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Photovoltaic conversion transforms the effective into DC electrical power at the output of each DC field. PlantPredict models this process in four stages: module temperature calculation, DC system loss application, electrical power calculation using the equivalent-circuit model, and time-dependent degradation.

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Models in This Section

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Cell Temperature Models

Cell temperature is a primary input to the electrical model—module power output decreases with increasing temperature. All temperature models use the front-side POA irradiance (GPOA,front,effG_{POA,front,eff}) before DC system losses. For monofacial modules this is identical to the effective POA irradiance, but for bifacial modules the contribution of rear irradiance to module heating is neglected. PlantPredict implements three temperature models:
  • Heat Balance (default): physics-based energy balance model adapted from the Faiman model, with empirically determined conductive and convective heat transfer coefficients. Aligned with PVsyst.
  • Sandia: empirical exponential model (King et al., 2004) with coefficients tabulated by module construction type.
  • NOCT-SAM: physics-based model anchored to the module’s experimentally measured Nominal Operating Cell Temperature (), with empirical scaling for irradiance and wind speed (Gilman et al., 2018).
  • Measured Surface Temperature: uses measured back-of-module temperature from a time series, overriding any of the above models.

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DC System Losses

Non-ideal effects that reduce power from the theoretical maximum: module mismatch, light-induced degradation (), module quality variation, and DC health. These are applied upstream of the light-to-electrical-power conversion, as a combined reduction of the effective irradiance before the electrical model is solved. DC wiring resistance is modeled within the single-diode model, as additional . Long-term time-dependent degradation is applied downstream of the single-diode model, either to DC power or to AC energy depending on the selected degradation model.

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Single-Diode Models

The electrical conversion uses a single-diode equivalent circuit model, available in two variants: the standard five-parameter model and a seven-parameter extension that adds a recombination current term for improved low-light accuracy. The conversion proceeds as follows:
  1. Parameter Translation: the diode parameters (characterized at ) are scaled to operating irradiance and cell temperature.
  2. Single Diode Solve: the single-diode equation is solved with the translated parameters to find the .

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Degradation (DC Applied)

Time-dependent power reduction due to module aging. Linear and non-linear degradation models are applied to DC power before inverter conversion.

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Calculation Sequence

  1. Module Temperature: Calculate cell temperature from ambient conditions, wind speed, and front-side POA irradiance
  2. DC System Losses: Apply combined loss coefficient to effective irradiance
  3. Parameter Translation: Scale diode parameters from STC to operating conditions (irradiance and cell temperature); add DC wiring resistance to module series resistance
  4. Single Diode Model: Solve the five- or seven-parameter equivalent circuit for maximum power point voltage, current, and power
  5. Degradation (if DC model selected): Apply time-dependent degradation to DC power