Overview
Bifacial photovoltaic (PV) modules represent a significant evolution in solar energy harvesting, designed to capture sunlight on both the front and rear surfaces of the cell structure. Unlike traditional monofacial modules that rely solely on direct and diffuse irradiance hitting the glass front, bifacial panels utilize the rear side to absorb reflected light, known as albedo, from the ground or surrounding environment. This dual-sided absorption increases the total energy yield per unit area, making them particularly effective in installations with high reflectivity surfaces, such as white concrete, gravel, or snow-covered ground. The efficiency gain is not uniform; it depends heavily on the installation height, tilt angle, and the specific albedo coefficient of the surface beneath the array.
The term "Schmal," which translates to "narrow" in German, refers to a specific form factor or design trend within the bifacial module market. These modules are characterized by a reduced width compared to standard industry sizes, often maintaining a similar length. This narrow profile offers distinct mechanical and electrical advantages. Mechanically, the reduced width lowers the wind load on the mounting structure, allowing for lighter and potentially cheaper racking systems. Electrically, shorter current collection paths within the cells can reduce resistive losses, enhancing the fill factor and overall performance under partial shading conditions. This design is particularly favored in tracking systems and rooftop installations where space and structural weight are critical constraints.
The fundamental principle behind bifacial gain is the addition of rear-side irradiance to the front-side input. The total energy yield Etotal can be approximated by the sum of front and rear contributions: Etotal≈Efront+(Erear×ηrear), where ηrear represents the rear-side efficiency relative to the front. This efficiency is influenced by the cell technology, such as PERC (Passivated Emitter and Rear Cell) or TOPCon (Tunnel Oxide Passivated Contact), and the quality of the rear contact grid. The albedo effect is quantified by the albedo coefficient α, which ranges from 0.1 for dark soil to 0.9 for fresh snow. Higher albedo values directly translate to increased rear-side current generation, boosting the module's overall power output.
Caveat: The actual bifacial gain is often lower than laboratory predictions due to self-shading between adjacent rows, module frame obstruction, and temperature variations. Real-world performance ratios typically range from 5% to 25%, depending on the specific installation geometry and environmental conditions.
Adoption of narrow bifacial modules has accelerated in recent years, driven by the need for higher energy density and reduced balance-of-system costs. These modules are increasingly common in utility-scale solar farms and commercial rooftops, where optimizing every watt-peak (Wp) is crucial for return on investment. The integration of bifacial technology with single-axis trackers further amplifies the gain, as the rear side captures more direct sunlight as the modules rotate throughout the day. This synergy between module design and system architecture exemplifies the ongoing optimization in solar photovoltaic infrastructure.
How does bifacial gain work in narrow modules?
Bifacial photovoltaic modules capture solar irradiance on both the front and rear surfaces, converting additional photons into electricity. The physics relies on light penetrating the front cell structure, reflecting off the ground or surrounding surfaces, and entering the rear side of the silicon wafer. This rear-side irradiance is not uniform; it depends heavily on the albedo of the surface beneath the module, the tilt angle, and the height of the module above the ground. Higher albedo surfaces, such as white gravel or concrete, reflect more light than grass or soil, significantly boosting the rear current.
The term "Schmal" refers to the narrow width of the module, typically around 1.1 meters compared to the standard 1.3 meters for monofacial panels. This dimension is critical in tracking systems and dense arrays. Narrower modules reduce the shading cast on adjacent rows, allowing for tighter spacing without significant inter-row shading losses. This is particularly important for single-axis trackers, where the module width directly influences the optimal row spacing to maximize land use efficiency.
| Parameter | Monofacial | Bifacial (Schmal) |
|---|---|---|
| Light Capture Sides | Front only | Front and Rear |
| Albedo Dependency | Moderate | High |
| Typical Width | ~1.3 m | ~1.1 m |
| Shading Impact | Higher inter-row shading | Reduced inter-row shading |
| Gain Factor | 1.0 (Baseline) | 1.05 - 1.25 |
The bifacial gain is quantified by the ratio of rear irradiance to front irradiance, often expressed as a percentage. This gain is not constant; it varies with the sun's position and the reflection properties of the ground. For a narrow module, the reduced width means that the rear side is less likely to be shaded by the front side of the adjacent module in a tracker array. This allows for better light trapping, where light enters the rear cell at more optimal angles, reducing reflection losses.
Caveat: Bifacial gain is not free. It requires careful system design. If the modules are spaced too far apart, the land use efficiency drops, negating the energy gain per square meter. Conversely, if they are too close, inter-row shading reduces the rear-side benefit.
Light trapping is enhanced by the texture of the silicon cells and the transparency of the rear glass. In narrow modules, the frame design is also optimized to minimize the shadow cast on the rear side. The frame must be thin enough to allow light to pass through but strong enough to support the module. This balance is crucial for maximizing the bifacial effect. The gain is also influenced by the height of the module above the ground. Higher modules capture more reflected light, but this increases structural costs and wind load.
The physics of rear-side irradiance involves complex interactions between direct, diffuse, and reflected light. Direct light hits the front, while diffuse light comes from the sky. Reflected light, or albedo, is the key differentiator for bifacial modules. The albedo value can range from 0.2 for grass to 0.7 for white concrete. This means that the same module can produce significantly different amounts of energy depending on the ground cover. Engineers must model these factors to predict the performance of a bifacial array accurately.
Narrow modules also offer advantages in installation and handling. They are easier to maneuver in tight spaces, such as rooftops or urban solar farms. This flexibility can lead to higher capacity factors in diverse environments. The reduced width also means that the thermal distribution across the module is more uniform, reducing hot spots and improving long-term reliability. These factors contribute to the overall efficiency and cost-effectiveness of bifacial technology.
What are the main types of bifacial module architectures?
Bifacial photovoltaic modules capture sunlight on both the front and rear surfaces of the solar cells, increasing energy yield by harvesting direct irradiance and albedo (reflected light). The structural architecture of these modules is critical for durability, weight, and optical efficiency. The three primary constructions are glass-glass (dual-glass), glass-plastic, and all-glass structures, each offering distinct trade-offs in cost and performance.
Glass-Glass vs. Glass-Plastic Constructions
Glass-glass modules, often referred to as dual-glass, consist of two sheets of tempered glass encapsulating the solar cells and the backsheet. This structure provides superior mechanical strength and resistance to environmental stressors, such as humidity ingress and UV degradation. The rear glass layer allows for high light transmission, typically exceeding 90%, which is crucial for maximizing rear-side energy capture. However, the added weight—often ranging from 20 to 25 kg per 200 W module—requires robust mounting systems, particularly in rooftop installations.
In contrast, glass-plastic modules use a polymer backsheet, usually made of Tedlar (PVF) or PET, on the rear side. This reduces weight significantly, making them easier to handle and install. However, the plastic backsheet is generally less transparent than glass, potentially limiting rear-side gain. Additionally, polymer backsheets may degrade faster under prolonged UV exposure, affecting the long-term reliability of the module. The choice between glass-glass and glass-plastic often depends on the specific installation environment and structural load capacity.
Cell Technologies in Schmal-Format Modules
Schmal-format bifacial modules, characterized by their narrower width compared to standard 120-cell modules, are designed to optimize shading tolerance and mounting flexibility. These modules commonly utilize advanced cell technologies such as PERC (Passivated Emitter and Rear Cell), TOPCon (Tunnel Oxide Passivated Contact), and HJT (Heterojunction Technology).
PERC cells are widely adopted due to their cost-effectiveness and moderate bifaciality, typically ranging from 70% to 80%. TOPCon cells offer higher efficiency and improved temperature coefficients, making them suitable for high-irradiance environments. HJT cells, known for their superior low-light performance and low temperature coefficient, provide excellent bifacial gains, often exceeding 85%. The bifaciality factor (β) is defined as the ratio of rear-side short-circuit current to front-side short-circuit current:
β=Isc,frontIsc,rearThis metric is essential for estimating the energy yield of bifacial modules under varying albedo conditions. Schmal-format modules, with their narrower width, can better accommodate partial shading, which is common in urban and rooftop installations. The combination of advanced cell technologies and optimized module formats enhances the overall performance and versatility of bifacial PV systems.
Caveat: While bifacial modules offer higher energy yields, their performance is highly dependent on the albedo of the surface beneath them. Low-albedo surfaces, such as dark roofs, may reduce the rear-side gain significantly.
The integration of bifacial technology with Schmal-format designs represents a strategic approach to maximizing energy production in diverse installation scenarios. By selecting the appropriate module architecture and cell technology, system designers can optimize performance, reliability, and cost-effectiveness for specific project requirements.
Design and engineering considerations
Mechanical and Thermal Profile
Narrow bifacial photovoltaic modules, often referred to as "Schmal" formats in European markets, present distinct mechanical challenges compared to standard 60- or 72-cell layouts. The reduced width, typically ranging from 900 mm to 1050 mm, alters the aspect ratio, influencing wind load distribution and mounting hardware requirements. Engineers must account for higher localized stress concentrations at the frame edges, particularly when using rail-less mounting systems common in agrivoltaics or vertical facades. The glass-glass construction standard for bifaciality adds weight, often exceeding 22 kg per module, necessitating robust racking solutions to prevent micro-cracking under thermal cycling.
Thermal performance is critical for efficiency retention. Bifacial modules benefit from enhanced rear-side heat dissipation, which can lower the operating temperature by 1–3°C compared to monofacial counterparts. This reduction improves the voltage coefficient, defined by the formula ΔVoc=Voc×γ×ΔT, where γ is the temperature coefficient of open-circuit voltage. However, narrow modules may experience edge effects that create thermal gradients, potentially accelerating encapsulant yellowing if the EVA or POE layers are not optimized for high UV exposure on both sides.
Caveat: Bifacial gain is highly dependent on albedo. On low-albedo surfaces like grass, the rear contribution may drop below 5%, whereas white gravel or snow can push it above 20%. Design assumptions must reflect local ground cover.
Electrical Characteristics and Mismatch
Electrical design for narrow bifacial modules requires careful attention to current and voltage matching. The reduced cell count per string (often 60 or 72 half-cut cells) results in slightly lower open-circuit voltage (Voc), which can optimize inverter input ranges in hot climates. However, the bifacial nature introduces a variable rear current (Irear), leading to potential mismatch losses if the rear illumination is not uniform across the module array. The effective maximum power point voltage (Vmp) shifts dynamically with the bifacial factor (β), calculated as β=Prear/Pfront. Mismatch losses can be quantified using the formula Lmismatch=1−∑Ii×Vi∑Pi, where variations in β across modules cause current divergence in series strings.
To mitigate these losses, module manufacturers often specify tighter tolerances for Vmp and Imp. System designers must ensure that the bifacial gain does not exceed the inverter’s maximum power point tracking (MPPT) range, especially in partial shading scenarios. The use of half-cut cells helps reduce resistive losses (I2R) by halving the current per cell, which is particularly beneficial for narrow modules where busbar layouts are optimized for lower current densities.
| Parameter | Typical Value (Narrow Bifacial) |
|---|---|
| Module Dimensions (W x H) | 1000 mm x 1722 mm |
| Rated Power (Pmax) | 400 – 450 Wp |
| Open-Circuit Voltage (Voc) | 45 – 50 V |
| Max Power Voltage (Vmp) | 38 – 42 V |
| Temperature Coefficient (γ) | -0.35 %/°C |
| Bifaciality Factor | 85 – 90 % |
Applications and use cases
Schmal bifacial modules are engineered for environments where the rear-side irradiance contribution is maximized by specific geometric arrangements. Their narrow width, typically ranging from 1.0 to 1.1 meters compared to standard 1.3-meter modules, reduces shading losses and allows for optimized packing in constrained spaces. This form factor is particularly advantageous for vertical bifacial installations, where modules are mounted at 90-degree angles to capture low-angle morning and evening sunlight. Such configurations are common in agrivoltaics, where vertical arrays allow crops to grow between rows, and in urban facades, where aesthetic integration is as critical as energy yield. The reduced width also minimizes the "edge effect," where the frame or busbars cast shadows on the rear cells, thereby enhancing the bifacial gain factor.
In carport structures, these modules provide dual utility: energy generation and shading for vehicles. The narrow profile allows for wider gaps between rows, improving airflow and reducing the "heat island" effect under the carport. This configuration often utilizes single-axis trackers tilted at low angles, which can increase annual energy yield by 15–25% depending on latitude and ground albedo. The bifacial gain, defined as the ratio of rear-side irradiance to front-side irradiance, is significantly influenced by the ground surface reflectivity. For example, a white gravel surface can increase rear-side yield by up to 30% compared to a grassy surface.
Caveat: Bifacial gain is not constant; it varies seasonally and diurnally. In winter, the lower sun angle increases rear-side exposure, while in summer, the higher sun angle may reduce it, depending on the tracker’s tilt strategy.
Logistics and mounting hardware present unique considerations for Schmal bifacial modules. Their narrower width allows for more flexible handling in tight spaces, such as rooftop installations with limited crane access. However, the increased number of modules required for the same capacity can raise balance-of-system (BoS) costs, particularly in wiring and racking. Specialized mounting systems, such as rail-less clamps or integrated rail-clamp systems, are often employed to reduce shading from the racking structure. These systems must account for the module’s specific thermal expansion coefficients to prevent mechanical stress over time.
Performance Metrics and Configuration
The energy yield of a bifacial module can be approximated using the formula: Etotal=Efront+Erear×ηbifacial, where ηbifacial is the bifaciality factor, typically between 85% and 95% for modern Schmal modules. This factor represents the ratio of rear-side power output to front-side power output under standard test conditions. In vertical installations, the rear-side contribution can account for up to 40% of the total energy yield, depending on the azimuth orientation and local albedo. For tracker systems, the optimal tilt angle is often calculated to maximize the sum of front and rear irradiance, which may differ from the optimal angle for monofacial modules.
Controversy exists regarding the long-term durability of bifacial modules in harsh environments. While the glass-glass construction offers superior mechanical strength and reduced potential-induced degradation (PID), the rear-side cells are more exposed to environmental stressors such as dust accumulation and bird droppings. Regular cleaning schedules are essential to maintain performance, particularly in arid regions where dust can reduce rear-side yield by up to 10%. Additionally, the increased complexity of the mounting systems can lead to higher initial installation costs, which must be weighed against the long-term energy yield benefits.
The choice between Schmal bifacial modules and standard monofacial modules depends on the specific project requirements. For large-scale utility projects with ample space, monofacial modules may offer a lower cost per watt. However, for constrained spaces, such as urban rooftops or agrivoltaic farms, the flexibility and higher energy density of Schmal bifacial modules make them a compelling option. As the technology matures, advancements in cell efficiency and mounting systems are expected to further enhance their competitiveness in the global solar market.
Worked examples
Bifacial modules capture light on both the front and rear surfaces. The "Schmal" (narrow) form factor typically refers to modules with a width around 1.1 meters, which allows for tighter row spacing or higher packing density on single-axis trackers compared to standard 2-meter-wide modules. This geometry influences shading and the effective albedo reaching the rear side.
Example 1: Basic Yield Calculation with Fixed Albedo
Calculate the expected energy yield for a 1 MWp array using Schmal bifacial modules on a single-axis tracker. Assume a front-side yield of 1,400 kWh/kWp, a bifaciality factor of 0.85, and a ground albedo of 0.35. Assume the effective rear irradiance is 60% of the front irradiance due to shading and cosine losses.
- Determine the rear-side energy contribution. The rear yield is calculated as: Front Yield × Bifaciality Factor × Albedo × Rear Irradiance Factor.
- Rear Yield = 1,400 kWh/kWp × 0.85 × 0.35 × 0.60 = 249.9 kWh/kWp.
- Calculate total bifacial yield. Add the front and rear yields: 1,400 + 249.9 = 1,649.9 kWh/kWp.
- Determine the percentage gain. (249.9 / 1,400) × 100 ≈ 17.85%.
The total expected yield is approximately 1,650 kWh/kWp. This represents a nearly 18% increase over the monofacial baseline.
Example 2: Impact of Higher Albedo Surface
Consider the same 1 MWp Schmal module array, but installed on a white gravel surface with an albedo of 0.45. Keep the bifaciality factor at 0.85 and the rear irradiance factor at 0.60.
- Recalculate the rear yield with the new albedo: 1,400 kWh/kWp × 0.85 × 0.45 × 0.60.
- Rear Yield = 333.9 kWh/kWp.
- Calculate total yield: 1,400 + 333.9 = 1,733.9 kWh/kWp.
- Determine the percentage gain: (333.9 / 1,400) × 100 ≈ 23.85%.
The yield increases to approximately 1,734 kWh/kWp. The gain rises to nearly 24%, demonstrating the sensitivity of bifacial gain to ground reflectivity.
Did you know: The narrow width of Schmal modules can reduce mutual shading between rows on single-axis trackers. This can increase the effective rear irradiance factor from the typical 0.60 to as high as 0.70 in dense arrays, further boosting yield.
Example 3: Adjusting for Shading in Dense Arrays
Using the white gravel scenario (albedo 0.45), assume the Schmal module's narrow width allows for tighter spacing, increasing the rear irradiance factor to 0.70. Recalculate the yield.
- Recalculate rear yield with increased irradiance factor: 1,400 kWh/kWp × 0.85 × 0.45 × 0.70.
- Rear Yield = 377.7 kWh/kWp.
- Calculate total yield: 1,400 + 377.7 = 1,777.7 kWh/kWp.
- Determine the percentage gain: (377.7 / 1,400) × 100 ≈ 26.98%.
The total yield is approximately 1,778 kWh/kWp. The gain is nearly 27%. This example highlights how the physical dimensions of Schmal modules interact with tracker geometry to enhance performance.
Market trends and economic analysis
Bifacial photovoltaic modules capture sunlight on both the front and rear surfaces, increasing energy yield compared to monofacial counterparts. As of 2026, the premium for bifacial technology has narrowed significantly, driven by economies of scale and improved cell efficiencies. The "Schmal" form factor, characterized by narrower, longer modules, introduces specific trade-offs in balance-of-system (BoS) costs. This configuration is particularly relevant for tracking systems and specific architectural integrations.
The economic viability of bifacial Schmal modules depends on the bifacial gain, which is the additional energy harvested from the rear side. This gain is influenced by albedo (surface reflectivity), mounting height, and row spacing. The effective capacity factor increases, but the initial capital expenditure (CapEx) is higher. Investors must weigh the incremental yield against the premium pricing of the modules and the structural requirements.
Cost-Benefit Analysis
Bifacial modules typically command a price premium of 5% to 15% over monofacial modules, depending on the market segment and volume. The Schmal form factor may add a further premium due to less standardized manufacturing processes. However, the energy yield increase can offset this cost. The levelized cost of energy (LCOE) is a critical metric, calculated as:
LCOE = (Total Lifetime Costs) / (Total Lifetime Energy Output)
For bifacial Schmal modules, the total lifetime energy output increases due to higher annual yield. The total lifetime costs include module cost, mounting structure, and inverter sizing. The narrower width of Schmal modules can reduce shading losses in dense arrays, improving the rear-side irradiance capture. This can lead to a more uniform current distribution, potentially reducing mismatch losses.
Impact on Balance-of-System Costs
The Schmal form factor affects BoS costs in several ways. Narrower modules allow for more flexible mounting configurations, which can optimize the rear-side exposure. This is particularly beneficial for single-axis trackers, where the module orientation can be adjusted to maximize bifacial gain. However, the increased length may require longer rails or more clamps, adding to the structural costs. The weight distribution also needs to be considered, as Schmal modules may have different load-bearing requirements.
Inverter sizing is another critical factor. Bifacial modules produce higher currents, which may require up-sizing of inverters or optimizers. The Schmal form factor, with its longer length, may lead to higher voltage drops along the module, necessitating careful string design. The use of micro-inverters or power optimizers can mitigate these issues, but adds to the initial investment.
Caveat: The economic advantage of bifacial Schmal modules is highly site-specific. High albedo surfaces, such as white gravel or concrete, maximize the rear-side gain. In contrast, low albedo surfaces, such as grass or soil, may reduce the benefit, making the premium less justifiable.
Market trends indicate a growing adoption of bifacial technology, with Schmal modules gaining traction in specific niches. The demand is driven by the need for higher energy density and flexibility in module layout. However, the standardization of module sizes remains a challenge, with manufacturers offering various dimensions. This lack of standardization can lead to higher BoS costs due to customized mounting solutions.
As of 2026, the bifacial premium is expected to continue declining, making bifacial Schmal modules more competitive. The key to maximizing economic returns lies in optimizing the system design to leverage the bifacial gain. This includes careful selection of mounting structures, inverter sizing, and site preparation. The integration of digital twins and simulation tools can aid in this optimization process, providing accurate predictions of energy yield and BoS costs.