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Looking for long-termperformance, durability, and safety for solar panels? Choose PV backsheet materials wisely!

Looking for long-termperformance, durability, and safety for solar panels? Choose PV backsheet materials wisely!

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Looking for long-termperformance, durability, and safety for solar panels? Choose PV backsheet materials wisely!

– By Rahul Khatri, Technical Manager, DuPont Photovoltaic Solutions, India

PV backsheets available in the market for solar panels can be broadly divided into two categories – fluoropolymer-based and non-fluoropolymer-based. Fluoropolymer-based backsheets consist of an outer layer (air side) only or outer and inner layer of fluoropolymer material such as Polyvinyl Fluoride (PVF) film, Polyvinylidene Fluoride (PVDF) film orFluoroethylene-Alkyl Vinyl Ether(FEVE) coating.

Non-fluoropolymer based backsheets consist of an outer layer of non-fluoropolymer material (does notcontain any fluorine atoms), such as Polyester (PET) and Polyamide (PA). Considering the importance of backsheet outer layers(exposure to climate) to protect the inner backsheet core and solar cells from harsh environmental stresses such as UV radiation, humidity, dust, chemicals, etc., fluoropolymers have shown consistentsuperior performance in the field over non-fluoropolymers,thanks to their excellent weather resistance properties.

There is sufficient data available demonstrating poor field performance of non-fluoropolymer-based backsheets,due to their poor resistance to outdoor stresses,which makes them unsuitable for harsh climate regions such as India.

Are all fluoropolymers good for PV backsheets?

PV backsheets need to have an optimum blend of three key properties – weatherability, mechanical strength, and adhesion – to perform in harsh climates for 30 years and enable reliable and safe operation of PV modules. Fluoropolymers, in general, provideexcellent weatherability. However, not all fluoropolymers possess good mechanical strength, adhesion or both.

Tedlar® PVF and PVDF fluoropolymer-based backsheets

Tedlar® PVF and PVDF are the two most commonly used fluoropolymerfilm-based backsheets. The former has been in use ever since NASA selectedthe material as part of its final recommended Bill of Materials in a decade long exercise (1975 – 1986) designed to identify the most reliablebacksheets for crystalline silicon PV module technology.Itis the only backsheet that has performed successfully (no backsheet degradation, low module power loss) in the field for more than 30 years.The superiority of Tedlar®-based solar backsheets is well establishedin field performance while PVDF has only been in use for around7-8 year, and isnot field proven.

Both PVF and PVDF sound similar and PVDF has been marketed as a product that performs like Tedlar® PVF in the field. However, there are many myths/ unknown factsassociated with PVDF material that are clarified below:

Myth 1:PVDF is a fluoropolymer

Fact: Most PVDF films are a blend of up to 30% acrylic (PMMA)polymer and are not composed of “pure”fluoropolymers; hence, they do not provide the benefits of a “pure” fluoropolymer.Tedlar® PVF, on the other hand, is >99% fluoropolymer and provides excellent weather resistance properties associated with fluoropolymers.

Myth 2: PVDF has higher fluorine content than Tedlar® PVF, therefore it is better

Fact: Fluorine atoms in the fluoropolymer do improve UV and chemical resistance but they also reduce its ability to adhere to other materials. As fluorine content increases, the adhesion of the fluoropolymer decreases. For example, DuPont™ Teflon® has very high fluorine content but very low adherability and thus it is used as a non-stick coating for multiple applications. Higher fluorine content in PVDF negatively impacts its adhesion properties.This is undesirable for PV backsheets as it increases the risk of delamination. On the contrary, proven field experience of Tedlar® PVF film-based backsheets in different climatic conditions demonstrate that the fluorine content in PVF is sufficient to provide adequate weather resistance and adhesion required for proper functioningof PV backsheets in any service environment.

Myth 3: All PVDF films are same

PVDF films are manufactured by multiple companies and have different compositions (acrylic content, structures) and quality levels. With the emergence of variouslow cost PVDF films in the market, PVDF material quality varies significantly, further increasing risks around PVDF based backsheets whichmight lead to much faster degradation in the field. Tedlar® PVF filmcomes from a singleknown and trusted source, which helps to control quality, uniformity and key properties of the material.

PVDF films can have very weak initial mechanical properties and the level varies with manufacturer. More importantly, the mechanical properties degrade very quickly under environmental stresses(addition of acrylic weakens ability of PVDF to withstand these stresses). PVDF can have very low elongation (stretch-ability) along transverse direction (TD) (figure 1).

When exposed to environmental stresses (damp heat, increased temperature, temperature cycles and UV exposure),the TD elongation drops to almost zero(figure 2& figure 3) for 7 different commercial PVDF films tested.This makes PVDF film brittle and prone to cracking in the field. This inherent weakness of PVDF + acrylic film cracking along transverse direction has also been analyzed and documented by third party research [1].

Tedlar® PVF filmhas significantly superior mechanical properties and more importantly retains them even after long-term exposure to environmental stresses and harsh climatic conditions(figure 2 & figure 3). The impact of weak mechanical properties of PVDF film have been seen in both lab as well as field analysis, where modules made with PVDF-based backsheets show cracks in PVDF layer (figure 4 & figure 5) along transverse direction.

Figure 1: Elongation of PVF and PVDF films

Conclusion

Fluoropolymer-based backsheets are widely chosen and used as the most reliable and best protection for PV modules. Within the fluoropolymer category, not all backsheets are equal. There are clear differences between Tedlar® PVF and PVDF-based backsheets. Tedlar® PVF-based backsheets are the gold standard for protection with more than 30 years of field proven use. PVDF-based backsheets suffer an inherent weakness in loss of mechanical properties and integrity.

In addition, there is large variability from multiple suppliers with different blends, compositions, and structures. The loss in mechanical properties in PVDF-based backsheets has been demonstrated in accelerated testing and is causing field failures. With only 7 years in the field, the concern is that even more failures will happen over time.

Making a prudent choice while selecting PV materials for solar panel backsheet will help mitigating backsheet failure risks and safeguard your investment.

References
1. Hosoda, T. and Yamada, T., “Effect of TiO2 on Morphology and Mechanical Properties of PVDF/PMMA Blend Films Prepared by Melt Casting Process”, J. Appl. Polm. Sci., 40454 (2014) Reprinted with permission. Copyright© 2015 Wiley Periodicals, Inc.

Anand Gupta Editor - EQ Int'l Media Network

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