Lightweighting vehicle-integrated photovoltaic modules

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To extend the driving range of electric vehicles, research groups are looking at integrating solar cells on the vehicle roof, the hood, the trunk and side body panels. But such vehicle-integrated photovoltaic (VIPV) applications will need new lighter-weight modules based on a deep understanding of potential failure mechanisms, and resilient module designs.

A European research team has reported some progress towards that goal after investigating interconnection and encapsulation strategies to improve  VIPV module reliability against damp heat and mechanical impacts.

“In developing lightweight PV modules with high reliability, mechanical properties of the bill-of-materials used were found to be crucial. We screened various composites and found carbon-fiber reinforced plastics to be promising,” the research's corresponding author, Bin Luo, told pv magazine. “The most challenging aspect was to understand the different degradation/failure mechanisms when using different materials.”

The team fabricated lightweight mini modules, weighing around 3.45 kg/m2. The backsheets selected were reinforced with either glass-fiber polypropylene or carbon-fiber polypropylene. The back encapsulant was made of polyolefin elastomer (POE) reinforced with randomly oriented short glass fibers in an 8.2 % weight ratio. Multi-wire connected heterojunction cell strings, a thermoplastic polyolefin (TPO) contact foil, a front encapsulant made of polyolefin elastomer (POE), and a polyethylene terephthalate front sheet completed the devices' structure.

The team then conducted damp heat and mechanical impact tests, which revealed failures, such as cracks in the solar cell, module delamination, and microcracks in the backsheets. These were then analyzed to arrive at a new bill of materials that included fiber reinforcement in the backsheet and thicker front encapsulant film.

Using a backsheet with a higher bending stiffness can reduce global bending, which reduces shear stresses and, therefore, could prevent cell cracks, explained the team. As for the damp heat resilience, which depends on the water vapor permeability of materials in the module stack, the team addressed it from the front and back.

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The result was a design that included carbon-fiber polyamide backsheets and new low-moisture transmission layers in the module. The design decisions increased the weight to 5.21 kg/m2 and raised the cost, but the strategy enabled increased resilience to the earlier observed failure modes.

In the discussion about using non-traditional materials to provide module protection and reliability, the team noted the tradeoffs between weight and cost. “Further testing and cost optimization are required to bring this to commercialization,” said Luo.

The study is described in “Encapsulation strategies for mechanical impact and damp heat reliability improvement of lightweight photovoltaic modules towards vehicle-integrated applications,” published in Solar energy materials and solar cells. The researchers came from Belgium’s KU Leuven, IMEC, Hasselt University, and EnergyVille, plus Switzerland’s Ecole Polytechnique Fédérale de Lausanne (EPFL), and the Austrian Research Institute for Chemistry and Technology.

Looking ahead at future research topics, the team sees a need for combined reliability tests, including hail impact, thermal cycling, and damp heat tests, in conjunction with a statistical analysis. More testing of curved lightweight PV modules in a larger size is also required.

Luo expanded on the future research outlook when he said, “We want to build further on our understanding of the failure mechanisms in different lightweight PV structures and the link to material properties under different stressing conditions that are relevant for PV in general but specifically also for different integrated PV applications.”

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