However, recent developments in module technology may have created this very opportunity. The use of another class of materials, polyolefins (POEs), has increased rapidly in recent years. IHS Markit put the market share of POE encapsulants at 20% last year and almost doubled from 11% in 2017. This share is expected to reach 30% by the end of 2022.
“The main drivers of POE growth will be the sharp rise in bifacial glass-glass technology (both p-type and n-type) and competitive prices achieved through increasing scaling in manufacturing,” says Karl Melkonyan, Senior Research Analyst at IHS Markit. “While EVA will continue to make up the majority of the market, it is not recommended to use it for bifacial technology or others like n-type and thin films.”
The growth of bifacial modules, most of which use glass front and back, presents encapsulation materials with multiple challenges. Despite estimates that they are at least 30% more expensive than EVA, module manufacturers are turning to POE-based encapsulants as the most attractive solution. “EVA naturally has its limitations, especially when a lifespan of 30 or 40 years is taken into account,” explains George Touloupas, Director of Technology and Quality at CEA. “In principle, POE is a better material and more stable than EVA. With the enlargement of the bifacial modules, their share has grown considerably. “
Over time, EVA decomposes in field conditions and produces acetic acid. In a glass backsheet stack, this acid can penetrate the backsheet and exit the module without causing damage. In the case of glass-to-glass modules, however, acetic acid is trapped and can build up, which ultimately leads to corrosion and significant loss of performance. This could have motivated some suppliers to investigate POE as an alternative to glass-to-glass modules. Manufacturers of transparent backsheet materials also cite the “breathability” of their backsheets and the prevention of the formation of acetic acid as an advantage for their products over glass-glass in bifacial modules.
However, the EVA penetration mechanism is powered by moisture, and glass-to-glass modules effectively prevent water ingress. “The absence of water in combination with EVA stabilizer additives should ensure that the glass-glass modules are not excessively corroded by the formation of acetic acid,” explains Touloupas. This has been shown to be sufficient for modules to pass manufacturer-run accelerated tests and third-party quality assurance. And EVA is still used in glass-to-glass lamination, although often in combination with POE encapsulation.
Potential-related degradation
The second challenge for bifacial modules, and the biggest driver of the recent increase in the use of POE encapsulants, is preventing a new potentially induced degradation (PID) mechanism that affects the back of bifacial modules. It is widely believed that a combination of new EVA formulations and frontside cell surface treatments “solved” PID problems in monofacial modules. However, with the recent growth of bifacial technology, a new PID mechanism has been observed, particularly affecting the back of cells (see pv magazine 02/2021, p.44).
Replacing EVA with a POE-based encapsulant is one of the few effective solutions for module manufacturers to secure products against PID risks and related reliability issues. Since the production of the materials in a POE encapsulation is inherently more expensive than that of EVA, and the large modules on the market today require a thicker layer of encapsulation and have to cover a larger area, innovations are already being made that aim at minimizing costs are aligned to be seen.
Some module providers choose to use the tried and tested EVAs for frontside encapsulation and a single POE film between the cells and the rear window as a PID prevention strategy. Encapsulation and backing film manufacturer First Applied Materials, based in Hangzhou, China, has patented a coextruded EVA / POE / EVA encapsulation film with a thin layer of POE in the middle ensuring good PID resistance. The lower cost, yet reliable EVA makes up most of the encapsulation layer, and these coextruded encapsulants are increasingly appearing on module manufacturers' roadmaps.
“Coextruded products not only offer the benefits of quick and easy processing when laminating with EVA, but also retain the strong anti-PID performance of POE,” said Kevin Wu, Overseas Marketing Director at First Applied Materials. “The caveat is that the properties of EVA and POE need to be fully understood and it is difficult to control the uniformity of the thickness of the middle POE layer.”
Process POU
Aside from the added cost of materials, POUs are also kept away from the mainstream by difficulties in processing them. Lower temperature and pressure requirements may be attractive to manufacturers, but long cycle times, a narrow temperature window and low glass adhesion are not. Until these issues are resolved, manufacturers will likely stick to EVA unless absolutely necessary.
Making POE encapsulants as a thermoplastic instead of a crosslinked polymer is one way to potentially simplify the processing of the materials. This is the approach that Austrian manufacturer Borealis prefers in its Quentys BPO range of POE-based encapsulants.
“BPO is a bespoke thermoplastic encapsulation where the adhesion functionality is built directly into the polymer chain as opposed to grafted solutions, resulting in excellent inherent adhesion,” said Alper Muraben, team leader for solar at Borealis. “In addition, due to the high melting temperature and the high viscosity of the polymer, it is no longer necessary for the encapsulant to be crosslinked during module lamination. This greatly simplifies both the resin-film extrusion step and the module lamination process. “
Muraben further explains that the Quentys encapsulant enables shorter lamination cycles compared to other POE materials and also allows processing at lower pressures and temperatures compared to EVA. This in turn reduces the stress on the cells during lamination. Finally, he notes that the high viscosity of the encapsulant minimizes the likelihood that cells or tapes can move during the lamination process – relevant for modules that use non-soldering connection technologies.
First Applied Materials offers both crosslinked and thermoplastic POE products, including a range of encapsulants with Dow Chemical's Enlight series of thermoplastic POE products. The company particularly recommends the connected product for modules that will be installed in high humidity environments and for extended PID resistance. For thermoplastic materials, according to Wu, “the non-crosslinking property can effectively shorten the lamination time of PV modules”. “And this helps improve customer productivity.”
The Italian manufacturer of laminators, TekniSolar, has now developed tools and processes that allow it to work effectively with both encapsulation material and others. “In our experience, the use of one or the other material depends mainly on the specific module technology,” explains Vittore De Leonibus, CEO of TekniSolar. “Thanks to our technology, all types of panels can be laminated with a very short cycle time thanks to the combination of multi-stage curing, fast vacuum system, even heat application and a flat press instead of the usual membrane.”
POE prospects
Bifacial is now a common PV technology and is expected to continue to grow rapidly over the next few years. And this should leave room for POE encapsulants to grow with you.
While EVA is likely to remain responsible for larger market share in the short to medium term, other recent developments in module technology suggest that a slow shift towards POE is underway. Apart from bifacial, the gentler processing by POE means that it is widely used for the lamination of heterojunction and TOPCon modules, the cells of which are very sensitive to elevated temperatures. Increasing module performance levels, as seen with the recent advent of larger wafer modules, could also be good news for POE.
“We assume that as the performance of solar modules increases, POE will become a more profitable option,” says De Leonibus. “In other words, if the solar module produces more, the production costs may exceed the upper limits that have been set for the most common types of solar module in production.”
With the Quentys Encapsulant, Borealis was also able to demonstrate an increase in module performance compared to modules encapsulated with EVA. In tests conducted by the company at a remote location in Abu Dhabi, modules encapsulated with Quentys have shown an energy yield of more than 2% over the reference EVA modules over a period of 12 months. Laboratory tests conducted by Fraunhofer ISE also showed significantly greater performance from the Quentys encapsulated modules, which were shown to lose 5.5% of their original rated output after a tough series of tests for moist heat, UV and thermal cycling , compared to 33.4% for an industry reference EVA. “Another growth path for Quentys is some module manufacturers who want to replace traditional EVA applications with Quentys because of the productivity gains and the associated higher performance retention,” said Muraben.
The backside PID protection has given polyolefins a boost in the encapsulant market. Longer life expectancies, higher power ratings and other developments in module technology mean manufacturers will keep an eye on them. The choice of material is only part of the puzzle, and the quality of the back panel or glass, as well as the lamination process itself, are equally important in ensuring that encapsulation provides good, long-term protection. “The bare truth is that POE is a better material than EVA because of its stability. provided, of course, that the specific formulations are good and the manufacture is correct, ”said CEA's George Touloupas. “POE needs to be at the same cost and ease of manufacture as EVA to completely replace it. This is not foreseen in the foreseeable future, at least not in the short term. “