Presented at the IEEE Photovoltaic Specialists Conference (PVSC) in June 2014. The better conclusion is to simply use the resultant modeled, "custom" IAM profile for PVsyst (modeled via the Fresnels equations & Snell's Law presented herein) & avoid the ASHRAE model altogether.

1. Modeling the Incidence Angle Dependence of Photovoltaic Modules in PVsyst
Junaid H. Fatehi, Kenneth J. Sauer
Yingli Green Energy Americas, Inc., San Francisco, CA 94108, USA
Abstract Results
References
Conclusion
Method
For reliable energy yield simulation, it is important to know how irradiance is
transmitted through the glass front cover of a photovoltaic module. In this work,
Fresnel reflection and Snell’s law are used to create a physical model to
determine transmission as a function of incidence angle. The physical model is
used to describe glass with and without an anti-reflective coating. An approach for
optimizing the ASHRAE incidence angle modifier parameter in PVsyst to match
the physical model output is presented and applied to both cases and the impact
on energy yield estimates is demonstrated for various geographical locations.
The results from the b0 optimizations using a year of incident irradiances and angles
from four geographic locations as inputs and physical models with and without ARC
are presented.
A plot displaying the difference in annual energy yield from PVsyst simulations using
the default and optimized b0 values is presented. When optimizing the b0 parameter
to a physical model that includes a typical glass ARC for Yingli Solar modules, the
simulation with the optimized b0 yields more energy than the simulation with the
default b0 for all locations. In all cases, optimizing b0 results in a less than +/- 1%
change in annual energy yield. However, for project financing these could already
be decisive differences.
A method for creating location and system specific incidence angle modifier
parameters in the context of the ASHRAE model used in PVsyst has been
described and applied to model a flat glass cover with and without an ARC. PVsyst
allows the b0 parameter to be specified to the thousandth decimal place in the
.PAN file as of version 6.23 of PVsyst so differences in the optimized b0 values
over different system designs and locations can be appreciated. The optimization
method introduced in this work is not limited to PVsyst and can be applied to any
simulation software that uses the AHSRAE incidence angle modifier model.
[1] Photovoltaic (PV) Module Performance Tasting and Energy Rating – Part 2: Spectral Response, Incidence Angle and Operating Temperature
Measurements, IEC 61853-2 Draft K1, 2010.
[2] A. Mermoud. (1994-2014). PVsyst (Version 6.23). [Computer Software]. Geneva, Switzerland: ISE, University of Geneva. Retrieved May 1, 2014.
Available from www.pvsyst.com.
[3] A. Mermoud and T. Lejeune, "Performance assessment of a simulation model for PV modules of any available technology," in 25th European
Photovoltaic Solar Energy Conference, Valencia (Spain), pp. 4786-4791, 2010.
[4] A. C. Hardy and F. H. Perrin, “General concepts,” in The Principles of Optics, 1st ed., New York and London, McGraw-Hill, 1932, pp. 25-28.
[5] Methods of Testing to Determine the Thermal Performance of Solar Collectors, ASHRAE Standard 93-77, 1978.
[6] Yingli Solar glass supplier, private communication, Jan., 2013.
Non-ARC ARC
b0
Opt. RMSD
[W/m2]
Def. RMSD
[W/m2]
b0
Opt. RMSD
[W/m2]
Def. RMSD
[W/m2]
Berlin 0.051 1.87 1.87 0.040 1.72 1.91
Antofagasta 0.045 3.10 3.18 0.035 2.74 3.54
Phoenix 0.056 4.62 4.78 0.045 4.31 4.47
Trivandrum 0.042 2.23 2.40 0.032 1.92 2.83
Physical
Model
Incident
Irradiance
Incidence
Angle
ASHRAE
Model
b0 = 0 to 0.1
Transmitted
Irradiance
Transmitted
Irradiance
RMSD
0 20 40 60 80
0
0.2
0.4
0.6
0.8
1
Angle of Incidence
Incidence Angle Modifier
ASHRAE b
0
= 0.05
Physical Model w/o ARC
Physical Model w/ ARC
-0.4%
-0.2%
0.0%
0.2%
0.4%
0.6%
0.8%
1.0%
Berlin Antofagasta Phoenix Trivandrum
Anual Energy Difference Between
Optimized and Default b0
Optimized without ARC
Optimized with ARC