Ray-Tracing, View-Factor, and DUET: Oh My!

BlogAbout DUET

Published: 6/6/2023

Updated: 6/15/2023

3 minute read

System modeling is important throughout the lifetime of a solar system. From site conception, design, and operation, the model is the baseline by which stakeholders evaluate system performance. However, not all models are created equal: this introduction to DUET will teach you the differences between various optical models and explain where our software model lies in this landscape. DUET’s unique algorithm makes simulating complex bifacial designs simple and worry-free for a clear line of sight towards optimal site performance. With optimal performance comes maximum profit with minimum maintenance costs! 

Let’s start with the basics: DUET’s shining star is our bifacial modeling capability. So what exactly are bifacial solar panels?

Traditional solar panels - like the ones you see on the top of houses or buildings, are known as monofacial: light incident on only the front face of the panel produces energy. If light incident on both sides of the panel produces energy, they are known as bifacial solar panels. Bifacial solar panels are becoming increasingly prevalent in the utility-scale market segment, as their costs of production are reducing and they can increase energy yield by up to 30%. 

With these dual-sided panels comes increased modeling complexity. To effectively capture how these panels - especially the rear faces - perform in a system, models must take into account shading, racking configuration, the surrounding environment, and more. With this in mind, let’s look at some of the types of models used in the industry today. 

Optical Ray-Tracing Models

Ray-tracing models, exactly like they sound, break down light into rays from the light source to the objects under investigation. This method allows for multiple reflections and absorptions of a single ray [1]. Ray-tracing simulations are highly computationally expensive, although the reward of this time and power is the ability to represent complex features of the solar system including shading, and edge effects [1]. 

The primary limitations of ray-tracing models are the duration of simulations and the computational power of simulating complex systems with the specificity that is required to capture the nuance of bifacial systems. 

2D and 3D Optical View Factor Models

In contrast, view factor (V-F) models are computationally inexpensive, meaning that the system can be modeled fairly quickly and without large amounts of computational power. V-F models calculate the irradiance on solar panels through integration - they assume isotropic scattering of reflected rays [1]. However, the price of this speed is oversimplification: these models can only represent the basic geometry of solar systems, in either 2D or 3D. 

DUET’s Optical Model

DUET is the best of both worlds: it is a numerical photovoltaic performance software that combines the computational efficiency of 3D V-F modeling with geographic and temporal specificity [2]. Our algorithm can capture the nuance of rear-side illumination through deterministic ray-object intersections [2]. DUET can model from the individual cell-level, all the way out to the full system. These capabilities are critical for an accurate assessment of the plant, including shading, time-varying albedo, and edge effects - factors that are exceptionally important for bifacial systems. 


[1] S Ayala Pelaez et al., Understanding Bifacial PV Modeling: Raytracing and View Factor Models, 2020.

[2] A. C. J. Russell et al, DUET: A Novel Energy Yield Model With 3-D Shading for Bifacial Photovoltaic Systems, 2022.

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