As varied as today’s backsheet materials are, they all share one thing in common: All types consist of multiple layers that are laminated together. These multi-layers might even be produced in a common so-called coextrusion process and laminated at the same time. Nevertheless, the layers are physically separated across the profile of the backsheet. Combined, these layers serve as the basic functionality of the backsheet, securing proper electrical insulation and blocking moisture ingress. Both functions are vital, because modules operate at high voltages and the metal connection inside is susceptible to corrosion. Backsheets also have an effect on the performance of the module: The higher their reflectivity, the more light is reflected back, and the higher the yield.
In recent years, Agfa Materials has developed an alternative to the industry standard. Its single-layer backsheet, named UNIQOAT, is a mono backsheet that the company says is more of a solution than a product, because it refers to a whole technology in polyester film manufacturing. This technology requires just one single extrusion die for producing a full backsheet film, and at no stage in the mono backsheet production process is more than one layer involved.
Thus, there is no interlayer lamination, and no coextrusion of different PET grades or other enhanced layers. By adopting this approach, Agfa is innovative without compromising the basic characteristics of a typical backsheet required for module stability, the company says, claiming that the product provides hydrolysis resistance and UV-resistance additional to low shrinkage PET and functional surface modifications.
The in-line PET surface modification is carried out during the PET extrusion process – a one-step process – in which all required functionality is added. The material composition is homogenous across the entire profile of the sheet. Cesar Campos, head of the functional foils business unit at Agfa, proudly labels the product “a real monolayer PET backsheet.” In his view, the traditional production technique of multi-layer is more of a “backsheet conversion.” Other products on the market use standard, non-modified PET that is laminated to other PET or fluoropolymer layers and so-called primers at cell side, which add functionality.
Whereas traditional laminated backsheets include a dedicated air-side layer protecting against UV and hydrolysis, the PET in Agfa’s product acts as an air-side layer in its full thickness. During the extrusion, ingredients are added that make the need for the traditional air-side lamination redundant. This makes the mono backsheet, in Agfa’s view, even more durable. In a three-layer backsheet, the dedicated air-side layer of 20 to 50 µm can erode or delaminate and the remaining core layer will offer no protection against UV and hydrolysis. In a mono backsheet, however, the full thickness of the material is hydrolysis and UV-resistant.
An obvious advantage of the mono-layer could be that by comprising a single layer, there is no risk of delamination to the backsheet itself. Therefore, adhesion between the layers of a multi-layer backsheet could be a weak link in a module. The adhesive strength between backsheet and encapsulant varies between 80 and 100 N/cm. “It doesn’t seem to make a lot of sense to require an adhesion between backsheet and EVA of > 100 N/cm, knowing that the adhesion between the layers within the laminated backsheet reaches a mere 4 to 5 N/cm,” says Agfa Materials Marketing Manager Werner Vandevelde.
Furthermore, the reflectivity of the mono-layer backsheet could lead to higher solar generation yields in modules that use these backsheets compared to modules made using standard backsheets. In a white paper, Agfa provides a measurement of the reflectivity of different backsheets that were available in 2016 (TPT, TPE, KPE, PPE, and PO) over the whole wavelength range. According to these measurements, across the entire light-spectrum the Agfa backsheets perform 20% better than the other backsheets chosen by Agfa for comparison. “UNIQOAT’s high reflectivity – varying with different UNIQOAT products – can increase power output by 0.3 to 0.5%,” adds Vandevelde.
Such stated advantages, as always in solar, have to prove their worth during mass production and in the field. How much the efficiency boost can lead to a reduction in LCOE will depend on how production costs of the various backsheets – and the modules they form part of – relate to the expected higher yield. Sraisth
Interview: Werner Vandevelde, Marketing Manager Functional Foils, and Cesar Campos, Head of the Functional Foils Business Unit at Agfa, discuss the advantages of the new mono backsheet.