By Vivek Chaturvedi, Business Leader Endurance Backsheets, DSM Advanced Solar
It is no secret that backsheets in PV modules are not always performing the way you expect. In all climates and in all types of modules, premature degradation can be, to a great extent, attributed to a faulty backsheet. This year’s DuPont analysis indicates that 16% of all modules inspected suffered backsheet failure. Dupont’s data supports previous research on the topic. According to a study by Solar Bankability, 1% of all modules exhibit backsheet failure, and out of all module failures, 9% are linked to the backsheet component.
Putting those percentages in real life application would mean that 1.1 GW of the forecasted 112-GW PV capacity installed in 2020 risks some degree of backsheet failure, which could potentially result in a repair bill of $500 million. It is clear that the widespread backsheet failure phenomenon is turning out to be extremely costly, not only because of the power loss, but mainly due to the accumulating bills for fixing or replacing the PV module.
Backsheet: The unsung hero of module protection
Before delving into analysing the reasons of modern backsheet underperformance, it is important we highlight their role in the solar module. The backsheet is the final layer on the back of a PV module, making it the first line of defense. Despite its role to protect the more fragile units of modules from ultraviolet radiation, moisture, wind, dust, sand and various mechanical impacts, traditional backsheets seem to be unable to provide long-term durability, resulting in underperforming PV modules and lower power outputs.
Adding to the complexity of the situation, while it’s often possible to spot a faulty backsheet in the field, it can be extremely difficult for solar park owners and module manufacturers to look into the ‘black box’ of backsheet failure and pinpoint how and why a fault happened. Usually, the backsheet gradually deteriorates to the point of exposing the core layer to moisture and air — causing corrosion of the electrical components, known as the ultimate common denominator behind every failure which marks the countdown in the PV module’s life expectancy. Common symptoms to look for include cracking, delamination, chalking and mechanical abrasion.
The culprit behind this phenomenon is what is known as “solar coaster.” The industry has been on a turbulent rollercoaster ride economically for more than a decade, where a significant growth in demand and manufacturing capacity has resulted in a tenfold decrease in solar panel prices, mainly due to the use of lesser quality materials and unreliable manufacturing techniques.
But why exactly are backsheets failing?
A weak core: preference for cheap materials such as the low stabilized PET for core layers is one of the key reasons behind backsheet failure. Despite being able to provide good electrical insulation, the PET polymer is extremely sensitive to moisture and sunlight – meaning its use in outdoor applications has huge limitations.
One of the solutions used in backsheet manufacturing is to add thin layers of fluoropolymers like PVF (Tedlar), PVDF or highly stabilized PET to protect the intrinsically weak core. This is considered a cost-effective alternative to stabilizing the full PET core layer, yet it makes its shelf-life dependent on the outer layers only, which real-life applications have shown to be a short-sighted tactic.
Cheaper, less materials: Under continuous cost pressure, manufacturers keep making the outer protective layer thinner and thinner, whilst the PET core remains still vulnerable to moisture. The market has been flooded with backsheets where the fluoropolymer outer film’s thickness is less than 20 microns, while in the past this layer was more than 40 microns thick. In some cases, the outer “protective” layer is as little as 10 microns, making the backsheet prone to bigger and faster damage.
Cost-driven manufacturing practices: Loopholes in the certification process allow for manufacturers to cut corners in production practices like sourcing parts of the bill of materials from different suppliers and laminate them with various adhesives, as these don’t require a re-certification according to the latest regulations. Unsurprisingly, such practices can result in faulty backsheets and, more importantly, in the inability to understand the precise reasons behind every failure.
The lack of transparency in materials and the inability of basic regulatory tests like the IEC especially, to predict long-term performance in the field are pushing for more traceability in sourcing and better-quality materials.
All three factors are obviously linked to the prioritization of cost reduction over robust materials that would guarantee the – previously — typical solar module lifespan of 25 to 30 years. Cheaper PV modules are proving to be a costly choice in the long run, as the constant need for replacement options leads to unplanned downtime, replacement and maintenance costs.
The financial benefit of high-quality materials
Considered in conjunction with other factors such as O&M reserves and appropriate scope, module manufacturing inspections as well as quality and design life of other components among others, it’s reasonable to assume that quality backsheets are likely to prolong the expected 30-year lifespan of PV modules.
Looking back at the initial question of whether the backsheet damages is a problem that can be avoided, the answer is clearly yes. Backsheets have only started massively failing when the prices of solar modules plummeted, and innovations in solar cells were prioritized above backsheet innovations, for which the cheaper quick fix was chosen. Now, with the proof that cheaper initial cost doesn’t necessarily outweigh the cost of sustaining and repairing, performance-focused manufacturers, such as DSM, have been rethinking how to support the industry with long-term reliable backsheets.
Co-extrusion and solar modules
One of the most successful processes to replace traditional lamination and the related cost and quality issues is co-extrusion, a manufacturing process commonly used in materials science applications like multi-layer polymer film processing and thermoplastics used in automotive parts. Despite not being a new process in manufacturing, bringing co-extrusion in the solar industry is an innovative approach to replace the outdated traditional lamination process. It is a single-step process that requires no adhesives, resulting in strongly coherent multi-material products.
Along with co-extrusion, the reinforcement of the core has huge potential too. Having established the relationship between quality materials and quality performance, the answer to the core conundrum lies in the material it is made from. Choosing to use a film made from modified polyolefin (PO) instead of the inferior PET will boost the core’s strength for instance. This solution is specifically designed for superior protection against moisture, excellent electrical insulation, crack resistance and protection against mechanical abrasion and UV – making the backsheet particularly well-suited for extreme environments like floating PV and tropical installations. When used in moderate weather conditions, it doesn’t need any extra protective layer.
For modules that need to operate in extreme weather conditions or that require an extended life, the layer that offers protection would have to be made by a material resistant to UV, abrasion and moisture. Looking more broadly to the automotive, aviation and energy industries, it’s clear that the Polyamide 12 is the ideal material for this type of function.
Furthermore, having complete control and traceability of the backsheet supply chain, is essential to give solar owners and operators peace of mind around their backsheet performance.
Moreover, a transparent supply chain gives visibility and credibility to sustainability credentials. The industry needs effective products that contain sustainable material and that can be economically recycled with a lower carbon footprint. DSM’s backsheet, for example, is fluorine-free with up to a 30% lower carbon footprint than traditional equivalents — and 100% recyclable with no production waste.
At DSM quality is not an alternative route, it’s a constant aspiration. Fifty years of experience and technical knowledge in polymers have led to a deep understanding of the industry, allowing us to leverage our resources to create cutting edge technology, environmentally friendly and efficient solar products. Products like the awarded DSM Endurance backsheet, which tackles the issue of failure using a co-extrusion process in manufacturing and the Polyamide 12 extra layer, are great examples of DSM’s leadership in solar innovation.