Methods used to address bottlenecks in the lamination process include the use of larger area laminators, multiple parallel laminators, stacked laminators (multiple lamination chambers in a vertical stack) and multi-stage laminators where the process steps are divided into different processing units.
Photovoltaic (PV) modules need to withstand harsh outdoor exposure in various climates for a long time (25 years or more) to convert sunlight into electricity at a reasonable cost. One of the keys to module longevity is the lamination process, which encapsulates the solar module laminator while attaching front and rear protective sheets. The materials, process technology and equipment described in this paper have been proven through more than 20 years of real-world field experience with crystalline silicon and thin-film modules.
Ethylene-vinyl acetate (EVA)-based sheets have been the industry standard encapsulation material since the 1980s, although these materials have improved significantly over time. The base EVA is used in the sheet extrusion process in combination with a variety of additives, including curing agents, UV stabilizers, antioxidants and primers for glass bonding. The additive package represents a few per cent of the final material. Early problems with EVA yellowing under UV and heat exposure were addressed by replacing the original curing agent with a more stable alternative, while faster reacting curing agents were developed to reduce process cycle times and increase the productivity of the laminating equipment.
Thermoplastic sealants such as polyvinyl butyral (PVB) and thermoplastic polyurethane (TPU) are also available in sheet form. Although these materials do not require curing, their melting points and viscosities are higher than those of EVA, so lamination process times are often similar to those of EVA.
Because the sealants are in sheet form, modular laminates can be easily assembled in layers for processing in a vacuum laminator. The most common module construction uses tempered low-iron glass as the transparent front structural member or cladding, followed by a layer of EVA, interconnected solar cell module laminates, another layer of EVA, and a UV-resistant plastic film as the backing, as shown in Figure 1. An optional thin non-woven fibreglass panel can be placed behind the cell to help remove air and prevent cell movement as the EVA melts and flows during the lamination process.
Other module designs include double-sided glass, which uses glass as the front and rear panels, and flexible, which uses flexible film as the front and rear panels. The former design is suitable for thin-film applications (CdTe and CIGS) that require an excellent moisture barrier to prevent film degradation, while the latter design is used where lightweight portable modules are required, as well as rooftop applications where modules are provided on the roof. Structural support.
The lamination process involves pulling air out of the module layers in a vacuum chamber, heating the layers to melt the sealant, and then pressing the layers together with a flexible diaphragm to embed the cells in the sealant and adhere to the front and back plates. Eva must withstand a temperature/time profile to obtain a minimum cure level of 80% for long-term module reliability. The lamination process is determined by visual inspection for voids, air bubbles, backplate folds and other defects; peel force measurements to determine the adhesion of the sealant to the individual layers in the laminate, and gel content testing to measure the amount of EVA cross-linking. Gel content testing is performed by extracting cured EVA from a sample module, weighing it, and immersing it in a hot solvent (toluene) to dissolve the uncured portion of the material. Less accurate but faster gel testing methods include thermal creep measurements and differential scanning calorimetry (DSC).
Module lamination is a critical process step that directly affects module reliability and longevity because it provides a weather barrier that protects solar module laminators from the environment. Sheet sealants allow for simple assembly of various module designs (glass laminate, double glazing and flexible) while providing good sealant thickness control with little to no material waste. Process control measures, such as peel force measurements and gel content testing, are critical to maintaining the quality of the components in production. While a variety of strategies can be used to alleviate bottlenecks in high-volume production lamination processes, the PV industry will benefit from the availability of faster curing sealants.