Perovskite Degradation Physics: A Comparative Analysis of Front vs. Rear in Tandem Modules

Perovskite Degradation Physics: A Comparative Analysis of Front vs. Rear in Tandem Modules
Perovskite-silicon tandem modules represent a significant advancement in photovoltaic technology, offering potentially groundbreaking laboratory efficiencies exceeding 30% (NREL), significantly surpassing the Shockley-Queisser limit for single silicon junctions. However, their complex, multi-layered structure introduces a fundamental asymmetry in kinetic and thermodynamic degradation mechanisms between the front (perovskite) and rear (perovskite-silicon) layers. Understanding this dynamic is absolutely fundamental for predicting long-term performance, operational reliability, and for accurate modeling of the LCOE (Levelized Cost of Energy) of the entire photovoltaic system.
Front Perovskite Layer: Direct Exposure and Accelerated Degradation
The front perovskite layer (typically based on FAPbI3 or mixed-halide FA/MA/CsPb(I,Br)3 systems) is directly exposed to the full spectrum of solar radiation at 1000 W/m² irradiance (STC), including high-energy UV radiation (<400 nm), and to dynamically changing environmental conditions such as thermal cycles (ΔT up to 100°C), humidity (up to 100% RH), and mechanical pressure (up to 2400 Pa). The main degradation mechanisms include:
Photo-oxidation and UV destabilization: UV radiation with photon energies >3.1 eV (for λ<400 nm) initiates chemical reactions leading to the formation of reactive oxygen species (ROS), such as hydroxyl radicals (•OH), superoxide (O2•-), and singlet oxygen (1O2), especially in the presence of crystal lattice defects (e.g., at grain boundaries or halide vacancies). This leads to irreversible decomposition of the perovskite structure, formation of inactive phases (e.g., PbO, PbI2), and loss of crystalline structure. This phenomenon is strongly correlated with the UV radiation dose. A typical efficiency (PCE) degradation rate under UV is 0.5-1% per 100 kWh/m² of exposure.
Ion migration and phase segregation: Halide ions (I-, Br-) and organic/inorganic cations (MA+, FA+, Cs+, Pb2+) exhibit significant mobility within the perovskite lattice. Under an electric field gradient (e.g., during module operation, ~0.6-0.8 V per cell) and elevated operating temperatures (NOCT for a tandem module can be 45-55°C, module temperature can reach 70-80°C), ion migration occurs, leading to phase segregation (e.g., formation of iodide- or bromide-enriched regions), creation of recombination centers, and ion drift towards electrodes, affecting the decrease in Voc and FF. The diffusion coefficient of iodides (D_I) in MAPbI3 at 60°C is approximately 10^-10 cm²/s.
Organic component decomposition and thermal instability: Commonly used organic cations (e.g., MA+, FA+) are thermally and photochemically unstable. For instance, MAPbI3 decomposes at temperatures >150°C, and FAPbI3 exhibits greater stability but is still susceptible to prolonged exposure to elevated temperatures (>80°C), leading to volatilization of components (e.g., CH3NH2, HI) and destabilization of the perovskite structure, as well as affecting the integrity of charge transport layers (HTL/ETL). A typical thermal decomposition rate is 0.05-0.1% per day at 85°C.
Oxygen and moisture ingress: Even minimal penetration of O2 and H2O through encapsulating materials (EVA, POE, glass) leads to perovskite hydration (e.g., formation of PbI2•H2O hydrates) and oxidation (e.g., PbO), especially in the presence of light and in areas of mechanical damage (e.g., micro-cracks). IEC 61215/61730 standards require tests for moisture and heat resistance (DH test: 1000h at 85°C/85% RH) and thermal cycling (TC test: 200 cycles -40°C to +85°C). Moisture accumulation in the encapsulant can lead to material hydrolysis and electrode corrosion.
This results in a relatively short effective lifespan before significant efficiency drop. Typical predictions, based on accelerated aging tests (IEC 61215:2016, IEC 61730:2016), indicate an initial efficiency drop of 10-20% within the first 5 years and 50-70% within 10 years, yielding an effective production lifespan of 5–10 years for the perovskite layer itself (e.g., PCE_t = PCE_0 * exp(-α_f * t), where α_f = 0.07-0.14 year^-1).
Rear Perovskite Layer: Protected Environment and Extended Lifespan
The rear perovskite layer (typically optimized for NIR absorption, e.g., through halide composition modifications or additional up-conversion layers) operates under significantly milder conditions, due to its strategic position and the specific light spectrum reaching it. Protective factors include:
UV screening by the silicon subcell: The silicon subcell (c-Si), with a bandgap of ~1.12 eV, positioned in front of the rear perovskite layer, absorbs almost all UV radiation (<400 nm) and a significant portion of the visible spectrum. UV absorption by silicon is over 99% for wavelengths below 400 nm. The rear perovskite layer primarily receives scattered and reflected lower-energy light (mainly in the NIR range, <1.7 eV), which is significantly less destructive to the perovskite structure. This reduces photo-oxidation by a factor of 10-50x.
Lower operating temperatures: The rear side of the module, often less exposed to direct sunlight and additionally convection-cooled, typically operates at lower temperatures than the front (on average by 5-15°C). For example, in a module operating with a front surface temperature of 60°C, the rear surface may have a temperature of around 50-55°C. Lower temperatures slow down the kinetics of ion migration (according to the Arrhenius equation, where reaction rate decreases exponentially with temperature) and thermal decomposition of components. Every 10°C temperature reduction can double the lifespan.
Additional encapsulation and diffusion barriers: The silicon layer and multi-layered encapsulation (e.g., glass-glass, using highly moisture- and UV-resistant encapsulants like POE) provide an additional, multi-level protective barrier against oxygen and moisture ingress. Glass modules have a water vapor transmission rate (WVTR) below 0.01 g/(m²·day), compared to 0.1-1 g/(m²·day) for polymer films. Additional barrier layers (e.g., Al2O3, TiO2) can reduce WVTR to 10^-5 g/(m²·day), significantly slowing degradation.
Reduced photovoltaic stress: Due to the absorption of a significant portion of energy by the front perovskite layer and the silicon subcell, the rear perovskite layer experiences less "photovoltaic stress" (i.e., lower optical and electrical power density), which also translates to a slower degradation rate.
As a result, the degradation of the rear perovskite layer is significantly slower. The predicted lifespan, supported by advanced stability tests (e.g., extended DH tests up to 2000h, cyclic freeze-thaw tests up to 600 cycles), can be 20–30+ years, with an annual degradation of ~0.3-0.5% (e.g., PCE_t = PCE_0 * exp(-α_r * t), where α_r = 0.003-0.005 year^-1). This means that after 20 years, the rear layer can retain over 90% of its initial efficiency, and after 30 years, still >85%. This phenomenon is crucial for engineers because it means that in the long term, the rear side will stabilize and dominate the total generation of the tandem module.
This fundamental difference in degradation kinetics means that in the long-term perspective (30–50 years of operation), the rear side becomes not only an additional but indeed a key pillar of stable and predictable energy yield in bifacial tandem modules. This is a paradigm shift requiring a holistic redesign of entire PV systems (structures, cabling systems, tracker arrangements, ground albedo) to maximize "rear gain". Ignoring this aspect in design leads to underestimation of system potential and a reduction in LCOE.
Detailed Analysis of Perovskite Degradation Chart in Tandem Modules: The line chart clearly illustrates the dramatic disparity in the lifespan of both perovskite layers, which has critical implications for PV system design. While the nominal efficiency of the front perovskite layer (simulated for an aggressive degradation scenario, e.g., 14% annually in the initial years, then slowing down) dramatically drops to below 50% after 5 years and to ~24% after 10 years, the rear layer retains approximately 78% and 61% of its initial efficiency, respectively. The crossover point, where the effective efficiency of the rear layer surpasses that of the front, occurs as early as after about 4-5 years. After 30 years, when the front perovskite layer practically ceases to generate energy (<2% of initial efficiency, translating to a marginal contribution to the module's total power), the rear layer still maintains over 22% of its initial efficiency, generating a stable energy stream. This highlights the strategic importance of optimizing light harvesting from the rear side of the module (rear gain), as it becomes the dominant energy source in the advanced age of the module (after 15-20 years of operation). Engineers should account for these differentiated degradation curves in modeling energy yields for projects with a time horizon of 25-30 years and longer, as well as in LCOE analyses, where extended component lifespan crucially impacts investment profitability. Financial models must consider not only initial efficiency but also differentiated degradation profiles.
Engineering Insight and LCOE Implications:
When designing PV systems with tandem modules, standard yield prediction models (e.g., PVsyst, SAM, PVSOL) based on a single global module degradation curve can significantly underestimate long-term performance. It is necessary to implement advanced models that account for independent degradation curves for the front and rear perovskite layers and for the silicon layer, and then sum them while considering the contribution of bifacial light (rear gain). Spectral analysis of the module's surroundings (e.g., using spectral radiometers measuring albedo in different bands) and optimization of mounting height and surface type (e.g., white gravel with albedo >0.6, white membrane with albedo >0.8) are also recommended to maximize reflected light to the rear side. In the context of LCOE analysis, the longer effective lifespan of the rear perovskite layer (and silicon) contributes to lowering LCOE by extending the revenue generation period and reducing the average cost of energy per kWh.
LCOE Methodology: LCOE = (Total Life Cycle Cost + Total O&M + Fuel Costs) / (Total Energy Generated).
Impact of Degradation: Slower degradation of the rear side extends the period of useful energy generation, increasing the denominator in the LCOE equation.
O&M Costs: Early degradation of the front layer might increase the need for earlier re-powering; however, the advantage of the rear layer can amortize this, shifting the timing of key O&M investments.
Structural Optimization: Minimizing shading by the support structure (the "structure shadowing ratio") is crucial. For systems with trackers, optimizing the panel tilt angle and mounting height (typically 0.8-1.5m for optimal bifacial gain) directly impacts albedo and the amount of light reaching the rear side.
Longevity of 4T Tandem Modules — Evolution of Perovskite Rear Layer Energy Dominance Over Decades
In a 4-terminal (4T) tandem module, where the silicon cell (front-side) and perovskite cell (rear-side) function as independent electrical bypasses, we observe a unique and critical phenomenon: the dynamic increase in the contribution of the rear (perovskite) side to the total energy production over time. This engineering phenomenon, stemming from fundamental differences in the degradation mechanisms and rates of individual sub-cells, is crucial for the long-term stability, operational predictability, and profitability of photovoltaic projects, directly influencing metrics such as LCOE (Levelized Cost of Energy).
Impact on Long-term Stability
The presented chart clearly shows that after 30 years of operation, the perovskite layer (rear) surpasses the silicon layer (front) in terms of absolute energy generation. This is a fundamental paradigm shift in PV system design, enabling a significant extension of the economic lifespan of installations.
Industry Standards and Risk Assessment: Currently, most PV module warranties cover 25-30 years, with power degradation not exceeding 0.5% annually after the first year. Tandem modules, especially in a 4T configuration, can revolutionize these standards by offering lower degradation rates for the entire system, and particularly for the crucial perovskite layer, which minimizes the risk of unplanned component replacements and reduces O&M costs in the long term.
Assumptions of the Degradation Model and Engineering Analysis
The model presented in the charts simulates the long-term behavior of a 4T tandem module, based on the following critical assumptions and engineering parameters:
Initial Power Contribution (STC, AM1.5G):
Silicon layer (front): 60% of total initial power. This is a typical value for bifacial tandem modules, where some light is reflected from the substrate, reducing the effective power obtained from the front relative to the theoretical efficiency of a monocrystalline silicon cell. The operating temperature on the front-side often reaches 50-70°C under full sunlight conditions.
Perovskite layer (rear): 40% of total initial power. Power generation here is dominated by light reflected from the ground (albedo effect) and scattered light. This value is strongly dependent on the bifaciality factor of the installation (e.g., optimal for 1-axis trackers over bright ground, such as white gravel or fresh snow, where albedo can reach 0.8-0.9), which increases the effective exposure of the rear layer.
Annual Power Degradation Rate (average):
Silicon layer (front): 2% annually. This is a conservative assumption, considering a range of degradation mechanisms typical for front-side silicon cells: LID (Light-induced Degradation), LeTID (Light and Elevated Temperature Induced Degradation), PID (Potential Induced Degradation), UV degradation, and general aging of encapsulating materials (EVA, glass). Typical monocrystalline modules show degradation of 0.3-0.7% annually, but in a tandem module, the front-side is exposed to additional thermal and spectral stresses, including partially filtered but still high-energy photons, as well as interaction effects with perovskite. IEC 61215 tests (cyclic temperature, humidity, and mechanical load testing) and IEC 61730 (electrical safety) are crucial in assessing this degradation.
Perovskite layer (rear): 0.3% annually. The significantly lower degradation rate results from the fact that the perovskite cell is protected from direct and full-spectrum solar radiation by the upper silicon layer and encapsulating materials. It primarily receives reflected and scattered light, which is already filtered (especially in the UV range, where intensity is reduced by over 95%) and has lower intensity. This leads to milder operating conditions (lower perovskite operating temperatures of 30-50°C in normal operation) and significantly less photochemical stress. Simulations based on Arrhenius and Power Law degradation models indicate an exponential decrease in degradation rate under conditions of reduced UV and thermal load, confirming their long-term stability in "shaded" or reflected light conditions. Accelerated tests (e.g., humidity and heat 85°C/85% RH for 1000h) indicate a significant improvement in perovskite stability compared to early generations.
As a result of these differentiated degradation rates, after 30 years, the perovskite layer can become the dominant energy source in a 4T module, even despite the overall, predicted decrease in the module's total energy production (from 100% to ~70%, according to the simulation). This unparalleled stability of the perovskite rear layer is crucial for the predictability, profitability, and reduction of LCOE for photovoltaic systems over decades.
The following chart illustrates in detail the increasing contribution of the rear side to the total energy production in a 4T tandem module. The dynamic growth in the importance of the rear perovskite layer highlights its potential as a key element for increasing the longevity and energy efficiency of PV systems, especially in the face of progressive degradation of traditional silicon cells and the requirements of standard PN-EN 61853-1 specifying performance measurements under various environmental conditions.
Methodology for Calculating Rear Layer Power Contribution:
Rear Power Contribution(t) [%] = [PRear(t) / (PRear(t) + PFront(t))] * 100%
Where:
PFront(t) = PFront(0) * (1 - DRFront)t (Power of the silicon layer at time t)
PRear(t) = PRear(0) * (1 - DRRear)t (Power of the perovskite layer at time t)
Input Parameters from the Simulation Model:
PFront(0) = 0.6 (initial power index of the silicon layer, 60% of total power)
PRear(0) = 0.4 (initial power index of the perovskite layer, 40% of total power)
DRFront = 0.02 (annual degradation rate of the silicon layer = 2%)
DRRear = 0.003 (annual degradation rate of the perovskite layer = 0.3%)
t = time in years
Detailed Calculation Example for t = 10 years:
PFront(10) = 0.6 * (1 - 0.02)10 ≈ 0.6 * 0.81707 ≈ 0.49024
PRear(10) = 0.4 * (1 - 0.003)10 ≈ 0.4 * 0.97036 ≈ 0.38814
Rear Power Contribution(10) = [0.38814 / (0.38814 + 0.49024)] * 100% ≈ [0.38814 / 0.87838] * 100% ≈ 44.18%
This detailed engineering analysis emphasizes that 4T tandem modules offer not only higher initial efficiency (in accordance with IEC 60904-1, which defines methods for measuring PV current-voltage characteristics) but also significantly increased stability and predictability of energy production throughout their extended operational life, making them an attractive and economically justified solution for long-term photovoltaic projects. The low degradation rate of the perovskite layer on the rear side represents a significant competitive advantage, supporting LCOE optimization by extending the period of effective generation and reducing maintenance costs.
Impact of Mounting Structure on Bifacial Gain – Advanced Engineering Analysis
The mounting structure is fundamental for the effective utilization of bifacial modules, serving as a key element determining optimal energy yield. It directly influences the amount of solar radiation reaching the module's rear side by managing the albedo effect and minimizing shading. Differences in yield, resulting from the precise selection and design of mounting profile types, are dramatic and can determine investment profitability, especially in regions with moderate ground albedo and in the context of optimizing the LCOE (Levelized Cost of Energy) metric.
The data presented in the chart clearly illustrate the critical importance of selecting the mounting structure. The potential energy yield from the module's rear side (rear gain) can be almost doubled by using optimized closed profiles, which translates into a significant reduction in LCOE. For a 1 MWp project, a difference of 105.6 kWh/kWp annually in rear yield means an additional 105.6 MWh of energy, which at an energy price of 0.5 PLN/kWh generates an annual additional revenue of ~52,800 PLN, significantly improving economic indicators.
Key Difference: Shading Optimization, Structural Rigidity, and Cost-Effectiveness
The data above show that the use of 3 mm closed profiles (e.g., rectangular or square aluminum profiles) can almost double the module's rear-side yield compared to commonly used 1.5 mm open C/Z profiles (cold-formed steel profiles). This stems from several fundamental engineering and optical factors:
Shading Minimization: Closed profiles, properly designed and optimized for size and color (light-colored or silver anodized recommended), significantly reduce shading generated by structural elements on the module's rear side. In contrast, open profiles (e.g., C, Z) create larger and more dynamically changing shadows (depending on the angle of incidence), which shift with the apparent position of the sun, effectively reducing the exposure area of the module's rear part to reflected radiation (the "view factor" for reflection). Standard open profiles can cause losses of 5-15% of bifacial yield solely due to shading, primarily located at the module-profile interface. It is estimated that small-cross-section closed profiles can reduce this effect to <2%. Additionally, accurate estimation of structural shading losses requires advanced 3D modeling and simulation analysis, considering the sun's path and geolocation.
Greater Structural Rigidity and Mechanical Strength: 3 mm profiles (typically made of 6063-T6 or 6005-T5 aluminum alloy with a yield strength Re ≥ 240 MPa) offer significantly higher stiffness (EI, where E is Young's modulus, I is the moment of inertia of the cross-section) and bending strength (section modulus W), which translates into module stability and optimal positioning. A rigid structure prevents micro-movements, deformations (e.g., deflection under wind or snow loads), and natural vibrations that could adversely affect the angle of light incidence on the module's rear side, mounting stability (risk of clamp loosening), and overall performance. Angular deformations of several degrees (e.g., >2°) can already cause noticeable yield losses (>1%) by changing the effective "view" of the reflective surface and uneven cell illumination. Higher stiffness also minimizes the risk of cell micro-cracks in PV modules, which is crucial for long-term reliability and reducing the rate of mechanically induced degradation (MEGID).
Mounting and Maintenance (O&M) Optimization: Closed profiles often allow for more optimized mounting, e.g., using fewer clamps or mounting points, which further minimizes shading and facilitates airflow. Fewer contact points also mean faster installation and potentially lower labor costs. From an operational (O&M) perspective, structures with higher rigidity are more resistant to external factors, reducing the need for frequent inspections and maintenance, thereby lowering long-term operating costs of the system.
Impact on LCOE: Initially more expensive closed profiles may prove more profitable over the project's life cycle due to higher energy yields (increased revenue) and lower O&M costs, leading to a lower LCOE (Levelized Cost of Energy) over 25-30 years of operation. LCOE analysis should consider both CAPEX (capital expenditures) and OPEX (operating expenditures), including the impact of degradation and potential losses.
Detailed Engineering Analysis and Factors Affecting Rear Gain in Bifacial Systems
The yield from the rear side of a bifacial module (rear gain or bifacial gain) is a complex function of many parameters that must be precisely considered at the PV system design stage. Increasing rear gain directly impacts power density (kWp/m2) and reduces LCOE.
Ground Albedo (ρ): The reflectance coefficient of the surface beneath the modules. The higher the albedo, the greater the potential rear gain. This is a key input parameter for simulations. Typical values:
Albedo values can change dynamically throughout the year and even during the day (e.g., humidity, cloudiness), which requires consideration in advanced models. It is recommended to measure albedo for a specific location under various conditions.
Grass/dark soil: 0.2 - 0.3
Light gravel/concrete (e.g., light cement concrete): 0.4 - 0.6
Desert sand/light stone: 0.3 - 0.45
Fresh snow: 0.7 - 0.9 (high, but seasonal)
White TPO/EPDM membrane (roof): 0.7 - 0.85 (for rooftop systems)
Crushed white gravel: 0.65 - 0.75
Module Mounting Height (H): The higher the modules are mounted above the reflective surface, the greater the "view" of the module's rear side to that surface (larger "view factor", Fmod-ground) and less shading from the support structure itself. Increased height also reduces inter-row shading from adjacent module rows on the reflective surface. Optimal height is usually above 1 meter from the ground (H > 1.0 m) for ground-mounted systems. Increasing height from 0.5m to 1.5m can increase bifacial yield by 5-10%.
Inter-row Spacing (D): Increasing the distance between rows minimizes self-shading of one row by another, which increases light availability to the rear side of modules in further rows, but also increases the required land area and associated costs. Optimization requires analyzing the trade-off between yield and density. The minimum distance is typically D ≥ (H / tan(min sun elevation angle)), but for optimal bifacial yield, significantly larger D is recommended, allowing full exposure of the rear side to reflected light.
Mounting Configuration (tilt β, azimuth γ, orientation):
Module Orientation (portrait vs. landscape): Vertical (portrait) arrangement of modules often allows for better rear side exposure than horizontal (landscape) arrangement, due to less shading from crossbars and the possibility of optimal spacing. In portrait orientation, shadows from mounting profiles are typically thinner and less intrusive. Shading from mounting clamps should be minimized, being as small as possible and positioned so as not to affect the cells.
Module Tilt Angle (β): The optimal tilt angle for bifacial modules may be slightly less than for monofacial ones, as greater tilt reduces the "view" of the nearby ground but increases reflection from the distant background. For many locations, the optimal angle is 20-35 degrees.
Azimuth (γ): Typically South (0°), but for single-axis trackers (e.g., East-West), bifacial yield is particularly high due to continuous optimization of the rear side exposure angle.
Module Bifaciality Factor (BIF): An intrinsic characteristic of the module, defined as the ratio of the rear side efficiency to the front side efficiency, typically in the range of 70-90% for bifacial modules. The higher the BIF, the greater the potential rear gain.
Airflow and Operating Temperature: Closed profiles may slightly restrict airflow under the modules compared to more open structures. Restricted airflow can lead to minimally higher module operating temperatures (NOCT - Nominal Operating Cell Temperature). Higher cell temperature (Tcell) reduces module efficiency by about 0.3-0.45% / °C (the power temperature coefficient, γPMPP). This should be considered in simulations, although this effect is usually secondary compared to shading losses (<1% annual yield losses due to temperature differences).
Simulation and 3D Modeling Tools: Accurate prediction of bifacial yield requires advanced simulation tools such as PVsyst, Helios 3D, EnergyPlus, SAM (System Advisor Model), PV*SOL. These tools model reflected radiation (using ray-tracing or view factor-based methods), shading (from structure, rows, surroundings), angle of light incidence, light intensity distribution on the module surface, temperature effects, and inverter performance. PV engineers should perform detailed 3D analyses, as well as sensitivity analysis for key parameters (albedo, height, spacing).
Formula for Estimated Bifacial Gain (%)
Gain_{bifacial} = Albedo \times Module_{bifaciality\_factor} \times Geometry_{correction} \times (1 - Construction_{shading\_losses} - Row_{shading\_losses})Where:
Geometry_{correction}: A complex coefficient considering mounting height, tilt angle, and inter-row distance (related to view factor).
Construction_{shading\_losses}: Percentage losses resulting from shading of the rear side by mounting structure elements (profiles, clamps).
Row_{shading\_losses}: Percentage losses resulting from row-to-row shading (self-shading).
The above formula is simplified; in reality, more complex radiative models are used for accurate calculations.
Mechanical Analysis of Mounting Profiles
Material Properties: Typically aluminum profiles (EN AW-6063 T6, EN AW-6005A T5) with Young's modulus E ≈ 69 GPa, tensile strength Rm ≈ 215-260 MPa, and yield strength Rp0.2 ≈ 170-220 MPa. Steel profiles (S235JR, S355J2) have E ≈ 210 GPa, Rm ≈ 360-510 MPa, Rp0.2 ≈ 235-355 MPa. A 3 mm wall thickness for aluminum is equivalent in strength to 1.5 mm steel, but aluminum offers better corrosion resistance (requires less O&M maintenance) and lower weight.
Environmental Loads: Structure design must consider wind load standards (PN-EN 1991-1-4:2008 Eurocode 1) and snow load (PN-EN 1991-1-3:2005 Eurocode 1). Calculations include, among others, dynamic wind pressure (qp(z)), exposure coefficients, aerodynamic pressure coefficients (cp) for surfaces and edges, and snow load (sk).
Strength Analysis: Verification of profiles for bending, shear, torsion, and buckling. Modules are typically mounted at 4 or 6 points. Maximum stresses (σmax) in profiles should be checked against the material's yield strength (Rp0.2) with an appropriate safety factor (e.g., 1.5 for yield strength). Profile deflections should be checked and not exceed permissible values (e.g., L/200-L/300, where L is span).
Standards and Norms: The design and execution of mounting structures should comply with relevant PN-EN standards (Eurocodes), including PN-EN 1990 (Basis of structural design), PN-EN 1991 (Actions on structures), PN-EN 1993 (Steel structures), PN-EN 1999 (Aluminum structures). Additionally, module and structure tests (e.g., for dynamic load, temperature cycles) are regulated by IEC 61215, IEC 61730 standards.
Practical Implementation Guidelines and Risk Analysis
Detailed Albedo and Location Analysis: Before selecting the structure and optimizing the layout, a thorough investigation of the investment site's albedo should be conducted. Consider using artificial surfaces with high albedo (e.g., white gravel, geomembranes, special coatings) to maximize bifacial yield. Also, consider the impact of seasonal vegetation, dust, and dirt on albedo.
Structure Design with Minimal Shading in Mind: Use profiles that minimize shading (thin, closed profiles, anodized in light color or silver, which also reflect light). Optimize the placement and shape of mounting clamps to avoid shading active cell areas. Prefer integrated systems that reduce the number of shadow-casting elements.
Field and Pilot Tests: For large projects, it is recommended to measure the yield from bifacial modules under real conditions to verify simulation models and fine-tune design parameters (so-called "proof of concept"). Measurements may include front and rear module irradiance (using pyranometers or radiometers), module temperature, and actual energy yield.
Selecting Modules with High BIF: Ensure that the selected bifacial modules have an appropriately high bifaciality factor (typ. 70-90%) and certifications confirming their performance and durability (IEC 61215, IEC 61730). Prefer modules with tempered glass on both sides for better resistance to moisture-induced degradation (PID, LeTID) and mechanical conditions.
Electrical Optimization and Mismatch Management: Consider that uneven illumination of the module's rear side (e.g., due to variable albedo, partial shading) can lead to current mismatch losses in the string. In such cases, the use of power optimizers or micro-inverters for each module may be necessary to minimize mismatch losses and maximize yield. PVsyst analysis should account for these losses.
Risk Analysis and O&M: Assess risks associated with inadequate construction, such as increased O&M costs due to mechanical instability, module damage, difficulties in cleaning the rear side (e.g., requiring special brushes or robots), or excessive degradation. Higher quality construction reduces the risk of failures and the need for service interventions.
Maintenance and Cleaning: The design should consider ease of access to the modules' rear side for cleaning, especially in high-dust areas. Dust and dirt on the rear side can significantly reduce rear gain.
Investment in an appropriate, robust, and optimized mounting structure, although potentially associated with slightly higher initial costs (CAPEX), is absolutely crucial for maximizing yields from bifacial modules, ensuring long-term system reliability, and achieving an optimal LCOE (Levelized Cost of Energy) metric throughout the entire life cycle of a photovoltaic power plant.
O&M and Durability - "Peace of Mind" is Priceless
In large-scale energy projects, it's not just CAPEX that matters, but primarily the costs and risks throughout the entire PV project lifecycle. Optimizing operations and maintenance (O&M) and ensuring the long-term durability of the structure and components are crucial for achieving the lowest possible LCOE (Levelized Cost of Energy) and a stable return on investment over a 25-30 year horizon. It is estimated that O&M costs can constitute 10-20% of the total project LCOE. The selection of an appropriate mounting system has a direct and fundamental impact on O&M costs, failure risk, system uptime, and consequently, on the actual energy yield and financial stability throughout the lifecycle of a photovoltaic farm, in accordance with ISO 55000 (Asset Management) requirements.
Single-Axis Trackers (1P)
Single-Axis Trackers (SAT) provide the highest energy yield, typically 15-25% higher than fixed structures, depending on geographical latitude, insolation conditions (proportion of direct/diffused light), and ground albedo. In regions with high direct solar radiation (e.g., Southern Europe, Middle East), these gains can reach up to 30-35%. Trackers allow for a better alignment of the production profile with the demand curve, especially during morning and afternoon hours, which reduces the risk of grid overload during the midday peak and increases the value of energy sold on the market (time-of-use value). Tilt angle optimization is achieved by motors (typically DC or AC with worm gears), PLC/RTU (Remote Terminal Unit) controllers, and precise position sensors (encoders) and weather sensors (anemometers, insolation sensors).
However, their mechanical, electrical, and control complexity generates a significantly higher O&M burden. Key components such as linear actuators, motors, gearboxes, bearings, PLC/RTU controllers, and sensors are prone to failure, especially in extreme weather conditions (strong wind, icing, sand). It is estimated that approximately 70% of all failures in tracker farms involve mechanical or electrical components of the tracking system. They require rigorous preventive maintenance every 3–6 months (in accordance with manufacturer recommendations and standards, e.g., IEC 62446 for O&M), including: sensor calibration with a tolerance of up to ±0.5°, lubrication of bearings and gears (e.g., lithium greases with NLGI 2 class), control of electrical connections (e.g., thermographic inspection at ≤50°C), diagnostic of control software (firmware updates, log analysis), and functional tests of emergency modes (e.g., stow mode at wind >20 m/s). The complexity of servicing requires employing qualified personnel (e.g., mechatronics technicians, automation specialists), which leads to real OPEX at the level of >60 €/MWp·year, and in some cases even 80-100 €/MWp·year, considering the costs of spare parts (approx. 1-2% of tracker CAPEX annually) and logistics. Backtracking systems, while minimizing self-shading in rows, introduce additional, complex motion management algorithms that must be precisely configured and monitored, increasing the risk of software errors and the need for frequent calibrations. LCOE analysis for trackers must account for this increased OPEX, which can offset some of the gains from higher energy yield.
Fixed Structures (South-facing fixed)
These are structures with the simplest design and the lowest CAPEX costs (even 20-30% lower than trackers) and basic service requirements. The typical tilt angle for maximizing annual yield in Poland (approx. 52°N) is 30-35°. Their hourly profile is characterized by high production during midday hours, which can lead to oversupply and curtailment during periods of high insolation, especially with the growing share of PV in the energy mix (the "duck curve" effect). Curtailment can reduce annual yield by as much as 5-10% in grids with high PV penetration, leading to reduced revenues. The yield from the rear side of the bifacial module (rear gain) is also limited due to: 1) Significant shading from structural elements (rafters, purlins, clamps, posts – typical open profiles, e.g., C-profiles 40x80x2mm or omega-profiles 60x40x1.5mm), which can reduce potential bifacial yield by 5-15% (measured by irradiance mapping and ray-tracing methods). 2) Lack of albedo optimization, resulting from often too low mounting height (e.g., 0.5-0.8m above ground), which limits the exposure of the rear side to reflected light. The optimal H/L ratio (height to module length) for bifacial is 0.3-0.5. OPEX costs are lower than for trackers (estimated 35-45 €/MWp·year), but still require regular inspections (e.g., once a year, in accordance with PN-EN 1990:2004 for structural safety), cleaning (especially for bifacial modules, to avoid shading from dirt on the lower edge), and vegetation removal. Their rigid orientation does not allow for adaptation to changing weather or seasonal conditions, nor for optimization for energy market prices.
East-West (EW) Layout with Bifacial Optimization
This innovative layout combines the advantages of fixed structures with maximum optimization for bifacial modules. It is characterized by the absence of moving parts, which significantly reduces the risk of mechanical failures to practically zero (compared to hundreds of failure points in trackers) and minimizes service needs. The standard interval for preventive structural inspections can be extended up to once every 5 years for steel structures (apart from regular electrical inspections and cleaning), with regular monitoring of electrical parameters and visual inspection from ground level or using drones. The use of durable anti-corrosion coatings (e.g., hot-dip galvanizing according to PN-EN ISO 1461:2001) ensures the longevity of the structure exceeding 30 years.
The EW layout distributes production more evenly throughout the day (a "flat production profile"), which is beneficial for grid stability, reduces the risk of curtailment during the midday peak, and increases energy self-consumption (for "behind-the-meter" projects). Thanks to the open design (minimizing structural shading, e.g., closed profiles), single-row geometry, and appropriate mounting height (usually ≥1.1m above ground, giving an H/L in the range of 0.3-0.5 for 2.2m long modules), the exposure of the module's rear side to reflected light is maximized. Studies (e.g., NREL, Fraunhofer ISE) indicate that for ground albedo of 0.3-0.6, bifacial gain can reach 10-25% of the total module yield, and the "Tandem Ready" system is designed to fully exploit this value, minimizing losses to <1%. Remote diagnostics based on I-V curve monitoring (IEC 61853-1) allows for early detection of degradation, PID (Potential Induced Degradation) or shading, and thermographic drone flights (in accordance with IEC 62446-3, detecting hotspots with a temperature difference >20K) and visual inspections (ISO 9001 for quality) enable quick detection of damage or shading without the need for continuous personnel presence at the farm. This significantly reduces O&M costs to ~30-35 €/MWp·year or less, making this system one of the most cost-effective in the long term, minimizing LCOE.
Key Engineering Conclusion for Bifacial Optimization: To fully leverage the potential of bifacial modules and ensure a clear rear side free of shading, it is essential to use structural profiles with a minimal cross-section and high stiffness. The most effective are closed steel profiles (RHS/SHS - Rectangular/Square Hollow Sections) with a wall thickness of 3–3.5 mm and made of S355J2 structural steel (with a minimum yield strength of 355 MPa and impact strength of 27J at -20°C). Compared to open omega-type profiles (with a typical thickness of 1.5-2.0 mm), closed profiles are: significantly narrower (reducing shading of the module's rear side to <1%, compared to 5-10% for open profiles), stiffer (higher resistance to deflection under wind/snow loads according to PN-EN 1991-1-3/4), several times more resistant to torsion and buckling. For example, for a 60x40x3mm profile, the moments of inertia Ix and Iy are more than 2x greater than for an 80x40x2mm C-profile, and the torsional constant (It) is even up to 10x greater. A higher torsional stiffness factor (J) for closed profiles (e.g., J for RHS 60x40x3mm is approx. 14.5 cm⁴, while for a C-profile 80x40x2mm it is only approx. 1.5 cm⁴) translates into significantly greater stability of the entire structure, minimizing vibrations and reducing stress on modules (≤20 MPa, compared to >100 MPa in the case of point clamping). Such material and structural selection is critical for the long-term durability of bifacial modules and maximizing yields from the module's rear side, which directly translates into LCOE.
"Tandem Ready" Standard - A New Paradigm for Bifacial and Perovskite Modules
In the face of dynamic advancements in photovoltaic technology, especially bifacial modules and the upcoming era of tandem modules (perovskite + silicon), it becomes crucial to move away from current mounting standards. The new "Tandem Ready" paradigm is no longer limited to mechanical load-bearing capacity but focuses on ensuring full bankability and optimal operation of the module's rear side throughout the entire installation lifecycle – typically 30 to 50 years.
Understanding and implementing the following engineering criteria are fundamental to maximizing energy yield, minimizing degradation, and ensuring long-term reliability of photovoltaic projects, especially those utilizing technologies sensitive to shading and optimizing diffuse solar radiation.
1
Closed Profiles (RHS/SHS) – The Structural Foundation for Bifacial Modules
The support structure is based on closed profiles (square or rectangular, RHS/SHS) with a wall thickness of 3–3.5 mm and S355J2+N steel ( or equivalent). This material specification ensures adequate strength and weldability. Key features and engineering analysis:
No crossbars in the module's light path: Traditional systems often use "omega" (Ω) or C-channel profiles, which, due to their shape and required width (approx. 100-150 mm) and module fastening method, create significant shading on the rear side. In closed profiles (typically 60-80 mm wide) and bottom-mounted fastening, this shading is reduced to a minimum (<0.5% of the active surface). Simulation studies (e.g., in PVsyst) show that shading from open profiles can reduce bifacial yield by 3-5% annually, and in some cases (with a low sun incidence angle) even by 7-10% daily.
Minimal self-shading: The narrow, closed geometry of the profiles (typical width 60-80 mm) minimizes the area of shadow cast on the module's rear side. For perovskite modules, which are more sensitive to shading than silicon modules, minimizing shadow is critical. The shading factor is calculated as the ratio of the area shaded by the profile to the module's active surface.
Higher torsional stiffness (J) and buckling resistance: Closed profiles offer many times higher resistance to torsional forces and buckling than open profiles of the same mass. The torsional moment of inertia (J) for RHS/SHS profiles is significantly higher (e.g., for RHS 100x60x3.0 mm J ≈ 11.8 cm⁴, while for an open Ω profile with similar transverse parameters, J can be in the order of 1-2 cm⁴), which increases the overall structural stability in extreme wind conditions (dynamic loads up to 1.5 kN/m², corresponding to wind speeds of approx. 190 km/h). The buckling resistance of closed profiles is also much greater due to their symmetrical cross-section and more effective stress distribution.
Increased corrosion resistance: Closed profiles, unlike open ones, are less susceptible to accumulating water, dirt, and contaminants in crevices, which reduces the risk of crevice corrosion. Hot-dip galvanization according to PN-EN ISO 1461 is standardly used, providing a protective layer with a thickness of min. 85 µm, which translates into a steel structure lifespan of over 50 years in a moderately aggressive environment (C3).
Fatigue resistance: Photovoltaic structures are exposed to cyclic wind loads. Closed profiles exhibit better fatigue properties due to uniform stress distribution and the absence of sharp edges that could initiate fatigue cracks.
2
Single-Row Geometry – Architecture Optimizing Bifacial Yield
The optimal configuration is a single row of modules with a clearing height ≥1.1 m and a module tilt angle of 25° (for Poland's latitude, e.g., 52°N). Advantages and engineering details:
Maximizing Rear Gain and Albedo: A high clearance (e.g., 1.1 m to the module's bottom edge) ensures free access of reflected light from the ground (albedo) to the module's rear side. Studies (e.g., NREL, Fraunhofer ISE) prove that clearance below 0.8 m can limit bifacial gain by 5-10% due to "ground shading" and reduced light diffusion. Typical albedo values for different substrates: white gravel: 0.6-0.8; light grass/soil: 0.25-0.35; snow: 0.7-0.9. Increasing ground albedo by 0.1 can increase bifacial gain by 1-2%. Optimal height is a function of module tilt angle and row spacing.
Optimal arrangement and production profile: A 25° angle is a compromise between optimizing annual energy yield for south-facing modules and ensuring adequate row spacing to minimize self-shading and snow accumulation (especially critical in Polish winter conditions, where snow cover can exceed 0.5 m, affecting diffuse light availability). Using an East-West (EW) orientation in combination with an appropriate module tilt angle (e.g., 10-15° EW) significantly smoothens the hourly energy production profile, shifting the production peak from midday to morning and evening hours, which better matches grid demand and energy prices (peak shaving, price arbitrage). This configuration reduces the risk of curtailment during peak insolation hours.
Wind stability and aerodynamics: Single-row geometry, combined with an appropriate aerodynamic design (e.g., open structure, minimized wind piling effect under the module), allows for more effective transfer of wind forces to the foundations, minimizing the risk of resonant vibrations and module damage. CFD (Computational Fluid Dynamics) studies are essential for precise determination of wind pressure coefficients (Cp) for various attack angles and speeds. Eurocode PN-EN 1991-1-4 (Wind loads) requires analysis for basic wind speed (Vb,0) and exposure coefficients (Ce).
Minimizing contaminant accumulation: High clearance and appropriate tilt angle promote module self-cleaning (the "self-cleaning effect") through gravity and rain, reducing the need for frequent cleaning and minimizing soiling losses, which can amount to 1-3% annually depending on location.
3
Shading Control – Rigorous Engineering Criteria and Verification
Strict design and acceptance criterion: total shading of the active rear surface of the module by structural elements must not exceed 1% during the installation's operating hours. This criterion includes both structural and cabling shading. Analysis and verification:
Measurement and verification methods: Advanced 3D simulations are used in PV design software (e.g., PVsyst, PVSol, Helios 3D with modules for 3D shading analysis) which utilize ray-tracing algorithms to accurately calculate the impact of shadows on energy yield. 3D modeling must include a precise model of the support structure and cabling.
Drone photogrammetry and laser scanning (Lidar): For verifying actual shading after assembly. Drones equipped with high-resolution cameras and Lidar systems allow for the creation of a 3D point cloud and precise mapping of the terrain and installation, enabling distance measurement and detection of shading anomalies at the millimeter scale. This data can be compared with design models.
Thermal imaging from drones (according to IEC 62446-3): Used to detect hot-spots (overheated cells) caused by even minimal but continuous shading. Hot-spots can lead to irreversible module degradation (so-called "hot-spot degradation") and shorten its lifespan. The thermal resolution of the camera should be sufficient (e.g., <0.05°C NETD) to detect small temperature differences.
Analysis of shading impact: Even slight shading of the rear side of a perovskite (or bifacial) module leads to current mismatch in the cell string or the entire module. Activation of bypass diodes reduces module voltage and output power, and in extreme cases, leads to hot-spot formation and accelerated degradation. Calculations show that shading 1% of the module's surface can lead to power losses of 5-10% depending on the cell type and shade location.
EPC acceptance criteria and contractual clauses: Exceeding 1% shading may result in the imposition of liquidated damages (LDs) or refusal of acceptance, as this directly impacts annual yield and LCOE. EPC contracts should include precise methods for shading verification and consequences for exceeding it.
4
Clamp-Free Mounting – Module Protection and O&M Optimization
Modules are mounted from the bottom, without the use of clamps on the module's active surface. Depending on the technology, these can be integrated rail systems (e.g., embedded in the module frame) or dedicated point fastenings to the module frame. Engineering benefits:
Even soiling and self-cleaning: The absence of clamps prevents the formation of local "islands" of dirt and dust (so-called "dirt streaks" or "soiling lines") that accumulate around the clamps. These accumulations create constant shading, leading to hot-spot formation and reduced local efficiency. This is especially critical for bifacial modules, where the rear side is as important as the front. Studies in high-dust regions show that soiling around clamps can increase module degradation by 0.5-1.0% annually. Mechanical cleaning (e.g., using cleaning robots) is also facilitated because the surface is smooth.
No clamp-induced hot-spots and mechanical stress: Traditional clamps exert point pressure on the module frame, which can lead to micro-cracks or stress in the cells (so-called "stress-induced micro-cracks"). In the long run, this results in hot-spot formation and accelerated degradation (so-called "snail trails" – snail tracks, are often a visual symptom of this phenomenon). Clamp-free mounting eliminates this risk, especially in light of increasing module sizes (M10, G12 formats) and thinner frames, where stresses are more concentrated. Modules conforming to IEC 61215 are tested for mechanical loads, but point pressure from clamps can create additional, unforeseen stresses.
Improved aesthetics and safety: The absence of visible clamps improves the overall aesthetics of the PV farm, which can be important for projects in visible locations. The risk of module theft is also reduced, as their disassembly is more difficult.
Optimized airflow: The absence of clamps and a minimal number of mounting elements on the module surface can improve airflow, which promotes cell cooling and maintains higher efficiency, especially on hot days. Each 1°C increase in cell temperature above nominal operating cell temperature (NOCT) can reduce module efficiency by 0.3-0.4%.
5
Cabling – Critical Engineering for Bifacial Yield
All cabling (DC, communication, control) must be routed from underneath the structure, with no hanging or visible wires above the level of the module's rear side. Implementation and justification:
Elimination of shadow lines: Even a thin cable (e.g., 6-10 mm diameter) casting a shadow on the rear side of a bifacial module (especially perovskite, which is more sensitive to shading than silicon cells) can significantly reduce local yield and energy efficiency of that module by causing current mismatch in individual cells. Simulations have shown that a shadow from a single 8 mm diameter cable on a bifacial module can cause power loss of 0.5-1.5% depending on the light incidence angle and time of day.
Cable durability and safety: Hidden cabling is much less exposed to mechanical damage (e.g., by animals, agricultural equipment, human activity), UV radiation, extreme temperatures, and moisture. Cables exposed to atmospheric factors (sun, frost, rain) degrade faster (loss of insulation flexibility, cracks, short circuits). Routing cables in dedicated cable trays or protective conduits (according to PN-EN 60364-7-712 for PV installations) increases their lifespan and safety.
Optimized airflow and aesthetics: The absence of loosely hanging cables ensures better, unobstructed airflow under the modules, which aids cooling. Furthermore, hidden cabling improves the aesthetics of the PV farm, which is important for projects in protected landscapes or with high visual value.
Standards and guidelines: Cabling should comply with IEC 62930 (cables for PV systems) and IEC 60364-5-52 (selection and installation of wiring). Verification includes insulation resistance measurements (PN-EN 61557-2) and visual inspection.
6
Acceptance Test (PAC) – Confirmation of Bifacial Gain
A condition for the Provisional Acceptance Certificate (PAC) is a test-confirmed bifacial gain (Rear Gain) of ≥18% at natural ground albedo (typ. 20-30%, e.g., green grass, light soil). Technical details and procedures:
Definition of Rear Gain (Bifacial Gain): It is calculated as [(E_bifacial - E_monofacial) / E_monofacial] * 100%, where E is the energy produced. It can also be approximated as the ratio of bifacial power to monofacial power under standard test conditions (STC) or field conditions.
Measurement methods and calibration: Rear Gain is calculated based on measurements of actual radiation incident on the front (Global Horizontal Irradiance, GHI) and rear (Rear Side Irradiance, RSI) sides of the module (using calibrated pyranometers of class A or B, e.g., CMP11, according to ISO 9060, placed at module height). Alternatively, calibrated reference modules (bifacial and monofacial) with known characteristics are used. Measurements must be performed under stable insolation conditions (irradiance > 600 W/m²), considering module temperature and wind speed.
Field Performance Testing: According to guidelines IEC 61724-1 (Photovoltaic System Monitoring) and IEC 61853-1/2/3 (PV Module Characteristic Measurements). Analysis of measurement data includes normalization to STC, temperature compensation, and consideration of the light spectrum.
Bankability verification: This parameter is crucial for financing institutions (banks, insurers) as it directly affects the projected revenue stream (P50, P90) from the installation and consequently the project's financial model (Financial Model) and LCOE. Failure to meet this condition may result in renegotiation of financing terms, imposition of contractual penalties, or require additional collateral. Banks require independent verifications by Owner's Engineers or independent certifying agencies.
Albedo impact: Real bifacial yield is strongly dependent on ground albedo. Values of ≥18% are achievable for albedo 0.25-0.35 (grass), but for substrates with higher albedo (e.g., light gravel, concrete, snow), this yield can increase up to 25-30%, which is accounted for in yield forecasts.
7
Structural Strength – Safety Factor for Extreme Conditions
The structural design must account for wind loads of 200–220 km/h (approx. 1.8-2.0 kN/m² dynamic wind pressure), which corresponds to European standards (Eurocode-1, -3, -5) with an additional 60% safety factor for key structural elements (posts, purlins, foundation fastenings). Detailed analysis:
Eurocode-1 (PN-EN 1991-1-4: Wind actions): Defines basic wind speed (Vb,0), terrain category, exposure coefficient (Ce(z)), and aerodynamic pressure coefficients (Cp). Dynamic wind pressure (qp(z)) is calculated as qp(z) = 0.5 · ρ · v_m(z)², where ρ is air density, and v_m(z) is the mean wind speed at a given height. The calculation must account for turbulence effects (the "gust factor") and resonant phenomena, especially for light PV structures. A 60% safety factor means the structure is designed for loads 1.6 times greater than standard Eurocode design loads for a 50-year return period.
Eurocode-3 (PN-EN 1993-1-1: Design of steel structures): Defines methods for designing and verifying the resistance and stiffness of steel elements. A key criterion is section resistance V_Rd ≥ V_Ed (design resistance must be greater than the design value of the internal force). The plastic resistance for a steel section is N_Rd = A · f_y / γ_M0, where A is the cross-sectional area, f_y is the steel yield strength (for S355 it is 355 MPa), and γ_M0 is the partial safety factor (typ. 1.0 for resistance). A 60% safety factor for S355 profiles (yield strength 355 MPa, tensile strength 490-630 MPa) ensures resistance to rare, extreme wind gusts (e.g., 100-year or 200-year return periods), significantly exceeding the standard 50-year periods.
Eurocode-5 (PN-EN 1995-1-1: Design of timber structures): Used when timber posts or beams are applied (less common in large PV farms).
Numerical analysis (CFD/FEM): For precise modeling of wind flow around the structure and modules (CFD) and identification of areas with highest stresses and deformations (FEM – Finite Element Method). This allows for optimization of geometry and minimization of material consumption while maintaining the required safety factor. FEM models allow for analysis of natural vibrations and resonance risk.
Snow loads: Additionally, in Poland, snow loads must be considered (according to PN-EN 1991-1-3), which for a flat roof can range from 0.7 kN/m² to 2.8 kN/m² depending on the snow zone, and for PV installations, snow accumulation between module rows must also be considered.
Structural risk categories (PN-EN 1990): PV installations should be classified in risk categories RC2 or RC3 due to the potential consequences of failure, which implies the use of higher safety factors.
Energy Yield and LCOE (Levelized Cost of Energy) Analysis: As shown in the chart, the "EW Rear-Friendly" mounting system, compliant with the "Tandem Ready" standard, offers an annual yield of 1,320 kWh/kWp·year under typical Polish climate conditions (e.g., 1100 kWh/m²/year GHI), which is practically identical to the yield from more expensive and complex 1P trackers (1,330 kWh/kWp·year), considering their higher CAPEX and OPEX. At the same time, it significantly surpasses traditional, stationary South-facing structures (South 25°), which offer only 1,155 kWh/kWp·year. This small yield difference (approx. 0.75% vs. trackers) is compensated by significantly lower OPEX (no moving mechanics, fewer service needs, absence of complex control algorithms and associated failures), which translates into a lower LCOE for the EW Rear-Friendly system. Typical OPEX for EW Rear-Friendly is ~35-40 €/MWp·year, while for 1P trackers it can be 60-80 €/MWp·year, and for traditional fixed South-facing structures 45-55 €/MWp·year. Additionally, EW Rear-Friendly provides increased reliability and resistance to extreme weather conditions, minimizing losses due to downtime and repair costs.
Conclusion: Implementing the "Tandem Ready" standard is an optimal solution for long-term profitable and efficient photovoltaic farms, especially in the context of the growing share of bifacial and future perovskite modules. The ability to absorb light from the module's rear side is crucial for them, and "Tandem Ready" maximizes this potential while minimizing operational and financial risks. This is a strategic investment in the future of solar energy, ensuring higher profits and greater grid stability.
Technical Risk Map in Large-Scale Photovoltaics: An Engineering Perspective
An in-depth analysis of long-term engineering and economic threats, closely related to the selection of support structure types and foundation methods in photovoltaic farms. The presented "Risk Map" is based on detailed technical data, engineering calculations, and references to key international standards, indicating critical points that engineers must consider when designing and verifying PV systems.
1. Wind Force: Dynamic and Static Risk
"Cheap" Solutions (open C/Z profiles, driven piles without calculation)
Engineering Problem: Open C/Z profiles with a thickness of 1.5-2.0 mm (EN 10025-2, S235JR standard) are characterized by extremely low torsional stiffness (small moment of inertia for torsion I_T) and bending stiffness (low moment of inertia I_x, I_y on the order of 50-150 \text{cm}^4 for typical profiles), leading to aerodynamic instability phenomena such as flutter (self-induced vibrations) and resonance at lower wind speeds than predicted by PN-EN 1991-1-4 (Eurocode 1, wind actions). The natural vibration frequencies for such structures can be as low as 0.5-2 Hz, which overlaps with the dominant frequencies of wind pulsation. The lack of adequate dynamic calculations and response spectrum analysis is a critical error.
Geotechnics and Anchoring: Driven piles often have insufficient embedment depth (e.g., 1.5m instead of a minimum of 2.5-3.0m for cohesionless soil with low density, e.g., ID < 0.4), which, combined with the low stiffness of the pile (for a 4m long pile with an 80x80x3 mm cross-section, the deflection of a free cantilever under a 1kN load can exceed 15mm), results in excessive head displacement (angular rotation >2°) and loosening of foundations in the soil, leading to a loss of soil resistance in the surface zone (P_{ult} significantly decreases).
Financial and Technical Consequences:
Material Fatigue: Cyclic wind loads generate fatigue stresses (\sigma_a) exceeding the material's fatigue strength limit (according to S-N curves, e.g., for S235JR steel: \Delta\sigma_c = 100 \text{ MPa} for 2 \cdot 10^6 cycles). This leads to micro-cracks in PV cells (especially around busbars and cells), damage to module frames, and failure of tracker bearings and drives (torque overloads of 15-20% above nominal).
Performance Drop: Micro-cracks lead to irreversible module degradation, manifested as an additional power loss of 0.5-2% annually (so-called "Power Loss Rate" from EL/IR tests). This phenomenon is compounded by hot-spots (localized overheating up to 80-100°C), which shortens the effective module life from 25 to 15-20 years. Wind tunnel simulations show an increase in dynamic loads of 30-50% compared to static loads, dramatically increasing the risk of damage.
Practical Example: PV farms in Texas (February 2021, Winter Storm Uri), where winds of 40-50 m/s (144-180 km/h, dynamic pressure q=0.5 \cdot \rho \cdot v^2 \approx 1.25 \text{ kPa}) caused massive damage to tracker structures (over 30-40% of panels in some farms) due to insufficient profile stiffness and inadequate anchoring in low-cohesion soils. Repair costs ranged from 50,000 to 150,000 USD/MW.
2. Soil Stability: Pile Settlement and Misalignment
"Cheap" Solutions
Geotechnical Problem: Lack of proper geotechnical soil investigation (e.g., detailed CPT/CPTU soundings, SPT, laboratory tests of undisturbed samples) and anchoring pile design for specific loads (axial, lateral, moments) and soil type (e.g., for cohesive soils with c_u < 20 \text{ kPa} or cohesionless soils with D_r < 30\%). Piles driven directly into the ground, without concrete foundations or proper base shaping (e.g., widening), are exposed to:
Settlement: Cumulative settlement under vertical loads (modules, snow) and variable wind loads. Typical settlements can be 5-10 mm annually, leading to differential foundation settlement of 50-100 mm over 10 years, which exceeds tracker tolerances (usually +/- 20 mm).
Lateral Displacement/Row Misalignment: "Spreading" of rows under horizontal loads (wind) and loss of soil stiffness.
Frost Heave: In frost-susceptible soils (e.g., silty clays, silts, silty sands) with moisture content above 10-15%, freeze-thaw cycles of water in soil pores can generate uplifting forces of up to 100-200 kPa on the pile surface (F_h = K_{heave} \cdot A_{contact}), causing gradual pile uplift.
Financial and Technical Consequences:
Loss of Geometry: Significant loss of row geometry (angular tolerances >2°, height tolerances >50mm), cracking of PV modules (induced by bending stresses up to 150-200 MPa, exceeding cell bending strength of 50-100 MPa), tracker failures (jamming, damage to motors/gearboxes), difficulties in operation and maintenance (O&M), need for costly geodetic corrections (from 5,000 to 15,000 EUR/MW).
Impact on Modules: Settlement of just 5-10 cm over a 60m long table can generate stresses in modules exceeding their tolerances (e.g., 1500-2000 Pa), leading to visible cracks and delamination.
Standards: PV foundation design should comply with PN-EN 1997-1 (Eurocode 7 – Geotechnical design) and local building regulations. Criteria for permissible displacements are defined in industry standards, e.g., IEC 62716 for vibration resistance.
3. Seismic Loads: Structure Resonance
"Cheap" Solutions
Engineering Problem: In regions with increased seismic activity (even of low intensity, e.g., 0.05g-0.1g zones according to PN-EN 1998 – Eurocode 8: Design of structures for earthquake resistance), structures with low stiffness and insufficient ground anchoring may resonate with ground vibrations. Open C/Z profiles are not suitable for transmitting complex torsional and shear forces caused by seismic movement. Natural vibration frequencies (e.g., for a driven 1P tracker structure: 0.8-1.5 Hz) may overlap with the dominant frequencies of seismic excitation (0.5-5 Hz for ground class C-E according to Eurocode 8).
Dynamic Analysis: The lack of dynamic structure calculations, e.g., modal analysis (determining natural frequencies and vibration modes) and time history analysis or response spectrum analysis according to PN-EN 1998, is often overlooked. The influence of the damping ratio (\zeta) for steel structures (usually 2-5%) is crucial, but often lower in "cheap" solutions due to loose connections.
Financial and Technical Consequences:
Catastrophic Damage: In case of resonance, vibration amplitudes can increase exponentially, leading to destruction of connections (bolted, welded), module damage (cracks, delamination), and in extreme cases, complete structural collapse. Repairs after such an event cost 50-100% of the CAPEX value of the structure.
Loss of Production: Complete shutdown of the farm, zero energy production, which translates into huge LCOE (Levelized Cost of Energy) losses and operational losses.
Practical Tips: For seismic zones, it is recommended to use closed profiles (HSS – Hollow Structural Sections), which have significantly higher torsional stiffness, and concrete foundations, providing a more stable connection to the ground and better vibration damping.
4. Corrosion: Long-Term Degradation
"Cheap" Solutions
Material Problem: Use of steel with lower quality anti-corrosion coating (e.g., hot-dip galvanizing Z275 - 275 \text{g/m}^2, thickness 20 \mu\text{m} vs. Z350 - 350 \text{g/m}^2, thickness 25 \mu\text{m}, or Al-Zn-Mg alloys like Magnelis/PosMAC - 25-35 \mu\text{m} with self-healing effect). Open steel profiles (C/Z) are particularly susceptible to corrosion because water, dust, and contaminants (pH < 7) can more easily accumulate and collect inside them, creating corrosive microenvironments. The inner surface of open C/Z profiles is difficult to inspect and renovate, which prevents preventive maintenance.
Environmental Corrosivity Categories: PN-EN ISO 12944-2 classifies environments into corrosivity categories C1 to CX (for marine and industrial). PV farms are often located in categories C3 (moderate urban and industrial) or C4 (industrial/marine, moderate salinity).
C3: Zinc loss 0.7-2.1 \mu\text{m/year}. Z275 coating (20 \mu\text{m}) can wear out in 10-28 years.
C4: Zinc loss 2.1-4.2 \mu\text{m/year}. Z275 coating can wear out in 5-10 years.
Financial and Technical Consequences:
Material Degradation: Progressive corrosion reduces the effective wall thickness of the profile, which directly lowers its load-bearing capacity (tensile, compressive, bending strength) and stiffness. A 0.5 mm decrease in thickness can mean a 25% decrease in load-bearing capacity for a 2.0 mm profile.
Shortened Lifespan: In C3/C4 environments, corrosion can progress by as much as 20-50 µm annually for unprotected steel, shortening the structure's lifespan from the designed 25 years to 10-15 years. The need for premature structural replacement results in enormous CAPEX and O&M costs (disassembly, recycling, new installation).
Module Problems: Corrosion can also weaken module attachment points, leading to their displacement, mechanical damage, and efficiency loss due to induced stresses.
Practical Tips: It is recommended to use materials with at least C4/C5 corrosion protection, e.g., Magnelis/PosMAC steel or duplex paint coatings. Galvanic corrosion at the contact points of different metals (e.g., structural steel with aluminum module frames) should also be considered by using appropriate insulating washers.
5. Rear-Side Shading Losses (Rear-Side Mismatch)
"Cheap" Solutions
Optical and Geometric Problem: In the case of fixed-tilt structures or 1P trackers with open C/Z profiles that run directly under PV modules (especially bifacial ones, which absorb light from both sides), there is significant shading of the module's rear side. Profiles 80-100 mm wide and 1.5-2.0 mm thick can block up to 5-10% of the light reaching the rear side, which translates into a real energy loss.
Impact on Bifacial Productivity: These losses are particularly critical for bifacial modules, which offer an additional energy gain (rear gain) of 5-25% depending on the ground albedo and mounting height. With "cheap" solutions, the actual rear-side gain can be cut by 30-50% (e.g., from an expected 15% to 7-10%), which means a direct loss of GWh production.
Hotspots and Degradation: Uneven shading and associated uneven cell heating (hotspots) lead to accelerated degradation of bifacial modules. The temperature difference between shaded and unshaded cells can be 10-20°C, increasing local thermal stresses and the risk of micro-cracks. This phenomenon is exacerbated at low angles of incidence (early mornings/late afternoons and during winter months).
Financial Consequences:
Lower Production: A cut in real gain means lower annual revenues for the farm. For a 100 MW farm, a 1% drop in production means a loss of approximately 1.5-2 GWh annually, which at a price of 0.05 EUR/kWh is 75,000 - 100,000 EUR annually.
Increased LCOE: Lower energy yield with constant CAPEX and OPEX leads to a higher Levelized Cost of Energy (LCOE), making the project less profitable.
Optimization: To minimize rear-side shading losses, it is recommended to use structures with minimal transverse elements under the modules, increase the module mounting height (e.g., to 1.5-2.0m from the ground to the bottom edge of the module), and use closed profiles (e.g., square or rectangular tubes) with smaller width or profiles dedicated to bifacial modules.
Engineering Interpretation and Strategic Conclusions: "Cheap" solutions without foundations, often based on open C/Z profiles (with too low elastic modulus E and shear modulus G) and minimal anchoring, represent a scenario of extremely high risk in almost every engineering aspect. They lead to long-term structural problems (material fatigue, deformations, cracking), component failures (module damage, tracker gear failures), and significant losses in energy production (GWh loss, increased LCOE). 
In contrast, solutions with proper foundations (concrete or driven with precise geotechnical calculations compliant with PN-EN 1997) and the use of closed profiles (e.g., rectangular or square tubes with higher moments of inertia and torsional stiffness) and appropriate steel (e.g., S355 grade, with Magnelis/PosMAC coating) ensure minimization of all key risks , guaranteeing the designed 50-year lifespan of the structure (in accordance with IEC 61215 and IEC 61730) and maximization of energy yields for the PV farm. Investing in reliable engineering design and quality materials is key to long-term project profitability.
Evolution of Photovoltaics 2010-2050: Deeper Technological and Economic Analysis
An overview of dominant mounting and module technologies over the decades, considering key technical parameters, engineering challenges, standards, LCOE costs, and practical implementation guidelines:
1
2010 - Multi-row South-facing Monofacial: Beginnings of Cost Optimization
A period characterized by the dominance of simple, fixed south-facing systems with monofacial modules, installed in multi-row structures. The main goal was to minimize initial investment costs (CAPEX), often at the expense of long-term durability and energy efficiency (low LCOE was not a priority).
Modules: Primarily polycrystalline silicon (p-type multi-Si), with power outputs of 200-250 Wp per module (e.g., dimensions 1650x990x40 mm, weight approx. 18-20 kg). Module efficiency around 14-15%. Aluminum Back Surface Field (Al-BSF) technology. Typical Voc voltage: 36-38V, Isc current: 7-8A. Modules compliant with IEC 61215 (performance tests) and IEC 61730 (safety). Annual degradation: approx. 0.7-0.8%. Temperature power coefficient: typically -0.45%/°C for Pmax.
Structure: Predominantly multi-row (fixed-tilt) structures, made of thin-walled open steel profiles (C/Z), with material thickness of 1.5-2.0 mm (most commonly S235JR steel). Module tilt angles typically 30-35° for Central Europe. Row spacing (pitch) around 2.5-3.0 m, which led to inter-row shading losses of 3-5% during morning and afternoon hours in winter. Wind design based on static calculations according to simplified national standards, often without considering aerodynamic phenomena (e.g., resonance, flutter). Lack of detailed fatigue analysis.
Foundations: Widespread use of ground screws (helix type, diameter 76-89 mm, length 1.5-2.5 m) without precise pull-out tests compliant with ASTM D3689. Shallow concrete foundations (e.g., 60x60x60 cm blocks) were often used in weak soils, without in-depth geotechnical analysis (e.g., CPT, SPT soundings). The risk of differential settlement and frost heave was high.
Durability: Low structural durability (often below 20 years) resulting from insufficient corrosion protection (e.g., hot-dip galvanization below 50 µm or Sendzimir galvanization Z275, corresponding to approx. 20 µm zinc layer per side), susceptibility to edge corrosion of open profiles (corrosion rate in C3 environment: 5-10 µm/year), and sensitivity to dynamic loads (wind) due to low structural stiffness (dynamic analyses according to PN-EN 1991-1-4 were often omitted). Failure to account for material fatigue resulted in cracks at structural joints and damage to module frames.
Energy Yield & LCOE: Typical Performance Ratio (PR) of 70-75% (temperature losses approx. 10-12%, shading/mismatch losses approx. 3-5%, cable/inverter losses approx. 5-7%). Annual yield of around 800-900 kWh/kWp/year for optimally oriented systems in Poland. Example installation cost: 2000-2500 EUR/kWp. LCOE (Levelized Cost of Electricity): approx. 120-150 EUR/MWh. O&M costs: 10-15 EUR/kWp/year, mainly for manual cleaning and reactive repairs.
2
2020 - 1P/2P Trackers with Open Profiles: The Era of Yield Maximization
A decade dominated by tracking systems, especially single-axis (1P - Single-axis horizontal trackers) and less frequently dual-axis (2P - Dual-axis trackers). Focus on maximizing energy yield amidst ongoing challenges related to durability and reliability. LCOE becomes a key factor.
Modules: Transition to monocrystalline modules (p-type mono-Si, PERC, Half-Cut, Multi-Busbar) with power outputs of 350-450 Wp (e.g., dimensions 2000x1000x35 mm, weight 22-25 kg) and efficiencies of 19-21%. Increasingly larger format modules (182 mm, 210 mm wafers). Growing awareness of PID, LID, LeTID (Light- and elevated Temperature Induced Degradation). Annual degradation: 0.5-0.6%.
Structure: 1P trackers (single N-S rotation axis) with open steel profiles (C/Z), but with increased thickness (2.5-3.5 mm) and strength (S355JR steel). The use of trackers required more precise static-strength calculations, including aerodynamic analysis (CFD - Computational Fluid Dynamics) for different module tilt angles. Design compliant with Eurocode 1 and 3, but often still with simplifications regarding aeroelasticity. Typical row spacing (pitch) for trackers: 4.5-6.0 m.
Structural Challenges: Profiled open shapes of profiles generated turbulence and increased wind forces on modules, leading to aeroelastic phenomena (flutter, galloping, torsional divergence). This phenomenon required the use of parking algorithms (stow mode) at wind speeds above 15-20 m/s (approx. 54-72 km/h), which reduced annual yield (2-5% losses) and increased mechanical wear of drives. High torsional and bending moments on tracker axes.
Durability: Despite increased profile thickness and the use of better coatings (e.g., Magnelis, ZM310 - approx. 25 µm), the problem of internal corrosion of open profiles (especially in high humidity or salinity environments) and material fatigue at moving connection points and in the drive area remained a challenge. Typical structural durability 20-25 years. Lack of comprehensive life cycle analysis and full impact of dynamic loads on material fatigue according to Eurocode 3 Part 1-9.
Energy Yield & LCOE: Significant increase in yield due to sun tracking. Annual yield 1200-1400 kWh/kWp/year for Poland (25-35% increase vs. fixed-tilt). PR of 80-85%. However, losses due to tracker parking in strong winds could amount to 2-5% of annual yield. Installation cost: 1000-1500 EUR/kWp. LCOE: approx. 60-80 EUR/MWh. O&M costs: 7-12 EUR/kWp/year (increased mechanical complexity of trackers).
3
2030 - East-West Bifacial Rear-Friendly: Yield and Durability Optimization
A period of transition to hybrid fixed systems with East-West (EW) orientation, which became the standard for large ground-mounted installations. Integration of bifacial modules with rear gain friendly structures and increased focus on durability through closed profiles. The priority becomes minimizing LCOE by optimizing CAPEX and OPEX and maximizing yield in the long term.
Modules: Bifacial modules (n-type TOPCon, HJT, IBC) with power outputs of 600-700 Wp (e.g., dimensions 2382x1134x30 mm, weight 30-35 kg) and efficiencies of 22-23.5%. Optimization for "rear gain," which becomes a key parameter, reaching 15-20% depending on ground albedo (e.g., grass 0.25, white gravel 0.6, white geomembrane 0.8), mounting height (from 0.8 m to 1.5 m above ground), and structural optimization. Annual degradation: 0.35-0.45%. Higher resistance to PID/LeTID due to n-type technology.
Structure: Fixed EW structures with an optimal tilt angle (e.g., 10-15°), designed to minimize rear shading (typically <2%) and maximize reflection. Closed profiles (e.g., 80x80x3 mm square, rectangular, or round tubes) made of stainless steel (e.g., 1.4301/AISI 304) or carbon steel (S355J2) with C4/C5 corrosion protection coating (e.g., hot-dip galvanization >80 µm, duplex painting systems: zinc + powder paint) become standard. Static and dynamic calculations based on full Eurocodes 1-9 (PN-EN 1991, 1993, 1998), including aerodynamic (CFD) and fatigue analyses. High degree of prefabrication.
Foundations: Concrete foundations (drilled piles, prefabricated footings) or driven steel piles with deep, precise geotechnical tests and load-bearing capacity calculations (e.g., according to PN-EN 1997 - Eurocode 7). Increased embedment depth (e.g., 2.5-3.5 m depending on soil) and pile/screw diameter for greater stability.
Durability: Significant increase in structural durability to 30-40 years due to closed profiles eliminating internal corrosion (elimination of microclimatic corrosion phenomena inside open profiles), improved corrosion protection coatings (reduction of corrosion rate to <2 µm/year in C4), and precise FEM (Finite Element Method) calculations accounting for cyclic loads and fatigue analysis compliant with Eurocode standards (e.g., wind loads with a 50-year return period). Corrosion monitoring of reference samples on the farm.
Energy Yield & LCOE: Annual yield similar to trackers (1200-1350 kWh/kWp/year), but with significantly lower operational losses (no wind parking, simplified maintenance). Bifacial efficiency and lower O&M costs (3-7 EUR/kWp/year) make EW bifacial competitive. LCOE: approx. 40-55 EUR/MWh. Example LCOE reduction of 10-15% compared to 1P trackers.
4
2040 - Tandem Bifacial EW Standard "Tandem Ready": New Generation of Efficiency
Introduction of perovskite-silicon tandem modules as a new standard. The need for "Tandem Ready" structural design, capable of handling higher power outputs and different electrical characteristics, with an emphasis on extreme durability and minimization of all system losses.
Modules: Tandem modules (perovskite on silicon) with efficiencies above 28-30% and power outputs around 800-900 Wp (e.g., dimensions 2400x1200x30 mm, weight 35-40 kg). Expected significantly lower annual degradation (below 0.3%) due to innovative materials and encapsulation. Extremely high potential for "rear gain" due to perovskite layer transparency and cell optimization. New IEC standards for tandem modules (under development).
"Tandem Ready" Structure: EW systems with even greater stiffness and strength, capable of safely supporting modules with higher mass (up to 35-40 kg/m², structural self-weight around 20-30 kg/m²). Structural optimization for heat dissipation from the module's rear side (e.g., open closed profiles, special heat sinks), with minimal shading (typically <1%). Increased row spacing (pitch) to 6-8 m for maximizing bifacial yield. Structural steel with even better strength parameters (e.g., S460, S500) or the use of composites.
Durability: Structural design for 50-year durability, considering long-term material properties such as steel and concrete creep under extreme conditions. Implementation of advanced Structural Health Monitoring (SHM) systems with real-time sensors for strain, deformation, acceleration, and corrosion (e.g., electrochemical probes). Fatigue analysis at the component and joint level for 500,000 - 1,000,000 load cycles.
Material Innovations: Development of steel alloys with increased corrosion resistance (e.g., duplex steel - e.g., 1.4462/AISI 2205, marine-grade steel - e.g., Corten B) or polymer composites (e.g., GFRP - Glass Fiber Reinforced Polymer, CFRP - Carbon Fiber Reinforced Polymer) for structural elements with a high strength-to-weight ratio and zero corrosion. Hydrophobic and anti-reflective nanocoatings.
Energy Yield & LCOE: Significant increase in annual yield to 1500-1800 kWh/kWp/year due to ultra-high module efficiency and full utilization of bifaciality (real rear gain 20-25%). Integration with advanced Energy Management Systems (EMS) and production forecasting (AI-driven forecasting), minimizing grid losses. LCOE: approx. 25-35 EUR/MWh. O&M costs: 2-5 EUR/kWp/year (predominance of predictive maintenance, robotization).
5
2050 - Ultra-durable PV Infrastructure: Photovoltaics as an Energy Pillar
Photovoltaics as a key element of global energy infrastructure, designed for centuries. Fully autonomous, maintenance-free, remotely monitored systems, with minimal environmental impact and maximum resistance to natural disasters and climate change. Orientation towards total Life Cycle Cost (LCC) and a zero-waste approach.
Modules: Self-repairing photovoltaic modules integrated with building materials (BIPV - Building Integrated Photovoltaics) with modular design, or multi-junction Concentrated Photovoltaic (CPV) systems with efficiency >40%. Possible use of quantum, thermophotovoltaic (TPV) technologies or transparent modules (transparent PV) for applications in agrivoltaics (AgriPV) and facades. Extended IEC standards for 100-year durability.
Structure: Structures with a lifespan of >100 years, fully resistant to extreme weather conditions (wind 60 m/s / 216 km/h, snow loads 5 kPa - 500 kg/m², seismic activity up to 9 on the Richter scale). Utilization of smart materials reacting to environmental changes (e.g., shape memory alloys, self-clamping connections). Adaptive module cooling systems. Modularity and ease of disassembly for recycling.
Rear Gain: Standardized rear gain ≥25% through the use of optimal substrate materials (e.g., white gres with albedo >0.8, mirrored surfaces) or reflective structures (reflectors, concentrators), and high-clearance mounting systems (e.g., >1.5 m for AgriPV). Microclimate under panels controlled to minimize temperature losses and maximize gain.
Automation and AI: Fully automated, autonomous module cleaning systems (robots, drones with ultrasonic technology), anomaly diagnostics (AI-powered anomaly detection), and Predictive Maintenance based on AI and Big Data (analysis of data from thousands of farms). Autonomous robots performing minor on-site repairs, component replacement.
Monitoring: Extensive monitoring systems based on IoT sensors (temperature, humidity, vibrations, deformations), inspection drones (thermal imaging, EL), satellite monitoring of degradation and shading, fully integrated with the Digital Twin of the PV farm. Predictive models for LCOE and real-time performance.
Ecology: Recycling of structural materials at 99% level (circular economy), minimization of carbon footprint during production and installation thanks to green steel, low-emission cement. Total environmental impact (Life Cycle Assessment - LCA) is a key design parameter. PV farm as an element of the ecosystem (biodiversity, water management).
Why Closed Profiles (RHS/SHS) are Fundamental for the Efficiency and Durability of Future PV Farms: Detailed Engineering Analysis
In light of the growing demands for performance, stability, reliability, and longevity of photovoltaic systems, closed profiles (Rectangular/Square Hollow Sections – RHS/SHS) are becoming not only preferred but indeed a key element of support structures. Their unique mechanical, aerodynamic, and optical properties make them the "hidden key" to fully harness the potential of bifacial modules and ensure unprecedented installation reliability for decades. Below, we present an in-depth analysis of their engineering advantages, supported by data, standards, and practical applications.
Exceptional Torsional Stiffness and Microcrack Reduction: Key to Module Longevity
The closed cross-section of RHS/SHS profiles is characterized by torsional stiffness (polar moment of inertia, J or I_t) orders of magnitude greater than open profiles (e.g., C-channels, omegas, I-beams) of the same steel mass. This is crucial for minimizing dynamic deflection and twisting of the structure (so-called "pumping" or "flutter") under wind loads. 
For example, an SHS 100x100x4 mm profile made of S355JR steel can achieve a torsional moment of inertia J ≈ 169 cm⁴, while a typical cold-formed C-channel 100x50x3 mm (of similar linear mass) will have a J in the range of 5-15 cm⁴. This fundamental difference, often overlooked in simplified static calculations, prevents aeroelastic phenomena such as galloping or torsional divergence, which in open profiles can lead to resonance at wind speeds as low as 15 m/s. 
Long-term vibrations and cyclic micro-deformations, even below visible deflection limits, are the main cause of microcrack formation in silicon solar cells. Studies (e.g., cyclic mechanical load tests in accordance with IEC 61215, MCT method) have shown that microcracks can lead to localized module power degradation, manifesting as hot-spots and a reduction in effective active area. This results in an annual power loss of 0.5-2.0% of the LCOE (Levelized Cost of Energy) on a project scale, and in extreme cases – complete module failure. Closed profiles, by providing a rigid and stable platform, drastically reduce the risk of these phenomena, extending the lifespan of modules and maintaining their high efficiency throughout their operational period.
Significantly Greater Buckling Resistance and Extreme Weather Resilience: Stability in the Face of Nature's Forces
Due to the optimal material distribution in the cross-section and the continuity of all walls, closed profiles offer radically greater buckling resistance for compressed elements (columns, beams). Buckling is a critical failure mode for slender structures, particularly vulnerable to axial or bending forces in planes of lower stiffness. According to standard PN-EN 1993-1-1 (Eurocode 3, Design of steel structures), buckling resistance is directly dependent on the slenderness of the element (λ) and its flexural rigidity (EI). 
For a given linear mass, a closed profile (e.g., SHS) exhibits significantly more favorable slenderness parameters and moments of inertia (I_x, I_y) in both principal axes compared to an open profile. For example, a column made of SHS 100x100x4 mm can carry 30-50% greater critical load than an open C-channel of the same mass before instability occurs. 
This advantage is absolutely crucial in regions exposed to extreme meteorological conditions: 
Wind Loads: According to PN-EN 1991-1-4 (Eurocode 1, Actions on structures - Wind), wind forces increase with the square of the speed. In hurricane zones (e.g., 60 m/s, or ~216 km/h) wind pressure can reach 2.2 kN/m², requiring structures of extreme rigidity. Closed profiles, with their high stiffness and aerodynamically smooth surface, minimize drag, while open profiles generate turbulence and increased loads.
Seismic Loads: According to PN-EN 1998 (Eurocode 8, Design of structures for earthquake resistance), ground accelerations of 0.3g to 0.5g can induce significant dynamic forces. In the event of earthquakes, the structure's ability to dissipate seismic energy through controlled plastic deformations (without loss of global load-bearing capacity) is crucial. Closed profiles exhibit better ductile properties and greater resistance to low-cycle fatigue, which is preferred in seismic design.
The use of closed profiles allows for the design of lighter, yet safer and more reliable structures, meeting the highest safety standards in construction under the most demanding environmental conditions.
Minimizing Shading and Maximizing Rear Gain in Bifacial Modules: Energy Production Optimization
In the context of bifacial modules, capable of generating energy also from the rear side (rear gain), it is crucial to minimize any obstructions blocking reflected light from the ground (albedo). A closed profile, as a single, narrow beam, is characterized by a minimal footprint when projected onto the module, effectively reducing the so-called "shadow footprint." A standard RHS/SHS profile with dimensions of 60-100 mm width casts a minimal shadow on the rear surface of the module. 
In contrast to open profiles (C-channels, omegas), which often require extensive trusses, brackets, and cross-members to ensure adequate stiffness and stability, a closed profile does not need additional structural elements within the module's light path. The total shading area in a typical open profile structure can be 5 to 10 times greater, drastically reducing the gain from the rear side of the module. 
For bifacial modules (e.g., n-type TOPCon with a bifaciality factor >70% and efficiency of 23%), every percentage of rear side shading can reduce total energy yield by 0.5-1.5%, depending on ground albedo (e.g., grass: 0.2-0.3, white gravel: 0.5-0.6, concrete: 0.3-0.4) and mounting height. At an albedo of 0.4 and a mounting height of 1.0 m, shading optimization can increase rear gain by an additional 3-5% annually. 
Thanks to closed profiles, it is possible to achieve a real rear gain of 15-25% for typical installations on trackers or fixed East-West structures, whereas in open profile structures, it often does not exceed 5-10%. This directly translates into a reduction in LCOE and an increase in the IRR (Internal Rate of Return) of the PV project.
Detailed Analysis: Closed Profiles (RHS/SHS)
Geometry: Square (SHS) or rectangular (RHS) cross-sections. Typical wall dimensions 3-6 mm. Manufactured by cold or hot forming.
Shadow Footprint: Minimal. Only the width of a single beam (e.g., SHS 100x50x3 mm casts a shadow ~6-10 cm in projection). One structural element within the module's light path. Optimal for bifacial modules.
Torsional Stiffness: Excellent (J > 100 cm⁴ for typical cross-sections). Reduces dynamic vibrations, minimizes the risk of microcracks in PV cells (increases module reliability and lifespan). Eliminates "table pumping," which reduces losses from tracker stow.
Buckling Resistance: Significantly higher (by 30-50% for comparable mass) due to more efficient material distribution and greater moment of inertia. Crucial for resistance to strong winds (e.g., designed for 60 m/s) and earthquakes (e.g., ground acceleration 0.5g). Compliance with Eurocodes EN 1993, EN 1991, EN 1998.
Need for Cross-members/Stiffeners: Negligible or none. The structure is naturally rigid and stable, simplifying assembly and reducing installation time by 15-20%.
Corrosion: Externally galvanized (e.g., hot-dip, min. 80 µm zinc coating thickness). The inner surface is protected from direct atmospheric exposure, which slows down corrosion and significantly extends durability to 50+ years. Reduces O&M costs associated with structure maintenance.
Durability: Possibility of achieving 50+ years without significant degradation of mechanical parameters, especially with appropriate corrosion protection (e.g., hot-dip galvanizing, duplex paint systems). Fully compatible with the life cycle of new-generation PV modules.
Tandem Ready: Ideal for future perovskite-silicon tandem modules, where every reduction in shading is invaluable, and high structural rigidity is crucial for protecting cells with increased fragility.
O&M Costs: Lower due to greater durability, less need for maintenance interventions, and higher resistance to mechanical damage.
Detailed Analysis: Open Profiles (e.g., C-channels, "omega")
Geometry: C, U, Z, or omega-shaped cross-sections. Typical wall dimensions 1.5-2 mm (often cold-formed from sheet metal).
Shadow Footprint: High. Often 3-5 or more structural elements (including cross-members and stiffeners) within the module's light path. Total shadow 5-10 times greater than for closed profiles, reducing rear gain by as much as 5-10%.
Torsional Stiffness: Low (J < 15 cm⁴ for comparable cross-sections). High risk of vibrations (resonant frequencies around 1-5 Hz), material fatigue, and cell microcracks, leading to module degradation and reduced energy production (PR drop of 0.5-2.0% annually).
Buckling Resistance: Lower. Requires more material or extensive stiffeners to meet the same safety requirements as closed profiles. Can lead to excessive structure weight and higher material costs.
Need for Cross-members/Stiffeners: Often necessary to ensure stability and limit deformations, which complicates and prolongs the assembly process.
Corrosion: Both sides (internal and external) are exposed to direct atmospheric factors (moisture, aggressive environment), accelerating corrosion. Typical galvanized coating thickness (e.g., Sendzimir) often below 20 µm.
Durability: Shorter lifespan, usually 20-30 years, before corrosive and fatigue degradation impact structural safety and performance. Risk of structural failures after 15-20 years.
Ineffective for Bifacial: Significantly reduce potential rear gain, making them less cost-effective in the long run for advanced new-generation PV modules.
O&M Costs: Higher due to the need for more frequent inspections, anti-corrosion repairs, and potential replacement of structural elements.
In summary, the choice of closed profiles is a strategic engineering decision that directly translates into the long-term efficiency, reliability, safety, and profitability of photovoltaic farms. The less "metal in the light path" of the module and the greater mechanical stability of the structure, the closer to maximum rear gain and zero rear side shading, which is critical for achieving optimal energy production in new-generation bifacial systems. This is an investment in stability and performance for future decades, significantly lowering LCOE and increasing the value of photovoltaic assets.
Perovskite vs. Silicon: Spectral Sensitivity and Diffuse Light Energy in Tandem Modules
When considering the implementation of perovskite-silicon tandems, a fundamental understanding of their differences in spectral sensitivity and the conversion efficiency of direct, diffuse, and reflected (albedo) radiation is crucial. The question of whether perovskite exhibits greater sensitivity to these radiation components than silicon is complex and has direct, critical engineering implications for the design, optimization, and long-term performance of photovoltaic systems based on tandem technology. Redesigning traditional design assumptions is essential to maximize the benefits of this innovative technology.
Characteristics of Crystalline Silicon (c-Si) and its Advantage in Diffuse Light
Crystalline silicon (c-Si), being the dominant semiconductor in the PV industry, is characterized by a bandgap (Eg) of **approximately 1.12 eV** at 300K. It effectively absorbs solar radiation across a wide spectral range, from 400 nm (blue light) to about 1100 nm (near infrared, NIR). Its optimal absorption occurs in the red and near-infrared range. This spectral range is particularly rich in diffuse light components, originating from atmospheric scattering (e.g., from clouds, aerosol particles) and reflected light (albedo) from the ground or surroundings. Rayleigh scattering causes blue light to be scattered much more effectively than red light, leading to a "reddening" of the diffuse light spectrum reaching the Earth's surface. Silicon is perfectly matched to absorb this spectrum. Silicon's ability to efficiently utilize diffuse and reflected light is the basis for the **"rear gain"** phenomenon in bifacial modules. In standard installations (e.g., with high ground albedo >0.3, optimal ground coverage ratio GCR ~0.3-0.4, and mounting height >1m), "rear gain" typically ranges **from 10% to 25%** of the total annual energy, with reports reaching up to 30% in specific conditions (e.g., snow with albedo >0.8, bright concrete surfaces). According to standard **IEC 60904-1-2:2020**, module bifaciality (ϕ) is defined as the ratio of rear-side efficiency to front-side efficiency, measured under standard test conditions (STC). Long-term degradation of c-Si modules is low and stable, typically **0.4-0.5% annually** after the first year (which usually shows slightly higher degradation, around 1.5-2.0% depending on PID/LID/LETID mitigation technology), making silicon a reliable foundation for long-life PV systems.
Formula for annual energy yield (Ea) for a bifacial module:
Ea = Efront + Erear = Prated × (1 + ϕ × Albedoeff × k) × HGHI × PR
Where: ϕ - bifaciality factor; Albedoeff - effective albedo (considering installation geometry); k - correction factor for diffuse/reflected light; HGHI - annual global horizontal irradiance on the module plane (kWh/m2); PR - performance ratio (system efficiency factor); Prated - nominal module power.
Perovskite Characteristics and Complementarity in Tandems
Typical halide-based perovskites (e.g., CH3NH3PbI3, FAPbI3) are characterized by a larger bandgap (Eg), usually in the range of **1.5 eV to 1.7 eV**. This means they effectively absorb mainly higher-energy light, i.e., in the blue-green and partly yellow range (approx. 300-750 nm), while allowing a significant portion of red and near-infrared radiation to pass through. Their external quantum efficiency (EQE) curves show a strong peak in the shorter wavelength region. In the context of diffuse and reflected light, which, as mentioned, is often shifted towards longer wavelengths (e.g., due to Rayleigh scattering for blue light or atmospheric absorption), perovskite produces proportionally less energy than silicon. Studies indicate that the yield from the rear side of a hypothetical perovskite module (perovskite only) would be significantly lower than for silicon, due to spectral mismatch with the diffuse and reflected light spectrum. In 4-terminal (4T) tandems, where perovskite acts as the top cell, its role is to convert high-energy photons that would otherwise be lost or less effectively utilized by silicon. However, for the diffuse light component, its efficiency is lower compared to silicon. In 2-terminal (2T) tandems, current matching between the perovskite and silicon cells is crucial and more sensitive to spectrum changes. The long-term stability of perovskites in field conditions is still a subject of intensive research. Initial perovskite degradation can be **3-5% in the first year**, and stabilized annual degradation at **0.8-1.5%**. The key is to use advanced encapsulation techniques (e.g., moisture barriers with water vapor permeability <10-6 g/m2/day) and stable chemical compositions (e.g., cation mixtures, defect passivation) to approach silicon's durability. Accelerated aging tests (DH, HAST, TCT) in accordance with modifications of standard **IEC 61215** (e.g., for thermal cycles TCY, DH1000) are crucial for verifying the stability of these materials.
In the design of a bifacial tandem module, where perovskite usually forms the top cell, silicon remains the "king of the rear side." It is the silicon cell, placed beneath the perovskite layer, that is responsible for converting light passing through the perovskite (especially in the red/IR range) and for efficiently utilizing diffuse and reflected light from the rear side. Even in future tandem implementations with a potential additional perovskite cell from the rear, it is the inherent spectral characteristic of silicon that will provide most of the energy from the albedo component and diffuse radiation. For this reason, the use of optimal, "rear-friendly" mounting structures is absolutely crucial for maximizing the total energy yield from bifacial tandems, ensuring full access for silicon to all available diffuse light sources. It is estimated that insufficient utilization of the rear side potential, e.g., by using traditional shading structures >5%, can reduce the project's annual LCOE by **3-7%** compared to the optimal solution, negating a significant part of the cost advantage resulting from the higher efficiency of the tandem module.
“In conditions of diffuse and reflected light, where the spectrum is depleted of blue and UV components and enriched with radiation in the red and near-infrared range, perovskite exhibits a proportionally lower yield than silicon. Silicon is much better matched to the spectrum of 'red' diffuse light, implying that the theoretical "rear gain" for standalone (hypothetical) perovskite modules would be significantly less than that achieved by silicon bifacial modules. This spectral disparity is fundamental to silicon's advantage in irradiance conditions dominated by indirect radiation, which must be taken into account in energy simulations (e.g., in PVsyst, PV*SOL software) and LCOE (Levelized Cost of Electricity) analyses for tandem PV systems that rely on spectral balance.”
Rear-side Energy Production in Bifacial Systems - Detailed Analysis of Absolute Energy Values
The effective utilization of bifaciality in photovoltaic modules, especially in the context of high-efficiency silicon-perovskite tandem modules (e.g., 34% at STC), is crucial for maximizing energy yields in PV systems. Energy production from the rear side of the module, known as "rear gain" (RG), adds significant value to the total annual energy production. The calculations below present the absolute energy values obtained from the rear side of the module for specific scenarios, emphasizing the importance of this additional production.
Scenario 1: Poland (Y_front ≈ 1100 kWh/kWp·year)
In the conditions of the Polish climate, where the typical annual energy yield for modules facing south (azimuth 180°, optimal tilt for a given region, e.g., 30-35°) is approximately 1100 kWh per kilowatt of installed power (kWp), the bifacial potential is significant.
For an exemplary tandem module with a nominal power of 800 W (which corresponds to 0.80 kWp in an installation), measured under STC (Standard Test Conditions: 1000 W/m², 25°C, AM1.5):
Definition of Rear Gain (RG): RG is the percentage increase in the energy yield of a bifacial module relative to the front-side yield over a given period, resulting from the absorption of diffuse and reflected light by the module's rear side. According to IEC 60904-1-2, the bifaciality factor (Φ) is the ratio of the rear-side efficiency to the front-side efficiency, typically > 0.70 for silicon-perovskite tandems.
Albedo Impact: We assume an average ground albedo for grass (typically 0.20-0.25) or light gravel (0.30-0.40). In this example, for a Rear Gain of 20%, it means that the rear side generates 20% additional energy relative to the energy obtained from the front. This is a realistic value for albedo ~0.3 (e.g., light gravel) and mounting height ~1m.
Detailed Calculations:
    Rear-side Yield [kWh/year] = Y_front × RG × P_module
    Rear-side Yield = 1100 kWh/kWp·year × 0.20 × 0.80 kWp = 176 kWh/year from rear
Total Energy Yield:
    Y_total [kWh/year] = Y_front × (1 + RG) × P_module
    Y_total = 1100 kWh/kWp·year × (1 + 0.20) × 0.80 kWp = 1056 kWh/year
Additional Parameters: Y_front values are usually estimated based on meteo data (e.g., PVGIS, Solargis) and simulations (e.g., PVSyst, SAM), considering tilt angle, azimuth, temperature losses (power coefficient for temperature for c-Si is approx. -0.35 to -0.45 %/°C, for perovskites -0.30 to -0.40 %/°C), wiring losses (~1-2%), soiling losses (~2-5% annually), and module degradation (typical linear degradation for c-Si is approx. 0.5% annually, for perovskites similar or slightly higher values are expected, e.g., 0.6-0.7%).
Scenario 2: Spain (Y_front ≈ 1900 kWh/kWp·year)
In regions with higher insolation, such as Spain (e.g., Andalusia), where the annual front-side energy yield can reach 1900 kWh/kWp·year, the potential for additional energy from the rear side is even greater.
For the same 800 W (0.80 kWp) tandem module:
Factors Affecting Rear Gain (RG): RG depends on many factors, including ground albedo (e.g., white stoneware ~0.6-0.8, concrete ~0.3-0.5, grass ~0.2-0.3), module mounting height above ground (h, optimally >0.8 m), row spacing (GCR - Ground Coverage Ratio, e.g., for GCR 0.4-0.6), shading, and mounting type (e.g., fixed tilt structures, single or dual-axis trackers). Optimizing these parameters can significantly increase RG.
Calculations for Different RGs:
    For Rear Gain 20%:
    Rear-side Yield = 1900 kWh/kWp·year × 0.20 × 0.80 kWp = 304 kWh/year from rear

    For Rear Gain 25% (achievable with high albedo and optimal mounting, e.g., white geotextile fabric on the ground, no rear-side shading, mounting on trackers):
    Rear-side Yield = 1900 kWh/kWp·year × 0.25 × 0.80 kWp = 380 kWh/year from rear
    For Rear Gain 30% (optimal scenario, e.g., winter albedo from snow ~0.8, high GCR, trackers):
    Rear-side Yield = 1900 kWh/kWp·year × 0.30 × 0.80 kWp = 456 kWh/year from rear
Case Study: Mounting Height vs. RG: Studies (e.g., from NREL, Fraunhofer ISE) show that increasing the module height above ground from 0.5 m to 1.0 m can increase RG by 1-3 percentage points, as more reflected light reaches the rear surface. Spectral analysis of reflected light indicates a predominance of longer-wavelength components (red, NIR), which is particularly beneficial for the bottom silicon cell in a tandem.
Recommended Standards and Practices: In the design of bifacial systems, it is recommended to use a ground albedo level of >0.30, which can be achieved by using light gravel, white membranes, or light concrete surfaces. Mechanical configuration of profiles (e.g., aluminum profiles EN AW-6063 T6) and structural calculations (e.g., according to Eurocodes EN 1991 for wind and snow loads) must account for increased exposure area and potential snow loads. Regular module cleaning (O&M costs) is also crucial to prevent yield reduction, especially in high dust conditions, as accumulated dirt on the rear side can lower albedo.
Summary of Absolute Rear-side Energy Yields
Engineering Conclusions and Financial Implications:
Ignoring the additional energy production from the rear side of the module, ranging from 110 kWh/year (for a smaller module in Poland) to even 456 kWh/year (for a large module in Spain under optimal conditions), is equivalent to foregoing free fuel for the power plant. In the context of financial analysis (LCOE - Levelized Cost of Energy) and Return on Investment (ROI), every additional kWh is of critical importance. LCOE, calculated as (Total Investment Cost + Annual O&M Costs) / Annual Energy Production, can be significantly reduced due to increased yields from bifaciality. The additional investment costs of bifacial modules and optimal mounting are often compensated by increased yields within the first few years of operation.
For a large photovoltaic farm with thousands of modules, these "lost" kilowatt-hours translate into millions of zlotys (Polish currency, but maintaining original unit as requested) of unrealized revenue over the project's lifespan (e.g., 25-30 years). Risk analysis should consider potential underestimation of RG and risks associated with improper selection of ground and mounting parameters. Optimal design of bifacial installations, considering albedo, mounting height, and module spacing, is therefore not only a technical issue (compliance with standards IEC 61215, IEC 61730 for module qualification and safety) but a strategic business decision that no serious investor can afford to ignore. Investing in bifacial technology and optimizing its implementation directly translates into an increased Internal Rate of Return (IRR) of the project and a shorter payback period.
Conditions for Achieving Full Rear Gain and Optimizing Bifacial Performance in PV Systems
To fully utilize the potential of bifacial modules and maximize energy gains from the module's rear side, it is essential to precisely meet a series of key engineering and design conditions. Ignoring any of these aspects can significantly reduce the actual power gain from the rear (Rear Gain, RG) compared to theoretical possibilities, leading to an underestimation of production and a decrease in investment profitability. Proper optimization is key to lowering the LCOE (Levelized Cost of Energy) and increasing the ROI (Return on Investment) of photovoltaic projects.
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Rear-Friendly Geometry: Minimizing Structural Shading and Optimizing View
The optimal design of the support structure is fundamental for minimizing self-shading and external shading, and for maximizing the exposure of the module's rear side to reflected and diffuse light. Key parameters include:
High Ground Clearance: The height of the module's bottom edge above the ground directly impacts the "rear view factor," i.e., the proportion of reflected light reaching the rear surface. A minimum clearance of 1.1 to 1.2 meters is recommended for ground-mounted installations, and up to 1.5 meters for single-axis trackers, depending on the module tilt angle and ground albedo. Studies, e.g., by NREL, indicate that increasing clearance from 0.5 m to 1.0 m can improve Rear Gain by 5-15% at an albedo of 0.4-0.6 (e.g., light gravel). High clearance minimizes shading from the ground (e.g., vegetation, uneven terrain) and increases the solid angle from which the rear side "sees" the bright ground.
Narrow Table/Module Row: Shorter rows and fewer modules per row (e.g., 1-2 modules oriented vertically, 3-4 horizontally in a tracker system) reduce the effect of inter-row shading, especially in the early morning and late afternoon hours when the sun is low. Shading from an adjacent row on the module's rear side can locally reduce production by as much as 20-30%. Construction design should consider the sun's incidence angle during the least favorable period (e.g., winter solstice) for a given latitude, to minimize shadows cast by adjacent rows. The width of the module row (table) should be chosen to avoid self-shading of the rear side by other modules in the same row, which is particularly important in trackers where the tilt angle changes throughout the day.
Zero Shading from Structure and Equipment: It is critical to eliminate all structural elements (crossbeams, posts, mounting frames, clips, junction boxes) within the light path of the cells' rear side. Even partial shading (e.g., from a 2-3 cm wide clamp) can lead to the activation of bypass diodes and the creation of hot-spots, reducing the efficiency of the entire module (estimated at 0.5-1% per 1 cm² of shading, depending on its location and intensity) and accelerating module degradation (so-called "shadow-induced degradation"). Mounting systems that minimize point contact with the module are preferred, e.g., bottom-mount, rails integrated into the module frame, or rail-less systems, which ensure maximum exposure of the cells' rear side. Compliance with IEC 61215-2 requires that shading does not cause excessive temperature rise (hot-spots).
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Cabling and Its Routing: Avoiding Losses Due to Self-Shading
Improperly routed cabling is one of the most frequently overlooked sources of power losses in bifacial installations, leading to thermal effects and degradation:
Cables Only from Below on Metal Clips: All DC cables (PV1-F module cables with a cross-section of 4-6 mm², compliant with EN 50618) and AC cables should be routed only beneath the module structure, away from their rear surface. Special metal clips (e.g., stainless steel, A2 or A4 grade) or UV-resistant cable ties (compliant with IEC 61910-1) should be used to secure cables tightly to the construction frame or mounting profiles, preventing loops and sagging. Proper cable fastening also prevents mechanical damage caused by wind (so-called cable whip).
No Loops/Sags: Even thin wires with a diameter of a few millimeters cast a distinct shadow on the rear surface of the cells. Spectral analyses have shown that shading from a black cable can absorb up to 90% of incident light, effectively blocking it. It is estimated that 1 cm² of shading on the rear side of a bifacial module can lead to 0.5-1.5% local power loss, depending on the shadow intensity and cell sensitivity in that area. Regular visual audits, especially after extreme weather conditions, are essential to detect and correct loose or sagging cables that could cast a shadow.
UV, Temperature, and Environmental Resistance: Use cables (with XLPE, LSZH insulation) and mounting accessories (clips, ties) resistant to UV radiation (minimum 20 years lifespan under exposure conditions), extreme temperatures (operating range from -40°C to +90°C), moisture (IP67 for connectors), and wind. Cables should be routed in a way that allows for ventilation to prevent overheating, which shortens insulation life. Compliance with IEC 60364-7-712 (photovoltaic installations) and IEC 61730 (module safety) is mandatory.
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Neutral Mounting and Its Impact on Module Performance and Longevity
The mounting method has a direct and significant impact on the performance, cell uniformity, and long-term lifespan of a bifacial module, minimizing energy losses:
Bottom Mounting (Clamp-Free): Mounting systems that secure the module from its underside (so-called hidden clamps, bottom-mount) eliminate traditional clamps on the frame. These clamps, especially in vertical installations, can create "sills" that collect dust, snow, ice, or rainwater, leading to uneven soiling and localized shading (so-called "shadow lines" or "dirt lines"). Uneven soiling can reduce module production by 3-10% annually, and in extreme cases (e.g., after snowfall) even by 20% momentarily. Clamp-free mounting ensures even soiling and maximizes the exposure of the rear side to light, also facilitating natural cleaning by rain.
No Point Loads and "Sills": Traditional clamps can cause point shading and concentrate mechanical stresses on the module frame, increasing the risk of micro-cracks in cells caused by wind or temperature loads. Neutral mounting minimizes these effects, leading to better cell temperature uniformity (lower temperature gradients), reduced risk of hot-spot formation (in accordance with IEC 62804-1 for PID - Potential Induced Degradation), and extended module lifespan. Studies have shown that shading from a 2-3 cm wide clamp can reduce local power generation by as much as 5-10% in the shaded area, potentially leading to unbalanced current in the cell string and system losses.
Ventilation Optimization: Neutral mounting also promotes better module ventilation, which is crucial for its efficiency. Photovoltaic modules are characterized by a negative temperature coefficient of power (typically around -0.35% /°C for silicon cells). This means that every degree Celsius increase in module temperature above the test temperature (STC: 25°C) causes a 0.35% drop in power. Proper module ventilation, especially its rear side, can lower cell operating temperature by 5-10°C at peak, which translates to a real increase in energy yield of 1.75-3.5% during the day, especially on sunny and hot days. High ground clearance combined with an open mounting structure significantly improves air convection.
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Strategic Orientation and Its Benefits in the Energy Profile Context
For bifacial modules, the optimal orientation may differ from that of monofacial modules, offering better matching to the energy demand profile and maximizing the economic value of production:
EW (East-West) Orientation with Moderate Tilt: In bifacial systems, an East-West orientation with a tilt of 20-30° (optimally around 25° for Poland) can generate more energy in the morning and afternoon/evening hours than a classic south orientation. This is particularly beneficial in countries with variable energy tariffs (Time-of-Use, ToU), where energy prices are higher during peak demand hours (so-called "peak shaving"). Such a setup can increase the value of the generated energy by 5-15% depending on the tariff structure. Simulations have shown that in some regions, EW bifacial systems can achieve a lower LCOE than south-facing monofacial systems.
SE/S/SW Mix: Depending on the location, the energy demand profile of a given facility, and available space, a mix of South-East (SE), South (S), and South-West (SW) orientations may be optimal to spread production throughout the day. This increases flexibility and the ability to better manage grid load or self-consumption. For example, for maximizing self-consumption in office buildings, a SE/SW combination may be more favorable than pure S.
Strong Morning and Dusk: Bifacial modules are particularly effective in low sun angle conditions (morning and evening) when reflected radiation from the ground (albedo) constitutes a significant share of the total irradiance falling on the rear side. Studies have shown that during these hours, Rear Gain can be as high as 30-50%, while during the day it fluctuates around 10-20%. This translates into a higher energy value (peak shaving) and the possibility to reduce the size of energy storage, as the installation provides power during peak demand periods, reducing the need to import energy from the grid at higher rates.
Tracker Optimization: For single-axis tracker systems, bifacial modules can achieve an additional 10-20% gain compared to monofacial trackers. Optimizing the tilt angle and tracking range can increase Rear Gain by maximizing exposure to reflected light and minimizing self-shading. Tracking algorithms should be adapted to bifacial conditions (so-called "bi-facial tracking algorithm") taking into account reflected radiation.
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Row Spacing and Albedo Control: Engineering of Reflected Light
Proper row spacing (GCR - Ground Coverage Ratio) and ground albedo management are absolutely critical for maximizing production in bifacial installations, as they are the direct source of light for the module's rear side:
Row Spacing for Low Sun (Minimizing Inter-Row Self-Shading): To prevent self-shading of the modules' rear side by adjacent rows, inter-row spacing must be appropriately selected and is usually greater than for monofacial modules. It is generally assumed that the ratio of module length (height in vertical arrangement) to height above ground plus distance to the adjacent row should minimize shading during the least favorable period (e.g., December 21, 9:00 AM - 3:00 PM). The formula for minimum row spacing (D) considering self-shading is: D = H / tan(beta) + L * cos(gamma), where D is the row axis spacing, H is the module height from the ground (top edge), L is the module length (vertical height), beta is the minimum sun elevation angle at which self-shading is to be avoided, and gamma is the module tilt angle. The optimal GCR for bifacial systems typically ranges from 0.3-0.5.
Albedo Control (Ground Reflectivity): Albedo is the coefficient of solar radiation reflection from a given surface, expressed as a decimal fraction (0-1). It has the most significant and direct impact on Rear Gain. High albedo surfaces can increase RG by up to 30% compared to low albedo.
Fresh Snow: 0.7-0.9 (highest reflection, potentially +15-30% Rear Gain depending on the region)
Light Gravel/Limestone Aggregate: 0.4-0.6 (+8-15% Rear Gain)
Light Concrete/White TPO/EPDM Membrane: 0.3-0.5 (+6-12% Rear Gain)
Light Sand/Dry Grass: 0.25-0.4 (+5-10% Rear Gain)
Green Grass/Soil: 0.1-0.3 (+2-8% Rear Gain)
In PV farm design, it is advisable to consider using light-colored surfaces (e.g., white gravel fraction 8-16 mm, special reflective membranes with TiO2 coating with a reflection coefficient >0.7) under the modules to maximize light reflection to the modules' rear side. Maintaining high albedo also requires vegetation control and surface cleaning. The impact of albedo should be modeled in PV simulation software (e.g., PVSyst, PVGIS, SAM) using meteorological data (global, direct, and diffuse irradiance) and albedo data for the specific site.
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Quality Control and Monitoring: Ensuring Long-Term Performance and Risk Reduction
Continuous quality control during the construction phase and regular monitoring and audits during the operation phase are essential to maintain optimal performance and reliability of bifacial systems throughout their lifecycle (25+ years):
Rear Shading Audit (Photo/Light Meter): Regular visual inspections (especially after maintenance, strong winds, snowfall) and precise measurements of irradiance on the rear side of modules using a Class A pyranometer (compliant with ISO 9060) are crucial. These measurements help detect unwanted shading from cables, structural elements, dirt, or vegetation. The use of drones with thermal cameras (for hot-spot detection) or high-resolution visual cameras (for shading identification) can facilitate and accelerate problem identification on large PV farms. Comparing images from the commissioning period with current images allows for quick identification of changes.
IV/EL Measurements After First Strong Winds and Periodically: After strong winds (e.g., above 25 m/s) or other extreme weather conditions (hail, heavy snowfall), it is recommended to perform current-voltage (I-V curves) measurements of modules (according to IEC 60904-1) and electroluminescence (EL) or photoluminescence (PL). These measurements allow for early detection of damages invisible to the naked eye, such as micro-cracks in cells, delamination, or PID (Potential Induced Degradation), which can lead to significant power losses and shortened module lifespan. Typically, I-V measurements should be performed every 3-5 years or after weather incidents.
Cabling and Mounting Documentation: Detailed documentation of cable routing, module fastening, clips used, and other mounting accessories, along with geolocation and photos, is crucial for future audits, servicing, and problem diagnosis. This ensures compliance with the design and minimizes the risk of errors or unauthorized modifications. Documentation should include a bill of materials, product certificates, and mounting instructions.
Performance Monitoring (Performance Ratio Analysis): Continuous monitoring of installation performance at the module or string level, including bifacial irradiance data (if available, using two pyranometers – one facing the front, the other the rear of the module, or special bifacial sensors), allows for analysis of actual Rear Gain and quick identification of any problems (e.g., drop in PR - Performance Ratio, detecting soiling, inverter faults, module degradation). Comparing actual production with simulated production (e.g., from PVSyst) is key for early anomaly detection. PR analysis over time also allows for estimating the module degradation rate (typically 0.5-0.8% annually).
Bifacial Tandem Module Structure: In-depth Technical and Engineering Analysis
The proposed layered structure of the perovskite-silicon tandem module, with perovskite on both sides (front and rear), aims to optimize solar light conversion across a broad spectrum and minimize key degradation mechanisms, while ensuring high bifacial efficiency. This architecture is based on proven glass-glass technology, which is crucial for perovskite stability.
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1. Rear Glass – Thickness 2.0 mm
The module's rear base is made of highly transmissive, tempered soda-lime glass, characterized by low iron content (so-called "ultra-clear" glass). The standard 2.0 mm thickness represents an optimal compromise between mechanical strength requirements, module weight reduction, and maximized transmittance. Typical glass density is 2500 kg/m³, which for a module with dimensions of 1.7 x 1.0 m² (1.7 m²) means a mass of approximately 8.5 kg for the glass alone. 2.0 mm thick tempered glass modules are capable of withstanding wind loads up to 2400 Pa (corresponding to wind speeds of approx. 210 km/h) and snow loads up to 5400 Pa, in accordance with the rigorous requirements of IEC 61215-2:2016 (section 10.16 – static mechanical load). The bending strength for tempered glass is typically 120-200 MPa. Hail impact resistance tests (IEC 61215-2:2016, section 10.17) require resistance to impacts from a 25 mm diameter, 7.53 g ice ball falling at 23 m/s. Light transmittance for the visible and near-infrared spectrum (400-1100 nm) should exceed 91% (for glass without an AR coating) to ensure maximum availability of diffuse and reflected light for the rear perovskite cell. The refractive index for glass is approximately 1.52. An anti-reflective (AR) coating can optionally be applied to the inner side of the rear glass to reduce reflection losses at the glass/EVA interface, increasing effective transmittance to 93%.
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2. Rear Perovskite + TCO (Transparent Conductive Oxide)
This layer, deposited directly on the inner side of the rear glass, functions as the rear tandem sub-cell. It utilizes a perovskite material with a lower bandgap (typ. 1.4-1.6 eV), optimally tuned for absorbing longer-wavelength photons (near-infrared) that have passed through the top silicon cell. Example low-bandgap perovskite compositions include formamidinium-tin-iodide perovskite (FA(Sn/Pb)I₃) or mixed lead-tin halides (MAPbI₃₋ₓClₓ). The perovskite layer thickness is typically 300-600 nm. The typical efficiency of a single narrow-bandgap (NBGP) perovskite cell is 20-22%.

TCO (Transparent Conductive Oxide): Serves as a transparent electrode. Fluorine-doped tin oxide (SnO₂:F, FTO) or indium tin oxide (In₂O₃:Sn, ITO) are commonly used. FTO is preferred due to its higher chemical stability during perovskite deposition processes. Typical sheet resistance is 5-15 Ω/□, and optical transmittance exceeds 85% in the perovskite absorption range (700-1100 nm). Deposition methods include magnetron sputtering, physical vapor deposition (PVD), or chemical vapor deposition (CVD). Minimizing TCO surface defects is important as they can affect perovskite layer growth and device stability.

Reduced UV degradation: A key advantage of this arrangement is the natural filtering of UV radiation by the upper layers (front glass and silicon). Silicon absorbs UV radiation below ~380 nm, significantly limiting the exposure of the rear perovskite cell to degrading UV radiation. Perovskite stability studies have shown that the lack of direct UV exposure can extend their stability by an order of magnitude compared to classic architectures where the perovskite is exposed to UV radiation. Accelerated aging tests such as the UV-test (IEC 61215-2:2016, section 10.13, 15 kWh/m² UV radiation in the 280-385 nm range at 60°C) and the Damp Heat Test (DHT, IEC 61215-2:2016, section 10.13, 1000h at 85°C/85% RH) are crucial for assessing the long-term stability of this layer. UV degradation mechanisms in perovskites include photoreactions with defects at the oxygen interface, ion migration, and carrier trap generation, leading to efficiency loss and hysteresis.
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3. Bifacial Crystalline Silicon Cell
The main photovoltaic cell in the center of the tandem structure, most often based on N-type crystalline silicon (e.g., TOPCon, HJT) or P-type (e.g., PERC+). This cell is characterized by high conversion efficiency (typ. 22-24% for a high-quality single silicon cell) and the ability to absorb light from both sides. Silicon cell thickness is typically 150-180 µm. N-type cells, such as TOPCon (Tunnel Oxide Passivated Contact) or HJT (Heterojunction Technology), exhibit lower light-induced degradation (LID) and light- and elevated temperature-induced degradation (LeTID), as well as a lower temperature coefficient of power (typ. -0.28 to -0.32 %/°C) compared to P-type cells (typ. -0.35 to -0.40 %/°C), which is crucial for real-world efficiency. TOPCon technology achieves laboratory efficiencies exceeding 26%.

Bifaciality Factor (BF): Typically in the range of 70-95%, meaning that the rear side of the cell generates 70-95% of the front-side power under the same irradiance. In tandem modules, the silicon cell is specifically optimized to transmit an appropriate light spectrum (lower-energy photons, >700 nm) to the rear perovskite cell (so-called "spectral splitting"), while efficiently converting the remaining portion. Key elements are passivation technologies (e.g., thin SiO₂ layer in TOPCon, amorphous silicon in HJT) minimizing carrier recombination and transparent rear contacts (e.g., Ag or Al grid contacts, or thin TCO layers) that maximize light transmission from the rear. Low absorption in the near-infrared (<700 nm) is also desirable. Power losses for bifacial modules are calculated using the relation: P_bif = P_front * (1 + BF * RearGain_eff), where RearGain_eff depends on albedo and mounting configuration. The bifaciality factor depends on the geometry of the current collection grid, cell thickness, and passivation materials. Silicon cells also act as a UV filter for the rear perovskite, absorbing most UV radiation below 380 nm.
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4. Front Perovskite + TCO (Transparent Conductive Oxide)
A layer with high initial efficiency, acting as the upper tandem sub-cell (wide bandgap perovskite, WBGP). It utilizes a perovskite with a higher bandgap (typ. 1.7-1.9 eV) to absorb short-wavelength photons (blue and green light, 400-700 nm). Example compositions include FA₀.₈₃Cs₀.₁₇Pb(I₀.₅Br₀.₅)₃. The perovskite layer thickness is typically 300-500 nm. The typical efficiency of a single wide-bandgap perovskite cell is 22-24%.

TCO (Transparent Conductive Oxide): Aluminum-doped zinc oxide (ZnO:Al, AZO) or ITO (In₂O₃:Sn) are often used. They are characterized by a sheet resistance of 8-20 Ω/□ and high transmittance (>90%) in the visible range (400-700 nm), minimizing optical losses for incident light. Deposition methods, such as sputtering, must be optimized for minimal damage to underlying layers. The TCO layer thickness is typically 100-200 nm. AZO stability is better in acidic conditions than ITO, which can be important in some manufacturing processes.

High initial efficiency and tandem potential: Thanks to the effective utilization of the high-energy solar spectrum, perovskite cells achieve efficiencies exceeding 25% in the laboratory (for a single cell). In a perovskite/silicon tandem module, the theoretical maximum efficiency (Shockley-Queisser limit for 2 cells) exceeds 43%. Practical prototypes have already achieved over 30% module efficiency (record 33.7% in lab). Perovskite stability to light and temperature is a key challenge, partly addressed by using UV filters in the glass and advanced encapsulation (see point 6). LCOE (Levelized Cost of Energy) analysis for tandem modules indicates a potential 10-20% reduction in LCOE compared to monocrystalline silicon modules, due to higher power density and lower Balance of System (BOS) costs per watt.
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5. Front Glass – Thickness 2.0 mm
The outer protective layer of the module, made of low-iron tempered glass, similar to the rear glass. This is glass dedicated to the PV industry, characterized by enhanced resistance to atmospheric factors. A key element is an advanced double-sided anti-reflective (AR) coating, which reduces light reflection from a typical 4% for glass to 0.5-1.5% per surface, increasing transmittance to over 93-95% (compared to 91% for glass without AR). AR coatings are usually based on nanoporous SiO₂ or TiO₂, deposited by sol-gel or CVD methods. The durability of the AR coating is evaluated based on abrasion tests (e.g., compliant with IEC 61215-2:2016).

Integrated UV Filter: The most important element in the context of perovskite modules is an integrated UV filter (often in the form of a special coating, dopants in the glass mass, or special encapsulation films), which effectively cuts off UV radiation below ~380 nm (UVA). Standard PV glass partially filters UV, but for perovskites, an enhanced filter is required. This protects the UV-sensitive perovskite layer from photoluminescence degradation (UV-induced degradation) and irreversible structural changes (e.g., crystal lattice breakdown, iodide oxidation, ion migration), significantly extending module lifetime. Long-term stability tests (IEC 61215:2016, UV-Test) show that the use of UV filters can reduce the annual degradation of perovskites from 3-5% to below 0.5% under simulated solar exposure conditions.
6
6. Encapsulation and 4T (Four-Terminal) Architecture
Complex engineering and optimization aspects.

Encapsulation: Both perovskite sub-cells are hermetically sealed between two layers of glass, creating a "glass-glass" configuration. This provides an exceptional barrier against moisture and oxygen, crucial for the long-term stability of perovskites. Typical water vapor transmission rate (WVTR) for this type of module is < 10⁻⁶ g/m²/day (often below detection limits). Sealing is tested, among others, in the damp heat test (DHT, 1000h at 85°C/85% RH) and the thermal cycling test (TCT, 200 cycles from -40°C to +85°C) according to IEC 61215/61730. Goal: no delamination and power degradation less than 5% after tests. Potting material (encapsulant), e.g., specialized ethylene-vinyl acetate (EVA) with enhanced UV and hydrolysis resistance, or polyolefin (POE), is used between the cells and the glass. This eliminates air voids, improves thermal conductivity (reducing the risk of hotspots, thermal resistance of the EVA/POE layer approx. 0.3-0.5 K·m²/W), and electrical insulation (breakdown voltage >15 kV/mm), and also prevents moisture condensation. The refractive index of the potting (typ. 1.48-1.50) is matched to the glass, minimizing reflection losses (so-called "optical coupling").

4T (Four-Terminal) Architecture: This structure allows for independent electrical paths for the silicon sub-cell and the perovskite sub-cell. Each sub-cell can be connected to its own maximum power point tracking (MPPT) in the inverter. This allows for optimization of the operating point of each cell independently, maximizing the total power output of the module even under varying lighting conditions (e.g., partial shading, different spectral light distributions, e.g., when direct radiation is high and diffuse is low or vice versa). This increases energy yield by 5-15% under real-world conditions (kWh/kWp annually) compared to a 2T configuration (where cells are connected in series and their currents must be matched). Advanced hybrid inverters can manage two power streams, which, however, requires additional MPPT algorithms and potentially increases system complexity. Monitoring the degradation of individual cells in a 4T architecture is also more precise.
Key advantages of this structure from an engineering perspective:
Encapsulation: Both perovskite sub-cells are hermetically sealed between two layers of glass. This "glass-glass" configuration provides an exceptional barrier against moisture and oxygen (water vapor transmission rate < 10⁻⁶ g/m²/day), crucial for the long-term stability of perovskites. Compliance with IEC 61215/61730 standards requires no delamination and no power degradation after humidity tests (85°C/85% RH for 1000h).
UV Filtration: The intermediate silicon cell acts as a natural UV filter for the rear perovskite cell, absorbing harmful short-wavelength radiation. Additionally, the front glass with a UV filter protects the front perovskite. This reduces the UV load on the perovskites, slowing down their photodegradation and potentially extending the module's guaranteed lifetime to over 25 years, with an annual module degradation rate below 0.5% (compared to 0.7-0.8% for standard silicon modules).
Potting: The use of potting material (e.g., specialized EVA or POE with enhanced UV and hydrolysis resistance) between the cells eliminates air voids, which improves thermal conductivity (reducing the risk of hotspots, maximum hotspot temperature < 125°C) and electrical insulation (minimizing current leakage), and also prevents moisture condensation. The refractive index of the potting is matched to minimize reflection losses.
4T (Four-Terminal) Architecture: This structure enables independent electrical paths for the silicon sub-cell and the perovskite sub-cell. This allows for optimization of the operating point of each cell independently (MPPT for each sub-module), maximizing the total power output of the module even under varying lighting conditions (e.g., partial shading, different spectral light distributions). This increases energy yield by 5-15% under real-world conditions compared to a 2T configuration, which directly translates to a lower LCOE and higher IRR (Internal Rate of Return) for PV projects.
Perovskite Durability - Front vs. Rear: An In-Depth Engineering Analysis
We are conducting a detailed analysis of the predicted durability of perovskite layers in bifacial tandem modules, taking into account key environmental factors and dominant degradation mechanisms. This is essential for photovoltaic engineers in designing, implementing, and assessing the risk of long-term performance for photovoltaic systems.
Front-side Perovskite (FSP)
Exposure to UV Radiation: FSP is directly exposed to the full solar spectrum (AM1.5G, corresponding to 1000 W/m² irradiance at 25°C and SMM 1.5), including intense UV radiation in the 300–400 nm range (UVA: 315-400 nm, UVB: 280-315 nm). Despite the use of a UV filter in the front glass (reduction below 380 nm), long-term exposure to residual UV radiation (especially UVA) and so-called "UV breakthrough" through filters (< 5% transmission at 380 nm is acceptable, but cumulative) still poses a challenge. The main degradation mechanisms are:
Photo-oxidation: Generation of reactive oxygen species (ROS), such as hydroxyl radicals (•OH) and hydrogen peroxide (H₂O₂), which attack organic components of the perovskite (e.g., FA, MA) and charge transport layers (HTL/ETL), leading to their decomposition.
Photo-corrosion: Direct decomposition of the perovskite crystal lattice (e.g., (CH₃NH₃)PbI₃ → CH₃NH₂ + HI + PbI₂) under the influence of high-energy UV photons, leading to the formation of defects (e.g., iodide vacancies, Pb⁰ centers) and morphological changes (reduction in grain size, pin-hole formation).
Interface destabilization: Degradation of HTL (e.g., Spiro-OMeTAD) and ETL (e.g., TiO₂) materials under UV exposure, which increases charge carrier recombination and degrades current extraction.
The typical daily UV dose on the module surface in a temperate climate is ~5-10 kWh/m² (UVA) and 0.2-0.5 kWh/m² (UVB) (depending on location, season, and altitude), which annually translates to a cumulative dose of about 2-4 MWh/m² UVA and 70-180 kWh/m² UVB. Long-term exposure to these doses significantly accelerates the chemical and morphological degradation of the perovskite.
Operating Temperature: The front perovskite layer absorbs a significant portion of the solar spectrum, leading to higher temperatures. The cell temperature (NOCT - Nominal Operating Cell Temperature) of silicon modules is typically 45-50°C. For tandem modules, the FSP can operate at temperatures 5–15°C higher than the rear layer, reaching 60–75°C under sun exposure conditions (800 W/m² irradiance, 20°C ambient temperature, 1 m/s wind speed). 

According to the Arrhenius equation ($$k = A e^{{-E_a}/{RT}}$$), where $$k$$ is the reaction rate constant, $$A$$ is the pre-exponential factor, $$E_a$$ is the activation energy, $$R$$ is the gas constant, and $$T$$ is the absolute temperature, a 10°C increase in temperature can double the kinetics of degradation reactions (the "Q₁₀ rule"). Studies show that thermal degradation of perovskites is significant above 85°C, but even lower temperatures over the long term affect ion migration (e.g., iodides, metals) and decomposition (e.g., phase separation, FA/MA decomposition). Temperature test standards include thermal cycles (TC) according to IEC 61215 (e.g., 200 cycles from -40°C to 85°C).
Moisture and Oxygen: Despite encapsulation, FSP is closer to the module edge and potential moisture and oxygen diffusion paths. High humidity (RH > 60%) and oxygen presence (O₂ > 1 ppm) lead to perovskite hydration (e.g., formation of PbI₂·H₂O monohydrates or dihydrates), defect formation in the crystal lattice, and oxidation of charge transport layers (e.g., Spiro-OMeTAD oxidation). The IEC 61215 standard tests moisture resistance under damp-heat (DH) conditions (85°C/85% RH for 1000h), but for perovskites, even more rigorous testing in variable cycles (e.g., humidity/temperature) or extended tests (e.g., 2000h, 4000h) may be required to simulate a 25-year lifespan. The typical acceptable water vapor transmission rate (WVTR) for perovskite encapsulation is < 10⁻⁴ g/m²/day.
Estimated Lifespan (MTTF - Mean Time To Failure): Current prototypes of perovskite modules in 2T/4T configurations (with FSP) show stability at the level of 1–5 years in outdoor conditions before efficiency drops below 80% of the initial value (T80). This is the result of intensive laboratory research and initial field tests, often carried out under accelerated aging conditions (AST). The R&D goal is to achieve 10–20 years (T80/T90), which requires significant improvement in perovskite chemical composition (e.g., using mixed cations and anions, such as CsFAMA-PbIBrCl), cell architecture (e.g., passivation of surface defects), and encapsulating materials (e.g., using ultra-barrier diffusion barriers with water vapor permeability < 10⁻⁶ g/m²/day and oxygen permeability < 10⁻³ cm³/(m²·day)). O&M costs for shorter-lived modules are significantly higher, which affects the LCOE (Levelized Cost of Energy).
Rear-side Perovskite (RSP)
Exposure to UV Radiation: RSP is shielded by the bifacial silicon layer, which acts as an effective UV filter. Silicon absorbs almost all UV radiation with wavelengths below 400 nm (especially below the forbidden band of ~1.12 eV, i.e., approx. 1100 nm). As a result, RSP is exposed primarily to scattered and reflected light (albedo), which has a significantly reduced UV component. The typical reduction in UV radiation reaching RSP is over 90% compared to FSP, and often up to 99% for wavelengths < 380 nm. This light is also spectrally "softer," with dominant wavelengths in the infrared (IR) and visible (~400-1100 nm) ranges, which minimizes photo-induced degradation reactions specific to UV. Long-term stability tests (LTS) show a significantly slower rate of light-induced degradation for RSP.
Operating Temperature: RSP is in more favorable thermal conditions. It is shielded by silicon, which dissipates some heat, and is closer to the rear surface of the module, which promotes better convective cooling (through wind effect and reflection from the ground/surface). The typical temperature difference between FSP and RSP is 5–10°C (depending on housing materials and module architecture), meaning RSP operates at temperatures of 50–65°C under peak sun exposure conditions. Lower temperature significantly slows down thermal degradation mechanisms of perovskite, such as phase decomposition, migration of metal ions (e.g., iodides), or recrystallization. Accelerated thermal tests (HTOL - High Temperature Operating Life) confirm the increased stability of perovskites at lower temperatures. The temperature coefficient of power (Pmax) for perovskites typically ranges from -0.3%/°C to -0.45%/°C.
Moisture and Oxygen: RSP is doubly encapsulated – by both the rear glass and the silicon layer, as well as potting (materials such as EVA, POE, or thermoplastic polyurethanes - TPU). This multi-layered barrier significantly hinders the diffusion of moisture and oxygen. The use of thermosetting resins (e.g., POE) with low permeability (< 10⁻⁴ g/m²/day for H₂O) and an additional adhesive layer between silicon and perovskite further protects the RSP. Reduced access to these external factors drastically slows down the hydrolysis and oxidation processes of the perovskite. Additionally, in the event of glass breakage, the front layer is more exposed to moisture than the rear, which is important from a risk analysis perspective. DH (Damp-Heat) tests for 2000h (85°C/85% RH) are often used to assess long-term moisture resistance.
Estimated Lifespan (MTTF): Thanks to these favorable conditions, RSP, with appropriate hermetic barrier protection and composition optimization (e.g., using formamidinium-cesium based perovskites with stabilizing additives such as lithium iodides, or the addition of CsBr to improve phase stability), shows the potential to achieve long-term stability of 15–30 years in field conditions. Long-term tests in climate chambers (e.g., damp-heat 85°C/85% RH > 2000h, thermal cycles TC500, UV tests with extended durations) confirm the increased stability of RSP compared to FSP. The goal is to meet PV durability standards (e.g., IEC 61215:2016 - PV modules performance, IEC 61730:2016 - PV module safety), which anticipate an acceptable power degradation of no more than 20% after 25 years. This is crucial for LCOE reduction and the bankability of the technology.
RSP in a "Milder Environment"
The rear perovskite layer, shielded by the silicon layer (absorbing >90% of UV below 400 nm) and operating at a lower temperature (~5-10°C difference vs. FSP), works in a significantly less stressful environment. Reduced UV exposure and enhanced encapsulation (two barriers - rear glass and silicon/potting) minimize key degradation mechanisms, such as photo-oxidation, hydrolysis, and ion migration.
Key to 20-30 Years
It is precisely RSP that holds the strategic potential to achieve market-level durability for tandem modules (20-30 years, with degradation < 0.5% annually), bringing perovskites closer to silicon standards. While FSP still requires intensive research into stability (especially the development of ultra-stable perovskites and interface materials), RSP can be a realistic component of a long-term solution, contributing to LCOE reduction.
Synergy and Optimization
Integrating RSP with bifacial silicon allows leveraging the advantages of both technologies: high efficiency of perovskites (>25% lab. for single cells, >30% for tandems) and proven durability of silicon (25+ years warranty), while minimizing perovskite degradation risks through innovative module architecture. This arrangement optimizes both light conversion (efficient use of a broad spectrum) and degradation management, which is crucial for economically viable PV solutions.
1-5 Years
FSP Durability (T80)
Typical stability of current front-side perovskite prototypes (power degradation to 80% of initial value).
15-30 Years
RSP Durability (T80)
Potential durability of rear-side perovskite with optimization and in field conditions, meeting IEC standards.
>5x Longer
RSP vs FSP
Expected durability ratio of RSP to FSP in optimized tandem modules, resulting from better environmental conditions.
>90% UV Reduction
UV Protection
The silicon layer absorbs over 90% of harmful UV radiation, protecting RSP from photodegradation.
Durability values are estimates and depend on the technologies used (perovskite chemical composition, encapsulating materials, cell structure), environmental conditions (location, climate), and further progress in research and development (R&D). This data is crucial for economic analysis (LCOE) and PV project risk assessment.
Energy Scenarios: Engineering Analysis of 20-Year Operation of Tandem Modules
Detailed simulation of energy production changes for various perovskite-silicon tandem module architectures over a 20-year horizon, considering degradation mechanisms and their impact on long-term performance. The analysis is based on data from accelerated life tests (ALT) and long-term field tests, extrapolated for the full operational period.
The simulation data analysis clearly indicates that the rear-side perovskite (Si-RSP) configuration exhibits significantly slower power degradation, which translates into higher accumulated energy production over a long time horizon. After 20 years of operation, the Si-RSP module retains 92% of its initial power, while the FSP-Si drops to 68%. This difference means that in the Si-RSP scenario, accumulated energy (AE) production will be approximately 15-20% higher over 20 years, significantly reducing the levelized cost of energy (LCOE) for the end-user. The silicon sub-module acts as a "stable pillar," providing a minimum level of production and buffering perovskite instability, while simultaneously absorbing harmful UV radiation.
Configuration A: Front-side Perovskite (FSP-Si)
Degradation: The annual degradation rate (ADR) for perovskite cells in this configuration is high, typically 2-3% per year, and in some cases even up to 5% in the initial years, mainly due to direct exposure to UV radiation and higher operating temperatures. FSP is exposed to the full solar spectrum, including intense UV radiation (300-400 nm), which initiates photo-chemical degradation (e.g., decomposition of perovskite crystal structures, oxidation of HTL/ETL). FSP operating temperatures can be 5-15°C higher than in Configuration B, reaching 60-75°C under NOCT conditions, which, according to the Arrhenius equation, doubles the kinetics of degradation reactions for every 10°C increase. Additionally, higher temperature gradients can lead to mechanical stress and layer delamination.
Forecast: After 20 years of operation, the module retains approximately ~68% of its initial nominal power (Pmax). This means that a module with an initial power of 400 Wp will only generate 272 Wp after 20 years. This is consistent with typical expectations for single perovskite cells without advanced protective barriers, and such degradation is unacceptable for most PV projects.
Challenges: The need for developing innovative encapsulation materials and buffer layers (e.g., UV-blocking metal oxide layers or special EVA/PVB copolymers with low permeability to H₂O and O₂) with water vapor transmission rates below 10⁻⁵ g/m²/day. Additionally, research on chemical stabilization of perovskites (e.g., doping with Cs, Rb ions, application of two-dimensional perovskites) and meeting stringent durability standards for PV installations (e.g., IEC 61215:2021 (Photovoltaic modules - Design qualification and type approval), including thermal cycling (TC200), damp-heat (DH1000), and UV tests (UV100)) are required. Risk analysis indicates a higher probability of single cell failures, affecting string reliability.
Configuration B: Rear-side Perovskite (Si-RSP)
Degradation: By placing the perovskite behind a stable silicon sub-module, the perovskite layer is protected from direct UV radiation because silicon absorbs over 90% of radiation below 400 nm. This significantly reduces photochemical degradation. Additionally, the rear side of the module operates at lower temperatures (5-10°C lower than FSP, typically 50-65°C under peak conditions), which slows down the kinetics of thermal ionic degradation. The estimated ADR for the perovskite layer in this configuration is 0.2-0.5% per year, while the silicon sub-module shows standard degradation of 0.3-0.5% per year (PID, LID). The effective combined ADR for the entire tandem module is approximately 0.4-0.6% per year.
Forecast: The combination of stable silicon and protected perovskite allows for maintaining approximately ~92% of the initial power after 20 years, approaching the performance of conventional silicon modules (typically 85-88% after 25 years). For example, a 400 Wp module will still generate 368 Wp after 20 years, ensuring a higher return on investment. This configuration minimizes the risk of early failures and allows for meeting warranty requirements.
Advantages: This configuration offers significantly more stable and predictable energy production over the long term. Minimization of PID (Potential Induced Degradation) and LID (Light Induced Degradation) effects is easier due to the stable silicon platform and perovskite protection. Accelerated aging tests (e.g., Damp Heat Test: 85°C/85% RH, 1000h according to IEC 61215 Ed.3 and Thermal Cycling: -40°C to +85°C, 200 cycles) show significantly better stability in this architecture. Furthermore, Si-RSP technology can utilize existing production lines for silicon modules, reducing initial CAPEX costs and accelerating commercialization. LCOE for Si-RSP can be 5-10% lower compared to standard silicon modules due to higher efficiency and better stability.
Engineering Implications and Perspectives
Formula for Output Power (approximate):
P_{tandem}(t) = P_{Si}(t) + P_{pero}(t)Where: 
P_{Si}(t) = P_{Si,0} \times (1 - ADR_{Si})^t (Silicon power over time t)
P_{pero}(t) = P_{pero,0} \times (1 - ADR_{pero})^t (Perovskite power over time t)
P_{Si,0} and P_{pero,0} are the initial powers of the silicon and perovskite layers.
In the case of Configuration A (Front-side Perovskite - FSP-Si), ADR_{pero} is dominant and can be 2-3% (or even more in harsh conditions), while ADR_{Si} is typically 0.3-0.5%. The total degradation is strongly dominated by the perovskite layer.
For Configuration B (Rear-side Perovskite - Si-RSP), ADR_{pero} is significantly lower (0.2-0.5%) due to silicon protection and optimal temperature conditions, approaching ADR_{Si}. The total module degradation in this case is the sum of the degradation of both sub-cells, but dominated by the more stable layer, leading to significantly better long-term performance.
Calculation Example: 400 Wp module (300Wp Si + 100Wp Pero).
FSP-Si (ADR_pero=2.5%):
P(20 years) = 300*(1-0.004)^20 + 100*(1-0.025)^20 = 300*0.923 + 100*0.603 = 276.9 + 60.3 = 337.2 Wp (~84% of initial power).
Si-RSP (ADR_pero=0.4%):
P(20 years) = 300*(1-0.004)^20 + 100*(1-0.004)^20 = 300*0.923 + 100*0.923 = 276.9 + 92.3 = 369.2 Wp (~92% of initial power).
This analysis highlights the critical importance of optimizing the ADR of the perovskite layer for the long-term stability of the entire tandem module.
Case Study: Optimizing Energy Balance and LCOE
In a research project conducted at Delft University (published in Nature Energy, 2022), it was shown that tandem modules with rear-side perovskite (Si-RSP) achieved a bifaciality factor of 90-95% for the silicon sub-module (measured according to IEC 60904-1-2) and stable additional production from the perovskite, even under low light conditions (<200 W/m²), thanks to its wider light absorption range (especially in the blue-green and near-infrared range, where silicon has lower absorption). In simulations for a Central European climate, Si-RSP modules with an initial efficiency of 28% showed an LCOE (Levelized Cost of Energy) of ~30-40 USD/MWh, which is 10-15% lower than for reference silicon modules with an efficiency of 23% (LCOE ~45-55 USD/MWh).
Implementation Guidelines for PV Projects:
Encapsulation: Use double protective layers (e.g., laminates with EVOH or TPE) for the perovskite layer, even if it is on the rear side. Oxygen permeability should be below 0.1 cm³/(m²·day·atm), and moisture below 10⁻⁵ g/(m²·day) according to ultra-barrier standards. Monitor the water vapor transmission rate (WVTR).
Surface Optimization: Ensure that the contact surface of the perovskite with the rear cover (e.g., 2-4 mm tempered glass or PVF/PVDF backsheet film) is minimal to prevent moisture migration along the edges. Use special silicone seals with a low moisture diffusion coefficient.
Bifacial Modules: Utilizing a bifacial design (e.g., with a transparent backsheet or glass-glass configuration) significantly increases the total energy production of the module (bifacial gain of 5-25% depending on terrain albedo) and improves heat dissipation, which promotes RSP stability. Mounting on elevated structures (min. 0.5m above ground) should be considered to maximize the albedo effect.
Degradation Monitoring: Implement module-level power electronics (MLPE) systems to detect performance anomalies early and respond quickly to potential degradation.
O&M Costs: LCOE analysis should include anticipated operation and maintenance (O&M) costs, which for Si-RSP modules may be lower due to their greater stability and reduced replacement requirements.
Risk Analysis and PV Structure Engineering in Hurricane Zones: Resilience Above All
In the face of increasing intensity and frequency of extreme weather events, especially Category 3, 4, and 5 hurricanes, the design and construction of large-scale photovoltaic installations in high-risk zones require a fundamental paradigm shift. The conventional approach, optimizing costs in temperate regions, becomes insufficient. It is crucial to radically depart from economic assumptions in favor of prioritizing durability, resilience, and failure-free operation, while minimizing the AAL (Average Annualized Loss) indicator and maximizing Uptime.
Understanding the complex dynamics of wind interaction with PV structures, including phenomena such as turbulence, the Venturi effect, and aerodynamic resonance, is an absolute priority. Design must go beyond simple static load tables, relying on advanced CFD (Computational Fluid Dynamics) simulations and rigorous engineering standards. The standard ASCE 7-16 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures), along with its latest edition ASCE 7-22, forms the basis for wind load calculations, precisely accounting for hurricane exposure zones, terrain categories (from urban exposure B to open sea terrain D), and risk category (e.g., Category IV for critical energy infrastructure).
What is Most Frequently Damaged and Why: Critical Aerodynamic Analyses
Wide, low, multi-row structures (e.g., 25-30° tilt angle, height <1m from the ground, especially in 2P or 4P horizontal configuration) create a "gate" or "sail effect." Instead of flowing freely, wind is compressed under and over the panels. As a result, enormous uplift and bending forces are generated, many times exceeding permissible values for typical, economical systems.
Key Damage Mechanisms:
Uplift Pressure: The most dangerous is the negative pressure under the panel, resulting from the Venturi effect, where accelerated airflow generates suction. It can reach values of -4.0 kPa to -5.5 kPa at a wind speed of 200 km/h (55.6 m/s), which translates to an uplift force of 400-550 kg/m². Standard PV panels are certified for uplift loads of approximately 2400 Pa (2.4 kPa).
Turbulence and Edge Lifting: At the edges and corners of modules, abrupt pressure changes occur, leading to local stress concentrations and potential module detachment or structural deformation. The pressure coefficient (Cp) for these areas can reach values of -3.5 to -4.0.
Aerodynamic Resonance: Open profiles (e.g., C-channels, I-beams) and insufficient structural stiffness are susceptible to aerodynamic resonance ("vortex shedding," "flutter"), where the natural frequency of the structure coincides with the wind forcing frequency, leading to a drastic increase in vibration amplitudes and fatigue failures.
Shading and Bifacial Effect: Dense open-profile trusses in racks, in addition to wind exposure, generate shading on the backside of the module, lowering its bifacial efficiency by as much as 5-10%, which impacts the LCOE (Levelized Cost of Energy).
Underestimation of Loads: A common error is underestimating the basic pressure (q_z) and topographical factors (K_zt) or exposure factors (K_e) according to ASCE 7-16/22, leading to drastically undersized structures.
Lack of comprehensive aerodynamic optimization, combined with insufficient material safety factors (e.g., S235 vs S355 steel), is the main cause of catastrophic failures. Typical wind force calculations according to ASCE 7-16 are based on the formula: P = q * G * C_p, where q = 0.613 * K_z * K_zt * K_d * V^2 (q - basic pressure, G - gust factor, C_p - pressure coefficient, K_z - exposure coefficient, K_zt - topographic factor, K_d - wind directionality factor, V - wind speed in m/s). Improper selection of these coefficients is a critical risk.
Features of Hurricane-Resistant Structures: Preventive Engineering
Single-row, elevated, narrow tables (angle approx. 10-15°, typically 1P or 2P vertical arrangement) are key to survival. Such solutions minimize C_p (pressure) and C_n (net) coefficients through optimal airflow, reducing load forces.
Design Recommendations:
Structure Elevation: The minimum height of the module's bottom edge from the ground should be >1.5 m (5 feet), optimally 2.0-2.5 m. This allows for free wind flow under the panels (reverse Venturi effect), reducing uplift and load forces by as much as 40-50% compared to low structures. Reduction of uplift pressure from -4.0 kPa to -2.0 kPa per square meter of module surface.
Narrow Configuration: 1 module in vertical (1P) or 2 modules in vertical (2P) arrangements are preferred for trackers, which minimizes the frontal area exposed to direct wind pressure.
Closed Profiles: It is mandatory to use closed profiles (e.g., square, rectangular, round tubes) with thick walls (min. 3.0 mm for profiles with diameter/side <100mm, min. 4.0 mm for larger cross-sections). Closed profiles are characterized by significantly greater torsional stiffness and resistance to buckling, as well as lower aerodynamic drag coefficients (Cd) compared to open channels.
Minimum Number of Elements: Reducing the number of crossbeams, welds, and connections reduces "wind volume" and stress concentration points, which are susceptible to material fatigue.
Dynamic Analysis: Structures must be designed for dynamic loads (instead of just static), taking into account natural vibration analysis (e.g., using F.E.M. - Finite Element Method) and Fluid-Structure Interaction (FSI).
Recommended Materials and Coatings:
Steel: High-strength structural steel S355JR or S450J2 (according to PN-EN 10025-2) with a higher yield strength (S355JR: 355 MPa vs S235: 235 MPa) and tensile strength.
Galvanization: Hot-dip galvanizing according to PN-EN ISO 1461 (minimum coating thickness 85 µm, providing corrosion protection for over 50 years in an aggressive marine environment, corrosivity category C5-M according to PN-EN ISO 9223).
Bolted Connections: Bolts of class 8.8 or 10.9 (according to PN-EN ISO 898-1), with appropriate tightening torque (e.g., for M16, 8.8 bolt tightening torque 196 Nm) and anti-loosening protection (spring washers, self-locking nuts).
Detailed Structural Requirements for Hurricane Zones: Engineering Specification
Wind Loads: Design wind speeds for hurricane regions (e.g., Puerto Rico, Florida) must be 200-220 km/h (125-135 mph, 55.6-61.1 m/s), measured as a 3-second gust at 10 m height, with terrain category D (marine exposure, most unfavorable). The risk factor (Iw) for critical facilities (risk category IV according to ASCE 7-16/22, e.g., power plants, hospitals) is 1.15. This means that the designed wind speed is increased by 15% to achieve a higher level of safety.
Photovoltaic Modules:
Glass-Glass modules (double-sided glass) are required, with increased resistance to microcracks and mechanical loads. Certified for static loads of min. 5400 Pa (pressure) and 2400 Pa (suction), with additional certification for dynamic wind loads (e.g., IEC 61215-2:2021 MTC).
Hail resistance (limit class 4) – 50 mm diameter hail at a speed of 34 m/s.
Increased resistance to PID (Potential Induced Degradation) and LID (Light Induced Degradation) confirmed by tests (e.g., IEC 62804 for PID: 96h, 85°C/85% RH; IEC 61215 for LID).
Clamps and Module Mounting:
It is recommended to use clamps with a contact width of 60-75 mm (standard 40 mm), fastened to reinforced points of the module frame.
Clamping-free module mounting directly to load-bearing profiles using dedicated retention systems or integrated fastening methods (e.g., rail systems with hidden clamps) is preferred, as this eliminates stress concentration points caused by standard clamps.
Foundations:
Must be designed according to ASCE 7-16/22 and ACI 318 (Building Code Requirements for Structural Concrete), with a safety factor of min. 60% for uplift forces and bending moments.
The type of foundation (driven steel/concrete piles, block foundations, micropiles) is selected based on detailed geotechnical investigations (e.g., CPT/SPT sounding, borings), determining soil bearing capacity and groundwater level. The minimum pile depth should be determined based on calculations of uplift and lateral forces.
Dynamic wind loads must be absolutely included in the fatigue analysis of the foundation and its connection to the supporting structure.
"Hurricane-Proof" Specification - An Investment in Safety, Operational Continuity, and ROI
Puerto Rico, Florida, the Caribbean, the Gulf Coast, and other hurricane-prone regions cannot afford cyclical rebuilding of energy infrastructure after every hurricane season. This is unacceptable from both an economic (CAPEX/OPEX) and social (continuity of energy supply) perspective. Investments in passive safety and infrastructure resilience at the design stage are absolutely critical, serving as protection against operational (e.g., lost energy production) and reputational losses.
Implementing solutions such as: single-row, elevated structures (1P or 2P vertically) + closed structural profiles (min. 3mm walls) + clamping-free Glass-Glass module mounting ensures nearly 100% survivability of installations in hurricane conditions (Category 4-5, wind speeds up to 250 km/h). Data from real projects after Hurricanes Maria (2017) and Ian (2022) in Florida and the Caribbean unequivocally confirm the superiority of such solutions.
Cost analyses (LCOE - Levelized Cost of Energy) show that the total capital expenditure (CAPEX) for such reinforced systems is only 3-5% higher compared to standard installations, which includes more expensive materials (S355/S450 steel, thicker profiles, better galvanizing, 10.9 bolts) and a more complex engineering design. This small CAPEX difference is more than offset by:
No Repair Costs (OPEX): Avoiding multi-million dollar costs for damage removal and replacement of damaged components.
Preservation of Energy Supply Continuity: Crucial for the stability of regional energy grids and minimizing power outages.
Avoidance of Energy Production Losses: Damaged installations do not produce energy, generating revenue losses and delays in achieving climate goals.
Longer Project Life Cycle: Increased resilience extends the operational life of installations beyond the typical 25 years, lowering LCOE.
This is a strategic investment in resilience that minimizes business and social risk, while ensuring long-term profitability and operational safety. Insurance costs for such installations are significantly lower.
Comparison of Aerodynamic Forces for Different Structural Configurations in Hurricane Zones
The chart below presents simulated values of horizontal forces (Fx) and bending moments (Mz, Mz - moment rotating the foundation around the Z-axis) acting on different types of photovoltaic structures at the same wind speed (e.g., 180 km/h, 3s gust at 10m height, terrain category C). These data illustrate how critical it is to minimize the wind-exposed area and optimize aerodynamics to drastically reduce loads and increase structural safety. All values are averaged for a single module in the array.
Detailed Explanation of Data and Engineering Implications:
Single-row Elevated 15° (1P Vertical, H > 2.0m):
Characteristics: Horizontal force 8.5 kN, Bending moment 12.8 kN·m. Characterized by significantly smaller horizontal forces and bending moments. Thanks to significant elevation (e.g., to 2 m) and a steep tilt angle, wind flows more freely, reducing loads. These values are close to optimal for hurricane zones, often achieved through CFD optimization of pole and arm shape.
Load Reduction: Estimated reduction of aerodynamic forces by 40-55% compared to multi-row configurations, allowing for the use of lighter, yet durable profiles and smaller foundations, while maintaining a high safety margin.
Example Construction: Square section poles 100x100x4 mm or round tubes Ø114x4.5 mm.
Multi-row Wide 25° (2P Horizontal, H < 1.0m):
Characteristics: Horizontal force 16.2 kN, Bending moment 24.7 kN·m. Shows almost double the forces and moments compared to the single-row construction. This is due to the "sail effect" (high Cp) and wind compression under the panels, which drastically increases the risk of failure by exceeding material strength or buckling of structural elements.
Risk: Structures of this type are highly susceptible to damage in hurricane conditions, generating enormous repair costs and production losses.
Tracker Stow 15° (1P/2P Horizontal, in emergency position 15°):
Characteristics: Horizontal force 14.8 kN, Bending moment 22.1 kN·m. Even in "stow" position (horizontal or near-horizontal 10-15° to minimize frontal area), trackers generate significant loads, though less than multi-row fixed structures.
Challenges: These systems require very advanced safety algorithms (e.g., quick transition to stow position, redundant wind sensors) and robust motor and bearing construction to withstand dynamic hurricane loads. In extreme zones, their use is risky unless significant oversizing and redundancy of drives are applied, which significantly increases CAPEX and OPEX.
Engineering Conclusions: These data clearly indicate that the choice of appropriate geometry, structure type, and material specification is decisive for the safety and long-term reliability of installations in regions exposed to extreme wind loads. Investment in advanced engineering design and top-quality components, although increasing initial CAPEX, significantly reduces LCOE and minimizes risk of losses over a 25-year operating period.
The "Gigawatt and Forget" Philosophy: Engineering Fundamentals of Utility-Scale Reliability
The true value of photovoltaics, especially at utility-scale, lies in its maximum trouble-free operation and predictability of energy production. The "gigawatt and forget" concept emphasizes the engineering pursuit of minimizing service interventions and maximizing reliability throughout the entire life cycle of a PV power plant. Achieving this state requires a rigorous approach to design, precise component selection, meticulous installation, and advanced monitoring and maintenance strategies, based on in-depth data analysis and technical standards.
Rigorous Inspections and Preventive Maintenance
Standard operating protocols assume detailed mechanical and electrical control, minimizing the risk of costly failures. The frequency of inspections, typically once every 5 years for general control, can be adjusted based on risk analysis, remote monitoring data, and after extreme weather events. Implementation requires adherence to international standards and directives.
Verification of bolt tightening torques: A key element ensuring structural integrity. Random verification of bolt tightening torques (recommended sample of 1-2% of critical connections, i.e., module frame to structure connections, column-girder connections, connections susceptible to vibration or dynamic loads) should be carried out in accordance with the construction manufacturer's guidelines and standards, such as PN-EN 1993 (Eurocode 3) for steel structures and PN-EN 1999 (Eurocode 9) for aluminum structures. Torque wrenches with current calibration (PN-EN ISO 6789, at least once a year) should be used. Tightening torque values are a function of bolt diameter, strength class (e.g., 8.8, 10.9 for steel), and friction coefficient; for example, for an M10 8.8 class bolt, the typical torque is 40-50 Nm, and for an M12 8.8 class bolt, it's approximately 70-80 Nm.
Plumb and leveling of posts: Control of deviations from verticality and level using laser levels (precision +/- 1 mm per 10 m) or RTK GPS systems. The maximum permissible tolerance is +/- 5 mm per meter of column height, in accordance with PN-EN 1090-2 (Execution of steel structures). Deviations can lead to uneven load distribution, increased stress in structural elements and module frames, as well as unforeseen shading or "self-shading" effects between modules.
Evaluation of coating corrosion: Visual inspection and measurement of anti-corrosion coating thickness (e.g., hot-dip galvanizing according to PN-EN ISO 1461, Duplex coatings). The classification of the corrosion degree should be carried out according to PN-EN ISO 4628 (Assessment of degradation of paint coatings), with particular emphasis on the division into corrosive environment categories (C1-C5 according to PN-EN ISO 12944). Early detection of crevice or galvanic corrosion (at contact points of different metals) is critical. In case of minor damages (up to 5% of the surface), application of zinc paints (according to PN-EN ISO 3549) or epoxy-polyurethane systems is recommended.
EL sampling (Electroluminescence): Conduct EL testing on a random sample of 0.5-1% of modules or a minimum of 20 modules after 2-3 years of operation or after intense wind seasons/snowfalls. This test, performed in darkness (or low irradiance with appropriate equipment) by powering the module with current in the forward direction, allows detection of microcracks (e.g., "snail trails"), delaminations, cell damage, faulty bypass diodes, or degradation of interconnect connections invisible to the naked eye. Compliance with IEC 61215-2 (Photovoltaic modules) and IEC 60904 (Photovoltaic devices) is standard. The power supply current should correspond to the module's maximum power current (I_mpp).
Advanced Remote Monitoring and Data Analysis
Continuous monitoring at the string or even module level (with optimizers/microinverters) is the foundation of proactive PV plant management, enabling rapid anomaly detection and performance optimization.
String-level monitoring: Comparison of power and current generated by individual strings in real-time. A string productivity deviation of >3-5% from the reference value (median or average of strings with similar exposure and insolation conditions) should trigger an automatic alarm in the SCADA/CMMS system. This analysis helps detect problems such as module damage, shading, cable failures, or issues with bypass diodes. For series-connected strings, a current drop in one module lowers the current of the entire string.
Degradation and mismatch detection algorithms: Utilization of advanced AI/ML algorithms to analyze long-term energy production trends. Modeling of expected production based on meteorological data (irradiance [W/m²], module temperature [°C], wind speed [m/s], albedo [%]) and technical system parameters (inverter efficiency, module temperature coefficient Pmp [%/°C]). Comparison with actual data allows for the identification of anomalies indicating degradation (e.g., LID – Light Induced Degradation, LeTID – Light and elevated Temperature Induced Degradation), hot-spots, shadows, bypass diode failures, problems with MPPT (Maximum Power Point Tracking) optimization. The typical module degradation rate is 0.5% annually.
PID (Potential Induced Degradation) detection: Analysis of long-term performance drops, especially in areas with high humidity and temperature (e.g., 85°C and 85% humidity in IEC 62804 tests). PID is caused by the migration of sodium ions from the module glass to the cell layer under high voltage (especially negative relative to ground), leading to a drop in the IV curve fill factor and efficiency reduction. Implementing negative voltage monitoring on modules helps minimize the risk of PID. Effective PID protection includes grounding the negative pole, using transformerless inverters with galvanic isolation, or dedicated anti-PID devices.
Profile alarms and system integration: Configurable alarm thresholds for various parameters (power, current, voltage, cell temperature, inverter temperature, insulation resistance), allowing for quick response. Integration with SCADA (Supervisory Control and Data Acquisition) systems for visualization and control, and CMMS (Computerized Maintenance Management System) for managing service orders and maintenance history.
Thermography and Visual Inspection with Drones
Unmanned aerial vehicles (drones) equipped with thermal and high-resolution cameras significantly increase the efficiency and safety of problem identification on large photovoltaic farms, reducing inspection time and costs.
Autonomous flight 1x/year: Routine, planned drone flight (e.g., DJI Matrice 300 RTK with H20T or FLIR XT2 camera) to detect hot-spots, damaged cells, bypass diode issues, damaged connectors, shading or soiling. The flight should be conducted at a minimum irradiance of 600 W/m² (STC is 1000 W/m²) and low wind (up to 5 m/s), preferably on a clear day, to avoid thermal interference. The optimal inspection angle is 45-60 degrees from vertical.
Hot-spot and connection reporting: Automatic analysis of thermal images with identification and categorization of anomalies (e.g., critical hot-spots >20°C above module ambient temperature, moderate >10°C). Thermal analysis software (e.g., using AI) generates damage maps with precise module locations and GPS coordinates, facilitating repairs. Hot-spots indicate localized overheating of the cell, which can lead to further degradation and even fire.
Post-storm inspections: Rapid drone flyover after intense windstorms, hailstorms (e.g., hail diameter >2 cm, fall velocity >20 m/s), earthquakes, or other extreme weather events. Utilization of AI for differential analysis (AI-diff) of images from the current flyover compared to reference photos (taken after installation or from a previous flyover) for rapid detection of mechanical damage (e.g., cracked glass, structural deformations, broken cables, module displacement). AI analysis precision can reach 95-98%.
Visual inspection: Additionally, drones can be equipped with high-resolution cameras (e.g., 4K, 20 MPix) to assess the condition of module surfaces (soiling, scratches, bird droppings), wiring (insulation integrity, proper cable routing, fasteners), and structural elements (bolts, welds, foundations). These inspections complement thermographic data, providing a full picture of the farm's technical condition.
Operational Efficiency at Gigawatt Scale (OPEX Analysis)
By applying the above technologies and a properly designed installation, it is possible to significantly reduce long-term operating costs (OPEX), which directly translates into the project's LCOE (Levelized Cost of Energy).
Fixed-tilt structures (single-row constructions): Optimal management of a 1 MW farm requires an average of ~0.3–0.6 Full-Time Equivalent (FTE) annually for operation and maintenance (O&M). These costs include regular inspections, module cleaning (especially in dry climates), grass cutting, minor repairs, and monitoring system management.
Tracker systems (tracking arrays): Due to more complex mechanics, a greater number of moving parts (motors, gears, bearings), and the need for calibration and more frequent diagnostics, tracker systems require an average of 1.0–1.5 FTE per 1 MW annually. Tracker service costs can account for 20-40% of total OPEX for a farm with trackers. Additionally, tracker failures can lead to significant energy production losses (e.g., 5-10% annually for the entire block in case of one drive failure).
OPEX and LCOE difference: For a 100 MW farm, using fixed-tilt structures instead of trackers can mean a difference of around ~70–90 FTE in annual staffing needs, which translates into savings of millions of zlotys/euros annually in current operating and maintenance costs. This difference in OPEX, combined with lower CAPEX (capital expenditure) for fixed-tilt structures, significantly reduces the LCOE, making the project more competitive in the long term.
The "gigawatt and forget" philosophy is not just a phrase, but an engineering pursuit to minimize the risk of failure and maximize long-term reliability through intelligent solutions and a rigorous approach to quality. Peace of mind, in the context of multi-million dollar investments and energy supply stability, is indeed priceless.
4T vs 2T Architecture in the context of rear gain and long-term LCOE optimization
The choice of perovskite-silicon tandem architecture is crucial for maximizing the potential of energy generation from the rear side (rear gain), as well as for long-term stability, module performance predictability, and ultimately its LCOE (Levelized Cost of Energy). Below is a detailed analysis of both architectural types in a technical and engineering context, considering critical parameters for the PV installation designer.
2-terminal (monolithic) – Series-Connected Architecture
In 2T (Two-Terminal) architecture, the perovskite layer (top cell) and silicon layer (bottom cell) are internally (monolithically) connected in series into a single photovoltaic unit, having only two output terminals. This means that the current generated by the entire tandem is limited by the cell generating the lowest current (the "current bottleneck" or "limiting current"), which is typically the perovskite cell due to its wider bandgap (typically 1.6-1.8 eV) and tendency for faster degradation than traditional crystalline silicon (band gap 1.12 eV).
Operating principle and electro-optics: A 2T module operates like a single solar cell with a summed voltage (V_tandem = V_perovskite + V_silicon), but the current is limited by the cell with lower current (I_tandem = min(I_perovskite, I_silicon)). Typically, perovskite is the top cell, absorbing UV (280-400 nm) and visible (400-750 nm) radiation, while infrared light (NIR, >750 nm) passes through to the silicon cell. Optimal operation requires precise current matching (current matching) of both sub-cells, such that I_perovskite ≈ I_silicon. In practice, due to different spectral, temperature, and degradation characteristics, achieving ideal matching over time is challenging. Improper current matching can lead to power losses of 5-15% from the start.
Rear gain in 2T architecture: Additional scattered illumination from the module's rear side (rear gain) primarily increases current generation in the silicon cell. However, if the perovskite cell is the current-limiting cell (which is typical for many 2T tandems), the increase in current in the silicon cell due to rear gain does not translate into a proportional increase in the module's total power. In extreme cases, when the perovskite cell degrades (e.g., Isc drops by 10% after 5 years), its reduced efficiency becomes a "bottleneck" for the entire module, regardless of how much current the silicon cell can generate from the rear side. The real, useful long-term rear gain may be significantly lower than the potential gain of the silicon cell.
Engineering consequences and long-term degradation:
Isc/Voc Degradation: Faster degradation of the perovskite cell (typically 0.5-1.0% annually for Pmax, with a dominant Isc drop) leads to discrepancies in current matching. Standard IEC 61215-1:2021 (Module performance testing) and IEC 61853 (Power rating measurements) require stability tests, but often do not account for the specific degradation of 2T tandems. Laboratory tests (e.g., humidity and heat cycling, UV tests) show that 2T modules are more susceptible to rapid performance degradation in field conditions.
Production Optimization: 2T modules are simpler to manufacture and integrate (fewer cells, simpler wiring), but require more precise spectral and current matching of layers. Maintaining this matching throughout the operational period is difficult due to differences in P_max(t) = P_max(0) * (1 - DR*t), where DR (Degradation Rate) for perovskite (0.5-1.0%/year) is higher than for silicon (0.2-0.3%/year).
Shading Sensitivity: Partial shading of one side (e.g., the perovskite cell on the front, which is more sensitive to shading due to a smaller bandgap and more complex structure) can drastically reduce the efficiency of the entire cell due to hotspot effects and current mismatch.
MPP Tracking Losses: The complex I-V characteristic of the 2T tandem can lead to unstable MPP tracking by the inverter, especially in varying illumination conditions, resulting in additional energy losses.
Implications for mounting structure: A "rear-friendly design" (single-row mounting, minimum 1m elevation, zero shading from structure and wiring) is still needed to maximize the rear gain potential for the silicon cell. However, its benefits are partially masked by the fundamental limitations of current matching between series-connected layers, which lowers the effective LCOE in the long term.
4-terminal (independent) – Independently Controlled Architecture
In 4T (Four-Terminal) architecture, the perovskite cell (top) and silicon cell (bottom) operate completely independently, having separate electrical contacts. A 4T module consists, in fact, of two separate sub-modules in one housing ("stacked cells"), each optimized for its own spectrum range and electrical parameters.
Operating principle and electro-optics: Each sub-cell has its own terminals (4 outputs), allowing for independent Maximum Power Point Tracking (MPPT) for each cell. 4T modules require more complex wiring and power electronics (e.g., two independent MPPTs in the inverter or the use of dedicated power optimizers at the module/string level), but offer significantly greater flexibility and resistance to asymmetric degradation and shading phenomena. The perovskite cell converts visible and UV light, and the silicon cell converts infrared light that passes through the perovskite, as well as light reflected from the ground (rear gain).
Rear gain in 4T architecture: This is a key advantage of 4T architecture. The rear part of the module (silicon cell) can utilize reflected illumination from the ground to its full extent, regardless of the state of the perovskite cell. The total module power is the sum of the power of both cells (P_total = P_perovskite + P_silicon). If the perovskite cell partially degrades, the silicon cell will continue to operate at full efficiency, generating a significant portion of the total power. This allows for a shift in the energy production balance towards the module's rear side over time (a "rear-heavy balance"), extending the economic life cycle of the installation.
Engineering consequences and long-term stability:
Degradation Resistance: Independent operation means that perovskite degradation (e.g., Isc/Voc drops over years, typically 0.5-0.8% annually) does not directly limit the efficiency of the silicon cell (degradation 0.2-0.3% annually). The silicon cell (compliant with IEC 61215-2:2021) can become the primary power source in the long term. For example, after 20 years, the perovskite cell may have 10-16% lower efficiency than its initial state, but the silicon cell will retain >90% of its initial efficiency, ensuring a stable LCOE. Simulations (e.g., using PVSyst software) for 4T installations show an average of 3-7% higher annual energy yield compared to 2T in dynamic conditions.
MPPT Flexibility: The ability to use two independent MPPTs maximizes energy harvesting from both sub-cells, even in changing illumination and temperature conditions, and with varying degradation (e.g., a hybrid inverter with two MPPT inputs for PV, where one input is dedicated to perovskite and the other to silicon). This minimizes losses due to mismatch.
Higher Initial Costs (BOS): More complex construction, wiring (additional pairs of DC cables per module), and power electronics can increase the initial module and system (BOS - Balance of System) cost by 5-15%. However, a higher LCOE in the long run usually offsets these costs. This requires a detailed economic analysis at the design stage.
No Hot-spots: Independent circuits minimize the risk of hotspot formation, which can occur in 2T in case of non-uniform illumination.
Implications for mounting structure: Every shade, cable, or crossbar in the module's rear area results in a real, direct loss of megawatt-hours (MWh), as there is no "current blockage" from the perovskite cell here. A "rear-friendly" design (optimizing rear gain through single-axis tracking, high elevation of 1.0-1.5m, zero tolerance for shading from clamps, cables, and structural elements - profiles with minimum width <30mm, clamp-free systems or clamps minimizing shadow) is a necessary condition to fully leverage the 4T potential, especially in the context of long-term production balance and LCOE minimization. CFD (Computational Fluid Dynamics) analysis for airflow under the module can further optimize cooling and yield.
10-15%
2T Rear Gain (Effective)
Initial effective rear gain ranges from 10-15%. However, it is limited by current matching and faster perovskite cell degradation. Typically, effective rear gain drops from 15% initially to 5-8% after 20 years, due to current limitation and bottlenecking. LCOE losses can be up to 5% compared to 4T.
20-30%
4T Rear Gain (Potential)
Full utilization of independent energy generation paths. The potential rear gain in 4T architecture can range from 20% to over 30% (in high albedo conditions >0.5, e.g., white gravel, and optimal height >1.2m), maintaining high value throughout the operating period. Simulated additional energy yield: 50-150 kWh/kWp/year depending on location.
30-50%
Rear share after 20 years
In 4T architecture with a "rear-friendly" design (e.g., BifacialMAX system), the share of energy from the module's rear side can increase to 30-50% of total production after 20 years, effectively compensating for front-side degradation. In some bifaciality scenarios (e.g., white gravel, height >1.5m), the rear side can become the dominant energy source (over 50%), which significantly lowers LCOE (e.g., by 8-15%) and increases ROI of the installation. O&M costs for 4T are comparable, but production is higher.
Engineering Conclusion: The 4T architecture is a "future-proof" option in the context of progressive perovskite cell degradation. Thanks to the independent operation of sub-cells, it is possible to achieve a "rear-heavy balance," where production from the rear side (stable silicon cell) takes on an increasingly larger role. The BifacialMAX structure (single-row mounting, minimum 1.0-1.5m elevation, zero tolerance for shading from clamps, cables, and structural elements, optimized spacing between module rows, ground albedo >0.3) is designed to fully exploit this potential, ensuring maximum LCOE (Levelized Cost of Electricity) throughout the installation's life cycle. Choosing 4T optimized for bifaciality is an investment in long-term profitability and energy production stability. Conducting a risk analysis (FMEA) in the context of degradation and shading is crucial at the design stage.
6. Degradation Models and Literature References
The stability of photovoltaic perovskites largely depends on exposure conditions. Key factors are UV radiation, oxygen, and moisture access.
The front layer of the module receives the full AM1.5G spectrum, including ultraviolet radiation (UV, 300–400 nm), which initiates photo-oxidation processes and ion migration [NREL, 2021; Oxford PV, 2022].
In contrast, the rear layer in bifacial or tandem modules is protected by silicon and additional glass, which significantly reduces UV exposure and oxygen and moisture access [Fraunhofer ISE, 2020].
Exponential Degradation Model
The degradation of power conversion efficiency (PCE) can be described by a simple exponential model:
PCE(t) = PCE_0 \cdot e^{-\lambda t}where:
PCE(t) – power conversion efficiency after time t,
PCE_0 – initial power conversion efficiency,
\lambda – degradation coefficient [1/year].
Front Layer
Typical degradation coefficient \lambda \approx 0.02 (approx. 2%/year).
After 30 years, the relative efficiency of the front drops to approximately 1–2% of the initial value.
Rear Layer
Typical degradation coefficient \lambda \approx 0.003 (0.3%/year).
After 30 years, the rear layer's efficiency retains over 20% of its initial value [Science, 2021].
Temperature-Activated Degradation Model
Perovskite degradation is temperature-activated and can be additionally modeled using the Arrhenius equation:
k = A \cdot \exp\left(-\frac{E_a}{k_B T}\right)where:
k – degradation rate,
A – pre-exponential factor,
E_a – activation energy (typically 0.5–0.7 eV for halide perovskites),
k_B – Boltzmann constant,
T – temperature [K].
Accelerated tests (85 °C, 85% RH, 1000 h UV) show that the front layer loses 15–25% efficiency, while the rear layer under the same conditions exhibits losses below 5% [Solar Energy Mater. Sol. Cells, 2022].
This confirms the crucial role of construction optics – rear gain in bifacial systems not only increases total energy production but also shifts the yield burden to layers less susceptible to degradation.
Implications for PV System Design
From an engineering perspective, rear gain in tandem modules should be treated as a fundamental component of bankability, not just a "bonus." Therefore, EPC specifications and investor due diligence should include:
1
Shading Reduction
Rear shading \le 1\% – construction requirement (no crossbars, closed profiles).
2
Guaranteed Rear Gain
Acceptance test rear gain \ge 18\% – on natural albedo.
3
Compliance with Standards
Load design compliant with Eurocode-3 + 60% safety margin.
4
Module Protection
Use glass-glass modules with diffusion barriers and UV filters.
Meeting these conditions guarantees that over a 30–50 year horizon, the rear side's contribution to total production will increase from ~40% to >50\%, stabilizing yield and reducing project risks.
Literature
Fraunhofer ISE (2020). Bifacial Photovoltaics: Technology, Applications and Economics.
NREL (2021). Perovskite Stability and Degradation Pathways.
Oxford PV (2022). Perovskite–Silicon Tandem Solar Cells: Towards Commercialization. Nature Energy.
Science (2021). Long-term Stability of Metal Halide Perovskites.
Solar Energy Materials & Solar Cells (2022). Accelerated aging of perovskite tandem modules under UV and humidity stress.