Bifacial Solar Panels: Electrical Design Considerations and Energy Yield Benefits

Bifacial solar panels have emerged as a game-changer in photovoltaic (PV) technology, offering higher energy yield by capturing sunlight on both the front and rear sides of the panel. With advancements in solar cell efficiency, mounting techniques, and optimization strategies, bifacial panels are increasingly becoming the preferred choice for utility-scale and commercial PV installations. This article explores the electrical design considerations, energy yield benefits, and analytical techniques used in bifacial PV system design.

Understanding Bifacial Solar Panel Technology

1. Structure and Working Principle

Bifacial solar panels differ from conventional monofacial panels as they can generate electricity from both direct and reflected sunlight. They typically consist of:

Bifacial solar cells: High-efficiency monocrystalline PERC (Passivated Emitter and Rear Cell) or heterojunction technology (HJT) cells.
Transparent backsheet or dual-glass: Allows light transmission to the rear side.
Frame and encapsulation: Designed to minimize shading and maximize durability.

The increased power generation is achieved through albedo, the reflectivity of the surface beneath the panels, which can enhance energy yield by 10-30% depending on installation conditions.

2. Types of Bifacial Solar Panels

Framed vs. Frameless: Frameless panels reduce rear-side shading but require specialized mounting.
Glass-Glass vs. Glass-Backsheet: Glass-glass panels offer better durability and longevity.

Electrical Design Considerations

1. Bifacial Gain and Irradiance Modeling

Bifacial Gain (BG) is the extra power generated from the rear side compared to a monofacial system. It is given by:

$BG = \frac{E_{bifacial} - E_{monofacial}}{E_{monofacial}} \times 100\%$

where:

$E_{bifacial}$ = Total energy yield of the bifacial panel (front + rear)
$E_{monofacial}$ = Energy yield of a monofacial panel under identical conditions

Simulation tools like PVsyst, Helioscope, and SAM (System Advisor Model) are used to model bifacial irradiance and optimize system design.

2. Albedo Factor Considerations

Albedo plays a critical role in bifacial performance. Common surface albedo values:

Fresh snow: 0.8 - 0.9
Concrete: 0.3 - 0.4
Grass: 0.2 - 0.25
Soil/Sand: 0.15 - 0.3

The equation for reflected irradiance $G_{rear}$ received by the panel’s rear side is: $G_{rear} = G_{front} \times Albedo \times (1 - Shadowing factor)$ where $G_{front}$ is the incident irradiance on the front side.

3. Tilt Angle and Row Spacing Optimization

Unlike monofacial PV arrays, bifacial panels require optimized tilt and spacing to maximize rear-side irradiance.

Tilt Angle: A steeper tilt increases rear-side exposure but reduces ground coverage ratio (GCR).
Row Spacing (Pitch): Wider row spacing allows more light reflection but reduces power density.

4. DC-AC Ratio and Inverter Selection

Bifacial modules produce higher energy, requiring optimized inverter sizing.

Recommended DC-AC ratios: 1.2 - 1.5
Maximum Power Point Tracking (MPPT): String inverters with multiple MPPTs are preferred.

5. Bifacial PV String Design

The rear-side contribution affects Isc (Short Circuit Current) but not Voc (Open Circuit Voltage) significantly.

Isc increases by ~10-20% due to bifacial gain, impacting conductor sizing and fuse ratings.
Voc remains nearly constant, allowing standard voltage ratings (e.g., 1000V or 1500V systems).

6. Shading and Bypass Diodes

Non-uniform rear-side irradiance can cause hotspots.
Bypass diodes mitigate partial shading effects but increase electrical loss.

Energy Yield Benefits of Bifacial Panels

1. Higher Energy Output

Bifacial modules deliver 10-30% more energy than monofacial counterparts, especially in high-albedo environments.

2. Lower Levelized Cost of Energy (LCOE)

Higher energy yield translates into reduced LCOE, making bifacial systems more cost-effective.

3. Improved Performance in Diffuse Light Conditions

Unlike monofacial panels, bifacial modules capture diffuse and reflected light, maintaining efficiency in cloudy conditions.

4. Longer Lifespan and Durability

Glass-glass bifacial modules have lower degradation rates (~0.5%/year) compared to glass-backsheet modules (~0.7%/year).
Better PID (Potential Induced Degradation) resistance.

Example Calculation

Consider a 100 kWp bifacial PV system installed on a concrete surface (albedo = 0.3). If monofacial energy yield is 150 MWh/year, the expected bifacial gain is: $BG = 150 \times 0.20 = 30 MWh/year$ Total energy output: $E_{bifacial} = 150 + 30 = 180 \text{ MWh/year}$ This results in a 20% increase in energy yield over monofacial panels.

Challenges and Mitigation Strategies

Challenge	Mitigation Strategy
Higher initial cost	Reduction in LCOE due to higher energy yield
Variability in albedo	Site-specific albedo enhancement techniques
Complex modeling	Advanced bifacial simulation tools
Rear-side shading	Elevated mounting structures, bifacial-friendly racking

Frequently Asked Questions (FAQs)

1. Are bifacial solar panels worth the extra cost?

Yes, the additional energy yield (~10-30%) justifies the marginally higher cost, leading to a lower LCOE over the system lifetime.

2. What is the best installation method for bifacial panels?

Ground-mounted systems with reflective surfaces and optimized tilt angles maximize bifacial gain.

3. How do bifacial panels perform in cloudy weather?

They perform better than monofacial panels due to their ability to capture diffused and reflected light.

4. Do bifacial panels require special inverters?

No, but inverters should accommodate increased Isc, with proper MPPT functionality for varying irradiance.

5. What is the impact of row spacing in bifacial installations?

Wider row spacing increases rear-side irradiance but reduces power density per unit area.

Conclusion

Bifacial solar panels offer substantial energy yield benefits, improved efficiency, and long-term cost savings. However, proper electrical design considerations, including inverter sizing, row spacing, and albedo management, are critical to maximizing their potential. With advancements in simulation tools and real-world performance validation, bifacial PV technology is set to redefine the future of solar energy.

Prasun Barua