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Optimization and Performance of Bifacial Solar Modules: A Global Perspective

Xingshu Sun, Mohammad Ryyan Khan, Chris Deline, Muhammad Ashraful Alam

arXiv:1709.10026v3physics.app-phphysics.optics

TL;DR

Bifacial PV lacks globally applicable optimization guidance because prior studies emphasize limited locations or specific configurations. This paper develops a comprehensive global opto-electro-thermal framework and empirical design rules, finding that albedo and elevation can raise worldwide bifacial gain to ~30%, while relative configuration performance depends on latitude. The study also identifies farm-level shading and land-cost tradeoffs as outside its scope.

  • Problem

    Prior research emphasizes optimized bifacial PV at a few locations or in specific configurations, leaving general global optimization guidance unavailable.

  • Method

    The paper combines global meteorological data with irradiance, light-collection, self-shading, and opto-electro-thermal models to optimize module configurations and derive empirical rules.

  • Results

    ~30% bifacial gain is achieved globally by increasing albedo to 0.5 and elevating ground-mounted modules 1 m above ground, versus less than 10% at albedo 0.25.

  • Takeaways & Limitations

    The empirical rules and online simulation tool support location-specific prediction and optimization of standalone bifacial modules worldwide.

  • Takeaways & Limitations

    The study does not address farm-level optimization, including mutual row shading or the tradeoff between module and land costs.

Abstract

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With the rapidly growing interest in bifacial photovoltaics (PV), a worldwide map of their potential performance can help assess and accelerate the global deployment of this emerging technology. However, the existing literature only highlights optimized bifacial PV for a few geographic locations or develops worldwide performance maps for very specific configurations, such as the vertical installation. It is still difficult to translate these location- and configuration-specific conclusions to a general optimized performance of this technology. In this paper, we present a global study and optimization of bifacial solar modules using a rigorous and comprehensive modeling framework. Our results demonstrate that with a low albedo of 0.25, the bifacial gain of ground-mounted bifacial modules is less than 10% worldwide. However, increasing the albedo to 0.5 and elevating modules 1 m above the ground can boost the bifacial gain to 30%. Moreover, we derive a set of empirical design rules, which optimize bifacial solar modules across the world, that provide the groundwork for rapid assessment of the location-specific performance. We find that ground-mounted, vertical, east-west-facing bifacial modules will outperform their south-north-facing, optimally tilted counterparts by up to 15% below the latitude of 30 degrees, for an albedo of 0.5. The relative energy output is the reverse of this in latitudes above 30 degrees. A detailed and systematic comparison with experimental data from Asia, Europe, and North America validates the model presented in this paper. An online simulation tool (https://nanohub.org/tools/pub) based on the model developed in this paper is also available for a user to predict and optimize bifacial modules in any arbitrary location across the globe.

I. INTRODUCTION

Bifacial modules can collect direct, diffuse, and ground-reflected light from both front and rear surfaces, but practical gains depend on deployment conditions. This paper develops a global optimization framework and empirical design rules for selecting module elevation, azimuth, and tilt.

  • Motivation: Bifacial modules collect light from both front and rear sides, including diffuse and ground-reflected albedo light.Their glass-to-glass structure also improves long-term durability relative to traditional glass-to-backsheet monofacial modules.
  • Research gap: Existing worldwide analyses focus on specific configurations, while general location-specific design guidelines for optimal tilt and azimuth are unavailable.Prior work includes worldwide studies of east-west-facing vertical modules, which are not necessarily optimal everywhere.
  • Approach: The paper provides a global analysis and optimization of varied module configurations using a comprehensive opto-electro-thermal simulation framework.The study aims to translate global modeling results into practical configuration rules.
  • Key results: 10% is the maximum worldwide bifacial gain for ground-mounted modules at albedo 0.25; albedo 0.5 raises it to ~20%, and 1 m elevation raises it to ~30%.The results identify reflective groundcover and module elevation as key optimization factors.
  • Key results: 15% is the maximum advantage of ground-mounted, vertical, east-west-facing modules over south-north-facing optimally tilted modules below latitude 30° at albedo 0.5.The relative energy-output relationship reverses above latitude 30°.

II. SIMULATION FRAMEWORK

The simulation framework combines irradiance, light collection, and electro-thermal models to estimate bifacial module output at arbitrary locations and times. It uses solar-position calculations, meteorological data, irradiance decomposition, geometric collection modeling, and temperature-dependent efficiency.

  • Framework overview: Three coupled components model geographic and temporal irradiance, geometric light collection, and physics-based electro-thermal PV output.The framework integrates these components sequentially to calculate electricity production from solar insolation.
  • Irradiance model: Solar position is calculated from zenith and azimuth angles using the NREL solar position algorithm for arbitrary times and locations.These angles provide the solar path needed to calculate insolation and its collection by modules.
  • Irradiance model: Monthly NASA insolation data are converted to minute-by-minute GHI, then decomposed into DNI and DHI using the clearness index and the Orgill and Hollands model.The model uses a clear-sky calculation for temporal irradiance and satellite-derived 22-year monthly averages as meteorological input.
  • Light-collection model: Diffuse irradiance is modeled through circumsolar, horizon-brightening, and isotropic components because their angular distributions require distinct collection treatments.The Perez model represents the angular contributions of diffuse irradiance, while view factors describe module-to-sky and ground interactions.
  • Light-collection model: The light-collection model accounts for direct and diffuse illumination on both module sides, including angle-dependent reflection and bifacial rear-side geometry.For bifacial modules, rear tilt equals 180° minus front tilt and rear azimuth is shifted by 180°.
  • Electro-thermal model: Output power sums front- and rear-side illumination weighted by their efficiencies, while module efficiency varies with operating temperature and wind-dependent convective cooling.The illustrative Prism Solar Bi60 parameters use 17.4% front-side STC efficiency, 90% bifaciality, and a temperature coefficient of −0.41%/K.
  • Validation and scope: The model matches reported bifacial gains within 6.4% while assuming NASA 22-year average meteorology, infinite-size ground reflectors, and obstruction-free shading.It can simulate and optimize module configurations including bifaciality, orientation, elevation, and albedo at arbitrary geographic locations.

A SUMMARY OF THE KEY RESULTS

The study maps worldwide bifacial performance and shows that natural groundcover limits gains, while reflective surfaces and module elevation substantially improve them. Bifacial gain also varies with local atmospheric clearness.

  • The worldwide optimization compares natural groundcover, reflective groundcover, and 1 m elevation to quantify deployment-dependent bifacial performance.The framework attributes improvements to higher albedo and reduced self-shading.
  • Less than 10% bifacial gain occurs globally for optimized, ground-mounted modules on natural groundcover with albedo 0.25.The limitation reflects both low ground albedo and self-shading.
  • ~20% bifacial gain is achieved globally by increasing ground albedo from 0.25 to 0.5 for ground-mounted modules.The result supports using cost-effective artificial ground reflectors.
  • ~30% bifacial gain is reached by elevating modules 1 m above ground while using albedo 0.5.The increase results from recovering self-shading-induced losses, although elevation adds installation cost.
  • Bifacial gain is ~5% higher in Shanghai than Cairo because Shanghai has more diffuse light despite lower total solar insolation.The annual clearness indexes are approximately 0.35 in Shanghai and 0.7 in Cairo.

IV. WORLDWIDE OPTIMIZATION OF BIFACIAL SOLAR MODULES: PHYSICS AND METHODOLOGY

The paper optimizes elevation, azimuth, and tilt through mutually dependent numerical analyses and derives empirical rules for arbitrary locations. These rules identify elevation thresholds that preserve energy yield while limiting self-shading and installation requirements.

  • Optimization parameters: Elevation, azimuth angle, and tilt angle are mutually dependent design parameters optimized for bifacial electricity yield.Optimal azimuth and tilt depend on elevation.
  • Empirical optimization: Empirical equations estimate optimal design values for an arbitrary geographic location.The equations summarize the numerical optimization results.
  • Elevation effect: Energy production plateaus at high elevation as self-shading losses diminish and eventually become negligible.Further elevation then provides no additional energy yield for infinitely large ground reflectors.
  • Elevation effect: E95 is the minimum elevation that achieves 95% of maximum self-shading-free energy production, and it decreases almost linearly with latitude.The latitude trend is attributed to reduced self-shading from higher optimal tilt angles.
  • Empirical optimization: Less than 1% relative error is reported for the empirical equations over realistic albedo coefficients from 0.25 to 0.75.The equations assume a ground reflector area exceeding 100 times the module area; otherwise E95 is expected to decrease.
  • Practical boundary: A complete elevation optimization must balance installation expenditure against energy yield because the empirical rules omit additional elevation costs.Elevated modules may also improve convective cooling and long-term durability.

B. Optimal Azimuth Angle (East-West vs. South-North)

The preferred azimuth depends on albedo, elevation, latitude, and self-shading. East-west vertical modules can outperform south-north modules at lower latitudes and higher albedo, whereas south-north configurations prevail at higher latitudes or sufficient elevation.

  • Optimal orientation: 90° is the globally optimal tilt angle for BiEW, corresponding to vertical installation.BiSN tilt varies with location and deployment conditions and is optimized separately.
  • Low Albedo: 15% is the maximum worldwide advantage of BiSN over vertical BiEW for ground mounting at albedo 0.25.Low albedo makes direct-light collection dominant, while vertical BiEW misses direct light at noon.
  • High Albedo: 15% is the maximum advantage of BiEW over BiSN within 30° latitude at albedo 0.5 and zero elevation.BiEW is less susceptible to self-shading, while self-shading reduces BiSN output.
  • High Albedo: Above roughly 30° latitude, BiSN outperforms BiEW because its increasing optimal tilt reduces self-shading losses.The critical latitude varies slightly with longitude because of the clearness index.
  • Elevation: Mounting modules more than 1 m above ground makes BiSN the globally optimal orientation by reducing bifacial self-shading.At elevation E95, with minimal self-shading, the optimum orientation is always south-north.
  • Optimization guidance: The analytical empirical rules reproduce numerically optimized energy production within 5% relative difference.The rules encode elevation, azimuth angle, and tilt angle for location-specific configuration.

V. DISCUSSION AND CONCLUSIONS

The paper develops a global optoelectro-thermal framework and applies it to bifacial-module optimization across deployment conditions. It reports global bifacial-gain patterns, provides analytical guidance, and identifies farm-scale, reliability, and local-data issues outside its scope.

  • Framework: The framework combines minute-by-minute irradiance modeling, front- and rear-side light collection, self-shading, and optoelectro-thermal conversion into annual electricity yield.It uses NASA’s 22-year average meteorological database and supports arbitrary locations and times.
  • Framework: The framework is incorporated into an online calculator for modeling and optimizing bifacial modules at arbitrary global locations.The tool is based on the paper’s global simulation framework.
  • Scope boundaries: Farm-level mutual shading and land-use economics remain outside the standalone-module optimization studied here.Future farm-level analysis must balance module costs against land costs.
  • Scope boundaries: Long-term in-field reliability is not discussed, although it is required for calculating bifacial-module LCOE.The paper notes that vertical installations may reduce soiling degradation, cleaning costs, and water usage.
  • Scope boundaries: Satellite-derived insolation has rRMSE = 10.25%, so the analytical guidance is preliminary and requires local meteorological and construction optimization.Actual design must examine local non-idealities such as obstruction shading and finite ground size.

APPENDIX

The appendix identifies the physical parameter definitions and the analytical equations used to optimize bifacial-module elevation and orientation.

  • Appendix: Fig. A1 provides the physical definitions of the parameters used in Table A1.The figure serves as the parameter reference for the appendix equations.
  • Appendix: The appendix links parameter definitions to analytical optimization of module deployment.Fig. A1 defines the parameters, while Table A1 organizes the equations that use them.
  • Appendix: Table A1 presents analytical equations for optimizing bifacial-module elevation and orientation.These equations provide the appendix’s configuration guidance.

Perspective

The supplementary information organizes global optimization and performance results by deployment scenario. It covers tilt, azimuth, annual energy yield, and bifacial gain for varying elevation and albedo.

  • Perspective: Four global-map tables summarize tilt angle, azimuth angle, annual energy yield, and bifacial gain across deployment scenarios.The scenarios vary module elevation and ground albedo.
  • Perspective: Table SI1 reports optimal tilt angles for a 1 m-high module across ground albedo and elevation values.The listed elevations include 0.5 m, 1 m, 1.5 m, and 2 m.
  • Perspective: Table SI2 reports optimal azimuth angles for a 1 m-high module across ground albedo and elevation values.The supplementary results use the same elevation set shown in the surrounding labels.
  • Perspective: Table SI3 reports maximum annual electricity yield for a 1 m-high module across ground albedo and elevation values.The table is part of the supplementary global performance maps.
  • Perspective: Table SI4 reports maximum bifacial gain for a 1 m-high module across ground albedo and elevation values.The table completes the supplementary optimization and performance set.
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