At HelioScope, we take pride in the accuracy of our simulations. DNV GL (formerly BEW Engineering) previously led a study that showed HelioScope production modeling to be within 1% of other commonly used models. Read the report here.
We are excited to announce that our new Bifacial Modeling feature upholds this same standard. This article outlines our modeling approach and the rigorous process we used to validate its accuracy.
Our Approach to Bifacial Modeling
To accurately model the additional energy production from the rear side of the module (bifacial gain), it is crucial to calculate the irradiance received by the back surface of the solar panel. Our model for calculating rear irradiance is the 2D view factor model, a widely used approach in solar simulation that considers the 2D cross-section of the solar array.
This approach, detailed in a research paper by NREL, calculates the rear-side irradiance by considering:
- Ground-reflected irradiance. Direct and diffuse irradiance reflected from the ground surface (albedo). This is the largest contribution.
- Diffuse irradiance arriving directly from the sky that is not obstructed by the module itself or the modules of the adjacent row.
- Reflected light (diffuse only) from the front of the adjacent rows of modules.
- Direct irradiance from the sun, although this is typically a small contribution.
We chose a 2D view factor model because it provides an excellent balance of precision and computational speed. While 3D ray-tracing models can offer slightly more detail, they are significantly more complex and computationally intensive. Our analysis, aligned with industry findings, shows that the 2D model delivers the accuracy needed for C&I project design without the performance overhead, allowing you to iterate on your designs quickly and confidently. NREL’s paper provides additional details on how they validated the 2D model against measured irradiance on a real installation, which gave us confidence that the model is accurate for real-world conditions.
Verifying Accuracy
We validated the full energy simulation against PVsyst in two parts.
In the first part, we built test cases using common C&I configurations, varying these key bifacial parameters:
- Ground Coverage Ratio (GCR): Smaller GCRs tend to increase the amount of ground-reflected light and therefore increase rear irradiance
- Module Height: Higher ground clearances tend to also increase the amount of ground-reflected light and rear irradiance
PVsyst published results in 2019 showing how varying GCR and height affect bifacial gain. We followed a similar setup for a series of designs in HelioScope. We found that bifacial gain for Fixed Tilt designs followed a very similar curve in both HelioScope and PVsyst, demonstrating that our model accurately captures the physical relationships between design parameters and bifacial production. Minor deviations in gain percentages are attributable to differences in the underlying weather files.
In the second part, we set up bifacial projects in HelioScope and PVsyst in both the Northern and Southern Hemispheres using common C&I configurations. We were careful to use equivalent configurations in both. We found that for both cases, the annual energy production simulated by HelioScope was within 1% of the results from PVsyst.
With our thorough approach to modeling and multiple methods of validation, you can be confident that HelioScope’s bifacial simulations will have the same accuracy you have always trusted for your monofacial designs.
We will be publishing further results in the coming weeks when we release Bifacial Modeling for Independent Tilt and Single-axis Trackers.