Impact of Rear-Hanging String-Cable-Bundle Shading on Performance Parameters of Bifacial Photovoltaic Modules

Abstract

The 2025 International Technology Roadmap for Photovoltaics (ITRPV) projects that bifacial modules will dominate the photovoltaic (PV) market, reaching roughly 60–80% global share between 2024 and 2035, while monofacial PV modules will steadily decline. Current industry practice is to route the cable bundles along structural members such as main beams or torque tubes, thereby preventing rear-side shading but resulting in two key drawbacks: increased cable length and decreased system reliability due to cable proximity with rotating members and pinch points. Both effects contribute to higher system costs and reduced cable reliability. An alternative method involves suspending cable bundles directly behind the modules using hangers. While this approach mitigates excess length and risk of cable snags, it introduces the possibility of partial rear-side shading, which could possibly cause performance loss and hot-spot formation due to shade-induced electrical mismatch. Experimental evidence indicates that this risk is minimal, as albedo irradiance typically represents only 10–30% of front-side irradiance as reported in the literature and is largely diffuse, thereby limiting the likelihood of significant directional shading. This study evaluates the performance and reliability impacts of hanger-supported cable bundles under varying experimental conditions. Performance metrics assessed include maximum power output (Pmax), short-circuit current (Isc), open-circuit voltage (Voc), and fill factor (FF), while hot-spot risk was evaluated through measurements of module temperature uniformity using infrared imaging. Each cable (1X) was 6 AWG with a total outer diameter of approximately 9 mm. Experiments covered different cable bundle counts/sizes (2X, 6X, 16X), mounting configurations (fixed-tilt and single-axis tracker), and albedo conditions (snow-covered and snow-free ground). Measurements were conducted hourly on clear days between 8:00 and 16:00 from June to September 2025. The results consistently show that hanger-supported cable bundles have a negligible shading impact across all hours of the day and throughout the measurement period. This indicates that rear-side cable shading can be safely and practically disregarded in performance modeling and energy-yield assessments for the tested configurations, including fixed-tilt systems and single-axis trackers with or without torque tube shading and with various hanger sizes and cable-bundle counts. Therefore, hanging cables behind modules is a cost- and reliability-friendly, safe and recommended practice.

1. Introduction

According to the ITRPV 2025 report, bifacial modules are expected to become the dominant PV technology, attaining an estimated 60–80% share of the global market between 2024 and 2035, while the share of monofacial modules is projected to steadily decrease [1]. Bifacial modules offer higher energy yield by harvesting irradiance from both the front and rear sides. While the front surface receives relatively uniform irradiance across all cells, the rear side is subject to spatially varying irradiance due to a range of albedo, angle of incidence, and structural factors.

Front-side shading due to uniform front irradiance has been extensively studied as indicated in the recent paper [2]. Rear-side irradiance non-uniformity can arise from several sources: (i) frame-lip shading of edge cells, (ii) inbuilt module cabling, (iii) array-level string-cable-bundle shading, (iv) shading from support beams or torque tubes, and (v) variations in the ground shading-to-unshaded ratio. The latter is particularly dynamic, changing throughout the day as the sun’s position alters the projected array geometry. Single-axis trackers partially mitigate this effect compared to fixed-tilt systems, owing to their reduced angle of incidence (reduced cosine loss). Such spatially heterogeneous rear illumination can cause current mismatch among cells, leading to performance loss and, in severe cases, localized heating (hot spots) that may reduce module lifetime.

Among the contributors listed above, shading due to string-cable-bundles represent a factor of growing importance. Industry-standard practice routes these bundles along beams or torque tubes to avoid rear shading. However, this approach increases cable length and risks related to pinch points, raising both system cost and reliability concerns. Suspending cables behind modules and away from rotating structural members using hangers offers a potential solution, but it may introduce rear-side shading effects that need to be carefully quantified to ensure that performance and reliability are not compromised.

To address this challenge, the present study introduces a novel metric, the Normalized Shading Ratio (NSR), designed to isolate the effect of array cable bundles from other confounding rear shading or non-uniform irradiance factors. By focusing on the NSR, the analysis specifically quantifies the performance impacts attributable to cable bundles alone, without interference from frame, structural, or ground-shading effects. The performance metrics evaluated include maximum power output (Pmax), short-circuit current (Isc), open-circuit voltage (Voc), and fill factor (FF), along with temperature uniformity (IR imaging) as an indicator of hot-spot risk.

This paper presents a comprehensive field investigation conducted under varying cable bundle sizes, array configurations, and albedo conditions, with measurements performed in Phoenix/Mesa, Arizona on clear summer days in 2025. The results provide insight into whether behind the module-supported cabling introduces measurable risks or whether it can serve as a cost-effective, reliable alternative to conventional routing practices in bifacial PV plants.

The state-of-the-art literature suggests that the associated cable shading risk remains minimal, as the red shifted albedo irradiance represents merely 10–30% of front-side irradiance is predominantly diffuse [3], thereby limiting directional shading effects by the rear side cable. Prior works have shown that cables laid under or near PV modules suffer elevated conductive or radiant heating, reducing ampacity and material lifetime [4]. Best-practice cable management documents, from LBNL, highlight risks from excessive cable length, UV/thermal exposure, and failure of supporting cable ties and hardware [5]. Studies of rear-side shading from structural components such as torque tubes and other near-field objects indicate that shading can significantly impact bifacial module performance [6]. Studies of PV array layout and cabling topology demonstrate that more direct routing reduces cable length, system cost, and resistive losses [7,8,9]. An optical simulation study conducted by Longi and the University of New South Wales found that rear-side components can introduce measurable shading on the bifacial PV modules; the rear frame lip caused 6–13% shading (depending on the distance between the frame lip and the outer string cell edge, ranging from 0 mm to 50 mm); a rear-hanging cable (10 mm in diameter, positioned 100 mm behind the module) produced approximately 3% rear-side shading, resulting in less than 1% impact on overall performance; and a rear torque tube (100 mm diameter, located 100 mm behind the cells) resulted in about 30% shading [10]. Based on NREL’s bifacial ray-tracing simulation model, Nextracker reported that rear-side shading from 2-inch (50.8 mm) wire bundles can reduce system-level specific yield by approximately 0.25% to 0.35% [11]. Based on the available literature, there is a lack of field-based experimental studies that quantitatively evaluate the effects of cable shading on key performance parameters (Pmax, Isc, Voc, and FF). This study addresses that gap by presenting field measurements of cable shading impacts, with and without torque/beam tube shading, in a single-axis tracker system under gravel albedo conditions and in a fixed-tilt system under both gravel and white surface albedo conditions.

Recent global trends in bifacial PV include the rapid adoption of high-efficiency cell architectures such as TOPCon, which may exhibit different shading responses compared with industry-dominant PERC modules used in this work. In parallel, the industry increasingly employs engineered cable-management solutions—including cable trays, spacers, and suspension blocks—to mitigate shading-related risks. Although these developments are outside the scope of the present study, they highlight both the novelty and limitations of our PERC-based shading analysis and motivate future work across a wider range of module technologies and cable-management practices.

2. Materials and Methods

2.1. Test Plan and Dimensions

The test plan is shown in Figure 1. Experiments were conducted on clear sunny days during summer 2025 using two bifacial PV modules (one served as the test module and the other served as reference module) with varying rear cable bundles (2X, 6X, and 16X) suspended on the modules’ frames using two hanger sizes as shown in

Table 1. Number of cables in each cable bundle, diameter of each cable bundle and dimensions of cable hangers.

Hanger Type	Hanger 1 (3”)	Hanger 2 (6”)	Hanger 2 (6”)
Number of cables in the bundle	2	6	16
Diameter of the cable bundle (mm)	12	20	30
Hanger Dimension: A (mm)	53.39	71.14	39.73
Hanger Dimension: B (mm)	14.22	17.14	17.70
Hanger Dimension: C (mm)	13.61	37.80	69.86

. Each cable was 6 AWG with a total outer diameter of approximately 9 mm. In this work, the notation ‘X’ refers to the number of 6 AWG cables in a bundle (e.g., 2X = two 6 AWG cables, 6X = six cables, and 16X = sixteen cables). The length of each cable bundle was approximately 125 cm. The overall outer diameters of 2X, 6X, and 16X cable bundles were approximately 12 mm, 20 mm, and 30 mm, respectively. The torque tube was square in the cross-section, with each side measuring approximately 100 mm. Both 1-axis tracker and fixed-tilt systems were used under different albedo conditions (ground and white surfaces). The STC (standard test conditions) data of these identically rated modules are provided in

Table 2. Standard Test Conditions (STC) rating of test and reference modules.

	Voc (V)	Isc (A)	Vmp (V)	Imp (A)	Pmax (W)
Test Module	49.45	13.79	41.47	12.9	535
Reference Module	49.45	13.79	41.47	12.9	535

.

2.2. Test Setup

The experimental setup involved bifacial PV modules mounted side-by-side, with one module serving as the reference (no rear cable bundle) and the other as the test module (with rear cable bundle). The cable bundle was suspended at 3/4th height of the modules using the 3” or 6” hangers. Photos of the test setups for both tracker and fixed-tilt configurations are shown in Figure 2(Top). Concrete pads and walkways of 1-axis tracker were covered with gravel and sand to minimize albedo differences between ground and concrete pads. Photos of the test setup for a fixed-tilt configuration with white polymeric sheet covered mimicking snow-covered ground is shown in Figure 2(Bottom).

The ground shading-to-unshaded ratio exhibits significant variation over the course of the day, as clearly seen in the morning, noon, and afternoon snapshots shown in Figure 2(Bottom). This variation is dynamic, driven by the sun’s changing position and the resulting shifts in the tracker geometry. While single-axis trackers help reduce cosine losses compared to fixed-tilt systems, they still produce time-dependent, spatially heterogeneous rear-side illumination. Such non-uniform albedo conditions can introduce current mismatch among cells, leading to measurable performance losses. Importantly, this dynamic shading pattern (consequently, rear illumination pattern) complicates the task of isolating and quantifying the specific impact of rear-hanging cable bundles. To address this challenge and other challenges indicated in the introduction section, the NSR approach presented in this paper provides a robust framework for separating cable-induced effects from all the rear irradiance influencing factors. In this study, detailed direct albedo measurements at all incident angles were not carried out using a reference cell, pyranometer, or albedometer; instead, albedo was inferred indirectly from front-covered and rear-side short-circuit current measurements together with the module’s known bifacial response at 1000 W/m². Since albedo varies substantially throughout the day and the goal of this work was to evaluate rear-side cable-shading impacts at different times of day, direct albedo measurements were not essential for achieving the study objectives.

2.3. Test Conditions

Tests were performed on-the-hour on clear sunny days between 8 am and 4 pm during summer 2025. The inbuilt module cabling was aligned between the cells along the line of junction boxes to avoid cell shading by the module cables. The 1-axis modules were aligned with and without the torque tube centerline shading on the rear sides of the modules. A Solmetric I–V curve tracer was used to obtain four I–V curves quickly and sequentially. Four I–V curves were sequentially measured from both modules in less than 2 min in the following order: Curve 1: With cable bundle on TEST module; Curves 2 and 3 (less than 1 min interval): Without cable bundle on REFERECE module; Curve 4 (less than 1 min interval): Without cable bundle on TEST module. The rear shading due to tracker torque tube, rear mounting structures, and suspended cables could potentially create hot-spots or hot-cells of the modules. These hotspots not only affect the performance due to cell mismatch but also would potentially affect the eventual lifetime of the modules. Elevated cell temperatures and hot spots accelerate degradation and can substantially shorten module lifetime. Using an Arrhenius model, a 10 °C local temperature raise can increase the degradation rate by a factor of ~1.6–3.5 depending on the activation energy of the failure mechanism; for the typical mechanisms with an activation energy near ~0.6 eV this corresponds to roughly a two-fold increase in degradation rate (about 50% lifetime reduction) [12,13]. Therefore, infrared (IR) imaging of the tracker mounted modules was also collected to show the hot-spot issues caused by the rear torque tube but not the suspended cable bundle (16X) investigated in this study. The front and rear IR images of the modules mounted on a 1-axis tracker are shown in Figure 3.

2.4. Data Processing

All four I–V curves were acquired within approximately two minutes to minimize potential variations in incidence angle and spectral composition affecting Pmax, Isc, Voc, and FF. Since the measurements were conducted under clear-sky conditions, the variation in front irradiance remained small (typically <4 W/m²), and no correction for irradiance change was strictly necessary. Nevertheless, a minor front-irradiance correction was applied to the Pmax and Isc data. No irradiance correction was necessary for Voc and FF, since these parameters are practically unaffected by such small front-side variations. Applying temperature corrections requires temperature coefficients for each of the four performance parameters (Pmax, Isc, Voc, and FF). However, these corrections were avoided by taking all four I–V curves nearly simultaneously within a few minutes.

After applying the front irradiance correction, the impact of cable shading can be derived using two different methods:

• Approach 1—One-Module Method

  ○

  Shading is calculated as the ratio between Curve 1 (test module with cable) and Curve 4 (same test module without cable).

  ○

  This approach assumes that any change in rear irradiance is directly proportional to the change in front irradiance.

  ○

  If Curves 1 and 4 were recorded at nearly the same temperature, no further correction is required.

• Approach 2—Two-Module Method

  ○

  Compute the Shading Ratio (SR) = Curve 1 (test module with cable) ÷ Curve 2 (reference module without cable).

  ○

  Compute the Identicality Ratio (IR) = Curve 4 (test module without cable) ÷ Curve 3 (reference module without cable).

  ○

  Calculate the Normalized Shading Ratio (NSR) = SR ÷ IR.

  ○

  In this approach, the actual rear irradiance variation is explicitly accounted for through normalization against the reference module, rather than being assumed proportional to the front irradiance (Approach 1).

The NSR method provides a more accurate assessment of cable-induced shading losses than the conventional Shading Ratio (SR) because it corrects for inherent performance differences and ground-shading geometry differences between the test and reference modules. Whereas SR simply compares the shaded test module to an unshaded reference module, it cannot account for module-to-module variability or differences in ground-reflected irradiance. By incorporating the Identicality Ratio (IR), which quantifies these baseline differences using measurements collected when both modules are unshaded, the NSR normalizes the shading effect so that the final value reflects only the true impact of cable shading. As a result, the NSR provides a more reliable and module-independent metric for evaluating rear cable shading effects.

Since all four I–V curves were collected within a very short time (≈2 min), both approaches yield nearly identical results (see Figure 4). In this study, the results are presented using Approach 2. Although it offers no significant advantage under tightly controlled conditions, it provides greater robustness when:

• rear irradiance changes independently from front irradiance,
• modules are not perfectly identical, or
• measurements are spread over longer periods.

3. Results

3.1. Normalized Shading Ratio (NSR)

This results section presents only the normalized shading ratios. The X-axis of all these plots is time (h) and the Y-axis is the NSR using Approach 2. The corresponding shading ratios (SRs) and identicality ratios (IRs) are provided in Appendix A (Figure A1, Figure A2, Figure A3, Figure A4, Figure A5 and Figure A6) to maintain clarity in the main text. Figure 5, Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10 show the NSR plots for all test configurations outlined in the flow diagram (Figure 1) of the Test Plan section. The plots consistently demonstrate that NSR values cluster around 1.0, indicating a negligible cable-shading effect regardless of the time of day or the month (June–September 2025). This clearly confirms the negligible, if any, impact of cable shading under the tested conditions, suggesting that its influence can be safely and practically disregarded in performance modeling and energy-yield assessments for the most likely configurations of 1-axis system with torque tube shading with different cable hangers and bundles (Figure 5, Figure 6 and Figure 7), 1-axis system without torque tube shading with 6” cable hangers and 16X cable bundles (Figure 8), and fixed-tilt system with ground and snow reflections (Figure 9 and Figure 10).

3.2. Infrared (IR) Imaging

The IR images of the two bifacial PV modules installed on a 1-axis tracker under short-circuit conditions provide critical insights into the impact of different shading elements. One module was tested with a rear-hanging 16X cable bundle, while the other was operated without such a bundle. The results clearly show that the presence of the cable bundle does not introduce any discernible hotspot effect on the modules, as the thermal distribution across both reference and test modules remains uniform in regions unaffected by other structural components. In contrast, a severe hotspot band is observed along the horizontal midsection of both modules, directly aligned with the torque tube of the 1-axis tracker. This hotspot persists on both the front and rear module surfaces, confirming that torque tube shading produces a localized heating effect due to reverse biasing, which is significantly more detrimental than cable shading (if any). The recorded hotspot temperatures exceed 95–100 °C, far higher than the surrounding cell regions (typically 58–70 °C), underscoring the seriousness of torque tube shading as a performance and reliability concern. Furthermore, the sequential IR snapshots taken only minutes apart (3:24 p.m. vs. 3:27 p.m.) reinforce the consistency of this observation, with no additional localized heating attributable to cable shading detected during the test period. These findings confirm that rear hanging cable bundles do not cause hotspot issues, whereas the torque tube is the dominant contributor to severe thermal non-uniformities and potential degradation risks in bifacial PV modules on tracker systems.

4. Summary

Across all test conditions, the Normalized Shading Ratio (NSR) remained very close to unity for Pmax, Voc, Isc, and FF, demonstrating that cable shading impact is ≤0.6%, which is within the measurement reproducibility error for the I–V tracer (+/−1%) allowed by the IEC 61215 design qualification standard of crystalline silicon PV modules [14].

Table 3. Average Pmax difference between with and without cable bundles (%).

Configuration	Result
1-axis-6X-3”-Beam shading	0.00%
1-axis-6X-6”-Beam shading	−0.20%
1-axis-16X-6”-Beam shading	0.60%
1-axis-16X-6”-No beam shading	0.00%
Fixed Tilt-16X-6”-Ground Reflection	−0.60%
Fixed Tilt-16X-6”-White Reflection	0.20%
Average	0.00%
Median	0.00%

summarizes the average Pmax difference between with and without cable bundles.

Consistency across all cases suggests that even with large cable bundles (16X) or reduced hanger clearance (3”), rear-side cable shading does not introduce any measurable PV performance losses.

5. Conclusions

• Rear-side cable bundles (2X, 6X, 16X) suspended from 3” and 6” hangers have negligible impact, if any, on PV module performance (≤0.6% Pmax from cable bundle vs. 3–30% from mounting structures).
• Key parameters (Pmax, Voc, Isc, FF) were unaffected within the margin of experimental uncertainty.
• Results hold true across 1-axis tracker and fixed-tilt systems, with both gravel and high-albedo surfaces.
• The data suggest that the evaluated cable hanger designs are unlikely to introduce practically noticeable PV performance losses.
• Although quantitative field data remain limited in the literature, industry experience and vendor evaluations strongly indicate that the behind-the-module cable hanger systems can reduce overall cable length (≈20–30%) and installation time (≈15–20%) relative to conventional torque-tube mounting, while eliminating pinch-points near torque tubes/rotators.
• The Normalized Shading Ratio (NSR) metric introduced in this study can potentially be extended to evaluate a wide range of rear shading conditions or albedo variations in bifacial modules and systems.
• This study was conducted under clear-sky conditions, representing a worst-case scenario for directional cable shading. Future work could be extended to different ground-cover albedos, such as grass, and to plant-design parameters, such as inter-array spacing, to further characterize rear-side shading behavior across diverse operating environments.
• This study evaluated cable-induced rear-side shading using PERC half-cell bifacial modules, which remain the dominant commercial technology; however, emerging architectures such as TOPCon may exhibit different shading sensitivities and therefore warrant future investigation. Our analysis focused specifically on suspended cable bundles and did not assess the shading impacts of alternative cable-management approaches. Future work should extend the NSR framework to newer module technologies, module architectures, and varied cable management configurations to more fully characterize rear-side shading behavior across modern PV systems.