Technical Reports

J. Stein, C. Reise, J. Bonilla Castro, G. Friesen, G. Maugeri, E. Urrejola, S. Ranta, with 49 other contributing authors, including A. C. J. Russell, C. E. Valdivia, K. Hinzer, “Bifacial Photovoltaic Modules and Systems: Experience and Results from International Research and Pilot Applications,” International Energy Agency - Photovoltaic Power Systems Programme, Task 13, IEA-PVPS T13-14:2021, Apr. 2021.
DOI: 10.2172/1779379. 
https://iea-pvps.org/key-topics/bifacial-photovoltaic-modules-and-systems

Within the framework of IEA PVPS, Task 13 aims to provide support to market actors working to improve the operation, the reliability and the quality of PV components and systems. Operational data from PV systems in different climate zones compiled within the project will help provide the basis for estimates of the current situation regarding PV reliability and performance. The general setting of Task 13 provides a common platform to summarize and report on technical aspects affecting the quality, performance, reliability and lifetime of PV systems in a wide variety of environments and applications. By working together across national boundaries we can all take advantage of research and experience from each member country and combine and integrate this knowledge into valuable summaries of best practices and methods for ensuring PV systems perform at their optimum and continue to provide competitive return on investment. Task 13 has so far managed to create the right framework for the calculations of various parameters that can give an indication of the quality of PV components and systems. The framework is now there and can be used by the industry who has expressed appreciation towards the results included in the high-quality reports. The IEA PVPS countries participating in Task 13 are Australia, Austria, Belgium, Canada, Chile, China, Denmark, Finland, France, Germany, Israel, Italy, Japan, the Netherlands, Norway, Spain, Sweden, Switzerland, Thailand, and the United States of America.

Conference Papers

A. C. J. Russell, C. E. Valdivia, J. E. Haysom, K. Hinzer, “Impact of snow depth on single-axis tracked bifacial photovoltaic system performance,” 49th IEEE Photovoltaic Specialists Conference (PVSC), Philadelphia, PA, USA, 5-10 June 2022. Oral presentation and paper.
DOI: 10.1109/PVSC48317.2022.9938750
https://ieeexplore.ieee.org/document/9938750

Photovoltaic (PV) capacity is rapidly expanding in mid-to-high latitude jurisdictions due to drastic decreases in PV system cost and global decarbonization efforts. However, uncertainty around performance impacts of latitude-specific conditions, such as ground-accumulated snow, contribute to investment risk. In this work, we employed DUET, our custom bifacial PV modelling tool, to study variable ground clearance resulting from snow accumulation. We model the impact of snow depth on a module in four generic, 2-in-portait single-axis tracked (SAT) systems with baseline ground clearances ranging from 1.6-2.8 m. Over the snowy season in Ottawa, Ontario (45°N) and Cambridge Bay, Nunavut (69°N), Canada, rear insolation and energy yield decrease by 3.4-9.6% and 0.36-0.69%, respectively - comparable to annual structure shading and electrical mismatch loss factors. The average daily energy yield loss in both locations is 0.034% per centimeter of accumulated snow. When hourly energy yield loss throughout the snowy season is binned by hour of day, hourly averages peak at 1.2-1.4% loss, suggesting implications for real-time and short-term forecasting.

T. J. Coathup, M. R. Lewis, A. C. J. Russell, A. Conesa, J. Guerrero-Perez, C. E. Valdivia, K. Hinzer, “Impact of reflective torque tube on rear side irradiance in bifacial photovoltaic modules,” SPIE Photonics West, OPTO; Physics, Simulation, and Photonic Engineering of Photovoltaic Devices XI; 119960A, San Francisco, CA, USA, 22-27 January 2022. Oral presentation and paper.
DOI: 10.1117/12.2615658
https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11996/2615658/Impact-of-reflective-torque-tube-on-rear-side-irradiance-in/10.1117/12.2615658.short

Non-uniform irradiance on the rear side of bifacial photovoltaic (PV) modules causes electrical mismatch between cells and energy loss across the module. Racking structures increase this non-uniformity through shadows and reflections that vary throughout the day. However, commercial software typically use constant values to estimate mismatch losses in annual simulations. We investigate the impact of torque tube shading and reflection on rear side irradiance mismatch in bifacial PV modules in one-in-portrait (1P) and two-in-portrait (2P) horizontal single-axis trackers with a range of ground albedos over a typical meteorological year in Livermore, California, USA. Irradiance simulations use a version of bifacial_radiance, the National Renewable Energy Laboratory’s python wrapper for the RADIANCE ray tracing software, which we modified for arbitrary 2D irradiance sampling of the module(s) under investigation. For a torque tube reflectivity of 0.745, torque tube reflection accounts for 3.0% and 5.5% of the annual rear insolation in 1P and 2P configurations, respectively, for a 0.2 albedo; or 2.9% and 3.1% for a 0.6 albedo. Torque tube reflection decreases annual rear insolation mismatch from 11.8% to 10.7% in 1P configurations, and from 11.5% to 9.8% in 2P configurations with 0.2 albedo. Similarly, with 0.6 albedo, annual rear insolation mismatch decreases from 12.6% to 11.6% in 1P configurations, and from 11.9% to 10.4% in 2P configurations. However, we demonstrate that annual figures are insufficient for capturing the impact of torque tube reflection; seasonal and diurnal variations must also be considered.

M. R. Lewis, A. C. J. Russell, C. E. Valdivia, J. E. Haysom, M. I. Bertoni, K. Hinzer, “Impact of air mass on energy yield calculation for bifacial silicon heterojunction photovoltaic modules in high-latitude conditions,” 47th IEEE Photovoltaic Specialists Conference (PVSC), virtual, 14-19 June 2020. Poster/video presentation and paper.
DOI: 10.1109/PVSC45281.2020.9300441
https://ieeexplore.ieee.org/document/9300441

At high latitudes, bifacial photovoltaics are expected to achieve significant bifacial gain due to the albedo of snow, but irradiance will include a wide range of incident angles and high air mass spectra. We studied the performance of bifacial silicon heterojunction solar modules with increasing angle of incidence and air mass to derive an incidence angle modifier and air mass modifier for short circuit current. We found that the incidence angle modifier remained constant with varied air mass, allowing the incidence angle modifier and air mass modifier to be applied independently. Module correction factors were applied to the SUNLAB's energy yield model, DUET. We demonstrate that the impact of air mass on energy yield increases with latitude and can reach >2.5% on an annual basis for single-axis tracked modules and >2% for fixed latitude-tilt modules in high-latitude locations. This is highly dependent on season, with greater impact in off-summer months, reaching >6.5% monthly air mass impact for a high-latitude location in winter. These results demonstrate that air mass effects are more significant for high-latitude locations, and should be considered in energy yield calculations.

C. E. Valdivia, C. T. Li, A. Russell, J. E. Haysom, R. Li, D. Lekx, M. M. Sepeher, D. Henes, K. Hinzer, H. P. Schriemer, “Bifacial Photovoltaic module energy yield calculation and analysis,” 44th Photovoltaic Specialists Conference (PVSC), Washington, D.C., USA, 25-30 June 2017. Poster presentation and paper.
DOI: 10.1109/PVSC.2017.8366206
https://ieeexplore.ieee.org/document/8366206

A new computationally-efficient algorithm has been developed for the evaluation of annual energy yields from bifacial photovoltaic panels. The model accounts for detailed anisotropic sky dome and albedo ray tracing with directional reflection, self-shading, and rack shading. The illumination profiles over both front and rear faces of bifacial and mono-facial panels provide realistic solar cell and panel performance calculations over various system configurations. Both landscape panel orientation and panel wiring with multiple parallel horizontal strings provide higher output powers, up to >3% for low ground clearances. Representative conditions for Ottawa, Ontario, Canada, resulted in up to 18% bifacial gain in annual energy yield, while instantaneous power production increased by 13-35% when sunny and 40-70% when cloudy.

Conference Presentations

C. E. Valdivia, E. M. Tonita, A. C. J. Russell, M. R. Lewis, K. Hinzer, “Impact of Quantum-Efficiency Weighted Albedo on Bifacial PV System Energy Yield Simulations,” 8th World Conference on Photovoltaic Energy Conversion (WCPEC), Milan, Italy, 26-30 September 2022. Late news poster presentation.

Bifacial photovoltaic (PV) systems collect more light from the environment, including ground reflection. Presently, most modeling software apply an albedo based solely on its broadband reflectivity with a wavelength range that is unrelated to the absorption range of the solar cell technology. In this work, we employ our custom software, DUET, to determine the impact of spectral albedo by comparing system performance using broadband albedo or an external quantum efficiency (EQE)-weighted albedo. Under the system, locations, and ground types simulated, spectral albedo effects caused only a minor difference for monofacial energy yield, but increased bifacial energy yields by up to 2.5%, and were higher for fixed-tilt compared to single-axis tracked systems. We also showed that EQE-weighted albedos varied by a factor of 2 across cell technologies for some ground types, comparing Si and non-Si materials, primarily relating to their bandgaps

A. Russell, C. E. Valdivia, C. Bohémier, J. E. Haysom, K. Hinzer, “Validation of novel bifacial photovoltaic performance model with 3D shading for fixed-tilt and single-axis tracked systems,” 49th IEEE Photovoltaic Specialists Conference (PVSC), Philadelphia, PA, USA, 5-10 June 2022. Oral presentation.
DOI: 10.1109/PVSC48317.2022.9938856
https://ieeexplore.ieee.org/document/9938856

Existing bifacial photovoltaic (PV) performance models fall primarily into two categories: (1) ray tracing models that capture complex shading but lack the computational efficiency required for optimization applications; and (2) view factor (VF) models that efficiently simulate energy transfer but rely on user-defined losses which neglect temporal variation in phenomena such as shading and electrical mismatch. Hybrid VF / ray tracing models selectively employ ray tracing while balancing computational efficiency. This paper describes the validation of a novel hybrid model, DUET, which combines a 3D VF model with deterministic ray-object intersections. The software provides 2D irradiance profiles and mismatch-inclusive current-voltage curves for each scale of components: from cells to the full array. Validation against open-access data from Denmark shows that DUET predicts bifacial energy yield at 0.76% and 0.65% lower than measured yield for fixed-tilt and horizontal single-axis tracked (HSAT) rows, respectively, over 3370 and 2731 daylight hours. Monthly relative error in bifacial energy ranges from < 1% to ~4.5% for both systems. The mean absolute error (MAE) in hourly bifacial power is 18.1 mW/Wp for fixed-tilt and 18.4 mW/Wp for HSAT. These errors fall below the lowest previously reported MAE for six software at the same field site by ~0.77 mW/Wp for fixed-tilt and ~1.1 mW/Wp for HSAT. Modelled average rear insolation agrees with pyranometer data within +4.4% for fixed-tilt and -0.76% for HSAT. For both configurations, rear irradiance MAE aligns with the lowest error previously reported for other software at the site.

M. R. Lewis, T. J. Coathup, A. C. J. Russell, J. Guerrero-Perez, C. E. Valdivia, K. Hinzer, “Racking reflection and shading effects on single axis tracked bifacial photovoltaic modules,” 49th IEEE Photovoltaic Specialists Conference (PVSC), Philadelphia, PA, USA, 5-10 June 2022. Oral presentation. [Winner: Best Student Presentation Award, Area 7].
DOI: 10.1109/PVSC48317.2022.9938693
https://ieeexplore.ieee.org/document/9938693

Bifacial photovoltaics (PV) is predicted to comprise 80% of the silicon PV share within the next ten years. However, bifacial energy yield models are still undergoing validation, and their uncertainty may slow adoption. One of the challenges of single-axis-tracked (SAT) bifacial PV performance modelling is accurately accounting for the effects of racking elements, such as the frame, module supports, and torque tube, on the rear irradiance. In this work, we calculated front and rear irradiances for the center modules of a 2-in-portrait SAT bifacial photovoltaic system from hourly typical meteorological year data for the Bifacial Test Evaluation Center (BiTEC) site in Livermore, California using bifacial_radiance ray tracing software. For every hourly timestamp, we calculated 2D front and rear irradiance maps in three cases: with no racking, absorptive racking, and reflective racking. From these, we calculated three racking effects: shading, reflection, and shading and reflection combined. We also calculated shading and reflection factors as well as rear irradiance non-uniformity for each case. For the PV system modelled, racking reflection is focused in the same areas of the module as racking shading, partially counteracting shading-induced irradiance reduction and irradiance non-uniformity. For example, for a winter day at noon, racking reflection reduces the rear shading factor from -18.4% to -10.8% and the irradiance non-uniformity from 14.8% to 10.8%. The effects of racking, including both shading and reflection, vary by time of day and year. Accounting for these variations, rather than using annual average correction factors, will improve energy yield prediction accuracy for bifacial PV, especially over short time periods.

M. R. Lewis, A. C. J. Russell, C. E. Valdivia, J. E. Haysom, M. I. Bertoni, K. Hinzer, “Impact of spectral correction on bifacial silicon heterojunction module energy yield in high latitude locations,” SPIE Photonics West, OPTO; Physics, Simulation, and Photonic Engineering of Photovoltaic Devices X; 116810R, San Francisco, CA, USA, 6-11 March 2021. Oral/video presentation.
DOI: 10.1117/12.2578783
https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11681/116810R/Impact-of-Spectral-Correction-on-Bifacial-Silicon-Heterojunction-Module-Energy/10.1117/12.2578783.short

Bifacial photovoltaics present a clean and cheaper alternative to diesel generators for high-latitude remote communities; however, solar cells are tested at air mass 1.5, while average air mass increases with increasing latitude. For example, Cambridge Bay (69°N) has an irradiance-weighted average air mass of 3.1. We demonstrate improved efficiency of bifacial silicon heterojunction modules under high air mass spectra due to reduced incident UV light. We implement air mass correction in our bifacial PV modelling software, and we quantify the impact of air mass on energy yield for fixed-tilt and tracked systems in high latitude locations.

M. R. Lewis, A. C. J. Russell, E. M. Tonita, C. E. Valdivia, J. E. Haysom, M. I. Bertoni, K. Hinzer, “Impact of bifacial photovoltaic cell characteristics on module energy yield in high latitude locations,” SPIE Photonics West, OPTO; Physics, Simulation, and Photonic Engineering of Photovoltaic Devices IX; 112750W, San Francisco, CA, USA, 1-6 February 2020. Oral presentation.
DOI: 10.1117/12.2546583
https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11275/112750W/Impact-of-bifacial-photovoltaic-cell-characteristics-on-module-energy-yield/10.1117/12.2546583.short

Bifacial photovoltaics present a clean and cheaper alternative to diesel generators for high-latitude remote communities; however, solar cells are typically tested at 0° angle of incidence, 25°C, and AM1.5, from which high-latitude conditions vary greatly. A bifacial silicon photovoltaic cell optimized for high-latitude conditions will improve energy yield for these systems. We integrate experimentally-derived cell parameters with a systems-level model capable of fixed-tilt and tracked energy yield predictions. We optimize to find the most efficient cell design for high-latitude environments in Sentaurus and SunSolve and determine the resulting improvement in energy yield for an entire panel in MATLAB.

A. C. J. Russell, C. E. Valdivia, M. R. Lewis, J. E. Haysom, K. Hinzer, “Modelling non-uniform irradiance and annual energy yield for single-axis tracked bifacial PV systems with racking,” BifiPV Workshop, 16-18 September 2019, Amsterdam, Netherlands. Poster presentation.
https://www.bifipv-workshop.com/2019amsterdamproceedings

Many bifacial PV energy yield tools (such as PVSyst, Plant Predict & CASSYS) require a structure shading factor to account for rack shading. Parameter tests are needed to quantify the impact of conventional single-axis tracker racking on bifacial irradiance non-uniformity and energy yield. We present DUET, an energy yield model for multi-row fixed-tilt and single-axis tracked bifacial PV, and apply it to a small 1-up bifacial single-axis tracked system in Ottawa, Canada. In this system, a 4-8 cm torque tube at 5 cm distance from the panel results in 6-11% rear-side irradiance loss and 0.7-1.5% annual energy yield loss. This bifacial SAT demonstrated a 27% energy yield gain over monofacial fixed tilt, a 14% gain over monofacial single-axis tracked, and a 10% gain over bifacial fixed-tilt.

A. C. J. Russell, C. E. Valdivia, M. R. Lewis, J. E. Haysom, K. Hinzer, “Modelling bifacial solar energy yield for single-axis tracked systems with racking,” 19th International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD), Ottawa, ON, Canada, 8-12 July 2019. Oral presentation.
DOI: 10.1109/NUSOD.2019.8807097
https://ieeexplore.ieee.org/document/8807097

This paper presents a bifacial PV energy yield simulation tool with highly-parameterized optical and electrical models. The resulting detailed irradiance profiles provide insight into the effects of rack shading. Horizontal parallel wiring architecture of cells in the module is found to mitigate nonuniform rear irradiance caused by torque tube shading in a single-axis tracked system for representative cloudy and clear days. Low cell shunt resistance is shown to amplify bifacial gain during low irradiance hours. Annual energy yield predictions for fixed and tracked bifacial PV systems for Ottawa, Ontario are also presented.

A. J. Russell, C. E. Valdivia, M. R. Lewis, J. E. Haysom, K. Hinzer, “Modelling energy yield including rack shading for single-axis tracked bifacial solar panels,” Photonics North, Québec City, QC, Canada, 21-23 May 2019. Poster presentation.
DOI: 10.1109/PN.2019.8819539
https://ieeexplore.ieee.org/document/8819539

Hourly and daily bifacial photovoltaic energy yields are calculated for a single-axis tracking system in Ottawa, ON. The proposed highly-parameterized, computationally-efficient model calculates detailed front and rear irradiance profiles allowing for the analysis of real-world shading losses due to PV panel racking. Annual energy yield calculations are also supported. Cloudy and clear day shading losses are found to increase linearly with torque tube radius for a single-tier (“1-up”) portrait configuration.

M. R. Lewis, C. E. Valdivia, C. T. Li, A. Russell, H. P. Schriemer, K. Hinzer, “Bifacial solar panel energy yield in northern Canada,” Photonics North, Montréal, QC, Canada, 5-7 June 2018. Poster presentation. [Winner: 1st prize, poster competition]. 

The SUNLAB’s bifacial modelling software is used to demonstrate the potential for high bifacial gain, between 24 and 46 percent, in Northern Canada. Calculations show that tilt angle shifts energy on a seasonal basis between summer and winter, and increasing ground clearance improves bifacial gain for heights of up to 2 meters.

A. Russell, C. T. Li, C. E. Valdivia, J. Haysom, K. Hinzer, H. P. Schriemer, “Modeling energy yields of bifacial solar panels,” Photonics North, Ottawa, ON, Canada, 6-8 June 2017. Oral presentation. 

Bifacial photovoltaic annual energy yields are calculated using a new computationally-efficient algorithm, accounting for detailed anisotropic sky dome, directional albedo, self-shading, and rack shading models. Ray tracing analysis computes the illumination profiles over both front and rear faces of a bifacial solar panel, allowing realistic investigations of solar cell and panel performance over various configurations. A set of conditions for Ottawa, Canada, resulted in a 15.6% bifacial gain in annual energy yield, while instantaneous power production increased by 13-35% when sunny and 40-70% when cloudy. Additional energy yield gains may also be possible when employing landscape orientation and/or series/parallel wiring architectures, especially for panels with low ground clearances.

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