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The advantages of double glass solar panels

Several solar panel manufacturers have shifted towards exclusively producing double glass solar panels &#; or plan to do this soon. Until now, this strategy was only a marginal phenomenon of single brands, but now Meyer Burger, Axitec, Luxor, and Trina are among the frontrunners in this transition. But what sets double glass solar panels apart from other types, and what are the advantages and disadvantages for electricians and solar professionals? In this blog post, we'll explore the world of double glass panels, their benefits, and how they compare to alternative solar panel types.

What are Double Glass Solar Panels?

Double glass solar panels replace traditional polymer backsheets with a glass layer on the back of the module. This design encapsulates the solar cells between two sheets of glass, providing unique advantages. While this technology can be used with both p-type and n-type cells, the latter tend to offer superior lifespan and performance. That&#;s why more and more glass-glass solar panels feature n-type cells with bifacial functionality, making them highly efficient.

The Advantages of Glass Glass Panels

Double glass solar panels offer a host of benefits:

• Material Durability: The primary advantage lies in the durability of the material itself. Glass has a remarkable resistance to aging, ensuring that these solar panels maintain their effectiveness over decades of use.
• Superior Solar Cell Protection: Double glass solar panels provide the best possible protection for solar cells. This robust shielding ensures that the modules continue to perform exceptionally well, even in the face of extended usage.
• Exceptional Robustness: These modules exhibit outstanding resilience against external factors and harsh environmental conditions like hail, making them a reliable choice for various installations.
• Transparency for Versatile Use: Double glass solar panels are particularly suitable for applications such as carports and conservatories, where aesthetics and functionality come together seamlessly.

In the following paragraphs, we will delve into a detailed comparison of the advantages of glass-glass solar panels in comparison to glass-backsheet panels.

Robustness

One of the standout features of double glass solar panels is their exceptional resistance to mechanical loads. Thanks to the equal thickness of the front and rear glass sheets, these modules experience minimal compressive or tensile stress, ensuring optimal protection for the embedded solar cells. This robustness makes them ideal for various installations, even in challenging environments.

The thermal expansion of both glass layers is identical. Thus the solar cells are exposed to less stress in hot or cold conditions, as both glass layers expand and contract at the same rate.

Performance and Efficiency

The longer lifespan of double glass solar panels compared to glass-backsheet panels results in significantly higher overall yields for the solar system over its lifetime. This is crucial in ensuring the long-term profitability. Bifacial solar panels, when appropriately installed and under favorable conditions, can achieve even higher efficiencies.

Safety

Despite the thinner front glass sheet, double glass modules maintain stability due to their total thickness of approximately 4 mm of glass. This design also offers superior protection from humidity, ammonia, salt spray, and even fire, making them a safer choice for long-term installations.

Eco-friendly

The longer lifespan of double glass modules results in proportionally less waste generation. Due to their construction as and the recyclability of the materials used, these modules are theoretically more environmentally friendly than conventional material combinations. However, in practice, recycling potential has been underutilized. Glass from solar panels is primarily used for producing glass wool, which cannot be further recycled.

If progress is made in the recycling of solar modules in the next 30 years, double glass solar panels are likely to become somewhat more environmentally friendly than other panel types. 

Disadvantages of double Glass solar panels

While double glass solar panels come with numerous advantages, it's essential to consider potential drawbacks as well:

• Higher weight: Glass glass solar panels tend to be heavier due to the double glass sheets. However, modern modules can feature thinner glass, mitigating this issue. In many cases, the weight difference isn't significant.
• Mounting challenges: Solar panels with higher weight can be more challenging to mount, potentially increasing installation costs.
• Cost consideration: Double glass solar panels can be more expensive than other solar panel types, especially when installation costs rise due to their weight.

Glass glass compared to other module types

Glass backsheet solar panels

Panels with backsheet were the standard for a long time. Some manufacturers are transitioning to glass glass due to its superior long-term performance, even though glass backsheet solar panels can be cheaper and lighter.

Bifacial solar panels

Both glass-glass and glass backsheet solar panels can utilize bifacial technology, collecting energy from the backside. This can be advantageous in solar parks. Also, terrace or carport roofs, exposed to the low-lying sun could benefit from bifacial panels. In these cases, however, double glass solar panels excel, due to safety and design reasons. 

The most important brands

Meyer Burger

Meyer Burger is a Swiss company with module and cell production in Europe. The premium manufacturer places great emphasis on research and sustainability. The double glass solar panels are among the most powerful, especially in the long term: Meyer Burger guarantees a performance of at least 93.2 percent after 30 years &#; as much as no other manufacturer in our portfolio.

Double glass solar panels from Meyer Burger

Luxor

The German manufacturer produces in Europe and Asia. The bifacial solar panels with heterojunction technology offer an especially high bifaciality factor of 95 percent. This means that the efficiency of the back side reaches 95% of the efficiency of the front side. With appropriate mounting, these modules can therefore yield particularly high performance. Moreover, the solar panels are considered to be PID-free and partially also LID-free. As such, they are only subject to age-related degradation.

Double glass solar panels from Luxor

DAS Solar

The DH108NA series panels are the lightest glass-glass modules in our portfolio and, at just over 20 kilograms, are almost ten kilos lighter than the heaviest ones. The efficiency under test conditions is at least 22.3 percent &#; the top value in our portfolio, which only two other manufacturers achieve.

Double glass solar panels from DAS Solar

Solitek

Solitek tests its solar panels with particularly high pressure and suction loads. The manufacturer offers a 30-year product warranty for its stable panels &#; not every manufacturer can keep up with that. The portfolio also includes a frameless solar panel. Due to the lack of a frame, it is only 7 millimeters thick, but the two glass panes, at 3 millimeters, are the thickest compared to the other modules in the Krannich Webshop. Thanks to the thick panes, the solar panel withstands particularly high loads, with test values of 10,500 Pa snow load and 5,400 Pa wind load, almost twice as high as most other solar panels.

Double glass solar panels from Solitek

Trina Solar

The double glass solar panels of the Vertex S and S+ series achieve good to very good results in all areas compared to other panels in the Krannich portfolio. For example, they are among the lightest solar panels and those with the best performance. Also, the promised performance until the end of the 30-year performance warranty is relatively high.

Double glass solar panels from Trina Solar

Talesun

With 450 watts, the TD6I72M series offers the highest nominal power among all glass-glass solar panels in our portfolio. However, the panels in this series are also particularly large and comparatively heavy.

Double glass solar panels from Talesun

Edited Monday, July 15

Two types of photovoltaic module structures coexist:

• Glass-polymer film (also called glass-backsheet) type modules. They are made of glass on the front side and polymer film on the rear side. Polymer film, also known as backsheet, is sometimes incorrectly called Tedlar, although this material, developed by Dupont, is only one of the components of polymer film among other options. The thickness of the front glass generally used for this type of structure is 3.2 mm.

• Dual-glass type modules (also called double glass or glass-glass) are made up of two glass surfaces, on the front and on the rear with a thickness of 2.0 mm each. Some manufacturers, in order to reduce the weight of the modules, have opted for a thickness of 1.6 mm. DualSun has chosen to stay with a thickness of 2.0 mm for reasons explained below.

In both configurations, the photovoltaic cells are laminated between the front and rear sides of the module using an encapsulation material. This is melted during the lamination process and helps preserve the integrity and performance of the cells.

There are several types of encapsulation material, specific to each structure:

• EVA (Ethylene Vinyl Acetate), commonly used for glass-backsheet modules

• POE (Polyolefin Elastomer), the preferred solution for dual-glass modules.

As these materials have different chemical compositions, the lamination parameters and expected properties, particularly in terms of moisture penetration and electrical insulation, differ for the two types of structures.

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Advantages of the dual-glass structure

Due to the ease of its manufacturing process, the glass-backsheet type structure was largely dominant during the period -. Certain durability problems reportedt from the field  after several years of installation for certain types of polymer films, coupled with the advent of bifacial cells, has led photovoltaic module manufacturers to rethink the design of their products.

It was from that dual-glass module technology really took off, continually gaining market share, thanks to three technological arguments:

• Hermetic encapsulation: the double glass modules offer a hermetic structure, resistant to aggressive weather conditions, the main one being moisture penetration highlighted during tests so-called Damp Heat, according to standard IEC -2: (clause MQT13). By testing a wide variety of PV modules in according to this test, but with a duration extended to hours, the PVEL laboratory in the United States concluded that glass-glass modules are more resistant to moisture penetration. In fact, the power loss measured after the heat humidity test was 1.0% for the bi-glass modules, compared to 1.9% for the glass-backsheet modules (source: PVEL scorecard )

• Bifaciality: The bifaciality of double glass modules produces a gain of around 10-11% compared to the power measured on the front panel alone, for TOPCon type modules under so-called BNPI (bifacial nameplate irradiance) test conditions. Being given that in the residential sector, the rear face of the modules is at a reduced distance from the roof, the bifaciality of the modules is not a preponderant factor in the choice of modules. However, part of the transmitted light is reflected and saves a few percent of electricity production. Even if bifaciality is only a minor factor in this segment, the choice of double glass for residential applications remains relevant.

> To go further: What is a bi-facial module? What performance gains does it allow?

• Mechanical constraints on cells: the fact that the structure of the double glass modules is symmetrical implies that the cells are located on a so-called neutral line, the upper part of the module being in compression during a downward mechanical load and the lower glass surface being in tension. In this configuration, the cells undergo very little tension-compression stress. This is not the case for glass-backsheet modules which asymmetrical structure places the cells in a tensile zone, increasing the risk of cracking (see details in the sectional views of PV modules below)

Mechanical stresses on a glass-backsheet module during a uniform mechanical load applied to the front face

Mechanical stresses on a dual-glass module during a uniform mechanical load applied to the front face

Furthermore, another point to take into consideration is the PID effect (or potential-induced degradation), mainly due to a migration of sodium ions from the glass towards the cells. Double glass modules, due to the hermeticity of their structure, present less risk of PID. This phenomenon can be avoided by the use of an appropriate encapsulation material and by quality control reinforced by tests in climatic chambers. According to tests carried out at the PVEL laboratory, there is no notable difference in terms of resistance to the PID effect between dual-glass and glass-backsheet modules.

To summarize the advantages cited above, the choice of a double glass structure means that the photovoltaic cells are better protected from external stress, in particular from the penetration of humidity and mechanical stress.

However, dual-glass modules have certain disadvantages that are important to take into consideration during the product design phase.

Disadvantages of the double glass structure and points of attention

One of the main disadvantages concerns the hail resistance.

To simplify, the hail resistance of a photovoltaic panel is mainly linked to that of its upper layer. In the case of a glass-backsheet module, not only is the upper glass layer thicker (3.2 mm versus 2.0 mm) but also this layer is fully tempered glass, whereas in the case of a thickness of 2.0 mm, the glass is only semi-tempered due to technical limitations of the tempering process. Complete tempering provides better mechanical characteristics, particularly with regard to impact resistance. Glass-backsheet type modules therefore naturally have better resistance to hail than dual-glass modules. Furthermore, dual-glass modules have a specific weakness located in the passage of conductors through the rear face of the glass before being fixed in the junction boxes. In fact at this location the total glass thickness (2.0 mm) of the laminate is lower than at the other locations of the module (where the total glass thickness is 4.0 mm) and constitutes a vulnerable point.

To ensure that its dual-glass panels meet hail resistance standards, DualSun has conducted extensive tests on its entire product range. The size of hailstones necessary to pass IEC standards is 25 mm. All DualSun modules meet this standard, including the doubleglass which pass the RG3 level, corresponding to hailstones with a diameter of 30 mm and 75% higher impact energy than the IEC standard. Glass-backsheet modules (with glass thickness 3.2 mm) such as Shingle technology modules even reach the RG4 level.

> To go further: Does the certification of DualSun panels cover hail risks?

It is also important to take into account the aspect of recyclability products during their design phase.

Current solutions for recycling glass-backsheet type modules are based on a heated blade process scraping the rear face of the modules in order to separate the glass from the other components. Regarding dual-glass modules, certain layer separation solutions by high temperature heating make it possible to separate the 2 layers of glass, but this process is currently not commonly applied and requires more research in order to recover the separated materials in a fair condition. By default, dual-glass modules which have reached the end of their life are currently (early ) crushed, which is far from being an ideal recycling solution. Research on the subject of recycling is currently very active and it is hoped that a recycling solution for all PV technologies will be implemented in the years to come.

Finally, the last disadvantage linked to dual-glass modules concerns their generally higher weight compared to glass-backsheet modules, which creates constraints on installation. For example, for a DualSun module with dimensions mm x mm, the weight in glass (2.8 mm) - backsheet configuration is 20.0 kg while it is 25.1 kg for bi-glass (2 x 2.0 mm), which corresponds to a weight increase of 25%. In the case of a comparison between a 2 x 2.0 mm dual-glass module and a glass (3.2 mm)-backsheet module, the excess weight of the dual-glass is approximately 20%.

Table of advantages and disadvantages

 Summary table comparing a dual-glass structure to a glass-backsheet structure

[1]: Cell technology type: Shingle, 3.2 mm thick glass

[2]: Glass thickness of glass-backsheet module 2.8 mm

Having chosen to base its range of photovoltaic products on TOPCon technology, DualSun favors, among the criteria in the table above, resistance to moisture penetration. Indeed, for TOPCon cells, there is a real risk of corrosion of the front face of the cells in the presence of humidity, due to a chemical reaction between certain types of encapsulation material and the silver paste used for cell metallization. This corrosion does not systematically affect all TOPCon module manufacturers, but DualSun judges that the risk is high enough to take protective measures and therefore naturally moved towards a dual-glass structure, guaranteeing hermetic encapsulation of the photovoltaic cells. Furthermore DualSun ensures that the disadvantages of dual-glass modules are minimized, for example by checking that our modules meet hail resistance criteria that exceed current standards. 

This particular attention to the durability of our products involves the desire to maintain a glass thickness of 2.0 mm on both the front and rear sides of our modules. In this, we stand out from certain manufacturers who have chosen a glass thickness of 1.6 mm to reduce the weight of their products but by accepting an increased risk of accelerated deterioration, particularly under the effect of hail and mechanical load induced by wind and snow.

Furthermore, experience has demonstrated that the two glass-backsheet and dual-glass variants largely satisfy the durability criteria for previous cell technologies, including PERC, which was at the heart of the DualSun offer over the period - and which is notably less sensitive to the effects of humidity than TOPCon.

> To learn more about TOPCon technology, access our article: What are the advantages of DualSun FLASH TOPCON panels? 

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