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 (present default in PVsyst).
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.
On this figure, we also show the ASHRAE parametrization, which is the default function in PVsyst. We see that this approximation is not satisfactory, it is here for historical reasons (chosen since the beginning of PVsyst in 1992). PVsyst will probably use the Fresnel's model as default in a near future.
File comment: Fresnel's laws and Ashrae parametrization
IAM_Ashrae_Fresnel.png [ 10.23 KiB | Viewed 5737 times ]
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:
- The indoor measurements
seem of very good quality, they usually give results which are very close to the Fresnel's model. The next figure gives an example.
File comment: IAM measurements, Indoor ans Outdoor
IAM_Measurements_Fresnel.png [ 11.55 KiB | Viewed 5737 times ]
- 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.
- 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 will not accept outdoor IAM measurements
for new modules of the PVsyst database. Only the Indoor data will be retained.
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%.
File comment: IAM losses for different profiles
IAM_Losses.png [ 12.18 KiB | Viewed 5736 times ]
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.