At HelioScope, we pride ourselves on 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 our new Bifacial Modeling feature upholds this same standard. We have used a widely accepted approach to modeling, and our testing shows that for bifacial fixed-tilt and single-axis tracker solar projects, HelioScope's simulations continue to be within 1% of PVsyst's annual production results.
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, based on the irradiance incident on the ground and the surface albedo. This is the largest contribution.
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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 in hours where the sun is behind the modules
We chose a 2D view factor model because it provides an excellent balance of precision and computational speed. While 3D view factor and 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 view factor 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 to real world conditions.
Verifying Accuracy
We validated the full energy simulation against PVsyst in two parts - a comparison of system behavior with differing row spacings and module-to-surface height, and an equivalent-system modeling comparison.
Parametric review of fixed-tilt systems
In the first part, we built parametric 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.
Equivalent models for fixed-tilt and single-axis trackers
In the second part, we set up bifacial projects in HelioScope and PVsyst in both the Northern and Southern Hemisphere using common C&I configurations, using equivalent settings for module layout and simulation inputs. We found that the rear-side bifacial gains were within 1 percentage point of PVsyst, and that 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 HelioScope’s bifacial simulations will have the same accuracy you have always trusted for your monofacial designs.
Independent tilt study
HelioScope introduced independent tilt in 2024, allowing modules to have a different orientation relative to their host surface. HelioScope’s implementation of the 2D bifacial view factor model incorporates the terrain tilt when determining rear-side irradiance.
Other industry implementations of 2D view factor bifacial models do not yet support independent tilt, and instead calculate rear-side irradiance assuming flat terrain, so a software-based comparison is not possible. Instead, we reviewed trends for the constituent components of rear-side irradiance and found consistency against known models or by testing against monofacial panels placed “upside down.”
- Ground surface irradiance: ground irradiance was consistent when tilting the surface up while simultaneously reducing the solar elevation by an equal amount, showing that the surface irradiance calculation behaves consistently with independent tilt enabled and disabled for equivalent setups
- View factors between ground surface and rear module area: the surface and modules were rotated with respect to a fixed point, so the relative geometry remained the same, and we observed minimal changes to the total view factors as expected
- Diffuse irradiance and diffuse shade factor: the contribution of diffuse irradiance behaved as expected over a range of terrain surface tilts, keeping the relative tilt of modules to the surface the same, confirming that shade factors were consistently calculated
- Direct beam contribution was found to be consistent for the rear side of bifacial panels and the front side of inverted monofacial panels