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: More information is available on our help page: https://www.pvsyst.com/help/project-design/array-and-system-losses/array-incidence-loss-iam.html

I agree that a model would be very useful. However this would require several input informations, namely in the meteo file, but not only. The effect of snow is not only the falling snow, but also the remaining snow on the PV modules (how much time will is stay? Partially or completely? Effect of the temperature, insolation, plane tilt, module's base geometry, etc. We will think about implementing a snow coverage ratio defined in hourly values, to be specified by the user. The difficulty will be to evaluate the electrical mismatch effect in case of partial covering.

Please carefully read the help "Project design > Shadings - General" and "Project design > Shadings > Horizon - Far shadings".

This limitation is for the parameter's definition safety. We think that a higher slope of the PNom variation as function of the inverter temperature doesn't make much sense.

The best way will be to measure the irradiance in the horizontal plane. Then the program will do the transposition to any plane. If you can register the diffuse irradiance, the transposition accuracy will still be much better !

This is a development that we should do rather shortly indeed. However it is not straightforward, this represents significant work for doing correctly. This will not be in the next main release.

In PVsyst you cannot specify different modules in a sub-array. Therefore you cannot define your systemwithout using Multi-MPPT. Now you can also redefine your inverter as a (dummy) Multi-MPPT device, and define 2 different sub-arrays. In this case you should use the button "Adjust" for correclty sharing the nominal powers of each MPPT input.

In PVsyst and the Northern hemisphere, the azimuth zero corresponds to the south. Therefore your 215° correspond to 35°. Please see our FAQ http://"How is defined the plane orientation ?"

It is extremely difficult to find an algorithm satisfying any situation and anybody. The tables are filled one after the other. If their size is changing, the number of modules on each table will vary and the result will be different for each table. What we could do would be to fill a selection of tables. We will think about that for a next version.

The module area is used essentially for the efficiency definition. PVsyst proposes 2 definitions for the area taken as reference for efficiency: - The rough module size, which gives the "usual" module efficiency, - The cell's area, which leads to an efficiency at the sensitive level, which characterizes the cell's technology (quality). The frame mentioned in the "Table" definition is an extra area representing eventual mechanical supports, which could produce shades on the next table. It is indeed null in many cases.

The Imp and Vmp values are used for establishing the one-diode model. Therefore they affect the PV production of the simulation. Now manufacturers use to take the positive sorting into account by altering the definition of Imp and Vmp. If you account for the positive sorting in the "Module Quality Loss" factor, you will account for this gain twice. Choosing one or the other option is your choice. PVsyst tries to stay coherent and relies to the PNom value for the "official" module efficiency.

This is indeed an error: this verification doesn't apply to the "Lake or River" configuration. This will be corrected in the next version 6.53. In the meantime, there is a workaround: in the "Hydraulic Pumping circuit" definitions, you should choose for Pumping system type "Deep well to storage", modify the parameter "Max. pumping depth" to a high value, and come back to your "Lake or River to Storage" option.

Thank you for your project. I have indeed found the error in the simulation. This affects the way of distributing the losses (some part of oveload is transferred to the efficiency loss), but doesn't affect the final result. I have corrected for the version 6.53.

The definition of the parameter "Limit overload Loss" has been moved to the Project's parameters (button "Albedo&Settings") since a long time. The value in thew Hidden parameters only acts as initial value for new projects.

The spectral effects are not considered for Crystalline modules. They are considered negligible. With amorphous modules, you have indeed an evaluation of the spectral effect, i.e. the deviation with respect to the AM1.5 conditions.

I don't know. Please attach an image of what is wrong.

Importing DXF files is not yet possible in PVsyst. The *.SHD files are meant for internal use, not for public modifications. You can sometimes modify something with some success, but the format has a strict structure suited for the internal organization in PVsyst. It is closely related to the object's defined in the 3D scene and their interactions. It is not possible to describe it in a simple way, and we don't intend to do so.

PVsyst tries to read your data in the american format MM/DD/YY instead of the European one DD/MM/YY. You should specify this format in the Format protocol.

Sorry, we have just discovered an error in the TArray evaluation: the averages of TArray in Daily or Hourly values don't work correctly, as the operating time is not well registered. If you want significant values of TArray, the only way is to analyze them in hourly values (in the CSV file). This will be corrected in the version 6.50.

This tool is behind the button "Miscellaneous tools". This will automatically be apparing on the Report as soon as you correctly define it (no red warning).

The fog is in principle included in the meteo data. There is no possibility of explicitly modifying the meteo data within PVsyst.

In the version 6, the "Module Layout" construction and calculation is based on the 3D shading scene. You cannot add customized shades.

This is also named "Irradiance loss". This represents the beam deficit due to shades. See the help "Project design > Shadings - General" and also "Project design > Shadings > Near Shadings: Main dialog > Shadings treatment of Beam, Diffuse and Albedo"

When the "Minimum voltage for PNom" is specified in the inverter's parameters, this corresponds indeed to an input current limitation. This current limit corresponds to the value I limit = PNom / VmppPNom Now when the Impp of the array is higher than this current Limit, the loss with respect to the available Pmpp is accounted in the IL_VMin loss.

A model for treating bifacial systems will ba available in the next "major" version 6.60, hopefully before the end of this year. However no manufacturer has submitted Bi-facial modules for the database up to now.