Optimal Interconnection Levels for Solar Modules Thanks to Optimized Quality Control

Currently, manufacturers of photovoltaic modules offer a performance guarantee of 25 years on 80 percent of the initial output. This reliability and service life can only be achieved if the encapsulation of the solar cells is perfectly processed, thus ensuring protection against external influences. "In places where encapsulation films are insufficiently cross-linked, delamination can occur over time, which impairs the service life of the module. Flawless processing is not always possible in production, which means that anomalies can lead to a drop in the performance of a solar module. This violates the product and performance guarantee, resulting in high module replacement costs," says Dr. Anton Mordvinkin, who heads the project at Fraunhofer CSP.

To prevent this, the solar cell array within a solar module is fixed in place and enclosed as tightly as possible to achieve maximum protection. For this encapsulation, the solar cells are enclosed in a plastic. Currently, ethylene vinyl acetate copolymer (EVA) is mainly used for this purpose. EVA is transparent, has rubber-like flexibility, good tear resistance and aging resistance, as well as good insulation and barrier properties. Depending on how it is modified, the material is also used, for example, as packaging material in households, as granulate in toothpaste, or in elastic midsoles for shoes. In solar module production, the two EVA films are heated in stages during a lamination process so that the EVA first melts, flows into the cavities between the cells, fills them, and then cross-links. This ensures long-term structural stability. The degree of cross-linking of an encapsulation film indicates how well this process has been carried out and can be characterized, whereby faulty lamination can significantly reduce the service life of modules.

Currently, too little is known in the industry about the relationship between encapsulation lamination and module lifetime, especially regarding the local distribution of the degree of cross-linking. The degree of cross-linking of EVA is insufficiently controlled in terms of its importance for module reliability. There is still no industry-wide accepted, non-destructive testing technology on the market. The only approach currently available is the X Link system from LayTec. This is where the partners are picking up in the three-year “EVAplus” project. Fraunhofer CSP is collaborating with Laytec AG, Sunset Energietechnik GmbH, ABO-Wind AG, PI Berlin AG, and Anhalt University of Applied Sciences to increase supply reliability, system performance, and productivity through lifetime-relevant investigations of the stability of photovoltaic module encapsulation. The existing approach will be further researched with the aim of quality control for encapsulation, which is expected to reduce encapsulation anomalies in the field by 20 percent and increase a module manufacturer's production yield by 2 percent. The latter is to be achieved by increasing speed through optimized lamination.

Fraunhofer CSP is contributing its expertise in polymer analysis and polymer evaluation and is working on questions that contribute to understanding material changes under realistic operating conditions, investigating the chemical and micromechanical properties of materials, and modeling the environmental stress-induced aging of laminates as well as the risk assessment of material and process parameter variations. "We use data from field returns and from climate chamber and weathering tests on specially manufactured test specimens. This enables us to develop a non-destructive measurement technology for determining the degree of cross-linking and other polymer film parameters, which also represents an Industry 4.0 approach to field-reliable lamination. However, this requires results-driven lamination instead of the time- and temperature-controlled process control currently in use. Ultimately, we want to be able to make better statements about the ideal degree of cross-linking for module reliability," adds Dr. Mordvinkin.