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Array incidence loss (IAM)

The incidence effect (the designated term is IAM for Incidence Angle Modifier) corresponds to the reduction in irradiance reaching the PV cell surface relative to irradiance at normal incidence. This reduction is primarily due to reflections on the glass cover, which increase with the incidence angle.

Transmission loss is a general phenomenon resulting from reflection and transmission of sun rays at each material interface (air-glass, glass-EVA, EVA-cell), as well as some absorption in the glass. This occurs for any incident ray. At normal incidence, reflection is around 5% and is included in the measured STC performance. The IAM only addresses the angular dependency of this effect—it is normalized to transmission at perpendicular incidence (0° incidence angle).

PVsyst uses an IAM function that describes transmission loss as a function of incidence angle. This function is applied to beam, diffuse, and albedo components using integration over all visible directions, assuming isotropic distribution of diffuse irradiance.

Physics Models

Several models predict the light effectively reaching the cell's surface as a function of the incidence angle. Since PVsyst 6.67, Fresnel's laws are used. In older PVsyst versions, the ASHRAE parametrization was used.

Fresnel's laws

This model is based on the physical behavior of light: Fresnel's Laws describe light transmission and reflection at the interface between two transparent materials with different refractive indices (n1 and n2). This is fundamental behavior derived from Maxwell's equations of electromagnetism.

Fresnel's laws are first applied to the air-glass interface, describing the reflected and transmitted rays. At the next interface (glass-EVA), Fresnel equations are applied again to the transmitted ray. Light trapping and multiple reflections are not modeled in PVsyst.

IAM_Reflexions

The exact IAM profile depends on the different material layers in the module. PVsyst considers:

  • Anti-reflective (AR) coating, if present
  • Glass
  • EVA

Reflections from the cell's silicon, busbars, or fingers are neglected in the IAM model. The figure below shows the IAM profile for normal and AR-coated glass and compares it to the ASHRAE parametrization. It shows that the ASHRAE parametrization underestimates IAM values at medium angles (30–60°) and overestimates them above 60°.

IAM_Ashrae_Fresnel

ASHRAE Parametrization (deprecated)

Historically, the IAM function was often estimated using the "ASHRAE" parametrization (proposed in the 1980s by this American standards organization), which depends on a single parameter bo:

\(FIAM = 1 - bo · (1/\cos i - 1)\) where i = incidence angle on the plane.

For single-glazed thermal solar modules, the typical value for bo is around 0.1. However, in PV modules, the lower interface contacting the cell has a high refractive index, and measurements on real crystalline modules indicate a value of bo = 0.05.

Sandia model (deprecated)

The Sandia IAM model has been used in the past for modules belonging to the Sandia database (not supported anymore). It is based on a 5th-order polynomial interpolation (hence relying on 6 coefficients from b0 to b5).

The model should not be used for modules outside the database, although the option is available in PVsyst. In case one selects the Sandia model nonetheless, the following default parameters are used:

$b0 = 1, b1 = -2.438E-3, b2 = 3.103E-4 , b3= -1.246E-5 , b4 = 2.11E-7, b5 = -1.36E-9$

Implementation in PVsyst

Definition in the PV module

Each module is associated with an IAM model or measurement. When opening a PV module file (.PAN file), the IAM is defined in "Additional data > Customized IAM".

IAM_Reflexions

  • When default is selected, Fresnel's equations are used and the front surface material should be selected from the list on the right. Refractive indices in Fresnel equations are set accordingly. Options include:
    • Normal glass
    • AR coating (glass + anti-reflective coating)
    • Textured glass (currently identical to AR coating)
    • Plastic (currently identical to normal glass)
    • Not defined (legacy modules; applies the deprecated ASHRAE parametrization)
  • The IAM can be defined from measurements by selecting User defined profile. We recommend caution when using a user-defined profile, as some manufacturers are known to overestimate IAM performance.
  • ASHRAE model (not recommended; available for backwards compatibility)

Note that:

  • For modules in our database, if a manufacturer wants to set their own IAM profile, PVsyst requires a detailed measurement report from an independent laboratory conducted indoors.
  • For bifacial systems, IAM losses on the rear side of PV modules are always calculated using the simple Fresnel model for glass without anti-reflective coating.

Use in the simulation

IAM losses are calculated by default from the PV module definition. However, you can select or compare the IAM model from the Detailed Losses window on the IAM losses tab. You can override the IAM definition from PV modules by unchecking "Uses definition of the PV module". Available options are:

  • Fresnel model (recommended with default coefficients)
  • User defined: IAM measurements can be entered manually. Use with caution.
  • ASHRAE (deprecated)
  • Sandia (deprecated)

Detailed_losses

Comparing models

The Detailed Losses dialog has a Detailed Study button that provides comprehensive analysis and helps you understand IAM effects based on irradiance angular distributions (beam, diffuse, and albedo).

You can study the differences between IAM models and their impact on full-year simulation in detail (shown below). We recommend using this tool before using a user-defined IAM profile. Energy yield should not differ significantly compared to the Fresnel model.

Note that differences in energy yield from IAM model choice also depend on weather data, plane orientation, etc. For example, a Geneva simulation showed:

  • Fresnel normal glass slightly lower than ASHRAE (−0.25%)
  • Difference between Fresnel AR coating and normal glass: 0.75%

Furthermore, differences in yearly energy yield between unrealistic user-defined IAM profiles and the default Fresnel model with AR coating can reach 2%.

Detailed_study

IAM Measurements

The overall trend in IAM measurements aligns well with the Fresnel model used in PVsyst. In their 2024 IAM test results (https://scorecard.pvel.com/iam/), PVEL measured 44 modules and compared them to Fresnel's law modeling for glass with AR coating, as shown in the figure below.

PVEL

AR coating lifetime

The lifetime of AR coatings and their degradation over time is not well documented. The 2023 ITRPV roadmap estimates it at 15 years in 2023, increasing to 25 years by 2031. PVsyst does not model any IAM changes from potential AR coating degradation.