How to determine the IAM profile ?

[André Mermoud]

The IAM (Incidence Angle Modifier) is the transmission deficit (up to the solar cell) due to the incidence angle. The transmission loss is a general phenomenon, due to the reflexion and transmission of the sun's ray at each material interface (air-glass, glass-EVA, EVA-cell), as well as some absorption in the glass. The IAM only concerns the angular dependency of this effect, i.e. it is normalized to the transmission at perpendicular incidence (0° incidence angle).

PVsyst uses an IAM function, which describes the deficit of transmission as a function of the incidence angle. This function is applied either to the beam component, and to the diffuse, using an integral over all "seen" directions, supposing an isotropic distribution of the diffuse irradiance.

The IAM function may be determined in different ways:

- By a physical calculation using the laws of optics (Fresnel laws),

- By measurements, either indoor (flash test) or outdoor (at sun),

- By a rough approximation proposed by the ASHRAE (old default in PVsyst until 2017).

Fresnel's laws

The Fresnel's laws describe the transmission and reflexion of a light ray at the interface of materials characterized by their Index of Refraction n. This calculation should be performed at each interface: Air-Glass (the dominant effect), Glass-EVA and EVA-Cell (less reflexion as the refraction indices are close to each other), secondary reflexion in the glass and its transmission to air, etc.

Globally, the next figure shows the Fresnel's laws for a single glass cover, as well as a glass with antireflective (AR) coating. This is the present default in PVsyst since 2017.

On this figure, we also show the old ASHRAE parametrization. We see that this approximation is not satisfactory, it is here for historical reasons (chosen since the beginning of PVsyst in 1992).

 

Fresnel's laws and Ashrae parametrization

 

Measurements

Now the measurements proposed by diverse Laboratories (manufacturers) are very difficult to interprete and understand. We have received many measurements, which are contradictory.

Normalized measurement procedures are described in the IEC 61853-2 directive. This document specifies 2 methods, either indoor measurements (flash tests) or outdoor measurements (at sun).

Among all measurements that we have received:

- Some indoor measurements seem of very good quality, they usually give results which are very close to the physical Fresnel's model. The next figure gives an example.

However we also receive sometimes measurements with much higher values, often erratic. We can't believe in these values, and we don't accept them for the database if they are too different than the fresnel's laws.

 

IAM measurements, Indoor ans Outdoor

 

- The outdoor measurements are much more erratic, and in most cases give much higher values than the Fresnel's laws. The methodology is not so "clean" as the indoor method, as the measurement is perturbated by the unavoidable diffuse and albedo parts. The reference beam component is indirectly evaluated by a pyrheliometer (beam) and a pyranometer (global). The diffuse is also suffering of the IAM effect, but this is not taken into account in the methodology.

Moreover:

- The angle determination is a source of uncertainty. The IEC 61863-2 requires an accuracy of +/- 1°. Now such a discrepancy involves an uncertainty on IAM of 1.4% at i = 70°, and over 5% at I = 80°.

- Remember that the sun moves by 1° every 4 minutes, and that the apparent diameter of the sun is 0.53°.

- There may also be an effect of the light polarization when the sun is low on the horizon: the Fresnel's laws show that the parallel and perpendicular components have very different angular behaviours (+/- 20% at 70°, with respect to the average).

When analysing the rough measurements of different modules (and +/- incidences), we notice a dispersion of the individual measurements of several percents, indicating a basic uncertainty in each measurement.

Such a systematic discrepancy between indoor and outdoor measurements lead to significant differences in the IAM loss calculations, which to our opinion is not justified. Therefore since January 2017 (version 6.53) we don't accept outdoor IAM measurements for new modules of the PVsyst database. Only the Indoor data are retained, as far as they are not too high.

Effect on the simulation

The IAM loss affects the 3 components of the irradiance, with almost half the contribution for the diffuse and albedo.

- The Diffuse and albedo contributions depend mainly on the geometry (plane tilt and shadings).

- The Beam contribution is related to the climate, namely the beam quantity along the year, and is computed for each simulation step.

The next figure (issued from the PVsyst tool "Detailed Losses > IAM > Detailed study") shows the contribution of different profiles, for the meteo of Marseille and a plane tilt of 20°.

- The Ashrae parametrization with bo = 0.05 corresponds about to the Fresnel's laws for single glass.

- The Ashrae parametrization with bo = 0.04 is close to the Fresnel's laws with AR coating, which is very close to the indoor measurement, and is probably the "modern" standard.

- The "User defined IAM profile" is the outdoor measurement mentioned above. We see that it overperforms the indoor measurement by more than 1%, and the PVsyst default by 1.8%.

 

IAM losses for different profiles

 

NB: Antireflective coatings are relatively fragile. Nothing ensures that they remain intact, especially in hard conditions (deserts, cleaning, etc). Therefore the real performance may derive from the nominal (outdoor measurements) to the single glass (PVsyst present default) along the life of the plant.