Hi Nathan,
The incidence angle modifier or IAM is a way to account for optical losses that occur when the solar angle of incidence on the array surface is greater than zero. In other words, when the array surface is not perfectly normal to the rays from the sun, some energy is lost to reflection from the module surface materials.
How SAM accounts for these losses depends on the module model that you choose. For a detailed description, please see the PV reference manual. (You can download it and other technical documents from the
Performance Model Documentation
page.)
Here's a summary:
The Sandia Module Model uses a set of empirically derived coefficients (b0...b5) and a fifth-order polynomial equation to account for the losses. These are based on measured data from modules in the field (see Equation 9.3 in the PV reference manual).
The CEC Module Model characterizes the losses based on the optical qualities of the module cover using physical equations for transmittance of surface normal, beam, sky diffuse and ground diffuse irradiance and an IAM equation (see Equations 9.17 and 9.18 in the PV reference manual).
The Simple Efficiency Module model does not account for these losses.
SAM reports these losses on the loss diagram as part of the "Module loss." Because of the way the models work, the IAM losses and other losses are not calculated as the model runs. There are ways to estimate the portion of the total module loss that can be attributed to specific factors like incidence angle, spectral losses, temperature effects, etc., but we chose not to try to estimate those contributions and instead just report the total module loss because it more accurately shows actual results from the model.
Best regards,
Paul.
