Optical losses: IAM and soiling

When rays hit the glass at a steep angle, part of the light is reflected and does not reach the cells. This is called the angle of incidence loss (IAM). Dirt on the glass adds another loss.

IAM models

Fresnel with anti-reflective coating (default)

Calculates the reflection with Fresnel's laws using the refractive indices of the glass (n = 1.526) and the anti-reflective coating (n = 1.290), with extinction coefficient K = 4 m⁻¹ and glass thickness L = 2 mm. It is the recommended model for modules with anti-reflective glass.

It is applied separately to each component:

• Direct and circumsolar: with the angle of incidence of each hour.
• Sky diffuse: with an effective value that integrates the whole sky (Marion integration) and depends on the tilt.
• Ground-reflected: with an effective angle calibrated against PVsyst simulations (θ = 71.0 − 0.435·tilt, between 20° and 75°).

The effective diffuse mixes the isotropic and circumsolar parts according to the circumsolar fraction of each hour.

Martin-Ruiz

Empirical single-parameter model (Ar) with integrated expressions for direct, diffuse and albedo (Martin and Ruiz, 2001). Simpler.

Soiling

Soiling is modelled with the parameters of the same Martin and Ruiz model: a transmittance (T_T) and an Ar parameter:

The dirtier the glass, the less it transmits and the more sensitive it is to the angle of incidence.

Application

The net irradiance is the sum of the three components (direct, sky diffuse, reflected), each multiplied by its IAM factor and by the soiling transmittance:

POA_net = T_T · ( POA_direct · IAM_dir + POA_diffuse · IAM_dif + POA_ground · IAM_alb )

Where to see it

In the loss waterfall it appears as «IAM loss» and «Soiling». With clean glass and well-oriented modules, IAM is usually around 2-3 % per year.