Performance and Reliability Considerations of PV Modules

Monofacial Glass–Backsheet vs. Bifacial Glass–Glass Solar Modules for Rooftop Applications

_{White paper prepared for industry stakeholders by Ramesh Shivanna, drawing upon field insights, global technology trends, and practical challenges observed in the Indian rooftop solar ecosystem.}

Abstract

The rapid evolution of solar photovoltaic (PV) technology has created both opportunities and challenges for developers, EPC contractors, and investors. While Chinese manufacturers dominate the market with continuous upgrades, downstream adopters in India and other regions often face uncertainty due to limited standardization and incomplete disclosure of trade-offs. This paper compares the performance, operating conditions, and reliability risks of monofacial glass–backsheet modules versus bifacial glass–glass (G2G) modules, with a specific focus on rooftop deployments. Drawing from material science, field experience, and engineering analysis, the paper highlights thermal behavior, bifacial gain potential, mechanical and degradation risks, and practical design and O&M recommendations.

Keywords

Solar modules, Monofacial, Bifacial, Glass–Glass, Rooftop PV, Reliability, Thermal performance, O&M

1. Introduction

The global PV industry is undergoing rapid technological shifts, with bifacial glass–glass (G2G) modules increasingly replacing conventional monofacial glass–backsheet designs. While each upgrade promises efficiency and durability improvements, the lack of standardized performance data and limited disclosure of drawbacks complicates decision-making. For rooftop projects—where higher module temperatures, limited airflow, and structural constraints apply—the choice between glass–backsheet and G2G modules has direct consequences for long-term energy yield and system reliability.

2. Performance Considerations

2.1 Temperature Coefficient

• Monofacial modules (glass–backsheet): -0.29% to -0.35%/°C typical for PERC/TOPCon.
• Bifacial G2G modules: No inherent advantage; improvements depend on cell technology (often n-type TOPCon/HJT).

2.2 Operating Temperature (NOCT)

• G2G modules radiate heat less efficiently and can operate 1–3 °C hotter on rooftops.
• This can translate to 0.3–1% power loss, assuming a -0.30%/°C coefficient.

2.3 Bifacial Gain

• Rooftop albedo is typically low (concrete/metal surfaces).
• 0–5% gain is common unless reflective roofing materials and ≥150–200 mm standoff are used, in which case 5–10% gain may be achievable.

3. Reliability and Failure Risks

3.1 Advantages of G2G

• Stronger protection against moisture/UV ingress.
•  Lower risk of backsheet cracking and yellowing.
•  Improved Potential Induced Degradation (PID) resistance.
• Lower annual linear degradation (≤0.40–0.45%/yr vs. 0.5–0.55%/yr).

3.2 Challenges with G2G

•  Higher operating temperature increases thermal cycling stress.
• Reliance on edge seals introduces new moisture ingress risks.
• Both sides are glass, leading to brittle failure risks (handling, footfall, point loads).
• Heavier weight increases structural load requirements.

4. Rooftop-Specific Issues with Bifacial G2G

• Rear-side shadows from HVAC, parapets, and rails reduce bifacial gains and can create localized hot spots.
• Limited clearance (<100 mm) negates the bifacial benefit and worsens heat buildup.
• String sizing must account for higher short-circuit current (Isc) under rear irradiance.

5. Practical Design and O&M Recommendations

1.     Thermal Management

• ≥150–200 mm clearance, reflective roofing materials, slotted mounting structures.

2.     Electrical Safety

•  Oversized cables/fuses; check inverter Isc limits with bifacial factors.

3.     Mechanical Handling

• Use G2G-rated clamps; avoid edge and mid-span stresses.

4.     Material Selection

• Prefer POE encapsulants, robust edge seals, n-type cells.

5.     O&M Practices

• Routine infrared (IR) thermography, rear-surface cleaning, and grounding verification.

Attribute	Monofacial (Glass + Backsheet)	Bifacial (Glass-to-Glass)
Temperature coefficient	-0.30%/°C typical	-0.29% to -0.30%/°C (n-type better)
Operating temp (rooftop NOCT)	Lower (~1–3 °C cooler)	Higher (~1–3 °C hotter)
Bifacial gain	None	+0–10% (depends on albedo & clearance)
Weight	Lighter	Heavier (structural checks needed)
Moisture / UV resistance	Moderate, risk of backsheet aging	Excellent, glass + POE stack
Long-term degradation	~0.5–0.55%/yr	~0.4–0.45%/yr
Mechanical risks	Backsheet cracking, UV	Glass edge seals, glass fracture

Table 1. Comparison Chart

6. Conclusion

For rooftop PV projects, monofacial glass–backsheet modules remain slightly cooler and simpler to handle, while bifacial glass–glass modules provide superior durability, reduced degradation, and bifacial gain potential when designed with sufficient ventilation and reflective surfaces. Developers and EPCs should balance thermal penalties, structural constraints, and rear irradiance conditions before making technology choices. With proper design and O&M practices, bifacial G2G modules can outperform conventional glass–backsheet modules in lifetime energy yield and reliability.