Backsheets & EVA: Enduring PV Production Behind the Scene

Most focus on solar generation is placed on the solar cells themselves, but attention should equally be paid to those components “Behind the Scenes.” What is so important about the back of a solar module? The Behind the Scene THINGs that are attached at the back of the module are one of the key process consumables in solar module manufacturing that influence both cost and quality of a solar panel, and are best referred as the Backsheet and EVA (ethylene vinyl acetate) Film. At first glance, the backsheet appears to be a simple layer of plastic film on the back surface of the module. However, when you consider that this is the only layer of protection from dangerous DC voltage, it’s clear why a quality backsheet is important. Photovoltaic backsheets play an important role in protecting solar modules over their lifetime. On the other hand, EVA is an encapsulant for solar Cells/ Modules. It is a copolymer film which acts as an essential sealant of photovoltaic solar modules for ensuring the reliability and performance.

The PV backsheet is on the outermost layer of the PV module. It is designed to protect the inner components of the module, specifically the photovoltaic cells and electrical components from external stresses as well as act as an electrical insulator. In order to accomplish this, the solar panel material must be a robust construction, typically a three layer laminate, and have high dielectric properties. Quality backsheets provide voltage protection and maintenance prevention and are equally as important as the glass covering the cells.

While the EVA Encapsulant sheets play an important role in preventing water and dirt from infiltrating into solar modules as well as protecting the cell by softening the shocks and vibrations to the cell. They should possess certain properties like excellent durability, adhesive bonding to the cell and glass, excellent optical transmission and transparency and flexibility. Cells are encapsulated before being laminated with glass and the backsheet. So, in a typical solar module, you have the glass on top, an EVA sheet after that, followed by the cells, one more layer of EVA sheet below the cell, and finally the backsheet. Solar panels have typically two layers of EVA-based encapsulants in a solar module.

ENDURING THE LIFE OF MODULES

Backsheets Allow Modules to Operate Safely:

Photovoltaic modules can operate at temperatures of 90ºC or higher. Backsheets insulate the electrical components of the module, ensuring they can produce electricity safely. A backsheet’s safety performance can be evaluated by looking at two key tests:

Breakdown voltage test: This test evaluates the voltage level that causes permanent failure of the backsheet and significant loss of electrical safety. The higher the breakdown voltage rating, the safer the backsheet. Low breakdown voltage can compromise module safety, resulting in potential shock or fire hazards

Wet leakage test: Measures the panel’s ability to keep electricity isolated within the proper components of the panel. If a panel fails a wet leakage test, it can mean that the module’s safety can become compromised in very wet conditions, such as melting snow, heavy rainfalls, condensing humidity and panel cleanings.

Backsheets Help Modules Produce Power Efficiently:

Photovoltaic (PV) modules need to be a reliable source of power for 25 years or more, so their components all need to work in concert to ensure the panel continues to perform. Backsheets help do that – they insulate the electrical components of the module, protecting them over their lifetime.

Backsheet performance can be analyzed by:

UV light can cause degradation: A UV aging test can determine whether a backsheet is more likely degrade and become yellow. Yellowing can also indicate a compromise in reflectivity, which can lead to decreased performance.

Embrittlement of PET layer: A brittle backsheet may crack under mechanical stress, causing the module to fail.

Delamination of backsheet and backsheet adhesive: Delamination of the backsheet layers or adhesive can decrease module performance, especially in harsh weather conditions.

SOLAR MODULE BACKSHEET AND ITS SERVICE LIFE ISSUES

During its lifetime, a solar module will be exposed to the environment and challenged by UV radiation, temperature cycles and mechanical loads, including snow and wind. The backsheet, is a critical component in a solar module, since it is responsible for protecting and electrically insulating the module itself. The defects of the most concern for backsheets include cracking, yellowing and delamination. The visual aspect of the solar modules as this can be an early and first indicator of failure that can lead to more serious problems. While modules containing backsheets that have cracked and delaminated, represent actual failures of the backsheets, yellowing of the backsheet known to be an early potential predictor of backsheet failure.

Typically, encapsulant yellowing is a result of UV light exposure. The encapsulant is formulated with cross-linking agents, free radical scavengers and UV blockers to prolong the life of the module. Although the EVA can yellow on its own, these additives have also been known to yellow. This yellowing phenomenon can reduce transmission by up to 5%, directly impacting power. It is typically associated with degradation of the polymer which, in severe cases, can result in the embrittlement of the backsheet. Day-night thermal cycles on a brittle backsheet can lead to cracking and/or delamination, resulting in loss of electrical protection of the module. A high rate of backside yellowing was observed in polyester-based backsheets, while front side yellowing occurred most prominently in PVDF-based backsheets. Yellowing typically represents degradation of the polymer that can lead to backsheet cracking, which can represent a safety hazard for anyone conducting operations or maintenance procedures. For standard modules that use EVA encapsulation, for the backing can be used a layer of Tedlar composite (Tedlar- Polyester-Tedlar), known as TPT.

Tedlar is the DuPont trade name of a film of polyvinyl fluoride, PVF, laminated with polyethylene terephtalate (PET), typically two layers of PVF on either side of a core layer of polyester. This backsheet, presents the lowest occurrence of the above mentioned defects, performing well with no major defects. TPT backsheets for PV modules present excellent strength, weather resistance, UV resistance and moisture barrier properties. These properties significantly improve module life, allowing module warranties up to 25 years v/s 10 years warranties of specially treated PET film laminates. So the use of TPT backsheets avoids the need to replace PV modules as frequently.

Module makers have a wide choice of backsheets. They make use of different components and constructions. Selecting the right backsheet for a specific module design is often complex, time-consuming and expensive. But, making the right choice can ultimately impact module’s ability to perform. Significant development, testing and standardization are involved in the design and testing of the materials used in backsheets. A poor quality backsheet can result in catastrophic failures and pose serious threats to installers and maintenance workers. Also, once the array is installed, a poor quality backsheet can lead to system degradation, unwanted or unplanned maintenance, and replacements and additional unplanned and unbudgeted costs. In regard to long-term protection of the panel, backsheets have two main functions – electrical insulation and to block moisture ingression. In the end, backsheets are a crucial part of the outer shell to protect the module components from environmental degradation.

Some of the key functions include:

• Physical protection
• Moisture protection
• Durability
• Electrical insulation
• Efficiency to help modules generate more power

HUGE TECHNOLOGY CHOICE

In general, backsheets can be divided into two groups- fluoropolymers and non-fluoropolymers. Among fluoropolymer backsheets either one or both of the protective films are fluorine based with module makers having the choice of products using Tedlar from DuPont, Kynar from Arkema, PVDF or coatings. While Kynar and Tedlar are exclusive brand products offered by DuPont and Arkema, PVDF is offered by various other suppliers. Tedlar was once the only choice for protective layers of backsheet compounds for solar modules – and it is still the only polymer that has proved its long-term reliability in the field. Kynar from Arkema is also interesting with its three-layer structure. It uses 100% PVDF film on the top and bottom, and PVDF along with other binders in-between. Coatings based solutions are also available among non-fluoropolymer backsheets. The material mostly used for the outer protective layer in the non-fluoropolymer segment is a PET film. The dominance of PET is much stronger as a core layer – most backsheets listed in our survey employ PET as core layer, only one company is promoting an atypical product that uses polyolefin as the core layer and polyamide as outer protection film.

A lot of changes have been happening in the backsheet segment over the last decade. Once a segment almost entirely dominated by Tedlar-based products, a supply shortage offered PVDF/Kynar as well as pure PET solutions the opportunity to enter the market. Then we have seen films getting thinner, fewer layers – and coatings gaining shares, both fluoro and non-fluoropolymer based.

The offer on backsheets technologies is probably wider than ever. Today, module makers can select from the classical TPT composition to pure polyester-based solutions and many different coating variations. There is basically almost any product available a module manufacturer’s heart is looking for – and in various quality levels. Backsheet technologies have clearly evolved during the past decades from pre-dominantly fluoropolymer based tri-layer laminates to innovative multi-material product offerings. By now PV industry has become familiar with many product acronyms representing different design constructions. Due to various industry dynamics including downward cost pressures, lower cost backsheet product variants have also entered into the solar market.

We are also seeing further advancements in the backsheet segment: Hangzhou First, for example, has introduced a transparent backsheet, which is currently being tested by customers. There will be more products suitable for 1500 V, according to the latest IEC 61730 standard. Coveme and Cybrid, for example, are working with highly reflective materials, and their existing product ranges can already be ordered with high reflectivity. Cybrid is developing a solution to replace the core PET film with a polymer “alloy” for both cost and quality reasons.

According to a research, backsheets with fluoropolymer based films (Tedlar & PVDF) owned around 50% of the market in 2016. Of this, PVDF composites had about 25% market share – the highest in the backsheet field. Pioneering product Tedlar was still used for around 15% of the backsheets. Pure PET backsheets accounted for 25%. The remaining 25% space was shared among the other configurations including coatings.

Solar photovoltaic (PV) technology is a recipe of highly engineered materials and components, sandwiched with specific functions and working together to harness sunlight and convert it into electricity. One of those layers is the EVA Encapsulation film which protects the solar cell and ensures its performance and reliability; its role is to provide optical and electrical transmissivity and keep out moisture. Ethylene vinyl acetate (EVA) has been the solar module encapsulant material of choice since the 1980s. It is as field-proven as any solar PV technology component. EVA still commands the vast majority of solar module encapsulation today: it has a proven track record and is a low-cost option. But the emergence of both thin-film and high-efficiency cells means a greater need for an alternative to EVA. As the market expands for more thin-film and higher-efficiency crystalline silicon solar cells, so too will the usage of alternative encapsulant materials.

Ionomer materials have been employed as encapsulants in niche applications for several years. PV ionomer encapsulants deliver long term protection for the most sensitive portions of PV modules. Choosing the right material not only increases module durability and production efficiency, but can also significantly enhance long term power generating efficiency. Chemicals and materials giant DuPont, knowing that PV module makers who wanted more varied encapsulant options came up with two ionomer-based sheeting products, DuPont PV 5400 Series and PV8600 Series. Both encapsulants are weather resistant and provide excellent strength, long-term durability, and dielectric properties to PV module manufacturers.

MARKET GLANCE

The market for solar backsheet has been segmented on the basis of material used for its construction into fluoride and non-fluoride types. Asia-Pacific is estimated to be the largest market for solar back sheet and accounts for around 30% of the global solar back sheet market. Developed markets of North America and Europe, also contribute a fair share to the solar back sheet market. Changing energy and power sector scenario with increasing importance towards renewable power generation, is one of the important factors enhancing the global solar back sheet market. Favorable energy policies undertaken as well as financial incentives provided by the governments are key drivers that have contributed significantly to the growth of solar back sheet market. Recent upsurge in off-grid solar applications from residential applications due to mandated net metering laws across developing countries have also contributed to the demand for solar back sheets. Various R&D initiatives undertaken by research institutes as well as technological advances achieved by back sheet manufacturing companies have aided in the reduction of costs for back sheets. With the demand set to increase in coming years as well as increase in the manufacturing set to be undertaken, economies of scale would help reduce the manufacturing cost, further subsiding cost as an obstacle for this market.

According to the report from Lucintel, the global solar PV backsheet market comprises two broad categories: crystalline silicon modules and thin film modules. Solar PV backsheets are used widely in crystalline silicon solar PV module. The backsheet market is expected to reach $1.6 billion, with a CAGR of 3% by 2017. The solar PV backsheet market has gone through several challenges recently. Manufacturers must develop products that meet strict performance requirements and industry standards while reducing costs, which becomes a challenge for them. New polymeric materials, which are the basic raw materials to manufacture backsheets, are subjected to extensive testing according to standards that ensure safety and reliability, sometimes resulting in delayed certification process. Another market report from TechNavio suggests that the Global Solar PV Backsheet market is expected to grow at a CAGR of 15 percent by 2018.

We have also seen a segment dominated by western players turning into a field strongly dominated by companies from China and India. Jolywood and Cybrid have shipped together about 28 GW in 2016 and plan to expand sales to around 38 GW in 2017, which would be around half of global solar demand this year. At the same time, we are seeing consolidation and new players entering the fray – after global market leader for anti-reflection module coatings DSM from Holland took over backsheet maker Sunshine New Materials Technology in China, it is assumed to push hard for winning market shares. Afga, a leading supplier of polyester for the core layer, is just launching a full backheeet product (though using Cybrid’s fluorine polymer skin layer).

Market consolidation in Europe and North America is due to increasing imports from Asia-Pacific, a result of the over-supply of materials manufacturing. The largest market, APAC constitutes over 70% of global PV materials consumption, yet growing adoption of premium products such as BIPV in Europe and NA is expected to boost revenues.

According to the datasheet of ENF Solar, more than half of the backsheet manufacturers are based in Asia, mainly China, reflecting how the PV industry supply chain has grown up around the Chinese module manufacturers that dominate the industry today. And in recent years, Indian manufacturers like Asian PV Technology, Garware Polyster and RenewSys have come under increasing demand for the production of backsheet. DuPont from United States has been a major supplier of both backsheet and EVA to Indian module makers.

With solar module production and installation taking place mostly China; it is no wonder Chinese companies dominate the backsheet segment. Cybrid and Jolywood claim that they were the market leaders with a global market share of around 20% each in 2016 – and both plan to increase shipments in 2017.

On the other hand, ReportsnReports.com predicted EVA demand increase – from 2011’s total of 2,286,193 tons to 2,966,078 tons in 2017, climbing at a Compound Annual Growth Rate (CAGR) of 4.4%. The huge economic progress of countries across the continent is the primary driver for the continued growth of the global Ethylene Vinyl Acetate (EVA) market. According to this research, Asia was accountable for 49% of the global EVA demand in 2011, with the market driven primarily by China (with a 62% share of the continent’s total). The growing solar power industry is therefore predicted to play an important role in the future production of the co-polymer, particularly in India and China – two major producer of solar power in Asia.

INNOVATIONS IN BACKSHEET

We are also seeing further advancements in the backsheet segment: Hangzhou First, for example, has introduced a transparent backsheet, which is currently being tested by customers. There will be more products suitable for 1,500 V, according to the latest IEC 61730 standard. Coveme and Cybrid, for example, are working with highly reflective materials, and their existing product ranges can already be ordered with high reflectivity. Cybrid is developing a solution to replace the core PET film with a polymer “alloy” for both cost and quality reasons.

At the occasion of SNEC 2017, Agfa Specialty Products announced the launch of UNIQOAT, its next generation of polyester-based backsheet products. Agfa’s UNIQOAT backsheet achieves a level of reflectivity that is probably the highest in the market today and thereby offers increased power output of solar modules. By design UNIQOAT eliminates the risk of backsheet delamination because it is conceived and manufactured as a single layer backsheet. During the extrusion process the hydrolysis and UV resistant polyester is surface-modified to face the challenges to which it will be exposed and to actively contribute to more reliable and durable solar modules. Unlike the laminated or offline coated backsheet structures, Agfa’s UNIQOAT is manufactured in a one-step-process and delivers a single layer sheet, which implies a great cost reduction potential. As an additional advantage of Agfa’s more efficient production process, the concept of UNIQOAT as a mono-backsheet eliminates the cost of a lamination adhesive and of managing a complex mix of lamination films from a variety of suppliers.

The guarantee throughout the industry is an installed solar system should survive past its 25th birthday before a drop-off in production is noticed. Backsheets are the final piece to the solar module puzzle, sealing a solar module from outside forces like moisture and UV light. And with the help of a lamination machine, the cells are laminated between films of EVA in a vacuum, which is under compression. Also with the help of the EVA, the solar cells ‘are floating’ between the glass and backsheet, helping to soften shocks and vibrations and therefore protecting the solar cells and its circuits. For that reasons, it is very likely that we see further consolidation in the sector – with manufacturers not comprising on the quality and willingly bringing in more innovative product line for the PV industry.