eff}), the NOCT-SAM model uses both the front-side POA irradiance (GPOA,frontG_{POA,front})—before module-level optical adjustments—and the effective front-side POA irradiance (GPOA,front,effG_{POA,front,eff})—after module-level optical adjustments. This allows explicit separation between thermal and optical contributions. The model starts from the temperature difference (NOCT−20)(NOCT - 20) between the nominal operating cell temperature and its reference ambient temperature (20 °C), then applies three scaling factors:

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Irradiance Scaling

GPOA,front800\frac{G_{POA,front}}{800} This factor scales the temperature rise by the ratio of actual to NOCT reference irradiance (800 W/m²).

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Absorptance and Efficiency Correction

1−ηSTCτα⋅GPOA,frontGPOA,front,eff1 - \frac{\eta_{STC}}{\tau\alpha} \cdot \frac{G_{POA,front}}{G_{POA,front,eff}} This factor reduces the temperature rise to account for the fraction of absorbed light converted to electricity rather than heat. The transmittance-absorptance product τα\tau\alpha represents the fraction of incoming light absorbed by the solar cell at normal incidence—accounting for transmittance through the front layers (glass, encapsulant), absorptance in the solar cell, and internal reflections at layer interfaces—while the ratio GPOA,front,eff/GPOA,frontG_{POA,front,eff}/G_{POA,front} adjusts for time-dependent optical effects (, , ).

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Heat Dissipation

9.55.7+3.8⋅0.51⋅vw\frac{9.5}{5.7 + 3.8 \cdot 0.51 \cdot v_w} This factor scales thermal losses relative to the NOCT reference wind conditions. The 0.51 factor accounts for the reduced air flow in PV power plants—compared to the open-rack configuration used for NOCT measurement—due to row-to-row shielding and other systemic effects. In PlantPredict, this parameter is hard-coded to 0.51, corresponding to ground-mount systems.

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Cell Temperature Equation

Tc=Ta+GPOA,front800⋅(NOCT−20)⋅(1−ηSTCτα⋅GPOA,frontGPOA,front,eff)⋅9.55.7+1.94⋅vwT_c = T_a + \frac{G_{POA,front}}{800} \cdot (NOCT - 20) \cdot \left(1 - \frac{\eta_{STC}}{\tau\alpha} \cdot \frac{G_{POA,front}}{G_{POA,front,eff}}\right) \cdot \frac{9.5}{5.7 + 1.94 \cdot v_w} where:
  • TaT_a is the ambient air temperature in °C (from weather data)
  • NOCTNOCT is the nominal operating cell temperature in °C (from DC field definition)
  • GPOA,frontG_{POA,front} is the front-side before module-level optical adjustments in W/m² (from irradiance calculation)
  • GPOA,front,effG_{POA,front,eff} is the front-side POA irradiance after module-level optical adjustments in W/m² (from irradiance calculation)
  • ηSTC\eta_{STC} is the module efficiency at (from module definition)
  • τα\tau\alpha is the transmittance-absorptance product at normal incidence, without soiling or spectral correction (from DC field definition)
  • τα⋅GPOA,front,eff/GPOA,front\tau\alpha \cdot G_{POA,front,eff} / G_{POA,front} is the effective absorptance under actual operating conditions (including soiling, IAM, and spectral effects)
  • vwv_w is the wind speed in m/s (from weather data)
The constants 800 W/m² and 20 °C are the NOCT reference irradiance and ambient temperature conditions. The general NOCT-SAM model also includes a modification to the base NOCT for roof-mounted systems with reduced standoff distance. PlantPredict does not include this adjustment, as it only models utility-scale PV power plants where the module’s NOCT value is used directly.

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Module Surface Temperature

Tm=Tc−GPOA,front,effGref⋅ΔTc−mT_m = T_c - \frac{G_{POA,front,eff}}{G_{ref}} \cdot \Delta T_{c-m} where:
  • GrefG_{ref} is the reference irradiance in W/m² (from module definition, typically 1000 W/m²)
  • ΔTc−m\Delta T_{c-m} is the cell-to-module temperature difference at the reference irradiance, in °C (from DC field definition)

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References

  • Gilman, P., Dobos, A., DiOrio, N., Freeman, J., Janzou, S., & Ryberg, D. (2018). SAM photovoltaic model technical reference update. NREL/TP-6A20-67399, National Renewable Energy Laboratory, Golden, CO.