Enduring to the end

As PV electricity generation closes the gap on grid parity in more regions worldwide, demand for large power generation projects in the utility and commercial markets is growing. Such large-scale projects often require project financing and access to capital markets to raise cash. Modules must guarantee performance for 25 years minimum to ensure banks, financiers and insurers that projects are bankable.
Module components in the form of backsheets, rightly, have come under increasing scrutiny of late. Broadly their role is to act as a skin, protecting the module’s cells from mechanical degradation and minimizing moisture penetration and damage from UV light. Backsheets also provide other critical functions, including electrical insulation at system voltage levels, maximizing emissivity as well as assisting with heat dissipation as temperature increases can impact a module’s ability to generate power.
Today’s leading backsheet manufacturers are from across the globe. According to rankings by ENF Solar, led by 3M in the U.S., the other top ten manufacturers include Agfa-Gevaert in Belgium, Alrack in the Netherlands, Aluminium Féron in Germany, followed by Chinese manufacturers Anhui Green Cosmotec Photoelectronics and Anhui JDPV New Material Technology. But of the hundred listed by ENF Solar, more than half 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. Therankings also tell an interesting story in terms of material constructions that have become more favored in recent years, showing how polyester – a highly transparent and cost-effective plastic film – has become a much more prevalent material in backsheet construction.
To meet requirements for 25+ year module warranties, but also to improve overall performance of the backsheet in terms of other functions in a more cost-effective offering, backsheet manufacturers have moved away from the standard formation of polyethylene terephthalate (PET) sandwiched between polyvinyl fluoride (PVF) film, known widely by DuPont’s branding, Tedlar. One such manufacturer is Isovoltaic, a long-running supplier to the industry that was one of the first companies to produce backsheets based on a three-layer construct of PET sandwiched between two layers of Tedlar, known as TPT.
Traditionally in backsheet manufacturing, polyester or PET film is required for electric insulation, while PVF film weathers exceptionally well. PVF remains very stable under UV light by not bubbling, coloring or cracking. Several manufacturers besides Isovoltaic, including Aluminium Féron as well as Taiwanese and Chinese backsheet producers such as Bocai New Energy, Fenghua Plastic Science and Taiflex Scientific, still use the TPT construct to produce highly durable backsheets for the PV industry.
In addition to its TPT and also its polyamide/PET/polyamide (APA) backsheet ranges, Isovoltaic introduced in early 2014 the latest version of its Tedlar/PET/polyamide (TPA) construct. The cell side has the high reflectivity polyamide (PA) film, additionally stabilized against UV radiation, oxidation and chemicals. Replacing the Tedlar layer nearest the cells with polyamide also results in a more cost-effective backsheet as well as reflectivity that is 15 – 20% higher than TPT versions. The outer layer retains the Tedlar for high UV stability and weathering resistance, explains Harald Lackner, Vice President of Sales and Marketing at Isovoltaic. The company developed the TPA HR backsheet based on data from a large power plant project in China, where sandstorms are very prevalent. “As we are facing grid parity, this emphasis on ‘bankability’ is growing. Investors are looking for products that have been proven to work out in the field for many years,” he said.
Indeed, it was in the early 1980s that TPT backsheets were initially used in a module in part of a terrestrial PV study by NASA, as part of a project to develop modules with a 30 year lifetime, and the glass/EVA/TPT-based backsheet as the standard, safe, reliable PV module construction was born.
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Quality issues

DuPont more recently has taken an aggressive stance on poor quality PV modules that the company believes have become more prevalent in installations that have occurred in recent years. “Modules installed and operational in the last five years are where we have seen the most issues with quality, especially around workmanship as well as the use of un-vetted and unproven products and materials,” says Conrad Burke at the company. The slip in quality DuPont is drawing attention to correlates with the tail-end of the biggest boom the PV industry has experienced in its history and also follows on from DuPont’s own shortages in production capacity for what had by that time become the dominant backsheet material, forcing many manufacturers to search out new contender materials, such as polytetrafluoroethylene (PTFE), polyamides and PET films engineered to be UV-resistant and durable compared with earlier off-the-shelf polyester films. Many resulting constructs either eschewed PVF completely or reduced PVF from two to one layer.
Regardless of the location, whether Spain, U.S. or Asia, poor quality has been in evidence in PV module installations of recent years, while DuPont has found that the best performing modules are generally the oldest ones in the field, according to Burke. As a further preemptive step to reverse the decline in quality, which included compiling a database of the results of backsheets tested from modules in the field, DuPont embarked on educating the downstream sections of the PV industry, including installers and project developers as well as financiers and insurers, at major solar shows, drawing a correlation between poorer quality, untried and tested backsheets and their impact on the economics of PV installations.
“Module makers were very receptive, but were facing relentless pressure from the downstream industry, so we knew that in speaking with the downstream part of the industry we could begin to instigate this refocus on quality.” Several tier-1 PV module manufacturers use backsheets with DuPont’s materials. One is Yingli, the world’s largest PV manufacturer, which in 2013 signed a new one-year $100 million strategic agreement with DuPont, an expansion on a previous $100 million supply agreement signed between the two in 2012.
Under the terms of the agreement, Yingli will buy materials including DuPont’s Tedlar film as well as the installation of a solar energy plant at a DuPont facility in China using panels made with advanced materials from DuPont, with potential to expand installation of similar plants to additional DuPont sites or other commercial projects in China. Just before the end of 2013 DuPont’s Tedlar film passed a stringent internal quality standard of Sharp’s. GD Solar in China is another company using DuPont’s materials in its backsheets. To ensure it can meet demand from PV customers DuPont recently invested in a Tedlar production site in Ohio, doubling capacity for making films and backsheets.

Quality control

There are few other industries that require their products to last over 20 years. The backsheet, which acts as the module’s protective layer, is exposed to the elements and electrical current, so is therefore a potential source of module failure that arguably requires exhaustive quality control. Much of the backsheet industry is proactively addressing this issue. Dunmore Corporation, for example, is developing a new testing procedure designed to test the resilience of PV backsheets. Many manufacturers, Isovoltaic included, put their backsheets through three times the amount of required damp heat testing for ensuring the durability of laminating adhesives. The objective is to simulate above and beyond the worst environmental conditions for the constructs in order to guarantee performance, reliability and 25 or more years of durability of modules.
Both DuPont as well as Isovoltaic are also pushing for tougher standards. DuPont, in a technical seminar, has recommended doses of 171 to 275 KWh of UV tests on backsheets, which is over 18 times TÜV’s requirement for the IEC 61215 standard. Isovoltaic wants to see standards that focus on the mechanical properties of the best performing materials after aging for many years out in the field.
But there are concerns from other quarters with regards to this approach. For instance, if standards are changed to emphasize materials proven in the field, how this might affect innovation within the backsheet industry is a concern. It would imply that only certain, field-proven materials can be used to make backsheets that are truly durable and, therefore, of the highest quality. What about alternative materials that meet all of the performance requirements yet do not have 30 years of data gathered from modules in the field to reference? As the costs of cells have come down, other module component manufactures, including backsheet producers, have also had to bring down the cost of their products, even though backsheets only account for around 3 – 5% of the total cost of a module.
TPT is the most expensive construct due to the cost of Tedlar. Shortages in the material, combined with pressure to make more cost-effective backsheets, helped spark a period of backsheet material innovation, especially as more cost-effective non-fluoropolymers, including PET engineered for high UV and moisture-resistance, were used. Over the past six years, Tedlar’s share in backsheets based on fluoropolymers has more than halved and has only recently begun to see an increase in demand again.
Coveme was one of the first companies to introduce non-fluoropolymer backsheets. The company’s dyMat PYE is made of two layers of high-grade PET laminated with special adhesives, each component developed for high levels of hydrolysis and UV resistance, while the laminate is finished off with a primer for enhanced adhesion to encapsulants such as EVA. Since Coveme launched dyMat PYE in 2008, several other manufacturers have begun producing PET-based backsheets, including Dunmore Corporation, Flexcon and Micel.

Monolayer backsheets

However, while a range of PET-based backsheets have been launched over the past few years, in terms of innovation, there has been little in terms of breaking away from offering a variation of the common standard of PET/PET/primer, with different properties enabled by varying backsheet thicknesses. The first generation of PET backsheets, which came out of Japan, comprised of a white PET layer and a primer layer sandwiching a standard PET layer. Innovation in the past four years has yielded a three-layer construct, again, based on a primer polyolefin layer, hydrolysis-resistant PET film in the middle and a white, hydrolysis-resistant and UV-stable PET film.
“Backsheet manufacturers add value through laminating different layers together, a process known as converting,” explains Cesar Campos at Italian backsheet maker Filmcutter. His company’s newest PET backsheet is a monolayer construct based on a polyolefin primer and a single layer of white, hydrolysis-resistant and UV-stable PET film. “At Filmcutter we identified the most important characteristics of the backsheet and also the weaknesses of the available configurations in order to develop a monolayer high-grade PET backsheet. We retain the performance, functionality, long-term resistance, but we have also eliminated the weak link in the current backsheet configurations, which is the adhesive layer,” Campos explains. “The adhesive layer has to hold the PET and primer layers intact for 25 years at least and be defect-free to avoid failure under partial discharge voltage (PDV). If the backsheet does not require the adhesive then you remove this potential weak link.” The monolayer PET backsheet, developed in conjunction with Japanese PET film manufacturer Toray, further pushes the price-to-quality ratio that conventional PET backsheets are able to achieve. TPT-based backsheets are the most expensive, typically costing in the region of €5 – 6/m² ($6.87 – 8.25), while three-layer PET backsheets cost about €3/m² ($4.12). The monolayer PET backsheet reduces this cost further to €2/m² ($2.75), which Campos believes the PV industry will appreciate as part of a relentless focus on reducing costs in all parts of the module’s construction.
Filmcutter’s monolayer backsheet is currently undergoing qualification and certification, a process which could take three to six months. Based on meetings with customers, Campos says they immediately grasp the value. Following on from existing customers, Filmcutter will promote its monolayer PET backsheet to tier-1 module manufacturers that are already using PET backsheets, before targeting other tier-1 manufacturers that use other backsheet materials. Filmcutter has been attending meetings with customers and PV manufacturers along with Toray as both companies help promote each other. This helps to ensure that Toray can further open up a downstream market for its high-grade PET films, while Filmcutter relies on Toray’s presence to show how critical PET is in an adhesive-free monolayer backsheet technology. “Filmcutter is a simple converter company, while Toray is one of the world’s leading PET manufacturers,” he says. By having Toray at meetings helps show the extent to which the PET film industry has invested in this product for PV applications.

Testing

“We are still fighting a misconception in the industry that PET backsheets do not last, that after 25 years in the field they result in bubbling and cracking. This may have been true of the standard PET film used in the mid-1980s, but the high-grade PET used today is a very different product,” says Campos.
He, like others in the backsheet industry, is concerned about how excessive testing of backsheets for specific degradation issues is little more than a marketing ploy. “If you apply doses of 171 to 275 kWh of simulated UV to backsheets, or you go above and beyond three times damp heat tests, what are you showing? These simulations go way beyond realistic weather conditions that would ever occur.” For module manufacturers the most important factor is the power output after 25 years, at the end of the module’s operational lifetime. Having combined tests designed to simulate real-life conditions, for example exposing backsheets to UV, humidity and temperature under electrical load, might be more useful to the industry than applying tests to expound on a specific weatherability characteristic of a backsheet material, such as UV stability, at the expense of other important characteristics that backsheet films also need to have. “Considering a backsheet has other critical functions, why not focus on hydrolysis-resistance to minimize moisture penetration to protect the EVA and cells? What about the electrical insulation function, after hydrolysis of electrical grade material between the PVF layers that compromises electrical insulation at system voltage? What about reflectivity that increases the cell’s efficiency, as PVF is opaque,” he says.
This is a pivotal time for the PV industry and also its suppliers, as the focus on performance and quality becomes more acute. In the backsheet industry a once-dominant material in the form of PVF has seen demand fall, partly at the expense of a more economical and increasingly high-performance product, in the form of PET.