Backsheets: From Conception to Finished Products
Mention solar panels or solar arrays to the average citizen, and thoughts of long, sunny days and moderate to high temperatures might enter their mind. Perhaps some might even envision a semi-tropical locale with gentle breezes wafting by. Most (if not all) solar panel installers wish that type of location was the reality. In truth, however, while sunlight is the prime factor in placing solar panels in order to maximize their usefulness as energy receptors, solar installation sites are not always warm and comfortable.
In many instances, solar installations are positioned in harsh climates and difficult locations. Because of this, all the non-solar-cell components have to be tested and found able to withstand conditions ranging from deep frosts to extreme heat, as well as high winds, ice, snow, sleet, rain, and any other type of potentially damaging moisture—even debris impingement. The various elements that make up the finished module must prove to be consistently able to perform, whether in an industrial array or for consumer use.
SOLAR MODULE COMPONENTS
Films and adhesives can play a critical role in the general construction and performance of solar modules (see Figure 1). A premask, or protective film, is applied to an anti-reflective glass panel. This premask protects against scuffing and scratching during assembly, transport, and installation. Since silicon, the primary component of solar cells, is very reflective, it can dissipate photons before they can be harnessed and converted into useable energy. Therefore, the panel of anti-reflective coated glass is important.
Solar cells are arranged in a pattern, usually forming a square or rectangle. They are held in place by a double-coated pressure-sensitive film. Once the cells are arranged and secured, they are placed in a frame, and a backsheet is applied to the underside. A junction box is then affixed to the solar panel to allow for connectivity with other panels.
The backsheet consists of multiple layers of barrier films and adhesives (see Figure 2). The construction is instrumental in protecting the solar panel from moisture, UV exposure and other environmental threats. Backsheets also improve solar module efficiency by helping to reduce partial discharge. Most constructions engineered for back laminates consist of combinations of films chosen for the different properties they exhibit. Examples of standard films include: polyester (PET) or ethylene vinyl acetate (EVA) and Tedlar® and Kynar®. Combinations can range from Tedlar/PET/Tedlar (TPT), Kynar/PET/Kynar (KPK) to PET/PET/EVA (PPE) or EVA/PET/EVA (EPE). Combinations can also include TPE (Tedlar/PET/EVA) and KPE (Kynar/PET/EVA), depending on what is needed for maximum efficiency and optimum performance.
Film constructions consisting of three layers are primarily used for photovoltaic backsheets and have been used in one form or another since the early 1960s, when Dupont’s Tedlar (polyvinyl fluoride) was the film of choice. Tedlar was first developed as a residential siding material, where it was shown to be able to withstand all of the elemental challenges of the outdoors and quickly became the “go-to” film for backsheet applications (along with a 25-year warranty). The evolution of these backsheet constructions started with a film sandwich, so to speak, consisting of two layers of the Tedlar material with a layer of PET in between. While it was thought that two layers were better than one, it also proved to be more difficult to engineer and more expensive to produce.
Eventually, fluoropolymer and non-fluoropolymer materials were tested and found to be highly effective in both single and double fluoropolymer constructions. Arkema offers a fluoropolymer called Kynar that offers equal performance to the increasingly-hard-to-obtain Tedlar. While Kynar has been providing surface protection with impressive longevity for over 40 years, it rapidly proved a popular alternative in both single- and double-layer constructions.
Higher demand for backsheets means an increase in the demand for higher performing, less costly materials. Some of the available films include:
- Double fluoropolymer—Generally constructed of two layers of either Tedlar or two layers of Kynar, it not only tops the performance charts, but the price list as well. As mentioned, both Tedlar and Kynar are similar in performance, while Kynar is more readily available.
- Single fluoropolymer—The construction choice of most major applications, it consists generally of one layer of either Tedlar or Kynar and still provides excellent durability and performance.
- Non-fluoropolymer—Cost-effective and durable, it is able to support varying warranties and is the affordable option to fluoropolymer. It is quickly gaining in popularity as manufacturers seek more economical alternatives.
TESTING AND CERTIFICATION
The films that are used for backsheets must offer protection against UV and moisture, durability, and the ability to provide electrical insulation. But the testing for these materials should go well beyond just these capabilities. Since solar panels and arrays can be installed almost anywhere in the world, strict testing protocols and adherence to a wide range of certification standards are crucial in an industry where non-compliance claims can irreparably damage the reputations and profitability of solar panel manufacturers. Certifications such as ISO and compliance with UL, IEC, and TUV quality standards usually mean that suppliers have established testing protocols to ensure the quality of their backsheet components.
Testing should consider the material’s direct support of live parts, its function as an outer barrier, enclosure protection and its role as a protective barrier. Some of the testing measures standards ranging from the comparative tracking index (measures the rates of resistance of an insulating material to electrical breakdown, which could cause a potentially hazardous conductive path on the surface) to high current arc ignition (determines the likelihood of a power surge by simulating a loose connection or broken lead). A partial discharge test can also verify that the backsheet is engineered with insulation properties to suit the voltage requirements of the module and UV resistance, thereby ensuring the appropriate resistance to UV degradation if the end use requires the backsheet be directly exposed to sunlight.
FINDING A SUPPLIER
Competent, trusted and well-established suppliers can provide the correct film constructions and adhesive combinations to make sure all aspects of solar module assembly meet end users’ requirements. With a fast-growing industry such as solar module construction and installation, it is best to be wary of the “supplier du jour,” who may not measure up to the standards required and the service and technical support that may be needed.
Solar manufacturers should look for a company that functions well beyond the confines of just sales and encompasses efficient, quality service and knowledgeable and dependable technical assistance. How long have they been in business? How would they find a solution to performance challenges, bonding or weather-related issues? Suppliers should have the ability to develop materials according to specifications and then test and deliver those products to optimize performance and minimize lost production hours and costs.
The availability of other related solar module products is another consideration when looking into a viable supplier. The concept of a one-stop shopping partner is a plus when the correct adhesives and other bonding, mounting and labeling materials and supplies are available, along with the technical expertise offered to make sure the correct components are being used.
Polymeric coated films can be ideal for many applications on a solar module. In addition to backsheets, high-performance adhesives in the form of transfer tapes and double-coated films can be used for cable, laminate and junction box fixation, along with cell placement. Durable film constructions provide brand identification and tracking label solutions.
The challenges faced by solar module designers and production staff are many and varied. It is important to find the right connections for all the elements that go into the finished product. Developing a relationship with a company that has a reliable, knowledgeable staff with quality, tested products is paramount in avoiding any unwanted production gaps or productivity lapses.
A proactive partnership should exist between all parties in helping to create viable product solutions to a variety of specifications, no matter how exacting. Working toward the development of next-generation products is an important factor in keeping any solar manufacturer’s products on the leading edge of the industry.
For additional information, contact FLEXcon at 1 FLEXcon Industrial Park, Spencer, MA 01562; phone (508) 885-8455; or visit www.flexcon.com/pv.
Note: Tedlar® is a registered trademark of E. I. DuPont Co. Kynar® is a registered trade name of Arkema.