Argus optical simulation overview

Overview

The optical simulation is the first computational stage of the Argus PV simulation chain, following the collection of simulation inputs. It takes the site parameters and energy system configuration established in the inputs stage and uses them to calculate the Global tilted irradiance (GTI) on the front and rear surfaces of PV modules — the foundational quantity for all subsequent electrical calculations.

This stage is critical because it determines how much solar radiation actually reaches each PV cell under real-world conditions. The Argus optical simulation goes significantly beyond industry-standard approaches by combining Perez's all-weather sky model with unbiased Monte Carlo backward raytracing, enabling cell-level accuracy in shading and irradiance calculations. The result is a highly detailed representation of direct, diffuse, and reflected radiation reaching each module, with all relevant optical losses applied sequentially.

The optical simulation results feed directly into the electrical simulation stage, where GTI values are converted into DC power output.

Processes included in this stage

The following processes are applied sequentially during the Argus PV simulator optical simulation:

• Sky irradiance model (Perez all-weather sky model, solar position via PSA model)

• Far horizon shading (View Factor model)

• Near shading — 3D backward raytracing (unbiased Monte Carlo path tracing)

• Soiling losses

• Angular reflection losses (Martin and Ruiz model)

• Spectral correction (Lee & Panchula model)

Optical simulation

Sky irradiance model

In Solargis Evaluate, satellite-derived Global horizontal irradiation (GHI) and Direct normal irradiation (DNI) variables are used to calculate the distribution of diffuse radiance on the sky dome. This is achieved through the implementation of Perez all-weather sky model. From these inputs, the Sun position and the configuration of the power plant, the theoretical Global tilted irradiation (GTI) without any losses is also calculated

Solar position calculation

The solar position is calculated using the PSA model from the geographical coordinates of the site and date-time information.

Far horizon shading

Far horizon shading effects are simulated using View Factor model. The default horizon from Solargis data is used at the project reference point. For energy systems with segments defined, the far horizon is pre-loaded for each segment’s reference point. Far horizon is defined by azimuth and height and can be edited in the Energy system designer.

Near shading

Near shading simulation is the most complex step in the Evaluate PV Simulator pipeline. To quantify near shading losses, all objects from the Energy system design and the surrounding terrain are put into a 3D calculation scene, in which direct and diffuse light simulation is run using backward raytracing.

3D calculation scene

The 3D calculation scene is an important component of the near shading simulation in Solargis Evaluate. It is constructed using inputs from the Energy system designer, terrain data, horizon data, and albedo values. The purpose of this scene is to accurately model the surfaces of PV modules and all objects in the area that can cast shadows or reflect solar radiation.

Components of the 3D scene:

• PV modules: These are the primary surfaces for which incident solar radiation is calculated.

• Objects: Include support structures, inverters, transformers, and any additional shading objects specified by the user, such as fences or buildings.

• Terrain: The terrain model is integrated to account for its impact on shading and reflections.

Simulation dynamics

The lighting and shading within the 3D scene are dynamic, depending on the solar position and solar radiation values. These factors vary over time and are recalculated for each time step of the simulation, ensuring that the simulation accurately reflects real-world conditions.

For a visual representation, refer to image below. This image illustrates how PV modules, objects, and terrain are integrated into the simulation environment.

Backward raytracing

Backward raytracing, specifically unbiased Monte Carlo path tracing, is a key method used in Solargis Evaluate to calculate the irradiation on PV modules. This process involves the following steps:

1. Direct illumination calculation: Determines whether sample points on PV cells are in direct sunlight or shaded, calculating a shadow ratio for partially shaded cells.

2. Diffuse radiation calculation: Generates random rays and traces them through a 3D simulation scene, recording ray direction and treating intersections as Lambertian reflections.

3. Post-processing: Denoises diffuse radiation values and resamples them per cell, summing them with direct radiation to obtain the final Global tilted irradiance (GTI).

For bifacial Energy systems, the simulation is done separately for front and rear side of the PV modules. This high accuracy method allows detailed simulation of light to an extent that even gaps between PV cells are considered.

Soiling losses

Soiling losses are an essential factor in the energy yield simulation, as they affect the amount of solar radiation that reaches the PV cells. In Solargis Evaluate, these losses are applied to the GTI calculated in the previous step. For bifacial energy systems, the simulation is done separately for the front and rear sides, while 15% of the front side loss value is applied to the rear side.

• Method of application: Soiling losses are typically applied as average monthly figures or as a single yearly figure. Users have the option to specify these values if needed.

Angular reflection losses

Angular reflection losses occur due to the angle of incidence effects on the surface of PV modules. These losses are significant because they affect the resulting radiation reaching the PV cell.

Solargis Evaluate employs the Martin and Ruiz model to estimate angular reflection losses. This model uses an angular loss coefficient, which is estimated by Solargis based on the properties of the PV module surface, particularly its soiling. For bifacial energy systems, the simulation is done separately for the front and rear sides.

Spectral correction

Spectral correction is an essential step in the simulation process, and Solargis Evaluate uses the Lee & Panchula model for this purpose.

The specific intensity of the spectral responsivity correction depends on two key atmospheric factors:

• Air mass: This represents the optical path length of sunlight through the Earth's atmosphere. It increases as the Sun's position moves closer to the horizon, affecting the spectral distribution of sunlight.

• Precipitable water content: This refers to the total amount of water vapor present in a column of the atmosphere.

Further reading

• A new simplified version of the Perez diffuse irradiance model for tilted surfaces. Richard Perez, Robert Seals, Pierre Ineichen, Ronald Stewart, and David Menicucci.

• All-weather model for sky luminance distribution—Preliminary configuration and validation. R. Perez, R. Seals, and J. Michalsky.

• Updating the PSA sun position algorithm. Manuel J. Blanco, Kypros Milidonis, and Aristides M. Bonanos.

• Comparison of ray tracing rendering technique with ground measurements for improved solar radiation modeling.  L. Dvonc, P. Orosi, T. Cebecauer, and B. Schnierer.

• Simple Model for Predicting Time Series Soiling of Photovoltaic Panels. M. Coello and L. Boyle.

• Calculation of the PV modules angular losses under field conditions by means of an analytical model. Martin N. and Ruiz J.M.

• Spectral correction for photovoltaic module performance based on air mass and precipitable water. Lee, M., & Panchula, A.

• Lambert W function for applications in physics. D.Veberič.