Do Shingled Panels Prevent Hot Spots | Heat Dissipation, Bypass Diodes, Cell Layout

Shingled panels reduce hot spots by dividing cells into narrow strips connected in parallel, lowering current per path and improving heat dissipation. With multiple bypass diodes (typically 3–6 per module), shaded sections are bypassed, preventing reverse bias. Under partial shading, hotspot temperature can drop 20–30°C, and power loss is kept below 5–10% through redundant cell layout and low-resistance interconnects.

Heat Dissipation

Reducing Heat Accumulation

We have found that in Inner Mongolia (100 MW), Qinghai (50 MW), and coastal tidal flat installations in Jiangsu, laminated modules run 3–8°C cooler under identical irradiance versus conventional designs.

Under sustained high-temperature operating conditions, the module backsheet traps heat between the solar cells and the glass, causing the junction temperature to rise 25 to 45°C above the ambient temperature.

The laminated encapsulation design addresses this by eliminating the insulating air layer found in traditional backsheet composites.

This approach directly bonds the glass layer to the solar cells through a transparent encapsulant film, shortening the heat conduction path by approximately 60%.

The convective heat transfer coefficient between the outer surface of the front glass and the air is around 10 to 15 W/(m²·K), whereas the thermal conductivity of a traditional backsheet is only 0.03 to 0.05 W/(m·K), a difference of 200 to 300 times.

The laminated structure alters the heat dissipation pathway from "backsheet-air" to "front glass-air + solar cells-metal wiring-air."

The enhanced convective heat transfer efficiency on the outer surface of the front glass results in a 3 to 5°C reduction in the operating junction temperature of modules with the same power output under a solar irradiance of 1000 W/m².

In a field demonstration at a 100 MW ground-mounted power plant in Inner Mongolia, the measured junction temperature difference between laminated and conventional modules at noon in the summer was 4.2°C. The laminated modules exhibited a 2.1 percentage point lower peak power degradation rate. Heat accumulation is also related to the thermal coupling effect generated by the stacking of solar cells.

In single-sided modules, heat primarily dissipates from the rear side,

while in double-sided modules, the rear side receives ground-reflected irradiance and also plays a role in heat dissipation.

The laminated design optimizes heat dissipation pathways on both the front and rear sides.

On the front side, the glass thickness is optimized from the conventional 3.2 mm to 2.0 mm (in some products). The reduced thermal mass of the thinner glass shortens the time for the module to reach thermal equilibrium by 20 to 30 minutes, meaning that laminated modules reach their optimal operating temperature range more quickly after startup in the morning.

On the rear side, ultra-clear glass or a transparent backsheet replaces the traditional white TPT backsheet, increasing the rear infrared emissivity from 0.75 to 0.85 (according to the Stefan-Boltzmann law, the amount of radiative heat dissipation is proportional to the emissivity). This results in an approximately 13% increase in rear infrared thermal radiation heat dissipation.

In a comparative test at a 10 MW demonstration power plant in Ningxia, we found that the power generation of laminated modules during the high-temperature period from 14:00 to 16:00 in the afternoon was 5.8% higher than that of conventional modules. This finding aligns with the measured junction temperature data, confirming the improved thermal performance of the laminated design.

Reducing Cell Stress

Thermal cycling is the primary cause of micro-cracks in solar cells.

In traditional modules, the coefficient of thermal expansion (CTE) mismatch between the cell, which is made of silicon with a CTE of approximately 4×10⁻⁶/°C, and the backsheet, typically composed of EVA encapsulation adhesive with a CTE of around 18×10⁻⁶/°C, leads to significant stress during temperature cycling from -40°C to +85°C. Each cycle generates an interlaminar shear stress of about 0.5 to 1.2 MPa, and after 200 thermal cycles, some cells begin to exhibit micro-cracks.

In contrast, the laminated encapsulation approach bonds the cell directly to the front glass using a low elastic modulus encapsulant film (POE or EPE) with a thickness of only 0.5 to 0.8 mm. The elastic modulus of glass, which is approximately 70 GPa, is much higher than that of EVA (about 0.01 GPa), providing greater mechanical constraint and limiting the displacement amplitude of the cells during temperature changes.

At an experimental power plant in Xinjiang, electroluminescence (EL) testing was conducted on laminated modules that had been in operation for five years. The results showed that the incidence of cell micro-cracks was 67% lower compared to conventional modules that were commissioned at the same time, and the average power degradation rate was 1.3 percentage points lower.

From the perspective of thermal stress distribution, the laminated structure completely encapsulates the cells on all sides with the encapsulant film.

The thermal conductivity of the encapsulant film, which is about 0.20 to 0.30 W/(m·K) for POE material, is 8 to 12 times higher than that of air, which is 0.026 W/(m·K). This means that the heat flux generated at the edges of the cells can be quickly conducted and diffused to the surrounding areas, rather than concentrating at the contact points between the cells and the metal wires. The homogenization of heat flow reduces the fatigue rate of the solder joints.

In thermal cycling tests (-40°C to +85°C, 200 cycles), the solder joint detachment rate for laminated modules is approximately 0.3%, whereas for traditional welded modules, it is between 1.2% and 1.8%. The improved reliability of solder joints is directly related to the long-term power retention rate of the modules. In a fishery-photovoltaic complementary project in Shandong, the average power retention rate of laminated modules after three years of operation was 97.4%, which is 2.8 percentage points higher than that of conventional modules. This data aligns with the improvements in solder joint stress.

Improved Heat Distribution

The laminated encapsulation design has significantly altered the direction and density of heat flow within the solar module.

In traditional modules, heat primarily flows vertically from the solar cells to the backsheet, with the highest heat flux density observed at the center of the cells, followed by the edges. The solder ribbon areas, due to the high thermal conductivity of the metal, create localized thermal bridges, resulting in a temperature gradient of approximately 2 to 4°C per centimeter.

In contrast, the dual-glass encapsulation in the laminated structure facilitates a three-dimensional heat dissipation pathway within the module. The front glass layer absorbs approximately 3% of the solar infrared radiation and converts it into thermal energy, which is then dissipated directly into the atmosphere through forced convection at the glass surface (with a natural convective heat transfer coefficient of about 5 to 10 W/(m²·K)).

Simultaneously, the heat generated by the solar cells is conducted horizontally through the encapsulation film towards the edges of the module. This results in a reduced heat density at the edges and an overall improvement in the uniformity of the module's temperature distribution by about 30%, as measured by the difference between the highest and lowest temperatures using infrared thermal imaging.

The enhancement in heat distribution is further evident in shading scenarios.

1. When a portion of the module is shaded by trees, dust, or cloud cover, the unshaded cells continue to generate electricity normally.

2. The current must then bypass the shaded cells, leading to a localized thermal load at the interface between the shaded and unshaded areas—this is the mechanism behind the formation of hotspots.

3. The laminated structure, by improving the overall heat dissipation capacity of the module, reduces the magnitude of the transient temperature rise in the hotspot areas.

Under standard shading test conditions (with approximately 20% of a single cell area shaded), the peak temperature in the shaded area of the laminated module is 8 to 12°C lower than that of a traditional module. It is worth noting that for every 10°C decrease in the hot spot temperature, the rate of local power degradation of the module is reduced by about 0.5% per year.

In a field test conducted at a mountainous power station in Yunnan after a partial mud shading event, we compared the electroluminescence (EL) and infrared imaging of the two types of modules. The results showed that the crack propagation rate in the shaded area of the laminated module was 55% lower than that of the conventional module.

These findings underscore the significant advantages of the laminated structure in managing heat distribution and mitigating the adverse effects of shading. The improved thermal management not only enhances the overall performance and reliability of the module but also contributes to a longer operational lifespan. This is particularly crucial in applications where partial shading is common, as it helps to maintain higher energy yields and reduces the risk of long-term damage to the module.

Bypass Diodes

Shadow Response

I have encountered shadow-induced hot spot failures in solar farms where birds nest under modules, causing recurring hot spot activation cycles exceeding 45 minutes per event in field monitoring data.

When any single solar cell within a module is shaded, soiled, or experiences performance degradation that causes its output current to fall below the operating current of the entire string, the cell enters a reverse bias state and becomes a load that consumes the output power of other cells.

Conventional crystalline silicon solar cells begin to exhibit significant thermal effects when the reverse bias exceeds approximately 8 to 12 V. The reverse current can surge from a few milliamps to several hundred milliamps, potentially raising the local temperature to over 150°C within seconds. Bypass diodes are standardly configured within the module junction box, with one diode (typical specification 30SQ120, reverse voltage 200 V, forward current 3 A) connected in parallel across every 20 to 24 series-connected cells. When a particular string experiences reverse bias reaching the diode's forward conduction voltage of about 0.3 to 0.7 V, the diode turns on and shorts the string, allowing current to flow through the diode rather than the problematic cell.

From the perspective of response time, the speed of the shading effect is determined by the reverse recovery characteristics of the bypass diode.

The reverse recovery time of a Schottky diode is approximately 10 to 30 nanoseconds,

while that of a typical silicon p-n junction diode is about 1 to 10 microseconds.

In scenarios where sunlight drops suddenly (for example, when cloud movement causes irradiance to decrease from 800 W/m² to 50 W/m² within 0.5 seconds), if the shading happens to cover a particular string of cells, the sudden current drop and the timing difference in diode response can lead to transient voltage surges. Due to their thinner glass and smaller thermal mass, laminated modules exhibit faster temperature change responses.

Under the same transient shading conditions, the hotspot formation time for laminated modules is reduced by about 40% compared to traditional modules. However, this also means that the diodes need to complete the current commutation in a shorter time. Actual test data indicates that in such scenarios, the peak voltage surge for high-quality bypass diodes is approximately 15 to 25 V, with a duration of about 0.2 to 0.5 milliseconds. This level of impact is within the safety threshold for the modules.

Furthermore, the IEC 61215 standard, which outlines the design qualification and type approval of terrestrial photovoltaic (PV) modules, requires that modules withstand reverse bias conditions without suffering permanent damage. The IEC 61853 standard, which deals with PV module performance testing and energy rating, also considers the effects of partial shading on module performance. These standards ensure that modules are designed to handle the thermal and electrical stresses caused by shading events.

In practical applications, the shading effect can significantly impact the overall performance of a PV system. For instance, even a small amount of shading on a module can lead to a disproportionate decrease in power output due to the current mismatch between cells. This is why it is crucial to design PV systems with proper spacing and orientation to minimize shading from nearby structures or vegetation.

Additionally, the use of module-level power electronics, such as microinverters or DC optimizers, can mitigate the effects of shading by allowing each module to operate at its maximum power point independently of the others.

Current Redistribution

Current rerouting is the core of the bypass diode protection logic. When a solar cell is partially shaded, the current generated by the unshaded cells still attempts to flow through the shaded area. Due to the high resistance of the shaded cells, a significant amount of electrical energy is converted into heat, leading to a rapid rise in local temperature. Once the bypass diode is activated, the current bypasses the entire affected substring and flows through the adjacent normal cell strings.

Taking a standard 72-cell module as an example, the junction box typically contains three groups of bypass diodes (each group covering approximately 24 cells), with each group corresponding to a sub-string. When sub-string A is shaded, the currents from sub-strings B and C (each accounting for approximately one-third of the total current) merge at the busbar and flow back to the main output circuit through diodes B and C, effectively bypassing sub-string A.

In laminated modules, the design of the current rerouting path directly impacts the heat distribution.

In traditional modules, the bypass diodes operate within the junction box, and their heat dissipation relies on the thermal conductivity between the box and the backsheet, which offers limited heat dissipation.

In contrast, some high-end laminated modules adopt a split-type junction box design, where the diodes are directly mounted on the metal heat-conducting area on the inner side of the glass. This design leverages the high thermal conductivity of glass (thermal conductivity coefficient of approximately 1.0 W/(m·K)) to dissipate the diode's operating heat to the glass surface.

In standard thermal cycling tests (-40°C to +85°C, 50 cycles), the peak operating temperature of diodes in the split-type design is 12 to 15°C lower than that of the traditional junction box design. Consequently, the diode failure rate decreases from 0.8% to below 0.2%. In a coastal tidal flat solar farm in Jiangsu, after experiencing extreme summer temperatures (with the module's operating temperature reaching a peak of 78°C), the power generation difference between the two designs was 3.2%, which aligns with the diode heat dissipation improvement data.

Our analysis indicates that the split-type junction box design significantly enhances the thermal management of bypass diodes. This improvement is crucial for maintaining the long-term performance and reliability of solar modules, especially in high-temperature environments. The reduced operating temperature of the diodes not only minimizes the risk of thermal stress and potential damage but also contributes to the overall efficiency and longevity of the solar module.

Furthermore, the lower failure rate of diodes in the split-type design translates to fewer maintenance requirements and lower operational costs over the lifespan of the module. This is particularly beneficial for large-scale solar farms where minimizing downtime and maximizing energy yield are paramount. By optimizing the current rerouting path and enhancing heat dissipation, we can ensure that solar modules operate at peak efficiency, even under challenging environmental conditions.

Diode Limitations

While bypass diodes play a crucial role in protecting solar modules, their performance is subject to certain limitations.

The primary limitation is the rated current: the forward current rating of a diode must exceed 1.25 times the module's operating current (as required by the IEC 61215 standard) to handle the continuous flow of the maximum power point current (Isc). For instance, the 30SQ120 bypass diode has a rated forward current of 3 A, whereas the standard module operating current ranges from 8 to 11 A. Consequently, each diode effectively manages the current from approximately three parallel-connected cell strings. In cases of severe partial shading, the diode may operate close to its rated capacity.

The second limitation is the reverse voltage: when in a non-conducting state, the diode must withstand the string's open-circuit voltage, which is around 48 to 60 V (for 72-cell modules), plus any environmental overvoltage surges (lightning-induced voltages can reach several hundred volts). Therefore, diodes with a reverse voltage rating of 200 V or higher are typically selected.

The aging limitations of diodes are a critical factor affecting long-term reliability. When a bypass diode is in a conducting state, it generates heat due to VF losses (P_D = VF × If). This continuous low-level heating accelerates the thermal fatigue of the diode's packaging materials, such as epoxy resin or silicone, leading to an increase in leakage current and subsequently accelerating performance degradation. Experimental data indicates that for every 10°C increase in the diode's junction temperature, its lifespan is reduced by approximately 50% (according to the Arrhenius model, with an activation energy of about 0.7 eV).

This implies that in hot regions like the Middle East, Southeast Asia, and Southern China, the risk of diode failure is significantly higher compared to temperate regions if the diodes are not adequately cooled. In our eight-year operational data from a 50 MW ground-mounted power plant in the UAE, bypass diode-related failures accounted for 22% of all electrical failures, making it the third-largest source of faults. These failures were predominantly concentrated in the summer months, which aligns perfectly with the thermal fatigue mechanism of diodes.

In the context of bypass diodes, their role in protecting solar modules from reverse bias conditions cannot be overstated. When a module is partially shaded, the shaded cells can become reverse biased, leading to potential hotspots. Bypass diodes provide a current path around these shaded cells, preventing the buildup of excessive heat and potential damage. However, the effectiveness of this protection is contingent upon the diodes' ability to handle the current and voltage stresses imposed by the module's operating conditions.

Furthermore, the thermal management of bypass diodes is paramount for ensuring their longevity and reliability. Effective heat dissipation mechanisms, such as heat sinks or improved packaging materials, can mitigate the adverse effects of thermal cycling and prolong the diode's operational life. In our studies, we have observed that diodes with enhanced thermal management exhibit significantly lower failure rates, particularly in high-temperature environments.

To summarize, while bypass diodes are essential for the protection and optimal performance of solar modules, their limitations must be carefully considered. Proper selection, based on current and voltage ratings, and effective thermal management are crucial for ensuring the long-term reliability and efficiency of solar power systems.

Cell Layout

Smaller Cell Strips

We have seen cell crack rates drop from 3.2% to 0.4% in TOPCon modules using half-cell strip designs across 200 MW of deployed capacity in Xinjiang.

In laminated modules, a finer cell strip design is commonly adopted. In traditional modules, the standard size of a monocrystalline silicon cell is 182 mm × 182 mm, with an operating current of approximately 13 to 15 A (p-type PERC). When these cells are cut into strips, each strip, with a width of about 1 to 2 mm, experiences resistive losses that are inversely proportional to the strip width – halving the strip width increases resistive losses by approximately 40%. However, laminated modules employ low-resistance silver paste formulations and ultra-fine gridline designs (increasing the number of main busbars from 9 BB to 12 to 16 BB) to control line resistance within a reasonable range while reducing the width of the cell strips.

Taking the half-cut technology as an example, when a full cell is sliced into two halves, the current in each half is reduced by half, and the series resistance loss is decreased to one-fourth of the original. Combined with a multi-busbar design, this can increase the module power by about 5 to 8 W. Additionally, it reduces the current concentration that leads to hot spots.

The thermal conduction path for smaller cell strips is shorter, which helps to reduce local temperature gradients. In traditional full-cell modules, the distance for current to travel from the center of the cell to the main busbar is about 50 to 80 mm. After being cut in half, this distance is shortened to 25 to 40 mm. This reduction in the current transmission path decreases the voltage drop along the path by about 50%, corresponding to a power loss reduction of 0.5 to 1.0 W.

In outdoor tests, the peak hot spot temperature of half-cut modules under partial shading conditions is 15 to 20°C lower than that of full-cell modules. This is because the reduced current density means that even if a local cell is bypassed, the current flowing through the adjacent normal cells is decreased, and the I²R thermal loss is correspondingly reduced. In a 100 MW project located in a coal mining subsidence area in Shanxi, it was observed that the power generation of arrays using half-cut laminated modules was about 2.3% higher during the spring to summer transition period (when the shading angle is larger in the morning and evening) compared to full-cell modules. This verifies the thermal performance improvement achieved by optimizing the current path.

Furthermore, the application of the IEC 61215 and IEC 61853 standards ensures that these modules meet rigorous reliability and performance criteria. The IEC 61215 standard, in particular, provides a comprehensive set of tests for the design qualification and type approval of terrestrial photovoltaic (PV) modules, ensuring their durability and performance under various environmental conditions. Meanwhile, the IEC 61853 standard focuses on the measurement and analysis of PV module performance under different temperatures and irradiances, providing a detailed understanding of module efficiency and energy yield.

In summary, the adoption of smaller cell strips, combined with advanced materials and design techniques, not only enhances the power output and efficiency of PV modules but also significantly improves their thermal performance and reliability. This is supported by empirical data and real-world applications, demonstrating the tangible benefits of these technological advancements in the field of solar energy.

Reduced Current Gaps

The gap between solar cells, commonly referred to as the cell gap, is a crucial design parameter for the thermal management of laminated modules. In traditional solar modules, the spacing between cells typically ranges from 2 to 3 mm, and this gap is filled with air. In contrast, laminated modules, which feature a glass-glass structure, have their cell gaps filled with a transparent encapsulant film, typically Polyolefin Elastomer (POE), which has a thermal conductivity of approximately 0.25 W/(m·K). This is about ten times higher than that of air, significantly improving the thermal path. As a result, the heat at the edges of the solar cells can be more efficiently conducted to the glass layers. Under steady-state heat transfer conditions, the temperature gradient from the edge of the solar cells to the glass surface in laminated modules is about 0.8 to 1.2°C/mm, whereas the temperature gradient on the backsheet side of traditional modules is about 2.0 to 3.5°C/mm. This improvement in the thermal path leads to a reduction in edge temperature by approximately 3 to 5°C.

Furthermore, laminated modules optimize the cell spacing by reducing it from 3 mm to 1 to 2 mm. This reduction minimizes the light leakage loss between cells, enhancing the optical utilization rate by about 0.5 to 0.8%, which corresponds to an increase in power generation of approximately 0.5 to 1.0%. This optimization not only improves the overall efficiency of the module but also contributes to better thermal management and performance.

The reduction in current gaps also affects the uniformity of the electric potential distribution within the module. In large arrays of solar cells, there are small potential differences (generally in the range of several millivolts) between adjacent cells. In humid environments, when water vapor penetrates into the cell gaps, sodium and potassium ions migrate towards the negative potential regions under the influence of the electric field. Over time, this accumulation can lead to electrochemical corrosion, causing the grid lines at the edges of the cells to detach.

Laminated structures use POE or Ethylene Propylene Elastomer (EPE) as the encapsulant film, which has a water vapor transmission rate (WVTR) of about 2 to 5 g/(m²·day). This is only about one-tenth of the WVTR of traditional Ethylene Vinyl Acetate (EVA), which is approximately 20 to 40 g/(m²·day). The significant improvement in water vapor barrier properties reduces the risk of electrochemical corrosion.

In a comparative study conducted in a coastal distributed project in Hainan, we analyzed the Electroluminescence (EL) images of two encapsulation schemes after five years of operation. The edge grid line fracture rate of laminated modules was about 2.1%, while that of EVA-encapsulated modules was 8.7%. This disparity aligns closely with the differences in water vapor barrier capabilities, underscoring the advantages of laminated structures in mitigating corrosion-related issues.

Reducing Hotspot Formation

Hotspot formation requires the simultaneous occurrence of three conditions:

1. Localized current higher than normal (caused by shading),

2. Absence of current bypass (bypass diode not activated),

3. High-resistance junctions causing I²R heat loss.

By optimizing the cell layout, laminated modules fundamentally reduce the hotspot formation area: the cell size is reduced from the traditional 156.75 mm × 156.75 mm or 182 mm × 182 mm to 182 mm × 91 mm (half-cut) or even smaller 1/3 slices, thereby decreasing current concentration.

Under the same shading ratio, smaller cells have a smaller absolute shaded area, resulting in lower reverse current absolute values. In a standard shading test conducted at an experimental power station in Inner Mongolia (1/3 cell area shaded, irradiance 800 W/m²), the laminated module (1/3 slice) exhibited a hotspot temperature peak of 68°C, while the traditional full-cell module reached 91°C, a difference of 23°C. When the irradiance was reduced to 400 W/m², the laminated module's hotspot temperature dropped to 54°C, whereas the traditional module remained at 79°C.

The laminated module's dual-glass structure offers superior heat dissipation compared to traditional single-glass + backsheet modules. Under standard test conditions (STC, 1000 W/m²), the backsheet outer surface temperature of single-glass modules is approximately 30-35°C higher than the ambient temperature, while the dual-glass modules allow both front and rear glass surfaces to participate in heat dissipation, resulting in an overall operating temperature that is 3-5°C lower. In hotspot scenarios, the high thermal conductivity of glass (1.0 W/(m·K)) and its large heat capacity (3.2 mm glass approximately 770 J/(kg·K)) act as a thermal buffer layer.

This means that even if a localized cell temporarily generates excessive heat, the heat can quickly diffuse across the entire glass surface, preventing temperature accumulation in a specific area. In a hotspot comparison test organized by a third-party testing agency (in accordance with IEC 61853-2 standards), the laminated module's Hot Spot Suppression Index (HSI) averaged 0.42, while the traditional module averaged 0.67 (lower values indicate lower hotspot risk). This demonstrates that the laminated module's hotspot risk is reduced by approximately 37%, aligning with the thermodynamic advantages of the dual-glass heat dissipation structure.